DEEP SEA DRILLING PROJECT LEG 761 · 2007. 5. 2. · DEEP SEA DRILLING PROJECT LEG 761 Felix M. Gradstein, Geological Survey of Canada, Bedford Institute of Oceanography, Dartmouth,

44. ON THE JURASSIC ATLANTIC OCEAN AND A SYNTHESIS OF RESULTS OFDEEP SEA DRILLING PROJECT LEG 761

Felix M. Gradstein, Geological Survey of Canada, Bedford Institute of Oceanography, Dartmouth, Nova Scotia,Canada, B2Y 4A2

andRobert E. Sheridan, Department of Geology, University of Delaware, Newark, Delaware

INTRODUCTIONIn this Initial Report of the Deep Sea Drilling Proj-

ect, detailed studies of Sites 533 (gas hydrates) on theBlake Outer Ridge and 534 (oldest ocean history) in theBlake-Bahama Basin have provided answers to manygeological and geophysical questions posed over the de-cade that deep drilling has been undertaken in this partof the western North Atlantic. The history of drillingand a historical review of key scientific accomplish-ments have been presented in the Introduction (Grad-stein and Sheridan, this volume). In this final chapterwe review highlights of new geological, geophysical andpaleoceanographic interpretations presented in this vol-ume, and offer a critical review of this information. Weconclude with a listing of some outstanding problemsand recommendations for future research, includingdata collection.

The following subjects are addressed: (1) summary ofSites 533 and 534—litho-, bio-, magneto-, and seis-mostratigraphic interpretations (Figs. 1-3); (2) the questfor the oldest oceanic sediments (Fig. 4); (3) the age ofbasement at Site 534; (4) the Jurassic time scale and astratigraphic synthesis chart for the western North At-lantic (Figs. 5, 6); (5) implications for early spreadingrates and the age of the Blake Spur Marine MagneticAnomaly (BSMA) (Fig. 7). (6) implication of the age ofthe Blake Spur Marine Magnetic Anomaly (BSMA) tothe North American margin; (7) pulsation tectonics; (8)160 m.y. of subsidence and sedimentation and the con-troversy around the Cenomanian-Miocene gap in theocean record (Figs. 9, 10); (8) sediment accumulation atSite 391 and at Site 534, as well as Late Cretaceous toPaleogene deposition and erosion, or nondeposition inthe Blake-Bahama Basin; (9) Jurassic paleogeographyand paleocirculation (Fig. lla-c); (10) black shales de-positional models and hydrocarbon source potential(Fig. 12); (11) seismic stratigraphy of the Blake-BahamaBasin; (12) origin of the mass flow deposits of the Mio-cene Great Abaco Member and the Eleuthera Fan com-plex; (13) gas hydrates on the Blake Outer Ridge; (14)heat flow in the Blake-Bahama Basin; (15) contouriteformation of the Blake Outer Ridge; and (16) recom-mendations for future drilling and research.

SITES 533 AND 534—SUMMARY OFACCOMPLISHMENTS

Hydraulic piston coring, rotary coring, and deploy-ment of the heat flow-pore water sample and pressurecore (PCB) barrel to a depth of 399.5 m at Site 533(Figs. 1, 2) on the Blake Outer Ridge has given us:

(1) A stratigraphically continuous record of middlePliocene-Holocene gray green mudstones used by Moul-lade (this volume) and Blanc-Vernet (this volume) toevaluate sedimentation rate, climatic fluctuations, anddeep circulation.

(2) A temperature gradient of 5.1°C/100 m near theseafloor to 3.6°C/100 m at the bottom of the hole,which agrees with the prediction that the strong bottomsimulating reflector at 0.60 s is the result of gas hydrateinversion.

(3) Direct observation of a few thin beds (3.5 cm) ofgas hydrate at 240 m sub-bottom, confirming that gashydrates exist in the Blake Outer Ridge sediments, but

Sheridan, R. E., Gradstein, F. M., et al., Init. Repts. DSDP, 76: Washington (U.S.Govt. Printing Office).

Figure 1. Location of Site 533, on the Blake Ridge, and Sites 534 and391, in the Jurassic Marine Magnetic Quiet Zone relative to themarine magnetic Anomalies M-25 and the Blake Spur. (BSMA= Blake Spur Magnetic Anomaly and ECMA = East Coast Mag-netic Anomaly.)

913

F. M. GRADSTEIN, R. E. SHERIDAN

possibly only in small lenses. There is at present no satis-factory chemical model with which to relate the lowmethane generation in Site 533 to the gas hydrate pres-ence, which requires higher CH4 concentrations thanwere observed (Claypool and Threlkeld, this volume).There is uncertainty about how the amounts of gas hy-drates present in the hydrated sediments produce theacoustic properties of slightly higher seismic velocitiesand apparent transparency.

(4) Indirect evidence of the presence of gas hydratethat comes from PCB retrievals in which the pressure de-cline survey shows the sawtooth pattern characteristicof hydrate decomposition (Kvenvolden and Barnard, thisvolume; Brooks et al., this volume).

(5) Molecular and isotopic ratios of gas samples fromthe hydrates and hydrated sediments that indicate a lo-cal biogenic source for the methane gas, the predomi-nant gas constituent. There appears no migration of gasfrom deeper horizons, derived from thermal maturation.

(6) A lack of obvious current-derived visual struc-tures in the calcareous clay and mud makes it difficult toidentify the Blake Outer Ridge sediments as contourites(Site 533 report, this volume).

Figure 2 (also in the Site 533 report) shows the compo-site core, biostratigraphic, and geochemical and physicalproperties record at Site 533.

The composite stratigraphy of Site 534 is shown inFigure 3. This site, in the Blake-Bahama Basin, was con-tinuously cored from 545 to 1666 m sub-bottom (56%recovery). The sediment record spans 160 m.y. of oceanhistory and cored the oldest (middle Callovian) oceanicsediments yet recovered. The Site 534 report and a largenumber of specialty chapters in this volume deal withthis excellent geological and geophysical record and in-clude, among other data:

(1) A Miocene record of extensive debris flow, grainflow, and turbidite deposition that can be traced to partof a deep-sea fan complex, funneling out of the BahamaCanyons.

(2) Evidence for hiatuses and nondeposition in theLate Cretaceous as well as in the Paleogene. Slow depo-sition coupled with nondeposition, rather than rapid de-position followed by drastic erosion in a single Oligo-cene event, may have caused the 60-m.y. gap in the rec-ord.

(3) A complete section of Cretaceous black shale thathas been more closely sampled and better dated than inany previous studies.

(4) Documentation of a more shaly upper section ofthe Neocomian Blake-Bahama Formation revealing a fa-des change with respect to nearby Site 391. This facieschange, related to increased influx of terrigenous com-ponents toward land, occurs all along the North Ameri-can margin.

(5) Dating of an excellent Middle and Upper Jurassicpelagic record of variegated shales, radiolarian silts, tur-biditic limestones, and reddish clays. The multiple bio-stratigraphic and paleoecologic studies (see specialtychapters in this volume and Site 534 report) greatly in-crease our insight into zonal correlation problems con-cerning stage boundaries of the Callovian, Oxfordian,

Kimmeridgian, and Tithonian, as recognized in a pelag-ic biofacies. It also has significantly increased our un-derstanding of biota distribution and oceanic paleoenvi-ronment in the Jurassic.

(6) Continuous coring of Jurassic strata that has ledto recovery of basal sediments, classified here as of pre-Cat Gap Formation character and age. It should bepointed out here that Ogg et al., (this volume) have pro-posed a change in the definition of the Upper JurassicCat Gap Formation as observed at Site 534. The newdefinition (Fig. 3) now includes the dark variegated clay-stones and limestones of Subunits 7a and b (Cores119-111) below the micritic-bioclastic limestone andgreenish gray clays in Subunit 6b (Cores 110-104) andthe variegated (reddish) claystones of Subunit 6a in theCat Gap Formation. This expanded section, which hasonly been observed at Site 534, now places all of the Ju-rassic sediments above seismic Horizon D in the CatGap Formation. The silty radiolarian-rich claystonesand pelletal limestones below Core 119 and older thanseismic Horizon D make up the basal, unnamed oceaniclithostratigraphic unit. This new definition makes theturbiditic limestones forming Horizon D an integralpart of the Cat Gap Formation. We are somewhat hesi-tant to accept redefinition of oceanic formations basedon single site criteria, without proof of mappability, butwe do show the new subdivision in Figure 3. Futuredrilling in the western North Atlantic will have to sub-stantiate this lithologic record on a regional scale.

(7) Better dating of the Jurassic-Neocomian geomag-netic record (J. Ogg, this volume) between M-24 andM-26. Seismic Horizon D has been shown to be earlyOxfordian (approximately equivalent in age to AnomalyM-27), and Horizon C has been confirmed to occur atthe Jurassic/Cretaceous boundary (M-18-M-19) (Fig. 3).

(8) Recovery of Callovian shales on ocean crust-typebasalt (Logothetis, this volume). These shales representthe oldest recovered sediments used to document ocean-ic spreading.

(9) Recovery of the first samples of Jurassic MagneticQuiet Zone oceanic basaltic basement. Steiner (this vol-ume) finds this Quiet Zone crust to be of uniform mag-netic susceptibility and constant polarity, which leads tothe assumption that the Jurassic Magnetic Quiet Zone istruly quiet.

THE QUEST FOR THE OLDESTOCEANIC SEDIMENTS

The principal objective of Site 534, Hole 534A was toextend significantly our knowledge of the Jurassic At-lantic Ocean beyond what was known from earlier drill-ing during Legs 1,11, and 44 in the western North At-lantic and Legs 41 and 50 in the eastern half, off north-west Africa. Prior to 1980 no ocean site had penetratedrocks much below seismic Horizon C, (i.e., older thanthe Late Jurassic) or closer to the continental marginthan M-25. In a nutshell this history is depicted in Fig-ure 4. Sites 100 and 105 (Leg 11, Hollister, Ewing, et al.,1972) and Site 367 (Leg 41, Lancelot, Seibold, et al.,1978) were drilled within the area of Anomaly M-25, orlandward of the edge of the so-called Jurassic Magnetic

914

SYNTHESIS OF RESULTS

Quiet Zone (JQZ), which is devoid of high-amplitudemagnetic anomalies. The sites bottomed in reddish cal-careous shale to gray, shaly limestone of the Oxfordian-Kimmeridgian. Both Site 391 (Leg 44, Benson, Sheri-dan, et al., 1978), which is 15 nautical miles (22 km)southwest of Site 534, and Site 416 (Leg 50, Lancelot,Winterer et al., 1980) drilled in the JQZ region, but failedto reach basement or extend far into Jurassic strata. Inthe Pacific, Site 462 (Leg 61) failed to reach JQZ-agerocks, but a new attempt was made to do so in 1982 dur-ing Leg 89 of the Deep Sea Drilling Project. In the Indi-an Ocean, off the western Australian passive margin,Site 261 (Leg 27), on about M-22, encountered Ox-fordian-Kimmeridgian shaly sediments on basement.

Site 534, on the flank of a small basement high in anegative lineation of weak magnetic amplitude in theJQZ (Fig. 1) called M-28 (Bryan et al., 1980), bottomedat 1666 m in tholeiitic pillow basalt, typical elsewhere ofoceanic basement. The basalt includes centimeter-thin,reddish shale intercalations, in microfacies and lithol-ogy similar to the sediment immediately overlying thebasalt. Microfossil recovery includes pelagic lamelli-branchs and some long-ranging benthic foraminifers.This shale testifies to the lack of a significant time inter-val between deposition of pillow lava flows on the Ridgeand the overlying sediments. Also the basal shale isslightly altered hydrothermally (Chamley, this volume),the result of higher heat flow during initial cooling ofthe crust.

Using radiolarian, dinoflagellate, and nannofossilbiostratigraphy, the basal sediments in Hole 5 34A wereunambiguously dated as middle (-late) Callovian.

AGE OF BASEMENT AT SITE 534One of the most significant results of drilling at Site

534 was the dating of the oceanic basement at the Site.To accept this dating as valid, we have to consider sev-eral factors that bear on the question. From the very be-ginning of the Deep Sea Drilling Project, there havebeen challenges to the assertion that the deep-sea drill-ing results truly date the basement. The most importantchallenge was the charge that the basalt recovered at thebottom of the holes was not basement but some sill thathas intruded the sediments. Critics complained that drill-ing had only continued a few meters farther, the oldestsediments at the site would have been recovered. As itturns out, this argument is countered at many sites wherethe extrusive nature of the pillow basalts recovered iswell documented. Site 534 is one such site. According tothe petrologists of the Shipboard Party (Site 534 report,this volume) and Logothetis (this volume), the pillowmargins are clearly evident from the contact relation-ships between the basalt and the thin (5-cm) layers ofslightly altered, intercalated sediments. The effects ofquick chilling by seawater of the rinds of the pillows, thethermal cracking, and the size of the pillows are all ra-ther typical of oceanic basalts of layer 2. These factorsrule out the intrusive nature of the basalt.

Another argument is that the basalt flows encoun-tered are interstratified with thick layers of even oldersediments, and thus the true base of the sequence -ould

not have been drilled. This point cannot be counteredwith the drilling data available at Site 534, because only30 m of basalt were cored. The decision to terminatedrilling was made in order to log the hole as early as pos-sible and finish Leg 76, which had already been extend-ed at the expense of Leg 77. Consequently, the possibil-ity of a pillow basalt flow sequence only 30 m thickabove older sediments cannot be refuted. However, theseismic reflection profiles at the Site (Site 534 report,this volume; Sheridan et al., this volume) reveal a typi-cal hyperbolic reflection pattern commonly recorded foroceanic basement. No indication of interstratified basaltflows and sediments is suggested, either in the sedimentsabove acoustic basement or as sub-basement reflectors.There are documented cases in which basalt flows inter-bedded with sediments have been detected on seismic re-flection profiles, and an obvious interlayering is resolv-able. In these examples the basalt layers have strongseismic amplitudes and a small-scale hummocky uppersurface, and are relatively planar reflectors that do notpersist over a wide area. None of these characteristicsare seen at Site 534. It appears that the basalt layer en-countered at the bottom of Hole 5 34A is truly the top ofseismic layer 2, oceanic basement. In any case, the Site534 basalt layer is not different in appearance and pe-trography from oceanic basement, and should be con-sidered no less valid as documented basement than thatat many other DSDP sites.

Given the well-established geologic age of the sedi-ments just above basement as no older than middle Cal-lovian (Site 534 report, this volume), the physical evi-dence of the penecontemporaneous deposition of thesesediments with the uppermost basalt layers supports theconclusion that the age of the sediments is the same asthe age of the basalt. The evidence for the continuum ofdeposition between the basalt and the sediments, with-out a major hiatus, is seen in the cores that were cut con-tinuously across this contact. The contact shows up with-in a single core section. Chamley et al. (this volume) re-port that the claystones directly in contact with the ba-salt have been altered by the thermal affects of the cool-ing lava, so sediment deposition closely followed lavaemplacement.

