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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↗

USGS1991

Tectonic evolution of submarine canyons along the California continental margin

The development of submarine canyons along active-plate margins commonly is influenced by tectonic processes. Recent studies of submarine canyons along the transform margin of western North America show that the origin and subsequent evolution of many canyons are correlatable with plate motion and plate-margin deformation. Elements of canyon morphology such as bends and meanders commonly are controlled by faults and folds that are relatable to the structural fabric of the continental shelf and slope. Some canyon heads that appear to be displaced from their lower reaches are explainable as the result of movement along strike-slip faults associated with the plate margin. Many submarine canyons along the California margin are not associated with large rivers and thus may owe their origins either to pre-Holocene fluvial or structural processes. Some modern canyons appear to be associated with pre-Pleistocene ancestral canyons. Because of both vertical and horizontal tectonic movements during the past 20 Ma, some California submarine canyons have been repeatedly filled and exhumed; the most recent exhumation began during the latest lowstand of sea level and continues today. Canyons that today have their upper reaches on the continental slope or outermost shelf, distant from large rivers or other sources of sediment supply, commonly appear to have been laterally displaced along offshore faults. Palinspastic reconstructions along these faults commonly reveal a genetic relation between such canyons and canyons heading nearshore, from which they were offset. For example, detailed studies of the Ascension-Monterey Submarine Canyon system in Monterey Bay suggest that several smaller canyons on the outer shelf and upper slope have been displaced northwestward from the headward part of Monterey Canyon by right slip along offshore faults of the Palo Colorado-San Gregorio, Ascension, and Monterey Bay fault zones. Many other canyons on the California margin have developed along, or had their courses abruptly altered by, structural zones, owing either to canyon cutting along a zone of weakness or to fault displacement. Mass wasting associated with zones of faulting and slumping, which may have been seismically induced, also may affect canyon form. Clearly, submarine canyons along the California margin commonly owe their origin and morphologic development to influences other than fluvial erosion during sea-level lowstands. A chief influence has been the San Andreas fault system.

California↗

USGS2009

Coastline shifts and probable ship landing site submerged off ancient Locri-Epizefiri, southern Italy

A geophysical survey provides new information on marine features located seaward of Locri-Epizefiri (Locri), an ancient Greek settlement on the Ionian coastal margin in southern Italy. The study supplements previous work by archaeologists who long searched for the site's harbor and recently identified what was once a marine basin that is now on land next to the city walls of Locri. Profiles obtained offshore, between the present coast and outer shelf, made with a high-resolution, seismic subbottom-profiling system, record spatial and temporal variations of buried Holocene deposits. Two of these submerged features are part of a probable now-submerged ship landing facility. The offshore features can be linked to coastline displacements that occurred off Locri: a sea-to-land shift before Greek settlement, followed by a shoreline reversal from the archaeological site back to sea, and more recently, a return landward. The seaward directed coastal shift that occurred after Locri's occupation by Greeks was likely caused by land uplift near the coastal margin and tectonic seaward shift of the coast, as documented along this geologically active sector of the Calabrian Arc. The seismic survey records an angular, hook-shaped, low rise that extends from the present shore and is now buried on the inner shelf. The rise, enclosing a core lens of poorly stratified to transparent acoustic layers, bounds a broad, low-elevation zone positioned immediately seaward of the shoreline. Close proximity of the raised feature to the low-elevation area suggests it may have been a fabricated structure that functioned as a wave-break for a ship-landing site. The study indicates that the basin extended offshore as a function of the coastline's seaward migration during and/or after Greek occupation of Locri.

