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C. W. Poag

Publications and source records attributed to C. W. Poag.

At least 19 recordsLinked to original sources
USGS2012

Structural outer rim of Chesapeake Bay impact crater: Seismic and bore hole evidence

Nine seismic-reflection profiles and four continuous core holes define the gross structural and stratigraphic framework of the outer rim of the Chesapeake Bay impact crater. The rim is manifested as a 90 km diameter ring of terraced normal-fault blocks, which forms a ∼320 m–1200 m high rim escarpment. The top of the rim escarpment is covered by a 20 m–30 m thick ejecta blanket. The escarpment encircles a flat-floored annular trough, which is partly filled with an ∼250 m thick breccia lens (Exmore breccia). The Exmore breccia overlies a 200 m–800 m thick interval of slumped sedimentary megablocks, which, in turn, rests on crystalline basement rocks. All postimpact strata (upper Eocene to Quaternary) sag structurally into the annular trough, and most units also thicken as they cross the rim into the crater. Postimpact compaction and subsidence of the Exmore breccia have created extensive normal faulting in overlying strata.

Chesapeake Bay↗
USGS2009

Paleoenvironmental recovery from the Chesapeake Bay bolide impact: The benthic foraminiferal record

The late Eocene Chesapeake Bay bolide impact transformed its offshore target site from an outer neritic, mid-shelf seafloor into a bathyal crater basin. To obtain a depositional record from one of the deepest parts of this basin, the U.S. Geological Survey (USGS) and the International Continental Scientific Drilling Program (ICDP) drilled a 1.76-km-deep core hole near Eyreville, Virginia. The Eyreville core and eight previously cored boreholes contain a rarely obtainable record of marine deposition and microfossil assemblages that characterize the transition from synimpact to postimpact paleoenvironments inside and near a submarine impact crater. I used depositional style and benthic foraminiferal assemblages to recognize a four-step transitional succession, with emphasis on the Eyreville core. Step 1 is represented by small-scale, silt-rich turbidites, devoid of indigenous microfossils, which lie directly above the crater-filling Exmore breccia. Step 2 is represented by very thin, parallel, silt and clay laminae, which accumulated on a relatively tranquil and stagnant seafloor. This stagnation created a dead zone, which excluded seafloor biota, and it lasted ~3-5 ka. Step 3 is an interval of marine clay deposition, accompanied by a burst of microfaunal activity, as a species-rich pioneer community of benthic foraminifera repopulated the impact site. The presence of a diagnostic suite of agglutinated foraminifera during step 3 indicates that paleoenvironmental stress related to the impact lasted from ~9 ka to 400 ka at different locations inside the crater. During step 4, the agglutinated assemblage disappeared, and an equilibrium foraminiferal community developed that contained nearly 100% calcareous species. In contrast to intracrater localities, core sites outside and near the crater rim show neither evidence of the agglutinated assemblage, nor other indications of long-term biotic disruption from the bolide impact.

Chesapeake Bay↗
USGS2005

Eastern rim of the Chesapeake Bay impact crater: Morphology, stratigraphy, and structure

This study reexamines seven reprocessed (increased vertical exaggeration) seismic reflection profiles that cross the eastern rim of the Chesapeake Bay impact crater. The eastern rim is expressed as an arcuate ridge that borders the crater in a fashion typical of the "raised" rim documented in many well preserved complex impact craters. The inner boundary of the eastern rim (rim wall) is formed by a series of raterfacing, steep scarps, 15-60 m high. In combination, these rim-wall scarps represent the footwalls of a system of crater-encircling normal faults, which are downthrown toward the crater. Outboard of the rim wall are several additional normal-fault blocks, whose bounding faults trend approximately parallel to the rim wall. The tops of the outboard fault blocks form two distinct, parallel, flat or gently sloping, terraces. The innermost terrace (Terrace 1) can be identified on each profile, but Terrace 2 is only sporadically present. The terraced fault blocks are composed mainly of nonmarine, poorly to moderately consolidated, siliciclastic sediments, belonging to the Lower Cretaceous Potomac Formation. Though the ridge-forming geometry of the eastern rim gives the appearance of a raised compressional feature, no compelling evidence of compressive forces is evident in the profiles studied. The structural mode, instead, is that of extension, with the clear dominance of normal faulting as the extensional mechanism.