Another argument has been suggested that might castdoubt on the age of anomaly M-28. This is the possi-bility that the basalt cored was an off-axis event, per-haps along a fracture zone, which extruded several mil-lion years after the true, older crust was formed at thesite. Although we have no data to contradict this sug-gestion, the probability of such an anomalous event hasto be considered in light of past drilling results. Lowrieet al. (1980) have dated many Tertiary and Cretaceousmagnetic reversals in the Tethyan stratigraphic sections.When comparing their data with the ages of sedimentson basalt in the DSDP sites on many equivalent seaflooranomalies, the DSDP ages are generally very agreeable(Channell et al., in press). Seismic mapping of the Ju-rassic Quiet Zone shows little evidence of extensive sea-mounts that might have been produced by late-stage,younger intraplate volcanism affecting this area of thewestern North Atlantic. We can only assume that the ba-salt at Site 534 is typical of that forming oceanic layer 2

915


CoreHole533

Hole533A

LithostratigraphySedimentation

Rate(cm/103 yr.)

Ageßiostratigraphy

Foraminifers Nannofossils

100

200

300

400

10

"'

I

#•'/

PCB

PCBj

PCB*

PCB;

Unit 1

Foraminifer-nannofossilclay and interbeddednannofossil silty claysand nannofossil silty marls(light gray greenand rose colored)

-. Holocene— ~\ late Pleistocene/

?E. hu×leyi

G. oceanica

P. finalis LP. lacunosa

8.3

G. tosaensis r "sma l l "

Gephyrocapsa

H. sellii

Unit 2

Homogeneous, fissile,hydrocarbon-richnannofossil (silty)clay

(dark greenish gray

colored)

-Gas hydrate observed

G. truncatulinoides\

G. miocenica

G. praemiocenica

G. tosaensis

C. mcintyrei

—I \LJ. orouweni i—

\D. pentaradiatus\

D. surculus

8.5 G. altispira

S. dehiscens s. s.

D. tamalis

G. margaritae

21.3

S. seminulina rS. abies\

Figure 2. Stratigraphic summary of Site 533, DSDP Leg 76, Blake Outer Ridge, western North Atlantic Ocean.

created at the rift axis. Nothing in the mineralogy ofthese typical tholeiitic basalts suggests otherwise (Logo-thetis, this volume).

The near fracture zone position of Site 534 does pre-sent a slight controversy. The engineering drill string

limit of the Glomαr Challenger (6800 m) required thatthe site be on the flank of a basement high where base-ment was mapped at a depth of 8.0 s two-way traveltime.A site near the fracture zone would have been prefer-able, because in this basement trough (8.2 s depth) the D

916


Velocity(km/s)

e> B

Failureby

fracture

cVc2

50

TemperatureGradient (°C)

100 10 202.4

10.3

h4.0

19.0

Alkalinity(meq/l)

Chlorinity(%)

18 19 20

100

200

300

-1400

Figure 2. (Continued).

to basement sediment interval (Sheridan et al., thisvolume) was more thickly developed, and it was clearthat seismic Horizon D could be penetrated and iden-tified. Consequently, there are about 200 m more sedi-ments below the D to basement interval in the center of

the fracture zone trough than on the flank where Site534 was located, which leads to the argument that evenolder sediments would have been recovered from the cen-ter of the trough if the drilling capability existed toreach them.

917


SITE 534 Hole 534A

y~• *'' »::>; ̂ iuw»;• ..;;.,„:';;;:‰.;> tf,»u:ff;»*f»;:; w a r * ;

ifflffl>>IWBf <w>H>WtwtXtiH>>fy j tBH

c —-

B 0 !

Age

earlyMiocene

lateEocene

Valanginian

Description

Intraclast chalk,green mudstone

Zeolite-siliceousmudstone; sandstone;, Porcellanite .

Var. claystone

Black greencarbonaceousclaystone

Calcareous claystone;nannofossil chalk;adiolarian/nannofossil

limestone

Reddish-green graycalc. claystone;micritic-faioclasticlimestone;dark claystone

Dark claystone;radiolarian claystone;pelletal limestone

1 I

GreatAbaco

Member

BermudaRise

Plantagenet

Blake-Bahama

CatGap

I i

696

« 724

<764

130

Figure 3. Stratigraphic summary of Site 534, DSDP Leg 76, Blake-Bahama Basin, western North Atlantic Ocean.

100

- : 135

150-

175-

200

Age

Cretaceous

Jurassic

— Keathley

— Sequence

NW Atlantic

100 105 391 534

% Quiet Zone

(predicted)

Oceanic crust

NE Atlantic

416 370 367

W Pacific

462

Indian

Ocean

261

Figure 4. The quest for the oldest oceanic sediments from 1972 to 1980—DSDP legs aimed at resolving earlyocean history.

918


Our only reply to such an argument is that the earliestsediments in contact with the basalt are of a pelagic andhemipelagic type that were draped over the basement,possibly by deep-sea currents, during a time when theSite was above the sill depth produced by the Blake SpurRidge (Sheridan et al., this volume) and at a time whenbasin-leveling turbidites could not reach it. If this inter-pretation is correct, and if the draped deposits are pref-erentially mounded on the north side of the fracture zonetrough, then conceivably the sediments in the center ofthe trough and on the flank are of essentially the sameage. In modern fracture zone environments where thetransparent drape sediments and contourites are found(Sheridan et al., this volume) the relief in the sedimentsof the same age can be as much as 100 to 200 m.

In our opinion, considering all these arguments, theage of the basement and of the crust at anomaly M-28 isdetermined by the age of the basal sediments drilled atSite 534—middle Callovian or approximately 153 Ma.

THE JURASSIC TIME SCALE AND A STRATI-GRAPHIC SYNTHESIS CHART FOR THE

WESTERN ATLANTIC

Organization of many paleontological and geologicalinterpretations in a linear, numerical fashion, and cali-brated to a geochronologic radiometric time scale servesmany purposes. The approach provides rates of changeof biological, geological, and Oceanographic processesand helps in clarifying trends. Two common approachesinvolving expression in a numerical scale are used in thisvolume; the calculation of early spreading rate and thesubsidence and sedimentation rates at Sites 533 and 534.Because both methods have been used to extrapolateback to the time of early opening (Vogt and Einwich,1979; Bryan et al., 1980) of the Atlantic Ocean, it mightbe instructive to analyze briefly the time scale itself. Inthe process we propose an update of a commonly usedJurassic scale and present a chart documenting key geo-logical, paleontological, and paleoceanographic eventsduring the long history of the western North Atlantic.

In their calculation of the Atlantic early spreadingrate Vogt and Einwich (1979) used the time scale pro-posed by van Hinte (1976a, b). This scale uses a multipleapproach to numerical calibration of Jurassic stages,rather than the arbitrary "equal duration of stages"model and a few key radiometric dates that are used as aworking hypothesis in the Geological Society of London(1964) scale.

Modern absolute time scales can make use of severalapproaches: (1) best fit of radiometric ages plotted ver-sus chronostratigraphic dates of the rocks in questionand adjusted to the standard set of decay constants andisotopic abundances (Armstrong 1978; Webb 1981); (2)linear scaling of stages using the concept of equal dura-tion of the biozones that characterize each stage (in thisway stage boundaries can be interpolated from few andfar-between numerical ages); (3) extrapolation to findnumerical ages of stage boundaries, using constancy ofaverage sedimentation rates in well-dated sections; (4)interpolation and calibration to find the age of geo-magnetic and biostratigraphic-chronostratigraphic

"events" and boundaries, using constancy of seafloorspreading to obtain a linear scale between marine mag-netic anomalies; these anomalies can also be recognizedas reversals in sedimentary sections.

The Jurassic scale of van Hinte (1976b) makes use ofthese approaches. In contrast, Armstrong (1978), in hispre-Cenozoic date file, only used interpolations betweenradiometric dates that were strictly based on accepteddecay constants of isotopic systems. For the Jurassic,few good dates are available and the Oxfordian-Callo-vian and Toarcian-Aalenian have no absolute dates tosupport this interpolation. Armstrong (1978) only usesmaximum age data of a series, that is, the oldest valueof age assignments to compensate for daughter-productloss. The Armstrong (1978) scale for the Jurassic isshown in Figure 7, together with an updated version ofthe van Hinte (1976b) scale. It is obvious that there aresubstantial discrepancies in numerical age, the first scalebeing older by 5 to 10 m.y. and having different stageduration proportions. The preferred (multiple) scale,updated from van Hinte (1976b), has been derived asfollows.

The Triassic/Jurassic boundary is best placed at about200 Ma rather than at 192 Ma. This value is a reasonablemidpoint of about 15 individual K/Ar and Rb/Sr dates,with a range of 215 to 185 Ma (Webb, 1981). These olderdata come close to Webb's (1981) age of 205 Ma.

Next, we have used Hallam's (1975) rather than Ar-kell's (1956) standard ammonite zonation for the Het-tangian to Bajocian (Fig. 5). If we retain 165 Ma for theupper limit of the Bajocian and use 30 Hettangian-Ba-jocian standard zones, a zone averages 1.2 × I06 m.y.long, and thus allows calculation of stage limits asshown. For the Bathonian through Tithonian, no timescale changes have been made, although the Bathonianmay be too long. This is because the zonation may havebeen oversplit, leading to stacking of subzones andzones (G. Westermann, personal communication, 1982)here assumed to be of equal duration. The Tithonianlower boundary is based on the appearance of Gravesia,which leads to the short (French) Kimmeridgian, rec-ognized by French workers.

The marine and sedimentary reversal scale has beencalibrated as follows: The geomagnetic polarity timescale of M-28 to M-25 to M-0 was initially stretched be-tween tie points, as discussed by J. Ogg and R. Sheridanin the Site 534 report. It was subsequently compared toand adjusted for stratigraphic assignments of sedimen-tary and ocean basement reversals in land sections andin DSDP Sites 166, 387, 307, 105, and 534. The M-0 dateof early Aptian, or ± 114 Ma, is based on Lowrie et al.,1980. The Cretaceous Magnetic Quiet Zone is Albianthrough Santonian. Anomaly M-7-8, based on DSDPSite 166, has been argued by van Hinte (1976a) to bepossibly 125 ±5 Ma, which is slightly older than fa-vored by Channell et al. (in press), who use interpola-tions between other reversals. Unfortunately the rangeof biostratigraphic assignments at Site 166 is such (Hau-terivian-Albian) that no firm conclusions are possible toverify Channell et al.'s interpolation. Anomaly M-16,based on Site 387, occurs at or below the Berriasian/

919


Valanginian boundary; M-17 is basal Berriasian in Ogg(1980) and Channell et al. (in press). If basement is Ti-thonian at DSDP Site 307, M-21 is 140 to 135 m.y. old,if Tithonian-Berriasian, it is slightly younger. For M-24and M-25 at Site 534 (Table 1), all reasoning presentedsuggests a position in or just below the Oxfordian/Kim-meridgian transition or near 143 Ma, an estimate alsofavored by Channell et al. (in press).

The small amplitude marine magnetic Anomaly 27,sensu Bryan et al. (1980), can be dated using the pinch-out of seismic reflector D against basement of the M-27signature. Drilling at Site 534 has shown D to be ear-ly-middle Oxfordian, and M-27 is placed in the earlyOxfordian. As shown by Site 534 drilling, small ampli-tude Anomaly M-28 is likely early Callovian.

Finally, the distances were (again) measured for themapped marine anomalies of M-0, M-7 to M-8, M-16,M-17, M-21, M-25, M-27, M-28, and the Blake SpurAnomaly (BSA) in the Blake-Bahama Basin. A spread-ing rate of 1.7 cm/yr. between M-0 to M-21 and of3.1 cm/yr. between M-21 and M-28 and BSA provides agood linear fit of the data and establishes the age of theBSA at the Bathonian/Callovian boundary.

The Figure 5 chart (see back pocket) combines thegeomagnetic scale, geological and paleontological (bio-stratigraphical) events or zonations used to reconstructthe history of the western North Atlantic Ocean and tosome extent the margin also. Emphasis is on "steppingstones" during the Mesozoic. Choice of standard am-monite zonation for the Jurassic has been discussedearlier. There is good calibration between the Early Ju-rassic benthic foraminiferal ranges studied in the GrandBanks and Portuguese basins (Gradstein, 1977; Extonand Gradstein, in press) and the ammonite zones. Thereis no Early and early Middle Jurassic foraminiferal rec-ord known from the western Atlantic oceanic realm.Few taxa (indicated with an * in Fig. 5) are probably ex-clusive of neritic biofacies and many probably occurredin deep, oceanic sediments. Increasing provincialismsince the Late Jurassic precludes recognition of many ofthe events in high latitudes, although along the westernAtlantic strong Tethyan influences extended to thenorthern limit of the marine Jurassic and Early Creta-ceous realm in the Newfoundland Basin.

The Jurassic planktonic foraminiferal record is wellestablished on the Grand Banks and probably includesthe first appearance of this group from an unknown an-cestor. The Atlantic pelagic record of this group issparse and confined to some specimens in Oxfordianstrata (see the summary in Gradstein, this volume). Thegroup was essentially confined to the epicontinental seasand ocean margins in the Jurassic and also in the Neo-comian.

The Cretaceous planktonic foraminiferal zonation,as expressed in the LC (Lower Cretaceous) and UC (Up-per Cretaceous) units adopted by van Hinte (1976a), isonly applicable in part. First, there is the extensive LateCretaceous hiatus in the bathyal and abyssal Atlanticrealm; second, LC14, UC4, UC8, UC11, and UC13 arehard to recognize, if they are recognizable at all, in theAtlantic region. Cretaceous benthic foraminifer events

largely follow studies by Moullade (1966), van Hinte(1976a), Ascoli (1977), Gradstein (1978, and unpub-lished data), and Drushtchitz and Gorbatschik (1978).

The Early and early Middle Jurassic palynologicalzonation developed by Bujak and Williams (1977) andS. Davies (personal communication, 1982) on the west-ern Atlantic margin is in reasonable agreement with thatusing foraminifers. It has been compared to the am-monite succession shown. For the younger Jurassic andCretaceous stratigraphic section the dinoflagellate bio-stratigraphy as developed by D. Habib for the pelagicrealm is shown (Habib and Drugg, this volume). Thisbiostratigraphy is explained by these authors in terms ofammonite zonations and stratotype coverage. The sub-division shown is reasonably in agreement with that ofP. Roth et al., (this volume) and P. Roth (1978) usingnannofossils, but only for parts of the section. There arecalibration problems in the Oxfordian, Kimmeridgian,early Tithonian, Hauterivian, and Aptian. Some of theseinaccuracies will be dealt with later. Jurassic nannofos-sil datums and their relation to ammonite zones as shownfollows Roth et al. (this volume) and D. Watkins (per-sonal communication, 1982).

