Geological Society of America Bulletin - Cascadia Geo€¦ · 180 Geological Society of America Bulletin, February 2002 GULICK et al. Figure 2. A composite onshore stratigraphic section

Geological Society of America Bulletin

doi: 10.1130/0016-7606(2002)114<0178:EOTNMM>2.0.CO;2 2002;114;178-191Geological Society of America Bulletin

Sean P.S. Gulick, Anne S. Meltzer and Samuel H. Clarke, Jr. River forearc basin, California: Stratigraphic developmentEffect of the northward-migrating Mendocino triple junction on the Eel

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GSA Bulletin; February 2002; v. 114; no. 2; p. 178–191; 11 figures.

Effect of the northward-migrating Mendocino triple junction on theEel River forearc basin, California: Stratigraphic development

Sean P.S. Gulick*Anne S. MeltzerDepartment of Earth and Environmental Sciences, Lehigh University, 31 Williams Drive,Bethlehem, Pennsylvania 18015-3188, USA

Samuel H. Clarke Jr.Coastal and Marine Geology, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA

ABSTRACT

The Eel River forearc basin, northernCalifornia, lies at the southern end of theCascadia subduction zone and at the lead-ing edge of the migrating Mendocino triplejunction. Stratigraphic relationships withinthe Eel River forearc basin suggest that thecurrent outer-arc high formed between ca.3 and 2 Ma when the margin switched froma nonaccretionary to an accretionary phaseand then uplifted to attain critical taper.Between ca. 2 and 1 Ma, an influx of sedi-mentation from the ancestral Klamath andEel River systems increased the width ofthe northern California margin and causedcontinued uplift followed by widespreaderosion of the western margin of the basinat ca. 1 Ma. In the northeastern part of theforearc basin, localized erosion of the shelfoccurred at ca. 500 ka. The arrival of thenorthward-migrating Mendocino triplejunction at ca. 500 ka is documented by up-lift, northward tilting, erosion of the mar-gin as much as 20 km north of Cape Men-docino, and reduced deposition within theforearc basin as much as 80 km north ofthe current position of the triple junction.Terrestrial sediments delivered to the con-tinental margin and eroded sediments nearthe triple junction largely bypassed thesouthern part of the basin and were likelydeposited in northern areas of the basin orflowed down the Eel Canyon to be depos-ited within the Gorda Fan.

*Present address: Institute for Geophysics, Uni-versity of Texas, Building 600, 4412 SpicewoodSprings Road, Building 600, Austin, Texas 78759,USA; e-mail: [email protected].

Keywords: Cascadia subduction zone,Mendocino triple junction, microplates,seismic reflection profiles.

INTRODUCTION

The Mendocino triple junction is the inter-section of the Pacific, North America, andGorda plates in the Cape Mendocino–PuntaGorda area of northern California (Fig. 1). Theorigin of the Mendocino triple junction beganbetween 28 and 24 Ma when the Pacific-Farallon spreading ridge encountered NorthAmerica effectively breaking up the Farallonplate (McKenzie and Morgan, 1969; Atwater,1970; Atwater and Stock, 1998). This breakupformed several microplates and a triple-junction system that has migrated northwardduring the past 24 m.y. owing to the relativemotions of the Pacific and North Americanplates (Atwater and Molnar, 1973; Dickinsonand Snyder, 1979; Lonsdale, 1991; Atwaterand Stock, 1998).

North of the Mendocino triple junction,subduction of the Gorda plate beneath NorthAmerica forms the southern part of the Cas-cadia subduction zone (Fig. 1A), resulting ina 75-km-wide accretionary prism offshorenorthern California (Fig. 1). Overlying this ac-cretionary prism is a forearc basin, the EelRiver basin (also called the Humboldt Basin).About 10% of the Eel River basin lies onshorein the Humboldt Bay region ;50 km north ofthe triple junction; the remaining 90% of thebasin lies offshore northern California andsouthern Oregon (Fig. 1A). The offshore EelRiver basin contains up to 2.5 km of primarilyupper Miocene to Holocene marine sedimentsdeposited above Middle Jurassic to early Ter-

tiary basement rocks of the Franciscan Com-plex (Ogle, 1953; Hoskins and Griffiths, 1971;Blake et al., 1984; Webster and Yenne, 1987;Clarke, 1992; McLaughlin et al., 1994) (Fig.2). The stratigraphic record in the basin re-cords the evolution of the margin and the ar-rival of the triple junction in the Cape Men-docino area.

In this paper, we describe the tectonic de-velopment of the continental margin over thepast ;5 m.y., and we highlight the arrival andinfluence of the northward-migrating triplejunction. We use multichannel seismic (MCS)profiles to examine the stratigraphic relation-ships within the Eel River basin to determinethe margin’s history. The results presented inthis paper, in addition to the regional seismicimages of the Eel River basin and southernCascadia subduction zone, include evidencefor (1) shifting loci of sediment depositionthroughout the late Miocene to Holocene, (2)uplift and subsidence of the western margin ofthe forearc basin related to the evolution ofthe accretionary prism, (3) middle Pleistoceneto Holocene uplift of the southern Eel Riverbasin related to the arrival of the Mendocinotriple junction, and (4) the current northernlimit of triple-junction influence on the strati-graphic development of the forearc basin. Anupcoming companion paper will discuss thetectonic implications of the structural relation-ships within the Eel River basin.