Chesapeake Bay↗
USGS1998

The Toms Canyon structure, New Jersey outer continental shelf: A possible late Eocene impact crater

The Toms Canyon structure [~20-22 km wide] is located on the New Jersey outer continental shelf beneath 80-100 m of water, and is buried by ~1 km of upper Eocene to Holocene sedimentary strata. The structure displays several characteristics typical of terrestrial impact craters (flat floor; upraised faulted rim: brecciated sedimentary fill), but several other characteristics are atypical (an unusually thin ejecta blanket; lack of an inner basin, peak ring, or central peak; bearing nearly completely filled with breccia). Seismostratigraphic and biostratigraphic analyses show that the structure formed during planktonic foraminiferal biochron P15 of the early to middle late Eocene. The fill unit is stratigraphically correlating with impact ejecta cored nearby at Deep Sea Drilling Project (DSDP) Site 612 and at Ocean Drilling Program (ODP) Sites 903 and 904 (22-35 km southeast of the Toms Canyon structure). The Toms Canyon fill unit also correlates with the Exmore breccia, which fills the much larger Chesapeake Bay impact crater (90-km diameter; 335 km to the southwest). On the basis of our analyses, we postulate that the Toms Canyon structure is an impact crater, formed when a cluster of relatively small meteorites approached the target site bearing ~N 50 E, and struck the sea floor obliquely.

New Jersey↗
USGS1997

Roadblocks on the kill curve: Testing the Raup hypothesis

The documented presence of two large (~100-km diameter), possibly coeval impact craters of late Eocene age, requires modification of the impact-kill curve proposed by David M. Raup. Though the estimated meteorite size for each crater alone is large enough to have produced considerable global environmental stress, no horizons of mass mortality or pulsed extinction are known to be associated with either crater or their ejecta deposits. Thus, either there is no fixed relationship between extinction magnitude and crater diameter, or a meteorite that would produce a crater of >100-km diameter is required to raise extinction rates significantly above a ~5% background level. Both impacts took place ~1-2 m.y. before the "Terminal Eocene Event" ( =early Oligocene pulsed extinction). Their collective long-term environmental effects, however, may have either delayed that extinction pulse or produced threshold conditions necessary for it to take place.

Palaios↗
USGS1997

Geophysical database of the east coast of the United States; southern Atlantic margin, stratigraphy and velocity in map grids

In 1990, the Naval Oceanographic Office and the U.S. Geological Survey agreed to develop a digital data base of stratigraphy and acoustic properties of sediments along the U.S. East Coast of the United States. The objective of this work was to utilize more than 25,000 km of publicly available multichannel seismic-reflection profiles (Sheridan et al., 1988) in order to assign acoustic properties to the continental margin postrift sediments in an internally consistent, geologically meaningful, regionally extensive, digital form. The acoustic properties of interest include thickness, depth, compressional- and shear-wave velocity, compressional- and shear-wave attenuation, density, and lithology. This data base subdivides the 0- to 14-km thick Jurassic and younger postrift deposits into 18 mappable horizons. The spatial scale of gridding is 5' latitude by 5 ' longitude, or about 9x8 km. This report describes the second part of developing the data base for the continental margin between Florida and Cape Hatteras: spatial gridding of the digital stratigraphic and velocity data, derivative calculations of density, shear-wave velocity, and attenuation, and construction of the final data base. The first report (Hutchinson et al., 1995) describes how the stratigraphy and velocity were digitized from the original profiles. Complementary reports that describe the data base for the area between Cape Hatteras and Georges Bank are given in Klitgord and Schneider (1994) and Klitgord et al. (1994).

