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At least 1,027 records · Page 57Linked to original sources
USGS1990

Berriasian (Early Cretaceous) radiometric ages from the Grindstone Creek Section, Sacramento Valley, California

The Grindstone Creek Section, Glenn County, Northern California is a sequence of hemipelagic mudstone, siltstone and sandstone interbedded with concretionary limestone and a few thin tuffs and bentonites. Two tuffs have been collected from a narrow interval of this sequence and subjected to mineralogical and isotopic analyses. U&z.sbnd;Pb isotopic analyses of zircon fractions from these volcanic horizons indicate an age of 137.1 + 1.6/−0.6 Ma. A detailed investigation has been conducted on the calcareous nannofossil stratigraphy of this section based on numerous samples with moderately preserved assemblages. The nannoflora is largely of Tethyan affinity, and allows direct correlation with the Berriasian stratotype section, with sections with published magnetostratigraphies and with a DSDP site drilled between known magnetic anomalies. The dated tuffs lie in the lower part of the upper Berriasian Cretarhabdus angustiforatus Zone ( Assipetra infracretacea Subzone) and within the narrow range of Rhagodiscus nebulosus . At three different sections, this subzone can be correlated with M-sequence Polarity Zones M16 and M16n. An independent magnetostratigraphic correlation is provided at DSDP Site 387, drilled between anomalies M15 and M16, where basal sediments contain R. nebulosus. Buchia collected within a meter of the lower tuff lie within the B. uncitoides Zone which is Berriasian in age. The upper tuff level, which occurs 65 m above the lower tuff, is situated within the overlying B. pacifica Zone. This zone had previously been correlated with the early Valanginian, but is clearly also partly of Berriasian age based on nannofossil stratigraphy. Our results allow an estimate of the age of the Berriasian-Valanginian and Jurassic-Cretaceous boundaries of 135.1 Ma and 141.1 Ma, respectively, and these fall within the range of, but differ significantiy from, several published time-scales.

California↗
USGS1992

Chronologic evidence for multiple periods of loess deposition during the Late Pleistocene in the Missouri and Mississippi River Valley, United States: Implications for the activity of the Laurentide ice sheet

The loess stratigraphy of the mid-continental U.S. is an important proxy record for the activity of the Laurentide Ice Sheet in North America. One of the most outstanding problems is deciphering the age of loess deposits in this area during the late Pleistocene. Radiocarbon dating of snails and thermoluminescence dating of the fine-silt fraction (4–11 μm) from loess at the Loveland Loess type section, Loveland, Iowa and a recent excavation at the Pleasant Grove School section. Madison County, Illinois provide new chronologic control on loess deposition in the Mississippi/Missouri River Valley chronology indicates that the Loveland Loess is Illinoian in age (135±20 ka) but is not correlative with the Teneriffe Silt which is dated to 77 ± 8 ka. Concordant radiocarbon and thermoluminescence age estimates demonstrate that the Roxana Silt and a correlative loess in Iowa, the Pisgah Formation, is probably 40-30 ka old. These age estimates in conjunction with previous results indicate that there were four periods of loess deposition during the last 150 ka at 25-12 ka, 45-30 ka, 85-70 ka and at ca. 135 ± 20 ka. This chronology of loess deposition supports the presence of both a late Illinoian and early Wisconsinan loess and associated soils. Thus, there may be more than one soil in the loess stratigraphy of the mid-continental U.S. with morphologies similar to the Sangamon Soil. The last three periods of loess deposition may be correlative with periods of elevated dust concentrations recorded in the Dye 3 ice core from southern Greenland. This is particularly significant because both areas possibly had the same source for eolian particles. Reconstructions of atmospheric circulation for glacial periods show a southerly deflected jet stream that could have transported dust from the mid-continental USA to southern Greenland. Lastly, the inferred record of loess deposition is parallel to a chronology for deglaciation of the Laurentide Ice Sheet deciphered from chronologic and stratigraphic studies of raised glacial and marine sediments in the Hudson Bay Lowlands, Canada. These chronologies indicate that the Laurentide Ice Sheet was quite dynamic during the late Pleistocene, advancing and retreating across North America at least four times during the last 150 ka.

