SEARCH · Geology Reports
Results for “Holocene”
Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Using geologic mapping to understand temporal and spatial relations of closely clustered to concurrent latest Holocene surface ruptures on two intersecting faults, south-central Mojave Desert, California
The Pinto Mountain Fault Zone (PMFZ) marks a major structural boundary between east-oriented sinistral faults of the eastern Transverse Ranges (to the south) and northwest-oriented dextral faults of the south-central Mojave Desert (to the north). These structural fault systems comprise sinistral and dextral deformational domains of the Eastern California Shear Zone (ECSZ) that intersect one another in the Copper Mountain and Twentynine Palms areas. The U.S. Geological Survey (USGS) is conducting detailed geologic mapping and geochronologic investigations designed to clarify geometric, kinematic, and temporal relations among the two domains, that are focused on the central portion of the left-lateral PMFZ near its intersection with the right-lateral Copper Mountain Fault (CMF) and Mesquite Lake Fault Zone (MLFZ).
Holocene barrier dynamics & management on the Eastern Shore of Virginia
No abstract available.
Stable carbon and oxygen isotope studies of the sediments of Elk Lake, Minnesota
Variations in the ratios of 18 O: 16 O and 13 C: 12 C in calcite throughout the Holocene in Elk Lake, Minnesota, are recorded in three varve-calibrated carbonate cores. Marl in a varved deep-basin (29.6 m) core consists mainly of calcite precipitated from surface waters during the summer and probably provides the least complicated isotope record. Marl in a sublittoral (10 m) core consists of calcite contributed from several inorganic and organic sources and probably is the most complicated of the three isotope records. Calcite from shells of the ostracod Candona ohioensis in the sublittoral core provides a record of shallow-water conditions in Elk Lake for the period between 10,500 and 5500 varve yr. Variations in the 13 C: 12 C ratio of organic carbon deposited in Elk Lake during the Holocene are recorded in organic matter in the deep-basin core. All three oxygen isotope records show that, in general, the 18 O: 16 O ratio in carbonate was enriched in 18 O by several parts per mil during the mid-Holocene relative to the past few thousand years. This pattern of oxygen isotope variation is similar to that observed for carbonate materials from other lakes in the northeastern and north-central United States. Oxygen isotope records from these other lakes also show that the 18 O: 16 O ratio during the early Holocene was lower than during the mid-Holocene, and this pattern has been interpreted as representing a response to a generally warmer and drier climate during the mid-Holocene beginning about 8000 varve yr (the so-called hypsithermal). Ostracod and diatom assemblages from Elk Lake cores show, however, that the lake was colder and more saline than at present until at least 6700 varve yr, with conditions similar to those that exist today in cold prairie lakes of Canada. It may be more appropriate, therefore, to refer to the mid-Holocene in northwestern Minnesota as the “prairie period” rather than the hypsithermal, indicating that the climate was drier, but with no connotation regarding temperature. The oxygen isotope data from the three Elk Lake records for this period are somewhat equivocal. Values of δ 18 O in the marl from the sublittoral core and shells of Candona increase from 10,000 to about 6800 varve yr. However, values of δ 18 O in the marl that accumulated in the deepest part of the lake over the same interval (10,000–6800 varve yr) are more or less constant and enriched in 18 O; this probably reflects the cold, saline prairie-lake conditions predicted from the ostracod and diatom assemblage data. All three oxygen isotope records show decreases in 18 O: 16 O ratios after about 6800 varve yr in response to an increase in temperature and decrease in salinity of the lake. The 13 C: 12 C ratios in carbonates from all three Elk Lake records show a distinct pattern; the ratio increased gradually from 10,000 to 8000 varve yr going into the mid-Holocene prairie period and then decreased gradually coming out of the prairie period between about 5500 and 2500 varve yr. These changes in the 13 C: 12 C ratio could have been related to temperature through its effect on solubility of carbon dioxide; however, this interpretation is not supported by the oxygen isotope data. Another possibility is that changes in the 13 C: 12 C ratio are related to organic productivity that removes 13 C-depleted organic carbon and results in 13 C-enriched surface waters. This interpretation implies that organic productivity was higher in Elk Lake during the mid-Holocene prairie period. Support for the high-productivity, 13 C-enriched surface-water model for the mid-Holocene prairie period in Elk Lake is provided by changes in the 13 C: 12 C ratio of organic carbon in the deep-basin core. These changes parallel almost exactly those in the 13 C: 12 C ratio of carbonate carbon, but are about 2% larger (about 6% as opposed to about 4% for carbonate carbon). The difference of about 2% may represent 13 C depletion due to CO 2 limitation. The percentage of organic carbon in the sediment did not increase during the prairie period because it was diluted by an increased flux of detrital clastic material. The ultimate burial rate of organic carbon increased considerably, however, indicating that organic productivity was higher and/or the degree of preservation increased. Diatom assemblages and plant-pigment concentrations indicate that productivity was higher during the prairie period. Pyrolysis hydrogen and oxygen indices show that the 13 C-enriched organic matter that accumulated during the prairie period was hydrogen rich and oxygen poor relative to organic matter that accumulated before and after. These two indices demonstrate that the organic matter that accumulated during the prairie period was much better preserved.