MCS DATA AND STRATIGRAPHICSEQUENCES

This paper uses ;750 km of the multichan-nel seismic (MCS) reflection data collected aspart of the 1994 Mendocino Triple JunctionSeismic Experiment (MTJSE) (Trehu et al.,



Geological Society of America Bulletin, February 2002 179

EFFECT OF THE MENDOCINO TRIPLE JUNCTION ON THE EEL RIVER BASIN

Figure 1. (A) Map view of thenorthern California study areawith inset showing location rel-ative to California. The EelRiver basin is shown over thesouthern Cascadia accretion-ary prism. The CSZ marks thedeformation front of the south-ern Cascadia subduction zone,and the dashed oval shows thegeneral location of the Men-docino triple junction (MTJ).(B) Perspective view of thestudy area as seen from thenorthwest. A mesh of 6 km by6 km cells is overlaid on inte-grated topography and ba-thymetry with a vertical exag-geration of 153 and with plateboundaries and key bathymet-ric features highlighted. TheEel River basin is the dashed-in region from the Oregon-Cal-ifornia border to its onshoreextension. Bold black arrowsare plate-motion vectors rela-tive to North America. Klam-ath and Eel Plateaus are ob-served as depressions on thewestern flank of the Eel Riverbasin, and the Gorda Fan isobserved as a lump in thesoutheast corner of the Gordaplate. Coastal locations:PSG—Point St. George, TH—Trinidad Head, HB—Hum-boldt Bay, CMPG—CapeMendocino–Punta Gorda, andSF—San Francisco Bay.

1995; Gulick et al., 1998) and 1850 km ofindustry MCS data that cover the offshore EelRiver basin to within 4.8 km (3 mi) of theCalifornia coast (Fig. 3). The MCS profileswere used to map four unconformity-bounded,seismically defined stratigraphic sequencesthroughout the basin (Fig. 4). In some loca-tions, subpackages are visible within these se-quences; however, these subpackages were notmappable throughout the basin and were in-cluded within the larger mappable sequences.

An acoustic-basement reflection, which sep-arates the Franciscan accretionary complexfrom the overlying forearc strata (Fig. 4), ischaracterized by lower frequencies and higheramplitudes than reflections within the forearcsedimentary units. The oldest mappable sedi-

mentary sequence, D, nonconformably over-lies the basement rocks and dips landwardalong the western flank of the basin (Fig. 4A).Sequence C dips landward at a shallower an-gle and locally onlaps the strongly tilted partsof sequence D in the western basin. SequenceC conformably overlies (Fig. 4A) or locallydownlaps onto sequence D in the central basin(Fig. 4B). The upper boundary of sequence Cis an angular unconformity in the western partof the basin (Fig. 4B). Sequence B progradesover and downlaps sequence C in the centralbasin (Fig. 4B). Sequence A is either flat lyingor westward prograding. Sequence A locallyonlaps sequence B in the central basin (Fig.4B), but conformably overlies sequence B inthe western basin (Fig. 4A).

Four offshore exploratory industry wellsdrilled in the Eel River basin, during the1960s, are crossed by several MTJSE and in-dustry transects (Fig. 3). The exploratorywells were tied to the industry and academicseismic data by using representative intervalvelocities converted from stacking velocitiesin MTJSE transect MTJ-8 (Gulick et al.,1998) (Fig. 5). Webster and Yenne (1987) andWebster et al. (1986) defined lithostratigraphicpackages in the four exploratory wells basedon changes in lithology and benthic forami-nifera assemblages. These authors assignedthe deeper lithostratigraphic packages to for-mations of the Wildcat Group observed in theonshore basin (Ogle, 1953), and the shallowerpackages were termed the upper and lower



180 Geological Society of America Bulletin, February 2002

GULICK et al.

Figure 2. A composite onshore stratigraphic section for the Eel River basin showing unitages, names, paleobathymetry (from McCrory, 1995) and lithology (from Clarke, 1992,and McCrory, 1995, 2000). Sea-level data adapted from Haq et al. (1988). The Bear Riverbeds are an informal unit (Haller, 1980). Unconformity names and ages are from McCrory(2000). The Falor unconformity is only found onshore (McCrory, 2000).

Quaternary (Fig. 5). All four wells bottomedin rocks identified as coming from the Fran-ciscan accretionary complex (Webster andYenne, 1987).

Given the .75 km distance from the wellsto the onshore basin (Fig. 3), the lithologicsimilarities among the Wildcat Group forma-tions (Ogle, 1953; McCrory, 1995), and thetime-transgressive nature of some of the on-shore geologic formations (Morrison and Sar-na-Wojcicki, 1981; Sarna-Wojcicki et al.,1987, 1991), assignment of onshore geologic

formation names to the lithostratigraphicpackages in the wells is difficult to do withcertainty (Clarke, 1992). However, the benthicforaminifera assemblages observed in thewells could be used to assign a general chron-ostratigraphy to the four mappable sequence-stratigraphic packages observed within theseismic data (Fig. 5). The benthic foraminiferaassemblages found in the industry wells forthese four sequences were Delmontian–Ven-turian for sequence D, Venturian–Wheelerianfor sequence C, Wheelerian–Hallian for se-

quence B, and Hallian for sequence A (Web-ster and Yenne, 1987). On the basis of theseassemblages, we assign sequence D to the up-per Miocene–middle Pliocene, sequence C tothe middle Pliocene–lower Pleistocene, se-quence B to the lower to middle Pleistocene,and sequence A to the middle Pleistocene-Ho-locene (Fig. 5).

Crouch and Bachman (1987) reported thepossibility of the Miocene Bear River beds ofHaller (1980) existing in the offshore Eel Riv-er basin. The Bear River beds were not en-countered in the well data in the northeasternpart of the forearc basin (Webster et al., 1986),and there are no separate seismically mappa-ble units within sequence D. These facts donot necessarily discount the possibility of theexistence of the Bear River beds in isolatedlocations in the southern Eel River basin.

