Snapshots of a Violent Impact:
The USGS-NASA Langley and Eyreville Deep Coreholes

Ammie Pascua, Laura Robertson, Emily Robinson
Department of Geology, College of William and Mary

Figure 1- Regional map showing the location of the CBIS and locations of the 2000 USGS-NASA Langley and 2005 Eyreville, Va. drill sites (From Gohn and others, 2005)

35 million years ago, a hurling mass ended its journey across the universe exploding into a shallow ocean, blasting out water, sand and rock, spawning giant tsunamis that radiated out in all directions before vaporizing. All it left was a mysterious scar that became slowly buried over millions of years. During the late 1980’s and 90’s, the seventh largest and probably best preserved impact crater on Earth was identified through analysis of samples from deep coreholes drilled by United States Geological Survey (USGS) and the Virginia Department of Environmental Quality, along with offshore drilling from Deep Sea Drilling Project Site 612, and interpretations of seismic reflection data.

The USGS-NASA Langley corehole was one of four deep coreholes drilled by the Chesapeake Bay Impact Crater Project from 2000-2002 in the western part of the outer ring, or annular trough in Hampton, Va. The goal of the project was to better understand the geology, structure and history of the Chesapeake Bay Impact Structure (CBIS), and post-impact sediments’ control on southeastern Virginia’s fresh groundwater aquifers. The result of studies from the Langley corehole was reported on in USGS Professional Paper 1688 in 2005. Drilling sponsored by the International Continental Scientific Drilling Program and USGS drilled another deep corehole in 2005 that was positioned over the central zone of the structure at Eyreville, Va. on the Eastern Shore (figure 1).

A conceptual model of crater formation was designed through analysis of deep coreholes and previous work. The model (figure 2) illustrates before and after stages of the CBIS formation. Before the impact, the local environment consisted of granite basement rocks, brachiopods and bivalves burrowing into poorly consolidated water soaked siliciclastic sediments, and ocean water deepening eastward. After the initial impact violently excavated a giant hole, the structure was modified by processes of collapse, slumping and water resurge — that is to say, what was blasted out came flooding furiously back in. The jagged and brutally broken mega-blocks and breccias were deposited during this time, eventually shrouded by quiet marine sediments, perfectly preserving the crater for later investigation.

The Langley corehole was drilled to a depth of 2083 feet (635 m) on the western outskirts of the annular trough. Gohn and others (2005) defined the stratigraphy in the Langley core and correlate the informal terms “Crater Unit A, Crater Unit B and Exmore beds” with observed lithology and interpretations of depositional environments. A Stratigraphic column linking the names, lithology and depositional environments is shown in figure 3.The oldest drilled sediments come from Crater Unit A, it is 603 feet (184 m) thick, composed of mainly Cretaceous aged, silty and sandy clays, clayey silts and muddy fine sands and gravels of the Potomac Formation. Crater Unit A is divided into two units: lower beds of undisrupted Cretaceous sedimentary sequences with horizontal or low-angle bedding and upper beds of highly fractured clays and massive, structure-less sands.
Figure 2 -Cross sections illustrating stages in a conceptual model of CBIS formation. Diagrams show western half of crater along west-to-east profile. A. Pre-impact target: before the projectile hit, the target site consisted of three main layers: crystalline basement rocks, siliciclastic sediments, and ocean water. B. Contact compression followed by excavation: Impact of the projectile produced shock waves, vaporizing the projectile and causing vaporization, melting and shock metamorphism in the target. Expansion of the shock wave excavated a bowl shaped transient cavity in the target, producing shack metamorphism, melts, ejecta,and an ejecta curtain. C. Crater modification: Collapse of the transient cavity was accompanied by inward collapse of slump blocks in poorly consolidated sediments of the annular trough. The collapse expanded the crater beyond the central excavation to a total width of about 53 miles. A violent resurge of ocean water and submarine debris flows filled the open cavity with water and debris. D. Post impact burial: after the resurge currents deposited the Exmore beds, the crater was buried by marine sediments. (From Horton and others, 2005)

 

 


Figure 3 – Stratigraphy and correlated nomenclature used in the USGS-NASA Langley core. Highlighted column displays data from references used. (From Gohn and others, 2005)

 

The Exmore beds are interpreted as ocean re-surge sediments deposited by repeated debris flows. The thinner, horizontal beds near the top represent a return to peaceful, normal continental shelf sedimentation. The Exmore beds contain un-shocked, pre-impact Cretaceous and Tertiary sediment clasts and shocked igneous and metamorphosed rock clasts suspended in a finer calcareous, muddy, quartz-glauconitic matrix (figure 7).

