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Results for “The Holocene”

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

USGS2004

40Ar/39Ar dating of the eruptive history of Mount Erebus, Antarctica: Summit flows, tephra, and caldera collapse

Eruptive activity has occurred in the summit region of Mount Erebus over the last 95 ky, and has included numerous lava flows and small explosive eruptions, at least one plinian eruption, and at least one and probably two caldera-forming events. Furnace and laser step-heating 40Ar/39Ar ages have been determined for 16 summit lava flows and three englacial tephra layers erupted from Mount Erebus. The summit region is composed of at least one or possibly two superimposed calderas that have been filled by post-caldera lava flows ranging in age from 17 ?? 8 to 1 ?? 5 ka. Dated pre-caldera summit flows display two age populations at 95 ?? 9 to 76 ?? 4 ka and 27 ?? 3 to 21 ??4 ka of samples with tephriphonolite and phonolite compositions, respectively. A caldera-collapse event occurred between 25 and 11 ka. An older caldera-collapse event is likely to have occurred between 80 and 24 ka. Two englacial tephra layers from the flanks of Mount Erebus have been dated at 71 ?? 5 and 15 ?? 4 ka. These layers stratigraphically bracket 14 undated tephra layers, and predate 19 undated tephra layers, indicating that small-scale explosive activity has occurred throughout the late Pleistocene and Holocene eruptive history of Mount Erebus. A distal, englacial plinian-fall tephra sample has an age of 39 ?? 6 ka and may have been associated with the older of the two caldera-collapse events. A shift in magma composition from tephriphonolite to phonolite occurred at around 36 ka. ?? Springer-Verlag 2004.

Bulletin of Volcanology↗

USGS2013

Long-range hazard assessment of volcanic ash dispersal for a Plinian eruptive scenario at Popocatépetl volcano (Mexico): implications for civil aviation safety

Popocatépetl is one of Mexico’s most active volcanoes threatening a densely populated area that includes Mexico City with more than 20 million inhabitants. The destructive potential of this volcano is demonstrated by its Late Pleistocene–Holocene eruptive activity, which has been characterized by recurrent Plinian eruptions of large magnitude, the last two of which destroyed human settlements in pre-Hispanic times. Popocatépetl’s reawakening in 1994 produced a crisis that culminated with the evacuation of two villages on the northeastern flank of the volcano. Shortly after, a monitoring system and a civil protection contingency plan based on a hazard zone map were implemented. The current volcanic hazards map considers the potential occurrence of different volcanic phenomena, including pyroclastic density currents and lahars. However, no quantitative assessment of the tephra hazard, especially related to atmospheric dispersal, has been performed. The presence of airborne volcanic ash at low and jet-cruise atmospheric levels compromises the safety of aircraft operations and forces re-routing of aircraft to prevent encounters with volcanic ash clouds. Given the high number of important airports in the surroundings of Popocatépetl volcano and considering the potential threat posed to civil aviation in Mexico and adjacent regions in case of a Plinian eruption, a hazard assessment for tephra dispersal is required. In this work, we present the first probabilistic tephra dispersal hazard assessment for Popocatépetl volcano. We compute probabilistic hazard maps for critical thresholds of airborne ash concentrations at different flight levels, corresponding to the situation defined in Europe during 2010, and still under discussion. Tephra dispersal mode is performed using the FALL3D numerical model. Probabilistic hazard maps are built for a Plinian eruptive scenario defined on the basis of geological field data for the “Ochre Pumice” Plinian eruption (4965 14 C yr BP). FALL3D model input eruptive parameters are constrained through an inversion method carried out with the semi-analytical HAZMAP model and are varied by sampling them using probability density functions. We analyze the influence of seasonal variations on ash dispersal and estimate the average persistence of critical ash concentrations at relevant locations and airports. This study assesses the impact that a Plinian eruption similar to the Ochre Pumice eruption would have on the main airports of Mexico and adjacent areas. The hazard maps presented here can support long-term planning that would help minimize the impacts of such an eruption on civil aviation.