Wildcat Group formations near HumboldtBay record an overall shallowing of the pre-sent onshore basin, from neritic to lowerbathyal and abyssal (;3000 m) depths duringthe late Miocene, to bathyal and then to lit-toral and nonmarine environments in the Pli-ocene–Holocene (Fig. 2) (Hopps and Horan,1983; Clarke, 1992; McCrory, 1995). De-creases and disappearances of deep-water ben-thic fauna up section at the onshore Center-ville Beach and Scotia sites demonstrate thatparts of the now-onshore basin were rapidlyuplifted to shallow depths at ca. 3 Ma(McCrory, 1989, 1995). Onshore, a basin-edge unconformity observed in the northeast-ern onshore part of the basin is termed theFalor datum and lies between the base of theFalor Formation, a unit coeval with some ofthe upper Wildcat Group sequences, and Fran-ciscan Complex rocks (Fig. 2). The Falor da-tum was determined to be ca. 2 Ma based ona radiometric date on the Huckleberry ash lo-cated near the base of the Falor Formation(Kelsey and Carver, 1988; McCrory, 2000).Although upper Wildcat Group strata are time-transgressive (Morrison and Sarna-Wojcicki,1981), the upper boundary of the WildcatGroup is a widespread middle Pleistocene un-conformity termed the Wildcat datum (Mc-Crory, 2000) (Fig. 2). This unconformity isdated at 1 6 0.3 Ma by extrapolating theamount of sediment accumulation up to theunconformity based on the sedimentation ratebetween the Rio Dell ash (1.48 Ma radiomet-ric date) (McCrory, 1995, 2000) and the Brun-hes-Matuyama boundary (0.78 Ma 6 0.01)(Kodama, 1979). Uranium-series dates of de-posits at Moonstone Beach indicate that anunconformity that crops out in the northeast-ern margin of the onshore Eel River basin(Carver, 1987; Kelsey and Carver, 1988) is





Figure 3. Seismic surveys (lines A–Z and AA–AD as well as the MTJ lines) and wells usedin this study superimposed on 500 m bathymetric contours and major synclines. Men-docino Triple Junction Seismic Experiment (MTJSE) transects MTJ-14, MTJ-8, and MTJ-6 are displayed in this paper. Industry wells P-007–1, P-012–1, P-014–1, and P-019–1 areshown as wells 7, 12, 14, and 19, respectively. White squares onshore denote the locationsof the Centerville Beach and Scotia (Eel River) sites of McCrory (1989).

correlative with the Wildcat datum andformed at ca. 1.0 Ma, as discussed byMcCrory (2000). Within the post-Wildcatgravels, sandstones, and siltstones of Qua-ternary age, a continental-shelf unconfor-mity—termed the Hookton datum (Mc-Crory, 2000) and formed by subaerialerosion at ca. 500 ka—is observed onshore(Sarna-Wojcicki et al., 1991; McCrory,1996, 2000) (Fig. 2).

Our age assignments for the offshore EelRiver basin sequences based on the well dataappear to correlate well with the younger da-tums mapped in the onshore Eel River basinsites (Figs. 2 and 5). The angular unconfor-mity that separates parts of sequence C fromsequence B and the one that separates parts ofsequence B from sequence A are consistentwith the ca. 1 Ma (Wildcat datum) and ca. 500ka (Hookton datum) unconformities mappedonshore (Fig. 4), respectively (McCrory,1995, 2000). The reflector that divides se-

quence D from sequence C is middle Pliocenein age, according to the well data, and isroughly correlative with the ca. 2 Ma uncon-formity (Falor datum) at the northeastern ba-sin edge.

In order to generate the isopach maps foreach sequence-stratigraphic unit presented inthis paper, structure-contour maps were cre-ated for each sequence boundary. Grid-basedinterpolation between transects was deter-mined by using a universal kriging algorithmwith a linear overlay (Olea, 1974). Standarddeviation of the variance with distance rangedfrom 0, within a transect, to 0.3, midway be-tween the widest-spaced transects. This erroranalysis shows that all kriging-calculated nodevalues in the structure-contour maps are with-in the 95% confidence interval (Olea, 1974)because of our good spatial data coverage(Fig. 3). Isopach maps were created from thestructure-contour maps by using the same ve-

locities that were used to tie the wells (Fig.5).

REGIONAL OBSERVATIONS

Observations from regional seismic tran-sects across the entire continental marginshow systematic variations in structures, de-gree of deformation, basin development, andprism geometry. Three dip-direction seismicprofiles from the MTJSE (MTJ-14, MTJ-8,and MTJ-6) lie north of the triple junction andimage from the abyssal plain to the continen-tal shelf (Fig. 3). The deformation-front regionof the prism has been discussed in detail byGulick et al. (1998), and a detailed analysis ofthe types and tectonic implications of thestructural deformation within the basin will bediscussed in an upcoming companion paper.Other academic and industry transects thatcross the deformation front (Fig. 3) showthese three transects to be representative of thenorthern, central, and southern sections of theEel River basin.