The Late Cretaceous nannofossil zonation is thatadopted from Tethyan standard zonations and used forthe western Atlantic margin (Doeven et al., 1982 andDoeven, in press). The calpionellid zonation follows Re-mane (1978); the cephalopod (ammonites and aptychi)correlation scheme in the pelagic biofacies is that ofRenz (this volume). The Tethyan radiolarian zonationapplicable to the western Atlantic was developed byP. Baumgartner (this volume).

Following this introduction, a presentation is madeon Late Jurassic stratigraphic inaccuracy in DSDP sites,with special reference to Site 534 (Table 1). The oldestforaminiferal assemblage in Hole 534A, which alsooccurs in other DSDP Sites, was found in Cores 102through 96, that is, the co-occurrence of Lenticulinaquenstedti and Epistomina aff. uhligi. This co-occur-rence correlates directly to the basal Cores 50 through52 in western Atlantic Hole 391C, to Cores 36 and 37(two cores above basalt) in western Atlantic Site 105,and to Cores 35 through 37 (immediately above basalt)in eastern Atlantic Site 367 (Fig. 6).

At Site 534 the interval between Cores 96 and 102contains magnetic reversals M-20 to M-24 (J. Ogg, thisvolume), which is consistent with the fact that at bothSites 105 and 367, basaltic basement is aproximatelyM-25 or slightly older, and the overlying sedimentsshould be of an age just post M-25. This also agrees withthe lithostratigraphic correlations of 534, 105, 367, and391 which, in the relevant intervals, either contain thelower part of the grayish red, calcareous claystone orthe upper part of the underlying grayish limestone andgreenish claystone, both of the Cat Gap Formation. Theco-occurrence of the two foraminifer taxa in theseDSDP Sites is estimated to be in the Kimmeridgian,probably ranging upward into the Tithonian (Gradstein,this volume). This age estimate agrees with the geomag-netic reversal age estimates proposed by J. Ogg (this vol-ume).

920

Table 1. Multiple bio-, chrono-, and physical stratigraphy of Jurassic strata at DSDP Site 534, western North Atlantic Ocean.

Biostratigraphy and age

Ostracodes

-? Kimm.—Tithon.-

IParanotacy there

Oxford ./Kimm.

Ammonites,aptychi

- IPaquiericeras _late Valanginian

L. beyrichiTithonian

Foraminifers

Valanginian 72

96Tithonian—

KimmeridgianL. quenstedtiE. aff. uhligi

102

Oxfordian

C. paraspisG. aff. o×fordiana

Calpionellids

87-6I e. Valanginian I

Berriasian 87-2Berriasian 90-1

Zone B=-, I. Tith.—e. Berr. ,-=

πH 9 2 • 2 fI ' 92-βl

Zone AI. Tithonian

93-4

Radiolarians

Kimm.—β. Tith.

e. Oxfordian

I. Cal.-e. Oxf.

m. Callovian

Dinoflagellates

D. deflandreiI. Val.-Haut. 76-3D. apicopaucicum

Valanginian81

82-1B. johewingii

86-187-6

e. Berriasian

91-3

Tithonian

104-2

105-1

Kimmeridgian

111-1

112-1

Oxfordian

121-1122-2

M

127-2

Nannofossils

85-3/4

early Berriasian

91 CC

C. me×icanaKimm.-Tithonian

101

V. stradneri

Kimmeridgian

Oxfordian

113

C. margareliI. Call.-e.Oxf.

121

S. he×um

m. Callovian127-4

Sub-bottomdepth(m)

1202

ΛOΛ.I R

1286

1331

1371.5

1410

1455

1495.5

1540.5

1581

1621.5

1666.5TD

—

—

=

Co

re

75

80

85

90

95

100

105

110

115

120

125

130

Physical stratigraphy

Lithologic units

5B

Blake-Bahama Fm.

5CWhite-gray laminatedchalk, bioturbated

calcareous siltstone,claystone

5D

Cat Gap Fm.Grayish-redcalcareousclaystone 6A

—

Limestone turbidites g ß

and greenish claystone

Variegated reddish to 7Agreenish claystone

Limestone turbidites 7 gdark claystone —Marly limestone.greenish red 7CclaystoneBlackish claystone -==rReddish claystone 7^

Basalt

Geomagneticreversals

M 16

M-17

10B

M-19M-20

M-22

M-23 or M 24

(Cande)

Seismicevents

—

c =

= D' =

i D =

i Basement =3ZHXmCO

CΛ

O

s85


105 391 534 367

43

Basalt

r 96

L102

110

127

130

Figure 6. Correlation of the co-occurrence of the foraminifers Lenti-culina quenstedti and Epistomina aff. uhligi (1) in DSDP Sites 105,391, 534, and 367, North Atlantic Ocean. (In Site 534 this intervalcontains magnetic reversals M-20 to M-24 [Ogg, this volume],which agrees with the fact that at both Sites 105 and 367 basementis of approximately M-25 age and the overlying sediments shouldbe slightly younger. The age of the L. quenstedti and E. aff. uhligiassemblage is estimated in these Sites to be Kimmeridgian, ex-tending into the Tithonian [Gradstein, this volume].)

Using dinoflagellate stratigraphy, the basal sedimen-tary assemblage at Site 105 correlates to the palynologi-cal Kimmeridgian in Site 534 (D. Habib, personal com-munication, 1982). Such a correlation fails to explainwhy at Site 534 the palynological Kimmeridgian (Cores111-105) occurs several cores below the interval with thementioned foraminifers. The answer to this discrepancyin correlation is not easily found. As a next step, itwould be useful to study point correlations of first andlast occurrences and of co-occurrences of many taxa indifferent microfossil groups together with geomagneticreversal sequences at the DSDP Sites. Such an approachmay tell if ranges are incomplete or if some correlationsare more likely to be "time lines" than others.

This correlation problem is but one of several thathamper a straightforward chronostratigraphic interpre-tation of the Oxfordian through Kimmeridgian intervalat Site 534 and other DSDP sites as well (see Table 1).The most striking discrepancy appears to be the offsetby five or more cores of the palynological and nanno-fossil late Oxfordian through early Tithonian interpre-tations, the palynological one being ahead in time.Again, no easy answers come to the fore, although itmay be of significance that three samples from the lateOxfordian Hauffianum Zone, in the Montejunto sec-tion in Central Portugal (collected by F. M. Gradstein),

were tentatively assigned to the early Kimmeridgian us-ing some dinoflagellate taxa, also known from westernEurope. The Portuguese Hauffianum Zone foraminiferassemblage, with a globigerinid resembling Globuliger-ina oxfordiana and Epistomina mosquensis, correlatesat Site 534 to Core 110, where it is assigned to the Ox-fordian (Gradstein, this volume). Geomagnetically thiscore is at the level of M-24 or M-25 of the latest Ox-fordian to the earliest Kimmeridgian (Channell et al., inpress). Such a correlation is not necessarily in disagree-ment with the V. stradneri nannofossil assemblage (Ox-fordian-Kimmeridgian) in Cores 113 through 101, orthe palynological Kimmeridgian as deep as Core 111.We may be dealing with the well-known boundary effectof biozonal stratigraphy, that is, uncertainty intervalsare not usually given to the boundaries of conventionalbiozones. It would be more realistic to use a notationthat would better reflect inaccuracies in the correlationof zonal limits.

The excellent fit of the Hole 534A sequence of Juras-sic age-depth assignments bears out this "inaccuracy"interpretation (see the section on subsidence and sedi-mentation; Fig. 9). A careful evaluation of the relativecorrelation of bio- and magnetostratigraphic events,zones, or assemblages in oceanic and land sites shouldbe helpful to understand this inaccuracy in time better.

IMPLICATIONS FOR EARLY SPREADING RATESAND THE AGE OF THE BLAKE SPUR MARINE

MAGNETIC ANOMALYUsing the Armstrong (1978) numerical scale, we found

that the seafloor magnetic reversals in Figure 7 are bestfitted using two doglegs. Spreading proceeds at 3 cm/yr.from the Callovian-Oxfordian, which coincides exactlywith the Jurassic Magnetic Quiet Zone (Channell et al.,in press), after which spreading slows down somewhatto 2.4 cm/yr. For the Cretaceous, the Armstrong (1978)absolute time scale does not have reasonable propor-tions, for example, the Albian is excessively long andthe Aptian almost "squeezed out." Therefore, the Arm-strong scale was not used for the Cretaceous in Figure 7.If the spreading rate curve is extrapolated using the up-dated van Hinte scale (1976a), spreading slows downeven more than 2.4 cm/yr. for the Early Cretaceous.

In the preferred new numerical scale (Fig. 5), earlyspreading appears as reasonably constant at 3.1 cm/yr.from M-28 to M-17 to M-16 at the onset of the Creta-ceous, after which the rate slows down to about 1.7cm/yr. until the Aptian.

In both scales extrapolation to the Blake Spur Mag-netic Anomaly (BSMA), 100 km landward of M-28 atSite 534, appears to yield the same ag;e. The age de-rived is near the Bathonian/Callovian boundary, or ap-proximately 155 Ma, which is about two stages or 20m.y. younger than thought in previous publications,such as the determinations of Vogt and Einwich (1979),Bryan et al. (1980), and Klitgord and Grow (1980). Ex-trapolation is aided by the pinch-out position of seismicHorizon D, dated at Site 534 as early Oxfordian. Be-cause Horizon D pinches out on basement just landward

922


E C M A S

500 - BS[NM

28 E

27

250

M-25

M-21

M-17|

M-16

M-7-8L3

D 3 cm/yr.

110

Sedimentary geomagneticreversal data

Sea-floor geomagneticreversal data

Extrapolated

Pinch-out of seismic Horizon Ddated as early Oxford ianat Site 534

2.4 cm/yr.

150 160 170 180 190 200 Ma

Albian | Aptian Barrem.| Haut. [Valang.| Berr.| Tithon. |<im|θxford,| Callov. Bathonian | Bajocian |Aal.| Toarcian | Pliensb. | Sinem. | Hett.|

Updated van Hinte (1976b) Armstrong (1978) JTithonian | Kimm. [Oxford.| Callov. | Bath.-Baj. | Aal.-Toar. | Pliensb. | Sinem.-Hett. |

Figure 7. Rates of early Atlantic Ocean seafloor spreading during the time of the Blake Spur Magnetic Anomaly (BSMA) to geomagnetic AnomalyM-0. (We assume a ridge jump at the time of the BSMA. The preferred time scale has been updated from van Hinte [1976b], which gives an earlyspreading rate of 3.1 cm/yr. and a Cretaceous rate of 1.7 cm/yr. Using the Armstrong [1978] scale, early spreading slows from about 3 to 2.4cm/yr. and becomes even slower in the Cretaceous. Both time scales predict the age of the BSMA to be of the Callovian/Bathonian boundary.)

of magnetic Anomaly M-27, as noted by Bryan et al.(1980), this aids in the extrapolation by providing an-other independent point in the Magnetic Quiet Zone.

The new age for the BSMA means that it might matchreversals older than the Jurassic Magnetic Quiet Zone(Channell et al. in press). However, the paleomagneticstratigraphy on the land section may produce a conflict,because reversals occurred during the Bathonian (Chan-nell et al., in press). As indicated by Sheridan (thisvolume), there is great uncertainty in the age of the in-ner part of the Jurassic Marine Magnetic Quiet Zone be-tween the Blake Spur Anomaly and the East Coast Mag-netic Anomaly. It could be anything older than Callo-vian, from Bathonian through Pliensbachian. The prob-lem is that there are no strong linear magnetic anomaliesin this corridor that resemble seafloor reversal anom-alies. Although the stratigraphy of the Bathonian toPliensbachian land sections is not precise, they can bedistinguished on a stage level, and there are enough sec-tions measured that most people agree that there aremagnetic reversals during these stages (Channell et al.,in press; Steiner, 1980).

Perhaps the easiest way to avoid the conflict is to ac-cept the possibility put forward by Sheridan (this vol-ume) that the inner quiet zone corridor could also havespread very rapidly, similar to the outer quiet zone. Be-cause this corridor contains both sides of an extinctspreading center, if the proto-Atlantic concept is right,only the upper part of the Bathonian stage might be rep-resented by this crust. From the data presented by Chan-nell et al. (in press), it is more certain that reversals are

measured in the bottom part of the Bathonian, but theage span of the upper part of their section is uncertain.

If we do not accept this alternative and interpret theECMA-BSMA (East Coast-Blake Spur magnetic anom-alies) corridor to be possibly as old as the Pliensbachian,then we must explain the lack of seafloor magneticanomalies. It might be that rapid sedimentation duringthe spreading created thick sediment and sill intercala-tions, as occurred in the Gulf of California, and becauseof the particular cooling regime of the sills, the magneticgrains formed with a low magnetization. Such an ex-planation was once invoked to explain the outer mag-netic quiet zone. With the recovery of basalt at Site 534,such explanations are no longer viable. Steiner (this vol-ume) finds that the basalts are well magnetized, withmagnetizations no different from typical oceanic basaltsof many ages. Unusually viscous remanence is probablynot the cause of the outer magnetic quiet zone. Anotherexplanation for the lack of magnetic anomalies in theECMA-BS corridor, and for the possibly equivalentcrust under the Gulf of Mexico, is that the basement isso deep that the amplitude of the anomalies is expectedto be low. However, recent magnetic surveys show thatthe M-sequence anomalies persist as much as 100 kmlandward of the Japan Trench axis where the basementsource must be some 15 km deep (von Huene et al.,1982). The resilency of these anomalies is thus demon-strated.

Another alternative is that the crust in the ECMA-BScorridor is of anomalous nature and not simply oceanic.Sheridan et al. (1979) considered the seismic refraction

923


data in the corridor and these seem to be compatiblewith those of the outer quiet zone, which is clearly ac-ceptable as oceanic crust. However, crustal velocitiesand thicknesses cannot be interpreted uniquely near tothe edge of a continent that has been rifted, with athinned and possibly dike-intruded crust forming thetransition to oceanic crust. Observations on stretched,listric-faulted continental crust show thicknesses ofabout 5 km and depths to mantle of around 13 km, whichare nearly identical to the same features of oceaniccrust. Thus crustal velocity and thickness and depth tomantle are not unique criteria to define oceanic crust.

Sheridan et al. (1979) also note that reflection pro-files in the ECMA-BS corridor reveal a "typical" hy-perbolic reflector characteristic of the oceanic basementof layer 2. This reflector might be stronger evidence thatthis corridor is truly oceanic. Certainly there are no indi-cations in the presently available data that tilted blocksof the listric-faulted type, often associated with conti-nental crust, are found in the ECMA-BSMA corridor.But until drilling is carried out west of the Blake SpurAnomaly this uncertainty about the nature of crust inthe ECMA-BSMA corridor will persist.

IMPLICATION OF THE AGE OF THE BLAKESPUR MAGNETIC ANOMALY FOR THE

EVOLUTION OF THE NORTHAMERICAN MARGIN

The new Callovian/Bathonian boundary age for theBlake Spur Marine Magnetic Anomaly has significantapplication to the evolution of the North Americanmargin. A spreading-center shift to this anomaly posi-tion, which would isolate the proto-Atlantic extinct rifton the North American plate, has been proposed as amechanism to explain the East Coast Magnetic Anoma-ly-Blake Spur corridor (Sheridan, this volume).