Popocatépetl Volcano↗

USGS1998

Tsunami generation by pyroclastic flow during the 3500-year B.P. caldera-forming eruption of Aniakchak Volcano, Alaska

A discontinuous pumiceous sand, a few centimeters to tens of centimeters thick, is located up to 15 m above mean high tide within Holocene peat along the northern Bristol Bay coastline of Alaska. The bed consists of fine-to-coarse, poorly to moderately well-sorted, pumice-bearing sand near the top of a 2-m-thick peat sequence. The sand bed contains rip-up clasts of peat and tephra and is unique in the peat sequence. Major element compositions of juvenile glass from the deposit and radiocarbon dating of enclosing peat support correlation of the pumiceous sand with the caldera-forming eruption of Aniakchak Volcano. The distribution of the sand and its sedimentary characteristics are consistent with emplacement by tsunami. The pumiceous sand most likely represents redeposition by tsunami of climactic fallout tephra and beach sand during the approximately 3.5 ka Aniakchak caldera-forming eruption on the Alaska Peninsula. We propose that a tsunami was generated by the sudden entrance of a rapidly moving, voluminous pyroclastic flow from Aniakchak into Bristol Bay. A seismic trigger for the tsunami is unlikely, because tectonic structures suitable for tsunami generation are present only south of the Alaska Peninsula. The pumiceous sand in coastal peat of northern Bristol Bay is the first documented geologic evidence of a tsunami initiated by a volcanic eruption in Alaska.

Alaska↗

USGS2021

Material properties and triggering mechanisms of an andesitic lava dome collapse at Shiveluch Volcano, Kamchatka, Russia, revealed using the finite element method

Shiveluch volcano (Kamchatka, Russia) is an active andesitic volcano with a history of explosive activity, dome extrusion, and structural collapse during the Holocene. The most recent major (> 1 km 3 ) dome collapse occurred in November 1964, producing a ~ 1.5 km 3 debris avalanche that traveled over 15 km from the vent and triggered a phreatic explosion followed by a voluminous (~ 0.8 km 3 ) eruption of juvenile pyroclastic material. Seismic records suggest that the collapse was likely triggered by a magnitude 5.1 earthquake associated with the ascent of magma into the edifice. The geomechanical properties of the pre-1964 dome are unknown; accordingly, the mechanics of the collapse are poorly understood. This project employs numerical slope stability modeling using the finite element method to constrain probable ranges of geomechanical properties for the materials involved in the collapse, considering earthquake loading as the most likely triggering mechanism. Model results show good agreement with the 1964 collapse geometry considering Geological Strength Index and horizontal pseudo-static seismic coefficient ranges of 30–60 and 0.05–0.15 g, respectively, representing variably fractured and altered dome rocks under moderate earthquake loading, confirming that ground acceleration alone could have triggered the dome collapse. Deep-seated rotational sliding is the dominant failure mode, but local extension within the dome during failure appears to play an important role in the development of the collapse. The findings of this work allow for better forward modeling of potential future collapses, the results of which can be incorporated into regional hazard and risk assessments.

Shiveluch Volcano↗

USGS2013

The transboundary non-renewable Nubian Aquifer System of Chad, Egypt, Libya and Sudan: Classical groundwater questions and parsimonious hydrogeologic analysis and modeling

Parsimonious groundwater modeling provides insight into hydrogeologic functioning of the Nubian Aquifer System (NAS), the world’s largest non-renewable groundwater system (belonging to Chad, Egypt, Libya, and Sudan). Classical groundwater-resource issues exist (magnitude and lateral extent of drawdown near pumping centers) with joint international management questions regarding transboundary drawdown. Much of NAS is thick, containing a large volume of high-quality groundwater, but receives insignificant recharge, so water-resource availability is time-limited. Informative aquifer data are lacking regarding large-scale response, providing only local-scale information near pumps. Proxy data provide primary underpinning for understanding regional response: Holocene water-table decline from the previous pluvial period, after thousands of years, results in current oasis/sabkha locations where the water table still intersects the ground. Depletion is found to be controlled by two regional parameters, hydraulic diffusivity and vertical anisotropy of permeability. Secondary data that provide insight are drawdowns near pumps and isotope-groundwater ages (million-year-old groundwaters in Egypt). The resultant strong simply structured three-dimensional model representation captures the essence of NAS regional groundwater-flow behavior. Model forecasts inform resource management that transboundary drawdown will likely be minimal—a nonissue—whereas drawdown within pumping centers may become excessive, requiring alternative extraction schemes; correspondingly, significant water-table drawdown may occur in pumping centers co-located with oases, causing oasis loss and environmental impacts.