Missouri and Mississippi River Valley↗
USGS1994

Third-order Middle Miocene-Early Pliocene depositional sequences in the prograding delta complex of the Pannonian Basin

Few studies exist in the geologic literature that show the distribution of seismic facies and depositional sequences within a lacustrine basin. The Pannonian Basin of Central Europe offers a unique opportunity to evaluate the influence of the eustatic signal on lacustrine deposition. Seismic stratigraphie and sedimentological studies indicate that the Middle Miocene-Early Pliocene infill of the transtensional Pannonian Basin was formed by large delta systems. Systematic sequence stratigraphie analysis of 6000 km of reflection seismic data and more than 100 hydrocarbon exploration wells in Hungary allowed the identification of twelve third-order sequence boundaries in the late Neogene sedimentary fill. This number of depositional sequences corresponds to that of the published global eustatic curve for this time period. Furthermore, based on magnetostratigraphic and radiometric data, the ages of these depositional sequences can be tentatively correlated with the global eustatic curve. The Pannonian Basin became isolated from the world sea at the Sarmatian/Pannonian (11.5 Ma) boundary and formed a large lake. The stratal patterns and sedimentary facies of individual systems tracts within the lacustrine sequences display the same characteristics as marine depositional sequences. The relatively low rate of thermal subsidence and the high rate of sediment supply resulted in a good sequence resolution. Within the third-order sequences higher-order sequences can be recognized with an average duration of about 0.1-0.5 Ma.

Tectonophysics↗
USGS2022

Isotopic analysis of radium geochemistry at discrete intervals in the Midwestern Cambrian-Ordovician aquifer system

Radium (Ra) is a geogenic radioactive contaminant that frequently occurs at elevated levels in the Midwestern Cambrian-Ordovician aquifer system (MCOAS). Geochemical indicators (e.g., redox conditions or total dissolved solids) can broadly characterize conditions associated with elevated Ra levels in groundwater, but do not consistently correlate to elevated Ra within specific stratigraphic horizons. A coupled geochemical and isotopic study of groundwater and aquifer solids for major and trace elements, Ra, and uranium (U) at discrete intervals in the MCOAS was used to elucidate processes that may be responsible for this disconnect, via analysis of groundwater as well as extracted and digested solid aquifer samples. We find that the potential for Ra mobilization varies by stratigraphic unit, as observed by whole-rock 226 Ra/ 238 U (dis)equilibrium. Overall, the examined aqueous geochemical characteristics (e.g., redox conditions, total dissolved solids) do not explain Ra concentrations within the system, suggesting that alternative factors, like solid-phase associations or the extent of alpha recoil damage, may be more important. A relation between aqueous 87 Sr/ 86 Sr and 226 Ra suggests that minerals with radiogenic 87 Sr/ 86 Sr are more likely to release 226 Ra to the aqueous system. Overall, the release of U and Ra due to water-rock interaction varies with discrete stratigraphy, depending on aqueous geochemistry and available mineral associations. Due to complex Ra-rock interactions and the heterogeneous geology of the MCOAS, aqueous geochemistry does not fully predict the mobilization and concentration of Ra in groundwater. As sources and sinks of Ra within the MCOAS vary across stratigraphy, knowledge of aqueous geochemistry, available solid-phase associations, and nuclide leachability all are important to consider for understanding elevated Ra occurrence in aquifer systems.

Wisconsin↗
USGS2006

The Cretaceous Tetori biota in Japan and its evolutionary significance for terrestrial ecosystems in Asia

Cretaceous nonmarine deposits are widely distributed on the Asian continent and include various kinds of zoo- and phyto-assemblages. The Tetori Group is one of the most important Mesozoic terrestrial deposits in East Asia, and for this reason its geology, stratigraphy, and biota have been studied intensively by our group for more than a decade. We present the main results herein. We confirm that formations as lithostratigraphic units are the best geological correlation tools for the Tetori Group and the best tools for a geological mapping of the group. Although subgroups have previously been used for correlation, proper designation and evaluation of subgroups is required if they are to be used effectively, and we show that previous geological correlation of the Tetori Group has been confused by inappropriate definition of these subgroups. We located fossil localities including reported zoo- and phyto-assemblages in the framework of formations correlated by our stratigraphy. The occurrence of zoo-assemblages was probably controlled by environments (i.e., most are in situ), but phyto-assemblages were mostly transported and rapidly buried by high-energy river systems. Although two distinct dinosaur faunas and four floras have been named for the zoo- and phyto-assemblages in the Tetori Group, in reality there is only one Tetori Dinosaur Fauna and one Tetori Flora, as proved by careful correlation. Two types of zoo-assemblages co-occur in the Tetori Group: vertebrate species whose ancestors flourished in the Jurassic (as found in China), and their descendants from the Late Cretaceous. As the latter modern type of assemblage is more abundant than the former, changeable environments at the continental margin probably accelerated evolution of more modern species. We can employ nonmarine molluscan species as geological correlation tools in some cases, i.e., when their taxon ranges are well-confirmed by independent evidence. However, because freshwater molluscan species and terrestrial vertebrate species had many opportunities to move to optimum habitat as environments changed through time on the Cretaceous Asian continent, their correlation potential is uncertain. Many nonmarine molluscan species from the Japanese and Chinese Cretaceous had their stratigraphic occurrences controlled by changing environments. ?? 2005 Elsevier Ltd. All rights reserved.

