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At least 991 records · Page 55Linked to original sources

USGS2010

Hydrothermal zebra dolomite in the Great Basin, Nevada--attributes and relation to Paleozoic stratigraphy, tectonics, and ore deposits

In other parts of the world, previous workers have shown that sparry dolomite in carbonate rocks may be produced by the generation and movement of hot basinal brines in response to arid paleoclimates and tectonism, and that some of these brines served as the transport medium for metals fixed in Mississippi Valley-type (MVT) and sedimentary exhalative (Sedex) deposits of Zn, Pb, Ag, Au, or barite. Numerous occurrences of hydrothermal zebra dolomite (HZD), comprised of alternating layers of dark replacement and light void-filling sparry or saddle dolomite, are present in Paleozoic platform and slope carbonate rocks on the eastern side of the Great Basin physiographic province. Locally, it is associated with mineral deposits of barite, Ag-Pb-Zn, and Au. In this paper the spatial distribution of HZD occurrences, their stratigraphic position, morphological characteristics, textures and zoning, and chemical and stable isotopic compositions were determined to improve understanding of their age, origin, and relation to dolostone, ore deposits, and the tectonic evolution of the Great Basin. In northern and central Nevada, HZD is coeval and cogenetic with Late Devonian and Early Mississippian Sedex Au, Zn, and barite deposits and may be related to Late Ordovician Sedex barite deposits. In southern Nevada and southwest California, it is cogenetic with small MVT Ag-Pb-Zn deposits in rocks as young as Early Mississippian. Over Paleozoic time, the Great Basin was at equatorial paleolatitudes with episodes of arid paleoclimates. Several occurrences of HZD are crosscut by Mesozoic or Cenozoic intrusions, and some host younger pluton-related polymetallic replacement and Carlin-type gold deposits. The distribution of HZD in space (carbonate platform, margin, and slope) and stratigraphy (Late Neoproterozoic Ediacaran–Mississippian) roughly parallels that of dolostone and both are prevalent in Devonian strata. Stratabound HZD is best developed in Ediacaran and Cambrian units, whereas discordant HZD is proximal to high-angle structures at the carbonate platform margin, such as strike-slip and growth faults and dilational jogs. Fabric-selective replacement and dissolution features (e.g., collapse breccias, voids with geopetal textures) are common, with remaining void space lined with light-colored dolomite crystals that exhibit zoning under cathodoluminescence. Zoned crystals usually contain tiny (<1–3 μm) fluid inclusions with vapor bubbles, requiring Th > ∼70 °C. The oxygen isotopic compositions of HZD are consistent with formation temperatures of 50–150 °C requiring brine circulation to depths of 2–5 km, or more. The few HZD occurrences with the highest concentrations of metals (especially Fe, Mn, and Zn) and the largest isotopic shifts are closely associated with Sedex or MVT deposits known to have formed from hotter brines (e.g., Th > 150–250 °C). These relationships permit that HZD formed at about the same time as dolostone, from brines produced by the evaporation of seawater during arid paleoclimates at equatorial paleolatitudes. Both dolostone and HZD may have formed as basinal brines, which migrated seaward from evaporative pans on the platform, with dolostone forming at low temperatures along shallow migration pathways through permeable limestones, and HZD forming at high temperatures along deeper migration pathways through basal aquifers and dilatant high-angle faults. The small MVT deposits were chemical traps where hot brines encountered rocks or fluids containing reduced sulfur. The abundant Sedex deposits mark sites where hot brine discharged at the seafloor in adjacent basins. Thus the distribution of HZD may map deep migration pathways and upflow zones between eastern shallow marine facies, where evaporative brine could have been generated, and western Sedex deposits, where heated brines discharged along faults into platform margin, slope, and basin facies. The small size and scarcity of Pb-Zn deposits and the abundance of barite deposits in the Great Basin suggests the brines were generally reduced, possibly due to reactions with carbonaceous rocks along deep migration pathways. While this scenario may have occurred at several times, the age and abundance of Sedex deposits suggest that such a hydrology was best developed in the Late Ordovician, Late Devonian, and Early Mississippian, possibly in response to episodes of extension and forebulge faults associated with the Antler orogeny. The improved understanding of HZD may aid future exploration for ore deposits in the Great Basin.

Nevada↗

USGS1983

Geology of the Volga-Ural petroleum province and detailed description of the Ramashkino and Arlan oil fields