Open-File Report↗
USGS1996

Impact origin of the Chesapeake Bay structure and the source of the North American tektites

Seismic profiles, drill core samples, and gravity data suggest that a complex impact crater ~35.5 million years old and 90 kilometers in diameter is buried beneath the lower Chesapeake Bay. The breccia that fills the structure contains evidence of shock metamorphism, including impact melt breccias and multiple sets of planar deformation features (shock lamellae) in quartz and feldspar. The age of the crater and the composition of some breccia clasts are consistent with the Chesapeake Bay impact structure being the source of the North American tektites.

Maryland, Virginia↗
USGS1995

Stratigraphic and palaeoenvironmental summary of the south-east Georgia embayment: a correlation of exploratory wells

A Continental Offshore Stratigraphic Test (COST) well and six exploratory wells have been drilled in the south-east Georgia embayment. The oldest rocks penetrated are weakly metamorphosed Lower Ordovician quartz arenites and Silurian shales and argillites in the Transco 1005-1 well and Upper Devonian argillites in the COST GE-1 well. These marine strata, which are equivalent to the Tippecanoe sequence in Florida, underlie the post-rift unconformity and represent part of a disjunct fragment of Gondwana that was sutured to the North American craton during the late Palaeozoic Alleghanian orogeny. The Palaeozoic strata are unconformably overlain by interbedded non-marine Jurassic (Bajocian and younger) sandstones and shales and marginal marine Lower Cretaceous sandstones, calcareous shales and carbonates, which contain scattered beds of coal and evaporite. Together, these rocks are stratigraphically equivalent to the onshore Fort Pierce and Cotton Valley(?) Formations and rocks of the Lower Cretaceous Comanchean Provincial Series. The abundance of carbonates and evaporites in this interval, which reflects marine influences within the embayment, increases upwards, eastwards and southwards. The Upper Cretaceous part of the section is composed mainly of neritic calcareous shales and shaley limestones stratigraphically equivalent to the primarily marginal marine facies of the onshore Atkinson, Cape Fear and Middendorf Formations and Black Creek Group, and to limestones and shales of the Lawson Limestone and Peedee Formations. Cenozoic strata are primarily semiconsolidated marine carbonates. Palaeocene to middle Eocene strata are commonly cherty; middle Miocene to Pliocene strata are massive and locally phosphatic and glauconitic; Quaternary sediments are dominated by unconsolidated carbonate sands. The effects of eustatic changes and shifts in the palaeocirculation are recorded in the Upper Cretaceous and Tertiary strata.

Georgia↗
USGS1995

Paleocene to Middle Miocene planktic foraminifera of the southwestern Salisbury Embayment, Virginia and Maryland: biostratigraphy, allostratigraphy, and sequence stratigraphy

The Paleocene to Middle Miocene sedimentary fill of the southwestern Salisbury Embayment contains a fragmental depositional record, interrupted by numerous local diastems and regional unconformities. Using planktic foraminiferal biostratigraphy, 15 unconformity-bounded depositional units have been identified, assigned to six formations and seven alloformations previously recognized in the embayment. The units correlate with second- and third-order sequences of the Exxon sequence stratigraphy model, and include transgressive and highstand systems tracts. Alloformation, formation, and sequence boundaries are marked by abrupt, scoured, burrowed, erosional surfaces, which display lag deposits, biostratigraphic gaps, and intense reworking of microfossils above and below the boundaries. Paleocene deposits represent the upper parts of upper Pleocene Biochronozones P4 and P5, and rest uncomformably on Cretaceous sedimentary beds of various ages (Maastrichtian to Albian). Lower Eocene deposits represent parts of Biochronozones P6 and P9. Middle Eocene strata represent mainly parts of Biochronozones P11, P12, and P14. Upper Eocene sediments include parts of Biochronozones P15, P16, and P17. Oligocene deposits encompass parts of Biochronozones. N4b to N7 undifferentiated, P21a, and, perhaps, N4a. Lower Miocene deposits encompass parts of Biochronozones N4b to N7 undifferentiated. Middle Miocene strata represent mainly parts of Biochronorones N8, N9, and N10. Nine plates of scanning electron micrographs illustrate the principal planktic foraminifera used to establish the biostratigraphic framework. Two new informal formine of Praeterenuitella praegemma Li, 1987, are introduced.

