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At least 1,189 records · Page 66Linked to original sources
USGS2004

Submarine landslides of San Pedro Escarpment, southwest of Long Beach, California

The coastal infrastructure of the southern greater Los Angeles metropolitan area would be profoundly affected by a large tsunami. Submarine slope failures and active faults, either of which could have generated a tsunami, are known on the shelf and slope near Long Beach. Large slope failures are present on the San Pedro Escarpment and on the basin slope adjacent to the San Pedro shelf. The southeastern part of the escarpment has had a long history of slope failure. The most recent failure, the Palos Verdes slide, is over 4.5 km long, has been dated as 7500 years old, and involved over 0.34 km 3 of material, which now litters the adjacent basin floor. Other, smaller, deposits from nearby failures are also present, as are buried wedges of debris that indicate slope failures have occurred locally throughout the Holocene and much of the late Pleistocene. Slope failures have occurred in response to continual Quaternary uplift of the Palos Verdes anticlinorium. The Palos Verdes slide could potentially have generated a failure-related tsunami with an amplitude in the range of 8-12 m because it apparently failed catastrophically, started in shallow water, evolved on low-drag bedding planes, had a long slide path, and involved high-strength lithified material. ?? 2003 Elsevier B.V. All rights reserved.

Conference Paper↗
USGS1997

Shallow subsurface geology of the continental shelf, Gulf of the Farallones, California, and its relationship to surficial seafloor characteristics

The Gulf of the Farallones is located on a continental margin that is tectonically active and has experienced eustatic fluctuations of sea level throughout the Quaternary. Bathymetry of the Gulf suggests that it is different from the average continental shelf off California in that instead of a seafloor that slopes gently seaward from the shoreline to the shelf edge (dominantly west to southwest), much of the Gulf shelf slopes to the northwest, a direction that is subparallel to that of the adjacent mainland shoreline. Isobaths are oblique to shore-normal for much of the Gulf shelf as opposed to the more typical shore-parallel isobaths. This northwest trend is parallel to that of the offshore granitic outcrops, the Farallon Island ridge and Cordell Bank. 3.5 kHz shallow subsurface profiles show a regional unconformity (basal unconformity of this study) that truncates bedrock and typically is overlain by a very thin veneer (1-2 m or less) of acoustically transparent unconsolidated sediment. In places, the basal unconformity appears to be coincident with the seafloor. Where the basal unconformity is within 1-2 m of the seafloor, side-scan sonar reveals that either bedrock pierces the seafloor or numerous linear depressions dissect the shelf seafloor. Side-scan sonar shows that the central part of the study area is characterized by numerous linear depressions that are on the order of 1-3 m deep, several meters to 5 km in length, and from 250 m to 2 km in width. The only Holocene unconsolidated sediment in excess of 1-2 m that has accumulated on the Gulf shelf is located where the bedrock surface is topographically depressed. The topographic lows are situated south of the Point Reyes headland and southwest of the Golden Gate. The sediment deposits that overlie these lows attain thicknesses up to 15-20 m; however, their average thickness is less than 10 m. Structure contours on the basal unconformity surface, essentially the top of the bedrock platform, also show that it is topographically high at its southern end and low at its northern end; the slope of the platform is therefore to the northwest. The seafloor mimics the attitude of the bedrock platform owing to the lack of appreciable sediment cover and hence also slopes northwest. This accounts for the oblique to shore-normal orientation of isobaths in the Gulf and the northwestward slope of the seafloor. The results of this investigation suggest that eustatic sea level fluctuations in conjunction with local tectonics had a profound effect on the deposition, erosion, and preservation of sediment on the Gulf shelf throughout the Quaternary and that the shallow subsurface greatly influences many aspects of the present surficial morphology.