The Volga-Ural petroleum province is in general coincident with the Volga-Ural regional high, a broad upwarp of the east-central part of the Russian (East European) platform. The central part of the province is occupied by the Tatar arch, which contains the major share of the oil fields of the province. The Perm-Bashkir arch forms the northeastern part of the regional high, and the Zhigulevsko-Orenburg arch makes up the southern part. These arches are separated from one another by elongate downwarps. The platform cover overlies an Archean crystalline basement and consists of seven main sedimentation cycles as follows: 1) Riphean (lower Bavly) continental sandstone, shale, and conglomerate beds from 500 to 5,000 m thick deposited in aulacogens. 2) Vendian (upper Bavly) continental and marine shale and sandstone up to 3,000 m thick. 3) Middle Devonian-Tournaisian transgressive deposits, which are sandstone, siltstone, and shale in the lower part and carbonates with abundant reefs in the upper; thickness is 300-1,000 m. In the upper carbonate part is the Kamsko-Kinel trough system, which consists of narrow interconnected deep-water troughs. 4) The Visean-Namurian-Bashkirian cycle, which began with deposition of Visean clastics that draped over reefs of the previous cycle and filled in an erosional relief that had formed in some places on the sediments of the previous cycle. The Visean clastics are overlain by marine carbonates. Thickness of the cycle is 50-800 m. 5) Early Moscovian-Early Permian terrigenous clastic deposits and marine carbonate beds 1,000-3,000 m thick. 6) The late Early Permian-Late Permian cycle, which reflects maximum growth of the Ural Mountains and associated Ural foredeep. Evaporites were first deposited, then marine limestones and dolomites, which intertongue eastward with clastic sediments from the Ural Mountains. 7) Continental redbeds of Triassic age and mixed continental and marine elastic beds of Jurassic and Cretaceous age, which were deposited on the southern, southwestern, and northern margins of the Russian platform; they are generally absent in the Volga-Ural province, however. The Volga-Ural oil and gas basin is a single artesian system that contains seven aquifers separated by seals. The areas of greatest hydraulic head are in the eastern parts of the basin near areas where the aquifers crop out on the western slopes of the Ural Mountains. The Peri-Caspian basin is the principal drainage area of the artesian system. Approximately 600 oil and gas fields and 2,000 pools have been found in the Volga-Ural province. Nine productive sequences are recognized as follows: 1) Upper Proterozoic (Bavly beds), which are promising but not yet commercial. 2) Clastic Devonian, which contains the major reserves and includes the main pays of the super-giant Romashkino field. 3) Carbonate Upper Devonian and lowermost Carboniferous, which is one of the main reef-bearing intervals. 4) Visean (Lower Carboniferous) elastics, which are the main pays in the super-giant Arian field. 5) Carbonate Lower and Middle Carboniferous. 6) Clastic Middle Carboniferous Moscovian. 7) Carbonate Middle and Upper Carboniferous. 8) Carbonate-evaporite Lower Permian, which contains the major gas reserves and the lower part of the Melekess tar deposits. 9) Clastic-carbonate Upper Permian, which contains the major part of the Melekess tar deposits. The Volga-Ural province is divided into several productive regions on a basis of differences in structure, distribution of reservoir and source-rock facies, and general composition of the petroleum accumulations. These regions are the Tatar arch, Birsk saddle, Upper Kama depression, Perm-Bashkir arch, Ufa-Orenburg monocline, Melekess-Sernovodsko-Abdulino basin, Zhligulevsko-Orenburg arch, Ural foredeep, and north borders of the Peri-Casplan depression. Exploration activity has declined in recent years; however, interest remains high in several parts of the province, particula

Open-File Report↗

USGS1995

Beach-steps: An evolutionary perspective

Field observation of contrasting beach-step behavior at Canaveral National Seashore on two subsequent days when incident-wave conditions in the inner surf zone were similar prompted this re-examination of our conceptual and quantitative understanding of beach steps. These lower-foreshore features are more complex than previously assumed, evolving through erosional as well as accretional phases, and displaying equifinality in geometric form but not necessarily internal sedimentary structure. Past and recent evidence is reviewed that links beach steps to incident waves at the surging-plunging transition and to the action of a backwash vortex. Tides and low-frequency waves likely play no direct role in beach-step initiation, although their presence can have pronounced influences on modulating nearshore hydrodynamics, and thus, on beach-step maintenance and evolution. A generalized, conceptual model capturing these aspects of beach-step dynamics is presented. Beach-step initiation proceeds via step “carving”, “excavation”, or “building” depending on the erosional-accretional character of the beach-foreshore system. Subsequent evolution of the step form may take one of several alternative morphodynamic pathways including stepface “retreat”, step “drag down”, or step “infilling/elimination” depending on tidal stage/range or wave set-up/setdown. Additional data on equilibrium beach-step forms and associated morphodynamic and hydrodynamic conditions in the field are necessary before quantitative models of beach-step existence and evolution can be formulated with realistic results.

Marine Geology↗

USGS1992

Accretionary margin of north-western Hispaniola: morphology, structure and development of part of the northern Caribbean plate boundary

Broad-range side-scan sonar (GLORIA) images and single- and multi-channel seismic reflection profiles demonstrate that the margin of north-western Hispaniola has experienced compression as a consequence of oblique North American-Caribbean plate convergence. Two principal morphological or structural types of accretionary wedges are observed along this margin. The first type is characterized by a gently sloping (≈4°) sea floor and generally margin-parallel linear sets of sea-floor ridges that gradually deepen towards the flat Hispaniola Basin floor to the north. The ridges are caused by an internal structure consisting of broad anticlines bounded by thrust faults that dip southwards beneath Hispaniola. Anticlines form at the base of the slope and are eventually sheared and underthrust beneath the slope. In contrast, the second type of accretionary wedge exhibits a steeper (≈6–16°) sea-floor slope characterized by local slumping and a more abrupt morphological transition to the adjacent basin. The internal structure appears chaotic on seismic reflection profiles and probably consists of tight folds and closely spaced faults. We suggest that changes in sea-floor declivity and internal structure may result from variations in the dip or frictional resistance of the décollement, or possibly from changes in the cohesive strength of the wedge sediments. The observed pattern of thickening of Hispaniola Basin turbidites towards the insular margin suggests differential southwards tilting of the Hispaniola Basin strata, probably in response to North America-Caribbean plate interactions since the Early Tertiary. Based upon indirect age control from adjacent parts of the northern caribbean plate boundary, we infer a Late Eocene to Early Miocene episode of transcurrent motion (i.e. little or no tilting), an Early Miocene to Late Pliocene period of oblique convergence (i.e. increased tilt) during which the accretionary wedge began to be constructed, and a Late Pliocene to Recent episode of increased convergence (i.e. twice the Miocene to Pliocene tilt), which has led to rapid uplift and erosion of sediment sources on the margin and on Hispaniola, generating a submarine fan at the base of the insular slope.

