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At least 1,711 records · Page 95Linked to original sources

USGS2024

Geologic input databases for the 2025 Puerto Rico – U.S. Virgin Islands National Seismic Hazard Model update: Crustal faults component

The last National Seismic Hazard Model (NSHM) for Puerto Rico and the U.S. Virgin Islands (PRVI) was published in 2003. In advance of the 2025 PRVI NSHM update, we created three geologic input databases to summarize new onshore and offshore fault source information in the northern Caribbean region between 62°–70° W and 16°–21° N. These databases, of fault sections, fault‐zone polygons, and geologic estimates of fault activity (fault‐slip rate and earthquake recurrence intervals) at specific sites, document updates to fault parameters used in prior seismic hazard models in PRVI. Fault sources were reviewed from published studies since 2003, which document substantial changes to the understanding of fault location, geometry, or activity. New fault section sources were added for features that meet the criteria of (1) length ≥7 km, (2) unequivocal evidence of recurrent tectonic Quaternary activity, and (3) documentation that is publicly available in a peer‐reviewed source. In addition, we revised several broad areal sources, such as the Mona and Anegada extensional zones. The 2003 model included three fault sections and two fault‐zone polygons (areal sources). These databases include 35 fault sections, 6 fault‐zone polygons, and 51 earthquake geology sites. To characterize fault activity rates, slip‐rate bins were assigned based on landscape expression and paleoseismic trench observations for faults without published slip‐rate sites. Additional fault sources were evaluated but not included in these databases due to a lack of published information about fault location, geometry, or recurrent Quaternary activity. The PRVI NSHM 2025 geologic input databases describe crustal faulting; the geometries and coupling of Puerto Rico subduction zone and Muertos Trough models are considered in a separate database. Updates to the fault sections, fault‐zone polygons, and earthquake geology databases can help inform the location and recurrence rate of damaging earthquakes in the PRVI NSHM implementation.

Puerto Rico, U.S. Virgin Islands↗

USGS2026

The Cascadia Region Earthquake Science Center (CRESCENT): Advancing understanding of Cascadia's earthquake hazards

The Cascadia Subduction Zone (CSZ) is capable of producing large megathrust earthquakes and tsunamis, posing significant risks to the Pacific Northwest. This paper examines the seismic characteristics of the CSZ, including its offshore coupling, sparse seismicity, and a complex paleoseismic record. While recent advances in geophysical methods and data collection have improved our understanding of subduction zone processes, important uncertainties remain regarding rupture dynamics, ground shaking, and tsunami impacts. The Cascadia Region Earthquake Science Center (CRESCENT) has been established to address these challenges through coordinated research, education, and community engagement. By integrating scientific studies, fostering collaboration, and leveraging new technologies, CRESCENT seeks to improve hazard assessment and resilience in the region. This paper outlines the primary challenges and opportunities in advancing subduction zone science and enhancing societal preparedness for future CSZ events.

California, Oregon, Washington↗

USGS2025

Earthquake recurrence estimates for northern Caribbean faults from combinatorial optimization

We use combinatorial optimization to find the optimal spatial distribution of random samples of earthquakes (≥6.5) that minimize the misfit in target slip rates for all faults in the northeast Caribbean, and we derive magnitude-frequency relationships with uncertainties for these faults. Slip rates for many faults are derived from GPS block models, not direct measurements, because of their underwater locations. Predicted recurrence rates for eastern Hispaniola and Puerto Rico Trench faults are 220-450 yr for M7 and 3-5 kyr for M8, with maximum feasible magnitude of M8.2. The most frequent earthquakes with magnitudes ≥7.0 are predicted on the large upper plate strike-slip faults, Enriquillo (EF) and Septentrional Fault, commensurate with the historical record. Calais et al. (2023) suggested that shortening in western Hispaniola is accommodated on the offshore Jérémie and onshore Malpasse faults north and south of EF, instead of on terrestrial faults in western Hispaniola and EF. Because of our system-modeling approach, such a configuration predicts less frequent earthquakes on EF and on western Hispaniola and Muertos convergent zones. Recurrence times of a few 100s yr for M6.7 earthquakes is predicted on the submerged faults in Mona Passage, and infrequent M>7 earthquakes are predicted on the Virgin Islands faults.

