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At least 685 records · Page 38Linked to original sources
USGS2018

Strain partitioning in southeastern Alaska: Is the Chatham Strait Fault active?

A 1200 km-long transform plate boundary passes through southeastern Alaska and northwestern British Columbia and represents one of the most seismically active, but poorly understood continental margins of North America. Although most of the plate motion is accommodated by the right-lateral Queen Charlotte–Fairweather Fault (QCFF) System, which has produced at least six M > 7 earthquakes since 1920, seismic hazard ass essments also include the Chatham Strait Fault (CSF) as a potentially active, 400 km-long strike slip fault that cuts northward through southeastern Alaska, connecting with the Eastern Denali Fault. Nearly the entire length of the CSF is submerged beneath Chatham Strait and Lynn Canal and has never been systematically imaged using high-resolution marine geophysical approaches. In this study we present an integrated analysis of new marine seismic reflectiondata ac quired across Lynn Canal and tectonic block modeling constrained by data from continuous and campaign GPS sites. Seismic profiles cross the CSF at twelve locations spanning ∼50 km of fault length; they reveal thick (up to 300 m) packages of glaciomarine sedimentary facies emplaced on an unconformity su rface that formed during the Last Glacial Maximum (LGM). Localized warping of post-LGM stratigraphy (∼13.9 kyr B.P. to present) appears to correlate with sediment drape on basement topography and current-controlled deposition. There is no evidence for an active fault along the axis of Lynn Canal in the seismic reflection data. Crustal block models constrained by GPS data allow, but do not require, a maximum slip rate o f 2–3 mm/yr along the CSF; higher slip rates on the CSF result in significant misfit to GPS data in the surrounding region. Based on the combined marine geophysical and GPS observations, it is plausible that the CSF has not generated resolvable coseismic deformation in the last ∼13 ka and that the modern slip-rate is <1 mm/yr. We propose that models for strain transfer between the QCFF and the Denali Fault, and seismic hazard maps in general, may need to be reevaluated.

Alaska↗
USGS2008

Application of sedimentary-structure interpretation to geoarchaeological investigations in the Colorado River Corridor, Grand Canyon, Arizona, USA

We present a detailed geoarchaeological study of landscape processes that affected prehistoric formation and modern preservation of archaeological sites in three areas of the Colorado River corridor in Grand Canyon, Arizona, USA. The methods used in this case study can be applied to any locality containing unaltered, non-pedogenic sediments and, thus, are particularly relevant to geoarchaeology in arid regions. Resolving the interaction of fluvial, aeolian, and local runoff processes in an arid-land river corridor is important because the archaeological record in arid lands tends to be concentrated along river corridors. This study uses sedimentary structures and particle-size distributions to interpret landscape processes; these methods are commonplace in sedimentology but prove also to be valuable, though less utilized, in geoarchaeology and geomorphology. In this bedrock canyon, the proportion of fluvial sediment generally decreases with distance away from the river as aeolian, slope-wash, colluvial, and debris-flow sediments become more dominant. We describe a new facies consisting of 'flood couplets' that include a lower, fine-grained fluvial component and an upper, coarser, unit that reflects subaerial reworking at the land surface between flood events. Grain-size distributions of strata that lack original sedimentary structures are useful within this river corridor to distinguish aeolian deposits from finer-grained fluvial deposits that pre-date the influence of the upstream Glen Canyon Dam on the Colorado River. Identification of past geomorphic settings is critical for understanding the history and preservation of archaeologically significant areas, and for determining the sensitivity of archaeological sites to dam operations. Most archaeological sites in the areas studied were formed on fluvial deposits, with aeolian deposition acting as an important preservation agent during the past millennium. Therefore, the absence of sediment-rich floods in this regulated river, which formerly deposited large fluvial sandbars from which aeolian sediment was derived, has substantially altered processes by which the prehistoric, inhabited landscape formed, and has also reduced the preservation potential of many significant cultural sites.