Hydrogeology Journal↗

USGS2017

New constraints on coseismic slip during southern Cascadia subduction zone earthquakes over the past 4600 years implied by tsunami deposits and marine turbidites

Forecasting earthquake and tsunami hazards along the southern Cascadia subduction zone is complicated by uncertainties in the amount of megathrust fault slip during past ruptures. Here, we estimate slip on hypothetical ruptures of the southern part of the megathrust through comparisons of late Holocene Cascadia earthquake histories derived from tsunami deposits on land and marine turbidites offshore. Bradley Lake in southern Oregon lies ~600 m landward of the shoreline and contains deposits from 12 tsunamis in the past 4600 years. Tsunami simulations that overtop the 6-m-high lake outlet, generated by ruptures with most slip south of Cape Blanco, require release of at least as much strain on the megathrust as would accumulate in 430–640 years (>15–22 m). Such high slip is inconsistent with global seismic data for a rupture ~300-km long and slip deficits over the past ~4700 years on the southern Cascadia subduction zone. Assuming slip deficits accumulated during the time intervals between marine turbidites, up to 8 of 12 tsunami inundations at the lake are predicted from a marine core site 170 km north of the lake (at Hydrate Ridge) compared to 4 of 12 when using a core site ~80 km south (at Rogue Apron). Longer time intervals between turbidites at Hydrate Ridge imply larger slip deficits compared to Rogue Apron. The different inundations predicted by the two records suggest that Hydrate Ridge records subduction ruptures that extend past both Rogue Apron and Bradley Lake. We also show how turbidite-based estimates of CSZ rupture length relate to tsunami source scenarios for probabilistic tsunami hazard assessments consistent with lake inundations over the last ~4600 years.

Oregon↗

USGS2011

Native and exotic plants of fragments of sagebrush steppe produced by geomorphic processes versus land use

Habitat fragmentation and invasion by exotic species are regarded as major threats to the biodiversity of many ecosystems. We surveyed the plant communities of two types of remnant sagebrush-steppe fragments from nearby areas on the Snake River Plain of southeastern Idaho, USA. One type resulted from land use (conversion to dryland agriculture; hereafter AG Islands) and the other from geomorphic processes (Holocene volcanism; hereafter kipukas). We assessed two predictions for the variation in native plant species richness of these fragments, using structural equation models (SEM). First, we predicted that the species richness of native plants would follow the MacArthur–Wilson (M–W) hypothesis of island biogeography, as often is expected for the communities of habitat fragments. Second, we predicted a negative relationship between native and exotic plants, as would be expected if exotic plants are decreasing the diversity of native plants. Finally, we assessed whether exotic species were more strongly associated with the fragments embedded in the agricultural landscape, as would be expected if agriculture had facilitated the introduction and naturalization of non-native species, and whether the communities of the two types of fragments were distinct. Species richness of native plants was not strongly correlated with M–W characteristics for either the AG Islands or the **kipukas. The AG Islands had more species and higher cover of exotics than the kipukas, and exotic plants were good predictors of native plant species richness. Our results support the hypothesis that proximity to agriculture can increase the diversity and abundance of exotic plants in native habitat. In combination with other information, the results also suggest that agriculture and exotic species have caused loss of native diversity and reorganization of the sagebrush-steppe plant community.

Idaho↗

USGS2014

Quantitative study of tectonic geomorphology along Haiyuan fault based on airborne LiDAR

High-precision and high-resolution topography are the fundamental data for active fault research. Light detection and ranging (LiDAR) presents a new approach to build detailed digital elevation models effectively. We take the Haiyuan fault in Gansu Province as an example of how LiDAR data may be used to improve the study of active faults and the risk assessment of related hazards. In the eastern segment of the Haiyuan fault, the Shaomayin site has been comprehensively investigated in previous research because of its exemplary tectonic topographic features. Based on unprecedented LiDAR data, the horizontal and vertical coseismic offsets at the Shaomayin site are described. The measured horizontal value is about 8.6 m, and the vertical value is about 0.8 m. Using prior dating ages sampled from the same location, we estimate the horizontal slip rate as 4.0 ± 1.0 mm/a with high confidence and define that the lower bound of the vertical slip rate is 0.4 ± 0.1 mm/a since the Holocene. LiDAR data can repeat the measurements of field work on quantifying offsets of tectonic landform features quite well. The offset landforms are visualized on an office computer workstation easily, and specialized software may be used to obtain displacement quantitatively. By combining precious chronological results, the fundamental link between fault activity and large earthquakes is better recognized, as well as the potential risk for future earthquake hazards.