Marine Geology↗
USGS1997

Transgressive systems tract development and incised-valley fills within a quaternary estuary-shelf system: Virginia inner shelf, USA

High-frequency Quaternary glacioeustasy resulted in the incision of six moderate- to high-relief fluvial erosion surfaces beneath the Virginia inner shelf and coastal zone along the updip edges of the Atlantic continental margin. Fluvial valleys up to 5 km wide, with up to 37 m of relief and thalweg depths of up to 72 m below modern mean sea level, cut through underlying Pleistocene and Mio-Pliocene strata in response to drops in baselevel on the order of 100 m. Fluvially incised valleys were significantly modified during subsequent marine transgressions as fluvial drainage basins evolved into estuarine embayments (ancestral generations of the Chesapeake Bay). Complex incised-valley fill successions are bounded by, or contain, up to four stacked erosional surfaces (basal fluvial erosion surface, bay ravinement, tidal ravinement, and ebb-flood channel-base diastem) in vertical succession. These surfaces, combined with the transgressive oceanic ravinement that generally caps incised-valley fills, control the lateral and vertical development of intervening seismic facies (depositional systems). Transgressive stratigraphy characterizes the Quaternary section beneath the Virginia inner shelf where six depositional sequences (Sequences I-VI) are identified. Depositional sequences consist primarily of estuarine depositional systems (subjacent to the transgressive oceanic ravinement) and shoreface-shelf depositional systems; highstand systems tract coastal systems are thinly developed. The Quaternary section can be broadly subdivided into two parts. The upper part contains sequences consisting predominantly of inner shelf facies, whereas sequences in the lower part of the section consist predominantly of estuarine facies. Three styles of sequence preservation are identified. Style 1, represented by Sequences VI and V, is characterized by large estuarine systems (ancestral generations of the Chesapeake Bay) that are up to 40 m thick, have hemicylindrical wedge geometries, and occur within large, coast-oblique trending depressions (paleo-estuaries). Style 1 is dominated by fluvial through estuary-mouth depositional systems (Seismic Facies 1-4). Style 2 sequence preservation, represented by Sequences III and II, is dominantly an inner shelf and shoreface succession with a seaward-thickening tabular wedge geometry that does not exceed 15 m in thickness. These shoreface and inner shelf depositional systems of the upper transgressive systems tract (Seismic Facies 9) and highstand systems tract (Seismic Facies 7 and 11) are not associated with paleo-estuaries. Style 3 sequence preservation is represented by Sequence 1, the Holocene Sequence. It consists of lower transgressive systems tract fluvial-estuarine, lagoonal, and tidal-inlet fill deposits (Seismic Facies 1-6, and 8) overlain by upper transgressive systems tract shelf and shoreface sands (Seismic Facies 9). Style 3 has a crenulated wedge geometry, and is thickest beneath and seaward of the modern Chesapeake Bay mouth. It thins northward and landward onto Late Pleistocene interfluvial highs on the basinward side of the southern Delmarva Peninsula.

Marine Geology↗
USGS1999

Seafloor environments in the Long Island Sound estuarine system

Four categories of modern seafloor sedimentary environments have been identified and mapped across the large, glaciated, topographically complex Long Island Sound estuary by means of an extensive regional set of sidescan sonographs, bottom samples, and video-camera observations and supplemental marine-geologic and modeled physical-oceanographic data. (1) Environments of erosion or nondeposition contain sediments which range from boulder fields to gravelly coarse-to-medium sands and appear on the sonographs either as patterns with isolated reflections (caused by outcrops of glacial drift and bedrock) or as patterns of strong backscatter (caused by coarse lag deposits). Areas of erosion or nondeposition were found across the rugged seafloor at the eastern entrance of the Sound and atop bathymetric highs and within constricted depressions in other parts of the basin. (2) Environments of bedload transport contain mostly coarse-to-fine sand with only small amounts of mud and are depicted by sonograph patterns of sand ribbons and sand waves. Areas of bedload transport were found primarily in the eastern Sound where bottom currents have sculptured the surface of a Holocene marine delta and are moving these sediments toward the WSW into the estuary. (3) Environments of sediment sorting and reworking comprise variable amounts of fine sand and mud and are characterized either by patterns of moderate backscatter or by patterns with patches of moderate-to-weak backscatter that reflect a combination of erosion and deposition. Areas of sediment sorting and reworking were found around the periphery of the zone of bedload transport in the eastern Sound and along the southern nearshore margin. They also are located atop low knolls, on the flanks of shoal complexes, and within segments of the axial depression in the western Sound. (4) Environments of deposition are blanketed by muds and muddy fine sands that produce patterns of uniformly weak backscatter. Depositional areas occupy broad areas of the basin floor in the western part of the Sound. The regional distribution of seafloor environments reflects fundamental differences in marine-geologic conditions between the eastern and western parts of the Sound. In the funnel-shaped eastern part, a gradient of strong tidal currents coupled with the net nontidal (estuarine) bottom drift produce a westward progression of environments ranging from erosion or nondeposition at the narrow entrance to the Sound, through an extensive area of bedload transport, to a peripheral zone of sediment sorting. In the generally broader western part of the Sound, a weak tidal-current regime combined with the production of particle aggregates by biologic or chemical processes, cause large areas of deposition that are locally interrupted by a patchy distribution of various other environments where the bottom currents are enhanced by and interact with the seafloor topography.

