In July 1983, the shipboard scientists of Deep Sea Drilling Project
Leg 95 found an unexpected bonus in a core taken 150 kilometers east of Atlantic
City, N.J. At Site 612, the scientists recovered a 10-centimeter-thick layer
of late Eocene debris ejected from an impact about 36 million years ago. Microfossils
and argon isotope ratios from the same layer reveal that the ejecta were part
of a broad North American impact debris field, previously known primarily from
the Gulf of Mexico and Caribbean Sea.
Since that serendipitous beginning, years of seismic reflection profiling, gravity measurements and core drilling have confirmed the source of that strewn field the Chesapeake Bay impact crater, the largest structure of its kind in the United States, and the sixth-largest impact crater on Earth.
C. Wylie Poag sampling core from Chesapeake Bay crater at NASA Langley Research Center, Hampton, Va. All images courtesy of C. Wylie Poag.
Residents living around the Chesapeake Bay are still feeling the effects of the impact event, millions of years later. The unusual chemical and physical processes associated with formation of the crater resulted in high-salinity groundwater in the coastal plain. Studies of the impact site are providing further explanations for this anomaly and its consequences, and have opened an ancient window to understanding large-scale impacts.
Bolide (exploding meteorites) impacts are mind-boggling geologic events, especially
because they exert stupendous energy bursts in extremely short time periods
virtually a geological blink of an eye. In order to understand the marine
impact process more fully, David Crawford, a numerical modeler at Sandia National
Laboratories, simulated the Chesapeake Bay impact and subsequent crater formation,
using a complex computer code (hydrocode). Based on the simulation, it takes
less than 10 seconds for an asteroid 3.3 kilometers in diameter and traveling
at 20 kilometers per second to plunge through 300 meters of seawater and 200
meters of sediments, and then to depress the underlying granitic basement rocks
to a depth of 15 kilometers below the seabed.
During the next three minutes the resultant crater reaches a maximum depth of 20 kilometers and maximum diameter of 40 kilometers. Immediately thereafter, the shock-fluidized granitic basement rebounds to form a mountainous central uplift, 30 kilometers high, then collapses and solidifies into a much smaller peak, surrounded by a modest circular ridge, called a peak ring. Meanwhile, the impact shock wave reverberates radially along the basement surface and destabilizes the base of the overlying sedimentary section. Blocks of collapsed sediments, including some kilometer-scale megablocks, slide along the basement surface, thereby widening the craters maximum diameter to 85 kilometers.
Approximately 36 million years ago, an extraterrestrial body struck the present-day Chesapeake Bay area, altering the evolution of the Virginia Coastal Plain. The sixth largest impact crater on Earth, the Chesapeake Bay crater affects almost 2 million people living in the region through subsidence, faulting and the presence of saline groundwater.
During this same three-minute interval, an ejected curtain of crushed rock debris, mixed with seawater, spurts 50 kilometers into the atmosphere, before raining back as incandescent ballistic missiles. The blast also creates a 300-meter-high wall of seawater around the briefly exposed seafloor. As that towering water column rushes back into the excavation, it scours the tops of the sedimentary megablocks, reduces the debris to smaller fragments, and begins to fill the excavation with the brecciated sediments.
Finally, a series of giant impact-generated tsunami waves rushes out across the shallow seafloor, runs up onto the adjacent coastal plain, and there scours and redistributes huge volumes of shock-weakened sediments. As the tsunami flow recedes, it washes much of this sedimentary debris back into the crater.
The evidence for this cataclysm at Chesapeake Bay is buried beneath a layer
of sediment hundreds of meters thick. However, extensive seismic reflection
surveying and deep, continuously cored boreholes have uncovered the principal
structural and stratigraphic features of the impact crater. A database of more
than 2,000 kilometers of seismic tracklines shows the crater as an irregularly
circular feature, 85 kilometers in average diameter. Its outer rim is a steep
sedimentary fault scarp, with structural relief of 300 meters on the west and
500 meters on the east (downdip). Inside this outer rim, a subcircular ridge
of uplifted crystalline basement rocks forms a peak ring (40 kilometers wide).
Inside the peak ring, an inner basin may be as deep as 1.5 to 2.0 kilometers.
At the very center of the crater, a series of rugged mountainous peaks towers
1 kilometer or more above the basin floor. While seismic profiles also exhibit
plausible evidence for most of the depositional units predicted to fill the
Chesapeake Bay crater, deep coring was required to be sure.
Researchers have drilled eight deep boreholes inside or near the Chesapeake Bay crater, through cooperation between the U.S. Geological Survey (USGS), the Virginia Department of Environmental Quality, the geology department at the College of William and Mary, the NASA Langley Research Center and the Hampton Roads Planning District Commission. The cores reveal a fascinating succession of granite basement overlain by upward-fining sedimentary deposits, primarily impact breccias containing mixed fragments of preimpact formations.
