SEARCH · Geology Reports

Results for “The Sedimentary Record”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 631 records · Page 35Linked to original sources

USGS1967

Time and space relationships of the taconic allochthon and autochthon

The Taconic rock sequence extends from near Sudbury, Vermont, to near Poughkeepsie, New York, a length of about 150 miles; and from just west of the Green Mountain Range and Berkshire Highlands to the valleys of the Hudson River and Lake Champlain, a width of about 20 miles. The Taconic rocks are now in the axial region of the Middlebury synclinorium and its southward extension. The Taconic sequence consists of about 2000 feet of slate, with subsidiary graywacke, quartzite, and limestone; many of the rock units are turbidites. Fossils from the rocks include forms of Early, Middle, and Late Cambrian and Early and Middle Ordovician age. Evidence of stratigraphic tops derived from the fossils and from primary sedimentary features agrees with structural data and demonstrates that the Taconic rocks are geometrically the highest strata within the Middlebury synclinorium. The underlying rocks of the synclinorium (the synclinorium sequence) are right side up; they also range in age from Early Cambrian to Middle Ordovician but belong to a different sedimentary lithofacies (dominantly carbonate and orthoquartzite). The relation between the Taconic sequence and the synclinorium sequence, therefore, is a baffling problem. Lithostratigraphically, the Taconic sequence falls into three groups: (1) the pre- Normanskill "low Taconic " sequence, occurring in the area between the main Taconic Range and the Hudson River, as far south as Rhinebeck, New York; (2) the Normanskill Shale in the same area, as well as in the area south of Rhinebeck at least as far as Poughkeepsie, New York, and also west of the Hudson River; and (3) the "high Taconic " sequence, occupying the main Taconic Range from Dorset Mountain, Vermont, south to Indian Mountain in Sharon, Connecticut, as well as Mount Greylock in Massachusetts. Rocks of (1) and (2) are fossiliferous, but to date no fossil has been found in (3). Rocks of (1) and that part of (2) areally coextensive with (1) thus are of known age but uncertain three-dimensional geometric configuration, whereas rocks of (3) are of known configuration (in the centers of open synclinoria) but unknown age. Only that part of (2) beyond the areal confines of (1) is both of known age and known configuration; these rocks are in sedimentary contact above the older rocks of the synclinorium sequence and are autochthonous. At the north end of the Taconic sequence in western Vermont, rocks of group (1) are beyond reasonable doubt allochthonous. Because of the southward geometric continuity of the structural elements, all the Taconic rocks of group (1), and that part of group (2) areally coextensive with it, are interpreted as allochthonous. The structure of group (3), the high Taconic sequence, is inferred by topography and by detailed lithostratigraphic matching with rocks of the east Vermont sequence; on this basis, as well as on the basis of the broad lithic similarity with rocks of group (1), rocks of group (3) are concluded to be also allochthonous. A discontinuous polymict conglomerate underlies and surrounds the allochthon on all sides and is interpreted here as a record that dates the imminent arrival of the allochthon at each locality. The conglomerate contains unsorted blocks of rocks of both the Taconic sequence and the synclinorium sequence; the matrix is the autochthonous upper Normanskill Shale or its equivalent. Fossils from the matrix shale date the event as Trenton, probably Sherman Fall in age. The geologic history of the area is reconstructed as follows: The pre-Normanskill Taconic rocks were deposited in the area of the present Precambrian massifs of the Green Mountains-Berkshire Highlands belt between the clastic, eugeosynclinal east Vermont sequence to the east and the miogeosynclinal synclinorium sequence to the west; they constitute the transitional facies between these two belts. Conditions were relatively stable until early Middle Ordovician time , when the Green Mountain- Berkshire Highlands area began to rise and the area of the present Middlebury synclinorium began to subside. Subsidence took place largely by a series of high-angle longitudinal faults that, as a whole, step down to the west. Argillaceous sediments (the Normanskill Shale) began to inundate the former miogeosynclinal area; because the conditions of sedimentation had become similar, the sediments resembled, in facies, the synchronous Taconic rocks that were being deposited to the east. Continued rise of the Green Mountains-Berkshire Highlands area led in middle Trenton time to the decollement of the Cambrian and Ordovician sediments into the area of the present Middlebury synclinorium in a series of giant submarine slides. Sedimentation continued at the receiving site throughout the event; sedimentation may also have persisted on the moving slides. The record is found today in the turbidite-laden shale and graywacke in the upper part of the Normanskill Shale of both the allochthon and the autochthon . Restoration of the allochthonous rocks to the original site of deposition leads to correlations between rocks of the Taconic sequence and of the largely autochthonous east Vermont sequence. The lithic correlation can be carried to the level of individual formations and is confirmed by a few known ages in the east Vermont sequence. Several lines of reasoning lead to a plausible correlation of part of the Cavendish Formation of southeastern Vermont with the oldest part of the Taconic sequence. This correlation leads further to the conclusion that in this area the contacts between the Green Mountain massif and the Cavendish Formation and between the Cavendish and the overlying east Vermont sequence must both be thrust faults of large displacements. This conclusion is in fact inevitable because one of the Taconic thrust slices that extends without interruption between the latitudes corresponding to the gap in the Precambrian massifs has been shown by local structural evidence to be allochthonous; an outside original depositional site must be found for it. The present Taconic allochthon is coextensive with an area of marked negative Bouguer gravity anomaly; the Green Mountains-Berkshire massifs constitute a belt of positive anomaly. It is here proposed that these anomalies resulted from a deepseated transfer of material; subcrustal addition of material caused the rise of the Green Mountains-Berkshire Highlands area, and the concurrent subtraction of material caused subsidence in the Middlebury synclinorium area through a series of faults which were the near-surface expression of an episode of crustal collapse. If this interpretation is correct, then the regional gravity anomaly represents an uncompensated feature that has persisted since Middle Ordovician time .

