Its not like a bus or a train schedule, it just happens that frequently on average," Mazanek said of objects entering Earths atmosphere. Okada, H., and Bukry, D., 1980, Supplementary modification and introduction of code numbers to the low-latitude coccolith biostratigraphic zonation (Bukry, 1973; 1975): Marine Micropaleontology, v. 5, no. 19). Interpretation of the York River seismic line (pl. The St. Marys consists of massive, mostly dense, well-sorted, dark-greenish-gray to dark-gray, micaceous, clayey silt to very fine, sandy clay and silt containing scattered shells, sparse to abundant burrows, fairly abundant iron sulfide (pyrite and chalcopyrite), and finely disseminated organic matter. The MW4-1 core and borehole geophysical logs provide the information needed to correlate stratigraphic units south of the James River. The seismic signature of the middle Miocene Calvert Formation is ill-defined outside the crater's outer rim (pl. 4b). Focazio, M.J., Speiran, G.K., and Rowan, M.E., 1993, Quality of ground water in the Coastal Plain physiographic province of Virginia: U.S. Geological Survey Water-Resources Investigations Report 92-4175, 20 p., 5 pls. The existing hydrogeologic framework and ground-water models were built upon a geological framework that described the Virginia Coastal Plain as an eastward dipping and thickening wedge of unconsolidated sediments, readily subdivided into uniform, homogeneous aquifers and confining units. The deepest unitinthe corehole is the NeoproterozoicLangleyGranite. One of the locations of highest relative sea-level rise is at Hampton Roads (the lower part of the James River), located over the crater rim. Known as the Chesapeake. Today, that 25-mile-wide (40 kilometers) crater is buried half a mile below the rocky basement of Chesapeake Bay the 200-mile-long (320 km) estuary linking Virginia and Maryland on the East. ', USGS, 'The Chesapeake meteorite: message from the past. As shown on the York River profile, post-impact deposits drape over the crater's outer rim and thicken in the crater; these deposits also sag and thicken into the inner basin, indicating ongoing crater subsidence during late Tertiary time. These faults are zones of crustal weakness and have the potential for continued slow movement, or sudden larger offsets if reactivated by earthquakes. Upper Cretaceous deposits south of the James River younger than pollen zone III (lower Cenomanian) are documented in the subsurface south and southwest and northeast of the outer rim of the crater extending westward to central Southampton County and eastern Surry County (Powars and others, 1992; fig. Back, W., 1966, Hydrochemical facies and ground-water flow patterns in northern part of Atlantic Coastal Plain: U.S. Geological Survey Professional Paper 498-A, 42 p., 1 plate. In addition to these specific consequences, the crater and the convulsive event that produced it, have widespread implications for traditional interpretations of certain structural and depositional features of the Atlantic Coastal Plain, particularly in southeastern Virginia. Rhonda Olson . 1). pp. 5). The relief of the buried escarpment ranges from 1,000 to nearly 4,000 ft, and its width varies from about 0.5 to 2 mi (Poag, 1996). Melosh, H.J., 1989, Impact cratering--A geologic process: New York, Oxford University Press, 245 p. Meng, A.A., III, and Harsh, J.F., 1988, Hydrogeologic framework of the Virginia Coastal Plain: U.S. Geological Survey Professional Paper 1404-C, 82 p. Mixon, R.B., Berquist, C.R., Newell, W.L., Johnson, G.H., Powars, D.S., Schindler, J.S., and Rader, E.K., 1989, Geological map and generalized cross sections of the Coastal Plain and adjacent parts of the Piedmont, Virginia: U.S. Geological Survey Miscellaneous Investigations Series Map I-2033, 2 sheets, scale 1:250,000. Resistivity logs reflect the very clayey nature of the Marlboro; however, the lowest Nanjemoy beds are often described as a gray clay, making differentiation between these two units difficult. Using this interpretation, the CBIC megablock beds range from about 700 to 2,500 ft in thickness. These investigations culminated in the discovery of the Chesapeake Bay impact crater (Powars and others, 1993; Poag, Powars, and Mixon, 1994; Poag, Powars, Poppe, and Mixon, 1994); the crater's association with the inland salt-water wedge (Powars and others, 1994, 1998; Bruce and Powars, 1995); and the crater's structural and stratigraphic effects on post-impact sediment distribution and on the development of the present-day landscape (Powars and others, 1993, 1998; Poag, Powars, and Mixon, 1994; Poag, Powars, Poppe, and Mixon, 1994; Poag, 1996, 1997c; Johnson and Powars, 1996; Riddle and others, 1996; Johnson, Kruse, and others, 1998; Johnson, Powars, and others, 1998). 