A 40,000-year woodrat-midden record of vegetational and biogeographical dynamics in north-eastern Utah
Aim A conspicuous climatic and biogeographical transition occurs at 40-45° N in western North America. This pivot point marks a north–south opposition of wet and dry conditions at interannual and decadal time-scales, as well as the northern and southern limits of many dominant western plant species. Palaeoecologists have yet to focus on past climatic and biotic shifts along this transition, in part because it requires comparisons across dissimilar records [i.e. pollen from lacustrine sediments to the north and plant macrofossils from woodrat ( Neotoma ) middens to the south]. To overcome these limitations, we are extending the woodrat-midden record northward into the lowlands of the central Rocky Mountains. Location Woodrat middens were collected from crevices and rock shelters on south-facing slopes of Dutch John Mountain (2000-2200 m, 40°57′ N, 109°25′ W), situated on the eastern flanks of the Uinta Mountains in north-eastern Utah. The site is near the regional limits for Pinus ponderosa, P. edulis, P. contorta, Cercocarpus ledifolius var. intricatus, Abies concolor, Ephedra viridis and other important western species. Methods We analysed pollen and plant macrofossils from the 40,000-year midden sequence. The middens represent brief, depositional episodes (mostly years to decades). Four middens represent the early to full-glacial period (40,000–18,000 cal-yr bp), eight middens are from the late-glacial/early Holocene transition (13,500–9000 cal yr bp), and 33 middens span the mid-to-late Holocene (last 7500 years). Temporal density of our Holocene middens (one every c. 210 years) is comparable with typical Holocene pollen sequences from lake sediments. Results Early to full-glacial assemblages are characterized by low diversity and occurrence of montane conifers ( Picea pungens, Pseudotsuga menziesii, P. flexilis, Juniperus communis ) absent from the site today. Diversity increases in the late-glacial samples with the addition of J. scopulorum, J. horizontalis, C. montanus, C. ledifolius var. intricatus and mesic understory species. The coniferous trees and J. communis declined and J. osteosperma appeared during the late-glacial/Holocene transition. Juniperus osteosperma populations have occupied the site throughout the Holocene. Pinus ponderosa was established by 7500 cal-yr bp, and has occurred at least locally ever since. Montane conifers and J. horizontalis persisted until c. 5500 cal-yr bp. The signature events of the late Holocene were the invasions of P. edulis and Ephedra viridis and establishment of pinyon–juniper woodland in the last 800 years. Main conclusions The Dutch John Mountain midden record adds to an emerging picture in which mid-elevation conifers ( P. flexilis, Pseudotsuga menziesii, Picea pungens, J. scopulorum, J. communis ) dominated vegetation over a wide area of the Colorado Plateau and adjacent Rocky Mountains. Rather than being fragmented, as often assumed in phylogeographical studies, these species had broader and more-connected distributions than they do in the region today. Paradoxically, subalpine conifers ( Picea engelmannii, A. lasiocarpa ) occurred at higher elevations to the south, possibly representing declining precipitation from south to north owing to southward displacement of the polar jet stream. The Dutch John Mountain record displays a series of extinction and invasion events. Most of the extinctions were local in scale; nearly all constituents of fossil midden assemblages occur within a few kilometres of Dutch John Mountain, and some occur at least locally on its slopes. The sole exception is J. horizontalis , which is regionally extinct. In contrast to extinctions, Holocene invasions were regional in scale; J. osteosperma, P. ponderosa, P. edulis and Ephedra viridis immigrated from glacial-age source populations far to the south.
Surface sedimentary units of the Gulf of Alaska continental shelf: Montague Island to Yakutat Bay
Four major sedimentary units occur on the sea floor of the continental shelf in the northern Gulf of Alaska. These units, defined on the basis of seismic and sedimentologic data, are: (1) Holocene sediments, (2) Holocene mind moraines, C3) Quaternary glacial marine sediments, and (4) Tertiary and Pleistocene lithified deposits. A wedge of Holocene fine sand to clayey silt covers most of the inner shelf, reaching maximum thicknesses of about 350 m seaward of the Copper River and about 200 m seaward of Icy Bay. Holocene end moraines are found at the mouth of Icy Bay, south of Bering Glacier, and at the mouth of Yakutat Bay. Quaternary glacial marine sediments are found in a narrow arc that borders, on the north and west side of Tart Bank and in a large arc 20 km or more offshore that parallels the shoreline between Kayak Island and Yakutat Bay. Tertiary or Pleistocene stratified sedimentary rocks, which in profile commonly are folded, faulted, and truncated, crop out on Tarr Bank, offshore of Montague Island, and in several localities southeast and southwest of Cape Yakataga. The lack of Holocene cover on Tarr Bank and Middleton, Kayak and Montague Island platforms may be due to the scouring action of swift bottom currents and large storm waves. West of Kayak Island the Copper River is the primary source of Holocene sediment. East of Kayak Island the major sediment sources are streams draining the larger ice fields, notably, the Malaspina and Bering Glaciers. Transport of bottom and suspended sediment is predominantly to the west. If deglaciation of the shelf was completed by 10,000 years B.P., maximum rates of accumulation of Holocene sediment on the inner shelf may be as high as 10-35 m per 1,000 years.