Lesser Antilles↗

USGS1986

Crustal structure of Oaxaca, Mexico, from seismic refraction measurements

Seismic refraction and gravity data have been analyzed to obtain a model of the compressional-wave velocity structure of the ocean-to-continent transition in the State of Oaxaca in southwestern Mexico. Crustal thickness on the continent at the latitude 18°N is 45 ± 4 km, based on reflected phases from the Moho discontinuity. The crust has been modeled with three layers, with velocities of 4.3 to 4.6, 5.0 to 5.7, and 6.85 to 7.0 km/sec, each with positive velocity gradient. The crust thins to 10 km at the coast near Pinotepa Nacional, where Precambrian metamorphic rocks are exposed 45 km from the mid-America trench. Offshore, the oceanic structure consists of an 8-km-thick crust with a normal crustal velocity structure (Spudich and Orcutt, 1980). The apparent dip of the subducting plate beneath western Mexico is 10°. On the oceanic side, strong reflections suggest a minimum depth of 35 km for the lithosphere-asthenosphere boundary. The asthenosphere has a seismic velocity of 7.6 km/sec, and a thin lid in which the velocity is 8.6 km/sec.

Oaxaca↗

USGS2003

Movements of Gulf sturgeon (Acipenser oxyrinchus desotoi) in nearshore habitat as determined by acoustic telemetry

Gulf sturgeon were tagged with telemetry tags and were tracked and relocated in fall and early winter of 1996 and 1998 to determine migration patterns and winter feeding habitats after they emigrated from the Suwannee River, Florida, into the Gulf of Mexico. We hypothesized that their migration would generally follow the drowned Suwannee River channel across the West Florida shelf. Fish left the river in late Oct. or early Nov., about the time river water temperatures fell below 20 C. Tracked and relocated fish moved slowly and remained offshore of Suwannee Sound in nearby shallow (<6 m) marine-estuarine habitats until at least mid or late Dec. The relatively small area (~115km 2 ) within which fish were consistently relocated in 1998 probably is a critically important feeding habitat because adult Gulf sturgeon, which do not feed while in the river, occupy it for up to half their short (4-5 mo) marine residency. The fish left the area in late Dec. or early Jan., most likely in response to powerful cold front-generated weather conditions (under which, boat-based acoustic tracking is infeasible). A large (1,760 km 2 ) adjacent area was searched for sonic-tagged sturgeon in early Jan. 1999, but only one was relocated (~50 km northwest of the Nov.-Dec. area). Although we were unable to address the hypothesis that their migration follows the Suwannee paleochannel, the results do indicate that Gulf sturgeon move to yet unknown, distant, late-winter feeding areas of the Gulf of Mexico before returning to the river in spring.