Geomorphology↗
USGS2016

Spectrally based mapping of riverbed composition

Remote sensing methods provide an efficient means of characterizing fluvial systems. This study evaluated the potential to map riverbed composition based on in situ and/or remote measurements of reflectance. Field spectra and substrate photos from the Snake River, Wyoming, USA, were used to identify different sediment facies and degrees of algal development and to quantify their optical characteristics. We hypothesized that accounting for the effects of depth and water column attenuation to isolate the reflectance of the streambed would enhance distinctions among bottom types and facilitate substrate classification. A bottom reflectance retrieval algorithm adapted from coastal research yielded realistic spectra for the 450 to 700 nm range; but bottom reflectance-based substrate classifications, generated using a random forest technique, were no more accurate than classifications derived from above-water field spectra. Additional hypothesis testing indicated that a combination of reflectance magnitude (brightness) and indices of spectral shape provided the most accurate riverbed classifications. Convolving field spectra to the response functions of a multispectral satellite and a hyperspectral imaging system did not reduce classification accuracies, implying that high spectral resolution was not essential. Supervised classifications of algal density produced from hyperspectral data and an inferred bottom reflectance image were not highly accurate, but unsupervised classification of the bottom reflectance image revealed distinct spectrally based clusters, suggesting that such an image could provide additional river information. We attribute the failure of bottom reflectance retrieval to yield more reliable substrate maps to a latent correlation between depth and bottom type. Accounting for the effects of depth might have eliminated a key distinction among substrates and thus reduced discriminatory power. Although further, more systematic study across a broader range of fluvial environments is needed to substantiate our initial results, this case study suggests that bed composition in shallow, clear-flowing rivers potentially could be mapped remotely.

Geomorphology↗
USGS2014

Geochemistry of hydrothermal alteration at the Qolqoleh gold deposit, northern Sanandaj–Sirjan metamorphic belt, northwestern Iran: Vectors to high-grade ore bodies

The Qolqoleh orogenic gold deposit in the northern part of the Sanandaj–Sirjan metamorphic belt in northwestern Iran is hosted by a steeply dipping sequence of greenschist facies Cretaceous volcano–sedimentary rocks, including mafic to intermediate metavolcanic rocks, sericite and chlorite schist, and marble. Geochemical and petrochemical data including the ∑ REE, (La/Yb) N and Eu/Eu* ratios were obtained from country rocks, ore-enveloping alteration zones, and mineralized zones to assess the nature of the trace element and rare earth element (REE) interaction between the wall rock and the mineralizing fluid. Quartz–sulfide veins at the deposit are characterized by a pyrite–pyrrhotite–chalcopyrite–sphalerite–arsenopyrite–native gold assemblage. Alteration halos border the mineralized zones and broadly comprise: (1) an outer carbonate–chlorite alteration zone in all rock types, particularly in chlorite schist; (2) a middle sericite–carbonate alteration zone in the sericite schist; and (3) an inner quartz–sulfide alteration zone in sericite schist and mafic to intermediate metavolcanic rocks. The geochemical data indicate that the concentrations of Al 2 O 3 , P 2 O 5 , TiO 2 , Y, and Zr are relatively constant, suggesting that these elements were the least mobile during hydrothermal activity. Using Al 2 O 3 as the immobile component, there is evidence for mobility of trace elements, particularly light REE, TiO 2 , and Zr in the altered wall rocks. The altered rocks show a relatively light REE depletion ((La/Yb) N ≅ 9.41), which clearly correlates with the grades of gold mineralization and intensity of the alteration (3 ppm Au). The depletion of light REE is best indicated by a decrease in (La/Yb) N as shown by ratios of 10.5 to 11.8. Wall rock decarbonation reactions during infiltration of the mineralizing fluid resulted in differential mobilization of REE, from a fluid with initially low REE content. The overall trace element geochemistry of the altered wall rock is controlled by the initial composition of the wall rocks and the ore-fluid composition. Hydrothermal ore-forming fluids are recognized as CO 2 -rich near-neutral reduced fluids with high values of H 2 S, K, and S content. Observed variability in alteration halos at the Qolqoleh deposit points to major differences in REE and trace element content in original host rocks that have interacted with a relatively similar ore fluid. Therefore, depending on the composition of each host rock lithology, the geochemistry of hydrothermal alteration (e.g., ∑ REE content and (La/Yb) N ratios) and alteration mineralogy including the carbonate–sericite–quartz–sulfide assemblages may be used as a primary tool for lithogeochemical exploration for gold deposits in northwestern Iran.

Journal of Geochemical Exploration↗
USGS2005

Cenozoic ice sheet history from East Antarctic Wilkes Land continental margin sediments