Postglacial response of a stream in central Iowa to changes in climate and Drainage basin factors
Postglacial geomorphic development of the Buchanan Drainage, a small tributary to the South Skunk River, is reconstructed by documenting relationships among four allostratigraphic units and 17 radiocarbon dates. Formation and headward expansion of the valley was both episodic and time-transgressive. Response to downstream conditions in the South Skunk River largely controlled the early formation of the basin. Downcutting through Pleistocene deposits produced a gravelly lag deposit that was buried by alluvium in the downstream portion of the valley during the early Holocene (10,500-7700 yr B.P.). Lag deposits formed in a similar manner continued to develop in the upper portion of the drainageway into the late Holocene (3000-2000 yr B.P.). Episodes of aggradation during the middle Holocene (7700-6300 yr B.P.) and late Holocene (3000-2000 yr B.P.) were separated by a period of soil formation. Holocene geomorphic events in the drainageway coincide with some vegetational and climatic changes as documented in upland pollen sequences from central Iowa. Analysis of plant macrofossil assemblages recovered from alluvium indicates that during the middle Holocene forest contracted and prairie expanded into the uplands within the basin. Vegetational changes within the basin apparently had only minor influence on rates of hillslope erosion, and the widely accepted relationship between prairie (versus forest) vegetative cover and increased rates of hillslope erosion did not hold. Instead, greater amounts of erosion occurred under forested conditions when local water tables were higher and seepage erosion was more effective. ?? 1990.
Timing and origin for sand dunes in the Green River Lowland of Illinois, upper Mississippi River Valley, USA
The recent increase in dune studies in North America has been heavily focused in the Great Plains, while less attention has historically been given to the dune fields east of the Mississippi River. Here we report ages and suggest a potential sediment source for sand dunes in the Green River Lowland, Illinois, which may provide a better understanding of the dynamic interactions between eolian, glacial, lacustrine and fluvial processes that shaped the landscapes of the upper Midwest. Seven coherent optically stimulated luminescence ages (OSL, or optical ages) obtained from four sites suggest that major dune construction in the Green River Lowland occurred within a narrow time window around 17,500 ago. This implies either an enhanced aridity or an episodic increase of sediment supply at 17,500 years ago, or combination of the both. Contrary to previous assertions that dune sand was sourced from the deflation of the underlying outwash sand deposited when the Lake Michigan Lobe retreated from the area, we propose that Green River Lowland dunes sand originated from the Green Bay Lobe through the Rock River. Specifically, sediment supply increased in the Rock River valley during drainage of Glacial Lake Scuppernong, which formed between ???18,000 and 17,000 years ago, when the Green Bay Lobe retreated from its terminal moraine. The lake drained catastrophically through the Rock River valley, providing glacial sediment and water to erode the preexisting sandy sediments. Throughout the remainder of the late Pleistocene, the Laurentide Ice Sheet drained into larger more northerly glacial lakes that in turn drained through other river valleys. Therefore, the dunes in the Green River Lowland formed only during the catastrophic drainage of Glacial Lake Scuppernong, but were stabilized through the remainder of the Pleistocene. This scenario explains the abrupt dune construction around 17,500 years ago, and explains the lack of later dune activity up to the Pleistocene-Holocene transition. OSL and radiocarbon ages also indicate that dunes were reactivated during the early, middle and late Holocene. Some eolian activation occurred within well-defined dry intervals in the upper Midwest, suggesting that increased aridity may have been the primary driver in mobilizing sand. However, many ages do not correspond to drier periods. In contrast to the relative coherency of the Pleistocene OSL ages from multiple study sites, the Holocene OSL ages do not overlap from one site to another, suggesting that increased aridity alone cannot explain the multiple phases of dune reactivation in the Holocene. Therefore, we conclude that the combined effect of localized disturbances and greater aridity acted in concert to increase eolian sand activity in the Holocene. The multiple periods of eolian activity during the Holocene suggest a high potential for future sand activation in the region, and these results are informative for environmental prediction and potential future mitigation.
An 11 000-year-long record of fire and vegetation history at Beaver Lake, Oregon, central Willamette Valley
High-resolution macroscopic charcoal and pollen analysis were used to reconstruct an 11??000-year-long record of fire and vegetation history from Beaver Lake, Oregon, the first complete Holocene paleoecological record from the floor of the Willamette Valley. In the early Holocene (ca 11??000-7500 calendar years before present [cal??yr??BP]), warmer, drier summers than at present led to the establishment of xeric woodland of Quercus, Corylus, and Pseudotsuga near the site. Disturbances (i.e., floods, fires) were common at this time and as a result Alnus rubra grew nearby. High fire frequency occurred in the early Holocene from ca 11??200-9300??cal??yr??BP. Riparian forest and wet prairie developed in the middle Holocene (ca 7500??cal??yr??BP), likely the result of a decrease in the frequency of flooding and a shift to effectively cooler, wetter conditions than before. The vegetation at Beaver Lake remained generally unchanged into the late Holocene (from 4000??cal??yr??BP to present), with the exception of land clearance associated with Euro-American settlement of the valley (ca 160??cal??yr BP). Middle-to-late Holocene increases in fire frequency, coupled with abrupt shifts in fire-episode magnitude and charcoal composition, likely indicate the influence anthropogenic burning near the site. The paleoecological record from Beaver Lake, and in particular the general increase in fire frequency over the last 8500??years, differs significantly from other low-elevation sites in the Pacific Northwest, which suggests that local controls (e.g., shifts in vegetation structure, intensification of human land-use), rather than regional climatic controls, more strongly influenced its environmental history. ?? 2010 Elsevier Ltd.