Special Paper of the Geological Society of America↗

USGS2006

Reconnaissance borehole geophysical, geological, and hydrological data from the proposed hydrodynamic compartments of the Culpeper Basin in Loudoun, Prince William, Culpeper, Orange, and Fairfax Counties, Virginia

The Culpeper basin is part of a much larger system of ancient depressions or troughs, that lie inboard of the Atlantic Coastal Plain, and largely within the Applachian Piedmont Geologic Province of eastern North America, and the transition region with the neighboring Blue Ridge Geologic Province. This basin system formed during an abortive attempt to make a great ocean basin during the Late Triassic and Early Jurassic, and the eroded remnants of the basins record major episodes of sedimentation, igneous intrusion and eruption, and pervasive contact metamorphism. Altogether, some twenty nine basins formed between what is now Nova Scotia and Georgia. Many of these basins are discontinuous along their strike, and have therefore recorded isolated environments for fluvial and lacustrine sedimentation. Several basins (including the Culpeper, Gettysburg, and Newark basins) are fault-bounded on the west, and Mesozoic crustal stretching has produced assymetrical patterns of basin subsidence resulting in a progressive basin deepening to the west, and a virtual onlap relationship with the pre-basin Proterozoic rocks to the east. A result of such a pattern of basin deepening is the development of sequences of sandstones and siltstones that systemmatically increase in dip towards the accomodating western border faults. A second major structural theme in several of the major Mesozoic basins (including the Culpeper) concerns the geometry of igneous intrusion, as discussed below. Froelich (1982, 1985) and Lee and Froelich (1989) discuss the general geology of the Culpeper basin, and Smoot (1989) discusses the sedimentation environments and sedimentary facies of the Mesozoic with respect to fluvial and shallow lacustrine deposition in the Culpeper basin. Ryan and others, 2007a, b, discuss the role of diabase-induced compartmentalization in the Culpeper basin (and other Mesozoic basins), and illustrate (using alteration mineral suites within the diabase and adjacent hornfels, among other evidence) how this process has played a role in organizing the paleo- and contemporary-flow of crustal fluids at local and regional scales. Within this report, the Newark Supergroup nomenclature of Weems and Olsen (1997) is adopted.

Virginia↗

USGS2026

Postglacial ecosystem development of a hydrothermal landscape in Yellowstone National Park

The Yellowstone geo-ecosystem has been the subject of much research, but the ecological history of the Yellowstone Plateau volcanic field and its iconic geyser basins is less known. In this investigation, paleoenvironmental analyses of sediment cores from lakes in Lower Geyser Basin were compared with regional records and paleoclimate model simulations to reconstruct the vegetation, wildfire, limnology, hydrothermal dynamics, and climate drivers since deglaciation, 15,000 to 14,000 years ago. Pollen data from Lower Geyser Basin lakes reveal the strong influence of infertile rhyolitic soils on vegetation history: an initial late-glacial steppe was replaced by lodgepole pine forest from 12.8 to 11.0 ka, with little change in forest composition or cover thereafter despite changing climate. This stability contrasts with the more dynamic vegetation response on nonrhyolite substrates in the Yellowstone region where nutrient and moisture availability is greater. Highest wildfire activity and low lake nutrient levels in Lower Geyser Basin occurred from 12 to 4 ka, when summers were substantially warmer and drier and fire-inducing vapor pressure deficits were 29 to 56% higher. The hydrothermal history, inferred from sedimentary arsenic and cesium abundances, was spatially and temporally variable but lake-forming hydrothermal events align with periods of abundant moisture. Thus, long-term changes in wildfire, limnology, and, to some extent, hydrothermal activity were governed by insolation-driven climate variations, whereas the vegetation response was muted and constrained by geologic processes. These findings suggest that warmer, drier conditions in the future could result in less hydrothermal activity yet little change in forest cover across the Yellowstone Plateau volcanic field despite more wildfires.