Marine Geology↗
USGS1999

Interaction of tectonic and depositional processes that control the evolution of the Iberian Gulf of Cadiz margin

This study provides an integrated view of the growth patterns and factors that controlled the evolution of the Gulf of Cadiz continental margin based on studies of the tectonic, sedimentologic and oceanographic history of the area. Seven sedimentary regimes are identified, but there are more extensive descriptions of the late Cenozoic regimes because of the larger data base. The regimes of the Mesozoic passive margin include carbonate platforms, which become mixed calcareous-terrigenous deposits during the Late Cretaceous-early Tertiary. The Oligocene and Early Miocene terrigenous regimes developed, in contrast, over the active and transcurrent margins near the African-Iberian plate boundary. The top of the Gulf of Cadiz olistostrome, emplaced in the Late Miocene, is used as a key horizon to define the 'post-orogenic' depositional regimes. The Late Miocene progradational margin regime is characterized by a large terrigenous sediment supply to the margin and coincides with the closing of the Miocene Atlantic-Mediterranean gateways. The terrigenous drift depositional regime of the Early Pliocene resulted from the occurrence of high eustatic sea level and the characteristics of the Mediterranean outflow currents that developed after the opening of the Strait of Gibraltar. The Late Pliocene and Quaternary regimes are dominated by sequences of deposits related to cycles of high and low sea levels. Deposition of shelf-margin deltas and slope wedges correlate with regressive and low sea level regimes caused by eustasy and subsidence. During the highstand regimes of the Holocene, inner shelf prograding deltas and deep-water sediment drifts were developed under the influence of the Atlantic inflow and Mediterranean outflow currents, respectively. A modern human cultural regime began 2000 years ago with the Roman occupation of Iberia; human cultural effects on sedimentary regimes may have equalled natural factors such as climate change. Interplay of tectonic and oceanographic controls dominated the evolution of the Cadiz margin during the Cenozoic. Depositional sequences formed where the tectonic setting provided the accommodation space and the shape of the deposits has been greatly influenced by the strong unidirectional Atlantic inflow currents on the shelf and Mediterranean outflow currents on the slope. The entire cycle of the inflow and outflow deposition along the margin has been controlled first by the tectonic evolution of the Betic and Rif gateways, which become closed during the Late Miocene, and after the Messinian by the opening of the Strait of Gibraltar. Strong current development during eustatic sea level highstands of the Pliocene and Quaternary has controlled deposition because of maximum sill depths at Gibraltar for water circulation. Lowstand sea levels slowed circulation and resulted in mud drapes over the slope and regressive stratigraphic sequences over the shelf. More recently, the human industrial revolution has caused heavy metal contamination of sediment and water over the Cadiz margin. Human activity also has affected sedimentation rates because of deforestation that caused increased depositional rates near undammed rivers and decreased rates where rivers have been dammed. Future research efforts will need to focus on: (1) the effect of increased Mediterranean outflow caused by river damming plus global warming and the increased outflow as a potential trigger for new ice ages; (2) assessments of geologic hazards for planning man-made shoreline structures, developing offshore petroleum resources and maintaining undersea communications cables; and (3) confirmation of the general geologic history of the Cadiz margin.

Marine Geology↗
USGS2000

Orbital and suborbital variability in North Atlantic bottom water temperature obtained from deep-sea ostracod Mg/Ca ratios