Cretaceous Research↗
USGS2008

Anatomy of a shoreface sand ridge revisited using foraminifera: False Cape Shoals, Virginia/North Carolina inner shelf

Certain details regarding the origin and evolution of shelf sand ridges remain elusive. Knowledge of their internal stratigraphy and microfossil distribution is necessary to define the origin and to determine the processes that modify sand ridges. Fourteen vibracores from False Cape Shoal A, a well-developed shoreface-attached sand ridge on the Virginia/North Carolina inner continental shelf, were examined to document the internal stratigraphy and benthic foraminiferal assemblages, as well as to reconstruct the depositional environments recorded in down-core sediments. Seven sedimentary and foraminiferal facies correspond to the following stratigraphic units: fossiliferous silt, barren sand, clay to sandy clay, laminated and bioturbated sand, poorly sorted massive sand, fine clean sand, and poorly sorted clay to gravel. The units represent a Pleistocene estuary and shoreface, a Holocene estuary, ebb tidal delta, modern shelf, modern shoreface, and swale fill, respectively. The succession of depositional environments reflects a Pleistocene sea-level highstand and subsequent regression followed by the Holocene transgression in which barrier island/spit systems formed along the Virginia/North Carolina inner shelf ???5.2 ka and migrated landward and an ebb tidal delta that was deposited, reworked, and covered by shelf sand.

Continental Shelf Research↗
USGS2015

Geologic control on the evolution of the inner shelf morphology offshore of the Mississippi barrier islands, northern Gulf of Mexico, USA

Between 2008 and 2013, high-resolution geophysical surveys were conducted around the Mississippi barrier islands and offshore. The sonar surveys included swath and single-beam bathymetry, sidescan, and chirp subbottom data collection. The geophysical data were groundtruthed using vibracore sediment collection. The results provide insight into the evolution of the inner shelf and the relationship between the near surface geologic framework and the morphology of the coastal zone. This study focuses on the buried Pleistocene fluvial deposits and late Holocene shore-oblique sand ridges offshore of Petit Bois Island and Petit Bois Pass. Prior to this study, the physical characteristics, evolution, and interrelationship of the ridges between both the shelf geology and the adjacent barrier island platform had not been evaluated. Numerous studies elsewhere along the coastal margin attribute shoal origin and sand-ridge evolution to hydrodynamic processes in shallow water (<20 m). Here we characterize the correlation between the geologic framework and surface morphology and demonstrate that the underlying stratigraphy must also be considered when developing an evolutionary conceptual model. It is important to understand this near surface, nearshore dynamic in order to understand how the stratigraphy influences the long-term response of the coastal zone to sea-level rise. The study also contributes to a growing body of work characterizing shore-oblique sand ridges which, along with the related geology, are recognized as increasingly important components to a nearshore framework whose origins and evolution must be understood and inventoried to effectively manage the coastal zone.