Journal of Foraminiferal Research↗
USGS1995

Stratigraphy and paleoenvironments of the Shell 410-1 well, Georges Bank Basin, US North Atlantic outer continental shelf

The Shell 410-1 well is the most downdip (seaward) hydrocarbon exploratory well in the Georges Bank Basin. It was drilled to a total depth of 4745 m RKB, and penetrated a section composed of Middle Jurassic to Quaternary sedimentary rocks. The lithostratigraphy of the section is described. The strata penetrated by the Shell 410-1 well are more marine than rocks at the updip (landward) COST G-1, Exxon 975-1, COST G-2, and Conoco 145-1 well sites. Limestones and calcareous mudstones dominate at the Shell 410-1 site. Dolomite and anhydrite are much more abundant in the Bajocian-Callovian strata of the Shell 410-1 well, which is evidence that the carbonate-bank palaeoenvironments recorded in the Iroquois and Abenaki Formations were more restricted (less marine) here than 47 km to the west-northwest at the Mobil 312-1 well site near the edge of the Jurassic carbonate platform. -from Authors

Northeastern Geology and Environmental Sciences↗
USGS1994

Meteoroid mayhem in Ole Virginny: Source of the North American tektite strewn field

New seismic reflection data from Chesapeake Bay reveal a buried, 85-km-wide, 1.5-2.0-km-deep, peak-ring impact crater, carved through upper Eocene to Lower Cretaceous sedimentary strata and into underlying pre-Mesozoic crystalline basement rocks. A polymictic, late Eocene impact breccia, composed mainly of locally derived sedimentary debris (determined from four continuous cores), surrounds and partly fills the crater. Structural and sedimentary characteristics of the Chesapeake Bay crater closely resemble those of the Miocene Ries peakring crater in southern Germany. We speculate that the Chesapeake Bay crater is the source of the North American tektite strewn field.

Geology↗
USGS1993

Mesozoic stratigraphy and paleoenvironments of the Georges Bank Basin: A correlation of exploratory and cost wells

The Exxon 975-1, Conoco 145-1, and Mobil 312-1 hydrocarbon exploratory wells and the Continental Offshore Stratigraphic Test (COST) G-1 and G-2 wells were drilled in the southeastern part of the Georges Bank Basin. We used drill cuttings and logs from these wells to describe and correlate the dominant lithostratigraphic units and to document lateral changes in the depositional environments. The strata penetrated by the Exxon 975-1 and COST G-1 wells are much more terrestrial than at the seaward (downdip) COST G-2, Conoco 145-1, and Mobil 312-1 wellsites. Oldest rocks penetrated by the exploratory wells represent a Middle Jurassic carbonate-evaporite sequence that correlates to the Iroquois Formation. The Iroquois records nonmarine to marginal-marine sabkha, tidal-flat, and restricted lagoonal paleoenvironments in the Exxon 975-1 and COST G-1 wells, but progressively changes to inner neritic, littoral, and lagoonal paleoenvironments at the Mobil 312-1 wellsite. The nonmarine deltaic siliciclastics of the overlying Mohican Formation, Misaine Shale, and Mic Mac-Mohawk Formations are thicker and the marine carbonates of the Scatarie and Bacarro Limestones are usually thinner in the Jurassic strata of the Exxon 975-1 and COST G-1 wells than at the other wellsites. Similarly, lower delta-plain and delta-front facies of the Early Cretaceous Missisuaga and Logan Canyon Formations at the Exxon 975-1 and COST G-1 wellsites reflect a greater terrestrial influence than the laterally-equivalent shallow marine to delta-front facies present at the downdip wellsites. ?? 1993.

