Eel River Basin Research Articles

Lower Pleistocene to present structural deformation and sequence stratigraphy of the continental shelf, offshore Eel River Basin, northern California

Seismic stratigraphic sequences and deformational features are mapped beneath the continental shelf of the offshore Eel River Basin of northern California, using a closely spaced, high-resolution multichannel seismic grid. Geometries of sequences and morphologies of bounding unconformities reflect competing tectonic and glacioeustatic influences, producing shifting sedimentation patterns in the offshore basin during the last ∼1 Myr. Estimated timing of unconformity formation correlates generally with a deep-ocean global δ 18O curve, both before and after a ∼500-kyr transition from 41-kyr to 100-kyr dominated cycles. We suggest that glacioeustatic fluctuations are the dominant control on unconformity formation basinwide. However, regional tectonic influences on sequence development are also observed. Folding associated with Gorda–North America Plate convergence ended progressively from south to north between ∼1.0 Ma and ∼500 ka, in response to northward migration of the Mendocino Triple Junction (MTJ). Since ∼500 ka, continued encroachment of the MTJ produces rotation of preexisting structures, uplift of the Table Bluff Anticline (TBA), related periods of channel incision south of the TBA, and generally reduced sediment preservation across the shelf. MTJ-related uplift may also have induced formation of Eel Canyon. Over the past ∼1.0 Myr, a dominant northern sediment source becomes progressively less important; a southern source, probably the paleo-Eel River, has become dominant since ∼750 ka. Seismic unconformities fall into two categories: (1) irregular surfaces of limited mappable extent, interpreted as incision/exposure surfaces forming during relative sea-level lowstands, and (2) smooth, laterally extensive regional unconformities, interpreted as ravinement surfaces that erode lowstand surfaces in all but deeply incised areas during sea-level transgressions. A sequence stratigraphic model for sediment deposition and preservation on the Eel River shelf predicts that preserved sediment on the Eel margin is dominated by fluvially derived highstand silts and muds, deposited by longshore-directed currents and waves. Lowstand sediments are preserved only in fluvial channels infilled during late lowstand/early transgression. Preserved transgressive sediments are likely limited to a thin veneer of well-sorted coarse sediment or shell debris, deposited above the transgressive ravinement. Future deep coring will be required to confirm these predictions.

Water column methane oxidation adjacent to an area of active hydrate dissociation, Eel river Basin

The role of methane clathrate hydrates in the global methane budget is poorly understood because little is known about how much methane from decomposing hydrates actually reaches the atmosphere. In an attempt to quantify the role of water column methanotrophy (microbial methane oxidation) as a control on methane release, we measured water column methane profiles (concentration and δ13C) and oxidation rates at eight stations in an area of active methane venting in the Eel River Basin, off the coast of northern California. The oxidation rate measurements were made with tracer additions of 3H-CH4.Small numbers of instantaneous rate measurements are difficult to interpret in a dynamic, advecting coastal environment, but combined with the concentration and stable isotope measurements, they do offer insights into the importance of methanotrophy as a control on methane release. Fractional oxidation rates ranged from 0.2 to 0.4% of ambient methane per day in the deep water (depths >370 m), where methane concentration was high (20–300 nM), to near-undetectable rates in the upper portion of the water column (depths <370 m), where methane concentration was low (3–10 nM). Methane turnover time averaged 1.5 yr in the deep water but was on the order of decades in the upper portion of the water column. The depth-integrated water column methane oxidation rates for the deep water averaged 5.2 mmol CH4 m−2 yr−1, whereas the upper portion of the water column averaged only 0.14 mmol CH4 m−2 yr−1; the depth-integrated oxidation rate for deep water in the 25-km2 area encompassing the venting field averaged 2 × 106 g CH4 yr−1. Stable isotope values (δ13C-CH4) for individual samples ranged from −34 to −52‰ (vs. PDB, Peedee belemnite standard) in the region. These values are isotopically enriched relative to hydrates in the region (δ13C-CH4 about −57 to −69‰), further supporting our observations of extensive methane oxidation in this environment.