Continental Shelf Research↗
USGS2012

High-resolution tephrochronology of the Wilson Creek Formation (Mono Lake, California) and Laschamp event using 238 U- 230 Th SIMS dating of accessory mineral rims

Sediments of the Wilson Creek Formation surrounding Mono Lake preserve a high-resolution archive of glacial and pluvial responses along the eastern Sierra Nevada due to late Pleistocene climate change. An absolute chronology for the Wilson Creek stratigraphy is critical for correlating the paleoclimate record to other archives in the western U.S. and the North Atlantic region. However, multiple attempts to date the Wilson Creek stratigraphy using carbonates and tephras yield discordant results due to open-system effects and radiocarbon reservoir uncertainties as well as abundant xenocrysts. New ion microprobe 238 U- 230 Th dating of the final increments of crystallization recorded by allanite and zircon autocrysts from juvenile pyroclasts yield ages that effectively date eruption of key tephra beds and delimit the timing of basal Wilson Creek sedimentation to the interval between 26.8±2.1 and 61.7±1.9 ka. Tephra (Ash 15) erupted during the geomagnetic excursion originally designated the Mono Lake excursion yields an age of 40.8±1.9 ka, indicating that the event is instead the Laschamp excursion. The new ages support a depositional chronology from magnetostratigraphy that indicates quasi-synchronous glacial and hydrologic responses in the Sierra Nevada and Mono Basin to regional climate change, with intervals of lake filling and glacial-snowpack melting that are in phase with peaks in spring insolation.

California↗
USGS2011

Variations in eruption style during the 1931 A.D. eruption of Aniakchak volcano, Alaska

The 1931 A.D. eruption of Aniakchak volcano, Alaska, progressed from subplinian to effusive eruptive style and from trachydacite to basaltic andesite composition from multiple vent locations. Eyewitness accounts and new studies of deposit stratigraphy provide a combined narrative of eruptive events. Additional field, compositional, grain size, componentry, density, and grain morphology data document the influences on changing eruptive style as the eruption progressed. The eruption began on 1 May 1931 A.D. when a large subplinian eruption column produced vesicular juvenile-rich tephra. Subsequent activity was more intermittent, as magma interacted with groundwater and phreatomagmatic ash and lithic-rich tephra was dispersed up to 600 km downwind. Final erupted products were more mafic in composition and the eruption became more strombolian in style. Stratigraphic evidence suggests that two trachydacitic lava flows were erupted from separate but adjacent vents before the phreatomagmatic phase concluded and that basaltic andesite lava from a third vent began to effuse near the end of explosive activity. The estimated total bulk volume of the eruption is 0.9 km 3 , which corresponds to approximately 0.3 km 3 of magma. Eruption style changes are interpreted as follows: (1) a decrease in magma supply rate caused the change from subplinian to phreatomagmatic eruption; (2) a subsequent change in magma composition caused the transition from phreatomagmatic to strombolian eruption style. Additionally, the explosion and effusion of a similar magma composition from three separate vents indicates how the pre-existing caldera structure controlled the pathway of shallow magma ascent, thus influencing eruption style.

Alaska↗
USGS2017

Paleoseismic potential of sublacustrine landslide records in a high-seismicity setting (south-central Alaska)

Sublacustrine landslide stratigraphy is considered useful for quantitative paleoseismology in low-seismicity settings. However, as the recharging of underwater slopes with sediments is one of the factors that governs the recurrence of slope failures, it is not clear if landslide deposits can provide continuous paleoseismic records in settings of frequent strong shaking. To test this, we selected three lakes in south-central Alaska that experienced a strong historical megathrust earthquake (the 1964 M w 9.2 Great Alaska Earthquake) and exhibit high sedimentation rates in their main basins (0.2 cm yr − 1 –1.0 cm yr − 1 ). We present high-resolution reflection seismic data (3.5 kHz) and radionuclide data from sediment cores in order to investigate factors that control the establishment of a reliable landslide record. Seismic stratigraphy analysis reveals the presence of several landslide deposits in the lacustrine sedimentary infill. Most of these landslide deposits can be attributed to specific landslide events, as multiple landslide deposits sourced from different lacustrine slopes occur on a single stratigraphic horizon. We identify numerous events in the lakes: Eklutna Lake proximal basin (14 events), Eklutna Lake distal basin (8 events), Skilak Lake (7 events) and Kenai Lake (7 events). The most recent event in each basin corresponds to the historic 1964 megathrust earthquake. All events are characterized by multiple landslide deposits, which hints at a regional trigger mechanism, such as an earthquake (the synchronicity criterion). This means that the landslide record in each basin represents a record of past seismic events. Based on extrapolation of sedimentation rates derived from radionuclide dating, we roughly estimate a mean recurrence interval in the Eklutna Lake proximal basin, Eklutna Lake distal basin, Skilak Lake and Kenai Lake, at ~ 250 yrs., ~ 450 yrs., ~ 900 yrs. and ~ 450 yrs., respectively. This distinct difference in recording can be explained by variations in preconditioning factors like slope angle, slope recharging (sedimentation rate) and the sediment source area: faster slope recharging and a predominance of delta and alluvial fan failures, increase the sensitivity and lower the intensity threshold for slope instability. Also, the seismotectonic setting of the lakes has to be taken into account. This study demonstrates that sublacustrine landslides in several Alaskan lakes can be used as reliable recorders of strong earthquake shaking, when a multi-lake approach is used, and can enhance the temporal and spatial resolution of the paleoseismic record of south-central Alaska.