Journal of Coastal Research↗

USGS2010

Alexandria's Eastern Harbor, Egypt: Pollen, microscopic charcoal, and the transition from natural to human-modified basin

Pollen and microscopic charcoal examined in Holocene sediment core samples record major environmental modifications affecting Alexandria's Eastern Harbor through time. We assess whether such changes on Egypt's coastal margin were influenced primarily by natural, or natural plus human, or primarily human factors. We focus on (1) the times when pollen assemblages and microscopic charcoal content changed in the core, (2) how they changed, and (3) why this occurred. The analysis takes into account the core's stratigraphy, regional climate variability, human history, and local archaeological record. Four pollenmicroscopic charcoal zones are identified. The earliest change occurred at ca. 6000 YBP, during Egypt's earlier Predynastic (Neolithic) period, coinciding with a lithologic break from sand to muddy sand. Pollen during this time indicates a transition to a much drier climate rather than effects of human activity. The second change in pollen occurred 3600-2900 YBP, during a period of continued aridity with no lithologic variation in this core interval. Pollen (cereal taxa, agricultural weeds, grape) and a sharp increase in microscopic charcoal indicate that human activity became prevalent at least 700 y before Alexander the Great's arrival in this region, and these results highlight the transition from a largely natural climatecontrolled environment to one influenced by both climate and anthropogenic activity. The third shift up-core in pollen assemblages is dated at ca. 2300 YBP, at the boundary between a sand and mud unit. It coincides with construction by the Ptolemies of the Heptastadion between Alexandria and Pharos Island. From this time onward, harbor sediment in the nearly enclosed catchment basin indicates a near-continuous record of dominant proximal human activity. ?? 2010 Coastal Education and Research Foundation.

Journal of Coastal Research↗

USGS2013

Geologic evidence for onshore sediment transport from the inner continental shelf: Fire Island, New York

Sediment budget analyses along the south shore of Fire Island, New York, have been conducted and debated in the scientific and coastal engineering literature for decades. It is well documented that a primary component of sediment transport in this system is directed alongshore from E to W, but discrepancies in volumetric sediment budget calculations remain. An additional quantity of sand, averaging about 200,000 m 3 /y is required to explain the growth of the western segment of the barrier island, a prograding spit. Littoral sediment derived from updrift erosion of the coast, addition of beach nourishment fill, and onshore transport of inner continental shelf, shoreface sediments, or both have all been proposed as potential sources of the additional sediment needed to balance the sediment budget deficit. Analysis of high-resolution seafloor mapping data collected in 2011, including seismic reflection profiles and inteferometric sonar acoustic backscatter and swath bathymetry; comparison with seafloor mapping data collected in 1996–1997; and shoreline change analysis from 1933 to 2011 support previous suggestions that the inner-shelf Holocene sedimentary deposit is a likely source to resolve this sediment budget discrepancy.

New York↗

USGS2013

Introduction to the special issue on “Understanding and predicting change in the coastal ecosystems of the northern Gulf of Mexico”

The coastal region of the northern Gulf of Mexico owes its current landscape structure to an array of tectonic, erosional and depositional, climatic, geochemical, hydrological, ecological, and human processes that have resulted in some of the world's most complex, dynamic, productive, and threatened ecosystems. Catastrophic hurricane landfalls, ongoing subsidence and erosion exacerbated by sea-level rise, disintegration of barrier island chains, and high rates of wetland loss have called attention to the vulnerability of northern Gulf coast ecosystems, habitats, built infrastructure, and economy to natural and anthropogenic threats. The devastating hurricanes of 2005 (Katrina and Rita) motivated the U.S. Geological Survey Coastal and Marine Geology Program and partnering researchers to pursue studies aimed at understanding and predicting landscape change and the associated storm hazard vulnerability of northern Gulf coast region ecosystems and human communities. Attaining this science goal requires increased knowledge of landscape evolution on geologic, historical, and human time scales, and analysis of the implications of such changes in the natural and built components of the landscape for hurricane impact susceptibility. This Special Issue of the Journal of Coastal Research communicates northern Gulf of Mexico research results that (1) improve knowledge of prior climates and depositional environments, (2) assess broad regional ecosystem structure and change over Holocene to human time scales, (3) undertake process studies and change analyses of dynamic landscape components, and (4) integrate framework, climate, variable time and spatial scale mapping, monitoring, and discipline-specific process investigations within interdisciplinary studies.