Gansu↗

USGS2013

Reconstructing vegetation response to altered hydrology and its use for restoration, Arthur R. Marshall Loxahatchee National Wildlife Refuge, Florida

We present reconstructed hydrologic and vegetation trends of the last three centuries across the Arthur R. Marshall Loxahatchee National Wildlife Refuge, Florida in order to understand the effects of 20th century water management. We analyzed pollen assemblages from cores at marsh sites along three transects to document vegetation and infer hydroperiod and water depth both before and after human alteration of Everglades hydrology. In the northern and central part of the Refuge, late Holocene water levels were higher and hydroperiods longer than the last 100 years. Post-1950 was a time of several different water management strategies. Pollen assemblages indicate drier conditions post-1950 in the northern and central parts of the Refuge, whereas sites in the southern Refuge are wetter and vegetation turnover is higher. Throughout the Refuge, Sagittaria pollen declines with the onset of water management, and may indicate a loss of greater variation in hydroperiods across years and water depths between seasons. Paleoecological evidence provides clear estimates of the vegetation response to hydrologic change under specific hydrologic regimes.

Florida↗

USGS2016

Impacts of shore expansion and catchment characteristics on lacustrine thermokarst records in permafrost lowlands, Alaska Arctic Coastal Plain

Arctic lowland landscapes have been modified by thermokarst lake processes throughout the Holocene. Thermokarst lakes form as a result of ice-rich permafrost degradation, and they may expand over time through thermal and mechanical shoreline erosion. We studied proximal and distal sedimentary records from a thermokarst lake located on the Arctic Coastal Plain of northern Alaska to reconstruct the impact of catchment dynamics and morphology on the lacustrine depositional environment and to quantify carbon accumulation in thermokarst lake sediments. Short cores were collected for analysis of pollen, sedimentological, and geochemical proxies. Radiocarbon and 210 Pb/ 137 Cs dating, as well as extrapolation of measured historic lake expansion rates, were applied to estimate a minimum lake age of ~1400 calendar years BP. The pollen record is in agreement with the young lake age as it does not include evidence of the “alder high” that occurred in the region ~4000 cal yr BP. The lake most likely initiated from a remnant pond in a drained thermokarst lake basin (DTLB) and deepened rapidly as evidenced by accumulation of laminated sediments. Increasing oxygenation of the water column as shown by higher Fe/Ti and Fe/S ratios in the sediment indicate shifts in ice regime with increasing water depth. More recently, the sediment source changed as the thermokarst lake expanded through lateral permafrost degradation, alternating from redeposited DTLB sediments, to increased amounts of sediment from eroding, older upland deposits, followed by a more balanced combination of both DTLB and upland sources. The characterizing shifts in sediment sources and depositional regimes in expanding thermokarst lakes were, therefore, archived in the thermokarst lake sedimentary record. This study also highlights the potential for Arctic lakes to recycle old carbon from thawing permafrost and thermokarst processes.

Alaska↗

USGS1987

The geochemistry of water near a surficial organic-rich uranium deposit, northeastern Washington State, U.S.A.

The chemistry of three stream, three spring and six near-surface waters in the vicinity of a Holocene organic-rich uranium deposit is described, with particular emphasis on the chemistry of U. Results characterize the solution behavior of uranium as U-bearing water interacts with relatively undecomposed, surficial organic matter. Of the measured major and trace chemical species, only U is consistently highly enriched (17-318 ppb) relative to reported values for regional waters, or to literature values for waters in largely granitic terrains. R-mode factor analysis of the chemical data suggests that most U is present in a soluble form, but that some U is also associated with fine suspended particulates of clay, organic matter, or hydrous oxides. Calculations that apply thermodynamic data to predict U speciation in solution indicate the relative importance of uranyl carbonate and uranyl phosphate complexes. Analysis of more finely filtered samples (0.05 ??m vs. 0.45 ??m), and direct radiographic observations using fission-track detectors suspended in the waters indicate the presence of some uraniferous particulate matter. Application of existing thermodynamic data for uranous- and uranyl-bearing minerals indicates that all waters are undersaturated with U minerals as long as ambient Eh ??? +0.1 v. If coexisting surface and near-surface waters are sufficiently oxidizing, initial fixation of U in the deposit should be by a mechanism of adsorption. Alternatively, more reducing conditions may prevail in deeper pore waters of the organic-rich host sediments, perhaps leading to direct precipitation or diagenetic formation of U4+ minerals. A 234U 238U alpha activity ratio of 1.08 ?? 0.02 in a spring issuing from a hillslope above the deposit suggests a relatively soluble source of U. In contrast, higher activity ratios of 234U 238U (??? 1.3) in waters in contact with the uraniferous valley-fill sediments suggest differences in the nature of interaction between groundwater and the local, U-rich source rocks. ?? 1987.