Alaska↗
USGS2025

Solution-collapse breccia pipe uranium deposits of the southern Colorado Plateau, northwestern Arizona, USA

Some of the highest-grade uranium deposits in the United States occur in breccia pipes that formed by solution and collapse of sedimentary strata, which occur in the southern portion of the Colorado Plateau in northwestern Arizona. The host breccia pipes are up to 1200 m in vertical extent, average about 90 m in diameter, and can cross-cut strata from their base in the Mississippian Redwall Limestone to as stratigraphically high on some plateaus as the Triassic Chinle Formation. These uranium-base metal deposits are up to 600 m thick and formed within the breccia pipes where they transect the Permian Coconino Sandstone, Hermit Formation, and the Esplanade Sandstone. Of the hundreds of breccia pipes identified across this region, only a small percentage are known to contain mineralization. The main uranium ore mineral is uraninite that is intergrown with at least 20 base-metal sulfide minerals, which contribute Fe, Cu, Co, As, Pb, Zn, Ni, and Ag to the deposits. This study considered regional stratigraphy, sulfur isotope systematics, mineralogy, in situ dating, and compilation and analysis of previous work on the deposits. A comprehensive deposit model has not been published for these deposits. This analysis identified new additions to update the deposit model for these unusual, possibly unique deposits. Proposed modifications to the model include: (1) the source, mechanisms, timing of the base-metal sulfide mineral assemblages, and (2) the source, mechanism, and timing of the uranium mineralization. Sulfide and uranium deposition are shown to be separate mineralization events. The study proposes the possible role of gypsum as a source of sulfur for the sulfide minerals in the deposits. Groundwaters carrying uranium encountered the preexisting sulfides in breccia pipes, reducing the uranyl ions, and precipitating U oxide (as uraninite). Analysis of the regional stratigraphy recognized that numerous beds of gypsum are in the strata that lie only tens of meters above the breccia pipe deposits. In the breccia pipe region, if these stratigraphic units (Toroweap and Kaibab Formations) do not contain gypsum layers then the underlying pipes are not mineralized; where these Permian gypsum layers do occur, breccia pipes can host mineralization. This new understanding should be useful in identifying the prospective region for mineralized pipes.

Arizona↗
USGS2016

Cenozoic sea level and the rise of modern rimmed atolls

Sea-level records from atolls, potentially spanning the Cenozoic, have been largely overlooked, in part because the processes that control atoll form (reef accretion, carbonate dissolution, sediment transport, vertical motion) are complex and, for many islands, unconstrained on million-year timescales. Here we combine existing observations of atoll morphology and corelog stratigraphy from Enewetak Atoll with a numerical model to (1) constrain the relative rates of subsidence, dissolution and sedimentation that have shaped modern Pacific atolls and (2) construct a record of sea level over the past 8.5 million years. Both the stratigraphy from Enewetak Atoll (constrained by a subsidence rate of ~ 20 m/Myr) and our numerical modeling results suggest that low sea levels (50–125 m below present), and presumably bi-polar glaciations, occurred throughout much of the late Miocene, preceding the warmer climate of the Pliocene, when sea level was higher than present. Carbonate dissolution through the subsequent sea-level fall that accompanied the onset of large glacial cycles in the late Pliocene, along with rapid highstand constructional reef growth, likely drove development of the rimmed atoll morphology we see today.

