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At least 1,045 records · Page 58Linked to original sources
USGS2002

New Mexico structural zone - An analogue of the Colorado mineral belt

Updated aeromagnetic maps of New Mexico together with current knowledge of the basement geology in the northern part of the state (Sangre de Cristo and Sandia–Manzano Mountains)—where basement rocks were exposed in Precambrian-cored uplifts—indicate that the northeast-trending Proterozoic shear zones that controlled localization of ore deposits in the Colorado mineral belt extend laterally into New Mexico. The shear zones in New Mexico coincide spatially with known epigenetic precious- and base-metal ore deposits; thus, the mineralized belts in the two states share a common inherited basement tectonic setting. Reactivation of the basement structures in Late Cretaceous–Eocene and Mid-Tertiary times provided zones of weakness for emplacement of magmas and conduits for ore-forming solutions. Ore deposits in the Colorado mineral belt are of both Late Cretaceous–Eocene and Mid-Tertiary age; those in New Mexico are predominantly Mid-Tertiary in age, but include Late Cretaceous porphyry-copper deposits in southwestern New Mexico. The mineralized belt in New Mexico, named the New Mexico structural zone, is 250-km wide. The northwest boundary is the Jemez subzone (or the approximately equivalent Globe belt), and the southeastern boundary was approximately marked by the Santa Rita belt. Three groups (subzones) of mineral deposits characterize the structural zone: (1) Mid-Tertiary porphyry molybdenite and alkaline-precious-metal deposits, in the northeast segment of the Jemez zone; (2) Mid-Tertiary epithermal precious-metal deposits in the Tijeras (intermediate) zone; and (3) Late Cretaceous porphyry-copper deposits in the Santa Rita zone. The structural zone was inferred to extend from New Mexico into adjacent Arizona. The structural zone provides favorable sites for exploration, particularly those parts of the Jemez subzone covered by Neogene volcanic and sedimentary rocks.

Arizona, California, Colorado, Idaho, Montana, New↗
USGS1994

Geologic map of the Krumbo Reservoir Quadrangle, Harney County, Southeastern Oregon

The Krumbo Reservoir 7.5-minute quadrangle encompasses parts of the Blitzen Valley and northwestern flank of Steens Mountain in the Basin and Range physiographic province of southeastern Oregon (fig. 1). The entire map area is underlain by a bimodal assemblage of middle and upper Miocene olivine basalt flows and rhyolite ash-flow tuffs (figs. 2 and 3). This assemblage is characteristic of volcanic rocks in the northern Basin and Range province and is thought to result from Cascade Range back-arc volcanic activity (Hart and Carlson, 1987). Locally derived Quaternary alluvium blankets part of the area. Faulting associated with Basin and Range extension has created the north-trending Blitzen Valley. In contrast, west-northwest-striking faults are parallel to and probably related to the Brothers fault zone, a northwest-trending zone of closely spaced, discontinuous fractures (Walker, 1969c; Lawrence, 1976).

Oregon↗
USGS2011

Estimate of tephra accumulation probabilities for the U.S. Department of Energy's Hanford Site, Washington