Lignin phenols in sediments of Lake Baikal, Siberia: Application to paleoenvironmental studies
Sediments from three cores obtained from distinct depositional environments in Lake Baikal, Siberia were analyzed for organic carbon, total nitrogen and lignin phenol concentration and composition. Results were used to examine changes in paleoenvironmental conditions during climatic cycles of the late Quaternary (< 125 ka). Average organic carbon, and total nitrogen concentrations, atomic C/N ratios and organic carbon accumulation rates were significantly higher in the Holocene compared with the late Pleistocene, reflecting overall warmer temperatures and increased runoff during the Holocene. A Holocene maximum in organic carbon was observed at about 6 ka, and may represent the warmest wettest period of the Holocene. At one site (Academician Ridge) pronounced late Pleistocene maxima in organic carbon and biogenic silica were observed at about 80-85 ka, probably indicative of an interstadial period with enhanced aquatic productivity. Total sedimentary lignin phenol contents were generally lower in the late Pleistocene compared to the Holocene, but with several peaks in concentration during the late Pleistocene. These late Pleistocene peaks in total sedimentary lignin content (dated at about 80, 50 and 30 ka) directly precede or occur during peaks in sedimentary biogenic silica contents. These periods likely represent relatively warm interstadial times, with increased precipitation producing the observed increase in terrestrial runoff and aquatic productivity. Lignin phenol ratios (S/V, C/V and P/V) were used to examine changes in terrestrial vegetation type resulting from changes in paleoenvironmental conditions during the late Pleistocene. A degree of caution must be used in the interpretation of these ratios with regard to vegetation sources and paleoenvironmental conditions, because of potential compositional changes in lignin resulting from biodegradation. Nevertheless, results show that long glacial periods were characterized by terrestrial vegetation composed of a mix of non-woody angiosperm vegetation and minor gymnosperm forest. Shorter interstadial periods are defined by a change to dominant gymnosperm forest and were observed at about 80, 75, 63, 50 and 30 ka, ranging from about 2-6 kyr in duration. These interstadial periods of the late Pleistocene defined by lignin phenol ratios generally occur during longer periods of enhanced sedimentary biogenic silica content (about 10-15 ka in duration), providing corroborative evidence of these warm interstadial periods.Sediments obtained in Lake Baikal were analyzed for organic carbon, total nitrogen and lignin phenol composition and used to study changes in paleoenvironmental conditions during climatic cycles of the late Quaternary. The organic carbon, total nitrogen concentrations, atomic C/N ratios and organic carbon accumulation rates were higher in the Holocene showing overall warmer temperatures and increased runoff. Total lignin phenol contents were lower in the Pleistocene representing relatively warm interstadial times with increased precipitation, runoff and aquatic productivity. Lignin phenol was used to examine vegetation changes due to paleoenvironmental conditions and showed that long glacial periods were characterized by terrestrial vegetation.
Changes in North Atlantic deep-sea temperature during climatic fluctuations of the last 25,000 years based on ostracode Mg/Ca ratios
We reconstructed three time series of last glacial-to-present deep-sea temperature from deep and intermediate water sediment cores from the western North Atlantic using Mg/Ca ratios of benthic ostracode shells. Although the Mg/Ca data show considerable variability (“scatter”) that is common to single-shell chemical analyses, comparisons between cores, between core top shells and modern bottom water temperatures (BWT), and comparison to other paleo-BWT proxies, among other factors, suggest that multiple-shell average Mg/Ca ratios provide reliable estimates of BWT history at these sites. The BWT records show not only glacial-to-interglacial variations but also indicate BWT changes during the deglacial and within the Holocene interglacial stage. At the deeper sites (4500- and 3400-m water depth), BWT decreased during the last glacial maximum (LGM), the late Holocene, and possibly during the Younger Dryas. Maximum deep-sea warming occurred during the latest deglacial and early Holocene, when BWT exceeded modern values by as much as 2.5°C. This warming was apparently most intense around 3000 m, the depth of the modern-day core of North Atlantic deep water (NADW). The BWT variations at the deeper water sites are consistent with changes in thermohaline circulation: warmer BWT signifies enhanced NADW influence relative to Antarctic bottom water (AABW). Thus maximum NADW production and associated heat flux likely occurred during the early Holocene and decreased abruptly around 6500 years B.P., a finding that is largely consistent with paleonutrient studies in the deep North Atlantic. BWT changes in intermediate waters (1000-m water depth) of the subtropical gyre roughly parallel the deep BWT variations including dramatic mid-Holocene cooling of around 4°C. Joint consideration of the Mg/Ca-based BWT estimates and benthic oxygen isotopes suggests that the cooling was accompanied by a decrease in salinity at this site. Subsequently, intermediate waters warmed to modern values that match those of the early Holocene maximum of ∼7°C. Intermediate water BWT changes must also be driven by changes in ocean circulation. These results thus provide independent evidence that supports the hypothesis that deep-ocean circulation is closely linked to climate change over a range of timescales regardless of the mean climate state. More generally, the results further demonstrate the potential of benthic Mg/Ca ratios as a tool for reconstructing past ocean and climate conditions.