Another way to expose the secrets of the CBIS is by interpreting seismic-reflection profiles. Catchings and others (2005) interpreted seismic-reflection profiles in order to reveal the stratigraphy. One way to create a seismic profile is to drill a number of holes along a planned transect, plant geophones and dynamite in them and then Bang! Bang! Bang! The reflected seismic waves are received, recorded and read. The resulting data produces a snapshot of an ancient earthen flesh wound dressed by multiple layers of marine sediments (figure 4).

The latest deep corehole to be examined from the CBIS came from a composite of three holes summed to a depth of 5794 feet. It was drilled at Eyreville, near Cape Charles, on Virginia’s Eastern Shore (figure 1). The core represents the most complete geologic section to be recovered from an impact structure to date. Studies from core samples will yield unprecedented insight into shallow marine impact events and local consequences on important current issues such as locations of fresh groundwater aquifers.


Figure 4- Interpreted seismic-reflection for the Langley profile showing major impact-related and post-impact seismic stratigraphic units. Unit Designations: A, crater unit A; Bl, lower beds of crater unit B; Bu, upper beds of crater unit B; Ex1…4, seismically defined subunits of the Exmore beds. (From Catchings and others, 2005)

The drill penetrated 4336 feet (1321 m) of impact related sediments and 1456 feet (444 m) of post-impact sediment sequences. Five major rock types were identified (figure 5). The oldest breccias are thought to come from the crater floor, the second oldest unit of lithic and impact breccias are interpreted as fall back or ground surge (figure 6). The granite and sediment clast breccia units are thought to be slump blocks that moved to their current position during late stage collapse of the crater. Post impact sediments are inferred as deposition from late stage collapse of the marine water column followed by a catastrophic flow of water into the crater (Fig. 2C).

Figure 5 – Stratigraphic column from the Eyreville corehole showing the five major rock types (from Gohn and others, 2005).

Geologists are better understanding the geometry of the CBIS through the USGS-NASA Langley, Eyreville coreholes, and on-going studies. This knowledge benefits science by bringing us a little bit closer in understanding the big picture of our universe and our planet. And, it is good for people - especially for people that like to drink water and live near our beautiful Chesapeake Bay. The CBIS is elemental in understanding the groundwater of Virginia’s Coastal Plain. Plus, it is a complete record of a violent impact entombed in our backyard! It is a crater-ghost screaming out from its watery grave to be understood and celebrated. Unlike most impact craters on Earth, it is still here — it has not been erased by surface processes.

Figure 6- Photo of the Eyreville corehole at a depth of ~2838 feet. The depth corresponds to the sediment–clast breccia and sediment megablocks from Figure 5. Photo by C.M. Bailey, College of William and Mary.

Figure 7- Snapshot of the Exmore beds at depths from 873.53 -862.15 feet from the Langley corehole. The Exmore beds are interpreted as debris flow deposits as water surged back into the crater. In both photos, broken clasts are chaotically suspended in a much finer matrix of sand. Photo by D.S. Powars, USGS.

 

 

The Eyreville, Va. drill site on the Eastern Shore. Photo taken in fall, 2005 by C.M. Bailey, College of William and Mary.


References


Catchings, R.D., Powars, D.S., Gohn, G.S., and Goldman, M.R., 2005, High-Resolution Seismic Reflection Image of the CBIS, NASA Langley Research Center, Hampton, Virginia, in Horton, J.W., Powars, D.S. and Gohn, G.S., eds., Studies of the CBIS - The USGS-NASA Langley Corehole, Hampton, Virginia, and Related Coreholes and geophysical Surveys: Reston, Virginia, United States Geological Survey, p. I1.

Gohn, G.S., Koeberl, C., Miller, K.G., Reimold, W.U., Cockell, C.S., Horton, J., J.W., Sanford, W.E., and Voytek, M.A., 2006, Chesapeake Bay Impact Structure Drilled: EOS Transactions American Geophysical Union, v. 87, p. 349.

Gohn, G.S., Powars, D.S., Bruce, T.S., and Self-Trail, J.M., 2005, Physical Geology of the Impact-Modified and Impact-Generated Sediments in the USGS-NASA Langley Core, Hampton, Virginia, in Horton, J.W., Powars, D.S. and Gohn, G.S., eds., Studies of the CBIS - The USGS-NASA Langley Corehole, Hampton, Virginia, and Related Coreholes and geophysical Surveys: p. C1.

Horton, J.W., Powars, D.S., and Gohn, G.S., 2005, Studies of the Chesapeake Bay Impact Structure-Introduction and Discussion, in Horton, J.W., Powars, D.S. and Gohn, G.S., eds., Studies of the Chesapeake Bay Impact Structure-The USGS-NASA Langley Corehole, Hampton, Virginia, and related Coreholes and Geophysical Surveys: Reston, Va, United States Geological Survey, .