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Gregory Gohn

Publications and source records attributed to Gregory Gohn.

At least 19 recordsLinked to original sources
USGS2019

Chesapeake Bay impact structure—Development of "brim" sedimentation in a multilayered marine target

The late Eocene Chesapeake Bay impact structure was formed in a multilayered target of seawater underlain sequentially by a sediment layer and a rock layer in a continental-shelf environment. Impact effects in the “brim” (annular trough) surrounding and adjacent to the transient crater, between the transient crater rim and the outer margin, primarily were limited to the target-sediment layer. Analysis of published and new lithostratigraphic, biostratigraphic, sedimentologic, petrologic, and mineralogic studies of three core holes, and published studies of a fourth core hole, provided information for the interpretation of the impact processes, their interactions and relative timing, their resulting products, and sedimentation in the brim. Most studies of marine impact-crater materials have focused on those found in the central crater. There are relatively few large, complex marine craters, of which most display a wide brim around the central crater. However, most have been studied using minimal data sets. The large number of core holes and seismic profiles available for study of the Chesapeake Bay impact structure presents a special opportunity for research. The physical and chronologic records supplied by study of the sediment and rock cores of the Chesapeake Bay impact indicate that the effects of the initial, short-lived contact and compression and excavation stages of the impact event primarily were limited to the transient crater. Only secondary effects of these processes are evident in the brim. The preserved record of the brim was created primarily in the subsequent modification stage. In the brim, the records of early impact processes (e.g., outgoing tsunamis, overturned flap collapse) were modified or removed by later processes. Transported and rotated, large and small clasts of target sediments, and intervals of fluidized sands indicate that seismic shaking fractured and partially fluidized the Cretaceous and Paleogene target sediments, which led to their inward transport by collapse and lateral spreading toward the transient crater. The succeeding inward seawater-resurge flow quickly overtook and interacted with the lateral spreading, further facilitating sediment transport across the brim and into the transient crater. Variations in the cohesion and relative depth of the target sediments controlled their degree of disaggregation and redistribution during these events. Melt clasts and shocked and unshocked rock clasts in the resurge sediments indicate fallout from the ejecta curtain and plume. Basal parautochthonous remnant sections of target Cretaceous sediments in the brim thin toward the collapsed transient crater. Overlying seawater-resurge deposits consist primarily of diamictons that vary laterally in thickness, and vertically and laterally in maximum grain size. After cessation of resurge flow and re-establishment of pre-impact sea level, sandy sediment gravity flows moved from the margin to the center of the partially filled impact structure (shelf basin). The uppermost unit consists of stratified sediments deposited from suspension. Postimpact clayey silts cap the crater fill and record the return to shelf sedimentation at atypically large paleodepths within the shelf basin. An unresolved question involves a section of gravel and sand that overlies Neoproterozoic granite in the inner part of the brim in one core hole. This section may represent previously unrecognized, now parautochthonous Cretaceous sediments lying nonconformably above basement granite, or it may represent target sediments that were moved significant distances by lateral spreading above basement rocks or above a granite megaclast from the overturned flap. The Chesapeake Bay impact structure is perhaps the best documented example of the small group of multilayer, marine-target impacts formed in continental shelves or beneath epeiric seas.

Chesapeake Bay↗
USGS2011

Drilling the central crater of the Chesapeake Bay Impact Structure: A first look

The late Eocene Chesapeake Bay impact structure is a well‐preserved example of one of Earth's largest impact craters, and its continental‐shelf setting and relatively shallow burial make it an excellent target for study. Since the discovery of the structure over a decade ago [ Edwards et al., 2004; Poag et al., 2004], test drilling by U.S. federal and state agencies has been limited to the structure's annular trough (Figure 1). In May 2004, the U.S. Geological Survey (USGS) drilled the first scientific test hole into the central crater of the Chesapeake Bay impact structure in Cape Charies,Virginia (Figure 1). This partially cored test hole, the deepest to date, penetrated postimpact sediments and impact breccias to a total depth of 823 m.