The long-term history of glaciation along the East Antarctic Wilkes Land margin, from the time of the first arrival of the ice sheet to the margin, through the significant periods of Cenozoic climate change is inferred using an integrated geophysical and geological approach. We postulate that the first arrival of the ice sheet to the Wilkes Land margin resulted in the development of a large unconformity (WL-U3) between 33.42 and 30 Ma during the early Oligocene cooling climate trend. Above WL-U3, substantial margin progradation takes place with early glacial strata (e.g., outwash deposits) deposited as low-angle prograding foresets by temperate glaciers. The change in geometry of the prograding wedge across unconformity WL-U8 is interpreted to represent the transition, at the end of the middle Miocene "climatic optimum" (14-10 Ma), from a subpolar regime with dynamic ice sheets (i.e., ice sheets come and go) to a regime with persistent but oscillatory ice sheets. The steep foresets above WL-U8 likely consist of ice proximal sediments (i.e., water-lain till and debris flows) deposited when grounded ice-sheets extended into the shelf. On the continental rise, shelf progradation above WL-U3 results in an up-section increase in the energy of the depositional environment (i.e., seismic facies indicative of more proximal turbidite and of bottom contour current deposition from the deposition of the lower WL-S5 sequence to WL-S7). Maximum rates of sediment delivery to the rise occur during the development of sequences WL-S6 and WL-S7, which we infer to be of middle Miocene age. During deposition of the two uppermost sequences, WL-S8 and WL-S9, there is a marked decrease in the sediment supply to the lower continental rise and a shift in the depocenters to more proximal areas of the margin. We believe WL-S8 records sedimentation during the final transition from a dynamic to a persistent but oscillatory ice sheet in this margin (14-10 Ma). Sequence WL-S9 forms under a polar regime during the Pliocene-Pleistocene, when most sediment delivered to the margin is trapped in the outer shelf and slope-forming steep prograding wedges. During the warmer but still polar, Holocene, biogenic sediment accumulates quickly in deep inner-shelf basins during the high-stand intervals. These sediments contain an ultrahigh resolution (annual to millennial) record of climate variability. Validation of our inferences about the nature and timing of Wilkes Land glacial sequences can be achieved by deep sampling (i.e., using IODP-type techniques). The most complete record of the long-term history of glaciation in this margin can be obtained by sampling both (1) the shelf, which contains the direct (presence or no presence of ice) but low-resolution record of glaciation, and (2) the rise, which contains the distal (cold vs. warm) but more complete record of glaciation. The Wilkes Land margin is the only known Antarctic margin where the presumed "onset" of glaciation unconformity (WL-U3) can be traced from shelf to the abyssal plain, allowing links between the proximal and the distal records of glaciation to be established. Additionally, the eastern segment of the Wilkes Land margin may be more sensitive to climate change because the East Antarctic Ice Sheet (EAIS) is grounded below sea level. Therefore, the Wilkes Land margin is not only an ideal location to obtain the long-term EAIS history but also to obtain the shorter-term record of ice sheet fluctuations at times that the East Antarctic Ice Sheet is thought to have been more stable (after 15 Ma-recent). ?? 2004 Elsevier B.V. All rights reserved.

Global and Planetary Change↗
USGS2025

The demise of an icehouse: Calibrating the end of the LPIA

Earth has experienced three complete icehouse-greenhouse turnovers in the Phanerozoic, with the Late Paleozoic Ice Age (LPIA) recognized as the last and most extreme icehouse. The nature, scale and dynamics of the LPIA are characterized by periods of intense glaciation, which are often interrupted by short-lived (1–2 Myrs) intervals associated with ice-free or distal from ice conditions. In this study, we focus on constraining the icehouse-greenhouse turnover across southcentral Gondwana (SCG) reporting new high-resolution U Pb zircon CA-ID-TIMS ages from immediate postglacial facies in the Kalahari and Karoo basins. We integrate these ages with published U Pb zircon CA-ID-TIMS ages ( n = 20) to build a stratigraphic framework for SCG, to investigate the duration and nature of the demise of the LPIA. We confirm the stepwise deglaciation of Gondwana over a ca. 20 Myr period, with deglaciation occurring first in the Paraná Basin at ca. 300 Ma and in the Karoo Basin by 282 Ma. Low-latitude marine carbonates deposited contemporaneously with the final demise of ice is characterized by a major shift towards isotopically depleted δ 13 C and δ 18 O values. We interpret the perturbations in stable isotopes records to be driven by either mantle outgassing or the release of methane and the addition of glacial melt water to the paleo-ocean during warming. The presented stratigraphic framework is built in intracratonic basins, far from any syn - tectonic affects, suggesting a largely climatic driver behind deglaciation events.

Global and Planetary Change↗
USGS2025

Multi-lithofacies alluvial characterization via airborne electromagnetic-borehole fusion using ordinary interval kriging and geologic constraints