Magnesium/calcium (Mg/Ca) ratios were measured in the deep-sea ostracod (Crustacea) genus Krithe from Chain core 82-24-4PC from the western mid-Atlantic Ridge (3427 m) in order to estimate ocean circulation and bottom water temperature (BWT) variability over the past 200,000 years. Mg/Ca ratios have been used as a paleothermometer because the ratios are controlled primarily by ambient water temperatures at the time the organism secretes its adult carapace. Over the past two glacial–interglacial cycles, Mg/Ca values oscillated between about 7 mmol/mol and 12 mmol/mol, equivalent to a BWT range of 0 to >3.5°C. The lowest values were obtained on specimens from glacial marine isotope stages (MISs) 2, 4 and 6; the highest values were obtained from specimens from the early part of the Holocene interglacial (MIS 1), and also from MISs 5 and 7. These trends suggest that BWTs in the North Atlantic Ocean fluctuate over orbital time scales. Suborbital variability in Mg/Ca ratios and BWT was also observed for the past 100,000 years. Ratios rose from ∼8 mmol/mol to ∼10 mmol/mol (implying a BWT increase of ∼1 to 3°C) during 14 Mg/Ca excursions. The highest ratios were found in Krithe dated at approximately 32, 36–38, 43, 48, 73, 85 and 93 ka. Although the age model for the Chain 82-24-4PC and temporal resolution do not allow precise correlation, some of these deep-sea bottom temperature excursions appear to correspond to Heinrich events recorded in other regions of the North Atlantic and perhaps Dansgaard–Oeschger interstadial events recorded in Greenland ice cores. If confirmed, this would support the hypothesis that millennial-scale oscillations of climate in the North Atlantic are capable of affecting global climate via thermohaline circulation changes.

Palaeogeography, Palaeoclimatology, Palaeoecology↗
USGS2003

A vegetation history from the arid prepuna of northern Chile (22-23°S) over the last 13,500 years

The Quaternary paleoclimate of the central Andes is poorly understood due to numerous discrepancies among the diverse proxy records that span this geographically and climatically complex region. The exact timing, duration and magnitude of wet and dry phases are seldom duplicated from one proxy type to another, and there have been few opportunities to compare climatic records from the same proxy along environmental gradients. Vegetation histories from fossil rodent middens provide one such opportunity on the Pacific slope of the Andes. We previously reported a vegetation history from the upper margin (2400–3000 m) of the absolute desert in the central Atacama Desert of northern Chile. That record identified a distinct wet phase that peaked between 11.8 and 10.5 ka, when steppe grasses and other upland elements expanded as much as 1000 m downslope, and a secondary wet period during the middle to late Holocene (7.1–3.5 ka). The latter wet phase remains controversial and is not as readily apparent in our low-elevation midden record. We thus sought to replicate both phases in a midden record from the mid-elevations (3100–3300 m) of the arid prepuna, where slight precipitation increases would be amplified. Midden records from these elevations identify conditions wetter than today at 13.5–9.6, 7.6–6.3, 4.4–3.2 and possibly 1.8–1.2 ka. Dry phases occurred at 9.4–8.4 ka and possibly at ca. 5.1 ka. Present floras and modern hyperarid conditions were established after 3.2 ka. The records from the two elevational bands generally match with some important differences. These differences could reflect both the discontinuous aspect of the midden record and the episodic nature of precipitation and plant establishment in this hyperarid desert.

Palaeogeography, Palaeoclimatology, Palaeoecology↗
USGS1999

Quaternary glacial and deglacial Ostracoda in the thermocline of the Little Bahama Bank (NW Atlantic): Palaeoceanographic implications

We determined faunal and oceanographic changes during the last glacial and deglacial in the Providence Channel, Little Bahama Bank (LBB), using modern ocean (from LBB, Florida-Hatteras Slope and Blake Plateau, western North Atlantic) and late Quaternary (LBB) distributions of the benthic ostracode genus Krithe from the mid-depth (300-1600 m) subtropical North Atlantic Ocean. Nine species of Krithe are limited in their bathymetric distribution by warm bottom water temperatures (or a temperature-related parameter) in the thermocline of the modern Atlantic. During the last glacial interval in the northwest Providence Channel of the Little Bahama Bank five species of Krithe (K. aequabilis, K. dolichodeira, K. gr. minima, K. reversa and K. trinidadensis) migrated upslope; conversely, during the deglaciation, most Krithe species migrated downslope, re-occupying their deeper niches. These vertical species migrations are attributed to decreased glacial bottom water temperatures and perhaps increased dissolved oxygen during the last glacial and warmer water temperatures during the deglacial. Based upon thermal values of recent depth ranges of selected species of Krithe, we estimate that glacial waters cooled about 4??C (shallower than 900 m) and about 2??C (deeper than 900 m) and deglacial waters warmed about the same values in shallow and mid-depth water masses, comparing to modern temperatures. The discovery of common Halocypris, a mesopelagic ostracode, in Little Bahama Bank glacial and deglacial sediments also suggests greater oxygenation relative to the late Holocene.