Palaeogeography, Palaeoclimatology, Palaeoecology↗
USGS1997

Correlation of Miocene sequences and hydrogeologic units, New Jersey Coastal Plain

We have developed a Miocene sequence stratigraphic framework using data from recently drilled boreholes in the New Jersey Coastal Plain. Sequences are shallowing upward, unconformity-bounded units; fine-grained shelf and prodelta sediments grade upward to delta front and shallow-marine sands, corresponding to confining bed-aquifer couplets. By dating Miocene sequences using Sr-isotope stratigraphy, and mapping with borehole data and geophysical logs, we can predict the continuity and effectiveness of the confining beds and aquifers. The following are illustrated on a 90-km basinward dip section: (1) the composite confining bed is comprised of the KwO and lower Kw1a (ca. 23.8-20.5 Ma) sequences downdip at Atlantic City, and the Kw1b, Kw1a and older sequences updip (ca. 69.3-20.6 Ma), and is continuous throughout most of the coastal plain; (2) the major confined aquifer, the Atlantic City 800-foot sand, is comprised of the upper Kw1a and Kw1b sequences (ca. 20.5-20.2 Ma) and is an areally continuous sand that is interconnected with the Kirkwood-Cohansey aquifer system updip of Mays Landing; (3) the confining bed above the Atlantic City 800-foot sand is comprised of the Kw2a, Kw2b, and Kw3 sequences (18.1-13.3 Ma) and is an extensive confining bed that pinches out updip. These sequences and aquifer-confining bed couplets are linked to global sea-level changes evinced by the ??18O record. We conclude that sequence stratigraphy is a powerful tool when applied to regional hydrogeologic problems, although basinal tectonic differences and localized variations in sediment supply can affect aquifer thickness and permeability.

New Jersey↗
USGS1999

Kinematic analysis of melange fabrics: Examples and applications from the McHugh Complex, Kenai Peninsula, Alaska

Permian to Cretaceous melange of the McHugh Complex on the Kenai Peninsula, south-central Alaska includes blocks and belts of graywacke, argillite, limestone, chert, basalt, gabbro, and ultramafic rocks, intruded by a variety of igneous rocks. An oceanic plate stratigraphy is repeated hundreds of times across the map area, but most structures at the outcrop scale extend lithological layering. Strong rheological units occur as blocks within a matrix that flowed around the competent blocks during deformation, forming broken formation and melange. Deformation was noncoaxial, and disruption of primary layering was a consequence of general strain driven by plate convergence in a relatively narrow zone between the overriding accretionary wedge and the downgoing, generally thinly sedimented oceanic plate. Soft-sediment deformation processes do not appear to have played a major role in the formation of the melange. A model for deformation at the toe of the wedge is proposed in which layers oriented at low angles to ??1 are contracted in both the brittle and ductile regimes, layers at 30-45??to ??1 are extended in the brittle regime and contracted in the ductile regime, and layers at angles greater than 45??to ??1 are extended in both the brittle and ductile regimes. Imbrication in thrust duplexes occurs at deeper levels within the wedge. Many structures within melange of the McHugh Complex are asymmetric and record kinematic information consistent with the inferred structural setting in an accretionary wedge. A displacement field for the McHugh Complex on the lower Kenai Peninsula includes three belts: an inboard belt of Late Triassic rocks records west-to-east-directed slip of hanging walls, a central belt of predominantly Early Jurassic rocks records north-south directed displacements, and Early Cretaceous rocks in an outboard belt preserve southwest-northeast directed slip vectors. Although precise ages of accretion are unknown, slip directions are compatible with inferred plate motions during the general time frame of accretion of the McHugh Complex. The slip vectors are interpreted to preserve the convergence directions between the overriding and underriding plates, which became more oblique with time. They are not considered indicative of strain partitioning into belts of orogen-parallel and orogen-perpendicular displacements, because the kinematic data are derived from the earliest preserved structures, whereas fabrics related to strain partitioning would be expected to be superimposed on earlier accretion-related fabrics.Permian to Cretaceous melange of the McHugh Complex on the Kenai Peninsula, south-central Alaska includes blocks and belts of graywacke, argillite, limestone, chert, basalt, gabbro, and ultramafic rocks, intruded by a variety of igneous rocks. An oceanic plate stratigraphy is repeated hundreds of times across the map area, but most structures at the outcrop scale extend lithological layering. Strong rheological units occur as blocks within a matrix that flowed around the competent blocks during deformation, forming broken formation and melange. Deformation was noncoaxial, and disruption of primary layering was a consequence of general strain driven by plate convergence in a relatively narrow zone between the overriding accretionary wedge and the downgoing, generally thinly sedimented oceanic plate. Soft-sediment deformation processes do not appear to have played a major role in the formation of the melange. A model for deformation at the toe of the wedge is proposed in which layers oriented at low angles to ??1 are contracted in both the brittle and ductile regimes, layers at 30-45?? to ??1 are extended in the brittle regime and contracted in the ductile regime, and layers at angles greater than 45?? to ??1 are extended in both the brittle and ductile regimes. Imbrication in thrust duplexes occurs at deeper levels within the wedge. Many structures within melange of the McHugh Complex are asymmetric and record