Airborne electromagnetic (AEM) survey provides extensive spatial coverage and detailed resolution at the near surface and can be used to develop hydrogeological models. However, utilization of AEM data is not straightforward because AEM resistivity is an indirect measurement for inferring sediment types. This study develops an ordinary interval kriging (OIK) algorithm and a resistivity-to-multi-lithofacies (R2ML) data fusion workflow for multi-lithofacies alluvial characterization. OIK utilizes irregular interval data to construct three-dimensional (3D) resistivity fields from one-dimensional inverted AEM resistivity models. The R2ML workflow maps the resistivity field generated from OIK into a multi-facies lithological model, incorporating geologic constraints derived from well logs and geological observations. The numerical and real-world cases demonstrate that OIK is computationally efficient, accounts for 3D anisotropy, and minimizes the smoothing effect, thereby preserving resistivity contrasts and reducing interpolation uncertainty. The methodology is applied to lithologic characterization of the Mississippi River Valley alluvial aquifer (MRVA) in the Shellmound area, Mississippi, U.S. A frequency-domain AEM survey was conducted to support groundwater studies for the managed aquifer recharge (MAR) to the MRVA. The resulting lithological model, including four types of lithofacies—clay, very fine sand, fine-medium sands, and graveliferous sands, illustrates the geomorphological processes of the MRVA and implies potential MAR. The alignment between the lithological model and existing geological and hydrogeological investigations demonstrates that OIK and R2ML workflow effectively capture the subsurface architecture of the MRVA. The methods have broad applicability for characterizing alluvial aquifers through AEM-borehole data fusion, supporting sustainable groundwater management.

Mississippi↗
USGS2004

Vestiges of an Iapetan rift basin in the New Jersey Highlands: Implfications for the Neoproterozoic Laurentian margin

Thin, discontinuous remnants of Neoproterozoic intracratonic rift-basin deposits of the Chestnut Hill Formation occur in the western New Jersey Highlands. These deposits form an important link between well-documented Iapetan rift-basins in both the northern and southern Appalachians. The close spatial relations of Chestnut Hill rocks to Paleozoic sedimentary rocks open the possibility that additional Iapetan rift-basins could be concealed beneath the rocks of the Valley and Ridge Province to the west indicating a much broader zone of rifting than has been previously proposed. The Chestnut Hill Formation is intermittently exposed along a 100 km-long band that extends northeast from Pennsylvania nearly to New York State. The lower part of the Chestnut Hill Formation is composed of interbedded lithic pebble- to boulder-conglomerate and feldspathic sandstone grading upward into interbedded phyllite, feldspathic and quartz sandstone, local paleosaprolite, quartz-pebble conglomerate, thin limestone lenses, volcanic, and volcaniclasic rocks, abundant bedded ironstone (hematite ore), and ultimately into diamictites that are interpreted as possible tilloids and containing rounded intra and extrabasinal clasts of the other lithologies. Extensive soft-sediment deformation, cross bedding, and clastic dikes are common in all but the lowest and upper facies. Banded hematite layers occur preferentially in fine-grained tuffs and tuffaceous sediments, but hematitization has affected most lithologies. Volcanic rocks consist of altered rhyolitic tuffs and lapilli tuffs that are interbedded with sediments. The Chestnut Hill Formation is interpreted to have been deposited in early alluvial, and later a complex of fluvial, lacustrine and deltaic environments. Provenance studies based upon petrographic and geochemical analysis of clastic rocks indicate that the sediments are predominantly immature and reflect derivation from local uplifted felsic basement sources in a rifted-margin tectonic setting. Low to moderate weathering of the source rocks is indicated by the geochemistry of most samples, as is the locally intense effect of hydrothermal alteration. Most occurrences of the Chestnut Hill Formation are associated with major faults that exhibit normal movement of apparent Neoproterozoic age. Rocks from the Morgan Hill fault near Easton, Pennsylvania display consistent normal shear sense and vary from low temperature S-C mylonites to breccia that contains deformed pieces of Chestnut Hill Formation. ?? 2004 Elsevier Ltd. All rights reserved.

Journal of Geodynamics↗
USGS2019

The Miocene Atastra Creek sinter (Bodie Hills volcanic field, California and Nevada): 4D evolution of a geomorphically intact siliceous hot spring deposit

The Atastra Creek siliceous hot spring deposit, or sinter, occurs in the Paramount-Bald Peak alteration zone, due north of the Bodie precious metals mining district in the Miocene Bodie Hills volcanic field, California and Nevada, U.S.A. Distinctive features include its geomorphically intact geyser vent mounds, the presence of growth-fault-stepped sinter terraces that developed westward along a NNW trending fault, and atypical Hg, Sb and As concentrations. High-temperature (>75 °C), subaerial, proximal apron sinter occurs in two settings – (1) radiating botryoidal geyserite in vent mounds and (2) bedded geyserite (columnar, nodular, size-sorted geyser eggs) intercalated with mid- to low-temperature (<60 °C), middle to distal apron terrace sinter. The active geyser vent mounds migrated from south to north across the apron terrace through time. A newly recognized, visually striking type of sinter – termed silica infiltrate herein – formed directly beneath the subaerial geyserite deposits. It is characterized by mainly parallel, thin, irregular beds of white silica with narrow borders of dark blue-gray silica, as well as swirly to globular white silica, all chaotically linked together by silica ‘necks’ and ‘bridges’. The silica infiltrate permeates the pre-existing sinter, and is interpreted to represent super-heated fluid injections into the immediately overlying geyser vent mounds and pools. The adjacent sinter sheets preserve fluid-flow directions of warm-water channels that traversed the discharge apron, including microbial streamer fabric and wavy laminated sinter with flattened bubbles. More distal sinter textures are recorded by rimmed terracettes with mammillated and palisade microbial fabrics and rare, warm pool-related stromatolites. However, plants are absent, probably due to the high metalloids, in particular As. Post-depositional events include an inferred hydrothermal eruption breccia and an interpreted drop in the water table accompanied by incursion of steam acid condensate, as evidenced by pervasive hydrothermal alteration of the Atastra Creek deposits. Hence, the well-exposed sedimentary facies distributions, well-preserved geomorphology, and stratigraphic and structural relationships together allow an unusually complete, paleoenvironmental reconstruction of the Atastra Creek paleohydrothermal deposit.