Marine and Petroleum Geology↗

USGS2020

The Cenozoic evolution of crustal shortening and left‐lateral shear in the central East Kunlun Shan: Implications for the uplift history of the Tibetan Plateau

The timing of crustal shortening and strike‐slip faulting along the East Kunlun Shan provides insight into the history of surface uplift and may constrain the time at which the Tibetan Plateau reached high elevations. We investigate a series of extensional basins and restraining bends along the Xidatan strand of the Kunlun strike‐slip fault, which provide an ideal setting to unravel the tectonic history of the northern plateau margin. We present new apatite (U‐Th)/He, apatite fission track, and zircon (U‐Th)/He ages and QTQt thermal modeling, 40 Ar/ 39 Ar fault gouge dating, and structural mapping from the central East Kunlun Shan. Our data suggest that the East Kunlun Shan experienced slow to negligible exhumation until late Cretaceous time, followed by an increase in rate by 65–50 Ma. Along with a ~47 Ma fault gouge age, we posit that the Paleocene–early Eocene was a time of crustal shortening along the northern plateau. Rapid exhumation along transpressional portions of the Xidatan fault initiated by 23–20 Ma, which we interpret as the local onset of strike‐slip faulting. An early Miocene transition from north‐south crustal shortening to left‐lateral shear along the East Kunlun Shan, the onset of normal and strike‐slip faulting in central and southern Tibet by 18 Ma, and lower crustal flow in eastern Tibet by 13 Ma suggest the establishment of orogen‐wide east‐west oriented extension and extrusion by the middle Miocene. The plateau‐wide shift in stress accommodation implies that high gravitational potential energy, and likely high elevation, was attained by the middle Miocene.

Tectonics↗

USGS1999

Cruise report for O1-99-SC Southern California Earthquake Hazards project

The focus of the Southern California Earthquake Hazards project is to identify the landslide and earthquake hazards and related ground-deformation processes occurring in the offshore areas that have significant potential to impact the inhabitants of the Southern California coastal region. The project activity is supported through the Coastal and Marine Geology Program of the Geologic Division of the U. S. Geological Survey (USGS) and is a component of the Geologic Division's Science Strategy under Goal 1—Conduct Geologic Hazard Assessments for Mitigation Planning (Bohlen et al., 1998). The project research is specifically stated under Activity 1.1.2 of the Science Strategy: Earthquake Hazard Assessments and Loss Reduction Products in Urban Regions. This activity involves "research, seismic and geodetic monitoring, field studies, geologic mapping, and analyses needed to provide seismic hazard assessments of major urban centers in earthquake-prone regions including adjoining coastal and offshore areas." The southern California urban areas, which form the most populated urban corridor along the U.S. Pacific margin, are among a few specifically designated for special emphasis under the Division's science strategy (Bohlen et al., 1998). The primary objective of the project is to help mitigate the earthquake hazards for the Southern California region by improving our understanding of how deformation is distributed (spatially and temporally) in the offshore with respect to the onshore region. To meet this objective, we are conducting field investigations to observe the distribution, character, and relative intensity of active (i.e., primarily Holocene) deformation within the basins and along the shelf adjacent to the most highly populated areas (Fig. 1). In addition, acoustic imaging should help determine the subsurface dimensions of the faults and identify the size and frequency of submarine landslides, both of which are necessary for evaluating the potential for generating destructive tsunamis in the southern California offshore. In order to evaluate the strain associated with the offshore structures, the initial results from the field mapping under this project will be used to identify possible sites for deployment of acoustic geodetic instruments to monitor strain in the offshore region. A major goal of mapping under this project is to provide detailed geologic and geophysical information in GIS data bases that build on the earlier studies and use the new data to precisely locate active faults and to map recent submarine landslide deposits.