Journal of Structural Geology↗
USGS1999

Stratigraphic constraints on the timing and emplacement of the Alika 2 giant Hawaiian submarine landslide

Previous work has found evidence for giant tsunami waves that impacted the coasts of Lanai, Molokai and other southern Hawaiian Islands, tentatively dated at 100 + and 200 + ka by U-series methods on uplifted coral clasts. Seafloor imaging and related work off Hawaii Island has suggested the Alika phase 2 debris avalanche as the source of the ~ 100 ka "giant wave deposits", although its precise age has been elusive. More recently, a basaltic sand bed in ODP site 842 (~ 300 km west of Hawaii) estimated at 100 ?? 20 ka has been suggested to correlate with this or another large Hawaiian landslide. Our approach to the timing and linkage of giant submarine landslides and paleo-tsunami deposits is a detailed stratigraphic survey of pelagic deposits proximal to the landslide feature, beginning with a suite of seven piston, gravity and box cores collected in the vicinity of the Alika 2 slide. We used U-series dating techniques, including excess 230 Th and 210 Pb profiling, high-resolution paleomagnetic stratigraphy, including continuous, U-channel analysis, δ 18 O stratigraphy, visual and X-ray sediment lithology, and the petrology and geochemistry of the included turbidites and ash layers. Minimum ages for the Alika phase 2a slide from detailed investigation of two of the cores are 112 ± 15 ka and 125 ± 24 ka (2σ) based on excess 230 Th dating. A less precise age for the Alika phase 1 and/or South Kona slide is 242 ± 80 ka (2σ), consistent with previous geological estimates. Oxygen isotope analyses of entrained planktonic foraminifera better constrain the Alika phase 2a maximum age at 127 ± 5 ka, which corresponds to the beginning of the stage 5e interglacial period. It is proposed that triggering of these giant landslides may be related to climate change when wetter periods increase the possibility of groundwater intrusion and consequent phreatomagmatic eruptions of shallow magma chambers. Our study indicates the contemporaneity of the Alika giant submarine landslides and distal deposits from enormous turbidity currents as well as coral clasts reported to be tsunami deposits on Lanai and Molokai through direct dating and compositional analysis of the landslide deposits.

Journal of Volcanology and Geothermal Research↗
USGS2003

The Pleistocene evolution of the East Antarctic Ice Sheet in the Prydz bay region: Stable isotopic evidence from ODP Site 1167

Ocean Drilling Program Leg 188, Prydz Bay, East Antarctica is part of a larger initiative to explore the Cenozoic history of the Antarctic Ice Sheet through direct drilling and sampling of the continental margins. In this paper, we present stable isotopic results from Ocean Drilling Program (ODP) Site 1167 located on the Prydz Channel Trough Mouth Fan (TMF), the first Antarctic TMF to be drilled. The foraminifer-based ??18O record is interpreted along with sedimentary and downhole logging evidence to reconstruct the Quaternary glacial history of Prydz Bay and the adjacent Lambert Glacier Amery Ice Shelf System (LGAISS). We report an electron spin resonance age date of 36. 9 ?? 3.3 ka at 0.45 m below sea floor and correlate suspected glacial-interglacial cycles with the global isotopic stratigraphy to improve the chronology for Site 1167. The ??18O record based on planktonic (Neogloboquadrina pachyderma (s.)) and limited benthic results (Globocassidulina crassa), indicates a trend of ice sheet expansion that was interrupted by a period of reduced ice volume and possibly warmer conditions during the early-mid-Pleistocene (0.9-1.38 Ma). An increase in ?? 18O values after ??? 900 ka appears to coincide with the mid-Pleistocene climate transition and the expansion of the northern hemisphere ice sheet. The ??18O record in the upper 50 m of the stratigraphic section indicates as few as three glacial-interglacial cycles, tentatively assigned as marine isotopic stages (MIS) 16-21, are preserved since the Brunhes/Matuyama paleomagnetic reversal (780 ka). This suggests that there is a large unconformity near the top of the section and/or that there may have been few extreme advances of the ice sheet since the mid-Pleistocene climate transition resulting in lowered sedimentation rates on the Prydz Channel TMF. The stable isotopic record from Site 1167 is one of the few available from the area south of the Antarctic Polar Front that has been linked with the global isotopic stratigraphy. Our results suggest the potential for the recovery of useful stable isotopic records in other TMFs. ?? 2003 Elsevier B.V. All rights reserved.