The northern transect MTJ-14 (Fig. 6; alltimes are two-way traveltimes [TWT], andhorizontal position is indicated by the com-mon midpoint [CMP] number) shows a well-developed accretionary prism ;85 km wide.Beneath the prism, MTJ-14 clearly images thesubducting Gorda plate beneath the entiremargin. MTJ-14 shows a broad synclinal EelRiver forearc basin that lies within the 65–70-km-wide region between the outer forearchigh and the emergent Franciscan terranes on-shore. The bathymetric profile along this tran-sect extends from the abyssal plain across theactively deforming lower slope of the accre-tionary prism to an ;750-m-deep, ;30-km-wide, plateau (the Klamath Plateau of Silver,1971), before shallowing to the continentalshelf (Fig. 6). Between the plateau and con-tinental shelf, the profile crosses a bathymetricrise termed the plateau slope by Silver (1971).From west to east it can be seen that structuraldeformation in MTJ-14 is primarily confinedto thrust faults in the deformation-front region(Gulick et al., 1998), a series of fault-relatedanticlines on the western edge of the forearcbasin, and high-angle faulting beneath thecontinental shelf (Fig. 6).

Widths of the accretionary prism (;80 kmwide) and forearc basin (;65 km wide) intransect MTJ-8 (Fig. 7) are similar to thoseobserved in the northern transect (MTJ-14).Beneath the deformation-front region and thecontinental shelf, MTJ-8 images clear reflec-tions from the top of the subducting plate;however, beneath the Klamath Plateau andplateau slope, only intermittent oceanic




GULICK et al.

Figure 4. Type sections used to define stratigraphic sequences D through A. (A) Sequence D is tilted eastward, sequence C onlapssequence D to the west, and sequences B and A are relatively flat lying. (B) Sequence D overlies acoustic basement and is tilted eastward,sequence C onlaps sequence D and is angularly truncated at the top in the western basin. Sequence B downlaps sequence C beneaththe eastern Klamath Plateau and is flat lying in the western basin. Sequence A downlaps sequence B beneath the eastern KlamathPlateau and is flat lying in the western basin. Locations in inset map: E—Eel River basin, PG—Point St. George, M—MendocinoFracture Zone, TH—Trinidad Head, CM—Cape Mendocino. Locations in inset maps: PG—Point St. George, TH—Trinidad Head, andCM—Cape Mendocino. Y-axes are in seconds of two-way traveltime.

crustal reflections are present. In MTJ-8, the35-km-wide Klamath Plateau, at ;1.5 kmwater depth, is deeper than in the MTJ-14profile and is separated from the continentalshelf by a steeper plateau slope, ;8.58 dip,than in MTJ-14, ;3.48 (Fig. 6–7). Structuresof the deformation-front region in the central

part of the margin are landward vergent forthe outer 6.5 km and seaward vergent farthereast (Fig. 7) (Gulick et al., 1998). As ob-served in MTJ-14 (Fig. 6), low-angle faultingis present from the deformation front to thewestern margin of the forearc basin, wherethe top of the inferred Franciscan Complex

shallows. High-angle structures are presentbeneath the continental shelf. The sedimen-tary section, within the central Eel River ba-sin, shows evidence of broad anticlinal fold-ing beneath the Klamath Plateau, butcontinues to be less deformed than the ba-sin’s margins (Fig. 7).





Figure 5. Correlations between offshore industry wells and coincident industry seismic transects. Industry wells are labeled with Websterand Yenne’s (1987) interpreted stratigraphy and velocities used to tie to seismic data. Webster and Yenne’s (1987) stratigraphic cor-relations of formations were not used in this study (see text for discussion), but the benthic foraminifera ages that defined their relativetiming allowed us to assign the chronostratigraphy. Acoustic basement is the top of the Jurassic to Early Tertiary Franciscan Complex.

MTJ-6 crosses a 45–55-km-wide accretion-ary prism in the southernmost Cascadia mar-gin and shows an intermittent reflection fromthe Gorda oceanic crust beneath the defor-

mation front and continental shelf (Fig. 8).Coincident OBS (ocean-bottom seismograph)refraction data recorded during the 1994 Men-docino Triple Junction Seismic Experiment

(Trehu et al., 1995) show the Gorda plate dip-ping ;68 beneath the accretionary prism. Theplate dip steepens beneath the coast to ;118–128 (Smith et al., 1993; Oppenheimer et al.,




GULICK et al.

Figure 6. Mendocino Triple Junction Seismic Experiment line MTJ-14, which images the northern Eel River basin (see Fig. 3 forlocation). Profile shows transition from the abyssal plain to the Klamath Plateau and continental shelf. Eel River basin sediments weredeposited above the Franciscan Complex of the accretionary prism. The subducting Gorda plate can be observed for the entire profile.Notice the pattern of lower-angle thrust faults to the west and higher-angle thrust faults to the east with little deformation in the centralsyncline.

1993; Cockerham, 1984). A second marginalplateau, the Eel Plateau (Silver, 1971), is;15–20 km wide in MTJ-6 (Fig. 3). Struc-tural deformation in MTJ-6 is not limited tothe foreslope of the prism and the basin edges(Fig. 8). In comparison with MTJ-14 andMTJ-8, which image the more northern partsof the northern California margin (Figs. 6 and7), the margin imaged in MTJ-6 appears lat-erally shortened or telescoped (Fig. 8). InMTJ-6, the Eel River basin sediments arethicker, and the basin no longer consists of onelarge syncline and some accompanying de-pocenters at the basin edges, but instead com-prises a series of smaller slope basins (Fig. 8).

PATTERNS OF DEPOSITION

Stratigraphic Observations

As discussed by Hoskins and Griffiths(1971) and Clarke (1992), the Eel River basintrends roughly south from southern Oregon toTrinidad Head, California (Fig. 3). South ofTrinidad Head, the trend curves east as the

basin comes onshore in the Humboldt Bay re-gion. The pattern of cumulative Neogene sed-imentary deposition closely follows the gen-eral trend of the regional basin axis (Figs. 3and 9E). The thickness of the sedimentarysection (sequence D–sequence A) overlyingacoustic basement ranges from ;50 to 2500m (Fig. 9E). Sedimentary sequences thinalong both flanks of the northern and centralparts of the basin owing to nondeposition anderosion. Sediment accumulation in the south-ern basin is greater than in the northern andcentral parts of the basin, reaching thicknessesof .2500 m in the Freshwater, South Bay, andEel River synclines (Fig. 9E).