In response to a request from the U.S. Department of Energy, we estimate the thickness of tephra accumulation that has an annual probability of 1 in 10,000 of being equaled or exceeded at the Hanford Site in south-central Washington State, where a project to build the Tank Waste Treatment and Immobilization Plant is underway. We follow the methodology of a 1987 probabilistic assessment of tephra accumulation in the Pacific Northwest. For a given thickness of tephra, we calculate the product of three probabilities: (1) the annual probability of an eruption producing 0.1 km 3 (bulk volume) or more of tephra, (2) the probability that the wind will be blowing toward the Hanford Site, and (3) the probability that tephra accumulations will equal or exceed the given thickness at a given distance. Mount St. Helens, which lies about 200 km upwind from the Hanford Site, has been the most prolific source of tephra fallout among Cascade volcanoes in the recent geologic past and its annual eruption probability based on this record (0.008) dominates assessment of future tephra falls at the site. The probability that the prevailing wind blows toward Hanford from Mount St. Helens is 0.180. We estimate exceedance probabilities of various thicknesses of tephra fallout from an analysis of 14 eruptions of the size expectable from Mount St. Helens and for which we have measurements of tephra fallout at 200 km. The result is that the estimated thickness of tephra accumulation that has an annual probability of 1 in 10,000 of being equaled or exceeded is about 10 centimeters. It is likely that this thickness is a maximum estimate because we used conservative estimates of eruption and wind probabilities and because the 14 deposits we used probably provide an over-estimate. The use of deposits in this analysis that were mostly compacted by the time they were studied and measured implies that the bulk density of the tephra fallout we consider here is in the range of 1,000-1,250 kg/m 3 . The load of 10 cm of such tephra fallout on a flat surface would therefore be in the range of 100-125 kg/m 2 ; addition of water from rainfall or snowmelt would provide additional load.

Open-File Report↗
USGS2001

Water Resources Data, Florida, Water Year 2000, Volume 4. Northwest Florida

This report series for the 2000 water year for the state of Florida consists of records for continuous or daily discharge for 355 streams, periodic discharge for 17 streams, continuous or daily stage for 211 streams, periodic stage for 1 stream, peak stage and discharge for 37 streams; continuous or daily elevations for 16 lakes, and periodic elevations for 45 lakes; continuous ground-water levels for 393 wells, and periodic ground-water levels for 1,003 wells; quality-of-water for 124 surface-water sites and 244 wells. This volume (Volume 4, Northwest Florida) contains records of continuous or daily discharge for 54 streams, periodic discharge for 1 stream, continuous or daily stage for 15 streams, periodic stage for 1 stream, peak stage and discharge for 30 streams; continuous or daily elevations for 1 lake, periodic elevations for 1 lake; continuous ground-water levels for 2 wells, periodic ground-water levels for 0 wells; quality-of-water for 4 surface-water sites and 0 wells. These data represent the National Water Data System records collected by the U.S. Geological Survey and cooperating local, State, and Federal agencies in Florida.

Water Data Report↗
USGS1981

Water resources data, Florida water year 1981, volume 4, northwest Florida

Water resources data for the 1981 water year in Florida consists of continuous or daily discharge for 279 streams, periodic discharge for 52 streams, continuous or daily stage for 64 streams, periodic stage for 82 streams, peak discharge for 78 streams and peak stage for 55 streams; continuous or daily elevations for 77 lakes, periodic elevations for 144 lakes; continuous ground-water levels for 488 wells, periodic ground-water levels for 575 wells, and miscellaneous water-level measurements for 2,317 wells; quality of water data for 444 surface-water sites and 613 wells. The data for northwest Florida includes continuous or daily discharge for 64 streams, periodic discharge for 19 streams, peak discharge for 41 streams, and continuous or daily stage for 8 streams; continuous elevations for 4 lakes and periodic elevations for 26 lakes; continuous ground-water levels for 38 wells, periodic ground-water levels for 154 wells, and miscellaneous water-level measurements for 264 wells; quality of water for 36 surface-water sites and for 113 wells. These data represent the National Water Data System records collected by the U.S. Geological Survey and cooperating local, state and federal agencies in Florida.

Florida↗
USGS1985

Water resources data, Florida, water year 1983. Volume 4. Northwest Florida

Water resources data for the 1983 water year in Florida consist of continuous or daily discharge for 299 streams, periodic discharge for 29 streams, miscellaneous discharge for 30 streams, continuous or daily stage for 76 streams, periodic stage for 47 streams, peak discharge for 86 streams and peak stage for 8 streams; continuous or daily elevations for 105 lakes, periodic elevations for 117 lakes; ground-water levels for 472 wells, periodic ground-water levels for 610 wells, and miscellaneous water-level measurements for 2,258 wells; quality of water data for 326 surface-water sites and 843 wells. The data for northwest Florida includes continuous or daily discharge for 46 streams, periodic discharge for 7 streams, peak discharge for 19 streams, and continuous or daily stage for 4 streams; continuous elevations for 5 lakes and periodic elevations for 4 lakes; continuous ground-water levels for 25 wells, periodic ground-water levels for 87 wells, and miscellaneous water-level measurements for 70 wells; quality of water for 27 surface-water sites and for 22 wells. These data represent the National Water Data System records collected by the U.S. Geological Survey and cooperating local, state and federal agencies in Florida.