Journal of Volcanology and Geothermal Research↗
USGS2005

Late Neogene and Quaternary evolution of the northern Albemarle Embayment (mid-Atlantic continental margin, USA)

Seismic surveys in the eastern Albemarle Sound, adjacent tributaries and the inner continental shelf define the regional geologic framework and provide insight into the sedimentary evolution of the northern North Carolina coastal system. Litho- and chronostratigraphic data are derived from eight drill sites on the Outer Banks barrier islands, and the Mobil #1 well in eastern Albemarle Sound. Within the study area, parallel-bedded, gently dipping Miocene beds occur at 95 to > 160 m below sea level (m bsl), and are overlain by a southward-thickening Pliocene unit characterized by steeply inclined, southward-prograding beds. The lower Pliocene unit consists of three seismic sequences. The 55–60 m thick Quaternary section unconformably overlies the Pliocene unit, and consists of 18 seismic sequences exhibiting numerous incised channel-fill facies. Shallow stratigraphy (< 40 m bsl) is dominated by complex fill patterns within the incised paleo-Roanoke River valley. Radiocarbon and amino-acid racemization (AAR) ages indicate that the valley-fill is latest Pleistocene to Holocene in age. At least six distinct valley-fill units are identified in the seismic data. Cores in the valley-fill contain a 3–6 m thick basal fluvial channel deposit that is overlain by a 15 m thick unit of interlaminated muds and sands of brackish water origin that exhibit increasing marine influence upwards. Organic materials within the interlaminated deposits have ages of 13–11 cal. ka. The interlaminated deposits within the valley are overlain by several units that comprise shallow marine sediments (bay-mouth and shoreface environments) that consist of silty, fine- to medium-grained sands containing open neritic foraminifera, suggesting that this area lacked a fronting barrier island system and was an open embayment from ∼10 ka to ∼4.5 ka. Seismic data show that initial infilling of the paleo-Roanoke River valley occurred from the north and west during the late Pleistocene and early Holocene. Later infilling occurred from the south and east and is characterized by a large shoal body (Colington Island and Shoals) and adjacent inlet fill. Establishment of a continuous barrier island system across the bay-mouth resulted in deposition of the latest phase of valley-fill, characterized by estuarine organic-rich muds.

Albemarle Embayment↗
USGS2008

Wave climate, sediment supply and the depth of the sand-mud transition: A global survey

The influences of wave climate and sediment supply on the depths of sand-mud transitions (hSMT) are investigated. Depths of sand-mud transitions (SMT) are based on published granulometric data from surface samples gathered from 14 sites in different wave-dominated coastal environments with fluvial input, including high energy (Columbia, Eel, Russian, San Lorenzo, Copper, and Nepean rivers), moderate energy (Ebro, Nile, Santa Clara, Tseng-wen and Kao-ping rivers), and low energy (Po, Pescara and Tronto rivers) regimes. Geometric mean diameter (GMD) and mud percent are compiled from samples along shore-normal transects, and significant correlation is found between these two textural descriptors. Nominally, the SMT is defined as the transition from GMD > 63????m to < 63????m. The correlation between mud percent and GMD permits an alternative, complementary definition of the SMT as the transition from < 25% mud to > 25% mud. This dual definition is applied to the 14 systems, and hSMT is tabulated for each system. Correlation is found between hSMT and the depth at which wave-induced bottom shear stress equals the critical erosion shear stress of the largest mud particles and also between hSMT and significant wave height. Lack of correlation between hSMT and sediment load of nearby rivers indicates either that the influence of sediment supply on depth of the sand-mud transition is small or is not adequately represented in this study. Shelf width and slope do not correlate with residuals from a formalized linear relationship between hSMT and significant wave height. The relationship between hSMT and wave climate is useful for calibration of numerical models of erosion and deposition in wave-dominated coastal environments, for prediction of seabed properties in remote or inaccessible areas, and for reconstruction of paleodepth based on facies changes from sand to mud in ancient rocks. ?? 2008.