Chemical Geology↗

USGS1988

Pedogenic replacement of aluminosilicate grains by CaCO3 in Ustollic Haplargids, south-central Montana, U.S.A.

A chronosequence of calcic soils formed on granitic glaciofluvial terrace deposits of Rock Creek and the Clarks Fork in south-central Montana shows progressive replacement of aluminosilicate parent-material grains by calcium-magnesium carbonate. The terraces range from late Pliocene to Holocene in age as dated by tephrochronology, correlation, and stream incision rates. Replacement is first seen in soils that are as old as 120,000 yr; the amount and degree of replacement increase in soils older than 120,000 yr along with the development of calcic horizons. Under the petrographic microscope, carbonate replacement of quartz, feldspars, and the groundmass of andesite grains in Rock Creek soils is shown by embayed grains, networks of carbonate along cracks and between parts of polycrystalline grains and optically aligned grain fragments within carbonate masses. Microprobe data suggest that silica is released by replacement because it is absent from carbonate-filled spaces and is depleted in corrosion pits. Little microscopic evidence exists to support displacement of framework grains by carbonate because fragments of a single grain are rarely rotated out of optical alignment. In the calcic soils of Rock Creek, K-fabric (grains floating in a carbonate matrix) may form by both replacement and displacement.

Montana↗

USGS1970

Rapid changes in the head of the Rio Balsas Submarine Canyon system, Mexico

The investigation of a river delta and the heads of several nearby submarine canyons in western Mexico produced evidence for rapid changes in the configuration and depth of the nearshore portions of canyon tributaries. General scarcity of data on the rates of submarine canyon formation and the relationship to river discharge should make these results of special interest. The Rio Balsas, one of Mexico's largest rivers, empties into the ocean near the heads of a large submarine canyon that terminates in the Middle America Trench. One of the distributaries of the Rio Balsas presently is discharging at the head of Cañón de la Necesidad, which is being eroded actively. Two inactive canyons are related to former discharge channels of the river. Their heads lie at some distance from shore and are being filled with sediment. The Cañón de Petacalco, not now receiving sediment directly from a Rio Balsas distributary, has remained active because the shoreline has not retreated far. Until about 100 years ago its head was being filled with fine-grained and highly organic sediments from a nearby rivermouth, while the coarse portion of the sediment supply joined the canyon via a tributary farther seaward. Since then the river has shifted away from this canyon, and the horizontally stratified sediments in the canyon head have been incised as much as 20–30 m, as evidenced by three 14 C dates of organic material exposed in the steep to overhanging canyon walls. The changes in the shallow portion of the Rio Balsas submarine canyons seem to be related to changes in river discharge pattern, either directly or indirectly. A shifting point source of sediment supply either activates a pre-existing, partly filled canyon, or erodes a new one near the new river mouth, whereas the canyon at the abandoned river mouth is deactivated following retreat of the shoreline. The heads of the different tributaries form a dendritic pattern in Holocene unconsolidated sediment. Subaerial processes are not involved in the formation of these submarine canyons. Thus, a dendritic pattern of submarine canyons is not necessarily indicative of subaerial erosion.