Journal of Coastal Research↗

USGS2013

Seasonal flux and assemblage composition of planktic foraminifers from a sediment-trap study in the northern Gulf of Mexico

Sediment-trap samples from the northern Gulf of Mexico reveal that Globorotalia truncatulinoides, Neogloboquadrina dutertrei, Pulleniatina spp. (includes P. obliquiloculata and P. finalis), and the Globorotalia menardii group (includes Gt. menardii, Gt. tumida, and Gt. ungulata) generally occur in cold months. Globigerinoides ruber (white and pink varieties) and Globigennoides sacculifer occur throughout the year. The seasonal occurrence of individual taxa of planktic foraminifers in the Gulf of Mexico have important differences with the seasonal occurrence of the same taxa observed in a 6-year sediment-trap dataset from the western Sargasso Sea. Thus information on the ecologic preferences of individual taxa determined in one region cannot necessarily be applied directly to another area. In the northern Gulf of Mexico 90% of the total flux of Globorotalia truncatulinoides tests to sediments occurs in January and February. Mg/Ca and d 18 Ο measurements indicate that nonencrusted forms of Gt. truncatulinoides calcify in the upper-surface-mixed zone. Thus, analyses of nonencrusted Gt. truncatulinoides in sediments of the northern Gulf of Mexico have potential for monitoring past conditions in the winter-surface-mixed layer. The relatively low overall abundance of Globigerinoides ruber (white) in sediment-trap samples is anomalous because Gs. ruber (white) is one of the most abundant foraminifers in>150 µm census data from northern Gulf of Mexico Holocene sediment core samples. Globigerinoides ruber (pink) is a relatively persistent and common component of the sediment-trap samples. Thus Gs. ruber (pink) has potential as a proxy for mean annual sea-surface temperature in the Gulf of Mexico

Journal of Coastal Research↗

USGS2004

Petroleum generation and migration in the Mesopotamian Basin and Zagros Fold Felt of Iraq: Results from a basin-modeling study

A regional 3-D total petroleum-system model was developed to evaluate petroleum generation and migration histories in the Mesopotamian Basin and Zagros fold belt in Iraq. The modeling was undertaken in conjunction with Middle East petroleum assessment studies conducted by the USGS. Regional structure maps, isopach and facies maps, and thermal maturity data were used as input to the model. The oil-generation potential of Jurassic source-rocks, the principal known source of the petroleum in Jurassic, Cretaceous, and Tertiary reservoirs in these regions, was modeled using hydrous pyrolysis (Type II-S) kerogen kinetics. Results showed that oil generation in source rocks commenced in the Late Cretaceous in intrashelf basins, peak expulsion took place in the late Miocene and Pliocene when these depocenters had expanded along the Zagros foredeep trend, and generation ended in the Holocene when deposition in the foredeep ceased. The model indicates that, at present, the majority of Jurassic source rocks in Iraq have reached or exceeded peak oil generation and most rocks have completed oil generation and expulsion. Flow-path simulations demonstrate that virtually all oil and gas fields in the Mesopotamian Basin and Zagros fold belt overlie mature Jurassic source rocks (vertical migration dominated) and are situated on, or close to, modeled migration pathways. Fields closest to modeled pathways associated with source rocks in local intrashelf basins were charged earliest from Late Cretaceous through the middle Miocene, and other fields filled later when compression-related traps were being formed. Model results confirm petroleum migration along major, northwest-trending folds and faults, and oil migration loss at the surface.

GeoArabia↗

USGS1983

The mobility and distribution of heavy metals during the formation of first cycle red beds

Holocene-Pliocene sequence sampled in northern Baja California. Geochemical data supported by petrographic, X-ray, and SEM observations of mineralogical transformations, fission-track radiography, and uranium decay series measurements. Results indicate that metal content of the studied samples is inherited from constituent detrital minerals and that reddening of whole-rock samples does not promote major open-system migration of heavy metals. The amount of secondary iron oxides and the fraction of whole-rock metals associated with the oxides increase during red-bed development. Results suggest that developed red beds which are well flushed by suitable pore fluids may be sources of significant quantities of heavy metals.