Idaho, Montana, Wyoming↗

USGS2000

Stratigraphy, correlation, depositional setting, and geophysical characteristics of the Oligocene Snowshoe Mountain Tuff and Creede Formation in two cored boreholes

Core descriptions and geophysical logs from two boreholes (CCM-1 and CCM-2) in the Oligocene Snowshoe Mountain Tuff and Creede Formation, south-central Colorado, are used to interpret sedimentary and volcanic facies associations and their physical properties. The seven facies association include a mixed sequence of intracaldera ash-flow tuffs and breccias, alluvial and lake margin deposits, and tuffaceous lake beds. These deposits represent volcanic units related to caldera collapse and emplacement of the Snowshoe Mountain Tuff, and sediments and pyroclastic material deposited in the newly formed caldera basin, Early sedimentation is interpreted to have been rapid, and to have occurred in volcaniclastic fan environments at CCM-1 and in a variery of volcaniclastic fan, braided stream shallow lacustrine, and mudflat environments at CCM-2. After an initial period of lake-level rise, suspension settling, turbidite, and debris-flow sedimentation occurred in lacustrine slope and basin environments below wave base. Carbonate sedimentation was initially sporadic, but more continuous in the latter part of the recorded lake history (after the H fallout tuff). Sublacustrine-fan deposition occurred at CCM-1 after a pronounced lake-level fall and subsequent rise that preceded the H tuff. Variations in density, neutron, gamma-ray, sonic, and electrical properties of deposits penetrated oin the two holes reflect variations in lithology, porosity, and alteration. Trends in the geophysical properties of the lacustrine strata are linked to downhole changes in authigenic mineralology and a decrease in porosity interpreted to have resulted primarily from diagenesis. Lithological and geophysical characteristics provide a basis for correlation of the cores; however, mineralogical methods of correlation are hampered by the degree of diagenesis and alteration.

Colorado↗

USGS2014

Distinguishing seawater from geologic brine in saline coastal groundwater using radium-226; an example from the Sabkha of the UAE

Sabkhat (Salt flats) are common geographic features of low-lying marine coastal areas that develop under hyper-arid climatic conditions. They are characterized by the presence of highly concentrated saline solutions and evaporitic minerals, and have been cited in the geologic literature as present-day representations of hyper-arid regional paleohydrogeology, paleoclimatology, coastal processes, and sedimentation in the geologic record. It is therefore important that a correct understanding of the origin and development of these features be achieved. Knowledge of the source of solutes is an important first step in understanding these features. Historically, two theories have been advanced as to the main source of solutes in sabkha brines: an early concept entailing seawater as the obvious source, and a more recent and dynamic theory involving ascending geologic brine forced upward into the base of the sabkha by a regional hydraulic gradient in the underlying formations. Ra-226 could uniquely distinguish between these sources under certain circumstances, as it is typically present at elevated activity of hundreds to thousands of Bq/m 3 (Becquerels per cubic meter) in subsurface formation brines; at exceedingly low activities in open ocean and coastal water; and not significantly supplied to water from recently formed marine sedimentary framework material. The coastal marine sabkha of the Emirate of Abu Dhabi was used to test this hypothesis. The distribution of Ra-226 in 70 samples of sabkha brine (mean: 700 Bq/m 3 ), 7 samples of underlying deeper formation brine (mean: 3416 Bq/m 3 ), the estimated value of seawater (< 16 Bq/m 3 ) and an estimate of supply from sabkha sedimentary framework grains (<~6 Bq/m 3 ) provide the first direct evidence that ascending geologic brine contributes significantly to the solutes of this sabkha system.

Abu Dhabi↗

USGS1998

The Purisima Formation and related rocks (upper Miocene - Pliocene), greater San Francisco Bay area, central California; review of literature and USGS collection now housed at the Museum of Paleontology, University of California, Berkeley