Global and Planetary Change↗
USGS2000

Ground penetrating radar imaging of cap rock, caliche and carbonate strata

Field experiments show ground penetrating radar (GPR) can be used to image shallow carbonate stratigraphy effectively in a variety of settings. In south Florida, the position and structure of cap rock cover on limestone can be an important control on surface water flow and vegetation, but larger scale outcrops (tens of meters) of cap rock are sparse. GPR mapping through south Florida prairie, cypress swamp and hardwood hammock resolves variations in thickness and structure of cap rock to ~3 m and holds the potential to test theories for cap rock-vegetation relationships. In other settings, carbonate strata are mapped to test models for the formation of local structural anomalies. A test of GPR imaging capabilities on an arid caliche (calcrete) horizon in southeastern Nevada shows depth penetration to ~2 m with resolution of the base of caliche. GPR profiling also succeeds in resolving more deeply buried (~5 m) limestone discontinuity surfaces that record subaerial exposure in south Florida. (C) 2000 Elsevier Science B.V. All rights reserved.Field experiments show ground penetrating radar (GPR) can be used to image shallow carbonate stratigraphy effectively in a variety of settings. In south Florida, the position and structure of cap rock cover on limestone can be an important control on surface water flow and vegetation, but larger scale outcrops (tens of meters) of cap rock are sparse. GPR mapping through south Florida prairie, cypress swamp and hardwood hammock resolves variations in thickness and structure of cap rock to approx. 3 m and holds the potential to test theories for cap rock-vegetation relationships. In other settings, carbonate strata are mapped to test models for the formation of local structural anomalies. A test of GPR imaging capabilities on an arid caliche (calcrete) horizon in southeastern Nevada shows depth penetration to approx. 2 m with resolution of the base of caliche. GPR profiling also succeeds in resolving more deeply buried (approx. 5 m) limestone discontinuity surfaces that record subaerial exposure in south Florida.

Conference Paper↗
USGS2022

Sedimentological and geochemical perspectives on a marginal lake environment recorded in the Hartmann’s Valley and Karasburg members of the Murray formation, Gale crater, Mars

This study utilizes instruments from the Curiosity rover payload to develop an integrated paleoenvironmental and compositional reconstruction for the 65-m thick interval of stratigraphy comprising the Hartmann's Valley and Karasburg members of the Murray formation, Gale crater, Mars. The stratigraphy consists of cross-stratified sandstone (Facies 1), planar-laminated sandstone (Facies 2), and planar-laminated mudstone (Facies 3). Facies 1 is composed of sandstone showing truncated sets of concave-curvilinear laminae stacked into cosets. Sets are estimated to be meter-to sub-meter-scale, consistent with low-height dunes. Thin stratigraphic intervals of Facies 1 and stacking patterns with Facies 2 and 3 support a wet aeolian dune interpretation. Meter-thick packages of planar-laminated sandstone (Facies 2) are interpreted to represent interfingering dune-interdune strata. Facies 3 consists of meter-thick packages of planar-laminated mudstone interpreted to represent lacustrine deposition with persistent standing water. Integration of geochemistry with each facies reveals some compositional control based on the depositional process. Models for source rock composition from Alpha Particle X-Ray Spectrometer measurements show that facies derived from a basaltic source. Alteration indices and geochemical trends provide evidence that moderate chemical weathering occurred before compositional changes due to diagenesis. Differences in wt% FeO (T) and TiO 2 between facies are minimal, though trends point to sediment sorting in transport. Comparisons to terrestrial basaltic sedimentary systems indicate that the Hartmann's Valley and Karasburg facies reflect deposition in an environment where diverse subaqueous and subaerial facies persisted adjacent to a long-lived body of water.

Journal of Geophysical Research Planets↗