Florida↗
USGS1987

Geohydrology of the unsaturated zone at the burial site for low-level radioactive waste near Beatty, Nye County, Nevada

Low-level radioactive solid waste has been buried in trenches at a site near Beatty, Nev., since 1962. In 1976, as part of a national program, the U.S. Geological Survey began a study of the geohydrology of the waste-burial site to provide a basis for estimating the potential for radionuclide migration in the unsaturated zone beneath the waste-burial trenches. Data collected include meteorological information for calibration of a long-term water-budget analysis, soil-moisture profiles, soil-water potentials, and hydraulic properties of representative unsaturated sediment samples to a depth of about 10 meters (m). The waste-burial facility is in the northern Amargosa Desert about 170 kilometers (km) northwest of Las Vegas, Nevo The region is arid; mean annual precipitation at Lathrop Wells, 30 km south of the site, is only 7.4 centimeters (cm). The mean daily maximum temperature at Lathrop Wells in July, the hottest month, is 37 ° C. The site is underlain by poorly stratified deposits of gravelly or silty sand and sandy gravel, and thick beds of clayey sediments. The total thickness of valley-fill deposits beneath the site is about 175 m; the unsaturated zone is about 85 m thick. Volumetric soil moisture to depths of 4 m ranges from 4 to 10 percent but commonly is in the range of 6 to 8 percent. Soil-water potential, measured to depths of 3 to 10 m, ranged from -10 to -70 bars. Unsaturated hydraulic conductivity computed from laboratory analyses of representative samples ranges from 10 -13 to 10 -14 centimeters per day (cm/d). Evaporation studies over a 2-year (yr) period were used to calibrate a numerical procedure for analyzing long-term precipitation data and estimating annual water budgets during the 15-yr period 1962-76. This analysis (1) demonstrated that a potential exists for deep percolation (greater than 2 m), despite high annual evaporation demands, and (2) provided predictions of the time of year and the antecedent conditions that enhance the probability of deep percolation. Soil-moisture profiles obtained monthly over an 18-month (mo) period demonstrate that deep percolation does occur. Soil-moisture conditions antecedent to an observed deep-percolation event, and the time of year when the percolation occurred, support the interpretations based on long-term meteorological records. Calculation of downward moisture movement through the waste-trench backfill material, on the basis of simplified assumptions, suggests that moisture could have penetrated as much as 6 m below land surface from 1963, when the oldest trenches were closed, to 1980, but that the moisture requirement for such penetration far exceeded the amount of moisture actually available. Steady-state downward movement of moisture at depths greater than 10 m and beneath the waste-burial trenches would be on the order of 4 cm per 1,000 yr, assuming a steady flux rate of 1x10 -5 cm/d.

Nevada↗
USGS1948

Submarine geology of Bikini atoll

The program of investigations made in connection with the atomic bomb tests in the summer of 1946 at Bikini Atoll provided a rare opportunity for obtaining information on the physiography and sediments of atolls. The outer slopes around Bikini Atoll rise from a depth of about 2500 fathoms in a long gradually steepening curve. Between 0 and 200 fathoms the average slope is 25°. On the windward side, the reef at the top of the slope is bordered by a narrow 15-fathom terrace, but on the leeward side by a steep, locally vertical, cliff extending to 35 fathoms. Contours of the outer slopes reveal that a flat-topped mass having nearly the area of Bikini lagoon but below 700 fathoms extends to the northwest. The lagoon itself is saucer-shaped and has a maximum depth of 34 fathoms. Around the lagoon and bordering the inner side of the reef and islands is a 12-fathom terrace. Coral knolls rise abruptly from the lagoon floor, and some of them reach to within a few fathoms of the surface. The middle of the lagoon is floored chiefly by algal debris, whereas the shallow edges are covered by algal and foraminiferal sand. Coral comprises a relatively small percentage of the bottom materials. The outer slopes consist of algal debris grading outward to fine sand and finally to Globigerina sand.