Late Pleistocene channel incisions in the southern Eel River Basin, northern California: implications for tectonic vs. eustatic influences on shelf sedimentation patterns

High-resolution multichannel seismic (MCS) profiles from the southern offshore Eel River Basin off northern California show multiple, superimposed buried channels beneath the continental shelf. These channels display consistent southwest orientations. Their dendritic map pattern and low but consistent cross-shelf gradients suggest that they formed by fluvial erosion during periods of shelf exposure associated with relative sea-level lowstands. All of the channels were incised after ∼500ka, the age of the onshore–offshore Hookton Datum. Their frequency of incision exceeds that of known glacioeustatic lowstands since ∼500ka, confirming that some of the surfaces must have formed during relative lowstands initiated by local tectonic uplift.Sedimentation patterns in the Eel River Basin shift dramatically in response to relative sea-level fluctuations. Today, sediments from the Eel River move north and west on the shelf, in response mainly to large winter storm events. In contrast, fluvial incision on the exposed shelf during relative lowstands directed sediments to the southwest, most likely into the head of the Eel Canyon. Shelf channels were probably fed not only by ancestral rivers, but also by local uplifts like the Table Bluff Anticline. Increases in gradient and channel depths at the southern end of the MCS coverage indicate that canyon-forming processes modified fluvial channels as they converge towards embayments near the head of the Eel Canyon. Cyclic recurrence and consistent orientation of these channels demonstrate that the Eel Canyon has been extant through numerous relative sea level cycles, and is perhaps as old as ∼500ka.

Biogeochemical and molecular signatures of anaerobic methane oxidation in a marine sediment.

Anaerobic methane oxidation was investigated in 6-m-long cores of marine sediment from Aarhus Bay, Denmark. Measured concentration profiles for methane and sulfate, as well as in situ rates determined with isotope tracers, indicated that there was a narrow zone of anaerobic methane oxidation about 150 cm below the sediment surface. Methane could account for 52% of the electron donor requirement for the peak sulfate reduction rate detected in the sulfate-methane transition zone. Molecular signatures of organisms present in the transition zone were detected by using selective PCR primers for sulfate-reducing bacteria and for Archaea. One primer pair amplified the dissimilatory sulfite reductase (DSR) gene of sulfate-reducing bacteria, whereas another primer (ANME) was designed to amplify archaeal sequences found in a recent study of sediments from the Eel River Basin, as these bacteria have been suggested to be anaerobic methane oxidizers (K. U. Hinrichs, J. M. Hayes, S. P. Sylva, P. G. Brewer, and E. F. DeLong, Nature 398:802-805, 1999). Amplification with the primer pairs produced more amplificate of both target genes with samples from the sulfate-methane transition zone than with samples from the surrounding sediment. Phylogenetic analysis of the DSR gene sequences retrieved from the transition zone revealed that they all belonged to a novel deeply branching lineage of diverse DSR gene sequences not related to any previously described DSR gene sequence. In contrast, DSR gene sequences found in the top sediment were related to environmental sequences from other estuarine sediments and to sequences of members of the genera Desulfonema, Desulfococcus, and Desulfosarcina. Phylogenetic analysis of 16S rRNA sequences obtained with the primers targeting the archaeal group of possible anaerobic methane oxidizers revealed two clusters of ANME sequences, both of which were affiliated with sequences from the Eel River Basin.

Sedimentary Carbon, Sulfur, and Iron Relationships in Modern and Ancient Diagenetic Environments of the Eel River Basin (U.S.A.)