Chesapeake Bay↗
USGS2010

Inside the crater, outside the crater: Stratigraphic details of the margin of the Chesapeake Bay impact structure, Virginia, USA

Two cores at the outer margin of the Chesapeake Bay impact structure show significant structural and depositional variations that illuminate its history. Detailed stratigraphy of the Watkins School core reveals that this site is outside the disruption boundary of the crater with respect to its lower part (nonmarine Cretaceous Potomac Formation), but just inside the boundary with respect to its upper part (Exmore Formation and a succession of upper Eocene to Pleistocene postimpact deposits). The site of the U.S. Geological Survey-National Aeronautics and Space Administration Langley core, 6.4 km to the east, lies wholly within the annular trough of the crater . The Potomac Formation in the Watkins School core is not noticeably impact disrupted. The lower part of crater unit A in the Langley core represents stratigraphically lower, but similarly undeformed material. The Exmore Formation is only 7.8 m thick in the Watkins School core, but it is over 200 m thick in the Langley core, where it contains blocks up to 24 m in intersected diameter. The upper part of the Exmore Formation in the two cores is a polymict diamicton with a stratified zone at the top. The postimpact sedimentary units in the two cores have similar late Eocene and late Miocene depositional histories and contrasting Oligocene, early Miocene, and middle Miocene histories. A paleochannel of the James River removed Pliocene deposits at the Watkins School site, to be filled later with thick Pleistocene deposits. At the Langley site, a thick Pliocene and thinner Pleistocene record is preserved. © 2010 The Geological Society of America. All rights reserved.

Virginia↗
USGS2009

Geologic columns for the ICDP-USGS Eyreville B core, Chesapeake Bay impact structure: Impactites and crystalline rocks, 1766 to 1096 m depth

The International Continental Scientific Drilling Program (ICDP)-U.S. Geological Survey (USGS) Eyreville drill cores from the Chesapeake Bay impact structure provide one of the most complete geologic sections ever obtained from an impact structure. This paper presents a series of geologic columns and descriptive lithologic information for the lower impactite and crystalline-rock sections in the cores. The lowermost cored section (1766-1551 m depth) is a complex assemblage of mica schists that commonly contain graphite and fibrolitic sillimanite, intrusive granite pegmatites that grade into coarse granite, and local zones of mylonitic deformation. This basement-derived section is variably overprinted by brittle cataclastic fabrics and locally cut by dikes of polymict impact breccia, including several suevite dikes. An overlying succession of suevites and lithic impact breccias (1551-1397 m) includes a lower section dominated by polymict lithic impact breccia with blocks (up to 17 m) and boulders of cataclastic gneiss and an upper section (above 1474 m) of suevites and clast-rich impact melt rocks. The uppermost suevite is overlain by 26 m (1397-1371 m) of gravelly quartz sand that contains an amphibolite block and boulders of cataclasite and suevite. Above the sand, a 275-m-thick allochthonous granite slab (1371-1096 m) includes gneissic biotite granite, fine- and medium-to-coarse-grained biotite granites, and red altered granite near the base. The granite slab is overlain by more gravelly sand, and both are attributed to debris-avalanche and/or rockslide deposition that slightly preceded or accompanied seawater-resurge into the collapsing transient crater.

Chesapeake Bay↗
USGS2009

Geologic columns for the ICDP-USGS Eyreville A and B cores, Chesapeake Bay impact structure: Sediment-clast breccias, 1096 to 444 m depth

The Eyreville A and B cores, recovered from the “moat” of the Chesapeake Bay impact structure, provide a thick section of sediment-clast breccias and minor stratified sediments from 1095.74 to 443.90 m. This paper discusses the components of these breccias, presents a geologic column and descriptive lithologic framework for them, and formalizes the Exmore Formation. From 1095.74 to ~867 m, the cores consist of nonmarine sediment boulders and sand (rare blocks up to 15.3 m intersected diameter). A sharp contact in both cores at ~867 m marks the lowest clayey, silty, glauconitic quartz sand that constitutes the base of the Exmore Formation and its lower diamicton member. Here, material derived from the upper sediment target layers, as well as some impact ejecta, occurs. The block-dominated member of the Exmore Formation, from ~855–618.23 m, consists of nonmarine sediment blocks and boulders (up to 45.5 m) that are juxtaposed complexly. Blocks of oxidized clay are an important component. Above 618.23 m, which is the base of the informal upper diamicton member of the Exmore Formation, the glauconitic matrix is a consistent component in diamicton layers between nonmarine sediment clasts that decrease in size upward in the section. Crystalline-rock clasts are not randomly distributed but rather form local concentrations. The upper part of the Exmore Formation consists of crudely fining-upward sandy packages capped by laminated silt and clay. The overlap interval of Eyreville A and B (940–~760 m) allows recognition of local similarities and differences in the breccias.