GSA Bulletin↗
USGS1998

Land cover mapping of the National Park Service northwest Alaska management area using Landsat multispectral and thematic mapper satellite data

A land cover map of the National Park Service northwest Alaska management area was produced using digitally processed Landsat data. These and other environmental data were incorporated into a geographic information system to provide baseline information about the nature and extent of resources present in this northwest Alaskan environment. This report details the methodology, depicts vegetation profiles of the surrounding landscape, and describes the different vegetation types mapped. Portions of nine Landsat satellite (multispectral scanner and thematic mapper) scenes were used to produce a land cover map of the Cape Krusenstern National Monument and Noatak National Preserve and to update an existing land cover map of Kobuk Valley National Park Valley National Park. A Bayesian multivariate classifier was applied to the multispectral data sets, followed by the application of ancillary data (elevation, slope, aspect, soils, watersheds, and geology) to enhance the spectral separation of classes into more meaningful vegetation types. The resulting land cover map contains six major land cover categories (forest, shrub, herbaceous, sparse/barren, water, other) and 19 subclasses encompassing 7 million hectares. General narratives of the distribution of the subclasses throughout the project area are given along with vegetation profiles showing common relationships between topographic gradients and vegetation communities.

Open-File Report↗
USGS2015

Aeromagnetic survey map of Sacramento Valley, California

Three aeromagnetic surveys were flown to improve understanding of the geology and structure in the Sacramento Valley. The resulting data serve as a basis for geophysical interpretations, and support geological mapping, water and mineral resource investigations, and other topical studies. Local spatial variations in the Earth's magnetic field (evident as anomalies on aeromagnetic maps) reflect the distribution of magnetic minerals, primarily magnetite, in the underlying rocks. In many cases the volume content of magnetic minerals can be related to rock type, and abrupt spatial changes in the amount of magnetic minerals commonly mark lithologic or structural boundaries. Bodies of serpentinite and other mafic and ultramafic rocks tend to produce the most intense positive magnetic anomalies (for example, in the northwest part of the map). These rock types are the inferred sources, concealed beneath weakly magnetic, valley-fill deposits, of the most prominent magnetic features in the map area, the magnetic highs that extend along the valley axis. Cenozoic volcanic rocks are also an important source of magnetic anomalies and coincide with short-wavelength anomalies that can be either positive (strong central positive anomaly flanked by lower-amplitude negative anomalies) or negative (strong central negative anomaly flanked by lower-amplitude positive anomalies), reflecting the contribution of remanent magnetization. Rocks with more felsic compositions or even some sedimentary units also can cause measurable magnetic anomalies. For example, the long, linear, narrow north-trending anomalies (with amplitudes of <50 nanoteslas [nT]) along the western margin of the valley coincide with exposures of the Mesozoic Great Valley sequence. Note that isolated, short-wavelength anomalies, such as those in the city of Sacramento and along some of the major roads, are caused by manmade features.