Economic Geology↗

USGS1990

Fine-grained rutile in the Gulf of Maine: Diagenetic origin, source rocks, and sedimentary environment of deposition

The Gulf of Maine, an embayment of the New England margin, is floored by shallow, glacially scoured basins that are partly filled with late Pleistocene and Holocene silt and clay containing 0.7 to 1.0 wt percent TiO 2 , chiefly in the form of silt-size rutile. Eleven basins in the gulf are estimated to contain 479 X 10 6 metric tons of TiO 2 (to a depth of 10 m) in the U.S. exclusive economic zone and 168 X 10 6 in Canada, based on analyses of surface sediment and of cores 10 to 20 m long. The U.S. annually consumes approximately 1 X 10 6 metric tons of TiO 2 , of which 73 percent is imported. The inferred amount of fine-grained TiO 2 in the basins is large, and we interpret it to be predominantly rutile; but no attempt has been made yet to mine and beneficiate the fine-grained ore.Sedimentary rocks of Nova Scotia, New Brunswick, and northern Maine contain fine-grained rutile crystals and have been the major source of the fine-grained sediment in the Gulf of Maine basins during and after the Wisconsinan glaciation. We conclude that much of the rutile in the Gulf of Maine mud formed diagenetically in poorly cemented Carboniferous and Triassic coarse-grained sedimentary rocks of Nova Scotia and New Brunswick after the dissolution of titanium-rich detrital minerals (ilmenite, ilmenomagnetite). Another major source of rutile is the generally finer grained Paleozoic sedimentary rocks of northern Maine (and possibly northwestern New Brunswick) which contain recycled diagenetic rutile that most likely originated in sandstones. Additional Wisconsinan sources of fine-grained, diagenetic TiO 2 probably include sedimentary rocks of inferred Triassic age that underlie the Gulf of Maine.The diagenesis of rutlie in coarse sedimentary rocks (especially arkose and graywacke) followed by erosion, segregation, and deposition (and including recycling of fine-grained rutile from shales) can serve as a model for predicting and prospecting for unconsolidated deposits of fine-grained TiO 2 .Gulf of Maine mud is comparable in TiO 2 content to typical shale. A determination of whether the fine-grained TiO 2 in shales primarily occurs as detrital titanium minerals from metamorphic and igneous sources or as recycled, diagenetic titanium oxides from sedimentary sources may assist in interpreting shale provenance and environment of deposition.

Economic Geology↗

USGS1992

Organic matter diagenesis as the key to a unifying theory for the genesis of tabular uranium-vanadium deposits in the Morrison Formation, Colorado Plateau

Interstitial, epigenetic amorphous organic matter is intimately associated with uranium in the Grants uranium region on the southern part of the Colorado Plateau in northwestern New Mexico and is considered essential to genetic models for these deposits. In contrast, uranium minerals are intimately associated with authigenic vanadium chlorite and vanadium oxides in amorphous organic matter-poor ores of the Slick Rock and Henry Mountains mining districts on the northern part of the plateau, and therefore, in some genetic models amorphous organic matter is not considered crucial to the formation of these deposits. Differences in organic matter content can be explained by recognizing that amorphous organic matter-poor deposits have been subjected to more advanced stages of diagenesis than amorphous organic matter-rich deposits, during which organic compounds were broken down, solubilized, and removed during various diagenetic stages (e.g., sulfate reduction, methanogenesis, and organic acid production). Most diagenetic alterations (e.g., dissolution of framework grains and cements and precipitation of coarse-grained coffinite, vanadium clays, and vanadium oxides) characteristic of amorphous organic matter-poor deposits occurred when temperatures exceeded 80 degrees C--a temperature typical of the organic acid stage of diagenesis.Two factors combined to assure that a high percentage of the amorphous organic matter was removed from the Morrison Formation during diagenesis: the fact that the organic matter was humic (and therefore highly oxygenated), and that the juxtaposition of authigenic smectite and amorphous organic matter on grain rims facilitated clay mineral-organic catalysis reactions leading to the breakdown of large organic molecules into smaller, soluble molecules.Evidence that amorphous organic matter was involved in the genesis of organic matter-poor, as well as organic matter-rich, deposits includes the presence of leached iron-titanium oxides and strongly etched garnets (both attributed to leaching by organic acids), the gradation from organic matter-rich to organic matter-poor deposits in the Grants uranium region, the inverse correlation between the amount of amorphous organic matter and the intensity of diagenetic alteration, the ubiquitous occurrence of amorphous organic matter inclusions in coffinite, and the similarity in geometry of tabular uranium-vanadium ores and Holocene humate deposits. By recognizing that amorphous organic matter was crucial to the initial concentration of uranium (even in those deposits that presently contain little amorphous organic matter) and by recognizing the effects of the various stages of diagenesis on the ore, one genetic model may be applied to all of the sandstone-hosted, tabular-type uranium-vanadium deposits in the Morrison Formation on the Colorado Plateau.