Sedimentary rocks more than 1.6 kilometers thick are attributed to the upper Miocene to upper Pliocene Purisima Formation in the greater San Francisco Bay area. These rocks occur as scattered, discontinuous outcrops from Point Reyes National Seashore in the north to south of Santa Cruz. Lithologic divisions of the Formation appear to be of local extent and are of limited use in correlating over this broad area. The Purisima Formation occurs in several fault-bounded terranes which demonstrate different stratigraphic histories and may be found to represent more than a single depositional basin. The precise age and stratigraphic relationship of these scattered outcrops are unresolved and until they are put into a stratigraphic and paleogeographic context the tectonic significance of the Purisima Foramtion can only be surmised. This paper will attempt to resolve some of these problems. Mollusks and echinoderms are recorded from the literature and more than 70 USGS collections that have not previously been reported. With the exception of one locality, the faunas suggest deposition in normal marine conditions at water depths of less than 50 m and with water temperatures the same or slightly cooler than exist along the present coast of central California. The single exception is a fauna from outcrops between Seal Cove and Pillar Point, where both mollusks and foraminifers suggest water depths greater than 100 m. Three molluscan faunas, the La Honda, the Pillar Point, and the Santa Cruz, are recognized based on USGS collections and published literature for the Purisima Formation. These biostratigraphically distinct faunas aid in the correlation of the scattered Purisima Formation outcrops. The lowermost La Honda fauna suggests shallow-water depths and an age of late Miocene to early Pliocene. This age is at odds with a younger age determination from an ash bed in the lower Purisima Formation along the central San Mateo County coast. The Pillar Point fauna contains only a single age diagnostic taxon, Lituyapecten purisimaensis (Arnold), which is reported as Pliocene in age, but it only occurs in the Purisima Formation, so its age here is an example of circular reasoning. However, based on tentative lithologic correlations this fauna may represent the same period of time as the upper part of the La Honda fauna. This fauna differs from either the La Honda or Santa Cruz faunas in that it represent significantly deeper water. The uppermost Santa Cruz fauna also suggests shallow-water depths and a possible age range of early to late Pliocene. The bivalve molluscan taxon Lyonsia, and gastropod taxon Rictaxis sp., cf. R. punctocaelatus (Carpenter) are reported here for the first time from the Purisima Formation.

Open-File Report↗

USGS2012

Spatial distribution of landslides triggered from the 2007 Niigata Chuetsu–Oki Japan Earthquake

Understanding the spatial distribution of earthquake-induced landslides from specific earthquakes provides an opportunity to recognize what to expect from future events. The July 16, 2007 M w 6.6 (M JMA 6.8) Niigata Chuetsu–Oki Japan earthquake triggered hundreds of landslides in the area surrounding the coastal city of Kashiwazaki and provides one such opportunity to evaluate the impacts of an offshore, magnitude 6 + earthquake on a steep coastal region. As part of a larger effort to document all forms of geotechnical damage from this earthquake, we performed landslide inventory mapping throughout the epicentral area and analyzed the resulting data for spatial, seismic-motion, and geologic correlations to describe the pattern of landsliding. Coupled with examination of a third-party, aerial-photo-based landslide inventory, our analyses reveal several areas of high landslide concentration that are not readily explained by either traditional epicentral and fault–plane-distance metrics or by recorded and inferred ground-motions. Whereas average landslide concentrations averaged less than 1 landslide per square kilometer (LS/km 2 ), some areas reached up to 2 LS/km 2 in the Nishiyama Hills to the northeast of Kashiwazaki and between 2 and 11 LS/km 2 in coastal areas to the north and south of the city. Correlation with seismometer-based and monument overturning back-calculated ground motions suggests that a minimum peak ground acceleration (PGA) of approximately 0.2 g was necessary for landsliding throughout the region, but does not explain the subregional areas of high landslide concentration. However, analysis of topographic slope and the distribution of generally weak, dip-slope, geologic units does sufficiently explain why, on a sub-regional scale, high landslide concentrations occurred where they did. These include: (1) an inland region of steep, dip-slope, anticlinal sedimentary strata with associated fold belt compression and uplift of the anticline and (2) coastal areas with generally weaker, weathered outcrop lithology and steeper slopes resulting from active marine and terrestrial cliff processes. The results offer lessons for understanding the effects of earthquakes on both regional and subregional scales with regard to the spatial distribution of landsliding.

Engineering Geology↗

USGS2009

Hydrogeologic Framework, Groundwater Movement, and Water Budget in Tributary Subbasins and Vicinity, Lower Skagit River Basin, Skagit and Snohomish Counties, Washington