California↗
USGS1972

Reconnaissance geology of the Precambrian rocks in the Wadi Mahraghah quadrangle, Kingdom of Saudi Arabia

The Wadi Mahraghah quadrangle covers an area of 2863 sq km at the eastern edge of the Precambrian Shield in Saudi Arabia. Precambrian rocks are exposed in the western quarter of the quadrangle. To the east the Precambrian rocks are overlain by limestone of Permian age and by Quaternary sand and gravel. The Precambrian rocks consist of granite intruded by a swarm of andesite dikes. These igneous rocks are overlain by sedimentary formations of the Precambrian Murdama Group comprising conglomerate of the Z'reiba Formation which grades upward into graywacke of the Abt Formation. Rhyolite dikes intrude the Abt. Possibly owing to these rhyolite dikes, or possibly as a result of weak mineralization along west-northwest- and east-trending faults related to the Najd fault zone, the area underlain by graywacke and conglomerate is characterized by threshold or anomalous amounts of beryllium in samples of wadi sand. Samples of sand from granitic areas contain threshold amounts of barium and lanthanum, but none is anomalous, no ancient mine nor other megascoplc evidence of mineralization was found; however,

Open-File Report↗
USGS1975

Tectonic framework of petroliferous rocks in Alaska

Alaska, comprising 3.6 X 10 6 sq km (about 28 percent) of the land, shelf, and upper continental slope of the United States, has been estimated by the U.S. Geological Survey (1974) to contain about 25 percent of the Nation's petroleum resources. Some 11 billion barrels of petroleum liquids and 31 trillion cubic feet of natural gas have been announced as discovered to date. In northern Alaska, Paleozoic and Mesozoic shelf and slope deposits of the Brooks Range orogen were thrust relatively northward over the depressed south margin of the Paleozoic and Mesozoic Arctic platform, upon which a foredeep (the Colville geosyncline) developed in earliest Cretaceous time. Detritus from the Brooks Range filled the foredeep and pro-graded northwest and northeast to fill the Cretaceous and Tertiary North Chukchi and Umiat-Camden basins and form the Beaufort shelf. In southern Alaska, a series of arc-trench systems developed on oceanic rocks during the Jurassic and Cretaceous. Between the arcs and the metamorphic (continental) terranes of east-central and northern Alaska, large back-arc and arc-trench gap basins received thick volcanic and detrital deposits. These deposits were extensively deformed and disrupted by mid-Jurassic to Tertiary plutonism, Laramide oroclinal bending, wrench faulting, and arc-related compression. The Laramide events 'continentalized' the late Mesozoic back-arc basin deposits and welded them to the older continental terranes to the north and east. Subsequent sedimentation was localized and nonmarine except in onshore and offshore coastal basins, where thick mixed marine and nonmarine sections were deposited. The Aleutian arc and associated Queen Charlotte transform fault system have dominated structural and depositional patterns in southern Alaska since the early Cenozoic. The largest petroleum reserves in Alaska (the Prudhoe Bay and associated fields) and the best prospects for additional large discoveries are in northern Alaska, where an extensive terrane is underlain by Upper Paleozoic to Tertiary carbonate and shelf, slope and delta clastic deposits. The pre-Tertiary back-arc and arc-trench gap basins in southern and interior Alaska are too intensely deformed or too low in porosity (because of diagenetic mobilization of labile constituents) to offer more than modest local prospects. The Tertiary coastal basins do, however, offer large tracts of thick marine and nonmarine clastic rocks and in some areas many large folds to exploration. Such basins are known to be petroliferous on Bristol Bay and the Gulf of Alaska and to contain major accumulations of oil and gas at Cook Inlet, but they are relatively little explored.

Alaska↗
USGS2002

Water Resources Data, Florida, Water Year 2001, Volume 4. Northwest Florida

This report series for the 2001 water year for the state of Florida consists of records for continuous or daily discharge for 387 streams, periodic discharge for 14 streams, continuous or daily stage for 147 streams, periodic stage for 2 streams, peak stage and discharge for 8 streams, continuous or daily elevations for 14 lakes, periodic elevations for 47 lakes, continuous ground-water levels for 429 wells, periodic ground-water levels for 1,257 wells, and quality-of-water for 112 surface-water sites and 248 wells. This volume (Volume 4, Northwest Florida) contains records of continuous or daily discharge for 65 streams, periodic discharge for 1 stream, continuous or daily stage for 18 streams, periodic stage for 2 streams, peak stage and discharge for 0 streams, continuous or daily elevations for 1 lake, periodic elevations for 1 lake, continuous ground-water levels for 2 wells, periodic ground-water levels for 3 wells, and quality-of-water for 3 surface-water sites and 0 wells. These data represent the National Water Data System records collected by the U.S. Geological Survey and cooperating local, State, and Federal agencies in Florida.