California↗

USGS2006

Geologic map of the west half of the Blythe 30' by 60' quadrangle, Riverside County, California and La Paz County, Arizona

The Blythe 30' by 60' quadrangle is located along the Colorado River between southeastern California and western Arizona. This map depicts the geology of the west half of the Blythe quadrangle, which is mostly in California. The map area is a desert terrain consisting of mountain ranges surrounded by extensive alluvial fans and plains, including the flood plain of the Colorado River which covers the easternmost part of the area. Mountainous parts of the area, including the Big Maria, Little Maria, Riverside, McCoy, and Mule Mountains, consist of structurally complex rocks that range in age from Proterozoic to Miocene. Proterozoic gneiss and granite are overlain by Paleozoic to Early Jurassic metasedimentary rocks (mostly marble, quartzite, and schist) that are lithostratigraphically similar to coeval formations of the Colorado Plateau region to the east. The Paleozoic to Jurassic strata were deposited on the tectonically stable North American craton. These rocks are overlain by metamorphosed Jurassic volcanic rocks and are intruded by Jurassic plutonic rocks that represent part of a regionally extensive, northwest-trending magmatic arc. The overlying McCoy Mountains Formation, a very thick sequence of weakly metamorphosed sandstone and conglomerate of Jurassic(?) and Cretaceous age, accumulated in a rapidly subsiding depositional basin south of an east-trending belt of deformation and east of the north-trending Cretaceous Cordilleran magmatic arc. The McCoy Mountains Formation and older rocks were deformed, metamorphosed, and locally intruded by plutonic rocks in the Late Cretaceous. In Oligocene(?) to Miocene time, sedimentary and minor volcanic deposits accumulated locally, and the area was deformed by faulting. Tertiary rocks and their Proterozoic basement in the Riverside and northeastern Big Maria Mountains are in the upper plate of a low-angle normal (detachment) fault that lies within a region of major Early to Middle Miocene crustal extension. Surficial deposits of the flanking alluvial fans and plains range in age from late Miocene to Holocene. Among the oldest of these deposits are limestone and fine-grained clastic sediments of the late Miocene and (or) Pliocene Bouse Formation, which is commonly interpreted to represent an estuary or marine embayment connected to the proto-Gulf of California. Most of the surficial deposits younger than the Bouse Formation are composed of alluvium either derived from local mountain ranges or transported into the area by the Colorado River. Large parts of the area, particularly near the northern margin, are covered by eolian sand, and small parts are covered by playa sediments.

Scientific Investigations Map↗

USGS1998

The ichnologic record of the continental invertebrate invasion; evolutionary trends in environmental expansion, ecospace utilization, and behavioral complexity

The combined study of continental trace fossils and associated sedimentary facies provides valuable evidence of colonization trends and events throughout the Phanerozoic. Colonization of continental environments was linked to the exploitation of empty or under-utilized ecospace. Although the nonmarine trace fossil record probably begins during the Late Ordovician, significant invasion of nonmarine biotopes began close to the Silurian-Devonian transition with the establishment of a mobile arthropod epifauna (Diplichnites ichnoguild) in coastal marine to alluvial plain settings. Additionally, the presence of vertical burrows in Devonian high-energy fluvial deposits reflects the establishment of a stationary, deep suspension-feeding infauna of the Skolithos ichnoguild. The earliest evidence of plant-arthropod interaction occurred close to the Silurian-Devonian boundary, but widespread and varied feeding patterns are known from the Carboniferous. During the Carboniferous, permanent subaqueous lacustrine settings were colonized by a diverse, mobile detritus-feeding epifauna of the Mermia ichnoguild, which reflects a significant palaeoenvironmental expansion of trace fossils. Paleozoic ichnologic evidence supports direct routes to the land from marginal marine environments, and migration to lakes from land settings. All nonmarine sedimentary environments were colonized by the Carboniferous, and subsequent patterns indicate an increase in ecospace utilization within already colonized depositional settings. During the Permian, back-filled traces of the Scoyenia ichnoguild record the establishment of a mobile, intermediate-depth, deposit-feeding in-fauna in alluvial and transitional alluvial-lacustrine sediment. Diversification of land plants and the establishment of ecologically diverse plant communities through time provided new niches to be exploited by arthropods. Nevertheless, most ot the evolutionary feeding innovations took place relatively early, during the Late Palaeozoic or early Mesozoic. A stationary deep unfauna, the Camborygma ichnoguild, was developed in Triassic transitional alluvial-lacustrinbe deposits. Terrestrial environments hosted the rise of complex social behavioral patterns, as suggested by the probable presence of hymenopteran and isopteran nests in Triassic paleosols. An increase in diversity of trace fossils is detected in Triassic-Jurassic eolian deposits, where the ichnofauna displays more varied behavioral patterns than their Paleozoic counterparts. Also, a mobile, intermediate-depth, deposit-feeding infauna, the Vagorichnus ichnoguild, was established in deep lake environments during the Jurassic. In contrast to Paleozoic permanent subaqueous assemblages typified by surface trails, Jurassic ichnocoenoses are dominated by infaunal burrows. High density of infaunal deposit-feeding traces of the Planolites ichnoguild caused major disruption of lacustrine sedimentary fabrics during the Cretaceous. Most insect mouthpart classes, functional feeding groups, and dietary guilds were established by the end of the Cretaceous. Diversification of modern insects is recorded by the abundance and complexity of structures produced by wasps, bees, dung-beetles, and termites in Cretaceous-Tertiary paleosols. The increase in bioturbation migrated from fluvial and lake-margin settings to permanent subaqueous lacustrine environments through time.