The oldest basin-wide sequence, D, rangesin thickness from 50 to 1500 m (Fig. 9D). Theaverage east-west width of these deposits is;45 km. Sediments are thickest along thecentral-basin axis. Principal depocenters arelocated in the northern and south-central partsof the basin. The south-central depocenter liesbeneath both the modern Klamath and EelPlateaus and an ;20 km2 region of the mod-ern continental shelf. Strata of sequence D

thin along the flanks of the basin (Fig. 9D).Along the western flank of the basin, sedi-ments are tilted eastward beneath an eastward-dipping unconformity (Fig. 10, D and E). Sed-iments in sequence C onlap this unconformityto the west in the northern and central partsof the basin. Beneath the continental shelfnear Humboldt Bay, this same eastward-dipping reflector forms the top of a packageof sediment that is tilted northeastward alonga seaward-vergent thrust (Fig. 10A).

Sediments in sequence C have a maximumthickness of 800–1000 m; much of the basincontains ,250 m of sequence C (Fig. 9C).The average east-west width of these depositsis ;40 km. Locally, along the western marginof the northern and central parts of the basin,sequence C is missing (Fig. 9C). The thickestdeposits are located in the northern part of thebasin, with some locally thicker deposits orlocal depocenters in the central and southernparts of the basin (Fig. 9C). Beneath theKlamath Plateau, a subhorizontal angular un-conformity, with a lateral east-west extent of;20 km, separates sequence C from sequence





Figure 7. Mendocino Triple Junction Seismic Experiment line MTJ-8, which images the central Eel River basin (see Fig. 3 for location).From west to east, notice the landward-vergent deformation front, the low-angle faulting on the western basin margin, the limiteddeformation within the central syncline, and the high-angle deformation on the eastern margin. Subducting Gorda crust was not imagedbeneath the Klamath Plateau.

B (Fig. 10, D and E). This angular unconfor-mity is also present beneath the Eel Plateauand the southeastern part of the continentalshelf (Fig. 10A). Sequences B and C are con-formable in the northeastern part of the basin(Fig. 10C).

Sediments of sequence B have an east-westlateral extent of 55 km (Fig. 9B). The mainlocus of deposition shifted to the south rela-tive to sequences C and D. Sequence B isthickest, 800–1000 m, in the central andsouthern parts of the basin. Sequence B thinsto ,250 m thick on the western flank of thebasin (Fig. 9B). In the northeastern basin, thehorizon separating sequence B from sequenceA is locally an angular unconformity (Fig.10C).

Compared to older strata, the sediments ofsequence A are deposited across a broader re-gion that extends 60 km from east to west(Fig. 9A). The sediments in sequence A arethickest (500–1000 m) in the northern andcentral parts of the basin, northwest of Trini-dad Head (Fig. 9A). Sediment thickness in se-quence A decreases southward, reaching a

maximum of only 500 m seaward of Hum-boldt Bay and 250 m still farther south (Fig.9A). For much of the southern basin, sequenceA appears as a thin layer overlying the muchthicker deposits of sequence B–sequence D(Fig. 10A).

A small region of sediments .2000 m thickthat likely consists of all four sequences isseparated from the rest of the Eel River basinby the southernmost basin fault zone and theEel Canyon (Fig. 10B). These strata are notmapped in Figure 9 because of uncertaintiesin interpreting sequence boundaries across theEel Canyon. Strikingly, MCS images fromthis region show that the entire sequence ofNeogene strata is tilted northward and trun-cated at the seafloor (Fig. 10B).

Depositional History and TectonicEvolution

If our age correlations are correct, the oldestregional basin fill (sequence D) was depositedthroughout the basin in late Miocene to mid-dle Pliocene time at an average accumulation

rate of 0.46 km/m.y.; the greatest accumula-tion was in the southern part of the basin (Fig.9D). This southern depocenter was probablythe primary depocenter for the majority of thesediment transported by the ancestral riversystem(s) of the Humboldt Bay region. UpperMiocene to middle Pliocene sequences havebeen tectonically tilted eastward along thewestern margin of the northern and central ba-sin (Fig. 10, D and E), whereas the overlyingupper Pliocene to lower Pleistocene strata arerelatively flat lying and, in places, onlap thetilted strata of sequence D. Recovery of lowerPliocene rocks in shallow core holes along thewestern margin of the basin is also consistentwith pre–late Pliocene uplift (Hoskins andGriffiths, 1971). These relationships indicatethat an episode of increased thrusting or upliftoccurred along the western margin during themiddle Pliocene (Fig. 11A). This episode ofuplift certainly ceased by ca. 2 Ma, but it mayhave been coeval with the rapid uplift of thepresent onshore basin at ca. 3.5 Ma (McCrory,1989, 1995). The increasing dip of the tiltedsedimentary units through time in sequence D




GULICK et al.

Figure 8. Mendocino Triple Junction Seismic Experiment line MTJ-6, crossing the southern Eel River basin (see Fig. 3 for location),shows the transition from the Gorda abyssal plain to the Eel Plateau and continental shelf. The Eel River basin lies above the FranciscanComplex rocks and is deformed on the western margin and beneath the plateau and shelf. The margin is 45 km wide, ;30 km narrowerthan in MTJ-14 and MTJ-8.

supports this possibility of a longer-term epi-sode of uplift that culminated in the ca. 2 Maevent.

