Eustatic Curve for the Middle Jurassic–Cretaceous Based on ...mmtk.ginras.ru/pdf/sahagianea1996.pdf · sedimentary units. Some of the grabens were inverted in the late Mesozoic

ABSTRACT

We have used the stratigraphy of the central partof the Russian platform and surrounding regions toconstruct a calibrated eustatic curve for theBajocian through the Santonian. The study area iscentrally located in the large Eurasian continentalcraton, and was covered by shallow seas duringmuch of the Jurassic and Cretaceous. The geo-graphic setting was a very low-gradient ramp thatwas repeatedly flooded and exposed. Analysis ofstratal geometry of the region suggests tectonic sta-bility throughout most of Mesozoic marine deposi-tion. The paleogeography of the region led toextremely low rates of sediment influx. As a result,accommodation potential was limited and is inter-preted to have been determined primarily byeustatic variations. The central part of the Russianplatform thus provides a useful frame of referencefor the quantification of eustatic variations through-out the Jurassic and Cretaceous.

The biostratigraphy of the Russian platform pro-vides the basis for reliably determining age andeustatic events. The Mesozoic section of the cen-tral part of the Russian platform is characterizedby numerous hiatuses. In this study, we filled the

sediment gaps left by unconformities in the cen-tral part of the Russian platform with data fromstratigraphic information from the more continu-ous stratigraphy of the neighboring subsidingregions, such as northern Siberia. Although thesesections reflect subsidence, the time scale of vari-ations in subsidence rate is probably long relativeto the duration of the stratigraphic gaps to befilled, so the subsidence rate can be calculatedand filtered from the stratigraphic data. We thushave compiled a more complete eustatic curvethan would be possible on the basis of Russianplatform stratigraphy alone.

Relative sea level curves were generated by back-stripping stratigraphic data from well and outcropsections distributed throughout the central part ofthe Russian platform. For determining paleowaterdepth, we developed a model specifically designedfor this region based on paleoecology, sedimentolo-gy, geochemistry, and paleogeography.

The curve describes a series of high-frequencyeustatic events superimposed on longer termtrends. Many of the events identified from ourstudy can be correlated to those found by Haq et al.(1988) and other sea level studies from other partsof the world, but there are significant differences inthe relative magnitudes of events. Because theeustatic curve resulting from this study is based ona stable reference frame, the curve can be used insedimentary basin modeling and as a tool for quan-tifying subsidence history from the stratigraphy ofpassive margins, basins, and other active regions.

INTRODUCTION

Eustasy is controlled by the relative volumes ofthe global ocean basins and global ocean water(Sahagian and Watts, 1991; Sahagian and Jones,1993). Long-term relative sea level changes (>1m.y.) have been observed qualitatively throughtheir effects on the depositional patterns and shore-line processes of marine facies. However, quantifi-cation of eustatic changes has been hampered by

1433AAPG Bulletin, V. 80, No. 9 (September 1996), P. 1433–1458.

©Copyright 1996. The American Association of Petroleum Geologists. Allrights reserved.

1Manuscript received May 18, 1995; revised manuscript received January8, 1996; final acceptance April 12, 1996.

2Institute for the Study of Earth, Oceans, and Space and Department ofEarth Sciences, University of New Hampshire, Durham, New Hampshire03824-3525.

3Centrgeologia, Washavskoje Shosse 39 A, 105103, Moscow, Russia.4Russian Academy of Sciences, Siberian Branch, Novosibirsk, Russia.

Russian platform stratigraphic data are offered under the AAPGBulletin Datashare program (Data 8). Users can obtain the data set in one oftwo ways: (1) write to AAPG Bulletin Datashare, P.O. Box 979, TulsaOklahoma 74101-0979, USA, and include US $5.00 for shipping andhandling, or fax your credit card number (please specify DOS or Macintosh);or (2) download the file for free from the Internet (http://www.geobyte.com/download.html).

This project was supported by NSF (EAR9218945) and a GSA studentresearch grant. The paper substantially benefited from discussions with S. Jacobson, A. Beisel, D. Naidin, J. Collinson, and H. Tischler. Thanks go toY. Bogomolov and B. Shurygin for field assistance. The authors are gratefulto AAPG reviewers W. Devlin, G. Ulmishek, and G. Baum for very helpfulcomments.

Eustatic Curve for the Middle Jurassic–Cretaceous Based on Russian Platform and Siberian Stratigraphy:Zonal Resolution1

D. Sahagian,2 O. Pinous,2 A. Olferiev,3 and V. Zakharov4

the lack of an adequate system of measure for thisrelationship. Although some workers have consid-ered it impossible to accurately determine eustaticvariations (Kendall and Lerche, 1988), other work-ers have suggested that a eustatic curve should ide-ally be based on the stratigraphy of a tectonicallyquiescent region for each time interval considered(Hallam, 1992). We have identified the central partof the Russian platform (Figure 1), based on its gen-erally flat-lying and otherwise relatively undisturbedstratigraphy, as a useful reference for eustatic sealevel quantification for the Middle Jurassic throughthe Cretaceous.

One way to observe sea level changes isthrough the effect on sedimentation patterns

along continental margins (Vail et al., 1977; Jervey,1988; Posamentier et al., 1988). Sea level changescan be estimated by applying backstripping tech-niques (Angevine et al., 1990). Because water-depth variations are an important term in back-stripping analyses, eustatic variations can bequantified only to the resolution of the availablepaleodepth indicators, which are most reliable inshallow environments. In most tectonic environ-ments, water depth is not solely dependent oneustasy (Baum et al., 1994), and in subsiding basinswhere subsidence depends on distance from ahinge, position also controls the relation betweeneustasy and water depth (Loutit et al., 1988). Inthis study, we examine the relationship between

1434 Mesozoic Eustasy

Figure 1—Sketch map of major structures of the Russianplatform.

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eustasy and sedimentation by quantifying waterdepth variations on a shallow platform sea.

Eustatic curves have been compiled in the past(Hallam, 1988; Haq et al., 1988; Harrison, 1988).Commonly, these curves have relied on methods ofcalculating tectonic activity (usually subsidence) toseparate tectonic and eustatic signals reflected instratigraphic sequences or paleohypsometric pro-files (Greenlee and Moore, 1988). The errors inher-ent in these calculations of passive-margin subsi-dence histories (e.g., Watts and Steckler, 1979) mayhave been larger than the total sea level signalinferred on time scales of tens of millions of years.Thus, these curves have necessarily been qualita-tive, at best can be used for only short time intervals(<10 m.y.), and are limited by cumulative errors insubsidence calculations. By using an epeirogenicallystable part of the lithosphere (central part of theRussian platform) as a reference frame, this limita-tion is eliminated because no discernible tectonicsubsidence or uplift can be documented through-out the time of deposition, and only local and dis-crete activity has occurred since deposition. Thestability and shallow-marine conditions of the cen-tral Russian platform throughout most of theJurassic and Cretaceous make this region useful fordetecting and quantifying eustatic variations. Inaddition, the Russian platform (Moscow depres-sion) is unique because during its stable period itwas very close to sea level, resulting in a relativelycomplete stratigraphic record compared to otherareas thought to have been stable during theMesozoic, such as North Africa and central NorthAmerica (Sahagian, 1987, 1988).