Quaternary soils and dust deposition in southern Nevada and California
Eolian dust constitutes much of the pedogenic material in late Pleistocene and Holocene soils of many arid regions of the world. Comparison of the compositions and influx rates of modern dust with the eolian component of dated soils at 24 sites in southern Nevada and California yields information on (1) the composition and influx rate of dust in late Pleistocene and Holocene soils, (2) paleoclimate and its effects on the genesis of aridic soils, especially with regard to dustfall events, (3) the timing and relative contribution of dust from playa sources versus alluvial sources, and (4) the effects of accumulation of dust in soil horizons. The <2 mm fractions of A and B horizons of soils formed on gravelly alluvial-fan deposits in the study area are similar to modern dust in grain size, content of CaCO 3 and salt, major oxides, and clay mineralogy; thus, they are interpreted to consist largely of dust. The major-oxide compositions of the shallow soil horizons are nearly identical to that of the modern dust, but the compositions of progressively deeper horizons approach that of the parent material. The clay mineralogy of modern dust at a given site is similar to that of the Av horizons of nearby Holocene soils but is commonly different from the mineralogies of deeper soil horizons and of the Av horizons of nearby Pleistocene soils. These results are interpreted to indicate that dust both accumulates and is transformed in Av horizons with time. Changes in soil-accumulation rates provide insights into the interplay of paleoclimate, dust supply, and soil-forming processes. Modern dust-deposition rates are more than large enough to account for middle and late Holocene soil-accumulation rates at nearly all sites. However, the early Holocene soil-accumulation rates in areas near late Pleistocene pluvial lakes are much higher than modern rates and clearly indicate a dust-deflation and -deposition event that caused rapid formation of fine-grained shallow soil horizons on uppermost Pleistocene and lower Holocene deposits. We interpret late Pleistocene soil-accumulation rates to indicate that dust-deposition rates were low during this period but that increased effective moisture during the late Wisconsinan favored translocation of clay and CaCO 3 from near the surface to deeper in the soil profile. Pre–late Pleistocene rates are very low in most areas, mainly due to a pedogenic threshold that was crossed when accumulations of silt, clay, and CaCO 3 began to inhibit the downward transport of eolian material, but in part due to erosion.
Geologic map of Colorado National Monument and adjacent areas, Mesa County, Colorado
New 1:24,000-scale geologic mapping in the Colorado National Monument Quadrangle and adjacent areas, in support of the USGS Western Colorado I-70 Corridor Cooperative Geologic Mapping Project, provides new interpretations of and data for the stratigraphy, structure, geologic hazards in the area from the Colorado River in Grand Valley onto the Uncompahgre Plateau. The plateau drops abruptly along northwest-trending structures toward the northeast 800 m to the Redlands area and the Colorado River in Grand Valley. In addition to common alluvial and colluvial deposits, surficial deposits include Holocene and late Pleistocene charcoal-bearing valley-fill deposits, late to middle Pleistocene river-gravel terrace deposits, Holocene to middle Pleistocene younger, intermediate, and old fan-alluvium deposits, late to middle Pleistocene local gravel deposits, Holocene to late Pleistocene rock-fall deposits, Holocene to middle Pleistocene young and old landslide deposits, Holocene to late Pleistocene sheetwash deposits and eolian deposits, and Holocene Cienga-type deposits. Only the lowest part of the Upper Cretaceous Mancos Shale is exposed in the map area near the Colorado River. The Upper and Lower? Cretaceous Dakota Formation and the Lower Cretaceous Burro Canyon Formation form resistant dipslopes in the Grand Valley and a prominent ridge on the plateau. Less resistant strata of the Upper Jurassic Morrison Formation consisting of the Brushy Basin, Salt Wash, and Tidwell Members form slopes on the plateau and low areas below the mountain front of the plateau. The Middle Jurassic Wanakah Formation nomenclature replaces the previously used Summerville Formation. Because an upper part of the Middle Jurassic Entrada Formation is not obviously correlated with strata found elsewhere, it is therefore not formally named; however, the lower rounded cliff former Slickrock Member is clearly present. The Lower Jurassic silica-cemented Kayenta Formation forms the cap rock for the Lower Jurassic carbonate-cemented Wingate Sandstone, which forms the impressive cliffs of the monument. The Upper Triassic Chinle Formation was deposited on the eroded and weathered Middle Proterozoic meta-igneous gneiss, pegmatite dikes, and migmatitic gneiss. Structurally the area is deceptively challenging. Nearly flat-lying strata on the plateau are folded by northwest-trending fault-propagation folds into at least two S-shaped folds along the mountain front of the plateau. Strata under Grand Valley dip at about 6 degrees to the northeast. In the absence of local evidence, the uplifted plateau is attributed to Laramide deformation by dated analogous structures elsewhere in the Colorado Plateau. The major exposed fault records high-angle reverse relationships in the basement rocks but dissipates strain as a triangular zone of distributed microfractures and cataclastic flow into overlying Mesozoic strata that absorb the fault strain, leaving only folds. Evidence for younger, probably late Pliocene or early Pleistocene, uplift does exist at the antecedent Unaweep Canyon south and east of the map area. To what degree this younger deformation affected the map area is unknown. Several geologic hazards affect the area. Middle and late Pleistocene landslides involving the smectite-bearing Brushy Basin Member of the Morrison Formation are extensive on the plateau and common in the Redlands below the plateau. Expansive clay in the Brushy Basin and other strata create foundation stability problems for roads and homes. Flash floods create a serious hazard to people on foot in narrow canyons in the monument and to homes close to water courses downstream from narrow restrictions close to the monument boundary.