Water Data Report↗
USGS2004

Water resources data, Florida, water year 2004, volume 4: northeast Florida

This report series for the 2004 water year for the state of Florida consists of records for continuous or daily discharge for 405 streams, periodic discharge for 12 streams, continuous or daily stage for 159 streams, periodic stage for 19 streams, peak stage and discharge for 30 streams, continuous or daily elevations for 14 lakes, periodic elevations for 23 lakes, continuous ground-water levels for 408 wells, periodic ground-water levels for 1,188 wells, and quality-of-water for 140 surface-water sites and 240 wells. This volume (Volume 4, Northwest Florida) contains records of continuous or daily discharge for 75 streams, periodic discharge for 2 streams, continuous or daily stage for 11 streams, periodic stage for 2 streams, peak stage and discharge for 22 streams, continuous or daily elevations for 1 lake, periodic elevations for 0 lakes, continuous ground-water levels for 3 wells, periodic ground-water levels for 0 wells, and quality-of-water for 3 surface-water sites and 0 wells. These data represent the National Water Data System records collected by the U.S. Geological Survey and cooperating local, State, and Federal agencies in Florida.

Water Data Report↗
USGS2005

Water resources data, Florida, water year 2005. Volume 4: Northwest Florida

This report series for the 2005 water year for the state of Florida consists of records for continuous or daily discharge for 429 streams, periodic discharge for 9 streams, continuous or daily stage for 218 streams, periodic stage for 5 streams, peak stage and discharge for 28 streams, continuous or daily elevations for 15 lakes, periodic elevations for 23 lakes, continuous ground-water levels for 401 wells, periodic ground-water levels for 1,098 wells, and quality-of-water for 211 surface-water sites and 208 wells. This volume (Volume 4, Northwest Florida) contains records of continuous or daily discharge for 76 streams, periodic discharge for 2 streams, continuous or daily stage for 14 streams, periodic stage for 0 streams, peak stage and discharge for 20 streams, continuous or daily elevations for 1 lake, periodic elevations for 0 lakes, continuous ground-water levels for 2 wells, periodic ground-water levels for 0 wells, and quality-of-water for 3 surface-water sites and 0 wells. These data represent the National Water Data System records collected by the U.S. Geological Survey and cooperating local, State, and Federal agencies in Florida.

Water Data Report↗
USGS1988

Water resources data for Florida, water year 1987. Volume 4. Northwest Florida

Water resources data for the 1987 water year in Florida consists of continuous or daily discharge for 320 streams, periodic discharge for 38 streams, miscellaneous discharge for 29 streams, continuous or daily stage for 94 streams, periodic stage for 48 streams, peak discharge for 76 streams and peak stage for 69 streams; continuous or daily elevations for 80 lakes, periodic elevations for 74 lakes; ground-water levels for 477 wells, periodic ground-water levels for 562 wells, and miscellaneous water-level measurements for 2886 wells; quality-of-water data for 223 surface-water sites and 900 wells. The data for northwest Florida include continuous or daily discharge for 49 streams, periodic discharge for 4 streams, miscellaneous discharge for 6 streams, continuous or daily stage for 9 streams, peak discharge for 30 streams, and peak stage for 30 streams; continuous elevations for 5 lakes and periodic elevations for 4 lakes; continuous ground-water levels for 16 wells, and periodic ground-water levels for 70 wells; quality of water for 22 surface-water sites and 20 wells. These data represent the National Water Data System records collected by the U.S. Geological Survey and cooperating local, state and federal agencies in Florida.