Assuming reasonably rigid plates in the reconstruc-tion of the Atlantic margin and the Gulf of Mexico, weexpected that when spreading began along the ECMAthat spreading should have occurred in the Gulf of Mex-ico as well. Thus the proto-Atlantic in the ECMA/BSMA corridor should have had an equivalent openingin the Gulf of Mexico. But most researchers studyingthe Gulf of Mexico believe the ocean spreading breakupof that basin occurred later in the Jurassic than the ageof the crust in the ECMA-BSMA corridor, previouslythought to go back to the Pliensbachian. To resolve thisdifference, Salvador and Green (1980) propose that thefirst opening of the ECMA-BSMA corridor was moreeast-west, and that a transform fault ran through theGulf of Mexico. This event would have allowed a proto-Atlantic Ocean to exist off the eastern United States be-fore the oceanic Gulf of Mexico. However, recent re-constructions of the African and North American con-tinents, based on possible fracture zone interpretationsin the ECMA-BS corridor, contradict the east-westmovement. K. D. Klitgord and H. Schouten (personalcommunication, 1981) prefer a northwest-southeastopening better.

Now that this Blake Spur event is made younger byour dating, it might be young enough to have occurred

after, rather than before, the breakup of the Gulf ofMexico, which formed a small ocean crust between thepreviously continuous Louann and Mexican salt basins(Salvador and Green, 1980). Although critical to datingthis breakup, the age of the Louann salt is uncertain. Itcould be Bathonian, as suggested by Todd and Mitchum(1977). The overlying marine limestone of the Smack-over Formation is part of the main transgressive se-quence. It is well dated as Oxfordian. Although theNorphlet Formation of sands, between the Smackoverand the underlying Louann, is poorly fossiliferous andits age is not well know, there are shaly facies that in-dicate the beginnings of the major transgression. P. R.Vail (personal communication, 1981) feels that theseshales represent the Callovian transgression seen on theNorth Atlantic margins. Thus the Callovian is thoughtto overlie the Louann salt. Pollen microfossils from theLouann give a general age of the Middle Jurassic andare not definitive to the stage level.

In another interpretation the age of the Louann salt isthought to be Callovian (A. Salvador, personal com-munication, 1981). If one assumes that the sea-watersource for the salt entered the Gulf of Mexico from thePacific rather than the Atlantic, the earliest marinelimestone above the salt in Mexico should be Callovian.If this Callovian limestone was deposited immediatelyafter the Louann salt, then the age of the salt can be in-terpreted to be Callovian (Salvador, personal communi-cation, 1981). Such a logical deduction is required, be-cause a hiatus in deposition above the salt would exposeit to solution in either marine or nonmarine conditions,so its mere preservation seems to require continuousdeposition.

Given the uncertainty about the age of the Louannsalt, it is possible that the opening of the Gulf of Mexicooccurred in the late Bathonian. Note that this is also thetime that could be assigned to the breakup along theECMA. Thus with the new dating of the Blake SpurAnomaly as earliest Callovian, this major spreadingshift could have occurred after the breakup of the Gulfof Mexico.

A major spreading center shift was needed to ter-minate spreading in the Gulf of Mexico and to allowbreakup along the southeast margin of the Yucatanpeninsula (Sheridan, this volume). Also, the breakupand spreading of the Yucatan away from South Ameri-can would have permitted the formation of a Late Ju-rassic Caribbean Sea, which most Caribbean expertswould like to postulate. It is now possible that thespreading center shift to the Yucatan margin to end theGulf of Mexico spreading was one and the same event asthe spreading center shift to the Blake Spur Anomalythat ended the proto-North Atlantic spreading. Furtherdrilling in the deep Gulf of Mexico and ECMA-BSMAcorridor is required to test these speculations.

PULSATION TECTONICS

Sheridan (this volume) has put forward a theory toexplain the origin of the magnetic quiet zones in termsof cyclic plume eruptions from the core/mantle bound-ary, and alternations of the heat transfer mechanisms in

924


the lower mantle between a conductive state (little or noplumes) and a convective state (many plumes). Thesevariations of plume convection altered the temperatureat the core/mantle boundary and, thus, the stability ofthe outer core. During plume eruptions the core iscooled while the asthenosphere is heated, which leads toa quiet magnetic field associated with a fast plate-spreading interval. In the absence of plume eruptions,the mantle is not convective but conductive and insu-lates the core, making it hotter. This leads to a cooler as-thenosphere and slower plate spreading during times ofa metastable, turbulent convecting core, which mani-fests itself in frequent magnetic field reversals.

A cooler core with more laminar core convectionwould be compatible with both an extended period ofconstant polarity and a weaker dipolar field measured atthe surface. There is evidence for both of these phenom-ena during the Jurassic Quiet Zone (Channell et al., inpress; Steiner, 1980). Also, in the Cretaceous QuietZone there is a well documented extended period of con-stant polarity.

SUBSIDENCE AND SEDIMENTATIONA detailed outline of the history of subsidence at Site

534 offers a unique opportunity to study ocean basindepositional history at an unprecedented scale. First,the sediment at Site 534, cored continuously from thelower Miocene through the middle Callovian, spans al-most 160 m.y. of ocean history. Second, it is only 22 kmaway from Site 391, where Tithonian through Recentsediments were recovered that comprise 140 m.y. ofhistory. A comparison of the subsidence curves for bothsites may be scrutinized for local variations in long-termdepositional history. The mapping and understandingof these variations ultimately will considerably refineMesozoic-Cenozoic paleoceanography.

Principal uncertainties in the determination of sedi-ment accumulation rates are (1) uncertainty in samplingand observers' bias in biostratigraphic or magnetostrati-graphic zone assignments, (1) uncertainty regarding therelation of the bio- or magnetostratigraphic zonationsto the numerical, geochronological scale, (3) uncertaintyof 5% or more in the radiometric dating of stratigraphichorizons, (4) uncertainty that the whole time interval ofa zone is represented at the site (i.e., was there continu-ous or episodic sedimentation?), (5) uncertainty aboutthe amount of erosion as opposed to nondeposition, and(6) uncertainty about the exact amount of compactionthrough time.

Error due to personal bias, reworking of micro fossiloccurrences, or sampling can be minimized through theuse of multiple biostratigraphy and decreases with an in-crease of sampling points. An arbitrary way of repre-senting this error and the numerical calibration error isby assuming that at each point on the average sedimen-tation path per zone (i.e., the diagonals AB and CD inthe zone-time interval boxes in Fig. 8), an error exists of50% of the duration of each zone. From this assump-tion follows the likely sedimentation rate pathway (ingray) in Figure 8. Within the pathway the rate fluctu-

Time

Figure 8. Likely pathway (in gray) of sediment accumulation per zone.(The square boxes represent the time-thickness range of biozones.For explanation see text.)

ates, but the average is given by inclination of the path-way.

Continuous versus episodic sedimentation and partic-ularly erosion are factors that are difficult to assess.The error is inversely related to the biostratigraphic res-olution. Average sedimentation rate per biozone is morerealistic the shorter the duration of the zone in time.Otherwise these factors will have to be evaluated indi-vidually at each site and are a function of the deposi-tional regime. Average sedimentation rates are clearlymore representative and meaningful for hemipelagicand pelagic sedimentation than for turbidites or debrisflows. This chapter only uses average rates of sedimen-tation.

The decompaction can be calculated from ht = h0(öo ~ 6w/ei ~~ 6w) per depositional interval, where h0 isthe measured thickness in m, ρ0 is the measured densityof the sediment in g/cm3, ρw is the density of seawater(1.028), and Q is the estimated original density of thecompacted sediment (-1.8). Residual errors may be ofthe order of 10%.

Probably the single, largest error in the technique ofestablishing sediment accumulation rates is in estimat-ing the numerical age of each biozone. The error in bio-chronology, that is, the fixation of successive zonal orfirst and last stratigraphic occurrence events to the or-dinal, time scale, is proportional to (1) radiometric er-rors, (2) the spacing of the events in time, and (3) thesynchroneity of the events and their correlative ones toradiometrically dated calibration points. The 30-50 mi-cropaleontological zones, aided by a detailed magneticreversal scale, in the Cenozoic (65 m.y.) theoreticallyallow for a local relative time stratigraphic resolution ofbetter than 1 to 2 m.y. In the Cretaceous and particular-ly in the Jurassic, far greater uncertainty exists. A 5%

925


radiometric error for numerical ages between 65 (topCretaceous) and 200 (base Jurassic) m.y. gives values of3 and 10 m.y.

One advantage in dealing with these errors is that thesediment accumulation technique is based on trends, thatis, a line is used to average changes. Gross rates arefixed between successive points in time. The more mid-points that are known in the sediment accumulationcurve, the better the determination of the successiverates. Single point errors in age or depth assignments donot detract much from the overall trends. Let us assumefor a moment that one tie point to the numerical timescale of a succession of biostratigraphic events is 5 m.y.in error, that is, too old. This is compensated for byshifting all previous points between the erroneous oneand the next firmly established younger one. The sum ofthe individual shifts of points is 5 m.y. Problems mainlyoccur when one tries to compare individual, short-termfluctuations in rate, within one site or between sites, atthe level of maximum stratigraphic resolution available.

Sediment Accumulation at Site 391The biostratigraphic criteria used to arrive at the sedi-

ment accumulation for Site 391 of Figure 9 are based onthe (1) nannofossil, (2) foraminifer, and (3) palynologi-cal zonations as compiled in Gradstein et al. (1978). Ad-vantage has been taken of minor new information. Cores391C-6 and -7 are now dated as late Albian through ear-ly Cenomanian rather than Albian (Jansa et al., 1979),and the zonation of P. Roth for Leg 76 (this volume) isused to update slightly the Leg 44 results by the same au-thor. Table 2 shows the numerical age of the top and

Table 2. Geochronology at Site 391, Blake-Bahama Basin.

Numerical age(m.y.)

01.84.568

111112.513.5

13-14.514.5

15-1615.515.517171719.519.522.5

23.524

Sub-bottomdepth(m)

0149150149210210210259320325326335364335421475500544573582

600649

Numerical age(m.y.)

92104107107108113

113113116119119121126128.5128.5130131133.5133.5136138138138142142

Sub-bottomdepth(m)

677780830925830905

990101010201000103010901125121012201200126012701300132513251340139014001412

bottom of the zones used to plot the multiple age-depthrelationship depicted in Figure 9. Precision of tops andbottoms of the zones was plotted to the nearest 0.5m.y., but is clearly arbitrary.

No significant compaction has been assumed for thelate Tertiary interval as densities are probably between1.85 and 2 g/cm3. The rates of accumulation of the de-compacted and the compacted sediment in cm/103 yr.are shown in Table 3. The rates are also plotted in Figure9, together with some error bars, as explained before.Clearly defined fluctuations in the late Mesozoic-Ceno-zoic sedimentation rate are indicated. For the Titho-nian-Barremian shaly limestones of the Cat Gap andBlake-Bahama Formations, rates are in the order of 1.7cm/103yr. (compacted) and 2.5 to 3 cm/103 yr. (decom-pacted). Because pelagic sedimentation plays an impor-tant role, this is a reasonable average figure. Sedimenta-tion drops to slightly lower values of 0.8 to 1.1 cm/103

yr. (compacted) and 0.9 to 2.7 cm/103 yr. (decom-pacted) in the mid-Cretaceous when the greenish blackand variegated colored clays were formed. Sedimenta-tion may have been relatively steady. The question ofthe Late Cretaceous-Paleogene sediment starvation and/or erosion is discussed later. The interclast chalk andsiliceous-calcareous mudstone interval of the MioceneGreat Abaco Member shows dramatic changes in ratesfrom 1.1 to 11.6 cm/103 yr. The sparse coring in this al-ternation of debris flows and background pelagites and"contourite" deposits prevents the determination of arealistic depositional rate for each component. Obvious-ly rapid changes in rate took place.

The Pliocene may have been a time of nondepositionand/or erosion. The uppermost Miocene to Pleistocenecontact is in Core 3 at 150 cm; it can be erosional. Thiserosional event may be the same as observed at Site 533on the Blake Ridge at the end of the Pliocene, and prob-ably is the result of an intensification of bottom cur-rents. The Pleistocene rate of 7.9 cm/103 yr. is almostidentical to the rate of 8 cm/103 yr. determined withgreater precision at Site 533, many miles to the north.

Table 3. Compacted and decompacted sedimenta-tion rates at Site 391, Blake-Bahama Basin.

Interval(m)

Sedimentation rate (cm/103 yr.)

Compacted Decompacted

1412-12001200-10301030-970970-780780-685685-649649-500500-325325-315315-205204-140

140-0

1.71.70.91.10.8—2.1

11.67.33.71.1

?Erosion7.9

3.02.81.42.70.9

2.111.67.33.71.1

?Erosion7.9

Note: Above 649 m, measured sediment densities ap-proach the uncompacted, average value of 1.8.—indicates no data available.

926


Sediment Accumulation at Site 534

The biostratigraphic criteria used in Figure 9 to arriveat the age-depth plot for Site 534 are based on nanno-fossil, foraminifer, radiolarian, calpionellid, dinoflagel-late, and magnetostratigraphy data. This multiple stratig-raphy and the resulting chronostratigraphic interpreta-tion are summarized in Table 4, which makes use of thetime scale constraints shown in Figure 5. Tables 4 and 5contain the body of biostratigraphic-geochronological

Table 4. Thickness and duration of biostratigraphic zones, assem-blages, and events in Hole 534A.

Zone, assemblage or eventaCore range

(core-section-core)

Sub-bottomdepth(m)

S. heteromorphus (N)G. insueta (F)H. ampliaperta (N)C. dissimilis (F)S. beiemnos (N)T. carinatus (N)G. kugleri (F)D. barbadiensis (N)D. saipanensis (N)G. stuarti (F)S. echinoideum (P)/. Albian (?) (F)S. vestitum (P)C. litterarius (N)S. periucida (P)W. oblonga (N)C. cuvillieri (N)T. verenae(N)R. neocomiana (N)H. sigali (F)mid-Valanginian (F)Zone A + 1. Tith. (C)TV. colomi (N)Zone B (C)Berr.-e. Valang. (C)C. mexicana (N)L. quenstedti (F)Kimmeridgian (P)C. paraspis (F)Oxfordian (P)e. Oxfordian (R)S. hexum (N)M. Call.-E. OxfordM-16 (M)M-18 (M)M-20 (M)M-22 (M)M-23/24 (M)M-24/25 (M)M-28 (M)

(R)

233,475,67-1810-1819-2119-2124-2627-302732-364436-4945-5859-7071-7778-8452-5371

92-6-93-485-93

90-7-92-287-2-90-191-10296-102105-111110112-121117-122

123-2-127-4122-126849096101-102104-105111-112127-130

545-555565555-574593574-593593-690593-660696-715696-715741-764764-802775812-859923-932850-972932-10531052-11571157-12181218-1277990-100811651347-13511277-13491322-13441297-13231331-14281371-14281446-150414951304-15901549-15941590-16381590-16301269-12761321-13281373-791409-14211436-14501495-15061636-1666

Duration(m.y.)