TECTONIC SETTING OF THE RUSSIANPLATFORM

The Russian platform is a large craton with anarea of 5.5 million km2 (Nalivkin, 1973). The plat-form generally consists of Precambrian crystallinebasement and Phanerozoic cover. The accumula-tion of presently unmetamorphosed sedimentsbegan in the late Proterozoic (Riphean), about1400 Ma, with the active formation of major aulaco-gens typical of the initial stages of platform devel-opment (Fedynsky et al., 1976). In the Vendian–Cambrian (650–550 Ma), the aulacogen stage ter-minated and sediments began to accumulate onthe relatively flat-lying surface (compared to themodern basement relief) (Milanovsky, 1987).During the Paleozoic, significant subsidence andgraben reactivation occurred throughout much ofthe platform, resulting in the deposition of thicksedimentary units. Some of the grabens wereinverted in the late Mesozoic or early Cenozoic.The structure relevant to the present study is the

Moscow depression, a subsiding graben complexwith thick Paleozoic sedimentary fill in the centralpart of the Russian platform (Figure 1). TheMesozoic stratigraphy analyzed here is based onwells and outcrops on and near the extinctMoscow depression, referred to as the “Moscowsyneclise” in the Russian literature.

By the Jurassic, the subsiding Moscow depres-sion stabilized. This interpretation is supported bythe uniformity and relatively thin nature ofMesozoic beds over great distances. For example,the Oxfordian shale facies between the Ryazanregion and Unzha River sections (located about 600km apart) displays similar isopachs down to theammonite zone level. The thickness of the entireMesozoic package increases in the marginal subsid-ing regions (e.g., Caspian depression, Dnepr-Donetsk depression).

The presence of a great number of uncon-formable surfaces and depositional hiatuses in theJurassic–Cretaceous section of the Moscow depres-sion represents additional evidence of stability. Dueto very flat hypsometry and the lack of subsidence,even minor sea level falls could have caused subma-rine erosion or subaerial exposure, resulting inunconformities. In contrast, the more continuousdeposition observed toward the southeast (lowerVolga River region and Caspian depression) indi-cates significant subsidence and deeper water.

During the Mesozoic, limited reactivation ofsome aulacogen structures began on the Russianplatform. However, the aulacogens in the Moscowdepression apparently remained stable from theMiddle Jurassic to at least the Santonian. Tectonicactivity resumed during the Campanian–Paleogenein some regions that had been stable since theJurassic. The inversion of Proterozoic graben struc-tures occurred in the Oksko-Tsinski swell andSukhonskii swell (Figure 1) through uplift of theaxis zones. Some minor local movements alsooccurred in other parts of the previously stablearea. All this resulted in local areas with differentaltitudes of Mesozoic strata; however, despite ofthe different vertical positions, the strata and faciesare very uniform, even on the Oksko-Tsinski swellwhere the relief of Mesozoic surfaces reaches 160 m.This observation demonstrates the stability of thebasin throughout deposition in the Mesozoic.

The interpretation that uniform sedimentationindicates stability is based on the assumption thatthe Russian platform did not rise or fall en masse.This assumption is supported by theoretical consid-erations and observations. The part of the Russianplatform that is covered by flat-lying Mesozoic sedi-ments is broader than the flexural half-wavelengthof the lithosphere (Watts, 1989; Sahagian andHolland, 1993), so any point source of uplift or sub-sidence would be expected to lead to large-scale

Sahagian et al. 1435

deformation, which is not observed. A broadsource of epeirogeny, such as low-order geoidalvariations, could affect the region’s utility as aeustatic reference frame, but tomographic modelssuggest that this region has not experienced anysignificant variations in the relationship betweendynamic topography and geoid since the Paleozoic(Gurnis, 1993). Consequently, we consider theMoscow depression as the most reliable sea levelreference frame possible for the Jurassic andCretaceous on an otherwise dynamic Earth.Although other regions on other continents suggeststability (Sahagian, 1987, 1988), the limited sizeand stratigraphic range make them inappropriatefor constructing a eustatic curve for any apprecia-ble interval in the Mesozoic.

PALEOGEOGRAPHY

Paleogeographic maps of the Russian platformhave been compiled based on preserved stratigra-phy (Naidin, 1959; Sazonov and Sazonova, 1967;Vinogradov, 1968; Naidin et al., 1986; Milanovsky,1987). However, the extent of Mesozoic marinedeposition was considerably greater than that ofpreserved strata because facies generally do notvary near the outcrop edge. Widespread post-Mesozoic erosion makes it difficult to make thor-ough paleogeographic reconstructions.

During the Triassic, the Russian platform was alow-elevation plain with deposition of alluvial andlacustrine sediments. Shallow-marine sediments ofEarly and earliest Late Triassic age are present onlyin the Caspian depression (Milanovsky, 1987). Inthe Early Jurassic, marine transgressions from theSouthern Tethyan basin are evident in the BlackSea depression, Dnepr-Donetsk depression, andCaspian depression. By the end of the Bajocian thecentral part of the Russian platform was a low-elevation plain with locally incised minor relief(Zhukov and Konstantinovich, 1951). The sites ofminor relief were the site of the first Bajocianmarine deposition, and after a period of incisedvalley fill, marine sedimentation spread uniformlyover the region during the Callovian. Marine con-ditions subsequently dominated until at least theend of the Santonian and were interrupted only forrelatively short periods during eustatic sea leveldrops. Thus, during the Jurassic–Cretaceous, theRussian platform was a shallow epicontinental seathat extended from Tethys to the Arctic with occa-sional marine connections to the west (Figures 2,3). Marine deposition occurred with limited sedi-ment supply from remote clastic sources. Thelack of subsidence and the low sediment supplycaused eustatic sea level to be the main factorcontrolling sedimentation. In our analysis, we use

paleogeographic reconstructions as a secondaryconstraint for estimating paleowater depth.

STRATIGRAPHIC DATA

Middle Jurassic–Upper Cretaceous strata are wide-ly distributed on the Russian platform. Many of thestratigraphic units in the Moscow depression arebounded both above and below by unconformities.The thickness of eroded sediments is impossible tocalculate using methods such as vitrinite reflectance(Armagnac et al., 1989) because of the thin overbur-den. In this study, we have filled the gaps left byunconformities on the Russian platform with strati-graphic information from the more continuousstratigraphy of the neighboring subsiding regions,such as northern Siberia. Although these sectionsreflect subsidence, the time scale of variations insubsidence rate are probably long relative to theduration of the stratigraphic gaps to be filled, so the(constant) subsidence rate can be reliably calculatedand filtered from the stratigraphic data (Sahagianand Jones, 1993). Northern Siberian stratigraphy hasbeen treated elsewhere (Sahagian et al., 1994), andso will not be repeated here, but is available as sup-plementary information, along with other specificinformation, through the AAPG Bulletin Datashareservice. The data from the western part of the near-by Ryazan-Saratov trough were incorporated directlyfor the lowermost Aptian and upper Bajocian–Bathonian because the similarity of overlying andunderlying intervals to correlative intervals of theMoscow depression suggests no significant subsi-dence during the short incorporated intervals.

Biostratigraphic zonation is based primarily onammonites, bivalves, forams, and palynomorphs(Sazonov, 1957; Gerasimov, 1962; Krymholts, 1972;Mesezhnikov, 1984; Naidin et al., 1984; Baraboshkinand Mikhailova, 1987; Mitta, 1993). The traditionalapproach taken by Russian geologists for subdivid-ing Russian platform stratigraphy was strictly bio-stratigraphic at the stage and zone level (Sazonov,1957; Gerasimov, 1971). More recently, Olferiev(Olferiev, 1986, 1988) subdivided units into forma-tions based on lithostratigraphy (and constrained bybiostratigraphy) and this is the approach of ourstudy. Data were obtained for this study from wells,cores, and correlated outcrops throughout the cen-tral part of the Russian platform (Figure 4), whereindividual units are continuous over great distances,allowing relatively reliable correlation of sections(Sazonov, 1957; Gerasimov, 1962, 1969; Krymholts,1972; Olferiev, 1986, 1988).

During times of lower sea level, the dominant clas-tic source was redeposition of previously depositedmaterial (Gerasimov, 1962). Sediment recycling mayhave contributed to the broad distribution of sand


across the Russian platform. Many examples havebeen found of fauna from several stages mixed togeth-er in redeposited strata on the Russian platform.