Chesapeake Bay↗
USGS2005

Studies of the Chesapeake Bay impact structure - Introduction and discussion

The late Eocene Chesapeake Bay impact structure on the Atlantic margin of Virginia is the largest known impact crater in the United States, and it may be the Earth's best preserved example of a large impact crater that formed on a predominantly siliciclastic continental shelf. The 85-kilometer-wide (53-milewide) crater also coincides with a region of saline ground water. It has a profound influence on ground-water quality and flow in an area of urban growth. The USGS-NASA Langley corehole at Hampton , Va., is the first in a series of new coreholes being drilled in the crater, and it is the first corehole to penetrate the entire crater-fill section and uppermost crystalline basement rock. The Langley corehole is located in the southwestern part of the crater's annular trough. A comprehensive effort to understand the crater's materials, architecture, geologic history , and formative processes, as well as its influence on ground water, includes the drilling of coreholes accompanied by high-resolution seismic-reflection and seismic-refraction surveys, audio-magnetotelluric surveys, and related multidisciplinary research. The studies of the core presented in this volume provide detailed information on the outer part of the crater, including the crystalline basement , the overlying impact -modified and impact -generated sediments (physical geology, paleontology, shocked minerals, and crystalline ejecta), and the upper Eocene to Quaternary postimpact sedimentary section (stratigraphy, paleontology, and paleoenvironments). The USGS-NASA Langley corehole has a total depth below land surface of 635.1 meters (m; 2,083.8 feet (ft)). The deepest unit in the corehole is the Neoproterozoic Langley Granite. The top of this granite at 626.3 m (2,054.7 ft) depth is overlain by 390.6 m (1,281.6 ft) of impact -modified and impact -generated siliciclastic sediments. These crater-fill materials are preserved beneath a 235.6-m-thick (773.12-ft-thick) blanket of postimpact sediments. A high-resolution seismic-reflection and seismic-refraction profile that crosses the Langley drill site is tied to the core by borehole geophysical logs, and it reveals the details of extensional collapse structures in the western annular trough. Electrical cross sections based on audio-magnetotelluric (AMT) soundings image a nearly vertical zone of high resistivity at the outer margin of the annular trough, possibly indicating fresh ground water at that location, and they show impedance trends that match the curvature of the structure . They also image the subsurface contact between conductive sediments and resistive crystalline basement , showing that the depth to crystalline basement is relatively constant in the western part of the annular trough. Chemical and isotopic data indicate that saline ground water of the Virginia inland saltwater wedge or bulge is a mixture of freshwater and seawater, and evidence for a mixing zone at the crater's outer margin supports the concept of differential flushing of residual seawater to create the bulge. Ground-water brine in the central part of the crater was produced by evaporation, and brine production from the heat of the impact is at least theoretically possible.

Chesapeake Bay↗
USGS2005

Petrography, structure, age, and thermal history of granitic coastal plain basement in the Chesapeake Bay impact structure, USGS-NASA Langley core, Hampton, Virginia

The USGS-NASA Langley corehole at Hampton , Va., was drilled in 2000 and was the first corehole to reach coastal plain basement in the late Eocene Chesapeake Bay impact structure . The Langley core provided samples of granite that had been concealed by 626.3 meters (2,054.7 feet) of preimpact, synimpact, and postimpact sediments. The granite, here named the Langley Granite, is pale red, medium grained, massive, and homogeneous in composition and fabric. It has a peraluminous composition (alumina saturation index 1.1) and a seriate-inequigranular, hypidiomorphic, isotropic fabric. A pervasive secondary mineral assemblage of chlorite + albite + clinozoisite is consistent with either deuteric alteration or lower greenschist-facies metamorphism. Chlorite, the principal mafic mineral, occurs as tabular masses that suggest pseudomorphous replacement of biotite. The top of the granite is weathered but not saprolitized and is nonconformably overlain by Lower Cretaceous clastic sediments. A SHRIMP 206 Pb/ 238 U weighted average zircon age of 612±10 Ma (2σ) indicates Neoproterozoic crystallization of the Langley Granite. The 40 Ar/ 39 Ar ages of microcline and plagioclase are consistent with regional cooling and uplift after the late Paleozoic Alleghanian orogeny. Zircon and apatite fission-track cooling ages of 375±44 Ma and 184±32 Ma (2σ), respectively, indicate no discernible impact -related thermal disturbance at the Langley corehole location in the annular trough of the structure about 19 kilometers (12 miles) outside the margin of the central crater. Modeling the apatite fission-track data places upper limits on the impact -related heating at this location. For an impact -related thermal disturbance equivalent to a modeled thermal spike having a duration of 1 to 0.1 million years, temperatures in this part of the impact structure could not have been higher than about 100°C-120°C. Most fractures, faults, and veins in the Langley Granite contain lower greenschist-facies minerals and are inferred to predate the impact . No shock-metamorphosed minerals or other features clearly attributable to the impact were found in the granite. Studies of the granite provide a glimpse into the nature of crystalline terranes beneath the Atlantic Coastal Plain and Chesapeake Bay and provide limits on the geographic extent of impact -generated shock and thermal effects.