15.5-1716-1717-18

18.5-19.518-1919-22.5

22.5-23.537.5-4337.5-4370-67100-103100-105103-106113-115106-115115-123123-126126-129129-13212128135-138132-135133-137130-135135-141139-143141-143143-146143-149146-149149-154147-154133135137140141-143144-142154-156

Note: Time scale is as used in Figure 5.a N = nannofossils; F = foraminifers; P = palynomorphs; R = radiolarians;

C = calpionellids; and M = geomagnetic reversals.

interpretations and the original and decompacted thick-ness and compacted and decompacted sedimentation ratefor average depositional intervals in Site 534. These in-tervals are determined from the points of gross changein rate. Further study will attempt to define the ratio ofturbiditic versus background (nepheloid, pelagic, trac-tion current) deposition to determine more realistic thanaverage sedimentation rate values. Measured densityvalues are as presented in the physical property studies(this volume); the values are corrected for postcoringdecompaction.

Essentially, the sediment accumulation curve showsthree broad trends. First, there is the remarkably con-stant average rate of 1.4 to 2.5 cm/103 yr. from the Cal-lovian through the Albian. In 60 m.y. (155-100 Ma),about two-thirds of the sediment (decompacted = 1520m) accumulated. This event was followed by a relativestandstill for another 80 m.y., after which the last one-third accumulated in less than 20 m.y. A more detailedexamination of the curve in light of known paleoceano-graphic and continental margin events over the last 150m.y. explains at least part of this sedimentation pattern.The broad Callovian to Kimmeridgian eustatic highstandof the sea (Hallam, 1975), as, for example, expressedin the significant Callovian-Oxfordian transgressions orfacies changes of part of the North Atlantic margins(Essaouira, Scotian, and Lusitanian basins), coupledwith a low relief hinterland, caused little or no, coarserthan clay-size, terrigenous clastic influx in the deepestpart of the Atlantic deep basin. Most of the sediment isclay or carbonate. The basal black, green, and red shaleswere deposited at an average rate of 0.7 cm/103 yr. Sucha rate compares well with the mid-Cretaceous blackshale rate; it agrees with the phosphatic content, the fishear bones, and high radiolarian content, which indicateslow sedimentation under possibly poorly oxygenatedconditions. The finding of nannofossil-rich fecal pelletsin thin section and poor benthic foraminiferal fauna in-dicates limited benthic life in the deep basin. Such limit-ed benthic life may be due to local depressions of theseafloor rapidly creating low oxygen conditions whenthe sedimentation was rich in organic material and thecirculation possibly restricted. The latter is quite con-ceivable in the earliest Central Atlantic Ocean, whichmight have had narrow passages to the Pacific in thewest and to Tethys in the east and not much of a verticaland lateral temperature gradient. Alternatively, as men-tioned in the discussion section of Habib's chapter (this

Table 5. Original and decompacted thickness, interval duration in time, and average compacted anddecompacted sedimentation rates for Site 534.

Interval(m)

1639-14951495-14201420-935935-775775-690690-593593-545

Measured density(g/cm3)

2.52.5

2.6-2.42.12.12.02.0

Decompactedthickness

(m)

27514392621611712160

Age

m. Callovian-OxfordianKimmeridgianTithonian-BarremianAptian-AlbianCenomanian-Oligoceneearly Mioceneearly-middle Miocene

Duration(m.y.)

154-144144-141141-115115-100100-22.5

22.5-1919-15.5

Sedimentation rate(cm/103 yr.)

Compacted

7.42.51.91.00.12.71.4

Decompacted

2.74.83.61.40.23.41.8

927


165 156 149 143 138 135 131 126 121 115 108

391 A, B,C

- J 3 -

Lithology

Silty clay

nannofossilooze

Chalk(no elastics)

Intraclasticchalk

Claystone

Intraclastchalk

Chalk, mudstone(with clastsl

Variegated 1

Olive grayblack claystone

Variegated II

Calcareous claystone.sparse

calcilutite

Calcarenite

mm laminatedgray, shalycalcilutite

White greenshaly calcilutite

White calcilutiteshaly interbeds

Variegated calcilutite

' Dark red ' •calcareous claystone

SbeJ È

Ss<o

c

O-

jjj

I

Im

1m

&

Sö

Cores1, ID

2A

1

3

^

5

67 8

910

11

12

13 1 4

19

2C20

3C4

5 , 6

7

8

9

10

11

12

13

14

2021

2223

54

160 Jurassic 140

Bathon. Callov. Oxf. Tithon.

130 120 110 Cretaceous 90

Berr. Valan. Haut. Barr. Apt. Alb. Cenom. Turon.

1219

1257

1371

1412 TD

Sub-bottom depth (m)

Decompacted - —

1412 1200 1030 970

Sub-bottom depth (m)

Figure 9. Sediment accumulation rates in DSDP Sites 534 and 391, Blake-Bahama Basin. (For explanation see text.)

928


Sant. Camp. Maest

53.5 49 43 37.5 22.5 16 11.540 Tertiary

Oligocen

2 Foraminifers

3 Calpionellids

4 Palynomorphs

5 Radiolarians

— I Geomagnetic reversals

Site 534Site 391

534 + 534ALithology Cores

Gray.silty

Not cored;(see 391 A)

Intraclastchalk,

siliceous

Variegatedclaystone chert

green-black 4fa

Variegated —green-black 4d

rbonaceclayston

Mi=B carbonu claystone 5b| laminatedi chalk _

-S Bioturbated

- siltstone

S Red claystone

-545-566-595

-696

-724

-764

-950 - B -

1-1340 - C -

-1612

-1639

1666.5 TD

Hole 534A

Virtual nondepos tion; some erosion

2.7

1.4

690 593

Virtual nondeposition; some en


929


volume), rapid deposition of terrigenous organic richsediments and fecal pellets might lead to preservation ofthe organic matter even in oxygenated bottom waters.Thus the Callovian black shales could have deposited inmore normal circulation conditions.

The overlying micritic and bioclastic limestones, red-dish to green gray claystone, and radiolarian silt of thelate Callovian through the Oxfordian bear strong evi-dence of turbiditic and probably current deposition. Pe-riodically, the sedimentation rate was high for oceanicconditions, although the average value is only 1.4 cm/I03 yr. The overlying Kimmeridgian through Tithonianred claystone only averaged 1.9 to 2.5 cm/103 yr. Thesequence of "boxes and error bars" suggests that at theJurassic/Cretaceous boundary sedimentation may havebeen even slower and, together with the low CCD, re-sults in a high biomass with relatively rich ammonitesand calcispherulid, calpionellid, and benthic foraminif-eral assemblages.

There is no change from the Jurassic average rate inthe Early Cretaceous when the pelagic and turbiditiclimestones, chalk, and terrigenous elastics of the Blake-Bahama Formation accumulated. Mid-Cretaceous ratesof sedimentation of the Hatteras dark clays is again onthe order of 1 cm/103 yr. (1.4 decompacted). By thetime of maximum transgression of the continents in theCenomanian, the Blake-Bahama Basin was a starved ba-sin, remaining so until the Miocene. As we will arguelater, it is unlikely that steady, Late Cretaceous-Pa-leogene accumulation occurred, only to be eroded in theOligocene. There was a return to relatively rapid ac-cumulation after the Oligocene, with large fluctuationsin rate due to the debris-flow nature of the deposits. It isprobably more than coincidence that somewhat similar"catastrophic" sedimentation is reported from the Mo-roccan margin (von Rad and Arthur, 1979).

The average rate of accumulation at Site 534 corre-sponds, within the range of error, to rates obtained fornearby Site 391. The meaning of (1) a slightly higheroverall rate in the Valanginian-Barremian Blake-Baha-ma Formation at Site 534, (2) a decrease in rate in theAptian-Albian, and (3) the presence of the Eocene Ber-muda Rise deposits will have to be evaluated after moredetailed study.

LATE CRETACEOUS TO PALEOGENE DEPOSI-TION OR NONDEPOSITION IN THE

BLAKE-BAHAMA BASIN

Maturation studies using vitrinite reflectance haveyielded conflicting results on what happened during theCretaceous-Miocene hiatus at Deep Sea Drilling Site391, 22 km southwest of Site 534. On the basis of a se-ries of vitrinite reflectance data of the Cretaceous blackshales and one data point in the immediately overlyingMiocene chalks and clays, Dow (1978) has hypothesizedthat 800 m of meterial were removed from the Creta-ceous section before the Miocene. An independent studyby Cardozo et al. (1978) provides three more points forthe Miocene interval that agree with the scant informa-tion from Dow (1978), but, unlike Dow, it fails to showa reflectance index in the Cretaceous strata higher than

compatible with the present stratigraphic thicknesses. Itis peculiar that Cardozo's et al. (1978) scatter of pointsin the Cretaceous shales shows a reflectance gradientwith a negative intercept at zero depth; maybe their datahas analytical errors. The lack of agreement between thetwo studies could be due to failure to differentiate satis-factorily between primary and secondary vitrinite parti-cles. Clearly, there is a need to reexamine the vitrinitereflectance measurements in an attempt to reconcile thetwo conflicting depth versus reflectance profiles. Creta-ceous palynomorphs at both Sites 391 and 534 are well-preserved and translucent, suggesting a low level ofthermal immaturity that requires no significant overbur-den. At best the 800 m of missing sediment should beconsidered an overestimate of the true value and is hardto reconcile with the slow net sedimentation rate for theLate Cretaceous-Paleogene found now at Site 534.

The new stratigraphic information from Site 534 pro-vides significant constraints on the amount of deposi-tion and erosion or nondeposition in the Late Creta-ceous-Paleogene. At Site 534 there is no Cretaceous-Miocene hiatus. The mid-Cretaceous dark shale lies dis-conformably under the Maestrichtian variegated shale,which in turn is disconformably overlain by late Eocenezeolitic clay and chert. Miocene (debris flow) interclastchalk disconformably overlies the Eocene cherty forma-tion. The total thickness of sediment present in thestratigraphic interval between the Albian and the Mio-cene is 68 m. Obviously the section is much condensedand riddled with hiatuses. The metalliferous content ofthe Eocene variegated, zeolitic clays indicates that sedi-mentation was very slow (see also Robertson, this vol-ume, on arguments against upward diffusion of metalsfrom the underlying Hatteras shales), much less than inthe mid-Cretaceous and Miocene.

What is now known of the Late Cretaceous-Paleo-gene paleoenvironment does not suggest that rapid de-position followed by drastic erosion occurred. The Mae-strichtian and Eocene sediments are apparently not rem-nants of a much thicker section. Even if some erosiontook place, for example, in the Oligocene when possiblythere were drastic local changes in Oceanographic condi-tions, it is extremely unlikely that prior buildup of muchsediment could have occurred after the late Eocene. Hy-pothetical deposition of 800 m from the Late Creta-ceous through the Paleogene would have required anaverage rate of sediment accumulation of 1.5 cm/103

yr., a rate similar to the earlier pre-Late Cretaceous.Rates of several orders of magnitude larger are in orderif the stripped sediment had to fit in the time gaps atthe disconformities. In all, the sedimentary informationfrom Site 534 does not corroborate steady average sedi-mentation followed by profound (Oligocene) erosion.Late Cretaceous through Paleogene "starvation" ap-pears more likely.

JURASSIC PALEOGEOGRAPHY ANDPALEOCIRCULATION

The stratigraphic, sedimentologic, and seismostrati-graphic record at Site 534 provides information on Ju-rassic water-mass circulation that will be reviewed in the

930


light of the position of the widening Atlantic Basin rela-tive to the Tethys and the Pacific Ocean. Figure 10shows a generalized Jurassic stratigraphic framework(Lancelot and Winterer, 1980) in circum-Atlantic mar-gin basins. Two regionally significant transgressions areshown, one in the postevaporite-dolomite basins of theGrand Banks, Portugal and Scotian Shelf, Morocco,dated as Sinemurian, and one in the Callovian-Ox-fordian.

The margin sedimentation was always shallow ma-rine; evidently sedimentation matched subsidence, asdocumented in Grand Banks wells (e.g., Gradstein etal., 1975). The sedimentary record in the marginal ba-sins shows that the deep part of the Jurassic basin wasconfined to the slope and rise areas of the present ocean.

On the basis of the discussions presented in this chap-ter on the age of marine magnetic Anomaly M-28 (earlyCallovian) and of the Blake Spur Anomaly (Callovian/Bathonian boundary), and on the nature and age of theJurassic (marine) Magnetic Quiet Zone, we believe thatthe Atlantic Ocean did not open much before the Cal-lovian. At that time the only deep oceanic basin wasconfined to the area of Site 534 in the western NorthAtlantic and to Site 416 in the eastern Atlantic. Thedeepening oceanic basin, then, was largely confined tothe outer marine Jurassic Quiet Zone. Based on thispostulate, we have reconstructed the Callovian throughKimmeridgian paleogeography of the central North At-lantic Ocean basin (Fig. 11A-C).

These reconstructions also show the extinct rift of theproto-Atlantic and Gulf of Mexico confined to theNorth American plate, and oceanic lithologies as foundat DSDP Sites 534, 105, 416, 100, and, for comparison,in the pelagic Lombardy Basin (J. Ogg, personal com-munication, 1981) and the Ligurian Tethys (Lemoine,this volume).

On these reconstructions we show that the Middle andLate Jurassic surface water exchange between Tethys inthe east, the incipient Central Atlantic Ocean, and prob-ably the Pacific as well was unimpeded and relativelystable. Such circulation is evidenced by the rich OldWorld-type Middle and Late Jurassic nannofossil andradiolarian assemblages and by the filament-type faciesat Site 534. The latter was even recorded from the silici-fied limestone laminae between individual basalt pillowsof basement. Coeval Callovian beds in the Mediter-ranean are also rich in these filaments, which are de-rived from a floating pelecypod. The presence of Ox-fordian planktonic foraminifers in Core 110, some ofthe oldest known (Gradstein, this volume), correlate totheir finding in Oxfordian limestone blocks dredgedfrom the Moroccan lower slope (Renz et al., 1975) andto an abundance peak in Oxfordian strata in the Medi-terranean basin margins. Clearly the Central Atlantic,however narrow, originally underwent surface water ex-change, as should be expected for a narrow ocean of thepresent Mediterranean Sea type.

The Saccocoma, Calcispherulid, Aptychi assemblageat the top of the Jurassic red claystones at Site 534 iswell known from other sites in the western and easternCentral Atlantic (100, 105, 367). It is typical of the pe-

lagic facies of Tethys and attests to further free surfacewater exchange in the expanding ocean basin.

