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At least 289 records · Page 16Linked to original sources
USGS2008

Trace fossils and sedimentary facies from a Late Cambrian‐Early Ordovician tide‐dominated shelf (Santa Rosita Formation, northwest Argentina): Implications for ichnofacies models of shallow marine successions

The Santa Rosita Formation is one the most widely distributed lower Paleozoic units of northwest Argentina. At the Quebrada del Salto Alto section, east of Purmamarca, Jujuy Province, it is represented by four sedimentary facies: thick‐bedded planar cross‐stratified quartzose sandstones (A), thin‐bedded planar cross‐stratified quartzose sandstones and mudstones (B), wave‐rippled sandstones and bioturbated mudstones (C), and black and greenish gray shales (D). Paleocurrent data, sandstone architecture, and sedimentary structures from faciès A and B indicate bipolar/bimodal paleoflows, suggesting the action of tidal currents. The succession is interpreted as that of a tide‐dominated shelf, with only secondary influence of wave processes. Trace fossils are restricted to facies B and C. The Cruziana ichnocoenosis is preserved on the soles of thin‐bedded planar cross‐stratified quartzose sandstones (faciès B). This ichnocoenosis consists of Conostichus isp., Cruziana omanica, C. semiplicata, C. cf. tortworthi, Cruziana isp. Helminthopsis abeli, Monomorphichnus bilinearis, M. multilineatus, Palaeophycus tubularis, Rusophycus carbonarias, R. latus , and R. isp. The occurrence of Cruziana semiplicata , C. omanica, C. cf. tortworthi , and Rusophycus latus supports a Late Cambrian‐Tremadoc age. Slabbing of Cruziana shows complex interactions between biologic and sedimentologic processes, and suggests a predominance of exhumed traces, washed out and recast by tractive sand deposition. Sandstone soles are densely packed with biogenic structures and exhibit distinctive clusters of Rusophycus isp. that most likely represent trilobite nesting burrows. The Cruziana ichnocoenosis records the resident fauna of a protected, lower intertidal to subtidal interbar setting. The Skolithos ichnocoenosis is represented by high to low density vertical burrows of Skolithos linearis , which extend downwards to the quartzose sandstone soles of faciès B and cross the Cruziana ichnocoenosis. The Skolithos ichnocoenosis represents colonization by suspension‐feeding organisms following a major change in environmental conditions, related to the migration of lower intertidal to subtidal sandwaves. The Planolites ichnocoenosis consists exclusively of Planolites montanus within mudstones overlying wave‐rippled sandstones (facies C). The Planolites ichnocoenosis records opportunistic colonization by inf aunal deposit feeders that mined the organic‐rich fine‐grained sediment during the waning phase of storms that scoured organic detritus from the sea bottom. The section records, from base to top, a Cruziana‐Skolithos ichnofacies zone, a Skolithos ichnofacies zone and an unbioturbated zone typified by the thick‐bedded cross‐stratified quartzose sandstone (fades A). This trend reflects progressively higher energy conditions linked to the establishment of a large sand wave complex. The presence of a mixed Cruziana‐Skolithos ichnofacies in the lower interval reflects changes in substrate and energy levels, rather than water depth. Accordingly, contrasting ichnocoenoses from interbars (Cruziana) and sandwaves (Skolithos) must be considered an example of ichnofacies controlled by local parameters instead of general bathymétrie trends. Conversely, the vertical replacement of the Cruziana ichnofacies by the Skolithos ichnofacies towards the middle interval of the section reflects the environmental changes associated with the transition between the intertidal and subtidal zones. As overall tidal energy increases from supratidal to subtidal settings, the Skolithos ichnofacies tends to occur seaward of the Cruziana ichnofacies in tide‐dominated shallow marine environments. Therefore, onshore‐offshore ichnofacies replacement in tide‐dominated shallow seas is opposite to that in wave‐dominated marine settings.