Florida↗

USGS2011

Polar bear population status in the northern Beaufort Sea, Canada, 1971-2006

Polar bears ( Ursus maritimus ) of the northern Beaufort Sea (NB) population occur on the perimeter of the polar basin adjacent to the northwestern islands of the Canadian Arctic Archipelago. Sea ice converges on the islands through most of the year. We used open-population capture–recapture models to estimate population size and vital rates of polar bears between 1971 and 2006 to: (1) assess relationships between survival, sex and age, and time period; (2) evaluate the long-term importance of sea ice quality and availability in relation to climate warming; and (3) note future management and conservation concerns. The highest-ranking models suggested that survival of polar bears varied by age class and with changes in the sea ice habitat. Model-averaged estimates of survival (which include harvest mortality) for senescent adults ranged from 0.37 to 0.62, from 0.22 to 0.68 for cubs of the year (COY) and yearlings, and from 0.77 to 0.92 for 2–4 year-olds and adults. Horvtiz-Thompson (HT) estimates of population size were not significantly different among the decades of our study. The population size estimated for the 2000s was 980 ± 155 (mean and 95% CI). These estimates apply primarily to that segment of the NB population residing west and south of Banks Island. The NB polar bear population appears to have been stable or possibly increasing slightly during the period of our study. This suggests that ice conditions have remained suitable and similar for feeding in summer and fall during most years and that the traditional and legal Inuvialuit harvest has not exceeded sustainable levels. However, the amount of ice remaining in the study area at the end of summer, and the proportion that continues to lie over the biologically productive continental shelf (<300 m water depth) has declined over the 35-year period of this study. If the climate continues to warm as predicted, we predict that the polar bear population in the northern Beaufort Sea will eventually decline. Management and conservation practices for polar bears in relation to both aboriginal harvesting and offshore industrial activity will need to adapt.

Ecological Applications↗

USGS2014

Nearshore energy subsidies support Lake Michigan fishes and invertebrates following major changes in food web structure

Aquatic food webs that incorporate multiple energy channels (e.g. nearshore benthic or pelagic) with varying productivity and turnover rates convey stability to biological communities by providing multiple independent energy sources. Within the Lake Michigan food web, invasive dreissenid mussels have caused rapid changes to food web structure and potentially altered the channels through which consumers acquire energy. We used stable C and N isotopes to determine how Lake Michigan food web structure has changed in the past decade, coincident with the expansion of dreissenid mussels, decreased pelagic phytoplankton production and increased nearshore benthic algal production. Fish and invertebrate samples collected from sites around Lake Michigan were analyzed to determine taxa-specific 13C:12C (delta 13C) and 15N:14N (delta 15N) ratios. Sampling took place during two distinct periods, 2002-2003 and 2010-2012, that spanned the period of dreissenid expansion, and included nearshore, pelagic and profundal fish and invertebrate taxa. Magnitude and direction of the 13C shift indicated significantly greater reliance upon nearshore benthic energy sources among nearly all fish taxa as well as profundal invertebrates. Although the mechanisms underlying this 13C shift likely varied among species, possible causes include the transport of benthic algal production to offshore waters and an increased reliance on nearshore prey items. Delta 15N shifts were more variable and of smaller magnitude across taxa although declines in delta 15N among some pelagic fishes may indicate a shift to alternative prey resources. Lake Michigan fishes and invertebrates appear to have responded to dreissenid induced changes in nutrient and energy pathways by switching from pelagic to alternative nearshore energy subsidies. Although large shifts in energy allocation (i.e. pelagic to nearshore benthic) resulting from invasive species appear to have affected total production at upper trophic levels, changes in trophic structure and utilization of novel energy pathways may help to stabilize food webs following species invasions.

Lake Michigan↗

USGS2017

Ecology of the Sand Roller (Percopsis transmontana) in a lower Snake River Reservoir, Washington

The Sand Roller ( Percopsis transmontana ), has not been abundant in the Snake River since it was first found in the system in the 1950s, but its population has apparently increased in recent years. As a result, we initiated a study to better understand its ecology in habitats of Lower Granite Reservoir. From November 2014 to October 2015, Sand Rollers were present along shorelines, with peak abundance being observed during spring months. Logistic regression analyses showed that Sand Rollers were more likely to be present in shoreline habitats at temperatures ≤18.4°C. Fish were found over a range of substrates, with the lowest odds of fish presence being associated with riprap, which is common in hydropower reservoirs. From length-frequency analysis, we suggest that Sand Roller spawning occurs primarily in May and early June. Assessment of Sand Roller diets found dipteran (chironomid) larvae and pupae were the most important prey consumed by all sizes of Sand Rollers, but Opossum Shrimp ( Neomysis mercedis ) were also prominent in diets of larger fish in shoreline and offshore habitats. At a time when the populations of so many native species are in decline, the increase of the Sand Roller population in the lower Snake River represents a positive, yet curious occurrence.