Palaeogeography, Palaeoclimatology, Palaeoecology↗
USGS2017

Unprecedented last-glacial mass accumulation rates determined by luminescence dating of loess from western Nebraska

A high-resolution chronology for Peoria (last glacial period) Loess from three sites in Nebraska, midcontinental North America, is determined by applying optically stimulated luminescence (OSL) dating to 35-50 ??m quartz. At Bignell Hill, Nebraska, an OSL age of 25,000 yr near the contact of Peoria Loess with the underlying Gilman Canyon Formation shows that dust accumulation occurred early during the last glacial maximum (LGM), whereas at Devil's Den and Eustis, Nebraska, basal OSL ages are significantly younger (18,000 and 21,000 yr, respectively). At all three localities, dust accumulation ended at some time after 14,000 yr ago. Mass accumulation rates (MARs) for western Nebraska, calculated using the OSL ages, are extremely high from 18,000 to 14,000 yr-much higher than those calculated for any other pre-Holocene location worldwide. These unprecedented MARs coincide with the timing of a mismatch between paleoenvironmental evidence from central North America, and the paleoclimate simulations from atmospheric global circulation models (AGCMs). We infer that the high atmospheric dust loading implied by these MARs may have played an important role, through radiative forcing, in maintaining a colder-than-present climate over central North America for several thousand years after summer insolation exceeded present-day values. ?? 2003 Elsevier Science (USA). All rights reserved.

Quaternary Research↗
USGS2003

High resolution seismic imaging of faults beneath Limón Bay, northern Panama Canal, Republic of Panama

High-resolution seismic reflection profiles from Limo??n Bay, Republic of Panama, were acquired as part of a seismic hazard investigation of the northern Panama Canal region. The seismic profiles image gently west and northwest dipping strata of upper Miocene Gatu??n Formation, unconformably overlain by a thin (<20 m) sequence of Holocene muds. Numerous faults, which have northeast trends where they can be correlated between seismic profiles, break the upper Miocene strata. Some of the faults have normal displacement, but on many faults, the amount and type of displacement cannot be determined. The age of displacement is constrained to be Late Miocene or younger, and regional geologic considerations suggest Pliocene movement. The faults may be part of a more extensive set of north- to northeast-trending faults and fractures in the canal region of central Panama. Low topography and the faults in the canal area may be the result of the modern regional stress field, bending of the Isthmus of Panama, shearing in eastern Panama, or minor deformation of the Panama Block above the Caribbean subduction zone. For seismic hazard analysis of the northern canal area, these faults led us to include a source zone of shallow faults proximal to northern canal facilities. ?? 2003 Elsevier B.V. All rights reserved.

Tectonophysics↗
USGS1999

An example of neotectonism in a continental interior - Thebes Gap, Midcontinent, United States

Some of the most intense neotectonic activity known in the continental interior of North America has been recently discovered on a fault zone in the Thebes Gap area, Missouri and Illinois. This faulting almost assuredly was accompanied by large earthquakes. The zone is located approximately 30 km north of the New Madrid seismic zone and consists of complex north-northeast- to northeast-striking, steeply dipping faults that have had a long-lived history of reactivation throughout most of the Phanerozoic. Geophysical studies by others suggest that the faults are rooted in the deeply buried Late Proterozoic and Early Cambrian Reelfoot rift system. Quaternary deposits are cut by at least four episodes of faulting, two of which occurred during the Holocene. The overall style of neotectonic deformation is interpreted as right-lateral strike-slip faulting. At many locations, however, near-surface displacements have stepped from one fault strand to another and produced normal and oblique-slip faults in areas of transtension and high-angle reverse faults, thrust faults, and folds in areas of transpression. There is evidence of reactivation of some near-surface fault segments during the great 1811-1812 New Madrid earthquakes. Quaternary faulting at Thebes Gap demonstrates that there are additional seismic-source zones in the Midcontinent, U.S., other than New Madrid, and that even in the absence of plate-margin orogenesis, intense neotectonic activity does occur over long time periods along crustal weakenesses in continental interiors.