Ichnos: An International Journal for Plant and Ani↗
USGS2017

Selective transport of palynomorphs in marine turbiditic deposits: An example from the Ascension-Monterey Canyon system offshore central California

The pollen assemblage of a deep-sea core (15G) collected at lower bathyal depths (3491 m) on a levee of Monterey Canyon off central California was investigated to gain insights into the delivery processes of terrigenous material to submarine fans and the effect this transport has on the palynological record. Thirty-two samples were obtained down the length of the core, 19 from hemipelagic and mixed mud deposits considered to be the background record, and 13 others from displaced flow deposits. The pollen record obtained from the background samples documents variations in the terrestrial flora as it adapted to changing climatic conditions over the last 19,000 cal yrs BP. A Q-mode cluster analysis defined three pollen zones: a Glacial Pollen Zone (ca. 20,000–17,000 cal yr BP), an overlying Transitional Pollen Zone (ca. 17,000–11,500 cal yr BP), and an Interglacial Pollen Zone (ca. 11,500 cal yr BP to present). Another Q-mode cluster analysis, of both the background mud and flow deposits, also defined these three pollen zones, but four of the 13 turbiditic deposits were assigned to pollen zones older than expected by their stratigraphic position. This was due to these samples containing statistically significant fewer palynomorphs than the background muds as well as being enriched (∼10–35% in some cases) in hydraulically-efficient Pinus pollen. A selective bias in the pollen assemblage, such as demonstrated here, may result in incorrect interpretations (e.g., climatic shifts or environmental perturbations) based on the floral record, indicating turbiditic deposits should be avoided in marine palynological studies. Particularly in the case of fine-grained flow deposits that may not be visually distinct, granulometry and grain size frequency distribution curves may not be enough to identify these biased deposits. Determining the relative abundance and source of displaced shallow-water benthic foraminifera entrained in these sediments serves as an excellent additional tool to do so.

California↗
USGS2013

The geologic records of dust in the Quaternary

Study of geologic records of dust composition, sources and deposition rates is important for understanding the role of dust in the overall planetary radiation balance, fertilization of organisms in the world’s oceans, nutrient additions to the terrestrial biosphere and soils, and for paleoclimatic reconstructions. Both glacial and non-glacial processes produce fine-grained particles that can be transported by the wind. Geologic records of dust flux occur in a number of depositional archives for sediments: (1) loess deposits; (2) lake sediments; (3) soils; (4) deep-ocean basins; and (5) ice sheets and smaller glaciers. These archives have several characteristics that make them highly suitable for understanding the dynamics of dust entrainment, transport, and deposition. First, they are often distributed over wide geographic areas, which permits reconstruction of spatial variation of dust flux. Second, a number of dating methods can be applied to sediment archives, which allows identification of specific periods of greater or lesser dust flux. Third, aeolian sediment particle size and composition can be determined so that dust source areas can be ascertained and dust transport pathways can be reconstructed. Over much of the Earth’s surface, dust deposition rates were greater during the last glacial period than during the present interglacial period. A dustier Earth during glacial periods is likely due to increased source areas, greater aridity, less vegetation, lower soil moisture, possibly stronger winds, a decreased intensity of the hydrologic cycle, and greater production of dust-sized particles from expanded ice sheets and glaciers.

Aeolian Research↗
USGS2008

Anatomy and dynamics of a floodplain, Powder River, Montana, U.S.A.

Centimeter-scale measurements on several Powder River floodplains provide insights into the nature of overbank depositional processes that created the floodplains; during a 20-year period after a major flood in 1978. Rising stages initially entered across a sill at the downriver end of the floodplains. Later, as stages continued to rise, water entered the floodplains through distinct low saddles along natural levees. The annual maximum depth of water over the levee crest averaged 0.19 in from 1983 through 1996, and the estimated flow velocities were approximately 0.15 m s-1. Water ponded in the floodplain trough, a topographic low between the natural levee and the pre-flood riverbank, and mud settled as thin layers of nearly constant thickness. Mud layers alternated with sand layers, which were relatively thick near the channel. Together, these beds created a distinctive natural levee. In some locations, individual flood deposits began as a thin mud layer that gradually coarsened upwards to medium-grained sand. Coarsening-upwards sequences form initially as mud because only the uppermost layers of water in the channel supply the first overbank flows, which are rich in mud but starved of sand. At successively higher stages, fine sands and then medium sands increase in concentration in the floodwater and are deposited as fine- and medium-sand layers overlying the initial mud layer. Theoretical predictions from mathematical models of sediment transport by advection and diffusion indicate that these processes acting alone are unlikely to create the observed sand layers of nearly uniform thickness that extend across much of the floodplain. We infer that other transport processes, notably bedload transport, must be important along Powder River. Even with the centimeter-scale measurements of floodplain deposits, daily hydraulic data, and precise annual surface topographic surveys, we were unable to determine any clear correspondence between the gauged flow record of overbank floods and the depositional layers mapped in the floodplain. These results provide a detailed example of floodplain deposits and depositional processes that should prove useful for interpreting natural levee deposits in a variety of geologic settings. Copyright ?? 2008, SEPM (Society for Sedimentary Geology).