Georges Bank Basin↗
USGS1993

Mesozoic stratigraphy and paleoenvironments of the Exxon 975-1 well, US North Atlantic shelf

The Exxon 975-1 well, located in the southeastern part of the Georges Bank Basin, was drilled to a total depth of 4452 m relative to the Kelly Bushing. About 6860 m of sedimentary rocks overlie the Paleozoic basement at the Exxon 975-1 wellsite. The Jurassic/Cretaceous boundary in the well is placed at 1673 m; the Cretaceous/Tertiary boundary occurs at 384 m. A carbonate-evaporite sequence dominates the section below 3966 m (Iroquois Formation); limestone is the predominant lithology in the intervals 3810-3246 m and 1897-1654 m. Siliciclastics of the Mohican, undivided Mic Mac-Mohawk, Missisauga, Logan Canyon, and Dawson Canyon Formations dominate the remainder of the Mesozoic section. Lagoonal, tidal flat and sabkha paleoenvironments dominated during deposition of the Iroquois Formation. Lower delta plain and delta front palaeoenvironments prevailed during deposition of the Mohican, Misaine Shale, Mic Mac-Mohawk, Missisauga, and most of the Logan Canyon units. -from Authors

Northeastern Geology↗
USGS1992

Mid-Mesozoic (Mid-Jurassic to Early Cretaceous) evolution of the Georges Bank Basin, U.S. North Atlantic outer continental shelf: Sedimentology of the Conoco 145-1 well

The Conoco 145-1 exploratory well, located in the southeastern portion of the Georges Bank Basin, was drilled to a total depth of 4303 m below the sea floor. The oldest sedimentary rocks sampled are of Middle Jurassic age (Late Bathonian-Callovian). A dolomite-limestone-evaporite sequence dominates the section below 3917 m; limestone is the predominant lithology in the intervals of 3271-3774 m, 2274-3158 m, and 1548-1981 m. Siliciclastics dominate the remainder of the drilled section. Calcite tightly cements most of the rocks below 1548 m; dolomite, silica, siderite, and diagenetic clay cements are locally important. Restricted inner marine environments, representing lagoonal and tidal flat conditions, prevailed at the wellsite during much of the deposition recorded by the Callovian-Bathonian age Iroquois Formation. These environments gave way to a carbonate platform, which formed part of the > 5,000 km long Bahama-Grand Banks gigaplatform that lasted through the end of the Late Jurassic (encompassing the uppermost portion of the Iroquois Formation and the Scatarie Limestone and Bacarro Limestone Members of the Abenaki Formation). The absence of a skeletal-reef association and the dominance of muddy limestone fabrics are evidence that the 145-1 wellsite was located on the platform interior. Major periods of siticiclastic deposition interrupted carbonate deposition, and they are recorded by stratigraphic equivalents of the Mohican Formation, Misaine Shale Member of the Abenaki Formation, and the Mohawk and Mic Mac Formations. A series of sustained prograding delta systems, the earliest of which is preserved as the Missisauga Formation, buried the carbonate platform following its drowning in the Early Cretaceous (Berriasian-Valanginian). The sparser, primarily allochthonous lignite content and better-sorted, glauconite-bearing sands of the Missisauga strata at the 145-1 wellsite suggest that shallow marine or barrier-bar environments were more prevalent than the low delta-plain facies recorded farther shoreward at the COST G-1 wellsite.

Sedimentary Geology↗
USGS1992

Lithology, stratigraphy, and paleoenvironments of the Mobil 312-1 well, Georges Bank Basin, US North Atlantic outer continental shelf

The Mobil 312-1 hydrocarbon exploratory well, southeastern Georges Bank Basin penetrated a section entirely composed of sedimentary rocks that range from Middle to Pliocene age. Carbonates are the dominant lithologies in the intervals at 6096-3444 m, 2560-2096 m and 1067-887 m; siliciclastics make up most of the remaining section. Although inferred paleoenvironments range primarily from supratidal to outer neritic, thin lignitic coal beds at 2204-2195 m and 1929-1920 m record brief periods of nonmarine sedimentation. Middle and Upper Jurassic calcarenites, the drilling targets of the well, have little or no visible porosity and underwent at least three episodes of cementation.

Northeastern Geology↗