Economic Geology↗

USGS1993

The Geysers-Clear Lake area, California: Thermal waters, mineralization, volcanism, and geothermal potential

Manifestations of a major thermal anomaly in the Geysers-Clear Lake area of northern California include the late Pliocene to Holocene Clear Lake Volcanics, The Geysers geothermal field, abundant thermal springs, and epithermal mercury and gold mineralization. The epithermal mineralization and thermal springs typically occur along high-angle faults within the broad San Andreas transform fault system that forms the western boundary of the North American plate in this area. The young volcanic rocks overlie Mesozoic marine rocks of the Great Valley sequence which have been thrust above the coeval Franciscan Complex and penecontemporaneously dropped back down along low-angle detachment faults.Many of the waters of the region are nonmeteoric as defined by their isotopic signature. One type of isotopically shifted water emerges from or near Great Valley sequence rocks and is the most chloride rich. It is interpreted to be evolved connate in origin. A second type, evolved meteoric water, has moderate chloride contents, high boron contents, and high B/Cl ratios and is found locally in Franciscan rocks, notably at the Sulphur Bank mercury mine, where it probably results from near-closed-system, repeated boiling of meteoric water in host rocks that also contribute organic components to the water. At the Sulphur Bank mine fracturing of otherwise impermeable Franciscan rocks by faulting has created a localized zone of permeability in which thermal water boils repeatedly with limited venting to the surface. Boron-rich fluids were apparently present at depth in The Geysers geothermal field when intrusion of silicic magma occurred, because the concealed intrusion of felsite is surrounded by a halo of tourmaline-bearing hornfels. The volume of this poorly dated early to middle Quaternary intrusive body probably exceeds the 100 km 3 of erupted Clear Lake Volcanics. Similar intrusions may have occurred in the eastern part of the area at Wilbur Springs and the McLaughlin mine, where gold deposition and evidence of hydrothermal phenomena suggest more magmatic activity than is indicated by the small exposed bodies of early Quaternary basaltic lava. The Clear Lake Volcanics are the present locus of volcanism in the northern Coast Ranges and progressively older volcanic centers occur to the south. Geophysical data suggest that a large silicic magma body may be centered north of The Geysers steam field, providing the heat for the geothermal field.Geothermal power production has peaked at The Geysers and pressure declines indicate significant depletion of the fluid resource. The vapor-dominated field evolved from a preexisting hydrothermal system within fractured, otherwise impermeable Franciscan metamorphic rocks. A deep water table of saline fluid has been postulated to be present under the steam field, but no chloride-rich water has been found at drillable depth. We propose that recently discovered, isotopically shifted steam in the northwest Geysers area indicates the presence not of deep connate water but xather of boiled-down, boron-rich Franciscan evolved meteoric water. This water is likely to be present in limited quantities and will not provide a significant hot water resource for geothermal power production at The Geysers field or from the main Clear Lake volcanic field.

Economic Geology↗

USGS2010

Probabilistic seismic hazard estimates incorporating site effects - An example from Indiana, U.S.A

The U.S. Geological Survey (USGS) has published probabilistic earthquake hazard maps for the United States based on current knowledge of past earthquake activity and geological constraints on earthquake potential. These maps for the central and eastern United States assume standard site conditions with Swave velocities of 760 m/s in the top 30 m. For urban and infrastructure planning and long-term budgeting, the public is interested in similar probabilistic seismic hazard maps that take into account near-surface geological materials. We have implemented a probabilistic method for incorporating site effects into the USGS seismic hazard analysis that takes into account the first-order effects of the surface geologic conditions. The thicknesses of sediments, which play a large role in amplification, were derived from a P-wave refraction database with over 13, 000 profiles, and a preliminary geology-based velocity model was constructed from available information on S-wave velocities. An interesting feature of the preliminary hazard maps incorporating site effects is the approximate factor of two increases in the 1-Hz spectral acceleration with 2 percent probability of exceedance in 50 years for parts of the greater Indianapolis metropolitan region and surrounding parts of central Indiana. This effect is primarily due to the relatively thick sequence of sediments infilling ancient bedrock topography that has been deposited since the Pleistocene Epoch. As expected, the Late Pleistocene and Holocene depositional systems of the Wabash and Ohio Rivers produce additional amplification in the southwestern part of Indiana. Ground motions decrease, as would be expected, toward the bedrock units in south-central Indiana, where motions are significantly lower than the values on the USGS maps.