Journal of Sedimentary Research↗
USGS2006

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

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

California↗
USGS2026

Geochronologic data reveal Late Pleistocene to Holocene debris-flow history and wildfire association within Whiskeytown National Recreation Area, Klamath Mountains, northern California

Understanding the local to regional history of extreme events such as debris flows and floods provides context to plan for and mitigate these hazards to life, property, and infrastructure. The Klamath Mountains of northwestern California have experienced both debris flows and devastating wildfires. Whiskeytown National Recreation Area (WHIS) is at the heart of this range and has a wealth of debris flow–related landforms. Gaining an understanding of prehistoric flows and their relationship with fire or other potential triggers can help mitigate future problems. Optically stimulated luminescence and radiocarbon analyses from sediment and entrained organics in undisturbed facies, including beneath partially buried boulders, establishes a chronology of paleo-events in WHIS. The levee deposits indicate a repetition of debris flows during the latest Holocene, every 125–150 years, since 850 yr. Larger flows occurred, with a record elucidated from debris-flow deposits along Clear Creek, with Middle Holocene ages, ca. 2600 to 5500 yr, most of which have sufficient concentrations of charcoal to indicate origins as postfire debris flows. Deposits at higher elevations show events from the latest Pleistocene ca. 13,000 yr. This geochronology indicates that these are not singular events but are relatively common and inherent to the geomorphic processes shaping this landscape.

California↗
USGS2013

Geologic map of MTM -15027, -20027, -25027, and -25032 quadrangles, Margaritifer Terra region of Mars

Mars Transverse Mercator (MTM) quadrangles −15027, −20027, −25027, and −25032 (lat 12.5°−28° S., long 330°−335° E. and lat 22.5°−28° S., long 324.5°−330° E.) in southwestern Margaritifer Terra include diverse erosional landforms, sedimentary deposits, and tectonic structures that record a long geologic and geomorphic history. The northeastern regional slope of the pre-Noachian crustal dichotomy (as expressed along the Chryse trough) and structures of the informally named Middle Noachian or older Holden and Ladon impact basins dominate the topography of the map area. A series of mesoscale outflow channels, Uzboi, Ladon, and Morava Valles, integrated these formerly enclosed basins by overflow and incision around the Noachian/Hesperian transition, although some flooding may have occurred earlier. The area includes excellent examples of Late Noachian to Hesperian valley networks, dissected crater rims, alluvial fans, deltas, and light-toned layered deposits, particularly in Holden and Eberswalde craters. Structural forms include Tharsis-radial grabens, Hesperian wrinkle ridges, floor-fractured impact craters, and severely disrupted chaotic terrains. These well-preserved landforms and sedimentary deposits represent multiple erosional epochs and discrete flooding events, which provide significant insight into the geomorphic processes and climate change on early Mars.