The basement rocks consist of fractured Proterozoic granites, about 600 million years old. Above the basement, the first, or lowermost crater-fill unit is composed of decimeter-to-kilometer-scale megablocks, which are confined to the region between the outer rim and the peak ring. The megablocks, composed mainly of nonmarine Lower Cretaceous sediments, formed by inward collapse of the craters outer walls (slumpback).
Above the megablocks is a clast-supported breccia, composed mainly of debris from the tops of the megablocks, crater walls and the surrounding seafloor, scoured by the marine water column as it surged back into the excavation.
Above this surgeback breccia is another breccia unit dominated by lithically varied, centimeter-to-millimeter-scale sedimentary clasts, which are supported by a distinctive matrix of glauconite and quartz sand. This glauconitic deposit constitutes a washback breccia, the pieces of which were scoured from the coastal plain and shallow seafloor by the run-up and washback of tsunami waves.
The slumpback, surgeback and washback deposits constitute the principal crater-fill units at Chesapeake Bay, ranging from 150 to 1,000 meters in combined thickness. A fourth thick breccia unit, composed mainly of crystalline clasts, may occupy the deepest part of the inner basin, but has not yet been cored. In addition, at the very top of the crater fill, two centimeter-scale deposits have been encountered at some sites. Above the sandy glauconitic sand, at two intracrater sites, is a silt-rich layer with inclined, multidirectional stratification. This unit appears to represent gravity flows triggered by postimpact storm activity. Some of the storms may have been super hurricanes, or hypercanes, resulting from the intense pulse of heat generated by this oceanic impact.
The final deposit of impact-derived debris, cored at a single site, is a fallout layer, composed of centimeter-scale, horizontal laminae of silt and clay. This silty bed contains pyrite microstructures with peculiar subspherical pits, interpreted to represent the impressions of millimeter-sized microspherules of impact glass called microtektites ejected, impact-melted droplets of crust-derived silica, which fell back out of the atmosphere to form a broad debris field. In this case, it is the North American tektite-strewn field, which covers more than 9 million square kilometers of the western North Atlantic Ocean, the Gulf Coastal Plain, the Gulf of Mexico and Caribbean Sea.
Ancient impact consequences
The Chesapeake Bay impact substantially altered the structural and depositional
evolution of the Virginia coastal plain. Because of its large size, some researchers
would expect the Chesapeake Bay impact also to have produced significant environmental
or climatic anomalies. So far, however, its immediate paleoenvironmental consequences
appear to have been limited to the impact site itself.
Initial postimpact sediments in intracrater cores record a seafloor paleoenvironment hostile to marine life. A 19-centimeter-thick interval of dark, clayey silt contains poorly preserved microfossil specimens reworked from the underlying impact breccia, but contains no indigenous biota. This dead zone represents, at most, about 3,000 years. Rich marine microfauna signal the return of amenable marine conditions during deposition of the succeeding Chickahominy Formation, which represents deposition during the final 2.1 million years of the late Eocene. Three negative excursions of oxygen isotope ratios (derived from benthic foraminifera) in Chickahominy time indicate that three pulses of relatively warmer climate followed the Chesapeake Bay impact. The warm pulses correlate with an interval of increased extraterrestrial helium in an equivalent late Eocene section at Massignano, Italy. This positive Helium-3 anomaly has been attributed to a long-term comet shower. These relations suggest that the Chesapeake Bay impact may have contributed to the long-term paleoclimatic excursions.
Modern impact consequences
The principal impact-generated changes were restricted to that moment approximately 36 million years ago; however, we have considerable evidence of important subsequent consequences. Some of the effects, including subsidence, faulting and saline groundwater, still influence the nearly 2 million citizens living around the mouth of Chesapeake Bay in Virginia in the cities of Virginia Beach, Norfolk, Hampton, Newport News, Chesapeake and Portsmouth.
Although the Chesapeake Bay crater is now buried 300 to 500 meters below ground, its presence is expressed on the modern land surface. Apparently, long-term differential subsidence of the crater-fill breccia has played a major role in controlling the maximum westward extent of two successive Pleistocene marine transgressions, creating erosional shoreline features.
The geologic map of Virginia clearly shows that sedimentary units inside the surface projection of the crater rim are of late Pleistocene and Holocene age, whereas most of those outside the projected crater rim are middle Pleistocene and older. Furthermore, the contact separating lower and middle Pleistocene units from upper Pleistocene units curves around the western perimeter of the crater (convex to the west) within approximately 1 kilometer of where the crater rim projects to the surface. Outside the crater rim, in contrast, the mapped contact between these older and younger units is nearly a straight line, which trends almost due north-south.
Similarly, on the east side of the bay, the contact separating middle Pleistocene sediments from upper Pleistocene sediments on the Delmarva Peninsula marks the surface projection of the crater rim. In addition, along the southern shore of Chesapeake Bay between Norfolk and Cape Henry, the surface projection of the crater rim is marked approximately by the contact between upper Pleistocene units and Holocene shoreline sands.