Environmental & Engineering Geoscience↗

USGS2014

Paleoseismology of the Southern Section of the Black Mountains and Southern Death Valley Fault Zones, Death Valley, United States

The Death Valley Fault System (DVFS) is part of the southern Walker Lane–eastern California shear zone. The normal Black Mountains Fault Zone (BMFZ) and the right-lateral Southern Death Valley Fault Zone (SDVFZ) are two components of the DVFS. Estimates of late Pleistocene-Holocene slip rates and recurrence intervals for these two fault zones are uncertain owing to poor relative age control. The BMFZ southernmost section (Section 1W) steps basinward and preserves multiple scarps in the Quaternary alluvial fans. We present optically stimulated luminescence (OSL) dates ranging from 27 to 4 ka of fluvial and eolian sand lenses interbedded with alluvial-fan deposits offset by the BMFZ. By cross-cutting relations, we infer that there were three separate ground-rupturing earthquakes on BMFZ Section 1W with vertical displacement between 5.5 m and 2.75 m. The slip-rate estimate is ∼0.2 to 1.8 mm/yr, with an earthquake recurrence interval of 4,500 to 2,000 years. Slip-per-event measurements indicate Mw 7.0 to 7.2 earthquakes. The 27–4-ka OSL-dated alluvial fans also overlie the putative Cinder Hill tephra layer. Cinder Hill is offset ∼213 m by SDVFZ, which yields a tentative slip rate of 1 to 8 mm/yr for the SDVFZ.

California;Nevada↗

USGS2001

Impact of climate and parent material on chemical weathering in Loess-derived soils of the Mississippi River valley

Peoria Loess-derived soils on uplands east of the Mississippi River valley were studied from Louisiana to Iowa, along a south-to-north gradient of decreasing precipitation and temperature. Major element analyses of deep loess in Mississippi and Illinois show that the composition of the parent material is similar in the northern and southern parts of the valley. We hypothesized that in the warmer, wetter parts of the transect, mineral weathering should be greater than in the cooler, drier parts of the transect. Profile average values of CaO/TiO 2 , MgO/TiO 2 , K 2 O/TiO 2 and Na 2 O/TiO 2 , Sr/Zr, Ba/Zr, and Rb/Zr represent proxies for depletion of loess minerals such as calcite, dolomite, hornblende, mica, and plagioclase. All ratios show increases from south to north, supporting the hypothesis of greater chemical weathering in the southern part of the valley. An unexpected result is that profile average values of Al 2 O 3 /TiO 2 and Fe 2 O 3 /TiO 2 (proxies for the relative abundance of clay minerals) show increases from south to north. This finding, while contrary to the evidence of greater chemical weathering in the southern part of the transect, is consistent with an earlier study which showed higher clay contents in Bt horizons of loess-derived soils in the northern part of the transect. We hypothesize that soils in the northern part of the valley received fine-grained loess from sources to the west of the Mississippi River valley either late in the last glacial period, during the Holocene or both. In contrast, soils in the southern part of the valley were unaffected by such additions.