Washington↗

USGS2006

High-resolution stratigraphy of a Mississippi subdelta-lobe progradation in the Barataria Bight, north-central Gulf of Mexico

The coastal zone of southeastern Louisiana is the product of numerous cycles of progradation, abandonment, and marine transgression of the Mississippi River delta. Currently, the shoreline in the Barataria Bight is undergoing significant erosion and retreat, and understanding its evolution is crucial in stabilization efforts. This study uses an extensive collection of geophysical and sediment core data from Barataria Bay and offshore to develop a geologic model of the shallow (< 10 m) subsurface. The purpose of the model is twofold: (1) establish the stratigraphic architecture of a subdelta lobe of the Bayou des Families delta, deposited by the Mississippi River approximately 4000 years before present; and (2) provide a high-resolution description of the geologic framework in a context that can be applied to coastal management issues in similar fluvially dominated coastal environments worldwide. The results of the study demonstrate how high-quality geologic data from the coastal environment can be used not only to further our understanding of shoreline evolution but also to provide pertinent information for coastal management needs.

Louisiana↗

USGS2015

Fluvial-aeolian interactions in sediment routing and sedimentary signal buffering: an example from the Indus Basin and Thar Desert

Sediment production and its subsequent preservation in the marine stratigraphic record offshore of large rivers are linked by complex sediment-transfer systems. To interpret the stratigraphic record it is critical to understand how environmental signals transfer from sedimentary source regions to depositional sinks, and in particular to understand the role of buffering in obscuring climatic or tectonic signals. In dryland regions, signal buffering can include sediment cycling through linked fluvial and eolian systems. We investigate sediment-routing connectivity between the Indus River and the Thar Desert, where fluvial and eolian systems exchanged sediment over large spatial scales (hundreds of kilometers). Summer monsoon winds recycle sediment from the lower Indus River and delta northeastward, i.e., downwind and upstream, into the desert. Far-field eolian recycling of Indus sediment is important enough to control sediment provenance at the downwind end of the desert substantially, although the proportion of Indus sediment of various ages varies regionally within the desert; dune sands in the northwestern Thar Desert resemble the Late Holocene–Recent Indus delta, requiring short transport and reworking times. On smaller spatial scales (1–10 m) along fluvial channels in the northern Thar Desert, there is also stratigraphic evidence of fluvial and eolian sediment reworking from local rivers. In terms of sediment volume, we estimate that the Thar Desert could be a more substantial sedimentary store than all other known buffer regions in the Indus basin combined. Thus, since the mid-Holocene, when the desert expanded as the summer monsoon rainfall decreased, fluvial-eolian recycling has been an important but little recognized process buffering sediment flux to the ocean. Similar fluvial-eolian connectivity likely also affects sediment routing and signal transfer in other dryland regions globally.

Indus Basin, Thar Desert↗

USGS2018

The tectonically controlled San Gabriel Channel–Lobe Transition Zone, Catalina Basin, Southern California Borderland