Florida↗
USGS2000

Water Resources Data for Florida, 1999, Volume 4: Northwest Florida

This report series for the 1999 water year for the State of Florida consists of records for continuous or daily discharge for 354 streams, periodic discharge for 17 streams, continuous or daily stage for 121 streams, periodic stage for 1 stream, peak stage and discharge for 38 streams; continuous or daily elevations for 21 lakes, and periodic elevations for 42 lakes; continuous ground-water levels for 408 wells, and periodic ground-water levels for 1,715 wells; quality-of-water for 131 surface-water sites and 198 wells. This volume (Volume 4, Northwest Florida) contains records of continuous or daily discharge for 54 streams, periodic discharge for 1 stream, continuous or daily stage for 15 streams, periodic stage for 1 stream, peak stage and discharge for 30 streams; continuous or daily elevations for 1 lake, periodic elevations for 1 lake; continuous ground-water levels for 1 well, periodic ground-water levels for 0 wells; and quality-of-water for 2 surface-water sites and 0 wells. These data represent the National Water Data System records collected by the U.S. Geological Survey and cooperating local, State, and Federal agencies in Florida.

Water Data Report↗
USGS2009

The Eocene-Oligocene sedimentary record in the Chesapeake Bay impact structure: Implications for climate and sea-level changes on the western Atlantic margin

A multidisciplinary investigation of the Eocene-Oligocene transition in the International Continental Scientific Drilling Program (ICDP)-U.S. Geological Survey (USGS) Eyreville core from the Chesapeake Bay impact basin was conducted in order to document environmental changes and sequence stratigraphic setting. Planktonic foraminifera and calcareous nannofossil biostratigraphy indicate that the Eyreville core includes an expanded upper Eocene (Biozones E15 to E16 and NP19/20 to NP21, respectively) and a condensed Oligocene-Miocene (NP24-NN1) sedimentary sequence. The Eocene-Oligocene contact corresponds to a =3-Ma-long hiatus. Eocene- Oligocene sedimentation is dominated by great diversity and varying amounts of detrital and authigenic minerals. Four sedimentary intervals are identified by lithology and mineral content: (1) A 30-m-thick, smectite- and illite-rich interval directly overlies the Exmore Formation, suggesting long-term reworking of impact debris within the Chesapeake Bay impact structure. (2) Subsequently, an increase in kaolinite content suggests erosion from soils developed during late Eocene warm and humid climate in agreement with data derived from other Atlantic sites. However, the kaolinite increase may also be explained by change to a predominant sediment input from outside the Chesapeake Bay impact structure caused by progradation of more proximal facies belts during the highstand systems tract of the late Eocene sequence E10.Spectral analysis based on gamma-ray and magnetic susceptibility logs suggests infl uence of 1.2 Ma low-amplitude oscillation of the obliquity period during the late Eocene. (3) During the latest Eocene (Biozones NP21 and E16), several lithological contacts (clay to clayey silt) occur concomitant with a prominent change in the mineralogical composition with illite as a major component: This lithological change starts close to the Biozone NP19/20-NP21 boundary and may correspond to sequence boundary E10-E11 as observed in other northwest Atlantic margin sections. It could result from a shift to more distal depositional environments and condensed sedimentation during maximum fl ooding, rather than refl ecting a climatic change in the hinterland. The distinct 1% increase of the oxygen isotopes may correspond to the short-term latest Eocene "precursor isotope event." (4) The abrupt increase of sediment grainsize, carbonate content, and abundance of authigenic minerals (glauconite) across the major unconformity that separates Eocene from Oligocene sediments in the Eyreville core refl ects deposition in shallower settings associated with erosion, winnowing, and reworking. Sediments within the central crater were affected by the rapid eustatic sea-level changes associated with the greenhouse-icehouse transition, as well as by an abrupt major uplift event and possibly enhanced current activity on the northwestern Atlantic margin. ?? 2009 The Geological Society of America.

Chesapeake Bay↗