Arkansas, Illinois, Iowa, Kentucky, Louisiana, Mis↗

USGS2011

Episodic soil succession on basaltic lava fields in a cool, dry environment

Holocene- to late Pleistocene-aged lava flows at Craters of the Moon National Monument and Preserve provide an ideal setting to examine the early stages of soil formation under cool, dry conditions. Transects were used to characterize the amount and nature of soil cover on across basaltic lava flows ranging in age from 2.1 to 18.4 ka. Results indicate that on flows <13 ka, very shallow organic soils (Folists in Soil Taxonomy) are the dominant soil type, providing an areal coverage of up to &sim;25%. On flows ≥13.9 ka, deeper mineral soils including Entisols, Aridisols, and Mollisols become dominant and the areal extent increases to ≥95% on flows older than 18.4 ka. These data suggest there are two distinct pedogenic pathways associated with lava flows of the region. The first pathway is illustrated by the younger flows, where Folists dominate. In the absence of a major source of loess, relatively little mineral material accumulates and soils provide only minor coverage of the lava flows. Our results indicate that this pathway of soil development has not changed appreciably over the past &sim;10 ka. The second pedogenic pathway is illustrated by the flows older than 13.9 ka. These flows have been subject to deposition of large quantities of loess during and after the last regional glaciation, resulting in almost complete coverage. Subsequent pedogenesis has given rise to Aridisols and Mollisols with calcic and cambic horizons and mollic epipedons. This research highlights the importance of regional climate change on the evolution of Craters of the Moon soilscapes.

Idaho↗

USGS2009

Hydrologic characterization of desert soils with varying degrees of pedogenesis: 1. field experiments evaluating plant-relevant soil water behavior

To assess the eff ect of pedogenesis on the soil moisture dynamics infl uencing the character and quality of ecological habitat, we conducted infi ltration and redistribution experiments on three alluvial deposits in the Mojave National Preserve: (i) recently deposited active wash sediments, (ii) a soil of early Holocene age, and (iii) a highly developed soil of late Pleistocene age. At each, we ponded water in a 1-m-diameter infi ltration ring for 2.3 h and monitored soil water content and matric pressure during and atier infi ltration, using probes and electrical resistivity imaging (ERI). Infi ltration and downward fl ow rates were greater in younger material, favoring deep-rooted species. Deep-rooted species tend to colonize the margins of washes, where they are unaff ected by sediment transport that inhibits colonization. The ERI results support important generalizations, for example that shallower than 0.5 m, infi ltrated water persists longer in highly developed soil, favoring shallow-rooted species. Soil moisture data for the two youngest soils suggested that saturation overshoot, which may have signifi cant but unexplored hydroecologic and pedogenic eff ects, occurred at the horizontally advancing weting front. Spatial heterogeneity of soil properties generally increased with pedogenic development. Evidence suggested that some early-stage developmental processes may promote uniformity; the intermediate- age soil appeared to have the least heterogeneity in terms of textural variation with depth, and also the least anisotropy. Lateral heterogeneity was pronounced in older soil, having a multitude of eff ects on the distribution and retention of soil water, and may facilitate certain water-conserving strategies of plants over what would be possible in a laterally homogeneous soil. ?? Soil Science Society of America.

California↗

USGS2009

High-resolution hydro- and geo-stratigraphy at Atlantic Coastal Plain drillhole CR-622 (Strat 8)

We interpret borehole geophysical logs in conjunction with lithology developed from continuous core to produce high-resolution hydro- and geo-stratigraphic profiles for the drillhole CR-622 (Strat 8) in the Atlantic Coastal Plain of North Carolina. The resulting hydrologic and stratigraphic columns show a generalized relation between hydrologic and geologic units. Fresh-water aquifers encountered are the surficial, Yorktown, Pungo River and Castle Hayne. Geologic units present are of the middle and upper Tertiary and Quaternary periods, these are the Castle Hayne (Eocene), Pungo River (Miocene), Yorktown (Pliocene), James City and Flanner Beach (Pleistocene), and the topsoil (Holocene). The River Bend Formation (Oligocene) is missing as a distinct unit between the Pungo River Formation and the Castle Hayne Formation. The confining unit underlying the Yorktown Aquifer corresponds to the Yorktown Geologic Unit. The remaining hydrologic units and geologic units are hydrologically transitional and non-coincident. The lower Pungo River Formation serves as the confining unit for the Castle Hayne Aquifer, rather than the River Bend Aquifer, and separates the Pungo River Aquifer from the upper Castle Hayne Aquifer. All geologic formations were bound by unconformities. All aquifers were confined by the anticipated hydrologic units. We conclude that CR-622 (Strat 8) represents a normal sequence in the Atlantic Coastal Plain.

Stratigraphy↗