Marine Geology↗
USGS2016

Accommodation space in a high-wave-energy inner-shelf during the Holocene marine transgression: Correlation of onshore and offshore inner-shelf deposits (0–12 ka) in the Columbia River littoral cell system, Washington and Oregon, USA

The Columbia River Littoral Cell (CRLC), a high-wave-energy littoral system, extends 160 km alongshore, generally north of the large Columbia River, and 10–15 km in across-shelf distance from paleo-beach backshores to about 50 m present water depths. Onshore drill holes (19 in number and 5–35 m in subsurface depth) and offshore vibracores (33 in number and 1–5 m in subsurface depth) constrain inner-shelf sand grain sizes (sample means 0.13–0.25 mm) and heavy mineral source indicators (> 90% Holocene Columbia River sand) of the inner-shelf facies (≥ 90% fine sand). Stratigraphic correlation of the transgressive ravinement surface in onshore drill holes and in offshore seismic reflection profiles provide age constraints (0–12 ka) on post-ravinement inner-shelf deposits, using paleo-sea level curves and radiocarbon dates. Post-ravinement deposit thickness (1–50 m) and long-term sedimentation rates (0.4–4.4 m ka − 1 ) are positively correlated to the cross-shelf gradients (0.36–0.63%) of the transgressive ravinement surface. The total post-ravinement fill volume of fine littoral sand (2.48 × 10 10 m 3 ) in the inner-shelf represents about 2.07 × 10 6 m 3 year − 1 fine sand accumulation rate during the last 12 ka, or about one third of the estimated middle- to late-Holocene Columbia River bedload or sand discharge (5–6 × 10 6 m 3 year − 1 ) to the littoral zone. The fine sand accumulation in the inner-shelf represents post-ravinement accommodation space resulting from 1) geometry and depth of the transgressive ravinement surface, 2) post-ravinement sea-level rise, and 3) fine sand dispersal in the inner-shelf by combined high-wave-energy and geostrophic flow/down-welling drift currents during major winter storms.

Oregon, Washington↗
USGS2018

Controls on submarine canyon head evolution: Monterey Canyon, offshore central California

The Monterey submarine canyon, incised across the continental shelf in Monterey Bay, California, provides a record of the link between onshore tectonism, fluvial transport, and deep-marine deposition. High-resolution seismic-reflection imaging in Monterey Bay reveals an extensive paleocanyon unit buried below the seafloor of the continental shelf around Monterey and Soquel canyon heads. Paleocanyons shifted position through numerous phases of cut-and-fill in response to Salinas, Pajaro, and San Lorenzo river extensions and avulsions across the continental shelf during high-frequency Pleistocene sea-level and climatic variations. Five seismic facies within the Monterey paleocanyon unit and below the modern canyon are defined to interpret canyon evolution during the Pleistocene. Repeated sea-level oscillations appear to have switched the main fairway(s) of sediment transport. Large-scale erosion and fill occurred in marine environments. Paleocanyon fill is characterized by paleo-axial channel deposits and mass transport deposits, followed by canyon head abandonment and marine sedimentation. The upper portion of the paleocanyon unit contains relatively small channels that were likely incised by erosion in the paleo-Salinas and Pajaro rivers and filled with a mix of nonmarine and marine deposits. Shifting position of submarine canyons over time is characteristic of Monterey Bay, east of the Monterey Bay Fault Zone, and is likely unidentified in other submarine canyon head regions that lack dense high-resolution seismic-reflection subbottom images. We show that canyon heads can be areas of sediment accumulation linked to sea-level oscillations, providing new insights into submarine canyon evolution and sequence stratigraphy.

California↗
USGS2023

Subaqueous clinoforms created by sandy wave-supported gravity flows: Lessons from the central California shelf

Subaqueous clinoforms are an important yet underappreciated shelf feature. Their origins are typically associated with subaerial deltas but recent work has identified similar features in settings without a significant fluvial source. These other studies have shown that such subaqueous clinoforms, also known as infralittoral prograding wedges (IPWs), are created largely by wave-induced processes. This study uses geophysical, sedimentological, and radiocarbon data to determine the sedimentary characteristics and genesis of a shore-parallel subaqueous clinoform developed far from any significant river on the central California continental shelf; a feature known locally as the Cross Hosgri Slope. Sediment cores through the feature reveal that it is composed of beds with an erosive base, followed by a thin coarsening upward sequence of shelly fine sands transitioning to a fining upward sequence marked by alternating parallel and ripple cross laminated very fine sands. The deposit is often capped by fine silts that are commonly interbedded with thin very fine sand beds. Radiocarbon dating of shells within the cores paired with seismic profiles indicate the subaqueous clinoform initiated progradation ~7 ka, nucleating on an older Younger Dryas relict shoreface. We suggest the CHS was created by winter-storm waves mobilizing sands in water depths up to ~ 70 m that transitioned into wave-supported gravity flows. The wave-supported gravity flows traveled downslope to water depths of up to ~85 m, corresponding to the foot of the subaqueous clinoform. They did not travel beyond this depth as wave influence at these depths is negligible and the shelf slope is insufficient to maintain movement of the load alone. Our work suggests that wave-supported gravity flows can entrain very fine sands and silts and build subaqueous clinoforms, even in the absence of a significant river source. Furthermore, we provide a facies model for sandy wave-supported gravity flow deposits.