Gold placer and Quaternary stratigraphy of the Jabal Mokhyat area, southern Najd Province, Kingdom of Saudi Arabia
An ancient gold placer at Jabal Mokhyat (lat 20?12.2'N., long 43?28'E.), about 90 km east of Qalat Bishah in the southern Najd Province, Kingdom of Saudi Arabia, was studied in 1973. Seven hundred and twenty-eight samples in 25 measured sections were collected along trenches and pits 2.5 m in depth and 2,600 m in total length. Alluvium was thicker than the excavation depth along about 50 percent of the trench length. The average gold content was 4.4 mg per m3, and the highest grade trench contained 40 mg gold per m 3. Because fine particulate gold is rare in the alluvium, a few large particles, 1 to 5 mm in diameter, greatly affected the sampling results. The ancient placer diggings are in small headwater wadis distributed over a 30-km 2 area, and the total dug area is about 1.2 km2. The placer produced an estimated 50 kg of gold and was worked about 2,600 + 250 years ago. The potential for a present-day placer operation is small. The gold is sparsely distributed in locally derived, flood-deposited, immature gravels throughout a stratigraphic section that consists of 1) calichified, saprolitic bedrock of Precambrian age; 2) basal, intensely calichified, saprolitic gravel (0-3 m thick) of Pleistocene age; 3) disconformable, slightly consolidated gravel and sand (0-1 m thick) of late Pleistocene age containing sparse, disseminated caliche; 4) firm loessic silt (0-1 m thick) of early Holocene age; and 5) loose sand and gravel (0.3-1 m thick) of late Holocene age. The loessic silt accumulated during the Holocene pluvial. The top of the loessic silt unit is dated at about 6,000 years B.P. by using charcoal from hearths of ancient man. Following the Holocene pluvial, the climate became arid, and extreme desiccation resulted in abundant eolian sand that progressively diluted the late Holocene gravels. The remnants of the pre-Holocene stratigraphy suggest similar climatic cycles during the Pleistocene. Abundant, sparsely mineralized, gold-bearing quartz veins (0-1 m wide) were the source of the placer gold. These late Proterozoic veins have hydrothermally altered wall-rock zones (1-5 m wide). The veins are dispersed over an area of 50 km 2. Though many veins were prospected in ancient times and some were slightly worked, only the Mokhyat ancient mine, located on a quartz-vein zone 30 m wide by 200 m long, was extensively worked. The quartz contains chalcopyrite, galena, sphalerite, tetrahedrite, an unidentified bismuth mineral, and small amounts of dispersed gold. The fissure quartz veins lie at the complexly splayed, terminal end of a small northwest-trending Najd fault that elsewhere along strike has ii km of left-lateral displacement. Most large veins are in north-trending vertical fractures where the stresses were distributed along an older, north-trending structural grain in andesitic greenstone terrane. Subhorizontal fracture sets contain conspicuous, well-developed gold-bearing quartz veins and associated alteration zones. These attest to the shallowness and youthfulness of mineralization during latest Precambrian time. Late Precambrian granitic plutons (625-600 m.y. old) had been deeply eroded before the gold minerals were emplaced; hence, the gold is not related to granitic plutonism. Abundant, widely distributed diabasic dikes associated with the Najd faulting event of latest Precambrian age were probably the heat source for the hydrothermal convection system and possibly the source of the gold.
Fate and pathways of injection-well effluent in the Florida Keys
Twenty-four wells (21 locations) were core drilled into the limestone beneath the Keys, reef tract, and outer reefs to determine if sewage effluents injected in Class V wells onshore are reaching offshore reef areas via underground flow. These wells were fitted with PVC casings and well screens and were sampled every three months for a period of one year. Analyses showed consistent hypersalinity in most wells and a marked increase in nitrogen (as ammonia) in offshore ground water. Other forms of nitrogen (NO 2 and NO 3 ) and phosphorous were not particularly elevated in offshore ground water but were above the levels found in surface marine water. The highest levels of nitrogen (NO 2 and NO 3 ) and phosphorous were in shallow onshore ground waters. Sources for the nutrients in the shallow onshore ground water consist of septic tanks and cesspools (@ 24,000 and 5,000 in the Florida Keys, respectively), agricultural fertilizers, and natural vegetation. Ammonia concentrations were low in shallow ground waters beneath the Florida Keys, probably because of oxidizing conditions. Tidal pumping is particularly active, especially nearshore. Hydraulic heads sufficient to elevate well water as much as 7 cm above sea level during falling tides were detected in all nearshore wells. During rising tides, the situation was reversed and water flowed into the wells. Tidal pumping implies considerable water movement both in and out of the upper few meters of limestone. Tidal pumping is a likely mechanism for mixing and transferring nutrient-rich ground water into the overlying marine waters. Although tidal pumping should cause rather complete mixing and dilution of any freshwater-based effluents entering the limestone via the more than 600 disposal wells in the Florida Keys, the ground waters in the 30- to 40-ft-depth range (9-12 m) nevertheless remained slightly hypersaline relative to sea water throughout the year. Fecal coliform and fecal streptococcal bacteria were associated with three Lower Keys offshore wells and two shallow onshore wells at Key Largo. On occasions, these bacteria were detected farther offshore, once in a well 4 miles off Key Largo. The bacterial analyses for Key Largo (both onshore and offshore) are supported by two independent bacteriological researchers using more sophisticated methods than the standard 100-ml membrane-filter method used in this study. Fecal bacteria can serve as tracers; thus, we conclude their presence is possible evidence for offshore transport of ground waters originating on Key Largo. Elevated nutrients (ammonia) and slightly elevated dissolved total phosphorous in offshore ground waters, however, cannot be tied to onshore sources with existing data. Rock analyses of material from our cores do not prove or disprove the hypothesis that limestone beneath the Keys or reef tract is serving as a sink for phosphorus or other nutrients. The data, however, do not rule out phosphorus uptake by limestone adjacent to disposal sources. For the purposes