Palaios↗

USGS2019

Life history, genetics, range expansion and new frontiers of the lionfish (Pterois volitans, Perciformes: Pteroidae) in Latin America

Pterois volitans (lionfish) is a midsize predatory fish commonly found in waters of the western Pacific and Indian Ocean. The species was first documented in Dania Beach, Florida (northwestern Caribbean) in 1985. Since that time the species has expanded its range rapidly to the Northwestern Atlantic Ocean, Gulf of Mexico, and Caribbean Sea. Since its introduction P. volitans has changed community structure and biodiversity of Caribbean reef communities and other coastal tropical ecosystems. Continuous introductions (accidental or intentional), limited natural predators, naïve-range prey behavior, high predation rates on competitors, continuous reproduction, and an extended period of larval dispersal have been the keys for successful invasion and rapid range extension of P. volitans . This invasion has become so severe that it has been recognized as one of the world’s top conservation issues. Here, we review the life history, behavior, and historical and contemporary genetic patterns that facilitate expansion and the colonization process. A greater understanding of lionfish biology, ecology, and the changes related to its present condition as a super-invader could improve current and future management strategies and new detection and response methodologies. We also examine new invasion frontiers that this species has the potential to colonize such as the eastern Pacific. This information will provide managers, the scientific community, and the civil society better tools for eradication, control and management of future invasions of this and other invasive species.

Caribbean Sea, Latin America↗

USGS2002

Flow and sediment suspension events on the inner shelf of central California

The US Geological Survey conducted a field experiment in the late spring and early summer of 1998 off northern Santa Cruz County, California, to study sediment fluxes along the central California shelf. As part of this study, a bottom-mounted instrument package was deployed in a sediment-filled paleo-stream channel (h=12 m) off a pocket beach to measure waves, currents, suspended-sediment concentration, temperature, salinity, and seabed-level changes. The observations of suspended-sediment concentration revealed intermittent, intense periods of sediment suspension near the bed that were associated with the passage of individual large waves or wave groups. We used an ensemble averaging technique to characterize the temporal structure of near-bed sediment suspension events (SSEs) defined as when the near-bed instantaneous concentration exceeded the mean by three standard deviations. During the 800 h of deployment, over 9500 SSEs occurred. The 3917 SSEs that began under onshore flow were found to have a different temporal structure than the 5650 events that began during offshore flow. The longer mean duration of the onshore flow events caused an average of 83% of the sediment suspended during the course of the events to be carried offshore after the reversal of flow. All of the sediment suspended during the offshore events was carried offshore due to the shorter mean duration of these events. SSEs are shown to contribute 12% on average and up to 95% of the total sediment suspended by mass, demonstrating the importance of these events to sediment transport in this type of environment. Suspended-sediment transport models developed for the shelf, which use the product of the mean current and the mean suspended-sediment concentration, cannot accurately model the magnitude and direction of suspended-sediment transport on this energetic inner shelf due to the lack of information regarding the instantaneous coupling between fluid flow and sediment suspension. We conclude that time-variant models must be used to accurately model suspended-sediment transport in this type of environment.

California↗

USGS2008

Geologic framework of the 2005 Keathley Canyon gas hydrate research well, northern Gulf of Mexico

The Keathley Canyon sites drilled in 2005 by the Chevron Joint Industry Project are located along the southeastern edge of an intraslope minibasin (Casey basin) in the northern Gulf of Mexico at 1335 m water depth. Around the drill sites, a grid of 2D high-resolution multichannel seismic data designed to image depths down to at least 1000 m sub-bottom reveals 7 unconformities and disconformities that, with the seafloor, bound 7 identifiable seismic stratigraphic units. A major disconformity in the middle of the units stands out for its angular baselapping geometry. From these data, three episodes of sedimentary deposition and deformation are inferred. The oldest episode consists of fine-grained muds deposited during a period of relative stability in the basin (units e, f, and g). Both the BSR and inferred gas hydrate occur within these older units. The gas hydrate occurs in near-vertical fractures. A second episode (units c and d) involved large vertical displacements associated with infilling and ponding of sediment. This second interval corresponds to deposition of intercalated fine and coarse-grained material that was recovered in the drill hole that penetrated the thin edges of the regionally much thicker units. The final episode of deposition (units a and b) occurred during more subdued vertical motions. Hemipelagic drape (unit a) characterizes the modern seafloor. The present-day Casey basin is mostly filled. Its sill is part of a subsiding graben structure that is only 10-20 m shallower than the deepest point in the basin, indicating that gravity-driven transport would mostly bypass the basin. Contemporary faulting along the basin margins has selectively reactivated an older group of faults. The intercalated sand and mud deposits of units c and d are tentatively correlated with Late Pleistocene deposition derived from the western shelf-edge delta/depocenter of the Mississippi River, which was probably most active from 320 ka to 70 ka [Winker, C.D., Booth, J., 2000. Sedimentary dynamics of the salt-dominated continental slope, Gulf of Mexico: integration of observations from the seafloor, near-surface, and deep subsurface. In: Proceedings of the GCSSEPM Foundation 20th Annual Research Conference, Deep-water Reservoirs of the World, pp. 1059-1086]. The presence of sand within the gas hydrate stability zone (in units c and d) is not sufficient to concentrate gas hydrate even though dispersed gas hydrate occurs deeper in the fractured mud/clay-rich sections of units e and f.