High-resolution geophysical data across the Catalina Basin, offshore southern California, USA, reveal a complex channel–lobe transition zone (CLTZ) and provide an opportunity to characterize an entire seafloor CLTZ in a tectonically active and confined-basin setting. The seafloor morphology, distribution of depositional and erosional features, and location of depocenters in the CLTZ are controlled by shifting confinement and seafloor gradient related to inherited basement structures, active faults, and basin margins. Below a Holocene hemipelagic drape, the Catalina Basin is dominated by CLTZ and lobe sedimentation from the San Gabriel Channel, with lesser accumulations from local sediment sources limited to basin margins. The San Gabriel Channel is structurally confined as it enters the Catalina Basin and appears unable to avulse; it continues into the basin as a channel that rapidly widens, decreases in relief, and becomes scoured at its margins. A CLTZ is imaged between the confined San Gabriel channel and its terminal lobes deposited > 50 km into the basin. Narrow, apparently disconnected channels with knickpoints occur throughout the proximal and mid-CLTZ and are concentrated near basement highs and basin-bounding Quaternary-active dextral strike-slip faults. A field of small-scale erosional crescent-shaped scours (∼ 100 m length, ∼ 200 m width, up to ∼ 10 m relief across ∼ 30 km 2 region) occurs above a partially buried basement high that creates perturbations in seafloor gradient. Likewise, above a buried basement structure that locally increases seafloor gradient (up to 0.4°), the distal CLTZ may contain sediment waves (∼ 2–4 m wave height and ∼ 200–300 m wavelength) that are smaller than many other CLTZ examples. This study of the San Gabriel CLTZ in Catalina Basin provides high-resolution geophysical data coverage of a complete CLTZ and illustrates a tectonically controlled end-member CLTZ from the modern seafloor.

Journal of Sedimentary Research↗

USGS2026

An integrated mudstone facies classification scheme and revised interpretation of the sedimentologic processes driving carbon burial in the Cenomanian–Turonian Greenhorn Formation, Colorado, U.S.A.

Standardizing facies descriptions has proven key to integrating interpretations of depositional processes and environments from sedimentologic observations with geochemistry data for mudstone lithologies. Because of their fine-grained nature, high degree of compaction, and heterogeneous composition, standardizing methods for mudstone descriptions has proven difficult, but it is critical to formulating meaningful interpretations of the processes that govern the accumulation of organic-rich lithologies and their role in both petroleum systems and the global carbon cycle. In this study, we have developed a modified facies classification scheme for mudstone lithologies that incorporates sedimentologic and compositional observation at the hand-sample and thin-section scales with geochemical measurements, including bulk organic and inorganic geochemistry, to characterize these rocks and their variability more completely for improved interpretations of depositional environments during a low-order sea-level transgression. The facies described in this study are of the Cenomanian–Turonian Greenhorn Formation in the USGS #1 Portland Core drilled in Fremont County, Colorado. Strata of the Greenhorn Formation span Oceanic Anoxic Event 2 (OAE-2) and the preceding interval. Lithologies range from organic-rich argillaceous mudstones with varied sedimentary structures to organic-lean, highly bioturbated limestones. Six facies were identified, each differentiated by varied sedimentary structures and geochemical composition. These facies occur in a predictable stratigraphic stacking pattern that represents a low-order sea-level transgression with interpreted depositional environments ranging from terrigenous-dominated pro-delta and muddy continental shelf at the base of the interval to pelagic offshore marine at the top of the Greenhorn Formation. Though the facies are consistent with previous interpretations of depositional environments at this locale in the Cretaceous Western Interior Seaway during the Greenhorn cyclothem, the sedimentary processes governing the accumulation of organic-rich strata that have defined this interval are significantly revised. Variability in the proximity and intensity of bottom currents driven by storms and geostrophic flows were key to the accumulation of each facies, with significant sediment transport occurring even through deposition in the most oxygen-depleted bottom waters. The methodology and interpretations provided here are now being employed to basin-scale predictions of organic enrichment utilizing calibrated petrophysical methods. The approach and results from this study improve understanding of how organic and inorganic carbon was sequestered during perturbations to the global carbon cycle associated with events such as OAE-2.