Scientific Investigations Map↗
USGS1976

Resource data bases-Resource assessment

The U.S. Geological Survey's Office of Resource Analysis is developing computer methods for the handling of mineral-resources data in order to provide improved means for addressing and manipulating data. These methods include: computerized data files and predictive resource models. Data files contain the raw or disaggregated information on mineral deposits and commodities. One operational data file is CRIB (Computerized Resources Information Bank) which is a general purpose inventory and reference file on metallic and nonmetallic mineral deposits. A computer file on resources should contain detailed information on the following main categories: record identification, name and location, description of deposit, analytical data, and production/reserves. A resource model employs postulates and inferences in conjunction with the data to make predictions about resources-as key variables concerning a mineral commodity are changed. The objective is to estimate the availability of minerals including: geological availability (occurrence models), technological availability (exploration and beneficiation models), and economic availability (economics models). ?? 1976.

Computers & Geosciences↗
USGS1954

Airborne radioactivity surveys for phosphate in Florida

Airborne radioactivity surveys totaling 5, 600 traverse miles were made in 10 areas in Florida, which were thought to be geologically favorable for deposits of uraniferous phosphate. Abnormal radioactivity was recorded in 8 of the 10 areas surveyed. The anomalies are located in Bradford, Clay, Columbia, DeSoto, Dixie, Lake, Marion, Orange, Sumter, Taylor, and Union Counties. Two of the anomalies were investigated briefly on the ground. One resulted from a deposit of river-pebble phosphate in the Peace River valley; the river-pebble samples contain an average of 0.013 percent equivalent uranium. The other anomaly resulted from outcrops of leached phosphatic rock containing as much as 0. 016 percent equivalent uranium. Several anomalies in other areas were recorded at or near localities where phosphate deposits have been reported.

Florida↗
USGS2007

Solar forcing of Gulf of California climate during the past 2000 yr suggested by diatoms and silicoflagellates

High-resolution records of the past 2000 yr are compared in a north–south transect (28° N to 24° N) of three cores from the eastern slopes of the Guaymas, Carmen, and Pescadero Basins of the Gulf of California (hereafter referred to as the “Gulf”). Evenly-spaced samples from the varved sediments in each core allow sample resolution ranging from ∼ 16 to ∼ 37 yr. Diatoms and silicoflagellates capture the seasonal variation between a late fall to early spring period of high biosiliceous productivity, that is driven by northwest winds, and a summer period of warmer, more stratified waters during which these winds slacken and/or reverse direction (monsoonal flow). As these winds decrease, tropical waters enter the Gulf and spread northward. Individual samples represent a composite of 7 to 23 yr of deposition and are assumed to record the relative dominance of the winter vs. summer floral components. Intervals of enhanced summer incursion of tropical waters, alternating with periods of increased late fall to early spring biosiliceous productivity are recorded in all three cores. Regularly spaced cycles (∼ 100 yr duration) of Octactis pulchra , a silicoflagellate proxy for lower SST and high productivity, and Azpeitia nodulifera , a tropical diatom, occur between ∼ A.D. 400 and ∼ 1700 in the more nearshore Carmen Basin core, NH01-21 (26.3° N), suggesting a possible solar influence on coastal upwelling. Cores BAM80 E-17 (27.9° N) and NH01-26 (24.3° N) contain longer-duration cycles of diatoms and silicoflagellates. The early part of Medieval Climate Anomaly (∼ A.D. 900 to 1200) is characterized by two periods of reduced productivity (warmer SST) with an intervening high productivity (cool) interval centered at ∼ A.D. 1050. Reduced productivity and higher SST also characterize the record of the last ∼ 100 to 200 yr in these cores. Solar variability appears to be driving productivity cycles, as intervals of increased radiocarbon production (sunspot minima) correlate with intervals of enhanced productivity. It is proposed that increased winter cooling of the atmosphere above southwest U.S. during sunspot minima causes intensification of the northwest winds that blow down the Gulf during the late fall to early spring, leading to intensified overturn of surface waters and enhanced productivity. A new silicoflagellate species, Dictyocha franshepardii Bukry, is described and illustrated.