A reasonable model of surface circulation is based onactualistic observations of wind and current patterns inthe equatorial belt. We use the reconstruction postulatedby Ager (1975). In its near equatorial position and neareast-west orientation, the embryonic North Atlanticformed an ideal passageway for the easterly trade windsof the Jurassic (Fig. 11). During the earliest stages, it ishere interpreted that the surface currents through theNorth Atlantic were wind driven from Tethys to the Pa-cific, and back into Tethys again with only a flow to thewest.

It is speculated here that during the late Callovianthrough the Oxfordian, the North Atlantic widened to apoint where the equatorial countercurrent was able tocreate return flow along the southeastern edge of theNorth Atlantic, which continued into the equatorialcountercurrent flow along the southern edge of Tethys(Fig. 11). This equatorial current pattern persisted, webelieve, until the overall Central Atlantic margin driftednorth of 30° latitude, perhaps in the Early Cretaceous,and then the effect of the westerly winds developed theclockwise North Atlantic gyre. Also, at about this timein the Cretaceous, access to the Pacific was closed by themovement of the Nicaraguan block into the Caribbean.This movement forced the trade wind currents to backup and veer northeast along the eastern North Americanmargin. This was the beginnings of the Gulf StreamCurrent.

Biogeographic studies on Jurassic mollusks (e.g.,Westermann & Riccardi, 1976), and the review by Hal-lam (1975) fail to show more than an increasing latitu-dinal differentiation of faunas from the Middle to LateJurassic. In the equatorial belt there was little or no evi-dence of a distinct eastern Pacific versus western Tethy-an (Mediterranean) province as far back as the earliestMiddle Jurassic. On the evidence of Middle and LateJurassic marine epicontinental deposits in Cuba andparticularly in Mexico (Barr, 1974; Kourdell, 1956), andLate Jurassic marine sediments in the Gulf of Mexico,Hallam (1975) and others favor a Central Atlantic path-way as early as the Bajocian. Rifting may have led to apre-Callovian ocean pathway, and it is reasonable to as-sume this to be the Bajocian-Bathonian one. We mayconclude that the new data from Site 534 agree reason-ably with the ocean connections in the Middle Jurassicsuggested by biogeographic data.

An assessment of Jurassic bottom circulation is morespeculative, but we are aided by the discovery of blackshales and current deposited or winnowed beds at Site534. Organic- and phosphatic-rich sediments of Callo-vian age occur in Cores 125 through 127. Callovian deepcirculation might have been weak and possibly tempo-rarily restricted, the result of geographic barriers. Asilled basin morphology due to the topographic relief ofthe then-existent ridge flank as seen on the seismic datais quite likely. Such a basin may have been a local fea-ture, and if the dark shales result from local anoxia,they may be local as well. At present it is not possible toassess if the barriers were local or affected the basin as a

931

SO

Age Gulf Coast BahamasPlatform

BaltimoreTrough

Georges BankScotian Shelf Central

Grand Banks

CentralPortugal

Lusitanian Basin

South PortugalAlgarve Basin

Morocco

EssaquiraBasin

Aaiun—TarfayaBasin

135

143

149

156

< 165

Berriasian

Tithonian

Kimmeridgian

Oxfordian

Evaporites, red bedsmarine carbonate

Smackovershallow carbonate

Shallow-watercarbonates, thin

beds of anhydrite

Deltaic elasticssandstone andsands and shalej

plus coal

Callovian NorphletSandstone

Interbedded shallow-water carbonates,

evaporites (?)

Shallow-watercarbonates, inter-bedded anhydrite,

and salt

Bathonian

Bajocian

Aalenian

Toarcian

LouannEvaporites

Eagle Millscontinental red beds

Terrestrial redelastics sands

and shales

192

Pliensbachian

Sinemurian

Hettangian

Triassic

Significant transgressionor deepening of basin

Open marineshales

Mohawk Fm.

Terrigenouselastics

(regressive)

Avalon Upliftand erosion

Whale Unit

shales, limestone

shallow marine

Gres sup.Estuarineelastics

Shallowcarbonates

Limestones,marls

(locally smallpatch reefs)

Clastics?

Limestonesmarls

Regressivefacies

Dolomitesred beds

Dolomiteslimestones

including reefs

CarbonatesMurre carbonates

including dolomites

DolomitesDolomites

Evaporites andred beds

Evaporites andred beds

Dagorda-Silvesevaporites, red

beds, shales Silves Red beds

Red beds

Evaporitesigneous rocks

Red beds

Clastics

Open marinecarbonates

Alteration ofneritic,

lagoonal,continental

facies

Red beds

Figure 10. Generalized Jurassic stratigraphy (Lancelot and Winterer, 1980) in circum-Atlantic margin basins.


whole. From seismic data it appears that the Callovianridge flank topography was quite irregular and couldhave contained poorly ventilated lows. Once these lowscould be reached by organic-rich debris from the mar-gin, either by turbidite or weak cross-slope or along-slope currents, black shales accumulated.

Alternatively, the organic rich Callovian shales couldhave been deposited under normal oxygenated bottomwaters (Habib, this volume) and in a formation analo-gous to the Hatteras Formation, which is of great re-gional extent. In fact the seismic data (Sheridan et al.,this volume) reveal evidence of current-deposited sedi-ments in the basement low near Site 534. Bottom cur-rents may have circulated through this fracture zonetrough.

Contrary to the poor ventilation hypotheses, the Cal-lovian and Oxf ordian ocean basin was not likely to havebeen inhospitable to bottom-dwelling organisms. Asmall-size, low-diversity agglutinating and calcareousforaminiferal fauna is seen in the cores, which is largelyindigenous rather than swept in from the margin (Grad-stein, this volume). The absence of provincialism in thisabyssal benthic fauna, which for the Oxfordian-Kim-meridgian was remarkably similar to that found inDSDP sites in the eastern Atlantic and Indian oceans,indicates that the Central Atlantic Basin was a relativelyopen connection with Tethys and possibly the Pacific aswell.

From the Callovian and lower-middle Oxfordian sed-iments comes sedimentological evidence for current de-position that indicates that at least temporary ocean-floor circulation and ventilation occurred. The sedimen-tary features that suggest transportation of the carbon-ate, organoclastic, and siliceous claystone "background"sediments of Unit 7 by weak bottom currents are: (1)more layers of well-sorted, silt-size particles, mainly ofradiolarian tests; (2) thin bedding less than 5 cm inthickness, and lack of massive bedding; (3) sharp topand bottom contacts; (4) lenticular beds possibly ofstarved ripples; (5) graded beds; (6) preferred grainorientation parallel to bedding; (7) placer deposits ofsize-sorted microfossils and grains, many of them worn;(8) ubiquitous lamination; (9) measured 5° to 10°primary dips, in sets of up to 10 cm thick; (10) herring-bone cross bedding; and (11) erosional surfaces.

All of these primary sedimentary features can be ex-plained by bottom-current deposition. The fine-grained,silt- and clay-size hemipelagic sediments of Unit 7 canbe interpreted to have deposited from suspension in aturbid bottom-water layer, such as a nepheloid layer,which dumped bed upon bed in local mud wave struc-tures creating low-angle relief on the seafloor. Penecon-temporaneous slumping from these mud ridges causedlocal redistribution in swales.

Significantly, the high-resolution seismic reflectionprofiles at Site 534 show hummocky reflectors in anasymmetrical elongate mound feature beneath HorizonD, which can be interpreted as current-deposited bedforms forming mud wave and ridge structures (Sheridanet al., this volume). The asymmetrical nature of this

elongate sediment mound (it is shallower on the north-east rim of the basement trough between Sites 534 and391) suggests geostrophic current control by the Coriolisforce. Bottom currents flowing through this trough,which is formed along an apparent fracture zone, wereaccelerated and deflected with the entrained muddy bot-tom water along the right side. There the sedimentmound eventually formed as an onlapping deposit. Al-though this interpretation fits the seismic and drillingobservations, it is actualistic in that such occurrencesare well documented today. In the modern oceans, bot-tom currents frequently follow the fracture zone de-pressions that allow access through the otherwise sub-stantial topographic barrier of the mid-ocean ridge. Sed-iment-drift ridges and elongate mounds formed alongthese troughs are also known to occur. Generally thesediment drifts formed by contourite deposits are pref-erentially deposited on the right side of the fracture zonein the northern hemisphere relative to the direction ofbottom flow. The bottom-flow arrows in Figure HBwere accordingly drawn to show a normal clockwisegyre of bottom flow around a bathymetric ridge in thenorthern hemisphere.

The evidence for bottom-water circulation in Unit 7corresponds to the increased abundance of the radio-larian silt layers. Generally, the more vigorous circula-tion in the embryonic Atlantic at that time could havebeen related to up welling, which could have increasedradiolarian productivity. Indeed, radiolarian cherts arecommon throughout the Tethyan realm in the Oxford-ian, and this event probably signaled the abrupt widen-ing of the Atlantic, the development of more complexsurface circulation and divergences, and the resultingup welling and bottom-water flow.

BLACK SHALE DEPOSITIONAL MODELS ANDHYDROCARBON SOURCE POTENTIAL

A subject of continuing controversy was addressedby drilling at Site 534: that of the origin of the blackshales. These interesting sediments were encountered ex-tensively in the Hatteras Formation and the Blake-Ba-hama Formation, as well as in the newly discovered Ju-rassic formation, as yet unnamed. Three contrasting ex-planations on the origin of these shales are included inthis volume (Habib; Katz; and Summerhayes and Mas-ran). We review these models, pointing out the salientfeatures of each and stressing their differences.

Katz (this volume) notes that several isolated strata inthe Hatteras and Blake-Bahama formations are hydro-gen rich and are interpreted to contain significant quan-tities of autochthonous marine organic matter. In hisopinion these occurrences are so restricted in time andspace (thickness) that they do not represent major ex-pansions of anoxic bottom-water masses, but rather lo-cal and brief, temporary occurrences of such reducingenvironments. Katz's opinion that the sediments rich interrestrial organic matter could survive oxidizing depo-sitional environments is accepted by many organic geo-chemists (Demaison and Moore, 1980; Tissot et al.,1980); however, this does not mean that they could not

933


Blake Spur Anomaly time

B

Surface flowBottom flowPaleogeography

M-26 Anomaly time

Figure 11. A-C. Paleogeography and schematic paleocirculation of the North Atlantic Ocean in the early Callovian (Blake SpurAnomaly), mid-Oxfordian (Anomaly M-26), and early Kimmeridgian (Anomaly M-24).

934


Surface flowPaleo geography

Spreading axis

M-24 Anomaly time


have been deposited in reducing environments as well.Other evidence would be needed to reach such a con-clusion about the conditions of the bottom water.

Summerhayes and Masran (this volume) have con-sidered this additional evidence by looking at the sedi-mentologic and age data as well. They conclude that theorganic matter reached the Site by turbidity currents ora slow-moving, near-bottom nepheloid layer. They notethe particular decrease in abundance of organic matterin the late Aptian versus the great abundance in theValanginian-Aptian and in the Albian-Cenomanian.Evidence of the well-preserved lamination and lack ofbiogenic burrowing in these black shales suggest mar-ginal living conditions on the bottom even while theremight have been high marine productivity in the surfacewaters. Summerhayes and Masran feel that there is ma-rine organic matter in these marine shales that requiredreducing conditions for preservation of both types oforganic matter, but that the marine organic matter wasdiluted at times of increased supply of terrestrial ma-terial. Alternations of marine and terrestrial dominanceof the organic matter is detected, with higher terrestrialfacies in the Valanginian and again in the Aptian-Al-bian. Citing the Vail et al. (1977) sea-level curve, theyconclude that an increase in the supply of terrestrial or-ganics occurred when the sea level dropped, because

rivers dumped more terrigenous elastics into the deepbasin, and that when the sea level rose there was adominance of marine organic matter because marineproductivity was higher when sea level was high. Re-gardless of the source of the organic matter, relativeabundances, and modes of deposition, these authorsfeel that reducing bottom water is necessary to explainthe preservation of this matter.

Accordingly, Summerhayes and Masran propose amodel that involves the influx of oxygen-poor waterfrom the Pacific at times of higher sea level and restric-tion of the influx during times of lower sea level. Withthe absence of the supply of oxygen-poor water, thenormal circulation of the Atlantic would cause the bot-tom to become oxidized during times of lower sealevel.Apparently, according to their model, when the Atlanticwas restricted it was more oxidizing, but when it waswell connected to the Pacific it was reducing (Fig. 12).

In contrast to this model, Habib (this volume) feelsthat all of the black shales, both with terrestrial and ma-rine organic matter and in all the formations includingthe Callovian, could have been deposited in the Atlanticwith oxidizing bottom-water conditions. He cites evi-dence in the form of black carbonized organic matterfound even in the red shale of the Cat Gap Formation.Habib feels that the presence of this organic matter indi-

935


Pacific Central North Atlantic Tethys

Sea level low; poorPacific connection:Oxidizing

Sea level high;good Pacific connection:Reducing

North America Central North Atlantic Africa

Trapped plant debris

J X X X X X X X

X X X X X XHigh x x x x x x x

productivity x π u x

Rapid depositionfecal pellets

Highest sea level;high marine organicmatter: oxidizing

Figure 12. Schematic models for Cretaceous black shale deposition.(A & B. After Summerhayes and Masran, this volume. C & D.After Habib, this volume.)

cates that even in the most oxidizing environments andat slow rates of deposition, some organic matter sur-vives. To him, the abundance of preserved organic mat-ter is then a function of the amount and rate of influx(the more rapid the influx the greater the preservation).Habib combines data on the organic microfacies thatdemonstrate transport sorting and the allochtonous na-ture of the organic matter with evidence about the strati-graphic age that demonstrates that the kind of organicmatter varies with the rate of influx. The influx rate ishigher when terrestrial plant material dominates, and itis lower when marine organic matter occurs. Habib alsonotes the alternations and cycles of occurrences of ter-restrial versus marine organic matter dominance.

Whereas Habib admits the preservation of organicsediments below the seafloor will lead to reducing con-ditions in the sediments, he feels there are cases in themodern oceans where this goes on with the overlyingwaters being normally oxidized. All that is required is arelatively rapid influx of organic-rich sediments. To ex-plain the preservation of the less resilient marine organicmatter, Habib cites evidence that the marine organicmatter has microscopic features resembling fecal pelletmaterial. He thus invokes the concept that the marine

organic matter is consumed and digested before beingrapidly deposited through the oxidizing water masses.

The cyclicity Habib observed between terrestrial ver-sus marine organic facies does correlate in time with theVail et al. (1977) sea-level curve. During times of risingsea level there is apparently a great influx of terrestrialplant debris (Fig. 12). The greater abundances of themarine microfacies apparently occur during times ofhighest sea level after the rise. A model to explain thiswould involve the greater influx of terrigenous materialfrom large coastal rivers during the beginnings of theworldwide sea-level rise, as it affected the continentsaround the central North Atlantic. As the sea levelreached its greatest height, the rivers dropped their ter-rigenous material, including much plant material, indeltaic estuaries on the continental shelves. At the sametime, during the high sea-level stands greater marineproductivity occurred, which gave rise to relatively rap-id sedimentation of fecal pellet debris.