Chesapeake Bay↗
USGS2005

High-resolution seismic-reflection image of the Chesapeake Bay impact structure, NASA Langley Research Center, Hampton, Virginia

A 1-kilometer-long (0.62-mile-long) seismic reflection and refraction profile collected at the National Aeronautics and Space Administration (NASA) Langley Research Center, Hampton, Va., provides a detailed image of part of the annular trough of the buried, 35-million-year-old Chesapeake Bay impact structure. This profile passes within 5 meters (m; 16.4 feet (ft)) of a 635.1-m-deep (2,083.8-ft-deep), continuously cored and geophysically logged test hole at the Langley Center (the USGS-NASA Langley corehole). High-resolution seismic reflection images (having a common-depth-point spacing of 2.5 m (8.2 ft)) of the upper 1,000 m (3,281 ft) along the seismic profile were generated by using refraction velocities and corehole sonic velocities to convert from time sections to depth sections. Time-distance, unmigrated depth-distance, and migrated depth-distance images show lateral variations in the geologic units observed in the USGS-NASA Langley corehole. A high-amplitude reflection at 630 to 625 m (2,067 to 2,051 ft) depth on the migrated depth image correlates with the top of weathered granite (the Langley Granite) at 626.3 m (2,054.7 ft) in the Langley core. Additional high-amplitude reflections below that depth likely represent a weathering profile developed in the upper part of the granite. Diffractions on the unmigrated images suggest that the granite contains numerous inhomogeneities that may consist of mineral veins and mineralized faults and fractures, as seen in the granite cores. Above the granite, crater unit A (minimally to moderately disturbed sands and clays of the Cretaceous Potomac Formation) is characterized by semicontinuous, horizontal and moderately inclined reflections that are broken by pervasive, subvertical, small-offset faults. Sediments of the lower beds of crater unit A below 558.1 m (1,831.0 ft) in the core have horizontal bedding and are nearly pristine. Above that depth, the upper beds of crater unit A contain thick fluidized sand intervals and fractured clay-silt beds. The contact between the granite and crater unit A is essentially horizontal on the migrated depth profile and shows minor relief produced by a few steeply dipping faults. Above crater unit A, the lower beds of crater unit B are lithologically similar to the upper beds of crater unit A and display similar impact-generated deformation. In the migrated depth image, crater unit A and the lower beds of crater unit B are combined into one unit. A thin zone (0.3 m (1.0 ft) thick) of injected glauconitic sediment at the base of the lower beds (at 442.5 m (1,451.7 ft) depth) is the only occurrence of exotic material in the lower beds of crater unit B in the core. The upper beds of crater unit B (above 427.7 m (1,403.3 ft) depth) are represented by discontinuous, locally weak, isolated, or inclined reflections on the migrated depth image. In the core, the upper beds of crater unit B are divided into megablocks and megablock zones that consist of fragmented sediments of the Potomac Formation. The megablocks are separated by matrix zones that consist of smaller blocks of sediments of the Potomac Formation suspended in a matrix of native disaggregated sediments of the Potomac Formation and injected, exotic disaggregated, glauconitic Upper Cretaceous and lower Tertiary marine sediments. Angular relationships and offsets of reflections across the high-relief contact between the upper beds of crater unit B and the underlying combined crater unit A and the lower beds of crater unit B suggest that the contact is a dip-slip fault locally. Above a contact with crater unit B at a depth of 269.4 m (884.0 ft), the Exmore beds are represented by strong, continuous and discontinuous, overstepping reflections that suggest division of the Exmore into four laterally discontinuous depositional subunits. Two of these subunits are present near the Langley corehole on the seismic images and are recognized in the core (Gohn and others, this volume, chap. C). In the Langley core, the Exmore beds consist of clasts of Cretaceous and Tertiary preimpact sediments and cataclastic, shocked, pre-Mesozoic igneous rocks suspended in a matrix of calcareous, muddy, quartz-glauconite sand and granules that contains shocked quartz. The dipping, truncated, and disrupted reflections within crater units A and B are interpreted to represent a 550-m-wide (1,805-ft-wide), stratabound collapse structure. This structure does not affect the underlying basement granite or the lower beds of crater unit A, nor does it affect the base of the Exmore beds above crater unit B. The collapse structure is not bounded laterally by major normal faults. Instead, structural displace ments appear to be distributed among abundant short, smalloffset faults and intervals of fluidized sediment. Fluidized sands above 558 m (1,831 ft) depth in crater unit A are interpreted as a low-strength zone that accommodated the widespread, latestage, gravitational collapse of the impact structure. The pro posed Langley collapse structure may be analogous to stratabound grabens in the outer zone of the Silverpit crater (North Sea). The Exmore beds are interpreted as impact-generated, ocean-resurge deposits. The upper contact of the Exmore section is a wavy, semicontinuous reflection that may represent large bedforms produced by resurge currents or returning impact-generated tsunamis, or it may represent the unmodified blocky or hummocky top of the final Exmore debris flow. Typically continuous, nearly horizontal reflections characterize the upper Eocene to Pleistocene postimpact section of dominantly marine sediments.