A study to characterize the groundwater-flow system in four tributary subbasins and vicinity of the lower Skagit River basin was conducted by the U.S. Geological Survey to assist Skagit County and the Washington State Department of Ecology in evaluating the effects of potential groundwater withdrawals and consumptive use on tributary streamflows. This report presents information used to characterize the groundwater and surface-water flow system in the subbasins, and includes descriptions of the geology and hydrogeologic framework of the subbasins; groundwater recharge and discharge; groundwater levels and flow directions; seasonal groundwater-level fluctuations; interactions between aquifers and the surface-water system; and a water budget for the subbasins. The study area covers about 247 mi2 along the Skagit River and its tributary subbasins (East Fork Nookachamps Creek, Nookachamps Creek, Carpenter Creek, and Fisher Creek) in southwestern Skagit County and northwestern Snohomish County, Washington. The geology of the area records a complex history of accretion along the continental margin, mountain building, deposition of terrestrial and marine sediments, igneous intrusion, and the repeated advance and retreat of continental glaciers. A simplified surficial geologic map was developed from previous mapping in the area, and geologic units were grouped into nine hydrogeologic units consisting of aquifers and confining units. A surficial hydrogeologic unit map was constructed and, with lithologic information from 296 drillers'logs, was used to produce unit extent and thickness maps and four hydrogeologic sections. Groundwater in unconsolidated aquifers generally flows towards the northwest and west in the direction of the Skagit River and Puget Sound. This generalized flow pattern is likely complicated by the presence of low-permeability confining units that separate discontinuous bodies of aquifer material and act as local groundwater-flow barriers. Groundwater-flow directions in the sedimentary aquifer likely reflect local topographic relief (radial flow from bedrock highs) and more regional westward flow from the mountains to the Puget Sound. The largest groundwater-level fluctuations observed during the monitoring period (October 2006 through September 2008) occurred in wells completed in the sedimentary aquifer, and ranged from about 3 to 27 feet. Water levels in wells completed in unconsolidated hydrogeologic units exhibited seasonal variations ranging from less than 1 to about 10 feet. Synoptic streamflow measurements made in August 2007 and June 2008 indicate a total groundwater discharge to creeks in the tributary subbasin area of about 13.15 and 129.6 cubic feet per second (9,520 and 93,830 acre-feet per year), respectively. Streamflow measurements illustrate a general pattern in which the upper reaches of creeks in the study area tended to gain flow from the groundwater system, and lower creek reaches tended to lose water. Large inflows from tributaries to major creeks in the study area suggest the presence of groundwater discharge from upland areas underlain by bedrock. The groundwater system within the subbasins received an average (September 1, 2006 to August 31, 2008) of about 92,400 acre-feet or about 18 inches of recharge from precipitation a year. Most of this recharge (65 percent) discharges to creeks, and only about 3 percent is withdrawn from wells. The remaining groundwater recharge (32 percent) leaves the subbasin groundwater system as discharge to the Skagit River and Puget Sound.

Scientific Investigations Report↗

USGS2009

The Sudbury impact layer in the paleoproterozoiciron ranges of northern Michigan, USA

A layer of breccia that contains fragments of impact ejecta has been found at 10 sites in the Paleoproterozoic iron ranges of northern Michigan, in the Lake Superior region of the United States. Radiometric age constraints from events predating and postdating deposition of the breccia are ca. 1875 Ma and 1830 Ma. The major bolide impact that occurred at 1850 Ma at Sudbury, Ontario, 500–700 km east of these sites, is the likely causative event. The Michigan sites described here, along with previously described sites in Minnesota and Ontario, define an extensive ejecta-bearing deposit throughout the Paleoproterozoic iron ranges of the Lake Superior region that we refer to as the Sudbury impact layer. The layer at the sites in Michigan exhibits a range of thicknesses, lithologic characters, and sedimentary settings. The diversity of rock types and internal stratigraphic details of the layer imply that several different processes of transport and deposition are represented, but the detailed investigations needed to document them are incomplete. Many of the sites had been described and interpreted previously as products of common terrestrial processes, but the presence of relict shock-induced planar deformation features in quartz indicates that the breccia layer is in fact the product of an extraterrestrial impact. At most localities, this layer also contains relict fragments of altered devitrified glass and/or accretionary lapilli. One immediate use of the impact layer is as an ultraprecise time line that ties together the well-known stratigraphic sequences of the various geographically separated iron ranges, the correlation of which has remained controversial for many decades. The Sudbury impact layer most commonly lies at a horizon that records a significant change in the character of sediments across the region. The impact layer marks the end of a major period of banded iron formation deposition that was succeeded by deposition of fine clastic rocks, commonly black shales. The impact may have produced regional, if not global, changes in the environment that resulted in this widespread synchronous change in sedimentation style.

Geological Society of America Bulletin↗

USGS2013

Morphology and stratal geometry of the Antarctic continental shelf: Insights from models

Reconstruction of past ice-sheet fluctuations from the stratigraphy of glaciated continental shelves requires understanding of the relationships among the stratal geometry, glacial and marine sedimentary processes, and ice dynamics. We investigate the formation of the morphology and the broad stratal geometry of topsets on the Antarctic continental shelf with numerical models. Our models assume that the stratal geometry and morphology are principally the results of time-integrated effects of glacial erosion and sedimentation related to the location of the seaward edge of the grounded ice. The location of the grounding line varies with time almost randomly across the shelf. With these simple assumptions, the models can successfully mimic salient features of the morphology and the stratal geometry. The models suggest that the current shelf has gradually evolved to its present geometry by many glacial advances and retreats of the grounding line to different locations across the shelf. The locations of the grounding line do not appear to be linearly correlated with either fluctuations in the 5 l s O record (which presumably represents changes in the global ice volume) or with the global sea-level curve, suggesting that either a more complex relationship exists or local effects dominate. The models suggest that erosion of preglacial sediments is confined to the inner shelf, and erosion decreases and deposition increases toward the shelf edge. Some of the deposited glacial sediments must be derived from continental erosion. The sediments probably undergo extensive transport and reworking obliterating much of the evidence for their original depositional environment. The flexural rigidity and the tectonic subsidence of the underlying lithosphere modify the bathymetry of the shelf, but probably have little effect on the stratal geometry. Our models provide several guidelines for the interpretation of unconformities, the nature of preserved topset deposits, and the significance of progradation versus aggradation of shelf sediments.