ABSTRACT Depositional and diagenetic controls on the distributions of carbon, sulfur, and iron (C-S-Fe) in modern sediments and upper Pleistocene mudrocks of the Eel River Basin (ERB), northern California continental margin, were investigated using a combination of geochemical, radioisotopic, and sedimentological methods. A mass balance based on down-core profiles of porewater and solid-phase constituents and diagenetic modeling suggests that only 12-30% of the pyrite-S produced via SO4-2 reduction during burial is retained in modern shelf and upper slope deposits of the ERB. Bioturbational reoxidation of initially reduced S is inferred to be the major control on S preservation, on the basis of an observed inverse relationship between pyrite-S retention and biological mixing intensity, estimated from profiles of excess 234Th. Importantly, these findings argue that massive depositional episodes on the shelf following floods of the Eel River have a negligible long-term impact on bioturbating macrofauna and the potential to affect geochemical properties of the sediments. Down-core profiles of reactive Fe3+ and Py-Fe(II) for the modern deposits suggest that highly reactive Fe phases are sulfidized well within 500-2000 years of burial, thereby limiting later pyritization, which could occur through sulfidation of less reactive phases. This result explains the low ( 0.4) degree of pyritization (DOP) values exhibited by both modern and ancient deposits of the ERB and lends support to the notion that pyritization in aerobic continental-margin sediments is largely associated with highly reactive detrital Fe oxides. Comparable mean C/S weight ratios for modern sediments (5.4 ± 3.3, 1) and mudrocks (6.9 ± 4.5) of the ERB suggest that the upper Pleistocene strata reflect a geochemical environment analogous to that of the modern margin. Specifically, the C-S-Fe signatures shared by the modern and ancient deposits are a consequence of similar detrital Fe mineralogies, initial organic-matter content (Corg 1%) and composition (C/N = 13 to 17, 13Corg = -22 to -25o/oo), burial rate, and importantly, bioturbation intensity. The findings of this study have important implications for the use of C-S-Fe signatures as indicators of diagenetic processes in dynamic, continental-margin environments.

Sedimentary Carbon, Sulfur, and Iron Relationships in Modern and Ancient Diagenetic Environments of the Eel River Basin (U.S.A.)

Sedimentary Carbon, Sulfur, and Iron Relationships in Modern and Ancient Diagenetic Environments of the Eel River Basin (U.S.A.)

Molecular and isotopic analysis of anaerobic methane-oxidizing communities in marine sediments

Convergent lines of molecular, carbon-isotopic, and phylogenetic evidence have previously indicated (Hinrichs, K.-U., Hayes, J.M., Sylva, S.P., Brewer, P.G., DeLong, E.F., 1999. Methane-consuming archaebacteria in marine sediments. Nature 398, 802–805.) that archaea are involved in the anaerobic oxidation of methane in sediments from the Eel River Basin, offshore northern California. Now, further studies of those same sediments and of sediments from a methane seep in the Santa Barbara Basin have confirmed and extended those results. Mass spectrometric and chromatographic analyses of an authentic standard of sn-2-hydroxyarchaeol (hydroxylated at C-3 in the sn-2 phytanyl moiety) have confirmed our previous, tentative identification of this compound but shown that the previously examined product was the mono-TMS, rather than di-TMS, derivative. Further analyses of 13C-depleted lipids, appreciably more abundant in samples from the Santa Barbara Basin, have shown that the archaeal lipids are accompanied by two sets of products that are only slightly less depleted in 13C. These are additional glycerol ethers and fatty acids. The alkyl substituents in the ethers (mostly monoethers, with some diethers) are non-isoprenoidal. The carbon-number distributions and isotopic compositions of the alkyl substituents and of the fatty acids are similar, suggesting strongly that they are produced by the same organisms. Their structures, n-alkyl and methyl-branched n-alkyl, require a bacterial rather than archaeal source. The non-isoprenoidal glycerol ethers are novel constituents in marine sediments but have been previously reported in thermophilic, sulfate- and nitrate-reducing organisms which lie near the base of the rRNA-based phylogenetic tree. Based on previous observations that the anaerobic oxidation of methane involves a net transfer of electrons from methane to sulfate, it appears likely that the non-archaeal, 13C-depleted lipids are products of one or more previously unknown sulfate-reducing bacteria which grow syntrophically with the methane-utilizing archaea. Their products account for 50% of the fatty acids in the sample from the Santa Barbara Basin. At all methane-seep sites examined, the preservation of aquatic products is apparently enhanced because the methane-oxidizing consortium utilizes much of the sulfate that would otherwise be available for remineralization of materials from the water column.