Virginia↗
USGS1997

Paleontology and physical stratigraphy of the USGS-Pregnall No. 1 core (DOR-208), Dorchester County, South Carolina

Pregnall No. 1, a 346-ft-deep corehole in northern Dorchester County, South Carolina, recovered sediments of late Paleocene, middle and late Eocene, and late Oligocene age. The core bottomed in the Chicora Member of the Williamsburg Formation (Black Mingo Group) of late Paleocene age (calcareous nannofossil Zones NP 7/8 (?) and NP 9). The Chicora (346 to 258 ft depth) consists of two contrasting lithologic units, a lower siliciclastic section of terrigenous sand, silt, and clay, and an upper carbonate section of moldic pelecypod limestone. The Chicora is overlain unconformably by the middle Eocene Moultrie Member of the Santee Limestone (Orangeburg Group). The Moultrie (258.0 to 189.4 ft) consists primarily of bryozoan-pelecypod-peloid packstones and grainstones, which are assigned to calcareous nannofossil Zone NP 16. Unconformably above the Moultrie are the locally shelly, microfossiliferous limestones of the Cross Member of the Santee Limestone (Orangeburg Group), which are assigned to middle Eocene Zone NP 17 and upper Eocene Zone NP 18. The Cross Member (189.4 to 90.9 ft) is unconformably overlain by a very thin, basal section of the upper Eocene Harleyville Formation (Cooper Group). The thin Harleyville section consists of fossiliferous limestone, primarily pelecypod-foraminifer-peloid packstones (90.9 to 85.8 ft), and is assigned to Zone NP 18, although samples from thicker Harleyville sections in the region typically are assigned to upper Eocene Zone NP 19/20. The Harleyville is overlain unconformably by the upper Oligocene Ashley Formation (Cooper Group). The Ashley Formation (85.8 to 30.0 ft) consists of a relatively homogeneous section of calcareous, microfossiliferous, silty and sandy clays assigned to Zones NP 24 and NP 25 (?). Neogene and (or) Quaternary deposits present in the upper 30 ft of the Pregnall section are assigned provisionally to an unnamed unit (30 to 22 ft) and to the Waccamaw Formation(?)(22 to 0 ft).

South Carolina↗
USGS1985

Geologic evidence for recurrent moderate to large earthquakes near Charleston, South Carolina

Multiple generations of earthquake-induced sand blows in Quaternary sediments and soils near Charleston, South Carolina, are evidence of recurrent moderate to large earthquakes in that area. The large 1886 earthquake, the only historic earthquake known to have produced sand blows at Charleston, probably caused the youngest observed blows. Older (late Quaternary) sand blows in the Charleston area indicate at least two prehistoric earthquakes with shaking severities comparable to the 1886 event.

South Carolina↗