An ;35-km-wide wedge of deformed sed-iment has accreted between the outer-arc highand deformation front (Figs. 6 and 7). We sug-gest that this accretion occurred entirely with-in the past 3 m.y. If instead accretion had beencontinuous through the Neogene, the modernwidth of the southern Cascadia accretionaryprism should be 60 km wider than it is today,if the same convergence rate, decollement po-sition, and sediment input are assumed. Sand-box analogue studies by Gutscher et al. (1998)suggest that convergent margins alternate be-tween intervals of accretion and nonaccretionthroughout their history. If it is assumed that(1) Gorda basin sediment entering the trenchduring the past ;3 m.y. has had an averagethickness of 1 km and that 90% of this sedi-ment has been accreted (Gulick et al., 1998),(2) the convergence rate was 3 cm/yr (Wilson,1993), and (3) the accreted wedge had a crit-ical taper of 7.58 (Biddle and Seely, 1983;Gulick et al., 1998), the time of onset of the

most recent phase of accretion that best bal-ances these factors with the observed width ofthe outer margin is ca. 3 Ma. The transitionfrom a nonaccretionary phase to an accretion-ary phase would require thrusting within theprism and a possible change in decollementposition near the toe of the prism (Davis etal., 1983; Dahlen et al., 1984). As the prismtaper increased toward critical, such changescould uplift the western margin of the Eel Riv-er basin and create an outer-arc high.

This postulated scenario explains the tiltedupper Miocene to middle Pliocene strata alongthe western basin margin (Fig. 10, D and E;Fig. 11A). McNeill et al. (2000) also observedthe formation of an outer-arc high offshorecentral and northern Oregon and similarly in-terpreted this high to have formed as a resultof the margin’s switching from nonaccretion-ary to accretionary; a switch to an accretion-ary regime at ca. 3 Ma may therefore havebeen a regional Cascadia event. The formationof the eastward-tilted unconformity at ca. 2Ma may signify the point where the youngaccretionary prism reached critical taper.

There is no evidence for out-of-sequencethrusting along the western margin of theprism (Figs. 6–8, Gulick et al., 1998), sug-gesting that critical taper of the outer part ofthe accretionary prism has been largely main-tained for ;2 m.y.

The age of the Falor datum onshore is alsoca. 2 Ma, but that datum is only found in thenortheastern part of the onshore Eel River ba-sin, where it is a nonconformity between rocksof the Franciscan Complex and the overlyingFalor Formation. The offshore equivalent tothis datum is probably only found at the north-eastern limit of the forearc basin where the ca.2 Ma and younger sediments lie directly overthe Franciscan Complex (Fig. 10C).

The angular unconformity that truncates themiddle Pliocene to lower Pleistocene section(sequence C) along the western margin of thebasin (Figs. 10A and E) appears to recordwidespread erosion of the western part of thebasin (Fig. 11B). Onshore, in the CentervilleBeach section examined by McCrory (1989,1995), a possibly correlative angular unconfor-mity, the Wildcat datum, is dated at ca. 1 Ma





Figure 9. Isopach maps determined from MCS data. Long, gray, double-headed arrows are the locations of the crosssections in Figure 11. Fill-pattern scale (lower right) is in meters; contour interval on maps is 250 m. Abbreviations:HB—Humboldt Bay, T—Trinidad Head, and PG—Point St. George. Black line segments are locations of seismicprofiles displayed in Figure 10. The seismic profile shown in Figure 10B lies just south of these isopachs. (A) Isopachmap for the middle Pleistocene to Holocene showing a depocenter shift to the north (compared to B) and thinneddeposits in the south (compare with the southern basin deposits in B). (B) Lower to middle Pleistocene isopach mapshowing thick deposits in the southern and central basin. (C) Middle Pliocene to lower Pleistocene isopach maprecords thin deposits along the western margin and thick deposits in the northern basin. (D) Upper Miocene to middlePliocene isopach map displaying depocenters in the southern and central basin. (E) Isopach map for entire upperMiocene to Holocene sequence.

6 0.3 (Fig. 2). This early Pleistocene angularunconformity is most likely the result of sub-aerial erosion. Benthic storms and contour cur-rents are unlikely to have the shear stress nec-essary to erode lithified sedimentary rocks oreven well-compacted sediments (Gross andWilliams, 1991), and the eastward tilt of themiddle Pliocene to lower Pleistocene strata be-neath the angular unconformity suggests thatthe strata were compacted or partially lithifiedbefore erosion occurred. Consequently, this ca.1 Ma episode of erosion would seem to requiresubaerial exposure and erosion of the outer;20 km of the basin during the early Pleisto-cene while the eastern basin, which shows noevidence of a early Pleistocene unconformity,was still submerged (Fig. 11B). The sequenceC strata along the western margin of the basinrepresent an accumulation rate of 0.16 km/m.y.,whereas the average accumulation rate for se-quence C was ;0.26 km/m.y. If the western

and central parts of the basin had equivalentsedimentation rates at this time, then an ;150m thickness of sequence C was removed by theca. 1 Ma erosional event. 1 Ma erosional event.This subaerial erosion may have been relatedto major sea-level fluctuations from the factthat a sea-level highstand (marine oxygen iso-tope stage MIS-23) and a sea-level lowstand(MIS-22) occurred within 0.3 m.y. of 1 Ma(Fig. 2) (Shackleton et al., 1990). A likely sce-nario is that following the formation of the out-er arc late in the middle Pliocene, continueduplift of the western part of the forearc basinin order to maintain critical taper eventuallybrought the western margin of the basin nearor above sea level during the middle Plioceneto early Pleistocene (Fig. 11B). Farther east, theabsence of the angular unconformity suggeststhat a seaway existed between the emergentouter-arc high and the continent during the ear-ly Pleistocene (Fig. 11B). The reason that co-