Tectonics of the Neogene Cascadia forearc basin: Investigations of a deformed late Miocene unconformity

Research Article| August 01, 2000 Tectonics of the Neogene Cascadia forearc basin: Investigations of a deformed late Miocene unconformity Lisa C. McNeill; Lisa C. McNeill 1School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK Search for other works by this author on: GSW Google Scholar Chris Goldfinger; Chris Goldfinger 2College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA Search for other works by this author on: GSW Google Scholar LaVerne D. Kulm; LaVerne D. Kulm 2College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA Search for other works by this author on: GSW Google Scholar Robert S. Yeats Robert S. Yeats 3Department of Geosciences, Wilkinson Hall, and College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Lisa C. McNeill 1School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK Chris Goldfinger 2College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA LaVerne D. Kulm 2College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA Robert S. Yeats 3Department of Geosciences, Wilkinson Hall, and College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA Publisher: Geological Society of America Received: 23 Nov 1998 Revision Received: 12 Jul 1999 Accepted: 04 Aug 1999 First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2000) 112 (8): 1209–1224. https://doi.org/10.1130/0016-7606(2000)112<1209:TOTNCF>2.0.CO;2 Article history Received: 23 Nov 1998 Revision Received: 12 Jul 1999 Accepted: 04 Aug 1999 First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Lisa C. McNeill, Chris Goldfinger, LaVerne D. Kulm, Robert S. Yeats; Tectonics of the Neogene Cascadia forearc basin: Investigations of a deformed late Miocene unconformity. GSA Bulletin 2000;; 112 (8): 1209–1224. doi: https://doi.org/10.1130/0016-7606(2000)112<1209:TOTNCF>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The continental shelf and upper slope of the Oregon Cascadia margin are underlain by an elongate late Cenozoic forearc basin, correlative to the Eel River basin of northern California. Basin stratigraphy includes a regional late Miocene unconformity that may coincide with a worldwide hiatus ca. 7.5–6 Ma (NH6). The unconformity is angular and probably subaerially eroded on the inner and middle shelf, whereas the seaward correlative disconformity may have been produced by submarine erosion; alternatively, this horizon may be conformable. Tectonic uplift resulting in subaerial erosion may have been driven by a change in Pacific and Juan de Fuca plate motion. A structure contour map of the deformed unconformity and correlated seaward reflector from seismic reflection data clearly outlines deformation into major synclines and uplifted submarine banks. Regional margin-parallel variations in uplift rates of the shelf unconformity show agreement with coastal geodetic rates.The shelf basin is bounded to the west by a north-south–trending outer arc high. Rapid uplift and possible eustatic sea-level fall resulted in the formation of the late Miocene unconformity. Basin subsidence and outer arc high uplift effectively trapped sediments within the basin, which resulted in a relatively starved abyssal floor and narrower Pliocene accretionary wedge, particularly during sea-level highstands. During the Pleistocene, the outer arc high was breached, possibly contributing to Astoria Canyon incision, the primary downslope conduit of Columbia River sediments. This event may have caused a change in sediment provenance on the abyssal plain ca. 1.3–1.4 Ma. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Tectonics of the Neogene Cascadia forearc basin: Investigations of a deformed late Miocene unconformity