California↗
USGS2016

Organic petrology and geochemistry of Eocene Suzak bituminous marl, north-central Afghanistan: Depositional environment and source rock potential

Organic geochemistry and petrology of Eocene Suzak bituminous marl outcrop samples from Madr village in north-central Afghanistan were characterized via an integrated analytical approach to evaluate depositional environment and source rock potential. Multiple proxies suggest the organic-rich (TOC ∼6 wt.%) bituminous marls are ‘immature’ for oil generation (e.g., vitrinite R o < 0.4%, T max < 425 °C, PI ≤ 0.05, C 29 ααα S/S + R ≤ 0.12, C 29 ββS/ββS+ααR ≤ 0.10, others), yet oil seeps are present at outcrop and live oil and abundant solid bitumen were observed via optical microscopy. Whole rock sulfur content is ∼2.3 wt.% whereas sulfur content is ∼5.0–5.6 wt.% in whole rock extracts with high polar components, consistent with extraction from S-rich Type IIs organic matter which could generate hydrocarbons at low thermal maturity. Low Fe-sulfide mineral abundance and comparison of Pr/Ph ratios between saturate and whole extracts suggest limited Fe concentration resulted in sulfurization of organic matter during early diagenesis. From these observations, we infer that a Type IIs kerogen in ‘immature’ bituminous marl at Madr could be generating high sulfur viscous oil which is seeping from outcrop. However, oil-seep samples were not collected for correlation studies. Aluminum-normalized trace element concentrations indicate enrichment of redox sensitive trace elements Mo, U and V and suggest anoxic-euxinic conditions during sediment deposition. The bulk of organic matter observed via optical microscopy is strongly fluorescent amorphous bituminite grading to lamalginite, possibly representing microbial mat facies. Short chain n- alkanes peak at C 14 –C 16 ( n- C 17 / n- C 29 > 1) indicating organic input from marine algae and/or bacterial biomass, and sterane/hopane ratios are low (0.12–0.14). Monoaromatic steroids are dominated by C 28 clearly indicating a marine setting. High gammacerane index values (∼0.9) are consistent with anoxia stratification and may indicate intermittent saline-hypersaline conditions. Stable C isotope ratios also suggest a marine depositional scenario for the Suzak samples, consistent with the presence of marine foraminifera including abundant planktic globigerinida (?) and rare benthic discocyclina (?) and nummulites (?). Biomarker 2α-methylhopane for photosynthetic cyanobacteria implies shallow photic zone deposition of Madr marls and 3β-methylhopane indicates presence of methanotrophic archaea in the microbial consortium. The data presented herein are consistent with deposition of Suzak bituminous marls in shallow stratified waters of a restricted marine basin associated with the southeastern incipient or proto-Paratethys. Geochemical proxies from Suzak rock extracts (S content, high polar content, C isotopes, normal (αααR) C 27–29 steranes, and C 29 /C 30 and C 26 /C 25 hopane ratios) are similar to extant data from Paleogene oils produced to the north in the Afghan-Tajik Basin. This observation may indicate laterally equivalent strata are effective source rocks as suggested by previous workers; however, further work is needed to strengthen oil-source correlations.

Marine and Petroleum Geology↗
USGS2019

India National Gas Hydrate Program Expedition 02 summary of scientific results: Numerical simulation of reservoir response to depressurization