The stable region of the Russian platform(Moscow depression) includes the provinces ofMoscow, Tver, Kostroma, Yaroslavl, Ivanovo,Vladimir, and Ryazan. Although individual strati-graphic sections preserve only portions of theMesozoic section, a relatively complete compositesection can be compiled from numerous sectionsthroughout the stable region (Figures 5, 6). Thiscomposite section provides the most reliableframework for building a eustatic curve.

Jurassic

The average thickness of preserved Jurassicdeposits is 120–140 m. The oldest marine section

on the central Russian platform was traditionallyconsidered to be Callovian, but more recently, upperBajocian fauna, including Parkinsonia donezianaBorissjak and forams (Lenticulina volganica) werediscovered in the basal oolitic sands of theVyazhnevskaya Formation (Figure 5) in the westernRyazan Saratov trough (A. Olferiev, 1993, personalcommunication; Meledina, 1994). The upper por-tion of Vyazhnevskaya Formation consists of claysand is assigned to lower Bathonian (Figure 5). Theconformably overlying middle to upper BathonianMokshinskaya Formation is represented by interca-lated lagoonal and lacustrine sands, silts, and clays,and dated by forams, bivalves, and palynomorphs(Olferiev, 1986).

The Elatminskaya Formation is composed of acoarsening-upward succession of clays, silts, andsands. The contact between the Mokshinskayaand Elatminskaya formations is an erosional


Figure 2—Late Oxfordian (Amoeboceras alternans) paleogeography of the Russianplatform (after Sazonov andSazonova, 1967).

unconformity overlain by an accumulation ofbelemnite fragments and pyritized wood. TheElatminskaya Formation has been considered a typi-cal transgressive-regressive cycle (Olferiev, 1986).The lower part of the middle Callovian is represent-ed by poorly sorted sands and silts with iron oolitesof the overlying Kriushskaya Formation. TheKriushskaya is conformably overlain by clays withabundant bivalves Posidonomya buchi (Roemer) ofthe Velikodvorskaya Formation. The Kriushskaya is atransgressive deposit, whereas the Velikodvorskayawas formed during highstand and subsequent sealevel fall (Olferiev, 1986). The overlying upperCallovian to lower Oxfordian PodosinkovskayaFormation consists of a basal oolitic marl overlain bylight-gray clay. The contact between thePodosinkovskaya Formation and the Velikodvorskayais unconformable in many locations.

The Podmoskovnaya Formation (middle toupper Oxfordian) is represented by fine-laminatedbituminous clays and shales with abundantammonite fauna. The erosional hiatus between the

Podosinkovskaya and Podmoskovnaya formations iswidespread, extending across and beyond theMoscow depression (Olferiev, 1986). The gray silty clays of the upper Oxfordian KolomenskayaFormation conformably overlie the PodosinkovskayaFormation, but lenses of coarse sand and accumula-tions of bivalve and gastropod shells in the basal partsuggest shallowing at the boundary of the two formations.

The Ermolinskaya Formation overlies theKolomenskaya with an erosional unconformity insome places and conformably, but with evidence ofshallowing, such as basal coarse-grained layers, inothers. The formation is generally composed ofdark-gray to black clays with abundant pyrite nod-ules and sparse fossils.

The upper Kimmeridgian has not been investigat-ed in as much detail as have other parts of theJurassic section because it has been almost complete-ly removed by erosion, being adequately preserved inonly several small areas in the Moscow depression.The Gorkinskaya Formation unconformably overlies


Figure 3—Hauterivian (Speetoniceras versicolor)paleogeography of the Russianplatform (after Sazonov andSazonova, 1967). See Figure 2 for legend.

the Ermolinskaya Formation and consists of darkclays with spongeolites and shell detritus.

The presence of biostratigraphically definedlower Volgian is reported only from the Kostromaprovince, where calcareous silty clays (3.5 m thick)have been encountered in wells. The age of theclay is indicated by forams (Pseudolamarckinapolonica (Biel. et Pos.), Planularia polenovaeKuzn., Marginulina gluschisaensis Dain.), but noammonites have been encountered (Olferiev,1986). Throughout most of the region, however,the middle Volgian overlies various Upper Jurassic

strata with a significant erosional hiatus (Figure 5).The middle Volgian is widespread throughout theMoscow depression and is characterized by severalmarine facies that are represented by four middleVolgian formations (Figure 5). The fine-laminatedbituminous shales and clays of the KostromskayaFormation overlie a basal layer of coarse glauconitesands containing numerous redeposited phospho-rite nodules with ammonites of different ages,including upper Oxfordian, Kimmeridgian, andmiddle Volgian. Note that there is no lower Volgianfauna present in this layer.


Figure 4—Locations of the mainwells and outcrops used for theconstruction of the quantifiedeustatic curve.

The Egorievskaya Formation that unconformablyoverlies the Kostromskaya Formation consists offine-grained glauconite sands with phosphoritenodules and pebbles. The gray-black clay-rich sandsand silts of the Philevskaya Formation overlie theEgorievskaya Formation with no sign of sedimenta-ry interruption. The Egorievskaya is interpreted torepresent transgressive deposition, and thePhilevskaya highstand and regressive deposition(Olferiev, 1986).

The Lopatinskaya Formation unconformablyoverlies the Philevskaya and is represented by veryhomogeneous gray-green fine quartz-glauconitesands with an abundance of sandy phosphoritenodules in the upper part. The top of the Jurassicsection is observed in outcrop in the vicinity of

Moscow, and consists of a thick section of white,fine, well-sorted quartz-glauconite sands with phos-phorite pebbles of the Kuntsevskaya Formation(Gerasimov, 1969).

Cretaceous

Cretaceous strata of the Moscow depression havean average thickness of 260–290 m. The sectionincludes several significant erosional hiatuses (Figure6) that developed local relief that was filled duringsubsequent transgressions (Figure 7). The marineCretaceous of the Russian platform began in theBerriasian (Ryazanian). The terms “Ryazanian” and“boreal Berriasian” are used synonymously in the


Figure 5—Middle and Upper Jurassic stratigraphy of the centralRussian platform.

Russian literature. The relationship of borealBerriasian biozones to Tethyan biozones and theposition of the Jurassic–Cretaceous boundary is aseparate issue that has been debated for over 30 yr(Krymholts, 1984) and will not be addressed inthis paper.

The most complete sections of the Berriasian arein the southern part of the Moscow depression inthe Oka River basin. At this locality, the lowerKuzminskaya consists of two beds of phosphoritesandstone separated by chamosite sands, all of veryshallow marine facies (Figure 6). The SvistovskayaFormation in some places unconformably overlies

the Kuzminskaya Formation. The SvistovskayaFormation is represented by chamosite sands withabundant phosphorite concretions and pebbles inthe lower part (Olferiev, 1988).

The Valanginian is absent in most of the Moscowdepression. Thin fragments of lower Valanginianare found in only two places, the Kostromaprovince and the northern part of the Oka-Donlowland (Ryazan province) where it uncon-formably overlies the Svistovskaya Formation.Because there is no useful Valanginian section, wedo not discuss Valanginian stratigraphy of theRussian platform, and filled this gap using the more


Figure 6—Cretaceous stratigraphyof the central Russian platform.


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complete northern Siberian sections along theBoyarka River (Zakharov and Judovnyi, 1974).

The overlying Hauterivian section is very wellrepresented in the central part of the Moscowdepression (Moscow and Vladimir provinces) andis divided into six formations. The RostovskayaFormation (Figure 6) is composed of predominant-ly fine-grained quartz sands and sandstones thatoverlie various Cretaceous and Jurassic strata withan erosional unconformity. The thin KrestovskayaFormation is composed of ferruginous sands. Thethin layer of poorly sorted coarse sand indicatesshallowing that occurred at the beginning ofKrestovskaya deposition. A subsequent sea level fallresulted in an extensive erosional unconformitythroughout the entire Moscow depression at thelower Hauterivian–upper Hauterivian boundary.