Habib's model fails to explain the higher-frequencycycles in the black shales manifested in the several-centi-meters-thick black and green bands (Site 534 report, thisvolume). Habib finds similar amounts of organic matterin both the black and green bands, and he concludesthat the color is caused by slightly more or less reducediron. This conclusion implies that the influx of organicmatter is not the only control of the sub-bottom Eh (oxi-dation potential). Rather, some other higher-frequencychange, on the order of 20,000 to 100,000 yr., must beinvoked. Slight global temperature variations due tovarious astronomic cycles are probably the causes ofthese cycles in the black shale.

Also, it is difficult to apply Habib's model to explainthe occurrences at about the same time in the Jurassicand Cretaceous of black shales at many other places onthe earth and in many different types of marine settings(e.g., in the continental interior seaways, which weremore like the Baltic Sea of today, and in the distantreaches of the deep Pacific Ocean). Black shales fromthe Cretaceous Pacific suggest widespread anoxia there,which Summerhayes and Masran (this volume) invoke asa means to reduce the Atlantic. Their model would ex-plain the correlation of Atlantic organic preservationevents with those occurring on a global scale. Habib'smodel, on the other hand, depends only on influx fromthe local Atlantic borderlands.

However, the organic shale events in the Habib mod-el depend ultimately on global sea level and global sur-face temperature and humidity, and consequently, theycannot help but correlate in time with other events of or-ganic carbon deposition in many other environments,even if the other carbonaceous deposits might or mightnot form in truly anoxic environments.

Regarding the petroleum potential of these blackshales, they all appear to be thermally immature (Katz,this volume). The terrestrial organic matter would be agood gas and coal source rock, but only a few levelshave high yields. The hydrogen-rich marine organicmatter has only limited potential to generate liquid hy-drocarbons because of its limited extent.

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SEISMIC STRATIGRAPHY AND CORRELATIONSAT SITE 534

Of importance to the seismic stratigraphy of the west-ern North Atlantic Ocean is the documentation of theage and origin of seismic reflection Horizon D (Sheri-dan et al., this volume). This was the "new ground"studied as part of Leg 76 and a major objective of thecruise. The correlation of this horizon with the bottom-levelling early Oxfordian limestone turbidites was thesuccessful completion of that objective.

One of the minor revelations was the discovery of Eo-cene porcellanites occurring as a relatively thin (27-m)unit; this unit is missing at nearby Site 391, only 22 kmaway. The cherts occurring near this interval indicatethat the zone of porcellanites and cherts that generallycause seismic Horizon Ac reaches to Site 534, fartherwest than heretofore mapped. Because of the thinnessof this unit, we interpret that seismic Horizons Au andAc are merged by convolution interference effects andare seismically irresolvable. This interpretation suggeststhat there might be more Eocene siliceous deposits exist-ing in the western North Atlantic west of the apparentAu/Ac truncation mapped by Tucholke and Mountain(1979).

There is apparent seismic evidence for possible deep-sea unconformities at newly defined seismic reflectionHorizons ß', C , and D' (Sheridan et al., this volume).Truncations of internal reflections below these hori-zons, and downlap above are the criteria for erosionaland nondepositional hiatuses (Vail et al., 1980). In fact,Vail et al. (1980) identified hiatuses at similar levels onseismic reflection profiles north of Site 534, and thoseidentified in this volume might be the correlative uncon-formities, although no direct seismic ties have beenmade to the Vail et al. (1980) profile. Abrupt changes indrilling rate and lithofacies, terminations of fossilzones, and changes in magnetization character are pres-ent at depths that correlate well with these seismic un-conformities and provide good supportive evidence oftheir occurrence. However, no biostratigraphic zonesare missing, so there is no paleontologic evidence forthese new unconformities.

These possible, new unconformities are unlike Hori-zon Au where several stages are missing between theMiocene and Eocene sediments. Also, there must exist ahiatus between the thin unit of Maestrichtian variegatedclaystones and the deeper Cenomanian black and greenclaystones, and between the Maestrichtian claystonesand the Eocene porcellanites (Fig. 5). These two uncon-formities, possibly of the mid-Cenomanian or Coniac-ian, and of the mid-Paleocene, might be correlative toones of Vail et al. (1980), but they are so close togetherphysically that they are merged into the same event asAu, the marked unconformity possibly of the Oligocene.Could this be true for the rest of the western AtlanticOcean as well?

There is no clear indication of seismic Horizon A* atSite 534, which Tucholke and Mountain (1979) correlatewith a Maestrichtian chalk layer farther seaward. Themerging of all these Au, Ac, and A* reflectors nearer the

margin makes the deciphering of the several possiblehiatuses difficult. Consequently, it is possible that sev-eral erosional events, all of equal importance in terms ofmaterial removed, could have occurred from the Creta-ceous through the Oligocene. This possibility contrastswith the previously held view that there might have beenmore continuous deposition in the Late Cretaceous andthe Paleogene, and then drastic erosion in the Oligo-cene.

Disagreements about the correlation of reflectors C',C, and D' have arisen between Sheridan et al. (this vol-ume) and Shipley (this volume). These differences resultfrom two different approaches applied to make the cor-relations: one approach uses only certain shipboard ve-locity measurements and physical properties data (Ship-ley, this volume); the other approach considers a widervariety of data, including shipboard velocities, limiteddensity logs, drilling rate logs, and geological evidencefor hiatuses and physical changes, as well as compari-sons with sonobuoy velocities from the site surveys (Site534 report, this volume; Sheridan et al., this volume).Although the correctness of either of these correlationscannot be uniquely demonstrated, because of the inabil-ity to get continuous acoustic and density logs and tocreate valid synthetic seismograms, there are somepoints about the correlations that should be pointed outto the reader.

One of the major reasons for the disagreement is theuncertainty about the position of the drill site with re-spect to the seismic data at the site. Although this un-certainty is less than 1 km, there is enough relief in thereflectors in question (i.e., C, C , and D') that onlyslight differences in the velocities assumed in the Shipleyanalysis, and a slight shifting in the possible position ofthe site, would lead to agreement with the Sheridan etal. (this volume) correlations.

We should state that the uncertainty of less than 1 kmin the position of the site relative to the seismic lines isbetter accuracy of positioning than any previous DSDPdrilling in the western North Atlantic and perhaps any-where else. The best available Loran C and satellitenavigation was used on the Robert Conrad during thesurveys and on the Glomar Challenger during the drill-ing, and the Loran C type of navigation is not availablein many other parts of the world.

The problem is that even with this relatively excellentpositioning, we are trying to discriminate between re-flectors in a thin sedimentary section where the mappedhorizons are only one wavelet apart. Thus, calculatedtime differences of only 0.04 s could shift the correla-tions by one wavelet. Uncertainties in the calculatedtimes to reflectors using assumed velocities, as Shipleyhas done (this volume), are very sensitive to uncertain-ties in those velocities.

For example, by increasing the assumed velocity inthe Au-j8' interval in the Shipley acoustic model by only7% and by shifting the possible location of the site byless than 1 km, there can be achieved good correlationof the C , C, and D' reflectors in agreement with thatof Sheridan et al. (this volume). This An-ß' interval isquite critical, for in this interval the Shipley (this vol-

937


ume) model assumes a velocity of 1.86 km/s, which isquite a bit lower than the 2.50 km/s velocity Bryan et al.(1980) reports from the same interval based on sono-buoy data. There are reasons to expect that the sono-buoy values are too high (Site 534 report, this volume),but the 1.86 km/s velocity of Shipley (this volume) isunusually low for this depth of burial. Apparently,these 1.86 km/s velocities are the same as the averagedvalues of those measurements made in the laboratory onboard the Challenger. No decompaction correctionswere applied by Shipley (this volume) for these measure-ments. This might be the source of the problem.

We note also that Shipley's correlation of C and Cplaces them at depths where the good density log doesnot indicate a drastic physical properties contrast. Thisis probably because Shipley accepted the boundaries ofthe sedimentologic units as the boundaries of his acous-tic units. Often, however, the sedimentologic bound-aries are based on the changes in composition, and thepercent of CaCO3 rather than on whether that CaCO3 ispoorly cemented or strongly consolidated, which is theproperty that causes an impedance contrast. This ispossibly true of the boundary of Lithologic Subunits 5cand 5d at 1268 m, which Shipley correlates with Hori-zon C. The gamma logs and drilling rate changes sug-gest that the physical break is shallower, at approxi-mately 1250 m. At this sub-bottom depth the nannofos-sil chalks become harder and cemented to form a mas-sive unit, and the clay content has decreased enough todecrease the gross fissility of the rocks. In certain casesthe physical contrast is not exactly at the boundary ofthe sedimentologic unit defined on other grounds. Mostof the time these are the same or are reasonably close indepth.

Shipley accepted the sedimentologic divisions in anattempt to avoid more subjective reasoning. One of hisgoals was to achieve a more objective means to establishseismic correlations using only the physical propertiesmeasurements. However, accepting the sedimentologicunit boundaries, while objective on his part, just in-volves the transferral of subjective decisions made byothers. This does not make the final correlation anymore correct.

ORIGIN OF THE GREAT ABACO MIOCENE MASSFLOW DEPOSITS AND THE ELEUTHERA

FAN COMPLEX

New information from seismic reflection profiles inthe area of Site 534 might clarify the presence of themassive, proximal turbidites, grain flows, and debrisflows deposited during the Miocene. Because the site isin the center of the Blake-Bahama Basin, more than 200km from the nearest possible sediment source (the BlakePlateau) and the nearest possible canyons (Great Abacoand Grand Bahama canyons), and the fades are socoarse, it is hard to imagine a sheet flow of this sedi-ment spreading out over the entire basin from a pointsource. This is especially true of individual coarse flowsthat are at least as thick as 10 m, and might be as thickas 30 m.

The synthesis of the seismic reflection profiles avail-able (Sheridan et al., this volume; Sheridan et al., 1981;and Sheridan, this volume) allows the mapping of a bur-ied Miocene ridge that creates a drastic, hummocky hy-perbolic reflection unit. This ridge occurs just west ofSite 534 and can be traced to the bight in the Blake-Ba-hama Escarpment where Great Abaco and Grand Baha-ma canyons enter the Blake-Bahama Basin from the Ba-hamas.

The interpretation is now made that this buried ridge,called Eleuthera Ridge, is a deep-sea fan complex (Blief-nick et al., this volume). The hummocky seismic patternis thought to be created by small-scale channels andlevees that crisscross the fan in a complicated distri-butary pattern. The confinement of an individual tur-bidite, debris flow, or grain flow in these narrow path-ways of the channels allows far greater velocities, andthe capacity of the flows therefore varies far from theirsource. A flow debouching from one of these fan chan-nels could then come from a point source only a fewtens of kilometers distant from Site 534, which could ex-plain the very proximal facies found in the MioceneGreat Abaco Member.

GAS HYDRATES ON THE BLAKE OUTER RIDGEGreat strides have been in understanding gas hydrates

in the marine environment as a result of drilling at Site533. Prior to drilling this hole the hydrate presence wasbased on theoretical considerations, the observed acous-tic properties, and conformity to geochemical princi-ples. Now, actual gas hydrates have been seen in thecores at Site 533, and various experiments, including thefirst successful deployment of the Pressure Core Barrel(PCB), measured geochemical parameters that givestrong evidence of hydrates. The most important ofthese is the measurements of gas volume and composi-tion of the recovered pieces of gas hydrate. These mea-surements indicate a gas volume of 20 times the normalpore volume present in the sample, requiring muchmore gas than could be present in normally saturatedpore fluid (Kvenvolden, this volume). Sublimation ofsolid gas hydrate must explain these large gas volumes.The composition of the gas involved in the hydrate ismostly methane, and no hydrocarbons of larger sizethan isobutane were found. This last factor agrees withthe theory about the size relationship of the gas hydratecrystal structure (Kvenvolden, this volume).

Pressure decline curves on two of the four PCB re-trievals had sawtooth patterns that indicate the presenceof small amounts of gas hydrates (Brooks et al., thisvolume). Apparently, the sublimating hydrate contri-buted gas pressures to merge with the normal, back-ground degassing of the core material in the PCB. Themerged volumes then caused the repeated pressure build-up with time after each venting. Quantitative estimatesof the portion of the pore volume in gas hydrate formcould not be gained from these PCB experiments, butthe acoustic velocities of the hydrated sediments mea-sured by sonobuoys, and the decrease in salinity of thepore water in hydrated cores suggest that 10 to 30% of

938


the bulk sediment pore space is hydrated (Site 533, re-port this volume). The physical distribution of the hy-drates, either as thin interstratified solid layers or assmall crystals at contacts of the mineral grains more ho-mogeneously distributed in all the pore spaces, is uncer-tain from the work at Site 533.

Isotopic studies of the gases trapped from the subli-mating hydrates indicate that little fractionation in com-position occurred as the gas hydrates decomposed(Brooks et al., this volume). The C13 isotopes and theCVC2 ratios down the hole indicate that there was nor-mal diagenesis of biogenic organic matter within thesediments, and that this was the source of the gastrapped in the hydrate form (Galimov and Kvenvolden,this volume). No indication of thermogenic gas wasfound.

Other isotopic studies of the gases from Site 533(Claypool and Threlkeld, this volume) reach negativeconclusions contradicting the evidence for the existenceof gas hydrates at the Site. These authors indicate thattheoretical calculations imply that not enough methanewas generated in the sediments to form stabilized gashydrate, which requires more methane than normallywould saturate the pore fluids. The calculations mightbe incorrect or the excess methane generation is nonuni-form in distribution. This latter possibility agrees withother indications that the sediments above the BSR areonly partially hydrated (10 to 30%). Partially hydratedsediments could indicate that the methane present is notfrom a source deeply buried under the continental risesediments, and this fact has implications for the pe-troleum potential of these thick sediments. Also, thepossibly heterogeneous distribution of the gas hydratesin the layer above the bottom simulating reflector leadsto the conclusion that the gas hydrates would not be agood source of commerical gas in themselves, andwould not perform well as sealants for gas accumulatingbelow the BSR (Kvenvolden, this volume). However,Dillon et al. (1980) have identified amplitude anomalies,bright spots, beneath the BSR where there is possiblestructural closure and gas could accumulate. In thesecases, the BSR seems to act as a sealant, which disagreeswith the conclusion of Kvenvolden (this volume).

HEAT FLOW IN THE BLAKE-BAHAMA BASIN

Downhole temperatures at Sites 533 and 534 weremeasured at much greater depths than any previous dataon this part of the margin. At Site 533, very good mea-surements to sub-bottom depths of 400 m yield a well-documented thermal gradient of 3.6°C/100 m near thebottom of the hole (Site 533 report, this volume). Usingthe average value of conductivity for Site 533 of 3.38mcal/cm°C s (Site 533 report, this volume) yields a heatflow of 1.22 HFU.

