Gregory S. Gohn

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.

Deep Drilling into the Chesapeake Bay Impact Structure

Samples from a 1.76-kilometer-deep corehole drilled near the center of the late Eocene Chesapeake Bay impact structure (Virginia, USA) reveal its geologic, hydrologic, and biologic history. We conducted stratigraphic and petrologic analyses of the cores to elucidate the timing and results of impact-melt creation and distribution, transient-cavity collapse, and ocean-water resurge. Comparison of post-impact sedimentary sequences inside and outside the structure indicates that compaction of the crater fill influenced long-term sedimentation patterns in the mid-Atlantic region. Salty connate water of the target remains in the crater fill today, where it poses a potential threat to the regional groundwater resource. Observed depth variations in microbial abundance indicate a complex history of impact-related thermal sterilization and habitat modification, and subsequent post-impact repopulation.

Chesapeake Bay impact structure drilled

The Chesapeake Bay impact structure was formed by a meteorite crashing to Earth during the late Eocene, about 35.5 million years ago (Ma). In May 2006, a scientific drilling project, sponsored by the International Continental Scientific Drilling Program (ICDP) and the U.S. Geological Survey (USGS), completed a deep coring program into the impact structure. The deep drilling produced one of the most complete geologic sections ever obtained in an impact structure, and studies of the core samples will allow scientists to understand a shallow-marine impact event and its consequences at an unprecedented level. This buried structure is the seventh largest, and one of the best preserved, of the known impact structures on Earth [Poag et al., 2004]. It consists of a 38-kilometer-wide, highly deformed central zone, which approximates the dimensions and location of the transient impact crater, surrounded by a shallower outer zone of sediment collapse known as the annular trough [Horton et al., 2005]. Together, these zones have a diameter of about 85 kilometers and a distinctive shape similar to an ‘inverted sombrero.’

Normal faulting and in situ stress in the South Carolina coastal plain near Charleston

In situ stress measurements were made to depths of 344 m in Atlantic Coastal Plain sediments near Charleston, South Carolina. The magnitudes of the least principal compressive stress were found to be considerably sublithostatic: 30.6 b at 193 m, 33.7 b at 209 m, 35.0 b at 297 m, and 41.6 b at 344 m. These data, combined with simple faulting theory, indicate that the least principal horizontal stress at depths of 297 and 344 m is sufficiently less than the lithostatic load to result in normal-type fault motion on favorably oriented faults. Stratigraphic evidence from three test wells and 20 auger holes in the area supports existence of at least one normal fault in the vicinity of the wells in which the stress measurements were made. We interpret these results to suggest that normal faults in coastal plain sediments near Charleston are currently active. Because the direction of relative horizontal extension appears to be northeast-southwest, or parallel to the trend of the continental margin, we infer that this stress field is of tectonic origin.

Regional implications of triassic or jurassic age for basalt and sedimentary red beds in the South Carolina coastal plain.

Whole rock potassium-argon ages for samples of subsurface basalt recovered near Charleston, South Carolina, are interpreted to indicate a Triassic or Jurassic age for the basalt and underlying sedimentary red beds. This age is consistent with existing evidence indicating that an early Mesozoic basin is present in the subsurface of a large part of the coastal plain of South Carolina, Georgia, Florida, and Alabama.

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 Earths 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 structures 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.

Impact Disruption and Recovery of the Deep Subsurface Biosphere

Although a large fraction of the worlds biomass resides in the subsurface, there has been no study of the effects of catastrophic disturbance on the deep biosphere and the rate of its subsequent recovery. We carried out an investigation of the microbiology of a 1.76 km drill core obtained from the ∼35 million-year-old Chesapeake Bay impact structure, USA, with robust contamination control. Microbial enumerations displayed a logarithmic downward decline, but the different gradient, when compared to previously studied sites, and the scatter of the data are consistent with a microbiota influenced by the geological disturbances caused by the impact. Microbial abundance is low in buried crater-fill, ocean-resurge, and avalanche deposits despite the presence of redox couples for growth. Coupled with the low hydraulic conductivity, the data suggest the microbial community has not yet recovered from the impact ∼35 million years ago. Microbial enumerations, molecular analysis of microbial enrichment cultures, and geochemical analysis showed recolonization of a deep region of impact-fractured rock that was heated to above the upper temperature limit for life at the time of impact. These results show how, by fracturing subsurface rocks, impacts can extend the depth of the biosphere. This phenomenon would have provided deep refugia for life on the more heavily bombarded early Earth, and it shows that the deeply fractured regions of impact craters are promising targets to study the past and present habitability of Mars.