of this study, monitoring wells were not positioned sufficiently close to injection wells to determine if uptake of phosphorous is taking place. Ground waters were found to contain more dissolved solids than could be accounted for if hypersalinity resulted from simple evaporation of sea water. These data indicate that ground waters in the vicinity of our wells are dissolving solids from the rock rather than precipitating material within the rock framework; however, as mentioned above, our wells were not positioned sufficiently close to disposal wells to determine if localized uptake is occurring. Examination of rock cores from these wells revealed a general distribution of reef- and grainstone-facies belts. The Upper and Middle Keys are composed of a thin coral reef facies that extends only a few hundred feet seaward of the Keys. Reef facies give way to mudstone facies within a few yards of shore on the Florida Bay side of the Keys. On the seaward side of the Keys, beneath Hawk Channel and White Bank, the Pleistocene limestone is a mixed grainstone, packstone, and wackstone facies. Corals are rare or absent. The Pleistocene limestone beneath the outer reefs 4 to 5 miles offshore, however, consists of reef facies with the same coral fauna as that found on Key Largo. This pattern of two major reef-facies belts separated by a 2- to 4-mile-wide belt of grainstone facies may have as yet undetermined effects on groundwater circulation beneath the Florida reef tract. Grainstone is approximately an order of magnitude less permeable than the coralline Key Largo Limestone facies. The Q3 surface, a major subsurface unconformity thought to form an effective confining zone elsewhere in south Florida, was not detected in wells drilled more than 1 mile from shore. This unconformity, however, was detected in all wells drilled on or near the Keys. What was found to be a more effective and widespread confining layer is the Holocene sediment deposited on the Pleistocene limestone during the past 6,000 to 7,000 years. These relatively impermeable sediments are extensive, forming a belt up to 5 miles wide beginning about 0.5 mile offshore. Holocene sediments generally consist of low-permeability lime mud just above the Pleistocene surface, overlain by more permeable carbonate sands and reefs. Leakage of ground water by tidal pumping is not likely to occur through lime-mud-dominated areas such as Hawk Channel but is likely to occur through isolated porous and permeable Holocene reefs situated on Pleistocene limestone highs, and in places where Holocene sediment does not cover the limestone bedrock. Leakage is therefore limited to 1) a shallow-water 0.5-mile-wide nearshore belt of exposed Key Largo Limestone, 2) Holocene patch reefs, which grow on mud-free topographic rock highs, and 3) along the seaward side of the outermost reef in 35 to 65 ft (10-20 m) of water, where Holocene reef and sediment accumulations are thin or absent. This study did not address direct measurements of lateral groundwater movement or a hydrologic mechanism for transporting hypersaline ground water away from the Florida Keys. More recent work, however (Halley et al., 1994), shows that sea level in Florida Bay is higher than on the Atlantic side of the Keys more than 50% of the time. Higher sea level on the bay side of the Keys provides a potential for groundwater flow toward the Atlantic most of the time. Use of tracers (dyes or harmless bacteriological tracers) injected into the center of tightly spaced clusters of monitoring wells is a simple way to ascertain the net direction and rate of groundwater movement. Knowing the direction and rate of groundwater movement is needed for prediction and modeling efforts in the future
Paleoliquefaction field reconnaissance in eastern North Carolina—Is there evidence for large magnitude earthquakes between the central Virginia seismic zone and Charleston seismic zone?
In June 2016, approximately 64 kilometers (km) of riverbank were examined along the Tar and Neuse Rivers near Tarboro and Kinston, North Carolina, for evidence of liquefaction-forming earthquakes. The study area is in the vicinity of the Grainger’s fault zone in eastern North Carolina. The Grainger’s fault zone is a fault zone in the inner Coastal Plain Province that has well-documented Paleogene and younger deformation of Cretaceous to Eocene strata. Low-magnitude earthquakes near the fault zone (for example, magnitude [M] 2.1 in 2013, 13 km south-southwest of Kinston) suggest larger earthquakes may have struck this region in the past. The study area is about equidistant from newly documented Holocene paleoliquefaction sites in the Central Virginia Seismic Zone (CVSZ) and liquefaction sites formed during the 1886 M7.1 Charleston, South Carolina earthquake. The northernmost Holocene paleoliquefaction features associated with the Charleston Seismic Zone (CSZ) are in Southport, North Carolina. Conditions suitable for liquefaction were identified at 38 sites on both rivers, but only one site was classified as highly susceptible. Stratigraphy consists of Paleozoic gneiss; Cretaceous sandstone/shale; Paleocene mudstone/claystone to Eocene fossiliferous limestone; Quaternary unconsolidated, crossbedded sand and gravel; and Holocene alluvium. Three sets of stratigraphic conditions suitable for liquefaction—unconsolidated source sand beneath capping strata—were identified in detailed examinations at 105 sites: (1) Holocene alluvial sand beneath alluvial silt and clay beds; (2) Quaternary terrace sand beneath beds of silt and clay; and (3) Holocene alluvial sand or Quaternary terrace sand capped by clay-rich Bt soil horizons. Weathered and unconsolidated Cretaceous sand capped by a Bt soil horizon was identified at one site, but the weathered sand is likely too compacted to liquefy readily. One outcrop containing three small sand dikes, and four outcrops of soft-sediment deformation features—mostly load casts—were observed, but none of these features could be conclusively established as seismogenic. A few examples of pseudo-sand-dikes were also identified: sand-filled cypress root casts and pedogenic weathering fronts created the appearance of sand dikes and sills. A comparable survey in 2015 of 119 km of riverbank exposures in the CVSZ yielded 19 paleoliquefaction sites of probable earthquake origin; these features formed from at least one M~6 earthquake in the past 6,000 years (6 ka). This survey in eastern North Carolina revealed no definitive paleoliquefaction features; earthquakes of sufficient magnitude to produce liquefaction likely have not affected this region during the Holocene.