For Site 534 the temperature measurements werequite disturbed by the cooler drilling fluids, and conduc-tivity measurements were only sporadically available.As an exercise, Henderson and Davis (this volume) at-tempt to allow for drilling disturbance of Hole 534Aand arrive at a minimum estimate of 30° C/100 m and amaximum estimate of 41 °C/100 m for the thermal gra-

dient in the hole. Also, using the few conductivity mea-surements available for Hole 534A, and relating these tothe more numerous measurements of porosity, they de-termine an algorithm between porosity and conductivitythat appears reasonable. Based on this porosity-con-ductivity relationship, they establish an estimated con-ductivity of 3.35 mcal/cm°C s. Henderson and Davis(this volume) note that the resulting range in heat flowvalues, 1.01 to 1.37 HFU, agrees with nearby pistoncore measurements made from the Knorr. We note herethat their estimates are also very close to the 1.22 HFUvalue determined with much better data from Site 533.Henderson and Davis (this volume) conclude, therefore,that their techniques of estimation might be useful inapplication to other DSDP holes where the temperatureand conductivities are only poorly known. We agree.

These heat flow values determined from Sites 533 and534 are typical for this part of the western AtlanticOcean. This is the first time such determinations weremade to such great depths below the seafloor, and it isimportant to note that these data conform closely toother measurements made by the shallow piston-coringtechniques.

Finding normal heat flow values in this area of theJurassic Magnetic Quiet Zone supports the other con-clusions made in this volume, namely that the OuterMagnetic Quiet Zone is composed of normal oceaniccrust. The character of the basalt cored at Site 534 indi-cates this fact, and the subsidence history of Site 534also appears normal.

CONTOURITE FORMATION OF THE BLAKEOUTER RIDGE

The results of Site 533 added important informationon the processes and timing of the formation of theBlake Outer Ridge. The seismicly identified unconform-ity at a depth of approximately 158 m is correlated withan abrupt change in shear strength (Site 533 report, thisvolume). Also, there is good evidence that the latestPliocene biostratigraphic zones are thinner than the ad-jacent older and younger ones, suggesting some conden-sation or hiatuses (Moullade, this volume). Although nozones are actually missing, the abrupt and unusual sedi-mentation rate change at the base of the Pleistocenecould also be interpreted as a hiatus.

The seismic reflection profiles across this unconform-ity show angular truncation of the underlying beds, as ifsignificant erosion has taken place. Also, the overlyingPleistocene sediments have downlapping and mud waveseismic bed forms, which suggest current deposition.The downlaps at the drill site suggest a period of nonde-position as well after the erosional surface formed.

Physical stratification in the rather homogeneous,green muds of Site 533 reveal little of the structure at theunconformity. This was a particular failing of the sedi-ments at the site where the seismic profiles reveal whatappear to be marked current stratification. The BlakeOuter Ridge has long been thought to have formed bythe accretion of hemipelagic mud deposited from nephe-loid layers carried by contour currents (Heezen et al.,1966). Seismic documentation of this process seems con-

939


vincing. Unfortunately the cores give no obvious visual(certainly not the "ubiquitous" stratification) evidenceexpected of contourites. Some occurrences of fissility inthe muds and claystones might reflect bedding struc-tures, and there is some coloration differences in thePleistocene deposits, but little else is observed. PerhapsX-ray studies could reveal hidden structures in the fine-grained muds and claystones.

Other supporting evidence of the unconformity atSite 533 is the mineralogy study of Matsumoto (this vol-ume). He notes that the sediments below the uncon-formity are rich in Fe and Mn (Zone III) and that this isnot a diagenetic effect. The sediments were originallyrich in these elements. Above the unconformity in thePleistocene sediments, there are a dolomite-rich (ZoneI) layer and a siderite-rich (Zone II) layer. Comparisonwith nearby Sites 102 and 103 on the crest of the BlakeOuter Ridge south of Site 533 reveals that the dolomitelayer and most of the siderite layer are missing. Thisleads Matsumoto (this volume) to conclude that deepsubmarine erosion in the Pleistocene was also a majorcontributor to the form of the Blake Outer Ridge, in ad-dition to the accretion of the hemipelagic mud.

RECOMMENDATIONS FOR FUTURE DRILLINGAND RESEARCH

This synthesis, the review in the Introduction to thisvolume, and the specialty chapters leave many questionsunanswered. Other questions are generated because theresults of Sites 533 and 534 in the western North At-lantic Ocean are definite, and the implications of theseresults conflict with existing hypotheses. One such ex-ample is the dating of the Blake Spur Marine MagneticAnomaly and the implications for the timing of laterbreakup of the eastern North American margin and theGulf of Mexico. Another is the evidence for sedimentstarvation, rather than erosion, as the principal causeof the widespread Cenomanian-Miocene hiatus in theBlake-Bahama Basin.

Consequently, the results of drilling at DSDP Sites533 and 534 have identified a number of key researchtopics that should be addressed to further our under-standing and better economic use of the geology of theeastern North American continental margin, and to de-velop better geochemical, sedimentological, paleonto-logical-stratigraphical, and geophysical models in pale-oceanography.

(1) Hole 534A is the first DSDP hole to reach base-ment well inside the Jurassic margin Magnetic QuietZone; further drilling in the outer marine Jurassic Mag-netic Quiet Zone should be done to document its age ofCallovian-Oxfordian. Such drilling will strengthen theestimate of relatively fast (~3 cm/yr.) early Atlanticspreading, which, in conjunction with revisions of Ju-rassic paleomagnetic and biochronostratigraphy as pre-sented in this volume, details early paleoceanographichistory. The age of the quiet zones should also be testedby drilling the basement in the Pacific Jurassic QuietZone.

We are aware that these suggestions are presently inthe program for future deep-sea drilling, but no one can

be sure that the planned drilling will be successfullycompleted or even started. Reiteration here of these pro-posals is therefore warranted.

(2) The small amplitude lineation M-28, as modelledin the marine Jurassic Magnetic Quiet Zone, has notbeen substantiated by drilling. The basalt at the bottomof Hole 534A was normal in its polarity of remanentmagnetization, rather than reversed. It is thought thatthe Jurassic Magnetic Quiet Zone is an extended periodof normal polarity, with possible short reversal eventsor intensity fluctuations as the cause of the weak linea-tions. The basalt at Site 534 that is near the easternboundary of M-28 could be a pillow flow, generated justa few kilometers from the Site during the time of thenext younger normal polarity. These normally magne-tized flows could then have laid on top of the reverselymagnetized flows deposited a short time before in theAnomaly M-28 zone.

Often at DSDP sites in the past drilling has pene-trated the basalt pillow lavas of alternating polaritieseven though they are located on well-defined seafloormagnetic anomalies. Such interlayering of alternatingpolarity basalts will produce no seafloor magneticanomalies measured at the surface, and this seems toimply that the source of the seafloor magnetic anoma-lies are deeper, perhaps in the dike layer (2b) below thepillow basalts (2a). A possible solution to this problemof the weak lineation will require a combined approachof further shipborne magnetic studies of the linearanomalies, documentation in the Oxfordian-Calloviansection on land of these past field variations, either ofreversals or intensity variations, and deeper drilling intolayers 2a and 2b at more than one location, on a well-mapped weak lineation.

(3) The Jurassic biochronology and magnetochronol-ogy that can be applied to Tethyan (and boreal) oceansites is still relatively undeveloped. Site 534 did not yieldgood paleomagnetic data for part of the Jurassic sec-tion, in part because of poor sediment recovery. Thesame is true for the biostratigraphy, which shows con-troversy on the position of the Oxfordian/Kimmeridgi-an and Kimmeridgian/Tithonian boundaries (Gradstein,this volume; and the section on Jurassic time scale inthis chapter). Some of the necessary studies are beingcarried out in Tethyan and boreal land-based sections,but, particularly in the case of foraminifers, nannofos-sils, and palynomorphs, unique oceanic zonations haveto be developed and properly calibrated to ammoniteand calpionellid standard zonations in land sections.The Jurassic magnetic reversal scale as shown in Figure5 is still untested for all but a few reversals in the earlyKeathley Sequence in deep-sea sites on seafloor magnet-ic anomalies. It is essential that further drilling takeplace to verify and strengthen the Jurassic time scale(see the section on Jurassic time scale in this chapter).

(4) Undisputed dating of the outer marine JurassicMagnetic quiet zone between M-25 and the Blake SpurAnomaly brings into question the age and nature of theinner Jurassic Quiet Zone between the Blake Spur andthe East Coast magnetic anomalies. Previously this cor-ridor was thought to be Jurassic oceanic crust, possibly

940


as old as the Pliensbachian. Now, however, the resultsat Site 534, yielding higher spreading rates and extra-polation of these into the ECMA/BSMA corridor, indi-cate that the inner Magnetic Quiet Zone might be upperBathonian oceanic crust. This would conform to newknowledge of the paleomagnetic time scale, includingpossible lower Bathonian reversals, and of the age of theGulf of Mexico, a possible continuation of the ECMA/BSMA corridor. Future drilling in the ECMA/BSMAcorridor could address this controversy about the char-acter and age of basement. Recently a basement knollwas found in seismic surveys of the inner MagneticQuiet Zone off New Jersey. The knoll appears to havebeen in place in the Jurassic, based on the onlap of theJurassic seismic reflectors (J. I. Ewing, personal com-munication, 1981). Could this be a Jurassic seamountwith a possible Bathonian reef on top? If so, drilling in-to this knoll, which is within reach of the Challengerdrill string, would give some knowledge of the age ofbasement and subsidence history of the oldest oceaniccrust, if indeed it is oceanic. Although the seismic re-fraction and other geophysical data in the inner QuietZone appear compatible with an oceanic origin, it is stillpossible that a thin transitional crust might exist there.

(5) Prior to drilling at Site 534, only seismostrati-graphic data existed with which to identify the paleoen-vironment of the Middle Jurassic oceans. Site 534 re-vealed Callovian-Oxfordian variegated and dark shale,radiolarite and limestone turbidite sediments of bathyal-abyssal depth, and evidence of contourites and Tethyansurface water exchange. Still, our knowledge of theMiddle Jurassic oceans using one data point is scant.Questions remain on the local or ocean-wide extent andnature of bottom currents, and the exact extent of sur-face water exchange with Tethys and the Pacific inparticular, using updated biogeographic and paleogeo-graphic reconstructions. Isotope studies should helpcharacterize Jurassic water-mass properties and distri-butions in time and space and may help to explain theremarkable affinity in composition of Jurassic benthicforaminifer al assemblages at Site 534 and at Site 261,Indian Ocean (Gradstein, this volume). Evidence for Pa-leobathymetric change-at ocean sites as Sites 534 largelyderives from backtracking. Independent evidence forthe depth of the Jurassic oceans should be sought thatwill aid in delineation of the lysocline and the hypso-metric curve.

The recalculation of Jurassic spreading rates with atendency to higher values than previously thought shouldlead to a reassessment of early ocean history and its im-plications for global transgression in the Callovian.

(6) A single drill site in the Middle Jurassic oceandoes not allow establishment of regional Jurassic abys-sal lithostratigraphy and seismostratigraphy. For ex-ample, Ogg et al. (this volume) discuss the problems indefining the lower boundary of the Cat Gap Formation,because of the different lithofacies in the lowermostcores of Hole 534A. How unique is this lithofacies andhow does it relate to its eastern Atlantic counterpart in-ferred from a seismic Horizon D equivalent (K. Hinz,personal communication, 1981)?

(7) Controversy over the correlation of the seismicprofiles and physical properties-lithology at Site 534 byShipley (this volume) versus Sheridan et al. (this vol-ume) can be resolved by deeper drilling into the reflec-tors C (J j), D (J2), and J3 (untested) at a place wherethey are not condensed in a profile only one waveletapart. Good density and acoustic logs, run continuouslythrough the Jurassic sediments as at Site 534, are alsoneeded. More detailed, calibrated seismostratigraphywill help in better mapping the paleoenvironment of theJurassic ocean.

(8) As discussed here (see the section on Late Creta-ceous to Paleogene deposition and erosion) and in theSite 534 report, the extent of abyssal sedimentation anderosion over an 80-m.y. period in the Cenomanianthrough the Oligocene is still largely unknown (see alsoRovertson, this volume). The evidence at Site 534 favorsstarvation rather than erosion, but both processes mayhave been active. Stratigraphic correlations in this in-terval in several Atlantic, Indian, and Pacific ocean sitesare questionable and need refinement through better re-covery of condensed sections (as at Site 534) and a moredetailed study of the cores, particularly geochemicallyand stratigraphically. Such studies will help to addressthe reasons for and extent of this prolonged sedimentstarvation in the deepest part of the basin, and assess be-low CCD dissolution as a means of balancing surfacewaters productivity at that time.

(9) Uncertainty remains on the origin, amount, andextent of the gas hydrates encountered at Site 533 on theBlake Outer Ridge (Claypool et al., this volume; Kven-volden et al., this volume). What is the precise vol-umetric percentage of the gas hydrate, and is micro-crystalline dissemination in tiny pore spaces realistic? Towhat extent is this gas hydrate a seal for gas mapped in abright spot below the gas hydrate bottom-simulating re-flector on the Blake Outer Ridge (Dillon et al., 1980)?

Further testing through deeper drilling in the gashydrate clays is desirable, in conjunction with furtheracoustic and physical properties mapping. Good acous-tic, density, and resistivity logs must be taken in the hy-drated sediments to measure in situ physical parameters.

(10) The Blake Outer Ridge has long been consideredthe classic sediment drift deposit of enormous size (seeSite 533 report, this volume). Seismicially the Ridge ap-pears as a contourite-built feature, but the detailed cor-ing at Site 533 failed to produce the expected stratifica-tion. The definition of contourites involving ubiquitouslaminations appears inapplicable at Site 533 and callsfor further investigation, which should involve X-raystudies, magneto stratigraphy, and isotope stratigraphyto infer sediment transport drift and nepheloid layering.Analog and digital modelling of drifts the size of theBlake Outer Ridge may clarify the nature of this enor-mous sedimentary feature.

In this chapter we have emphasized the extraordinaryaccomplishments of Deep Sea Drilling Project Leg 76,after only a decade of Atlantic Ocean drilling. Yet theseaccomplishments will wither on the tree of cultural andeconomic growth and accomplishment if not followedup by further testing and exploration. In this sense Leg

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76 is typical of all successful scientific research. New an-swers breed new questions to challenge and enlightenour and the next generation of mankind.

ACKNOWLEDGMENTS

J. Hall (Dalhousie University, Halifax, N. S.) critized a version ofthis manuscript; Carol Mitchell and G. Cook (Bedford Institute ofOceanography, Dartmouth, N. S.) provided typing and drafting ser-vices. We thank them for their support.

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Date of Initial Receipt: November 15, 1982

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DEEP SEA DRILLING PROJECT LEG 761 · 2007. 5. 2. · DEEP SEA DRILLING PROJECT LEG 761 Felix M. Gradstein, Geological Survey of Canada, Bedford Institute of Oceanography, Dartmouth,