Research Article| August 01, 2000 Tectonics of the Neogene Cascadia forearc basin: Investigations of a deformed late Miocene unconformity Lisa C. McNeill; Lisa C. McNeill 1School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK Search for other works by this author on: GSW Google Scholar Chris Goldfinger; Chris Goldfinger 2College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA Search for other works by this author on: GSW Google Scholar LaVerne D. Kulm; LaVerne D. Kulm 2College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA Search for other works by this author on: GSW Google Scholar Robert S. Yeats Robert S. Yeats 3Department of Geosciences, Wilkinson Hall, and College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2000) 112 (8): 1209–1224. https://doi.org/10.1130/0016-7606(2000)112<1209:TOTNCF>2.0.CO;2 Article history received: 23 Nov 1998 rev-recd: 12 Jul 1999 accepted: 04 Aug 1999 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Lisa C. McNeill, Chris Goldfinger, LaVerne D. Kulm, Robert S. Yeats; Tectonics of the Neogene Cascadia forearc basin: Investigations of a deformed late Miocene unconformity. GSA Bulletin 2000;; 112 (8): 1209–1224. doi: https://doi.org/10.1130/0016-7606(2000)112<1209:TOTNCF>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The continental shelf and upper slope of the Oregon Cascadia margin are underlain by an elongate late Cenozoic forearc basin, correlative to the Eel River basin of northern California. Basin stratigraphy includes a regional late Miocene unconformity that may coincide with a worldwide hiatus ca. 7.5–6 Ma (NH6). The unconformity is angular and probably subaerially eroded on the inner and middle shelf, whereas the seaward correlative disconformity may have been produced by submarine erosion; alternatively, this horizon may be conformable. Tectonic uplift resulting in subaerial erosion may have been driven by a change in Pacific and Juan de Fuca plate motion. A structure contour map of the deformed unconformity and correlated seaward reflector from seismic reflection data clearly outlines deformation into major synclines and uplifted submarine banks. Regional margin-parallel variations in uplift rates of the shelf unconformity show agreement with coastal geodetic rates.The shelf basin is bounded to the west by a north-south–trending outer arc high. Rapid uplift and possible eustatic sea-level fall resulted in the formation of the late Miocene unconformity. Basin subsidence and outer arc high uplift effectively trapped sediments within the basin, which resulted in a relatively starved abyssal floor and narrower Pliocene accretionary wedge, particularly during sea-level highstands. During the Pleistocene, the outer arc high was breached, possibly contributing to Astoria Canyon incision, the primary downslope conduit of Columbia River sediments. This event may have caused a change in sediment provenance on the abyssal plain ca. 1.3–1.4 Ma. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Abstract: Gas Occurrence from the Subsurface to the Water Column: An Integrated Study in the Offshore Eel River Basin, California

Abstract: Gas Occurrence from the Subsurface to the Water Column: An Integrated Study in the Offshore Eel River Basin, California

Subsurface gas offshore of northern California and its link to submarine geomorphology

The northern California continental margin contains evidence of abundant subsurface gas and numerous seafloor features that suggest a causative link between gas expulsion and geomorphology. Analyses of seismic reflection, sidescan sonar, and high-resolution multibeam bathymetric data show that the occurrence of subbottom gas and the migration processes beneath the shelf differ from those beneath the slope. Subsurface gas, inferred from enhanced reflectors and other geophysical indicators, is spatially variable and related more to total depth and stratigraphy than to underlying structure, with the exception of one band of gas that follows the regional structural trend. Shallow depressions on the seafloor (pockmarks) are used to infer expulsion sites. The largest zone of acoustically impenetrable subsurface gas occurs between water depths of 100 m and 300 m, where expulsion features are rare. The upper slope (water depths 400–600 m) has a high concentration of pockmarks (diameter 10–20 m), in contrast to a near-absence of pockmarks at water depths shallower than 400 m. Of nearly 4000 pockmarks observed on sidescan sonar records, more than 95% are located in water depths deeper than 400 m. Bottom simulating reflectors (BSRs) on some seismic reflection profiles indicate the possible presence of gas hydrate. We find that gas and pore-fluid migration in the offshore Eel River Basin is: (1) correlated to surface morphology; (2) a contributor to seabed roughness; (3) a significant mode of sediment redistribution on the upper slope; and (4) potentially a factor in large slope failures.

Abstract: Fluid Flow and Carbonate Precipitation in the Eel River Basin and Monterey Bay - Two examples from the California Margin&amp;nbsp;

Abstract: Fluid Flow and Carbonate Precipitation in the Eel River Basin and Monterey Bay - Two examples from the California Margin&amp;nbsp;

Abstract: California Cold Seeps Past and Present: Monterey Bay, the Eel River Basin, and Inland Paleo-Seeps&amp;nbsp;

Abstract: California Cold Seeps Past and Present: Monterey Bay, the Eel River Basin, and Inland Paleo-Seeps&amp;nbsp;

Abstract: Gas Seeps and Associated Gas Hydrate of the Okhotsk Sea Near Sakhalin Island, Russia, and Eel River Basin, Northern California - A Comparison of Gas and Carbonate Geochemistry&amp;nbsp;