Rio Balsas Submarine Canyon system↗

USGS1979

Small-scale slump deposits, Middle Atlantic Continental Slope, off eastern United States

Analyses of 24 high-resolution seismic-reflection profiles that were collected during local and regional surveys show that small-scale slump deposite are ubiquitous whthin the intercanyon areas of the Continental Slope of the Middle Atlantic Bight. The deposits involve the upper 10-90 m of sediments, extend downslops for 1.8-7.2 km, and are present at water depths ranging from 545 to 1500 m. The characteristics of the deposits vary from thin, homogeneous or fairly regularly bedded lenses of sediment, to masses of intermediate thickness with contorted bedding, to relatively large slump blocks. A detailed survey of one slump mass just south of Hudson Canyon (by means of close-spaced Minisparker profiles and sediment cores) showed that it had a thickness of about 30 m and a volume of at least 0.4 km3 and consisted of homogeneous clay which accumulated rapidly during the late Pleistocene or Holocene. Although some of the slump deposits undoubtedly are relict, stemming from sediment instability porduced by rapid deposition during Pleistocene sea-level regressions, others were formed relatively recently. Possible causes of modern slumps include gas generation in the sediments, bottom-water turbulence on the upper slope, and shallow faulting. This study indicates that small-scale slumping in the intercanyon areas may be an important process in transporting sediments to the deep sea and suggests that recent mass movements may constitute a geologic hazard to future economic development of this part of the Continental Slope.

Middle Atlantic Bight↗

USGS1980

Current-controlled, abyssal microtopography and sedimentation in Mozambique Basin, southwest Indian Ocean

The Antarctic Bottom Water (AABW) activity and the variations in the abundance and grain size of the terrigenous sediments, derived from Africa and Madagascar land masses, are reflected in different types of microtopography in the Mozambique Basin. In southerly areas, where the sediment supply is much less, the bottom-current activity has resulted in the presence of manganese nodules, a thin veneer of sediments, and the absence of sediment waves. Farther north, along the marginal areas of the basin where the fine-grained sediments from the Africa—Madagascar source have been supplied in abundance, wavy bedforms have been generated by AABW. Wavy bedforms do not exist even in the northerly areas if coarse-grained, turbidite sediments are present on the sea floor. The continuation of acoustic reflectors from the zone of turbidites in the central areas of the basin into the zone of sediment waves along the margins, and the lithology and structures in sediment cores from these zones suggest that the turbidity-current-fed, fine-grained sediments were deposited as wavy bedforms by AABW flow. Thus, sediment waves formed readily during Pleistocene times. The enrichment of quartz and displaced Antarctic diatoms, and the relatively low kaolinite/chlorite ratios in the sediments, the north-pointing current lineations on the sea floor, the lack of any perceptible sedimentary fill in the troughs of waves, and the dense nepheloid layer in the westerly areas of the Mozambique Basin, attest to the current-controlled sedimentation and generation of wavy bedforms during Holocene time also. The formation of sediment waves in the Mozambique Basin can be modeled after a fluvial antidune mechanism. This model envisages that internal waves, focused on a benthic boundary layer cap, have been locked in phase with sediment waves in the presence of an 8–10 cm/sec current in the Mozambique Basin. A density contrast of 2·10 −6 g/cm 3 appears to exist at the tops of benthic boundary layers in the Mozambique Basin and is quite sufficient for supporting the internal waves. The densiometric Froude number calculated for a 60–280 m thick boundary layer in the basin is close to unity or greater, and is compatible with the antidune model.

Marine Geology↗

USGS1980

Sedimentology and geochemistry of surface sediments, outer continental shelf, southern Bering Sea

Present-day sediment dynamics, combined with lowerings of sea level during the Pleistocene, have created a mixture of sediments on the outer continental shelf of the southern Bering Sea that was derived from the Alaskan Mainland, the Aleutian Islands, and the Pribilof ridge. Concentrations of finer-grained, higher-organic sediments in the region of the St. George basin have further modified regional distribution patterns of sediment composition. Q-mode factor analysis of 58 variables related to sediment size and composition - including content of major, minor, and trace elements, heavy and light minerals, and clay minerals - reveals three dominant associations of sediment: 1. (1) The most significant contribution, forming a coarse-grained sediment scattered over most of the shelf consists of felsic sediment derived from the generally quartz-rich rocks of the Alaskan mainland. This sediment contains relatively high concentrations of Si, Ba, Rb, quartz, garnet, epidote, metamorphic rock fragments, potassium feldspar, and illite. 2. (2) The next most important group, superimposed on the felsic group consists of andesitic sediment derived from the Aleutian Islands. This more mafic sediment contains relatively high concentrations of Na, Ca, Ti, Sr, V, Mn, Cu, Fe, Al, Co, Zn, Y, Yb, Ga, volcanic rock fragments, glass, clinopyroxene, smectite, and vermiculite. 3. (3) A local group of basaltic sediment, derived from rocks of the Pribilof Islands, is a subgroup of the Aleutian andesite group. Accumulation of fine-grained sediment in St. George basin has created a sediment group containing relatively high concentrations of C, S, U, Li, B, Zr, Ga, Hg, silt, and clay. Sediment of the Aleutian andesite group exhibits a strong gradient, or "plume", with concentrations decreasing away from Unimak Pass and toward St. George basin. The absence of present-day currents sufficient to move even clay-size material as well as the presence of Bering submarine canyon between the Aleutian Islands and the outer continental shelf and slope, indicates that Holocene sediment dynamics cannot be used to explain the observed distribution of surface sediment derived from the Aleutian Islands. We suggest that this pattern is relict and resulted from sediment dynamics during lower sea levels of the Pleistocene.