California↗
USGS2001

Vegetation and paleoclimate of the last interglacial period, central Alaska

The last interglacial period is thought to be the last time global climate was significantly warmer than present. New stratigraphic studies at Eva Creek, near Fairbanks, Alaska indicate a complex last interglacial record wherein periods of loess deposition alternated with periods of soil formation. The Eva Forest Bed appears to have formed about the time of or after deposition of the Old Crow tephra (dated to ??? 160 to ??? 120 ka), and is therefore correlated with the last interglacial period. Pollen, macrofossils, and soils from the Eva Forest Bed indicate that boreal forest was the dominant vegetation and precipitation may have been greater than present around Fairbanks during the peak of the last interglacial period. A new compilation of last interglacial localities indicates that boreal forest was extensive over interior Alaska and Yukon Territory. Boreal forest also extended beyond its present range onto the Seward and Baldwin Peninsulas, and probably migrated to higher elevations, now occupied by tundra, in the interior. Comparison of last interglacial pollen and macrofossil data with atmospheric general circulation model results shows both agreement and disagreement. Model results of warmer-than-present summers are in agreement with fossil data. However, numerous localities with boreal forest records are in conflict with model reconstructions of an extensive cool steppe in interior Alaska and much of Yukon Territory during the last interglacial. ?? 2000 Elsevier Science Ltd.

Quaternary Science Reviews↗
USGS2017

Tectonic Speleogenesis of Devils Hole, Nevada, and Implications for Hydrogeology and the Development of Long, Continuous Paleoenvironmental Records

Devils Hole, in southern Nevada, is a surface collapse into a deep, planar, steeply dipping fault-controlled fissure in Cambrian limestone and dolostone. The collapse intersects the water table about 15 m below land surface and the fissure extends at least 130 m deeper. Below water, most of the fissure is lined with a >30-cm-thick layer of dense maxillary calcite that precipitated continuously from groundwater for >500,000 yr. The thick mammillary calcite coat implies a long history of calcite-supersaturated groundwaters, which, combined with the absence of dissolutional morphologies, suggests that Devils Hole was not formed by karst processes. Devils Hole is located in a region of active extension; its tectonic origin is shown by evidence of spreading of its planar opening along a fault and by the orientation of its opening and others nearby, perpendicular to the northwest-southeast minimum principal stress direction of the region. Most Quaternary tectonic activity in the area, including seismicity and Quaternary faults and fractures, occurs on or parallel to northeast-striking structures. The hydrogeologic implications of this primarily structural origin are that fracture networks and caves opened by extensional tectonism can act as groundwater flowpaths functionally similar to those developed by karst processes and that, during active extension, transmissivity can be maintained despite infilling by mineral precipitation. Such extensional environments can provide conditions favorable for accumulation of deposits preserving long, continuous paleoenvironmental records. The precipitates in Devils Hole store chronologies of flow system water-level fluctuations, hydrochemistry, a half-million-yr proxy paleoclimate record, evidence of Devils Hole's tectonic origin, and probably atmospheric circulation.

Quaternary Research↗
USGS2018

Postglacial eruptive history and geochemistry of Semisopochnoi volcano, western Aleutian Islands, Alaska