Tectonophysics↗
USGS1996

Use of liquefaction-induced features for paleoseismic analysis

Liquefaction features can be used in many field settings to estimate the recurrence interval and magnitude of strong earthquakes through much of the Holocene. These features include dikes, craters, vented sand, sills, and laterally spreading landslides. The relatively high seismic shaking level required for their formation makes them particularly valuable as records of strong paleo-earthquakes. This state-of-the-art summary for using liquefaction-induced features for paleoseismic interpretation and analysis takes into account both geological and geotechnical engineering perspectives. The driving mechanism for formation of the features is primarily the increased pore-water pressure associated with liquefaction of sand-rich sediment. The role of this mechanism is often supplemented greatly by the direct action of seismic shaking at the ground surface, which strains and breaks the clay-rich cap that lies immediately above the sediment that liquefied. Discussed in the text are the processes involved in formation of the features, as well as their morphology and characteristics in field settings. Whether liquefaction occurs is controlled mainly by sediment grain size, sediment packing, depth to the water table, and strength and duration of seismic shaking. Formation of recognizable features in the field generally requires a low-permeability cap above the sediment that liquefied. Field manifestations are controlled largely by the severity of liquefaction and the thickness and properties of the low-permeability cap. Criteria are presented for determining whether observed sediment deformation in the field originated by seismically induced liquefaction. These criteria have been developed mainly by observing historic effects of liquefaction in varied field settings. The most important criterion is that a seismic liquefaction origin requires widespread, regional development of features around a core area where the effects are most severe. In addition, the features must have a morphology that is consistent with a very sudden application of a large hydraulic force. This article discusses case studies in widely separated and different geological settings: coastal South Carolina, the New Madrid seismic zone, the Wabash Valley seismic zone, and coastal Washington State. These studies encompass most of the range of settings and the types of liquefaction-induced features likely to be encountered anywhere. The case studies describe the observed features and the logic for assigning a seismic liquefaction origin to them. Also discussed are some types of sediment deformations that can be misinterpreted as having a seismic origin. Two independent methods for estimating prehistoric magnitude are discussed briefly. One method is based on determination of the maximum distance from the epicenter over which liquefaction-induced effects have formed. The other method is based on use of geotechnical engineering techniques at sites of marginal liquefaction, in order to bracket the peak accelerations as a function of epicentral distance; these accelerations can then be compared with predictions from seismological models.

Engineering Geology↗
USGS1986

Cyclic terpenoids of contemporary resinous plant detritus and of fossil woods, ambers and coals

Cyclic terpenoids present in the solvent extractable material of fossil woods, ambers and brown coals have been analyzed. The sample series chosen consisted of wood remains preserved in Holocene to Jurassic sediments and a set of of ambers from the Philippines (copalite), Israel, Canada and Dominican Republic. The brown coals selected were from the Fortuna Garsdorf Mine and Miocene formations on Fiji. The fossil wood extracts contained dominant diterpenoid or sesquiterpenoid skeletons, and aromatized species were present at high concentrations, with a major amount of two-ring aromatic compounds. Tricyclic diterpenoids were the predominant compounds in the ambers. Aromatized derivatives were the major components, consisting of one or two aromatic ring species with the abietane and occasionally pimarane skeletons. The saturated structures were comprised primarily of the abietane and pimarane skeletons having from three to five carbon (C 1 , C 2 , etc.) substituents. Kaurane and phyllocladane isomers were present in only minor amounts. Bicyclic sesquiterpenoids as saturated and partial or fully aromatized forms were also common in these samples, but only traces of sesterterpenoids and triterpenoid derivatives were found. The brown coal extracts were composed of major amounts of one- and two-ring aromatized terpenoids, with a greater proportion of triterpenoid derivatives than in the case of the woods and ambers. This was especially noticeable for the German coal, where the triterpenoids were predominant. Open C-ring aromatized structures were also present in this coal. Steroid compounds were not detectable, but some hopanes were found as minor components in the German brown coal. An overview of the skeletal structure classes identified in each sample, as well as the general mass spectrometric characteristics of the unknown compounds are included in the present paper. It can be concluded from these structural distributions that aromatization is the main process for the transformation of terrestrial cyclic terpenoids during diagenesis, constituting a general pathway for all terpenoids.