Gulf of Mexico↗

USGS2012

Archive of digital boomer subbottom profile data collected in the Atlantic Ocean offshore northeast Florida during USGS cruises 03FGS01 and 03FGS02 in September and October of 2003

In September and October of 2003, the U.S. Geological Survey (USGS), in cooperation with the Florida Geological Survey, conducted geophysical surveys of the Atlantic Ocean offshore northeast Florida from St. Augustine, Florida, to the Florida-Georgia border. This report serves as an archive of unprocessed digital boomer subbottom profile data, trackline maps, navigation files, Geographic Information System (GIS) files, Field Activity Collection System (FACS) logs, and formal Federal Geographic Data Committee (FGDC) metadata. Filtered and gained (a relative increase in signal amplitude) digital images of the seismic profiles are also provided. Refer to the Acronyms page for expansions of all acronyms and abbreviations used in this report. The USGS St. Petersburg Coastal and Marine Science Center (SPCMSC) assigns a unique identifier to each cruise or field activity. For example, 03FGS01 tells us the data were collected in 2003 as part of cooperative work with the Florida Geological Survey (FGS) and that the data were collected during the first field activity for that project in that calendar year. Refer to http://walrus.wr.usgs.gov/infobank/programs/html/definition/activity.html for a detailed description of the method used to assign the field activity identification (ID). The naming convention used for each seismic line is as follows: yye##a, where 'yy' are the last two digits of the year in which the data were collected, 'e' is a 1-letter abbreviation for the equipment type (for example, b for boomer), '##' is a 2-digit number representing a specific track, and 'a' is a letter representing the section of a line if recording was prematurely terminated or rerun for quality or acquisition problems. The boomer plate is an acoustic energy source that consists of capacitors charged to a high voltage and discharged through a transducer in the water. The transducer is towed on a sled floating on the water surface and when discharged emits a short acoustic pulse, or shot, which propagates through the water, sediment column, or rock beneath. The acoustic energy is reflected at density boundaries (such as the seafloor, sediment, or rock layers beneath the seafloor), detected by hydrophone receivers, and recorded by a PC-based seismic acquisition system. This process is repeated at timed intervals (for example, 0.5 seconds) and recorded for specific intervals of time (for example, 100 milliseconds). In this way, a two-dimensional (2-D) vertical profile of the shallow geologic structure beneath the ship track is produced. Refer to the handwritten FACS operation log (PDF, 442 KB) for diagrams and descriptions of acquisition geometry, which varied throughout the cruises. Table 1 displays a summary of acquisition parameters. See the digital FACS equipment logs (PDF, 9-13 KB each) for details about the acquisition equipment used. The archived trace data are in standard Society of Exploration Geophysicists (SEG) SEG Y (Barry and others, 1975) format (rev. 0), except for the first 3,200 bytes of the card image header, which are stored in ASCII format instead of the standard EBCDIC format. The SEG Y files may be downloaded and processed with commercial or public domain software such as Seismic Unix (SU) (Cohen and Stockwell, 2005). See the How To Download SEG Y Data page for download instructions. The printable profiles provided here are Graphics Interchange Format (GIF) images that were filtered and gained using SU software. Refer to the Software page for details about the processing and links to example SU processing scripts and USGS software for viewing the SEG Y files (Zihlman, 1992).

Florida;Georgia↗

USGS1992

New links between the Chicxulub impact structure and the Cretaceous/Tertiary boundary

The 200-km-diameter Chicxulub structure 1–3 in northern Yucatan, Mexico has emerged as the prime candidate for the Cretaceous/Tertiary (K/T) boundary impact crater 3–6 . Concentric geophysical anomalies associated with enigmatic occurrences of Upper Cretaceous breccias and andesitic rocks led Penfield and Camargo 1 to suspect that this structure was a buried impact basin. More recently, the discovery of shocked quartz grains in a Chicxulub breccia 3 , and chemical similarities between Chicxulub rocks and K/T tektite-like glasses 3–6 have been advanced as evidence that the Chicxulub structure is a K/T impact site. Here we present evidence from core samples that Chicxulub is indeed a K/T source crater, and can apparently account for all the evidence of impact distributed globally at the K/T boundary without the need for simultaneous multiple impacts or comet showers. Shocked breccia clasts found in the cores are similar to shocked lithic fragments found worldwide in the K/T boundary ejecta layer 7,8 . The Chicxulub melt rocks that we studied contain anomalously high levels of iridium (up to 13.5 parts per 10 9 ), also consistent with the iridium-enriched K/T boundary layer 9 . Our best estimate of the crystallization age of these melt rocks, as determined by 40 Ar/< 39 Ar analyses, is 65.2 ±0.4 (1σ) Myr, in good agreement with the mean plateau age of 64.98 ± 0.05 Myr recently reported 10 . Furthermore, these melt rocks acquired a remanent magnetization indicating that they cooled during an episode of reversed geomagnetic polarity. The only such episode consistent with 40 Ar/< 39 Ar constraints is chron 29R, which includes the K/T boundary.