The crater rim also is expressed in Virginias coastal topography. Two Pleistocene shoreline erosional features, known as the Suffolk and the Big Bethel scarps, mark the contact between older and younger Pleistocene units on the western side of the bay. Thus the positions of these scarps (with topographic relief as great as 22 meters) also approximate the western boundary of the crater. The buried southern rim of the crater is marked at the surface by the southern shore of Chesapeake Bay and by the parallel slope of the Diamond Springs scarp. A comparable landform on the Delmarva Peninsula is Ames Ridge; relief there is about 5 meters.
The lower courses of the four largest rivers in southeastern Virginia bear signs that long-term differential subsidence also has determined the locations of their channels. The James, York, Piankatank and Rappahannock rivers all make sharp bends approximately at the outer rim of the crater. The upstream courses of the James and York head directly southeastward to the Atlantic; but in each case, the river turns abruptly to the northeast near the crater rim (an acute angle in the case of the James), and its channel heads toward the center of the crater. In the case of the Piankatank, its channel makes a right-angle turn to the northeast and parallels the outer rim for about 5 kilometers before turning 90 degrees back to the southeast toward the craters center.
Long-term tide (sea-level) gauges in the Chesapeake Bay region indicate that during the last 70 years, the rate of relative sea-level rise in this area has been among the highest in the continental United States. Long-term compaction of the crater-fill deposits may have contributed an additional component to these high relative sea-level rise values, by differentially lowering the ground surface and bay floor over the crater.
At 4 millimeters per year, this rate is more than twice the global average (1.8 millimeters per year). About half of this high value can be attributed to structural relaxation and rebound of a large regional bulge created by Wisconsinan ice sheets during the last glacial maximum in eastern North America. Furthermore, as most of these gauges are near population centers, some of the relative rise may be the result of land subsidence due to groundwater extraction.
Nevertheless, after correction for periglacial rebound, some sites inside and near the Chesapeake Bay crater still average 4 millimeters per year relative sea-level rise. Rates at all four sites inside the crater are higher than the global average, and at Gloucester Point, on the York River, the value is as high as 6.7 millimeters per year.
A second potential geologic hazard for the region around the lower Chesapeake Bay arises from the presence of an extensive system of impact-related, near-surface normal faults. Most of these faults begin at the base of the Chickahominy Formation and extend upward into the Miocene, Pliocene and Pleistocene sections. On some high-resolution seismic reflection profiles, the faults can be traced to within 15 meters of the bay floor. The presence of these faults creates a zone of structural weakness over the crater, which is more susceptible to crustal adjustments, such as earthquake displacement, than areas outside the crater. Although historical earthquakes in southeastern Virginia have been rare and relatively mild, surface projections of the epicenters of all four significant historical earthquakes in this region (in 1884, 1899, 1918 and 1995) were near or inside the trace of the crater rim.
Perhaps the most significant modern consequence is the presence of unusually salty groundwater at shallow depths in coastal-plain wells. Hydrogeologists have known of this anomalous salinity for more than 50 years but have lacked a compelling explanation for its presence. Discovery of the Chesapeake Bay crater has provided a plausible answer.
Cores in and around the crater show that areal distribution of the anomalous groundwater is constrained by the crater rim. Salinity increases markedly in intracrater boreholes within the impact breccia, and reaches 1.5 times that of normal seawater at one site near the center of the crater. Researchers have not yet firmly established the original source of the brine. Preliminary analyses indicate, however, that a large volume of late Eocene seawater evaporated during the impact, leaving solutes in the breccia. Subsequently, elevated temperatures in the crater-fill breccias estimated to be more than 400 degrees Celsius could have been maintained for approximately 10,000 years by the superheated crystalline basement, resulting in production of a residual brine.
Regardless of how it formed, the presence of the shallow brine reservoir in the crater has two significant implications for the citizens living around the southern margin of the Chesapeake Bay. The shallow stratigraphic position of the brine, coupled with the existence of hundreds of faults in the overlying strata (shown on the seismic profiles), creates a contamination hazard for overlying freshwater aquifers. And the shallow depth to the top of the brine reservoir (the impact breccia) and its great thickness inside the crater limit the availability of fresh groundwater to communities overlying the crater.
To clarify these implications and resolve the groundwater-management problems they pose, the USGS and its collaborators are placing special emphasis on analysis and interpretation of porewater properties in breccia cores. The objectives are to determine the chemical properties of the brine, ascertain its origin, and measure the directions and rates of flow within the breccia and across the crater rim. The results are especially crucial for upgrading computer models of the groundwater systems in southeastern Virginia. Future core drilling will focus on the center of the crater, where high salinity in the breccia poses the greatest risk.
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