Book chapter↗

USGS2010

The North American Midcontinent rift beneath Lake Superior from GLIMPCE seismic reflection profiling

The Midcontinent rift system is a 1.1-b.y.-old structure extending from Kansas, through the Lake Superior region, and into southern Michigan. The rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments. For most of its extent it is buried beneath Paleozoic rocks but can be traced by its strong gravity and magnetic anomalies. The rocks of the rift system are exposed only in the Lake Superior region and comprise the Keweenawan Supergroup. Much of the geology of the Keweenawan is beneath Lake Superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on-shore geology. Seismic reflection surveys by the Great Lakes International Multidisciplinary Program on Crustal Evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail. The reflection profiles across the rift reveal a deep, asymmetrical central graben whose existence and magnitude was not previously documented. They show that, in addition to crustal sagging documented by previous investigations, normal faulting played a major role in subsidence of the axial region of the rift. A sequence of volcanic and sedimentary rocks, in places greater than 30 km thick, fills the graben. Thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben. Near the axis, the prerift crust is thinned to about one fourth of its original thickness, apparently by a combination of low-angle extensional faulting and ductile stretching or distributed shear. The sense of asymmetry of the central graben changes along the trend of the rift, documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments. The location of the accommodation zones is inferred from abrupt disruptions in the Bouguer gravity signature of the rift. Uplift of the central graben occurred when the original graben-bounding normal faults were reactivated as high-angle reverse faults with throws of 5 km or more in places. The Midcontinent rift has some striking similarities to some younger passive continental margins. We propose that it preserves a record of nearly complete continental separation which, had it not been arrested, would have created a Middle Proterozoic ocean basin.

Lake Superior↗

USGS2019

Constraining parameter uncertainty in modeling debris-flow initiation during the September 2013 Colorado Front Range storm

The occurrence of debris flows during the September 2013 northern Colorado floods took the emergency management community by surprise. The September 2013 debris flows in the Colorado Front Range initiated from shallow landslides in colluvium. Most occurred on south- and east-facing slopes on the walls of steep canyons in crystalline rocks and on sedimentary hogbacks. Previous studies showed that most debris flows occurred in areas of high storm-total rainfall and that strength added by tree roots accounts for the low number of landslides in densely forested areas. Given the lack of rainfall thresholds for debris flow occurrence in northern Colorado, we want to parameterize a numerical model to assess potential for debris flows in advance of heavy rainfall. Natural Resources Conservation Service (NRCS) soil mapping of the area, supplemented by laboratory testing and field measurements, indicates that soil textures and hydraulic properties of landslide source materials vary considerably over the study area. As a step toward modeling storm response, available soil and geologic mapping have been interpreted to define zones of relatively homogeneous properties. A new, simplified modeling approach for evaluating model input parameters in the context of slope and depth of observed debris flow source areas and recorded debris-flow inducing rainfall helps narrow the range of possible parameters to those most likely to produce model results consistent with observed debris flow initiation. Initial results have narrowed the strength parameters to about one third of possible combinations of cohesion and internal friction angle and narrowed hydraulic conductivity to a range spanning slightly more than one order of magnitude.

Colorado↗

USGS2006

Overview of the Opportunity Mars Exploration Rover Mission to Meridiani Planum: Eagle Crater to Purgatory Ripple

The Mars Exploration Rover Opportunity touched down at Meridiani Planum in January 2004 and since then has been conducting observations with the Athena science payload. The rover has traversed more than 5 km, carrying out the first outcrop‐scale investigation of sedimentary rocks on Mars. The rocks of Meridiani Planum are sandstones formed by eolian and aqueous reworking of sand grains that are composed of mixed fine‐grained siliciclastics and sulfates. The siliciclastic fraction was produced by chemical alteration of a precursor basalt. The sulfates are dominantly Mg‐sulfates and also include Ca‐sulfates and jarosite. The stratigraphic section observed to date is dominated by eolian bedforms, with subaqueous current ripples exposed near the top of the section. After deposition, interaction with groundwater produced a range of diagenetic features, notably the hematite‐rich concretions known as “blueberries.” The bedrock at Meridiani is highly friable and has undergone substantial erosion by wind‐transported basaltic sand. This sand, along with concretions and concretion fragments eroded from the rock, makes up a soil cover that thinly and discontinuously buries the bedrock. The soil surface exhibits both ancient and active wind ripples that record past and present wind directions. Loose rocks on the soil surface are rare and include both impact ejecta and meteorites. While Opportunity's results show that liquid water was once present at Meridiani Planum below and occasionally at the surface, the environmental conditions recorded were dominantly arid, acidic, and oxidizing and would have posed some significant challenges to the origin of life.