Colorado↗

USGS2008

Sand composition and transport history on a fringing coral reef, Molokai, Hawaii

Composition of sand grains from the beaches, reef flat, and fore reef of south Molokai, Hawaii, provides key information about the origin and transport history of sediment on the reef and adjacent beach. The most common grain types include coralline algae, coral, chemically altered carbonate, and siliciclastic grains. Minor components include calcareous algal plates (Halimeda), mollusk fragments, and foraminifera; in selected areas, these components are abundant. Similarities in composition indicate that sand grains are freely transported between the beach and the reef-flat environments, whereas the fore reef has limited exchange of sand with either the beach or the reef flat. Overall, the calcium carbonate fraction of the sand, silt, and clay increases with distance offshore from 400 to 650 m, where the percentages plateau and remain relatively stable across the fore reef. The calcium carbonate content of bottom sediment indicates that sand transport on the reef flat is generally shore parallel with little shore-normal movement. This study has implications for management of beaches and coastal resources along fringing coral reefs in that it documents the effectiveness of the wide reef flat and reef crest in blocking sand transport with the fore reef. It also indicates that sediment introduced to the inner reef flat is not quickly dispersed seaward and therefore has a relatively high residence time in that setting.

Conference Paper↗

USGS2004

Influence of near-surface stratigraphy on coastal landslides at Sleeping Bear Dunes National Lakeshore, Lake Michigan, USA

Lake-level change and landslides are primary controls on the development of coastal environments along the coast of northeastern Lake Michigan. The late Quaternary geology of Sleeping Bear Dunes National Lakeshore was examined with high-resolution seismic reflection profiles, ground-penetrating radar (GPR), and boreholes. Based on sequence-stratigraphic principles, this study recognizes ten stratigraphic units and three major unconformities that were formed by late Pleistocene glaciation and postglacial lake-level changes. Locally high sediment supply, and reworking by two regressions and a transgression have produced a complex stratigraphy that is prone to episodic failure. In 1995, a large landslide deposited approximately 1 million m3 of sediment on the lake floor. The highly deformed landslide deposits, up to 18 m thick, extend 3-4 km offshore and unconformably overlie well-stratified glacial and lacustrine sediment. The landslide-prone bluff is underlain by channel-fill deposits that are oriented nearly perpendicular to the shoreline. The paleochannels are at least 10 m deep and 400 m wide and probably represent stream incision during a lake-level lowstand about 10.3 ka B.P. The channels filled with sediment during the subsequent transgression and lake-level highstand, which climaxed about 4.5 ka B.P. As lake level fell from the highstand, the formation of beach ridges and sand dunes sealed off the channel and isolated a small inland lake (Glen Lake), which lies 5 m above the level of Lake Michigan and may be a source of piped groundwater. Our hypothesis is that the paleochannels act as conduits for pore water flow, and thereby locally reduce soil strength and promote slope failure.

Michigan↗

USGS2017

Coastal eolian sand-ramp development related to paleo-sea-level changes during the Latest Pleistocene and Holocene (21–0 ka) in San Miguel Island, California, U.S.A.

Coastal eolian sand ramps (5–130 m elevation) on the northern slope (windward) side of the small San Miguel Island (13 km in W-E length) range in age from late Pleistocene to modern time, though a major hiatus in sand-ramp growth occurred during the early Holocene marine transgression (16–9 ka). The Holocene sand ramps (1–5 m measured thicknesses) currently lack large dune forms, thereby representing deflated erosional remnants, locally covering thicker late Pleistocene sand-ramp deposits. The ramp sand was initially supplied from the adjacent island-shelf platform, extending about 20 km north of the present coastline. The sand-ramp deposits and interbedded loess soils were 14 C dated using 112 samples from 32 archaeological sites and other geologic sections. Latest Pleistocene sand ramps (66–18 ka) were derived from across-shelf eolian sand transport during marine low stands. Shoreward wave transport supplied remobilized late Pleistocene sand from the inner shelf to Holocene beaches, where dominant NW winds supplied sand to the sand ramps. The onset dates of the sand-ramp deposition in San Miguel are 7.2 ± 1.5 ka (sample n = 14). The internal strata dates in the vertically accreting sand ramps are 3.4 ± 1.7 ka ( n = 34). The sand ramps in San Miguel show wide-scale termination of sand supply in the latest Holocene time. The sand-ramp top dates or burial dates are 1.7 ± 0.9 ka ( n = 28). The latest Holocene sand ramps are truncated along most of the island's northern coastline, indicating recent losses of nearshore sand reserves to onshore, alongshore, and, possibly, offshore sand sinks. The truncated sand ramps in San Miguel Island and in other sand-depleted marine coastlines provide warnings about future beach erosion and/or shoreline retreat from accelerated sea-level rise accompanying predicted global warming.