Abstract: Gas Seeps and Associated Gas Hydrate of the Okhotsk Sea Near Sakhalin Island, Russia, and Eel River Basin, Northern California - A Comparison of Gas and Carbonate Geochemistry&amp;nbsp;

Abstract: Gas Occurrance From the Subsurface to the Water Column: An Integrated Study in the Offshore Eel River Basin, CA

Abstract: Gas Occurrance From the Subsurface to the Water Column: An Integrated Study in the Offshore Eel River Basin, CA

Swath Mapping on the Continental Shelf and Slope: The Eel River Basin, Northern California

Swath Mapping on the Continental Shelf and Slope: The Eel River Basin, Northern California

Evolution of a trench‐slope basin within the Cascadia subduction margin: the Neogene Humboldt Basin, California

ABSTRACTThe Neogene Humboldt (Eel River) Basin is located along the north‐eastern margin of the Pacific Ocean within the Cascadia subduction zone. This sedimentary basin originated near the base of the accretionary prism in post‐Eocene time. Subduction processes since that time have elevated strata in the south‐eastern portion of the basin above sea level. High‐resolution chronostratigraphic data from the onshore portion of the Humboldt Basin enable correlation of time‐equivalent lithofacies across the palaeomargin, reconstruction of slope‐basin evolution, and preliminary delineation of climatic and tectonic influence on lithological variation. Emergent basin fill is divided into five lithofacies which clearly document shoaling of the inner trench slope from deep‐water environments in early Miocene time to paralic environments in Pleistocene time. The oldest strata consist of hemipelagic mudstones and minor debris‐flow breccias deposited in a deep‐water setting during elevated sea level. These strata are overlain by glauconite‐rich, fine‐grained turbidites which heralded an increasing influx of terrigenous detritus. Water depths shoaled earlier in the eastern basin area as the palaeoshoreline prograded seaward. Turbidite deposition ceased in the eastern basin area at about 2‐2 Ma, whereas 22 km to the west, turbidite deposition continued until about 1‐8 Ma. Lithofacies at the western study site change abruptly across a middle Pleistocene unconformity from outer shelf to paralic deposits. In the east, a more complete Pleistocene section records transition from outer to inner shelf, beach and fluvial environments.The Humboldt Basin lithofacies sequence is overprinted by eustatic control of sediment source. Comparison of sediment character with palaeoceanographic conditions indicates dominance of hemipelagic facies during periods of elevated sea level in the middle Miocene and early Pliocene when depocentres were isolated from terrigenous sediment. Glauconite‐rich facies were mobilized from an upper slope setting following these periods of elevated sea level and redeposited in a deep‐marine environment. Pleistocene shoreline lithofacies display glacio‐esutatic control of depositional environment by recording several cycles of nearshore to fluvial progressions.General models of accretionary prism behaviour and trench‐slope basin evolution are compatible with the overall coarsening‐upward lithofacies sequence filling the Humboldt Basin. Early structural barriers precluded deposition of terrigenous material except from locally derived debris flows; subsequent shoaling and burial of deactivated thrust‐folds enabled turbidity flows to reach the basin floor.However, late‐stage tectonism apparently controlled the onset of coarse‐grained deposition in this sequence. Significant sand‐rich turbidite deposition began in the middle Pliocene, synchronous with tectonic uplift of the southern basin margin. Conversely, cessation of turbidite deposition in the eastern basin area in latest Pliocene time was synchronous with growth of anticlinal structures which again blocked widespread dispersal of turbidity flows. This middle Pliocene to Holocene period of crustal shortening is synchronous with continued reduction in spreading rate along the southern Juan de Fuca ridge, and probably reflects partial coupling between the subducting lithosphere and the overlying accretionary prism.