northern Yucatan↗

USGS2006

A stage-based model of manatee population dynamics

A stage-structured population model for the Florida manatee (Trichechus manatus latirostris) was developed that explicitly incorporates uncertainty in parameter estimates. The growth rates calculated with this model reflect the status of the regional populations over the most recent 10-yr period. The Northwest and Upper St. Johns River regions have growth rates (8) of 1.037 (95% interval, 1.016?1.056) and 1.062 (1.037?1.081), respectively. The Southwest region has a growth rate of 0.989 (0.946?1.024), suggesting this population has been declining at about 1.1% per year. The estimated growth rate in the Atlantic region is 1.010 (0.988?1.029), but there is some uncertainty about whether adult survival rates have been constant over the last 10 yr; using the mean survival rates from the most recent 5-yr period, the estimated growth rate in this region is 0.970 (0.938?0.998). Elasticity analysis indicates that the most effective management actions should seek to increase adult survival rates. Decomposition of the uncertainty in the growth rates indicates that uncertainty about population status can best be reduced through increased monitoring of adult survival rate.

Marine Mammal Science↗

USGS1998

Cruise report for a seismic investigation of gas hydrates in the Mississippi Canyon region, northern Gulf of Mexico: Cruise M1-98-GM

During June 1998, the U.S. Geological Survey (USGS) and the University of Mississippi Marine Minerals Technology Center (MMTC) conducted a 12-day cruise in the Mississippi Canyon region of the Gulf of Mexico (Fig. 1). The R/V Tommy Munro, owned by the Marine Research Institute of the University of Southern Mississippi, was chartered for the cruise. The general objective was to acquire very high resolution seismic-reflection data across of the upper and middle continental slope (200-1200-m water depths) to study the acoustic character, distribution and potential effects of gas hydrates within the shallow subsurface, extending from the sea floor down to the base of the gas-hydrate stability zone. The Gulf of Mexico is well known for hydrocarbon resources that include petroleum and related gases. Areas of the Gulf that lie in waters deeper than about 250 m potentially have conditions (e.g., pressure, temperature, near-surface gas content, etc.) that are right for the shallow-subsurface formation of the ice-like substance (gas and water) known as gas hydrate (Kvenvolden, 1993). Gas hydrates have previously been sampled in sea-floor cores and observed as massive mounds in several parts of the northern Gulf, including the Mississippi Canyon region (e.g., Anderson et al., 1992). Extensive seismic data have been recorded in the Gulf, in support of commercial drilling efforts, but few very high resolution data exist in the public domain to aid in gas-hydrate studies. Studies of long-term interest include those on the resource potential of gas hydrates, the geologic hazards associated with dissociation and formation of hydrates, and the impact, if any, of gas-hydrate dissociation on atmospheric warming (i.e., via release of methane, a "greenhouse" gas). Several very high resolution seismic systems (surface-towed, deep-towed, and sea-floor) were used during the cruise to test the feasibility of using such data for detailed structural (geometric) and stratigraphic (physical property) analyses based on the acoustic data. The cruise was conducted in two regions, on opposite flanks of the Mississippi Canyon, where gas hydrates are known and suspected from prior coring and seismic operations (e.g., Neurauter and Bryant, 1989). The regions are also characterized by thick surficial, relatively young (Pleistocene and younger) sediments. Swath-bathymetry data (Fig. 2) show extensive sea-floor faults, piercement features, and slumps—features whose development could potentially be related to gas hydrates. The specific objectives of the cruise were (a) to image the gas-hydrate stability zone across the continental margin to document bottom-simulating reflections (BSRs) and changes in geometry of the hydrate stability zone; (b) to image known hydrate features (with several seismic systems) to estimate physical properties for hydrate and non-hydrate areas; (c) to outline the shallow structures of the hydrate stability zone to ascertain their potential effects on the formation/distribution of hydrates and on stability of the sea floor; and (d) to estimate, if possible, the amounts of hydrates present in the shallow sub surface. During the cruise about 850 km of multichannel and single-channel seismic data were recorded. Seismic measurements at nine ocean-bottom seismometer (OBS) stations were recorded for several of the multichannel tracklines (see Fig. 3 in report). The following report describes the field operations and equipment systems employed, gives two examples of ship-board seismic records, and outlines a few preliminary results.

Open-File Report↗

USGS2012

Patterns in species richness and assemblage structure of native mussels in the Upper Mississippi River

1. To evaluate patterns in mussel assemblages in the Upper Mississippi River (UMR), data from systematic surveys of mussels conducted in three large reaches (Navigation Pools 5, 6, and 18) from 2005–2007 were analysed. 2. Nonmetric multi-dimensional scaling analyses and permutation tests indicated that assemblages differed among reaches. The mussel assemblage in Pool 18 was substantially different from the assemblage in Pool 5 and moderately different from the assemblage in Pool 6, whereas assemblages in Pools 5 and 6 were similar. Assemblages in broadly defined, flowing aquatic habitats did not substantially differ. 3. The dissimilarity of Pool 18 was primarily the result of Pool 18 having higher abundances of three Quadrula species (Q. quadrula, Q. pustulosa, and Q. nodulata), and lower abundances of Amblema plicata and Fusconaia flava. 4. Rarefaction analyses showed that species richness and species density were higher in Pool 18 compared with the other two pools. 5. Large-scale patterns in mussel assemblages may be related to other longitudinal trends in the system including geomorphology, water quality, and abundances of fish species that serve as hosts for glochidial larvae. 6. The results suggest that management goals and actions in the UMR may need to account for important differences in mussel assemblages that occur among reaches.