The overlying four formations reflect the trans-gression of the late Hauterivian sea. The basal bedsare composed of sands of the Sobinskaya Formationthat fill depressions and the incised valleys of theeroded surface. The overlying clays and silts of theSavelievskaya Formation and shoreface sands andsilts of the Gremyachevskaya Formation spreadmore uniformly across the region. The dark-grayclays and silts of the Kotelnikovskaya Formation arethe most widespread, and probably represent the highest sea level stand in the uppermostHauterivian. Sands and silts of the lower BarremianButovskaya Formation conformably overlie theKotelnikovskaya formation. The upper Barremian isabsent from the Moscow depression and representsan erosional hiatus throughout the region.

The earliest marine Aptian sediments in the cen-tral part of the Russian platform are documented inthe western Ryazan-Saratov trough where unsortedsands and silts fill incised valleys. The Aptian in theMoscow depression includes three formations. Thedeepest formation is the Ikshinskaya, which uncon-formably overlies Carboniferous, Jurassic, andLower Cretaceous strata. The Ikshinskaya is com-posed of quartz sands with abundant fossil flora,and grades upward from terrestrial to lagoonal, andfinally to marine sediments. The Ikshinskaya gradu-ally transitions to the Vorokhobinskaya Formation,which is composed of offshore marine sands, silts, and clays. The overlying upper AptianVolgushinskaya Formation is composed ofshoreface to lagoonal clays and silts with sideriteconcretions. The Volgushinskaya includes Aptian–Albian spore and pollen assemblages and isassigned to the upper Aptian based on its positionin the section. Significant regression occurred inthe second half of the late Aptian that resulted inerosion of the upper contact of the VolgushinskayaFormation. The marine uppermost Aptian is pres-ent only in the Caspian depression and some smallareas of the eastern part of the Ulyanovsk-Saratov

trough, and represents an erosional hiatus through-out most of the Russian platform (Sazonova, 1958),suggesting one of the most significant sea level fallsof the Lower Cretaceous on the Russian platform.

The lower Albian is represented by theKolokshinskaya Formation of light-purple silts andsands with ammonites and palynomorphs (Bara-boshkin, 1992). The unconformably overlying mid-dle Albian Gavrilkovskaya Formation is composedof offshore-marine glauconite quartz and fine andmedium sands with phosphorite nodules.Shallowing and subaerial exposure occurred in theuppermost middle Albian, as is indicated by anunconformity and mud cracks at the top of theGavrilkovskaya Formation.

The clays of the Paramonovskaya Formation con-sist of three parts from base to top: (1) intercala-tions of sands and silts with basal coarse sands, (2) massive silts and black clays, and (3) silts andsands. The Paramonovskaya Formation is the thick-est Lower Cretaceous unit preserved in the Moscowdepression (up to 57 m thick) and represents thetime of highest Early Cretaceous sea level. Theupper Albian age of the Paramonovskaya is indicat-ed by radiolarians and forams. The green-gray fineglauconite shoreface sands of the JakhromskayaFormation overlie the Paramonovskaya clays withan erosional unconformity having relief of up to 20m. The Lyaminskaya Formation consists ofshoreface fine glauconite quartz sands with quartzgravels and phosphorite pebbles. In some places,the Lyaminskaya overlies the Jakhromskaya withsome evidence of erosion (Olferiev, 1988).

A large stratigraphic gap includes the middleCenomanian to lower Turonian. The gap is mani-fest as an erosional unconformity throughout theMoscow depression, but has been identified as acondensed section on the Voronezh arch and theeastern Ulyanovsk-Saratov trough (Naidin, 1981).The middle–upper Turonian overlies the unconfor-mity and consists of marls of the ChernevskayaFormation. Another gap exists in the lowerConiacian (Figure 6). We have bridged both ofthese gaps with stratigraphic data (Zakharov et al.,1989a, b) from the Ust-Yenisey depression ofnorthern Siberia (Sahagian and Jones, 1993;Sahagian et al., 1994).

The upper Coniacian Zagorskaya Formationunconformably overlies the Albian, lower Cen-omanian, and middle–upper Turonian. In the west-ern Moscow depression (Moscow province), theZagorskaya is composed of intercalated quartzglauconite shoreface sands and silts with radiolari-ans. To the east (Vladimir province), this formationconsists of offshore and deep-marine silicic clays,tripoli, and silts with radiolarians of late Coniacianand early Santonian age. The unconformably over-lying Dmitrovskaya Formation consists of quartz


glauconite shoreface sandstone. The TentikovskayaFormation also overlies an erosional unconformity,and is composed of offshore to deep-marine silts andsiliceous oozes. The Upper Cretaceous section of theMoscow depression is capped by the quartz glau-conite sands and sandstones of the GodunovskayaFormation that unconformably overlies theTentikovskaya Formation. The GodunovskayaFormation is preserved only in the central part of theMoscow depression. The Godunovskaya’s age isuncertain, but it is conditionally placed in the upperSantonian based on its stratigraphic position. TheCampanian and Maastrichtian are absent from theMoscow depression, probably due to subsequent(until present) subaerial erosion.

WATER DEPTH SCHEME

Water depth is an important component inreconstructing past sea level variations, but it is dif-ficult to estimate paleowater depth accurately forbackstripping analyses. In the case of extremelyslow subsidence and deposition rates, water depthvariations may be the sole expression of eustasy.Thus, a basis is necessary for the most accurate esti-mate of water depth, particularly when, as is usual-ly the case on the Russian platform, water depthvariations for a given interval are of greater magni-tude than sediment thickness of the same interval.

Determining Paleowater Depth

Many different methods have been attempted forreconstructing paleowater depth, but none are uni-versally applicable (Hallam, 1967a; Eicher, 1969;Benedict and Walker, 1978; Clifton, 1988; Brett etal., 1993). Most analyses use some form of geologicdata (lithological, chemical, paleontological, etc.)and their typically associated water depths in mod-ern marine environments. However, no universalmodel exists that takes into account all environmen-tal factors, nor is there a generally accepted schemefor shallow shelf environments. Consequently, anyattempts to apply water depth schemes based onmodern conditions to paleowater depth are limitedby the differences in the relationships betweenwater depth and the geologic indicators used toestimate it. To accurately determine paleowaterdepth, one must account for all possible factors,including types and rates of sedimentation, climate,bottom geometries, and tectonic conditions, on thebasis of all available geologic data.

Paleobathymetric Model for the Jurassic andCretaceous Seas, Central Russian Platform

A few workers have attempted to estimate paleo-depth on the Russian platform (Sazonov and

Sazonova, 1967). However, these attempts haveinvolved very low (100 m) depth resolution. Forexample, Sazonov and Sazonova (1967) groupedall Jurassic and Cretaceous facies of the entireRussian platform into two “magnafacies,” one inshallow water (0–100 m) and one in deep water(100–200 m). In our analysis, we attempt to obtainthe maximum possible depth resolution by devel-oping a consistent quantitative model of waterdepth for the central Russian platform.

The paleodepth model for Jurassic and Cretac-eous seas of the Russian platform is summarized inFigure 8. The model is based on the depths thatcorrespond to deposition of different kinds of sedi-ments, formation of sedimentary structures, miner-alization, different kinds of taphonomic conditions,trace fossil distribution (ichnofacies), and bottomcommunities.