Semisopochnoi Island, located in the Rat Islands group of the western Aleutian Islands and Aleutian volcanic arc, is a roughly circular island composed of scattered volcanic vents, the prominent caldera of Semisopochnoi volcano, and older, ancestral volcanic rocks. The oldest rocks on the island are gently radially dipping lavas that are the remnants of a shield volcano and of Ragged Top, which is an eroded stratocone southeast of the current caldera. None of these oldest rocks have been dated, but they all are likely Pleistocene in age. Anvil Peak, to the caldera’s north, has the morphology of a young stratocone and is latest Pleistocene to early Holocene in age. The oldest recognized Holocene deposits are those of the caldera-forming eruption, which produced the 7- by 6-km caldera in the center of the island, left nonwelded ignimbrite in valleys below the edifice, and left welded ignimbrite high on its flanks. The caldera-forming eruption produced rocks showing a range of intermediate whole-rock compositions throughout the eruption sequence, although a majority of clasts analyzed form a fairly tight cluster on SiO 2 -variation diagrams at 62.9 to 63.4 weight percent SiO 2 . This clustering of compositions at about 63 weight percent SiO 2 includes black, dense, obsidian-like clasts, as well as tan, variably oxidized, highly inflated pumice clasts. The best estimate for the timing of the eruption is from a soil dated at 6,920±60 14 C years before present underlying a thin facies of the ignimbrite deposit on the island’s north coast. Shortly after the caldera-forming eruption, two scoria cones on the northwest flank of the volcano outside the caldera, Ringworm crater and Threequarter Cone, simultaneously erupted small volumes of andesite. The oldest intracaldera lavas, on the floor of the caldera, are andesitic to dacitic, but are mostly covered by younger lavas and tephras. These intracaldera lavas include the basaltic andesites of small Windy cone, as well as the more voluminous basaltic andesites of three-peaked Mount Cerberus, which takes up most of the west half of the caldera and has erupted lavas that flowed to the sea on the southwestern coast of the island. Apparently active at the same time as Mount Cerberus, extracaldera Sugarloaf Peak at the southern point of the island has exclusively erupted basalts. Its young satellite peak, Sugarloaf Head, has erupted morphologically young lavas and cinder cones and may be the source of the last historical eruption in 1987. Several tephra sections on the east half of the island record as many as 50 tephras, mostly from Mount Cerberus, Sugarloaf Peak, and Sugarloaf Head, over the past several thousand years. Eruptive products of Semisopochnoi Island show an overall compositional range of basalt to dacite, though basaltic andesite and andesite constitute the largest proportions of rock types. They are tholeiitic, low to medium K, and have geochemical characteristics typical of magmatic arcs. The earliest Pleistocene lavas are mostly basalts that show the greatest geochemical diversity, as illustrated by, for example, La N /Yb N ratios of 1.9 to 3.5, suggesting fluctuations in the magma source region over the hundreds of thousands of years recorded by these older lavas. The Holocene rocks, in contrast, follow arrays in compositional space that suggest crystallization differentiation from discrete, subtly different batches of magma under varying pressure and temperature conditions. Increasingly negative Eu anomalies and an only modestly increasing alumina saturation index value with differentiation suggest that plagioclase and mafic silicates (amphibole and pyroxene) were involved to varying degrees in fractional crystallization to produce Semisopochnoi’s magmatic diversity. The crystal-poor, andesitic magmas that erupted during caldera formation likely separated from a plagioclase-, amphibole-, and clinopyroxene-dominated crystal residue in the upper crust at less than 900 °C, possibly following a period of decreased magmatic flux. During the Holocene, basaltic Sugarloaf Peak appears to bypass any upper crustal magmatic storage region and erupt crystal-rich basalts. Recent seismic swarms and long-lived warm springs attest to ongoing magmatic activity. The Holocene eruptive record at Semisopochnoi volcano is one of diverse eruptive styles as well as frequent eruptions from multiple vents located within and outside the caldera. The number and diversity of postcaldera vents means that the sites of future eruptions cannot be predicted with certainty. Future eruptions of ash similar in magnitude to the VEI 3 or less eruptions recorded in the documented tephra deposits would pose a hazard to aircraft in the region.

Alaska↗
USGS2024

Polar science results from Mars Reconnaissance Orbiter: Multiwavelength, multiyear insights

Mars Reconnaissance Orbiter (MRO), with its arrival in 2006 and nearly continuous operation since, has provided data for the study of martian polar processes spanning nine Mars years. Mars' polar deposits have long been thought to preserve records of past climates, potentially readable like terrestrial ice cores. However, unraveling millions of years of history in the ice depends on understanding Mars' current and recent-past climate, including the interactions of atmospheric and surface processes . MRO has allowed for revolutionary discoveries, long-term monitoring of ongoing processes, and multiple complementary datasets to address the question of how the polar ice deposits reflect climatological changes. In part, MRO has been able to do this from its variety of instrumentation simultaneously observing interannual changes in geomorphology of the surface in up to ∼25 cm/pixel detail, repeatable processes and changes in the atmosphere, and surface composition, as well as investigating signs of past changes recorded in the icy polar layered deposits. In this paper, we summarize the contribution of MRO to our current understanding of Mars polar science, and in particular how MRO's long-duration mission has improved our understanding of the fundamental volatile cycles on Mars.