Organic Geochemistry↗
USGS2001

Recent formation of arroyos in the Little Missouri Badlands of southwestern North Dakota

In the Little Missouri Badlands of southwestern North Dakota, the channels of ephemeral streams are incised 2 to 10 m or more into mid-to-late Holocene alluvium. The objectives of this study were to determine the timing and cause(s) of the most recent episodes of fluvial incision and to develop a process-response model that illustrates the formation and evolution of arroyos in this region. The purpose was to distinguish natural from anthropogenic changes to the landscape and to discriminate allogenic from autogenic causes of incision, thereby gaining a greater sense of how steep, relatively small, ephemeral streams evolve. Dendrochronologic and dendrogeomorphic analyses of riparian cottonwoods provide an inexpensive, high-resolution dating method to constrain the time of incision, thereby permitting determination of the cause(s) of incision by evaluating environmental conditions prior to and at the onset of fluvial incision. An examination of seven small (10 to 100 km2) drainage basins indicated ephemeral streams have undergone a four-stage cycle of change within the past 200 years, comprising(i) an initial period of relative geomorphic stability with pedogenesis on the flood plain and low rates of lateral channel migration, (ii) a period of channel incision with subsequent widening of the flood plain through lateral corrasion along middle and upstream reaches, (iii) a concomitant period of aggradation along downstream reaches and, finally, (iv) a period of downstream incision. Dendrochronologic data and dendrogeomorphic relations indicate there have been three distinct periods of fluvial incision in the past 200 years. The first period of incision began in the 1860s and 1870s prior to the onset of European settlement and intensive grazing by domesticated cattle in the area. This period of incision occurred along the middle reaches of all seven of the streams examined and coincided with a severe, protracted drought, suggesting an allogenic cause. The second period of incision occurred during the end of the 19th century and into the 20th century along the upstream reaches of streams. This period of incision was mentioned in some written accounts of early settlers and coincided with the introduction of cattle and historical overgrazing of the region. However, overgrazing may not have caused the incision of this period, but instread exacerbated or accelerated the upstream migration of knickpoints formed earlier along middle reaches. A third period of incision has occurred in the past 20 years along the lower reaches of two of the seven streams examined. This incision was caused by the process of meander cutoff. This autogenic process decreases sinuosity and increases channel slope, stream power, and the potential to incise along adjacent stream reaches. ?? 2001 Elsevier Science B.V. All rights reserved.

Geomorphology↗
USGS2002

A comparison of factors controlling sedimentation rates and wetland loss in fluvial-deltaic systems, Texas Gulf coast

Submergence of coastal marshes in areas where rates of relative sea-level rise exceed rates of marsh sedimentation, or vertical accretion, is a global problem that requires detailed examination of the principal processes that establish, maintain, and degrade these biologically productive environments. Using a simple 210Pb-dating model, we measured sedimentation rates in cores from the Trinity, Lavaca-Navidad, and Nueces bayhead fluvial-deltaic systems in Texas where more than 2000 ha of wetlands have been lost since the 1950s. Long-term average rates of fluvial-deltaic aggradation decrease southwestward from 0.514 ?? 0.008 cm year -1 in the Trinity, 0.328 ?? 0.022 cm year -1 in the Lavaca-Navidad, to 0.262 ?? 0.034 cm year -1 in the Nucces. The relative magnitudes of sedimentation and wetland loss correlate with several parameters that define the differing fluvial-deltaic settings, including size of coastal drainage basin, average annual rainfall, suspended sediment load, thickness of Holocene mud in the valley fill, and rates of relative sea-level rise. There is some evidence that upstream reservoirs have reduced wetland sedimentation rates, which are now about one-half the local rates of relative sea-level rise. The extant conditions indicate that fluvial-deltaic marshes in these valleys will continue to be lost as a result of submergence and erosion. ?? 2002 Elsevier Science B.V. All rights reserved.