eval unconformities formed at ca. 1 Ma alongboth the western margin of the basin and onthe flanks of structures in the southeastern basin(Fig. 10, A and D) is likely that the currentposition of the southern basin structures east ofthe modern outer-arc high (Fig. 8) is due to thetriple-junction–induced eastward bending of thesouthern Eel River basin (Clarke, 1992; Gulick,1999). There was some continued uplift of thewestern part of the accretionary margin be-tween 2 and 1 Ma as shown by the tilted strataof sequence C (Fig. 10D), and structures of thewestern margin continue to deform the seafloortoday (Figs. 6–8). However, the western marginof the northern and central parts of the basinnow lies at 750 m below sea level. The ces-sation of subaerial erosion probably occurred asa combined consequence of rising sea level andbroad subsidence in the central basin that ex-ceeded the uplift required to maintain criticaltaper (Fig. 11C). We are currently investigating




GULICK et al.

Figure 10. Prominent uncon-formities observed in the MCSprofiles. Scale bars and verticalexaggeration are shown aboveeach profile, and the locationsof the specific parts of the linesdisplayed are shown in insetand in Figure 9. Small blackarrows show reflection trunca-tions. Abbreviations: PG—Point St. George, T—TrinidadHead, and CM—Cape Men-docino. (A) Ca. 2 and ca. 1 Maunconformities observed onthe flank of a structure in thesoutheastern part of the EelRiver basin. (B) Seafloor un-conformity showing truncationof tilted strata south of Eelcanyon. (C) Ca. 0.5 Ma uncon-formity observed beneath thecontinental shelf in the north-eastern part of the basin. (D)Ca. 1 Ma angular unconfor-mity showing erosional trun-cation of middle Pliocene tolower Pleistocene strata. (E)Ca. 2 Ma unconformity show-ing tilted upper Miocene tomiddle Pliocene strata overlainby onlapping middle Plioceneto lower Pleistocene strata.

the cause of this broad subsidence that mayhave ultimately formed the submerged KlamathPlateau.

The thinness of the deposits observed alongthe western margin of the basin above the 1Ma unconformity (Fig. 10, D and E) indicatesthat, following erosion of the western marginof the basin at ca. 1 Ma, a hiatus in deposition(due to lack of accommodation space on topof the recently emergent outer-arc high) musthave persisted during some part of the earlyto middle Pleistocene (Fig. 11C). Depositionof the lower to middle Pleistocene sedimen-tary units (sequence B) appears concentratedalong a northward trend located east of thepreviously exposed western basin margin (Fig.9B). Deposition of up to 1 km of sediment in0.5 m.y. (maximum accumulation rate of 2

km/m.y. and average rate of 0.8 km/m.y.) in-dicates a very high rate of sediment influx tothe margin at this time. This increased influxof terrigenous sediment could represent theonset of the glacial-interglacial cycles (Emi-liani, 1955; Shackleton, 1969) and/or the up-lift of the source region. Widening of the de-positional basin from 40–45 km in lateMiocene–early Pleistocene time to 60 km dur-ing the early Pleistocene to Holocene (Figs. 9and 11) suggests that the increased sedimentflux to the margin resulted in an increased rateof prism building. Beneath the Klamath Pla-teau, deposits of lower to middle Pleistocenesediment (sequence B) are ,250 m thick andprograde gently westward (Fig. 4B). These de-posits support the idea of recent subsidence(Fig. 11C).

Locally, on the continental shelf, angularunconformities truncate lower to middle Pleis-tocene strata (Fig. 10C). These unconformitiesmay be correlative with a ca. 500 ka uncon-formity onshore that was interpreted byMcCrory (1995) to be due to eustatic changesin sea level. The fact that a pronounced sea-level lowstand (MIS-12) occurred ca. 450 ka(Fig. 2), after a highstand at ca. 500 ka (MIS-13) (Shackleton et al., 1990) possibly supportsthis interpretation; local structural growth mayalso have contributed to the development ofthe unconformity in some places (e.g., Fig.10C).

Average middle Pleistocene to Holoceneaccumulation rates in the Eel River basin were0.64 km/m.y., implying that the high rates ofsediment delivery to the margin continued





Figure 10. (Continued.)

(Figs. 9A and 11D). Although the unconfor-mity at ca. 500 ka may have been eustatic innature, this time horizon also serves as amarker in the seismic data for the onset of anorthward shift in deposition away from thesouthern basin. The middle Pleistocene to Ho-locene depocenter is shifted strongly north-ward relative to the lower to middle Pleisto-cene depocenter, resulting in a pronouncedsouthward decrease in depositional thickness-es in the southern part of the basin (Figs. 9Aand 11E). The southward thinning of sedi-ments suggests that regional uplift and tiltingof the southern part of the basin has occurredsince the middle Pleistocene (ca. 500 ka),causing terrigenous sediment to largely bypassthe shelf offshore Humboldt Bay.

INFLUENCE OF THE MENDOCINOTRIPLE JUNCTION

The Mendocino triple junction has influ-enced sediment deposition within the Eel Riv-er basin in two primary ways. (1) Proximityof the triple junction has caused broad areasof the southern Eel River basin to be uplifted,tilted northward, and, in some places, eroded.(2) The uplifting of the southern part of the

Eel River basin has caused the sediment tobypass this region and either flow out the EelCanyon or to more northern parts of the basin.