Journal of Geophysical Research E: Planets↗

USGS1991

Sonobuoy seismic studies at ODP drill sites in Prydz Bay, Antarctica

Five sonobuoy seismic-refraction records were collected along the Leg 119 geophysical transect across the Prydz Bay shelf. Velocity-depth profiles are computed from the sonobuoy data and are used to produce a depth section for the principal acoustic unit boundaries observed in the seismic-reflection data along the transect. Traveltime curves generated by ray-tracing for models constructed from downhole velocity logs are compared to curves generated for models based solely on the sonobuoy data. This comparison reveals that sonobuoy data are less reliable for analysis of lithostratigraphy in vertically and laterally complex areas; however, the sonobuoy data can be used to accurately estimate the depth to a specific horizon in the reflection data to within 10 m. Near-surface velocities exceed 2.0 km/s at all sites, indicating likely overconsolidation of sediments due to glacial loading and erosion during periods of grounded ice sheets on the shelf. Sedimentary rock velocities exclusively were observed beneath the shelf to depths of 3 km. At the landward site a deep refraction is observed with a velocity and vertical gradient indicative of basement rock.

Conference Paper↗

USGS2004

Assembly of the Pamirs: Age and origin of magmatic belts from the southern Tien Shan to the southern Pamirs and their relation to Tibet

Magmatic rocks and depositional setting of associated volcaniclastic strata along a north-south traverse spanning the southern Tien Shan and eastern Pamirs of Kyrgyzstan and Tajikistan constrain the tectonics of the Pamirs and Tibet. The northern Pamirs and northwestern Tibet contain the north facing Kunlun suture, the south facing Jinsha suture, and the intervening Carboniferous to Triassic Karakul-Mazar subduction accretion system; the latter is correlated with the Songpan-Garze-Hoh Xi system of Tibet. The Kunlun arc is a composite early Paleozoic to late Paleozoic-Triassic arc. Arc formation in the Pamirs is characterized by ???370-320 Ma volcanism that probably continued until the Triassic. The cryptic Tanymas suture of the southern northern Pamirs is part of the Jinsha suture. A massive ??????227 Ma batholith stitches the Karakul-Mazar complex in the Pamirs. There are striking similarities between the Qiangtang block in the Pamirs and Tibet. Like Tibet, the regional structure of the Pamirs is an anticlinorium that includes the Muskol and Sares domes. Like Tibet, the metamorphic rocks in these domes are equivalents to the Karakul-Mazar-Songpan-Garze system. Granitoids intruding the Qiangtang block yield ???200-230 Ma ages in the Pamirs and in central Tibet. The stratigraphy of the eastern Pshart area in the Pamirs is similar to the Bangong-Nujiang suture zone in the Amdo region of eastern central Tibet, but a Triassic ocean basin sequence is preserved in the Pamirs. Arc-type granitoids that intruded into the eastern Pshart oceanic-basin-arc sequence (???190-160 Ma) and granitoids that cut the southern Qiangtang block (???170-160 Ma) constitute the Rushan-Pshart arc. Cretaceous plutons that intruded the central and southern Pamirs record a long-lasting magmatic history. Their zircons and those from late Miocene xenoliths show that the most distinct magmatic events were Cambro-Ordovician (???410-575 Ma), Triassic (???210-250 Ma; likely due to subduction along the Jinsha suture), Middle Jurassic (???147-195 Ma; subduction along Rushan-Pshart suture), and mainly Cretaceous. Middle and Late Cretaceous magmatism may reflect arc activity in Asia prior to the accretion of the Karakoram block and flat-slab subduction along the Shyok suture north of the Kohistan-Ladakh arc, respectively. Before India and Asia collided, the Pamir region from the Indus-Yarlung to the Jinsha suture was an Andean-style plate margin. Our analysis suggests a relatively simple crustal structure for the Pamirs and Tibet. From the Kunlun arc in the north to the southern Qiangtang block in the south the Pamirs and Tibet likely have a dominantly sedimentary crust, characterized by Karakul-Mazar-Songpan-Garze accretionary wedge rocks. The crust south of the southern Qiangtang block is likely of granodioritic composition, reflecting long-lived subduction, arc formation, and Cretaceous-Cenozoic underthrusting. Copyright 2004 by the American Geophysical Union.