Contrasting levels of wave activity due to differentgeographic conditions in different basins result in sig-nificant variations in depth indicators. Wave base,however, remains at a depth of one-half the wave-length. On the basis of several factors (e.g., size,shape, depth, etc.), the Russian platform sea can becompared to the present Baltic Sea. Observationaland theoretical studies indicate that Baltic stormwave base is 20–30 m (Zakharov, 1966). Mild Jurassicand Cretaceous climates may have had winds weakerthan present winds (Hallam, 1967b). In addition, a cli-matic model for the Kimmeridgian–Volgian (Moore etal., 1992) shows relatively low annual wind activityfor the region of the Moscow depression. As such,we take Jurassic and Cretaceous fair-weather wave-length on the Russian platform to be 20 m (wavebase of 10 m). The maximum wavelength for majorstorms would have been 40 m (wave base 20 m).Fair-weather wave base (5–15 m, depending on localconditions), can be considered to divide sand-dominated shoreface sediments from mud-dominatedoffshore sediments (Walker and Plint, 1992). We thusdefine a relationship between lithology and waterdepth in our model.

Structural evidence may provide important infor-mation for paleodepth reconstruction, especiallyfor shallow-marine environments (Clifton, 1988).Sedimentary structures closely connected to waveactivity include cross-bedding, swaly cross-stratification, and various types of wave ripplesabove fair-weather wave base. Parallel laminationsand hummocky cross-stratification are predominantbelow fair-weather wave base, and fine laminationsof sediments rich in organic carbon are normallyfound below storm wave base (Walker, 1984).

Oolites are chemically formed in warm, oxygen-saturated, turbulent shoreface water above fair-weather wave base and are most abundant indepths of less than a few meters (Kump and Hine,1986). The abundance of iron oolites and oolitic


marls in Jurassic and Cretaceous strata of theMoscow depression makes them an exceptionallyuseful paleobathymetric indicator.

Authigenic minerals (particularly iron group min-erals and phosphates) form at specific depths.Ghoethite in tropical seas is formed in extremelyshallow (0–10 m) water. Chamosite forms in shal-low (10–50 m) very warm (up 20°C) and activewater. Glauconite is deposited at the deeper part ofthe shelf (150–250 m) at temperatures below 15°C(Porrenga, 1967). Most recent investigators haveinterpreted a shallow-water origin for phosphorites(30–150 m) (Bushinski, 1964; Bromley, 1967;

Benedict and Walker, 1978). Carbon is of particularinterest as a paleodepth indicator because it is wellpreserved in anoxic sediments (Black Sea, Med-iterranean Sea depressions, Norway fjords). Carbonis usually reduced with Fe+2 (marcasite, pyrite),which is present in sediment and is well preservedin rocks. The presence of sulfides in rocks indicatesrelatively deep water conditions (below stormwave base).

Benthic communities are useful depth indica-tors. Regular patterns emerge in the range of mod-ern marine benthic communities with respect tothe alternation of the suspension feeders and


Figure 8—Paleodepthmodel for Jurassic andCretaceous seas of the central Russian platform.

deposit feeders throughout the shelf zone(Zenkevitch, 1977). Two zones of suspension feed-ers can be defined parallel to the shoreline in turbidwater along typical continental margins: one zone inshallow water near the shoreline, and the other zonein considerably deeper water at the shelf–slopeboundary (shelf break). Deposit feeders, however,are concentrated in oxygen-poor water on the deep-er shelf and slope (Zenkevitch, 1977). No strictbathymetric control exists for the distribution ofmodern bottom communities in boreal seas, but sus-pension feeders are generally found in the nearshoreand deposit feeders offshore.

Trace fossils are used widely for paleodepthinterpretation (Seilacher, 1967; Ekdale, 1988). Fourichnofacies are identified from the shoreline sea-ward, and include Scolithos, Cruziana, Zoophycos,and Nereites. The only two trace fossil facies thathave been considered as being depth constrainedare the shallow-water Scolithos facies and the deep-er water Nereites facies (Seilacher, 1967). Othertypes of ichnofacies can be found at differentdepths. In our model, we use the approximationsuggested by Walker and Plint (1992).

Taphonomic data based on marine invertebratesis useful for quantitative paleobathymetric interpre-tation for depths of from 0 m to fair-weather wavebase. Depth is indicated by preservation, separation,selection, and orientation of hard parts of these ani-mals (bivalves, brachiopods, ostracods) (Yanin,1983). Taphonomic investigation of bivalves in Peterthe Great Bay of the Sea of Japan shows the follow-ing results. At depths of 0–5 m the species from dif-ferent communities are mixed, the shells are broken,and there are shell banks. At depths of 5–15 m,valves are separated and moved from life position.Occasionally, the shells accumulate in spots andlenses. At depths greater than 15 m entire shells areburied in life positions (Evseev, 1981). Two types ofaccumulations are formed above fair-weather wavebase. The “rose-type” or “shingled” accumulation isformed at the surf zone above 2.0 m water depth(Zakharov, 1966, 1984). This type consists of a nest-ed and vertically oriented accumulation of largevalves. “Shell pavement” accumulations are formedat between 2 and 15 m water depth by separatevalves of bivalves (or brachiopods) that are convexupward and lying in close contact with a cobble-stone appearance (Maksimova, 1949).

We assume that the maximum depth of the lightpenetration to support macroalgae is 40 m and that temperature decreased with depth in theMesozoic seas; however, microalgae (e.g., calcare-ous cyanophycean) depths can reach 80 m. Theseassumptions are based on low water clarity in theRussian platform seas interpreted from terrigenousfine-grained sedimentation that predominatedthroughout the Jurassic and most of the Cretaceous.

Subaerial unconformities were assumed to repre-sent at least some time of 0 m water depth (expo-sure), unless further sea level fall could be estimat-ed on the basis of the amplitude of subaer ialincision.

Our generalized scheme is adapted for “normal”shallow-marine systems and cannot be accuratelyapplied to deltas, lagoons, estuaries, etc., and issubject to the vagaries of bottom relief, currents,variations in sediment supply, and other factors.Clearly, all local environmental factors must betaken into account before the model is applied.Because this paleodepth model was specificallyconstructed for the Moscow depression epiconti-nental sea, it should not be applied to other envi-ronments and hypsometries, or errors will result.However, the methods by which the model wasconstructed can be applied to other times andplaces, and can be used to develop models specificto other basins or platforms.

DATA ANALYSIS

Data analysis consisted of two main parts: geo-logical analysis (isopach, lithology, facies, subsur-face geometry, areal distribution, paleodepth inter-pretations, etc.) and backstripping. We studiedover 50 wells and outcrops from this region and 32wells were specifically used for constructing thecomposite curve (Figures 4, 9). We carefully exam-ined five main regional stratigraphic profiles andseries of lithologic-paleogeographic maps and ana-lyzed subsurface stratal geometries and facies distri-butions throughout the region. Part of the regionalprofile IV-IV is shown in Figure 7. The results of thegeological analysis were a database for use in back-stripping routines that are used to construct sealevel curves.

Backstripping involves three primary variablesobtained from stratigraphic data: (1) thickness ofeach sedimentary unit, (2) reconstructed deposi-tional water depth, and (3) lithology. Our simpleAiry backstripping routine accounted for com-paction and loading of sediments and water (Watt,1988; Angevine et al., 1990; Sahagian, 1993).Coefficients from Angevine et al. (1990) were usedfor decompaction. A potential error can arise fromassigning compaction coefficients to poorly sortedunits with variable grain size, such as sandy shalesor silty sands. However, the thin nature and mini-mal overburden of Mesozoic strata make decom-paction and associated potential errors relativelysmall. Because thicknesses of individual deposition-al units were relatively constant across the stablepart of the Russian platform, a distance greater thanthe lithospheric flexural wavelength, Airy isostaticresponse to lithospheric loading is interpreted to


have been maintained. The compaction of the rela-tively thicker Paleozoic section is considered to beunaffected by the thin Mesozoic–Cenozoic over-burden. Backstripping resulted in a series of rela-tive sea level curves.