The India National Gas Hydrate Program Expedition 02 (NGHP-02) discovered gas hydrate at high saturation in sand reservoirs at several sites in the deepwater Bay of Bengal. To assess the potential response of those deposits to scientific depressurization experiments, comprehensive geologic models were constructed to enable numerical simulation for two sites. Both sites (NGHP-02-09 and NGHP-02-16) feature thick sequences of thinly-interbedded reservoir and non-reservoir facies at sub-seafloor depths of less than 300 m and sub-sea depths of 2,400 m or more. These settings pose significant challenges to current modeling capabilities. First, the thinly-bedded reservoir architecture complicates the determination of basic reservoir parameters from both log and core data due to measurement resolution issues. Secondly, the fine-scale variation in sediment properties imparts great contrasts in key parameters over very short distances, creating high gradients at multiple scales and varying orientations that necessitate careful design of high-definition simulation grids. Thirdly, the deposits include internal sources of water, as well as a range of complex boundary conditions, including variable permeability within the overlying mud-rich “seals”, that complicate reservoir depressurization. Lastly, because of the unique combination of great water depth and relatively shallow sub-seafloor depth, models designed to maximize the dissociation rate impose large pressure drawdowns on relatively low-strength sediments. This condition renders the proper evaluation and integration of the geomechanical response to hydrate dissociation critical. In this report, we review the history of gas hydrate reservoir simulation, discuss methods for creating geologic input models, and summarize the key findings and implications of the collaborative NGHP-02 numerical simulation effort. Together, the studies confirm the viability of the modeled accumulations for scientific testing and identify key challenges related to the selection of specific test sites and the design of test wells.

Journal of Marine and Petroleum Geology↗
USGS2019

National Gas Hydrate Program expedition 02: Identification of gas hydrate prospects in the Krishna-Godavari Basin, offshore India

After completing the first expedition of India's National Gas Hydrate Program (NGHP-01) in 2006, it was concluded that for the next expedition (National Gas Hydrate Program 02; NGHP-02), a new drill site review effort should focus on identifying potential deep-water offshore gas hydrate accumulations in sand dominated depositional environments. Therefore, geological and geophysical data analysis and 3D seismic data interpretation along with associated seismic modeling were carried out in three areas of the Krishna-Godavari Basin: Areas B, C, and E. Conventional petroleum exploration approaches of seismic amplitude evaluation were adapted to prospect for potential sand-rich depositional systems within the gas hydrate stability zone. Subsequently, these prospective areas were further assessed through the geological and geophysical evaluation of depositional setting, gas sources, and gas migration pathways. In Area B, prospecting focused on a large anticlinal structure with a prominent bottom-simulating reflector and several key horizons that indicated evidence for potential sand-hosted hydrate occurrences. In Area C, the prospects were distributed throughout various settings within a very large deep-water channel-levee-fan system with complex indications of potential gas hydrate occurrence in sand-prone seismic facies. In Area E, prospects were associated with high amplitude events within inferred channel-levee sequences. Based on the pre-expedition/onboard drill-site evaluation, the 22 most promising sites in the Krishna-Godavari Basin were identified and prioritized to investigate and delineate a total of 17 identified gas hydrate prospects. This paper describes the geo-scientific studies carried out prior to NGHP-02 for site identification, evaluation and prioritization. An important outcome of this study is the identification of two potentially producible gas hydrate systems inferred to host significant quantity of gas hydrate in stratigraphic-structural traps.

Journal of Marine and Petroleum Geology↗
USGS2022

Geochemical characterization of natural gases in the pre-salt section of the Santos Basin (Brazil) focused on hydrocarbons and volatile organic sulfur compounds

The objective of this work is to characterize the geochemistry of a suite of natural gas samples from five fields in order to improve the understanding of the lacustrine petroleum system of the pre-salt section from the Santos Basin (Brazil). Additionally, the distribution of volatile organic sulfur compounds (VOSC) in petroleum reservoirs was examined to investigate possible applications to petroleum systems assessments. The hydrocarbon gases were generated by thermogenic processes associated with the oil window. The 13 C-enriched values for C 1 (>−40‰) were interpreted as an organic source signature rather than an indication of thermal maturity, except for the oil occurrence (Field B), where a different fluid charge mainly composed of methane and CO 2 from a minor external kitchen area in the Santos Basin was identified. The molecular composition and the carbon and hydrogen isotopic data of the hydrocarbon gases, when combined with the VOSC molecular compositional data, allow the identification of four gas families associated with different kitchens and/or migrations pathways. The total VOSC concentrations range from 0.7 to 23.9 ppm by volume (ppmV). The organic sulfides are mainly composed of carbonyl sulfide (COS) and diethyl sulfide. The main thiol compound is ethanethiol. The cyclic VOSC are primarily composed of thiophene, with a negligible amount of branched thiophenes (<0.1 ppmV). H2S showed a strong positive Pearson's correlation with COS and methanethiol (MeSH) concentrations (r = 0.943 and 0.807, respectively). This suggests that COS and MeSH formation was linked to H 2 S generation and/or post-catagenetic interactions between hydrocarbons and H 2 S, mainly related to thermochemical sulfate reduction (TSR). In contrast, the distribution of higher molecular weight VOSC seems to be controlled by source rock facies, rather than H 2 S concentration. Principal component analysis of the VOSC compositional data identified some subgroups within the gas families mainly associated with TSR. The results presented in this work reveal that VOSC can be an important auxiliary tool in petroleum system studies.

Atlantic Ocean, Santos Basin↗