Tectonics↗

USGS2015

Regional seismic-wave propagation from the M5.8 23 August 2011, Mineral, Virginia, earthquake

The M5.8 23 August 2011 Mineral, Virginia, earthquake was felt over nearly the entire eastern United States and was recorded by a wide array of seismic broadband instruments. The earthquake occurred ~200 km southeast of the boundary between two distinct geologic belts, the Piedmont and Blue Ridge terranes to the southeast and the Valley and Ridge Province to the northwest. At a dominant period of 3 s, coherent postcritical P-wave (i.e., direct longitudinal waves trapped in the crustal waveguide) arrivals persist to a much greater distance for propagation paths toward the northwest quadrant than toward other directions; this is probably related to the relatively high crustal thickness beneath and west of the Appalachian Mountains. The seismic surface-wave arrivals comprise two distinct classes: those with weakly dispersed Rayleigh waves and those with strongly dispersed Rayleigh waves. We attribute the character of Rayleigh wave arrivals in the first class to wave propagation through a predominantly crystalline crust (Blue Ridge Mountains and Piedmont terranes) with a relatively thin veneer of sedimentary rock, whereas the temporal extent of the Rayleigh wave arrivals in the second class are well explained as the effect of the thick sedimentary cover of the Valley and Ridge Province and adjacent Appalachian Plateau province to its northwest. Broadband surface-wave ground velocity is amplified along both north-northwest and northeast azimuths from the Mineral, Virginia, source. The former may arise from lateral focusing effects arising from locally thick sedimentary cover in the Appalachian Basin, and the latter may result from directivity effects due to a northeast rupture propagation along the finite fault plane.

GSA Special Papers↗

USGS2003

Tidal-bundle sequences in the Jordan Sandstone (Upper Cambrian), southeastern Minnesota, U.S.A.: Evidence for tides along inboard shorelines of the Sauk Epicontinental Sea

This study documents for the first time tidal bundling in a lower Paleozoic sheet sandstone from the cratonic interior of North America, providing insights into the hydrodynamics of ancient epicontinental seas. The Jordan Sandstone (Upper Cambrian) in the Upper Mississippi Valley contains large-scale planar tabular cross-sets with tidal-bundle sequences, which were analyzed in detail at an exceptional exposure. Tidal-bundle sequences (neap-spring-neap cycles) were delineated by foreset thickening-thinning patterns and composite shale drapes, the latter of which represent accumulations of mud during the neap tides of neap-spring-neap tidal cycles. Fourier analysis of the bundle thickness data from the 26 measurable bundle sequences revealed cycles ranging from 15 to 34 bundles per sequence, which suggests a semidiurnal or mixed tidal system along this part of the Late Cambrian shoreline. We extend the tidal interpretation to widespread occurrences of the same facies in outcrops of lesser quality, where the facies is recognizable but too few bundles are exposed for tidal cycles to be measured. By doing so, this study shows that tidally generated deposits have a significant geographic and temporal extent in Upper Cambrian strata of central mid-continent North America. The deposition and preservation of tidal facies was related to the intermittent development of shoreline embayments during transgressions. The tidally dominated deposits filled ravined topographies that were repeatedly developed on the updip parts of the shoreface. Resulting coastal geomorphologies, accompanied perhaps by larger-scale changes in basinal conditions and/or configuration, led to changes in depositional conditions from wave-dominated to tide-dominated. Outcrops of the Jordan Sandstone tidal facies in the Upper Mississippi Valley represent the farthest inboard recorded transmission of ocean-generated tides in the Laurentian epicontinental seas, demonstrating that tidal currents were significant agents in the transport of sand along the far cratonic interior shorelines of Cambrian North America. The results of this study improve the facies-level understanding of the genesis of sheet sandstones. Furthermore, tidalites documented here occur in a specific position within a sequence stratigraphic architecture for the Jordan Sandstone. This provides a framework to compare these ancient deposits and processes to younger (e.g., Carboniferous) epicontinental systems where stratal and sediment dynamics are better documented.

Illinois, Indiana, Iowa, Michigan, Minnesota, Miss↗

USGS1999

Climate and lake-level history of the northern altiplano, Bolivia, as recorded in holocene sediments of the Rio Desaguadero

Strata exposed in terraces and modern cutbanks along the Rio Desaguadero contain a variety of lithofacies that were deposited in four distinct facies associations. These facies associations document a history of aggradation and downcutting that is linked to Holocene climate change on the Altiplano. Braided-stream, meandering-stream, deltaic and shoreline, and lacustrine sediments preserved in multi-level terraces in the northern Rio Desaguadero Valley record two high-water intervals; one between 4500 and 3900 yr BP and another between 2000 and 2200 yr BP. These wet periods were interrupted by three periods of fluvial downcutting, centered at approximately 4000 yr BP, 3600 yr BP, and after 2000 yr BP. Braided-river sediments preserved in a single terrace level in the southern Rio Desaguadero Valley record a history of nearly continuous fluvial sedimentation from at least 7000 yr BP until approximately 3200 yr BP that was followed by a single episode (post-3210 yr BP) of down-cutting and lateral migration. The deposition and subsequent fluvial downcutting of the northern strata was controlled by changes in effective moisture that can be correlated to Holocene water-level fluctuations of Lake Titicaca. The deposition and dissection of braided-stream sediments to the south are more likely controlled by a combination of base-level change and sediment input from the Rio Mauri.

Journal of Sedimentary Research↗