The individual relative sea level curves fromeach well or section were compared to check forinconsistencies. Most curves coincided, indicatingclose agreement in timing and magnitude of sea level variations. Minor deviations were exam-ined and the supporting data were scrutinized.

Variations between curves were generally causedby uncertain depth interpretations, erosion, andminor variations of sedimentation rates. Despitethe minor differences, general agreement betweenthe wells and outcrop sections analyzed supportedour interpretation of a stable region in the Russianplatform, and further validated its use as a refer-ence frame for eustatic quantification.

We incorporated data from northern Siberiansections to fill gaps in the Russian platform stratig-raphy by matching highstands in the two regions


Figure 9—Stratigraphicdata sources used for constructing the quantifiedeustatic curve. Numberedsections include both welland outcrop data. See Figure 4 for locations.

immediately before and after each Russian platformunconformity. To accomplish this, we backstrippedthe appropriate intervals of northern Siberian datato generate relative sea level curves. This proce-dure accounted for sediment fill, compaction, load-ing, and water depth variations and loading, andresulted in the sum of tectonic subsidence andeustasy (Steckler and Watts, 1978; Angevine et al.,1990). The relatively short durations of the inter-vals made it possible to assume that Siberian subsi-dence rates were constant within the intervals

(usually <5 m.y.). Thus, the tectonic subsidencedefined a straight-line trend, the deviations fromwhich could be attributed to eustatic variations. Tomerge Russian platform and Siberian data, we chosehighstand tie points in the Russian platform eustaticcurve immediately before and after each hiatus tobe filled, and correlated them to their northernSiberian equivalents. The correlations were basedon biostratigraphic control at biostratigraphic zonalresolution. For example, for the Valanginian hiatus,we correlated highstands in the upper Berriasianammonite zone Peregrinoceras albidum/Bojarkiamesezhnikovi and lower Hauterivian zoneHomolsomites bojarkensis (Figure 10). The differ-ence in sea level rise in this interval between thetwo curves was attributed to northern Siberian sub-sidence, and the slope defined by this differencewas subtracted from the northern Siberian strati-graphic data. This procedure resulted in a subsi-dence-corrected northern Siberian sea level curve.Once this was obtained, we could simply use thenorthern Siberian data to fill the gap in the stratigra-phy of the Moscow depression to complete themissing intervals in the eustatic curve.

SEA LEVEL CURVE

Constructing the Quantified Eustatic Curve

On the basis of stratigraphic continuity and reli-able geologic age control, the relative sea levelcurves from two wells, M1 (Teplyi Stan) and M456(Varavino), were chosen as a frame for the eustaticcurve. Data from the various other well, core, andoutcrop sections of the Russian platform wereincorporated both as a check for consistency and asa source for additional stratigraphic detail. The finaleustatic curve was constructed as a composite ofthe individual relative sea level curves (Figure 11)using the Harland time scale (Harland et al., 1990).The thickness of the error band reflects uncertain-ties in water depth analysis as well as other smallerfactors. The data for upper Bajocian–Bathonian andtwo zones of the basal Aptian were taken from thewestern Ryazan-Saratov trough region of theRussian platform (Figure 1). We filled the gapscaused by the most prominent unconformities(Valanginian, upper Cenomanian–lower Turonian,lower Coniacian) with north Siberian data. UpperBarremian and upper Aptian sediments are absentthroughout most of the Russian platform (Sazonovand Sazonova, 1967) and are also poorly controlledbiostratigraphically or are completely absent inSiberian sections. These intervals, in addition tothe lower Volgian and middle Cenomanian, areshown with dashed lines on the eustatic curve(Figure 11).


Figure 10—Example of incorporation of Siberian (Boy-arka River) stratigraphic data to fill Valanginian uncon-formity of the Russian platform. (A) Relative sea levelcurve for Boyarka River section. (B) Boyarka River dataincorporated into Russian platform curve. Incorporationwas accomplished by matching the elevations of thehighstands in the two regions immediately before andafter the Russian platform unconformity. Note that high-er order cyclicity is superimposed on Hauterivian sealevel rise in the Boyarka River section.

Relation of the Eustatic Curve to Present Sea Level

Although we find stratigraphic evidence for stabil-ity of the Moscow depression throughout Mesozoicdeposition, we also find evidence of significant post-Santonian rejuvenation of pre-Mesozoic structures.Consequently, we do not use the present elevationof the central Russian platform strata as an absolutemeasure of eustatic change after the Mesozoic, butrestrict its use to the variations within the Mesozoic.To tie the Mesozoic curve to present sea level andset the elevation scale in Figure 11, we had tochoose a point that can be considered as havingbeen stable throughout the Cenozoic. On thePaleozoic Voronezh arch (Figure 1), the top of theCenomanian is flat-lying at 214–225 m above presentsea level across a region spanning 250 km and trend-ing northwest-southeast (Blank et al., 1992).However, south of the Voronezh arch is the Dnepr-Donetsk depression, in which the Cenomanian hassubsided by 500–600 m. North of the Voronezharch, the base of the Aptian lies at an elevation of260–270 m, indicating significant uplift that causederosion of the Cenomanian strata. This setting com-promises the reliability of the Voronezh arch fortying the Mesozoic to present sea level.

A region in Minnesota (United States) also has flat-lying Cenomanian–Turonian strata and has beeninferred to be tectonically stable since that time(Sloan, 1964; Sleep, 1976; Sahagian, 1987). Theregion extends through Minnesota and into Iowa,and may have extended across an eroded or non-deposited region as far as Tennessee (Marcher andStearns, 1962). There is no evidence for anyepeirogenic activity in the region aside from glacialrebound and regional erosion (Sahagian, 1987).However, the limited stratigraphic range of thesedeposits (only Cenomanian–Turonian) does notallow construction of a continuous Mesozoic sealevel curve. If the elevation of the Minnesota strata is used as a baseline, with the elevation of theCenomanian–Turonian highstand at 270 m abovepresent sea level (Sahagian, 1987), 160 m should beadded to the arbitrary elevation scale in Figure 11.The shape and magnitude of variations within theeustatic curve in Figure 11 were determined solelyon the basis of Russian platform stratigraphy (withsome short sections from Siberia). The Minnesotahorizon can be used only for placing the zero pointof the elevation scale (vertical axis) relative to pres-ent sea level.

Comparison with Other Sea Level Curves

Several sea level curves in recent decades havebeen based mainly on the stratigraphy of passive

margins or otherwise-subsiding basins. The mostcited curve is that of Haq et al. (1987, 1988). Thiscurve, however, has been criticized by some work-ers (Sloss, 1991; Miall, 1992) for not taking suffi-cient account of the tectonic and sedimentary processes that dominate the mainly passive margin environments upon which it was based.Nevertheless, the curve was based on a large bodyof stratigraphic and biostratigraphic data, and pro-vides an interesting comparison for the eustaticcurve generated from the Russian platform (Figures12, 13). The most obvious result of this comparisonis that both curves have similar long-term trends.Haq et al. (1988) indicated a general sea level risefrom the Bathonian (Middle Jurassic) to the Volgian(Late Jurassic) punctuated by several sea level falls.We obtained a similar pattern (Figure 12), althoughthe magnitude of the long-term rise is less than that indicated by the Haq et al. (1988) curve. Both curves indicate a long-term lowstand in theBerriasian and Valanginian and sea level rise in theHauterivian that continued to a maximum in theearly Turonian.

We also found similarities and some significantdifferences between the curves at a finer scale, par-ticularly with respect to magnitude of eustaticevents. The Bathonian portions of both curves arealmost identical, showing slow eustatic fall, and thesimilarity continues through the Upper Jurassic.Although the number of small-scale eustatic eventsis similar in the two curves, we found some dis-crepancies in the timing of events at the biostrati-graphic zonal level, possibly owing to biostrati-graphic correlation uncertainties.