Alaska↗

USGS1986

Ascension Submarine Canyon, California - Evolution of a multi-head canyon system along a strike-slip continental margin

Ascension Submarine Canyon, which lies along the strike-slip (transform) dominated continental margin of central California, consists of two discrete northwestern heads and six less well defined southeastern heads. These eight heads coalesce to form a single submarine canyon near the 2700 m isobath. Detailed seismic stratigraphic data correlated with 19 rock dredge hauls from the walls of the canyon system, suggest that at least one of the two northwestern heads was initially eroded during a Pliocene lowstand of sea level ∼3.8 m.y. B.P. Paleogeographic reconstructions indicate that at this time, northwestern Ascension Canyon formed the distal channel of nearby Monterey Canyon and has subsequently been offset by right-lateral, strike-slip faulting along the San Gregorio fault zone. Some of the six southwestern heads of Ascension Canyon may also have been initially eroded as the distal portions of Monterey Canyon during late Pliocene-early Pleistocene sea-level lowstands (∼2.8 and 1.75 m.y. B.P.) and subsequently truncated and offset to the northwest. There have also been a minimum of two canyon-cutting episodes within the past 750,000 years, after the entire Ascension Canyon system migrated to the northwest past Monterey Canyon. We attribute these late Pleistocene erosional events to relative lowstands of sea level 750,000 and 18,000 yrs B.P. The late Pleistocene and Holocene evolution of the six southeastern heads also appears to have been controlled by structural uplift of the Ascension-Monterey basement high at the southeastern terminus of the Outer Santa Cruz Basin. We believe that uplift of this basement high sufficiently oversteepened submarine slopes to induce gravitational instability and generate mass movements that resulted in the erosion of the canyon heads. Most significantly, though, our results and interpretations support previous proposals that submarine canyons along strike-slip continental margins can originate by tectonic trunction and lateral offset.

California↗

USGS1987

Ice erosion of a sea-floor knickpoint at the inner edge of the stamukhi zone, Beaufort Sea, Alaska

In 1981 and 1982, detailed bathymetric and side-scan sonar surveys were made of an area of the sea floor north of Prudhoe Bay, Alaska, to study the changing characteristics of the seabed at the inner boundary of the stamukhi zone, the coast-parallel zone of grounded ice ridges that occurs in water depths between 15 and 50 m in the arctic. The fathograms and sonographs resolved 10-cm features and electronic navigation gave relocations accurate to about 10 m. Year after year an ice boundary develops at the inner edge of the stamukhi zone where major shear and pressure deformation occur in about the same location. Associated with this ice boundary, the bathymetry shows a pronounced break in slope — the knickpoint — on the shelf profile at about 20 m depth. The 2–3 m-high knickpoint is cut in a consolidated gravelly mud of pre-Holocene age. A well-defined gravel and cobble shoal a few meters high usually occurs at the inshore edge of the knickpoint. The sonograph mosaic shows that seaward of the knickpoint, ice gouges saturate the sea floor and are well defined; inshore the gouges are fewer in number and are poorly defined on the records. Few gouges can be traced from the seaward side of the knickpoint across the shoals to the inshore side of the knickpoint. Studies of ice gouging rates in two seabed corridors that cross the stamukhi zone reveal the highest rates of gouging seaward of the knickpoint. We believe that the knickpoint results from ice erosion at the inner boundary of the stamukhi zone. Intensified currents associated with this boundary winnow away fine sediments. Ice bulldozing and currents shape the shoals, which perch atop the knickpoint. The knickpoint helps to limit ice forces on the seabed inshore of the stamukhi zone.

Marine Geology↗