Physical stratigraphy, paleontology, and magnetostratigraphy of the USGS-Santee Coastal Reserve core (CHN-803), Charleston County, South Carolina

4 Introduction 4 Acknowledgments 6 Unit conversions 6 Methods 6 Physical stratigraphy and lithology 6 Paleontology 6 Calcareous nannofossils 6 Palynology 6 Foraminifera 7 Strontium-isotope measurements 7 Paleomagnetic measurements 7 Results and stratigraphic discussions 7 Stratigraphy 7 Paleontology 11 Strontium-isotope results 14 Paleomagnetic results 14 Donoho Creek Formation (Black Creek Group) 14 Physical stratigraphy and lithology 14 Paleontology 15 Magnetostratigraphy 15 Peedee Formation 15 Physical stratigraphy and lithology 15 Paleontology 17 Strontium-isotope stratigraphy 21 Magnetostratigraphy 21 Rhems Formation (Black Mingo Group) sensu stricto 21 Physical stratigraphy and lithology 21 Paleontology 23 Magnetostratigraphy 23 Upper part of the Rhems Formation (Black Mingo Group) sensu Bybell and others (1998) 23 Physical stratigraphy and lithology 23 Paleontology 24 Magnetostratigraphy 26 Lower Bridge Member of the Williamsburg Formation (Black Mingo Group) 26 Physical stratigraphy and lithology 26 Lower beds 26 Upper beds 27 Paleontology 27 Magnetostratigraphy 27 Chicora Member of the Williamsburg Formation (Black Mingo Group) 27 Physical stratigraphy and lithology 27 Paleontology 28 Magnetostratigraphy 28 Mollusk-bryozoan limestone 28 Physical stratigraphy and lithology 28 Paleontology 29 Magnetostratigraphy 29 Wando Formation 29 Physical stratigraphy and lithology 29 Paleontology 30

Liquefaction Evidence for Repeated Holocene Earthquakes in the Coastal Region of South Carolinaa

Features thought to have originated from earthquake-induced liquefaction have been discovered throughout much of the coastal region in South Carolina and in extreme southeastern North Carolina. Nearly all these liquefaction features are sandblows presently manifested as filled craters. Prehistoric craters near Charleston formed in long-separated episodes at least three times within the past 7200 years. Ages of dated craters far from Charleston, beyond the farthest 1886 earthquake sandblows, differ from ages of craters near Charleston. Insufficient data have been collected to determine whether ages of all craters far from Charleston differ from ages of craters near Charleston. Both the size and relative abundance of pre-1886 craters are greater in the vicinity of Charleston (particularly in the 1886 meizoseismal zone) than elsewhere, even though the susceptibility to earthquake-induced liquefaction is approximately the same at many places throughout this coastal region. These data indicate that, in this coastal region, the strongest earthquake shaking during Holocene time has taken place repeatedly near Charleston.

Vertical crustal movements in the Charleston, South Carolina-Savannah, Georgia area

(Accepted for publication April 4, 1978) ABSTRACT: Lyttle, P.T., Gohn, G.S., Higgins, B.B. and Wright, D.S., 1979. Vertical crustal movements in the Charleston, South Carolina—Savannah, Georgia area. In: C.A. Whitten, R. Green and B.K. Meade (Editors), Recent Crustal Movements, 1977. Tectonophysics, 52: 183-189. First-order vertical level surveys (National Geodetic Survey) repeated between 1955 and 1975 suggest that modern vertical crustal movements have taken place in the Atlantic Coastal Plain between Charleston, South Carolina and Savannah, Georgia. The relative sense of these movements correlates with the sense of displacement of Tertiary strata on known geologic structures. Whereas regional dip of strata in most of the Atlantic Coastal Plain is southeasterly, the regional dip of Tertiary strata in this part of the Coastal Plain averages 2 m/km to the south or southwest. Positive structural features disturb this regional dip along a poorly defined zone, about 25 km wide, parallel to the coast between Savannah and Charleston. Structural relief on these features is as much as 20 m. Repeated level lines that cross the Atlantic Coastal Plain elsewhere generally show an increase in modern relative subsidence from west to east. However, in the Charleston—Savannah area, the amount of relative subsidence remains fairly constant or decreases from west to east across the structural highs. At two localities near Charleston, where Tertiary beds are offset by faults roughly on strike with one another, an abrupt break in a repeated level line occurs where the level line crosses the probable extensions of these faults. The average modern rates of relative uplift and subsidence (assuming they are constant) are compatible with rates noted throughout the Coastal Plain. Long-term extrapolation of modern rates appears unreasonable; episodic or oscillatory movements are much more likely.