Paleoseismic investigation of the Taylorsville fault at the Airport East site, West Valley fault zone, Salt Lake County, Utah
The West Valley fault zone (WVFZ) and Salt Lake City segment (SLCS) of the Wasatch fault zone comprise Holoceneactive normal faults that bound an intrabasin graben in northern Salt Lake Valley, Utah. Both fault zones have evidence of recurrent Holocene surface-faulting earthquakes. A topic of recent research is the seismogenic relation of the antithetic (subsidiary) WVFZ to the Wasatch fault zone—specifically, to what degree are WVFZ earthquakes independent of slip on the SLCS, or other adjacent segments, of the Wasatch fault zone. To improve paleoseismic data for the WVFZ and better understand the seismogenic relation between the WVFZ and Wasatch fault zone, we conducted a fault-trench investigation at the Airport East site, developed new earthquake recurrence and fault sliprate estimates for the WVFZ, and compared WVFZ earthquake timing data with data from the Wasatch fault zone. The Airport East site is near the northern end of the easternmost traces of the WVFZ, collectively referred to as the Taylorsville fault. At this site, we excavated two parallel trenches across a small (~0.5-m high) east-facing fault scarp. Shallow groundwater severely limited trench depth, and we were able to expose deposits only as old as mid-Holocene. However, the late Holocene section contained evidence for three surfacefaulting earthquakes on the Taylorsville fault, as well as earthquake-related deformation (liquefaction and folding) from a fourth earthquake that was likely sourced elsewhere. Based on OxCal modeling of radiocarbon and optically stimulated luminescence ages, the most recent earthquake, AE1, occurred at 0.4 ± 0.2 ka (mean modeled time ± 2σ), earthquake AE2 occurred at 0.6 ± 0.2 ka, and earthquake AE3 occurred at 2.0 ± 0.3 ka. Timing data from the Airport East site indicate the liquefaction and folding event (LE1) occurred at 5.1 ± 0.3 ka. Net vertical displacement across the fault is 0.6–1.1 m, and calculations of mean per-event displacement range from 0.20 to 0.37 m. Inter-event recurrence intervals for the Taylorsville fault at the Airport East site vary from 200 to 1400 yr, and the mean late Holocene (post-2 ka) recurrence interval is 800 yr. Paleoseismic (closed-interval) slip rates range from 0.23 to 1.4 mm/yr, and geologic (open-interval) slip rates range from 0.1–0.2 mm/yr over the past ~5000 yr to 0.2–0.4 mm/yr over the past ~2500 yr. Combining our new Airport East data with previous paleoseismic data for the WVFZ shows that during individual late Holocene earthquakes, surface faulting has occurred on either the Taylorsville or Granger fault, but not both faults at the same time (at least not on the parts of the faults that have been trenched). However, five mid- to late Holocene WVFZ earthquakes have mean modeled times that are either very similar or identical to mean modeled times of SLCS and Weber-segment earthquakes, and temporal correlations of two additional late Pleistocene WVFZ earthquakes with SLCS earthquakes cannot be ruled out. When comparing the earthquake chronologies of the WVFZ and Wasatch fault zone, our new data lend support to the idea that, more often than not, some part of the WVFZ moves in response to, and possibly synchronously with, slip on the Wasatch fault zone.
Estimated post-Messinian sediment supply and sedimentation rates on the Ebro continental margin, Spain
Because of the extensive data base of seismic profiles, radiometric ages, and stratigraphic time markers such as the subaerial Messinian surface, sedimentation rates and Ebro River sediment discharge can be estimated for different periods and environments of the Ebro continental margin. New values for sediment discharge (i.e., 6.2 versus previous estimates of 2–3.5 million t/yr) for the Holocene highstand are more reliable but remain minimum estimates because a small proportion of Ebro sediment advected to the Balearic Rise and Abyssal Plain cannot be accounted for, especially during lowstands. The general highstand conditions of the Pliocene, which were similar to those of the Holocene, resulted in a low discharge of Ebro River sediment (ca. 6.5 million t/yr) and an even thickness of sediment across the margin that deposited at rates of about 24–40 cm/ky. In contrast, sediment supply increased two-three times during the Pleistocene, the margin prograded rapidly and deposition occurred at rates of 101–165 cm/ky on the outer shelf and slope, but basin floor rates remained anomalously low (21–26 cm/ky) because sediment was drained and broadly dispersed eastward in Valencia Trough. During the late Pleistocene rise of sea level, the main depocenters progressively shifted shoreward and sedimentation rates greatly decreased from 175 cm/ky on the upper slope during the early transgression to 106 cm/ky on the outer shelf and then to 63 cm/ky on the mid-shelf during the late transgression as the river sediment discharge dropped to half by Holocene time. Maximal sedimentation rates occurred in active depocenters of sediment dispersal such as the Holocene delta (370 cm/ky) or the youngest Pleistocene Oropesa channel-levee complex (705 cm/ky) where deposition rates increased by an order of magnitude or more compared to average Ebro shelf (38 cm/ky) or base-of-slope rates in the Pleistocene (21 cm/ky). The sedimentation rates verify the importance of sea-level control on the progressive change in location of depocenters and amount of sediment supply, but Pleistocene climatic change and deforestation alone can be observed to double river sediment discharge. The latter observation helps explain the anomalously high deposition rates in Pleistocene turbidite systems compared with older systems that may be controlled more by tectonic and sea-level changes alone. During the past 2000 years, in contrast, man has controlled deposition in the Ebro margin system, first by deforestation that more than doubled river sediment discharge and shelf deposition rates to equal those of Pleistocene time; and second by dam contruction that reduced sediment discharge to less than 5% of the normal Holocene discharge. Similar recent discharge reductions from the Nile and Rhone Rivers suggest that loss of the majority of the river sediment supply in the Mediterranean Sea may result in significant erosion of biologically and agriculturally important lobate delta areas.