Irregular geometry of the Gorda subduction and deep structure of the Eel River basin determined from teleseismic P delays

Nearly 1000 observations of travel-time residuals for teleseismic P arrivals across the Humboldt Bay Seismic Network near Cape Mendocino, California, are analyzed for a study of 3-D geometry of the Gorda plate near Mendocino Triple Junction (MTJ), by the application of a three-dimensional velocity inversion procedure. The area is underlain by the subducted edge of the Gorda plate and the eastward extension of the Mendocino Fracture Zone. The observed variation of the travel-time gradient with azimuth favors either deepening of the slab southward, or southward increase of dilatational velocity across the Mendocino Fracture Zone at depths of more than 50 km. The topographically corrected mean travel-time delays at individual stations suggest that the Eel River basin is deep and tectonically controlled. Further support for this view is provided by the inversion results, which point to the possible extension of the low-velocity material through the entire crustal section. These results also suggest that from the south, the Eel River basin may be confined at depth by a steeply northward-dipping sliver of high-velocity material (possibly a detached oceanic slab). This feature may be responsible for the intense seismic activity reported beneath the eastern end of the Mendocino Fracture Zone (MFZ). Thus, the role of the Eel River basin in accommodating the relative motion between the adjacent plates, may be significant. The leading edge of the downgoing slab appears irregular and at some latitudes may not extend beyond the coast line. The estimated velocity anomalies vary by as much as 20% in the near-surface blocks and nearly half as much at depth. The pattern may result from intense tearing and deformation of the slab, producing buckling, folding and compression ridges.

Geology of the Eel River Basin and Adjacent Region: Implications for Late Cenozoic Tectonics of the Southern Cascadia Subduction Zone and Mendocino Triple Junction (1)

Two upper Cenozoic depositional sequences of principally marine strata about 4000 m thick overlie accreted basement terranes of the Central and Coastal belts of the Franciscan Complex in the onshore-offshore Eel River basin of northwestern California. The older depositional sequence is early to middle Miocene in age and represents slope basin and slope-blanket deposition, whereas the younger sequence, late Miocene to middle Pleistocene in age, consists largely of forearc basin deposits. Youthful tectonic activity related to Gorda-North American plate convergence indicates an active Cascadia subduction zone and strong partial coupling between these plates. Structures of the northeastern margin of the Eel River basin are principally north-northwest-trending, east-northeast-dipping thrust and reverse faults that form imbricate thrust fans. Many faults merge downward into sole thrusts that extend to or near the Gorda-North American plate interface. In the southern part of the region, these structures have been modified by northward encroachment of the Pacific plate. The Coastal belt fault, the early Tertiary accretionary suture between the Franciscan Central and Coastal belts, can be traced from Arcata Bay northward offshore to the southern Oregon border. It is tentatively extended farther northward based on aeromagnetic data to an offshore position west of Cape Blanco. Thereafter, it may coincide with the offshore Fulmar fault. If so, the Franciscan Coastal belt in California is correlative with the offshore Fulmar terrane in Oregon. The Cascadia subduction zone (CSZ) does not join the Mendocino transform fault at the commonly depicted offshore location of the Mendocino triple junction (MTJ). Instead, the CSZ extends southeastward around the southern Eel River basin and shoreward along Mendocino Canyon to join the Petrolia shear zone. Similarly, the Mendocino fault may extend shoreward via Mattole Canyon and join the Cooskie shear zone. These two shear zones intersect onshore north of the King Range, and the area of their intersection is the probable location of the MTJ.

Observations of gas hydrates in marine sediments, offshore northern California

Biogenic gas hydrates were recovered in shallow cores (< 6 m deep) from the Eel River basin in offshore northern California between 40°38′ and 40°56′N. The gas hydrates contained primarily methane ( δ 13C = −57.6 to −69.1‰) and occurred as dispersed crystals, small (2–20 mm) nodules, and layered bands within the sediment. These hydrates, recovered in sediment at water depths between 510 and 642 m, coincide nearly, but not exactly, with areas showing bottom-simulating reflectors (BSRs) on seismic-reflection records. This study confirms indirect geophysical and geologic observations that gas hydrates are present north of the Mendocino Fracture Zone in sediment of the Eel River basin but probably are absent to the south in the Point Arena basin. This discovery extends the confirmed sites of gas hydrates in the eastern Pacific region beyond the Peruvian and Central American margins to the northern California margin.