We found significant discrepancies in theBerriasian–Valanginian interval. A sea level fall atthe Jurassic–Cretaceous boundary caused a region-al erosional unconformity throughout the Moscowdepression; however, the Haq et al. (1988) curveshowed a sea level highstand at this time. Further-more, we found no evidence for the major sea leveldrops of more than 100 m indicated by Haq et al.(1988) at 128.5 and 126 Ma on either the Russianplatform or the more continuous record of westSiberia [note different time scales on our quantifiedeustatic curve (Figure 11) and Haq et al. (1988)curves].

In the Cretaceous, both our eustatic curve andthat of Haq et al. (1988) show a general lowstand inthe Valanginian. On our quantified eustatic curve(Figure 11), the Valanginian lowstand is followedby early Hauterivian eustatic rise with a magnitudeof at least 30 m. After a sea level fall at the end ofthe early Hauterivian, there is another rise of atleast 60 m in a time interval of at most 2 m.y. Thetotal magnitude of sea level change during theHauterivian is estimated to have been at least 60 mbetween a low in the uppermost Valanginian to a


high in the basal Barremian. This generally agreeswith the results of Haq et al. (1988), who place alowstand in the Valanginian and a highstand in thebasal Barremian, but the curves differ in detail.

There is little agreement between the twocurves for the Barremian–Aptian interval. For theAlbian both our curve and the Haq et al. (1988)curve indicate an overall sea level rise with ahighstand in the upper Albian. In the UpperCretaceous, both curves agree with respect to afew important events, but again show differentamplitudes that include middle Cenomanian andmiddle Turonian sea level falls, and a maximumhighstand just after the Cenomanian–Turonianboundary.

Hallam (1988) obtained an Upper Jurassic curvesimilar in general form to ours, but again the curvesdiffer in detail. For example, Russian platform andSiberian data do not reflect important sea level fallsduring the basal Callovian and basal Oxfordian.Hallam (1988) summarized transgressive/deepen-ing events recorded on various continents andregressive/shallowing events of regional impor-tance in Europe. Note that most of these eventscorrespond to those of our quantified eustaticcurve and include transgressive episodes from thelate Bajocian, late Bathonian, early Oxfordian, mid-dle Oxfordian, late Oxfordian, early Kimmeridgian,and middle Volgian. Regressive events occurredduring the latest early Callovian, latest earlyOxfordian, latest middle Oxfordian, and earlyKimmeridgian (submutabilis zone).

In their general trend, our results agree particu-larly well with those of Weimer (1984) (using theObradovich and Cobban time scale) for the UpperCretaceous of the United States Western Interior.Our quantified eustatic curve includes some high-frequency oscillations that do not appear on theWeimer (1984) curve, but at the level of majorevents, the agreement is excellent.

DISCUSSION

The relationship between eustatic variations andsedimentation is quite different for cratonic areas

than it is for passive margins. This difference is aresult of contrasting geometries (hypsometries),sedimentation rates, and tectonics. High rates ofsubsidence and sediment supply on passive mar-gins result in relatively continuous deposition ofthick (>100 m) sedimentary units. Shoreline shiftsare primarily caused by variations of rates of eustat-ic change, and relative sea level change can beinterpreted through change of coastal onlap.However, even though eustatic change is generallyconsidered as the main factor that drives coastalonlap shifts, transgressions and regressions on pas-sive margins remain very sensitive to subsidenceand sedimentation rates. For example, Pitman(1978) demonstrated that on passive margins,transgressions and regressions may occur as a resultof variations in the rate of sea level fall, which mayresult in potential error in reconstructing eustasyon the basis of changes in coastal onlap unless thetime and space distribution of tectonic subsidenceis known. This problem does not arise in cratonicregions such as the Moscow depression, whichlacks discernible syndepositional tectonics and hasextremely slow sedimentation rates. In these envi-ronments, transgressions and regressions are drivenonly by eustatic rise and fall, respectively.

An important difference in the phase relation-ship between eustasy and shoreline shifts (trans-gressions-regressions) exists between passive mar-gins and cratonic basins. Angevine et al. (1990)showed that maximum regression may occur any-where from the point of maximum rate of eustaticfall to lowstand, depending on tectonics and geom-etry. This results in different timing of shorelineshift episodes in different basins caused by thesame eustatic change. Angevine et al. (1990) calcu-lated that for third-order eustatic variations, trans-gressive/regressive episodes may vary in timing byas much as 2.5 m.y. in different basins, resulting inuncertainties of dating eustatic events inferred fromsubsiding basins, but are avoidable only in the unusu-al case in which the time and space distribution oftectonic subsidence rate is well documented for theperiod of deposition. Some workers have argued thatthe most meaningful way to view subsidence, eusta-sy, and sedimentation on passive margins is on the


Figure 11—Quantified eustatic curve for late Bajocian–Santonian. The time scale is from Harland (1990). The eleva-tion scale is arbitrary, with 0 set at the lowest point on the curve. The position of the curve relative to present sealevel can be established if one can identify a frame that has been stable between the Mesozoic and the present. Abaseline in Cenomanian–Turonian strata of Minnesota is considered best suited for this purpose, and indicates that160 m should be added to the eustatic scale (see discussion in text). The width of the band defined by high and lowestimates reflects the various sources of error in the analysis, including interpretations of water depth, decom-paction, lithology, stratal thickness, and correlation. Subaerial unconformities are assumed to represent at leastsome time of 0 water depth (exposure), and are indicated by convergence of high and low estimates. The curve rep-resents eustasy to the extent that the Russian platform frame of reference is sensitive to the relationship betweenocean basin volume and ocean water volume.


9698

100

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L.tradefurcata L.tradefurcata

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Relative eustatic change (m)

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158

160

162

164

166

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S.niortense

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Low

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Q.lamberti

Cal

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Bat

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basis of rates rather than amounts (Pitman, 1978;Posamentier et al., 1988; Christie-Blick, 1990).However, if the rate of subsidence is zero and therate of sedimentation is low, as on the Russian plat-form, it may be useful to consider amounts ofeustatic change instead.

The tectonic stability and very slow sedimenta-tion rates of the Moscow depression make a phaserelationship such that eustasy and shoreline shiftschange in parallel. Thus, assigning eustatic eventsto stratigraphic elements observed on the Russianplatform is much more precise than would be pos-sible on passive margins. Another advantage of theRussian platform is its well-documented ammonitebiostratigraphy (for most of the section), whichprovides the most reliable correlation possible.

Potential errors in eustatic reconstructions arisefrom different sources on passive margins and sta-ble cratons (e.g., Moscow depression). On passivemargins, large potential errors may occur from sub-sidence, sedimentation, and water depth (all termsof the backstripping equation), whereas erosion is

a negligible factor compared to sedimentary thick-nesses. On the Russian platform, potential errorsmay occur from estimating water depth and erosion, with smaller errors arising from minorvariations of sedimentation. These errors can beminimized by the use of multiply redundant strati-graphic data.

The greatest potential source of error for eustaticreconstruction based on the stratigraphy of theMoscow depression arises from paleodepth inter-pretations. This is a universal problem, but com-pared to other basins, the Moscow depression iswell suited for paleodepth analysis as a result ofshallow water, and thus relatively preciselyestimable paleodepths. Unlike previous sea levelanalyses based on passive margins, in which poten-tial errors from estimating thermotectonic subsi-dence are so great that water depth variations aresmall in comparison, in our analysis these samewater depth variations are the largest potentialsources of error and have been carefully accountedfor to achieve the greatest eustatic interpretive reli-ability.