California↗

USGS2016

Topobathymetric elevation model development using a new methodology: Coastal National Elevation Database

During the coming decades, coastlines will respond to widely predicted sea-level rise, storm surge, and coastalinundation flooding from disastrous events. Because physical processes in coastal environments are controlled by the geomorphology of over-the-land topography and underwater bathymetry, many applications of geospatial data in coastal environments require detailed knowledge of the near-shore topography and bathymetry. In this paper, an updated methodology used by the U.S. Geological Survey Coastal National Elevation Database (CoNED) Applications Project is presented for developing coastal topobathymetric elevation models (TBDEMs) from multiple topographic data sources with adjacent intertidal topobathymetric and offshore bathymetric sources to generate seamlessly integrated TBDEMs. This repeatable, updatable, and logically consistent methodology assimilates topographic data (land elevation) and bathymetry (water depth) into a seamless coastal elevation model. Within the overarching framework, vertical datum transformations are standardized in a workflow that interweaves spatially consistent interpolation (gridding) techniques with a land/water boundary mask delineation approach. Output gridded raster TBDEMs are stacked into a file storage system of mosaic datasets within an Esri ArcGIS geodatabase for efficient updating while maintaining current and updated spatially referenced metadata. Topobathymetric data provide a required seamless elevation product for several science application studies, such as shoreline delineation, coastal inundation mapping, sediment-transport, sea-level rise, storm surge models, and tsunami impact assessment. These detailed coastal elevation data are critical to depict regions prone to climate change impacts and are essential to planners and managers responsible for mitigating the associated risks and costs to both human communities and ecosystems. The CoNED methodology approach has been used to construct integrated TBDEM models in Mobile Bay, the northern Gulf of Mexico, San Francisco Bay, the Hurricane Sandy region, and southern California.

Journal of Coastal Research↗

USGS2005

Mercury- and silver-rich ferromanganese oxides, southern California Borderland: Deposit model and environmental implications

Mercury- and silver-enriched ferromanganese oxide crusts were recovered at water depths of 1,750 tol,300 m from La Victoria knoll, located about 72 km off the coast of northern Baja California. No other ferromanganese precipitate found so far in the modern ocean basins is similarly enriched in Hg and Ag. The precipitates consist of submetallic gray, brecciated, Mn oxide layers overlain by brown earthy, laminated Fe-Mn oxide crusts. Both oxide types are rich in Hg (to 10 ppm) and Ag (to 5.5 ppm). The Mn-rich layers are composed of ??MnO2, with lesser amounts of 10A?? and 7A?? manganates, whereas the Mn phase in the Fe-Mn crusts is solely ??MnO2. The Fe phase in both layers is X-ray amorphous. Established criteria for distinguishing hydrothermal versus hydrogenetic crusts indicate that the Mn-rich layers are predominantly of low-temperature hydrothermal origin, whereas the Fe-Mn crusts are hydrogenetic, although there is some overlap in the source of chemical components in both types. La Victoria knoll is uplifted continental basement rock with basalt, andesite, and schist cropping out at the surface; the knoll may have an intrusive core. The Hg and Ag were derived from leaching by hydrothermal fluids of organic matter-rich sediments in basins adjacent to La Victoria knoll and, to a lesser extent, from continental basement rocks underlying the knoll and adjacent basins. Both rock types are notably enriched in Ag and Hg. Faults were the main fluid transport pathway, and hydrothermal circulation was driven by high heat flow associated with thinned crust. Other elements derived from the hydrothermal fluids include Tl, Cd, Cr, and Li. The main host for Hg and Ag is FeOOH, although MnO2 likely hosts some of the Ag. Minor sulfide and barite also may contain small amounts of these metals. Possible analogs in the geologic record for this deposit type are found in the Basin and Range province of the western United States and Mexico. The discovery highlights the fact that fluids circulating along faults in the offshore California borderland are transporting potentially toxic metals (Hg, Ag, Tl, As, Cd, Cr, Pb, and Ni) and depositing them on and just below the ocean floor. ?? 2005 Society of Economic Geologists, Inc.