Geomorphology↗
USGS2002

Regional controls on geomorphology, hydrology, and ecosystem integrity in the Orinoco Delta, Venezuela

Interacting river discharge, tidal oscillation, and tropical rainfall across the 22,000 km2 Orinoco delta plain support diverse fresh and brackish water ecosystems. To develop environmental baseline information for this largely unpopulated region, we evaluate major coastal plain, shallow marine, and river systems of northeastern South America, which serves to identify principal sources and controls of water and sediment flow into, through, and out of the Orinoco Delta. The regional analysis includes a summary of the geology, hydrodynamics, sediment dynamics, and geomorphic characteristics of the Orinoco drainage basin, river, and delta system. Because the Amazon River is a major source of sediment deposited along the Orinoco coast, we summarize Amazon water and sediment input to the northeastern South American littoral zone. We investigate sediment dynamics and geomorphology of the Guiana coast, where marine processes and Holocene history are similar to the Orinoco coast. Major factors controlling Orinoco Delta water and sediment dynamics include the pronounced annual flood discharge; the uneven distribution of water and sediment discharge across the delta plain; discharge of large volumes of water with low sediment concentrations through the Rio Grande and Araguao distributaries; water and sediment dynamics associated with the Guayana littoral current along the northeastern South American coast; inflow of large volumes of Amazon sediment to the Orinoco coast; development of a fresh water plume seaward of Boca Grande; disruption of the Guayana Current by Trinidad, Boca de Serpientes, and Gulf of Paria; and the constriction at Boca de Serpientes. ?? 2002 Elsevier Science B.V. All rights reserved.

Geomorphology↗
USGS2003

Stratigraphy and palaeoclimatic significance of Late Quaternary loess-palaeosol sequences of the Last Interglacial-Glacial cycle in central Alaska

Loess is one of the most widespread subaerial deposits in Alaska and adjacent Yukon Territory and may have a history that goes back 3 Ma. Based on mineralogy and major and trace element chemistry, central Alaskan loess has a composition that is distinctive from other loess bodies of the world, although it is quartz-dominated. Central Alaskan loess was probably derived from a variety of rock types, including granites, metabasalts and schists. Detailed stratigraphic data and pedologic criteria indicate that, contrary to early studies, many palaeosols are present in central Alaskan loess sections. The buried soils indicate that loess sedimentation was episodic, or at least rates of deposition decreased to the point where pedogenesis could keep ahead of aeolian input. As in China, loess deposition and pedogenesis are likely competing processes and neither stops completely during either phase of the loess/soil formation cycle. Loess deposition in central Alaska took place before, and probably during the last interglacial period, during stadials of the mid-Wisconsin period, during the last glacial period and during the Holocene. An unexpected result of our geochronological studies is that only moderate loess deposition took place during the last glacial period. Our studies lead us to conclude that vegetation plays a key role in loess accumulation in Alaska. Factors favouring loess production are enhanced during glacial periods but factors that favour loess accumulation are diminished during glacial periods. The most important of these is vegetation; boreal forest serves as an effective loess trap, but sparsely distributed herb tundra does not. Thus, thick accumulations of loess should not be expected where tundra vegetation was dominant and this is borne out by modern studies near the treeline in central Alaska. Much of the stratigraphic diversity of North American loess, including that found in the Central Lowlands, the Great Plains, and Alaska is explained by a new model that emphasizes the relative importance of loess production factors versus loess accumulation factors.

Quaternary Science Reviews↗
USGS1997

A diatom record of climate and hydrology for the past 200 KA from Owens Lake, California with comparison to other Breat Basin records

Diatoms from lake sediments beneath Owens Lake playa, Inyo County, California, document a nearly continuous paleolimnological record of climate and hydrologic change since the penultimate glacial-interglacial cycle based on a chronology established by radiocarbon, tephrochronology, and paleomagnetic control. Freshwater planktic diatoms (especially species of Stephanodiscus), plagioclase feldspar-rich sediments with high magnetic susceptibility, and Juniperus-type pollen characterized the penultimate glaciation at Owens Lake. Saline diatoms dominated in the following interglacial period, and there are several episodes during which freshwater planktic diatoms became abundant between 100 and 50 ka that may represent interstadial climatic conditions. Saline diatoms fell to low values after 50 ka, but warm-season Aulacoseira species indicate episodes of significant summer precipitation in the hydrologic balance of Owens Lake prior to the last glacial maximum. By 25 ka, glacial environments were again characterized by abundant Juniperus, plagioclase feldspar, and Stephanodiscus species. Generally and Holocene climates were recorded in Owens Lake by short-term fluctuations of saline and freshwater diatoms, desiccation, and oolitic sediments barren of diatoms. Comparison to paleoclimate records both north and south of Owens Lake suggest a southerly displacement of storm tracks originating from the Aleutian Low during glacial episodes.

Quaternary Science Reviews↗