Erosion in the Moscow depression resulted invery limited distribution of certain stratigraphicunits, making it more difficult to constrain paleo-depth estimates on the basis of paleogeographyand stratal geometry. The areal extent of youngersediments on the Russian platform is more limitedthan that of older sediments due to extensivepostdepositional (including Quaternary) erosion.For example, Upper Cretaceous strata are onlylocally preserved, although there is ample evi-dence that indicates deposition throughout mostof the Late Cretaceous. Thus, our facies analysisand sea level curve based on the Russian platformare more precise for earlier times than for later.The lack of proximal margins (shoreline deposits)of many units due to erosion also results in diffi-culties with paleogeographic control for unitsdeposited during highstands of sea level. Duringsignificant regressions, the Russian platform wascompletely exposed, resulting in unconformitiesdue to erosion or nondeposition. Thus, the leastaccurate depth estimates are for times of extreme-ly high or extremely low sea level. During high-stands, the deep water is difficult to quantify, and the proximal margins of highstand depositsare generally eroded. During lowstands, previous-ly deposited sediments may be eroded. In-corporating intervals from Siberian basins can fillthe gaps, but represent an additional potentialsource of error. Nevertheless, the magnitude ofthis error is only a few meters, whereas the errorfor eustatic reconstructions from passive marginsor subsiding basins (with kilometers of subsi-dence and sedimentation) can be considerablygreater.


Bathonian

Bajocian

Haq et al., 1988 Russian Platform (this study)

Mid

dle

Jura

ssic

1: 5750

1: 2300

sea level rise

Callovian

Oxfordian

Upp

er J

uras

sic

Kimmeridgian

Volgian

Berriasian

Low

er

Cre

tace

ous

50 m 0100 m 50 0

Figure 12—Comparison of Jurassic results of our studywith those of Haq et al. (1988). Amplitudes from Haq etal. (1988) are 240% greater than those established onthe basis of Russian platform stratigraphy (rescaled forillustration purposes).

The quantification of eustatic variations on timescales of less than 1 m.y. bears on the issue of ratesand causes of eustatic changes. Because we usestratigraphic techniques, deposition rates must beconsidered minimum estimates. Thus, rates of sealevel rise are minimal. During the late Hauterivian,sea level rose by at least 60 m in at most 2 m.y. Thisrate of greater than 30 m/m.y. is normally consid-ered greater than that attributable to the mecha-nisms accepted for causing sea level variations dur-ing the Mesozoic. In the absence of majorcontinental ice sheets (Barron et al., 1981; Barronand Washington, 1985), the mechanisms for large,rapid eustatic variations are poorly understood.Some explanations have been offered that may bearon the problem (Karner, 1986; Cloetingh, 1988;Jacobs and Sahagian, 1993), but no single mecha-nism has been accepted that can account for theinferred magnitudes and rates.

CONCLUSIONS

The Russian platform provides a stable frame ofreference for the quantification of Mesozoic eustat-ic changes. Our quantified eustatic curve so con-structed should be more reliable than curvesinferred from passive margin and subsiding basinstratigraphies because of better tectonic control. Inour analysis, our largest source of error is paleo-depth, which is relatively well constrained due toshallow water depths. Other, larger sources oferror, such as thermal subsidence or sedimentationrate variability, are eliminated by the stable tectonicenvironment of the Russian platform. A well-established biostratigraphic framework, basedmostly on ammonites and good correlation withwestern Europe, makes it possible to accuratelyassign ages to the interpreted eustatic events. Therelatively small potential errors result mainly frompaleodepth interpretations and erosion.

The fortuitous hypsometry and elevation of theRussian platform during the Mesozoic led to shallow-marine deposition useful for accurately determin-ing paleowater depth. However, these same condi-tions also led to the development of many uncon-formities throughout the Jurassic and Cretaceoussections. The largest of these stratigraphic gaps(zone-to-substage duration) have been filled withstratigraphic data from adjacent subsiding regionswith more continuous sections.

The quantified eustatic curve shows a generaleustatic rise from the Middle Jurassic through theUpper Cretaceous, punctuated by many higherorder events. The highest sea level was reached inthe basal Turonian at an elevation of 270 m abovepresent sea level (using Minnesota as a datum).Comparison of the quantified eustatic curve withother published sea level curves shows low-ordersimilarities, but many differences at a higher order.In many cases, events can be correlated betweencurves, but magnitudes are different.

The quantified eustatic curve may be a useful toolapplicable to any basin globally. The West Siberianbasin may be a particularly well-suited case for initialapplication because of its well-established inter-regional biostratigraphic correlation with theRussian platform. Our eustatic curve (Figure 11) canbe applied to problems of basin subsidence andbasin modeling where eustatic input is necessary.Thus, our curve may potentially be used as a tool to(1) estimate the influence of local factors (subsidenceand sedimentation rates) by removing the eustatic sig-nal from stratigraphic data, (2) improve geologicalcorrelation (where there is poor biostratigraphic con-trol), and (3) constrain the eustatic parameter forcomputer models of basin sedimentation. By subtracting the eustatic signal from backstrippingresults from any basin, tectonics and sedimentation


sea level rise100 m 50 0 50 m 0

Valanginian

Hauterivian

Barremian

Aptian

Albian

Haq et al.,1988 Russian Platform (this study)

Low

er C

reta

ceou

s

Berriasian

1:45501:10900

Cenomanian

Turonian

Coniacian

Santonian

Upp

er C

reta

ceou

s

Figure 13—Comparison of Cretaceous results of ourstudy with those of Haq et al. (1988). Amplitudes fromHaq et al. (1988) are 250% greater than those estab-lished on the basis of Russian platform stratigraphy(rescaled for illustration purposes).

can be quantified at a resolution and reliability previ-ously unattainable. This improved data source maylead to a better understanding of depositional andthermal histories of specific sedimentary basins.

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Dork Sahagian

Dork Sahagian is the executivedirector of the Global Analysis,Interpretation and Modelling Task Force of the International Geosphere-Biosphere Program.Having received his Ph.D. inepeirogeny and sea level from theUniversity of Chicago in 1987, hehas since conducted a many facetedresearch program in sea level, basinanalysis, tectonics, global change,and volcanology. His stratigraphic research has latelyfocused on the quantification of eustatic variations usingstratigraphic data from the Russian platform.

Oleg Pinous

Oleg Pinous is a graduate studentat the University of New Hampshireand is working on the developmentand application of quantifiedMesozoic eustatic curves based onRussian stratigraphy. He receivedsecond-place and then first-placeresearch presentation awards fromthe International Scientific StudentConference (Novosibirsk) in 1992and 1993, respectively, and re-ceived the Outstanding Student Research Award from theSedimentary Geology Division of the Geological Societyof America in 1994.

Alexander Olferiev

Alexander G. Olferiev was bornin 1936 and graduated from theMoscow Institute of the GeologicalSurvey in 1959. He then workedfor the Russian Geological Surveyand Mapping in various regions ofEuropean Russia, the Ural Moun-tains, and Krasnoyarsk. He hasfocused on the Mesozoic stratigra-phy of Russia and Bulgaria. Olferievis author or co-author of numerousmaps and articles, including the Uniform (Standard)Stratigraphic Scheme for the Jurassic and Cretaceous ofthe Russian platform.

Victor Zakharov

Victor Zakharov is the chief ofthe Laboratory of Mesozoic Stra-tigraphy and Paleontology, andchair of Paleontology and HistoricalGeology at Novosibirsk StateUniversity. He is author or co-author of more than 170 scientificpublications in Russian, English,French, and German. His researchhas focused on the Jurassic andCretaceous, with emphasis on thesedimentary basins of the northern territories of Russia.His studies include paleontology and paleoecology ofbivalves, Jurassic and Cretaceous boreal biostratigraphy,and paleobiogeography.

ABOUT THE AUTHORS

Documents

Eustatic Curve for the Middle Jurassic–Cretaceous Based on ...mmtk.ginras.ru/pdf/sahagianea1996.pdf · sedimentary units. Some of the grabens were inverted in the late Mesozoic