Illinois, Iowa, Minnesota, Wisconsin↗

USGS2009

Eocene total petroleum system — North and East of the Eocene West Side Fold Belt Assessment Unit of the San Joaquin Basin Province

The North and East of Eocene West Side Fold Belt Assessment Unit (AU) of the Eocene Total Petroleum System of the San Joaquin Basin Province comprises all hydrocarbon accumulations within the geographic and stratigraphic limits of this confirmed AU. Oil and associated gas accumulations occur in Paleocene through early middle Miocene marine to nonmarine sandstones found on the comparatively stable northeast shelf of the basin. The assessment unit is located north and east of the thickest accumulation of Neogene sediments and the west side fold belt. The area enclosed by the AU has been affected by only mild deformation since Eocene time. Traps containing known accumulations are mostly low-relief domes, anticlines, and up-dip basin margin traps with faulting and stratigraphic components. Map boundaries of the assessment unit are shown in figures 19.1 and 19.2; this assessment unit replaces the Northeast Shelf of Neogene Basin play 1006, the East Central Basin and Slope North of Bakersfield Arch play 1010, and part of the West Side Fold Belt Sourced by Pre-middle Miocene Rocks play 1005 considered by the U.S. Geological Survey (USGS) in their 1995 National Assessment (Beyer, 1996). Stratigraphically, the AU includes rocks from the uppermost crystalline basement to the topographic surface. In the region of overlap with the Central Basin Monterey Diagenetic Traps Assessment Unit, the North and East of Eocene West Side Fold Belt AU extends from basement rocks to the top of the Temblor Formation (figs. 19.3 and 19.4). In map view, the northern boundary of the assessment unit corresponds to the northernmost extent of Eocene-age Kreyenhagen Formation. The northeast boundary is the eastern limit of possible oil reservoir rocks near the eastern edge of the basin. The southeast boundary corresponds to the pinch-out of Stevens sand of Eckis (1940) to the south, which approximately coincides with the northern flank of the Bakersfield Arch (fig. 19.1). The AU is bounded on the southwest by the limit of major west side structural deformation and to the northwest by the San Andreas Fault and the limit of hydrocarbon-prospective strata in the Coast Ranges. As described by Gautier and others (this volume, chapter 2), existing oil fields in the San Joaquin Basin Province were assigned to assessment units based on the identified petroleum system and reservoir rocks in each field. Vallecitos oil field in the extreme northwest corner of the basin was assigned to the Eocene Total Petroleum System, because oil analyses conducted for this San Joaquin Basin assessment indicate that Eocene oil charged the reservoir rocks (Lillis and Magoon, this volume, chapter 9). Some literature classifies the Vallecitos oil field as part of the northernmost fold of the basin’s west side fold belt (see, for example, Rentschler, 1985; Bartow, 1991), but because of the oil field’s spatial separation and differing trend from the west side fold belt, Vallecitos field was considered here to be within the North and East of Eocene West Side Fold Belt Assessment Unit rather than in the other assessment unit in the Eocene Total Petroleum System, the Eocene West Side Fold Belt. Primary fields in the assessment unit are defined as those containing hydrocarbon resources greater than the USGS minimum threshold for assessment (0.5 million barrels of oil); secondary fields contain smaller volumes of oil but constitute a significant show of hydrocarbons.

California↗

USGS2017

High-resolution seismic characterization of the gas and gas hydrate system at Green Canyon 955, Gulf of Mexico, USA

The Pliocene and Pleistocene sediments at lease block Green Canyon 955 (GC955) in the Gulf of Mexico include sand-rich strata with high saturations of gas hydrate; these gas hydrate accumulations and the associated geology have been characterized over the past decade using conventional industry three-dimensional (3D) seismic data and dedicated logging-while-drilling (LWD) borehole data. To improve structural and stratigraphic characterization and to address questions of gas flow and reservoir properties, in 2013 the U.S. Geological Survey acquired high-resolution two-dimensional (2D) seismic data at GC955. Combined analysis of all available data improves our understanding of the geological evolution of the study area, which includes basin-scale migration of the Mississippi River sediment influx as well as local-scale shifting of sedimentary channels at GC955 in response to salt-driven uplift, structural deformation associated with the salt uplift, and upward gas migration from deeper sediments that charges the main gas hydrate reservoir and shallower strata. The 2D data confirm that the sand-rich reservoir is composed principally of sediments deposited in a proximal levee setting and that episodes of channel scour, interspersed with levee deposition, have resulted in an assemblage of many individual proximal levee deposit “pods” each with horizontal extent up to several hundred meters. Joint analysis of the 2D and 3D data reveals new detail of a complex fault network that controls the fluid-flow system; large east-west trending normal faults allow fluid flow through the reservoir-sealing fine-grained unit, and smaller north-south oriented faults provide focused fluid-flow pathways (chimneys) through the shallower sediments. This system has enabled the flow of gas from the main reservoir to the seafloor throughout the recent history at GC955, and its intricacies help explain the distributed occurrences of gas hydrate in the intervening strata.

Gulf of Mexico↗