Economic Geology↗

USGS1954

Sedimentary facies of iron-formation

The sedimentary iron -formations of Precambrian age in the Lake Superior region can be divided on the basis of the dominant original iron mineral into four principal facies : sulfide, carbonate, oxide, and silicate. As chemical sediments, these rocks reflect certain aspects of the chemistry of the depositional environments. The major control, at least for the sulfide, carbonate, and oxide types, probably was the oxidation potential. The evidence indicates that deposition took place in restricted basins, which were separated from the open sea by thresholds that inhibited free circulation and permitted development of abnormalities in oxidation potential and water composition. The sporadic distribution of metamorphism and of later oxidation permits description of the primary facies on the basis of unoxidized, essentially unmetamorphosed material. The sulfide facies is represented by black slates in which pyrite may make up as much as 40 percent of the rock. The free-carbon content of these rocks typically ranges from 5 to 15 percent, indicating that ultra-stagnant conditions prevailed during deposition. Locally, the pyritic rocks contain layers of iron -rich carbonate. The carbonate facies consists, in its purer form, of interbedded iron -rich carbonate and chert. It is a product of an environment in which oxygen concentration was sufficiently high to destroy most of the organic material but not high enough to permit formation of ferric compounds. The oxide facies is found as two principal types, one characterized by magnetite and the other by hematite. Both minerals appear to be of primary origin. The magnetite-banded rock is one of the dominant lithologies in the region; it consists typically of magnetite interlayered with chert, carbonate, or iron silicate, or combinations of the three. Its mineralogy and association suggest origin under weakly oxidizing to moderately reducing conditions, but the mode of precipitation of magnetite is not clearly understood. The hematite-banded rocks consist of finely crystalline hematite interlayered with chert or jasper. Oolitic structure is common. This facies doubtless accumulated in a strongly oxidizing, probably near-shore, environment similar to that in which younger hema-titic ironstones such as the Clinton oolite were deposited. The silicate facies contains one or more of the hydrous ferrous silicates (greenalite, minnesotaite, stilpnomelane, chlorite) as a major constituent. Granule structure, similar to that of glauconite, is typical of some varieties; others are nongranular and finely laminated. The most common association of the silicate rocks is with either carbonate- or magnetite-bearing rocks, which suggests that the optimum conditions for deposition ranged from slightly oxidizing to slightly reducing. The relationship between the iron -rich rocks and volcanism, stressed by many authors, is considered by the writer to be structural, not chemical: in the Lake Superior region both iron -deposition and volcanism are believed to be related to geosynclinal development during Huronian time. In Michigan, the lower Huronian rocks are iron -poor quartzite and dolomite-typical "stable-shelf" deposits; much of the upper Huronian consists of iron -poor graywacke and slate with associated volcanic rocks -a typical "geosynclinal" assemblage. Thus the iron -rich beds of the middle Huronian and lower part of the upper Huronian were deposited during a transitional stage in structural history. The major environmental requirement for deposition of iron - formation is the closed or restricted basin; this requirement coincides in time with what would be a normal stage in evolution of the geosyncline: namely, structural development of offshore buckles or swells that subsequently develop into island arcs characterized by volcanism.

Lake Superior↗