Icarus↗
USGS1999

7Be as a tracer of flood sedimentation on the northern California continental margin

Sediment inventories of the cosmogenic radionuclide 7 Be (t 1/2 =53 d) were measured on the Eel River shelf and slope (northern California continental margin) to investigate sedimentation processes associated with coastal river flooding. Seabed coring shortly after major riverflow events in 1995 and 1997 documented a shelf-wide flood deposit, and subsequent radionuclide studies determined 7 Be to be a powerful tracer of fine-grained river sediment. In addition, distinctive signatures of 234 Th and 210 Pb were observed in oceanic flood deposits and provided additional information regarding depositional processes. During the 1995–1997 monitoring period, 7 Be was present (2–35 dpm cm -2 ) in shelf and slope sediments only after periods of high rainfall and river runoff during the winter months. It is suggested that fluvial input was the primary source of 7 Be in shelf sediments after the floods. 7 Be sediment inventories and sediment-trap fluxes determined after the 1997 flood revealed that fine-grained fluvial sediments were rapidly (within one month) broadcast over the continental margin, to the 500 m isobath. Dispersal was apparently facilitated by energetic storm waves, which resuspended and redistributed some fraction of the suspended load residing on the shelf prior to accretion as flood deposits. These observations illustrate that floods are an important sedimentary process for modern environments of the Eel shelf and slope, and perhaps for other fluviomarine sedimentary systems of the northern California continental margin. Ratios of the 210 Pb sediment-accumulation rate (100 yr average) to the 7 Be deposition rate (1–2 month average) for shelf sites illustrate the episodic nature of shelf sedimentation, and suggest that a minimum of 3–30 depositional events complete the most recent stratigraphic record. This observation is consistent with the magnetude and frequency of fluvial sediment input, as Eel River floods with return periods of 3–33 yr (3% of the time of record) have supplied >80% of the total 85 yr suspended load. Based on radionuclide and hydrologic data, it can be concluded that a small number of flood depositional events have had a disproportionate impact on the sedimentary record of the Eel shelf.

California↗
USGS1997

A diatom-based paleohydrologic record of climate change for the past 800 k.y. from Owens Lake, California

A 323-m (~800 k.y.) core of lake deposits beneath Owens Lake playa, Inyo County, California, contains a nearly continuous paleolimnological record based on diatom assemblages. The core chronology is anchored by the Matuyama/Brunhes magnetostratigraphic boundary and the Bishop ash near the base of the record and by radiocarbon dates near the top. Throughout most of its history, Owens Lake was characterized by fresh-water diatoms, indicating a positive hydrologic input from the Owens River and overflow to lake systems downstream. Both benthic and planktic freshwater diatoms dominate in ashy and sandy sediments between 800 ka and 440 ka and suggest shallow, open-water environments in a basin where sedimentation and subsidence were approximately balanced. After 440 ka, freshwater planktic diatoms dominate, implying that the Owens basin became deeper, perhaps as a result of increased rates of tectonic subsidence. The stratigraphic distribution of saline benthic and planktic diatoms record comparatively short intervals when the lake was shallow and saline. Nevertheless, periodic overflow during these times prevented deposition of evaporites. According to a chronology based on sediment mass-accumulation rates, the alternation of saline and freshwater diatom assemblages approximately tracks the progression of oxygen isotope stages recorded in marine deposits. Even-numbered isotope stages representing glacial conditions are matched by episodes where freshwater planktic diatoms dominate, indicating abundant precipitation in the Sierra Nevada in response to a southward shift of storm tracks originating in the North Pacific around the Aleutian Low.