South of the Eel Canyon, Neogene sedi-ments, which are tilted northward and trun-cated against the seafloor (Fig. 10B), are pos-sibly the erosional remnant of a former, moreextensive Eel River basin (Nilsen and Clarke,1987; Clarke, 1992). The angular unconfor-mity that truncates these sediments is at theseafloor and lies beneath ,100 m of water.This relationship indicates that uplift and tilt-ing has caused erosion of the shelf during re-cent sea-level lowstands (Fig. 11E). The up-lift, tilting, and erosion extends ;20 km northof the triple junction.

The isopach data suggest that the upperPleistocene to Holocene sediments (Fig. 9A)are largely bypassing the uplifting southernEel River basin. The bypassing sedimentswere likely deposited either farther north with-in the Eel River basin or flowed out the EelCanyon in order to build the .2-km-thickGorda Fan located on the southeastern Gordaplate (Fig. 1) (Gulick et al., 2001). The upliftand northward shift of sediment depositionpersist in the offshore Eel River basin as muchas 80 km north of the triple junction. The up-

lift, northward tilting, and erosion of the upperCenozoic strata south of the basin and the by-passing of the sediments across the southernEel River basin, due to uplift, but continuingdeposition in the central and northern parts ofthe basin are most logically related to the ar-rival of the Mendocino triple junction fromthe south.

Recent geodynamic modeling by Furlongand Govers (1999) and numerous field studiesfrom the triple-junction region (McLaughlin etal., 1983; Carver et al., 1986; Merritts andVincent, 1989; Merritts and Bull, 1989; Aaltoet al., 1995; McCrory, 2000) suggest that thenorthward migration of the triple junction hascaused uplift and crustal shortening in ad-vance of its passage. The only depositional se-quence showing sediments bypassing thesouthern part of the basin is the middle Pleis-tocene to Holocene section (Fig. 9A), sug-gesting that shortening and uplift related to thenorthward-migrating triple junction reachedthe southern Eel River basin at ca. 500 ka(Fig. 11E). Carver (1992) and McCrory(1995) suggested that the 1 Ma unconformityon land was related to the arrival of the triplejunction. We consider this unlikely as the 1Ma unconformity in the offshore strata is pre-




GULICK et al.

Figure 11. Cartoon of tectonic evolution of the Eel River basin approximately drawnacross regions defined by the large, gray, double-headed arrows in Figure 9; major struc-tures are displayed. A–D are drawn from west to east across the central basin, whereasE is drawn 5 km offshore Humboldt Bay parallel to the coastline from southwest tonortheast. Single-headed arrows show uplift, subsidence, and oblique-slip motion through-out basin evolution, and long, double-headed black arrows show width of basin fromouter-arc high to near the coastline. (A) Late Miocene to middle Pliocene deposition show-ing start of subsidence and formation of outer-arc high. (B) Middle Pliocene to earlyPleistocene deposition recording continued uplift of outer basin, subsidence, and uplift tothe east. A broad area of the western basin was elevated above sea level and was subaer-ially eroded. (C) Lower to middle Pleistocene evolution with resubmergence of the westernmargin while thrusting on eastern margin continued. Greater contrast has developed be-tween the central and eastern parts of the basin through subsidence and uplift, respec-tively, and uplift in shelf region has brought parts above sea level to be eroded. The basinwidened from 40 to 55 km at this time. (D) Middle Pleistocene to Holocene central basinhistory with continued western thrusting. (E) Northeast-southwest profile for the middlePleistocene to Holocene showing uplift and erosion south of Eel Canyon, leading to littledeposition within the southern Eel River basin.

sent nearly as far north as Oregon along thewestern side of the Eel River basin and doesnot show any evidence of being related to anorthward-directed tectonic component. Also,during the time interval between 1 Ma and500 ka, the most deposition occurred in thesouthern part of the basin, and it was not untilafter 500 ka that sediment started bypassingthe region as the welt of uplift that precedesthe triple junction’s arrival reached the Eel

River basin (Fig. 9E). It is possible that someof the southern Eel River basin structuresstarted responding to the changing stress fieldsbetween 1 and 0.5 Ma prior to the arrival ofthe welt of uplift.

CONCLUSIONS

Upper Miocene to Holocene sediments ofthe Eel River basin record the tectonic history

of the northern California margin before andafter the arrival of the Mendocino triple junc-tion at Cape Mendocino. The modern forearchigh may have begun developing at ca. 3 Ma,coincident with the start of a modern episodeof prism accretion that likely reached criticaltaper at ca. 2 Ma. Continued uplift of thewestern margin of the basin resulted in sub-aerial exposure of this area at ca. 1 Ma whilea coastal seaway developed to the east. Theonset of Pleistocene sedimentation widenedthe forearc basin and resulted in rapid accre-tion at the toe of the accretionary prism thatcontinues today. A welt of uplift that precededthe northward-migrating triple junctionreached the Eel River basin at ca. 500 ka. Thisuplift since ca. 500 ka has resulted in erosionof the shelf as far as 20 km to the north; fur-thermore, sediment has largely bypassed themargin for as far as 80 km north of the triplejunction. From ca. 500 ka to today, the erodedsediment and the sediments that are bypassingthe southern part of the basin have been andare being deposited farther north within thebasin and flowing out of the Eel Canyon tocontribute to the Gorda Fan.

ACKNOWLEDGMENTS

We thank the crew of the R/V Maurice Ewingand J. Diebold for aid in seismic data collection. P.Mann, C. Goldfinger, and an anonymous reviewerprovided excellent reviews. This manuscript bene-fited from several discussions with P. McCrory.Supported by National Science Foundation grantsEAR-9219598 and EAR-9526116 to LehighUniversity.

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