California↗
USGS1984

Accumulation of organic matter in Cretaceous oxygen-deficient depositional environments in the central Pacific Ocean

Complete records of organic-carbon-rich Cretaceous strata were continuouslycored on the flanks of the Mid-Pacific Mountains and southern Hess Rise in the central North Pacific Ocean during DSDP Leg 62. Organic-carbon-rich laminated silicified limestones were deposited in the western Mid-Pacific Mountains during the early Aptian, a time when that region was south of the equator and considerably shallower than at present. Organic-carbon-rich, laminated limestone on southern Hess Rise overlies volcanic basement and includes 136 m of stratigraphic section of late Albian to early Cenomanian age. This limestone unit was deposited rapidly as Hess Rise was passing under the equatorial high-productivity zone and was subsiding from shallow to intermediate depths. The association of volcanogenic components with organic-carbon-rich strata on Hess Rise in the Mid-Pacific Mountains is striking and suggests that there was a coincidence of mid-plate volcanic activity and the production and accumulation of organic matter at intermediate water depths in the tropical Pacific Ocean during the middle Cretaceous. Pyrolysis assays and analyses of extractable hydrocarbons indicate that the organic matter in the limestone on Hess Rise is composed mainly of lipid-rich kerogen derived from aquatic marine organisms and bacteria. Limestones from the Mid-Pacific Mountains generally contain low ratios of pyrolytic hydrocarbons to organic carbon and low hydrogen indices, suggesting that the organic matter may contain a significant proportion of land-derived material, possibly derived from numerous volcanic islands that must have existed before the area subsided. The organic carbon in all samples analyzed is isotopically light ( δ 13 C − 24 to − 29 per mil) relative to most modern rine organic carbon, and the lightest carbon is also the most lipid-rich. There is a positive linear correlation between sulfur and organic carbon in samples from Hess Rise and from the Mid-Pacific Mountains. The slopes and intercepts of C-S regression lines however, are different for each site and all are different from regression lines for samples from modern anoxic marine sediments and from Black Sea cores. The organic-carbon-rich limestones on Hess Rise, the Mid-Pacific Mountains, and other plateaus and seamounts in the Pacific Ocean are not synchronous but do occur within the same general middle Cretaceous time period as organic-carbon-rich lithofacies elsewhere in the world ocean, particularly in the Atlantic Ocean. Strata of equivalent age in the deep basins of the Pacific Ocean are not rich in organic carbon, and were deposited in oxygenated environments. This observation, together with the evidence that the plateau sites were considerably shallower and closse to the equator during the middle Creataceous suggests that local tectonic and hydrographic conditions may have resulted in high surface-water productivity and the preservation of organic matter in an oxygen-deficient environment where an expanded mid-water oxygen minimum developed and impinged on elevated platforms and seamounts.

Organic Geochemistry↗
USGS2019

A river is born: Highlights of the geologic evolution of the Colorado River extensional corridor and its river: A field guide honoring the life and legacy of Warren Hamilton

The Colorado River extensional corridor, which stretched by a factor of 2 in the Miocene, left a series of lowland basins and intervening bedrock ranges that, at the dawn of the Pliocene, were flooded by Colorado River water newly diverted from the Colorado Plateau through Grand Canyon. This water and subsequent sediment gave birth, through a series of overflowing lakes, to an integrated Colorado River flowing to the newly opened Gulf of California. Topock Gorge, which the river now follows between the Chemehuevi and Mohave Mountains, is a major focus of this field guide, as it very nicely exposes structural, stratigraphic, and magmatic aspects of the Miocene extensional corridor, a core complex, and detachment faults as well as a pre-Cenozoic batholith. Topock Gorge also is the inferred site of a paleodivide between early Pliocene basins of newly arrived Colorado River water. Overspilling of its upstream lake breached the divide and led the river southward. The Bouse Formation in this and other basins records the pre–river integration water bodies. Younger riverlaid deposits including the Bullhead Alluvium (Pliocene) and the Chemehuevi Formation (Pleistocene) record subsequent evolution of the Colorado River through a succession of aggradational and re-incision stages. Their stratigraphic record provides evidence of local basin deepening after river inception, but little deformation on a regional scale of the river valley in the last 4 m.y. except in the Lake Mead area. There, faults interrupt both the paleoriver grade and incision rates, and are interpreted to record 100’s of m of true uplift of the Colorado Plateau. Warren Hamilton’s insightful work beginning in the 1950s helped set the stage for interpretation of Mesozoic orogeny and Cenozoic extension in this region, as well as the record of the Bouse Formation.

Arizona, California, Nevada↗