3). Cape Charles, a sleepy town located at the mouth of the bay in eastern Virginia, holds the secret to the origins of the Chesapeake Bay region. The top of the crystalline basement, marked by two closely spaced, parallel, high-amplitude reflectors, sharply contrasts with the overlying chaotic seismic signature of the Exmore tsunami-breccia deposits. In the absence of alternative water supplies, water utilities in this region have begun to develop projects that withdraw brackish ground water along the edge of the CBIC. Recently, investigators have refined the hydrogeology of the shallow aquifer system of York County (the top two aquifers and one confining unit of the RASA study) into a more local aquifer-confining unit subdivision, adding one more aquifer and confining unit beneath the Lackey Plain (Brockman and others, 1997) (pl. Emplacement of a mixture of the lithically heterogeneous Exmore tsunami-breccia with seawater, and its subsequent burial by fine-grained deposits in the structural low, likely altered regional flow paths, resulting in differential flushing of freshwater over and/or around the primarily fine-grained deposits found inside the crater. Recent investigations have focused on refinement of the shallow geohydrologic framework (Brockman and Richardson, 1992; Brockman and others, 1997). The submarine crater, discovered through oil drilling exploration, was, like Spider . In Large Meteorite Impacts III: Geological Society of America Special Paper 384, 2005a, pp. The Holocene transgression, along with higher subsidence rates over the crater, has generally produced the highest measured rates of subsidence in the mid-Atlantic region (Nerem and others, 1998), which possibly account for the abundant swamps that border the lower Chesapeake Bay. A comprehensive effort to understand the crater's materials, architecture, geologichistory, 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. "ICDP-USGS Workshop on Deep Drilling in the Central Crater of the Chesapeake Bay Impact Structure, Virginia, USA: Proceedings volume." According to Erwin, however, approximately the top 0.9 second twt represents Coastal Plain deposits; therefore, 1.5 seconds twt is most likely equivalent to between 6,000- to 8,000-ft-depth, and 2.0 seconds twt is equivalent to 10,000- to 12,000-ft-depth. Another example is the overall fine-grained nature of the Calvert, St. Marys, and lower Eastover Formations, as reflected by low single-point resistance and multipoint resistivity-log responses (deflection to the left; figs. Various structural and stratigraphic complexities related to the CBIC and its burial have altered the hydraulic characteristics of the aquifers and confining units inside and adjacent to the crater and have apparently retarded the flushing of salt water from inside the crater. Powars D. S., Johnson G. H., Bruce T. S. (1998) Stratigraphic, structural, and hydrogeological complexities related to the outer rim of the Chesapeake Bay impact crater (abstract). Laczniak, R.J., and Meng, A.A., III, 1988, Ground-water resources of the York-James Peninsula of Virginia: U.S. Geological Survey Water-Resources Investigations Report 88-4059, 178 p. Larson, J.D., 1981, Distribution of saltwater in the Coastal Plain aquifers of Virginia: U.S. Geological Survey Open-File Report 81-1013, 25 p., 2 pls. (2004) The Chesapeake Bay Crater: Geology and Geophysics of a Late Eocene Submarine Impact Structure, Springer-Verlag, Berlin, 2004. 1730 East Parham Road Powars, D.S., Poag, C.W., and Bruce, T.S., 1991, Uppermost Mesozoic and Cenozoic stratigraphic framework of the central and outer Coastal Plain of Virginia [abs. to lowermost middle Miocene] truncate the Delmarva beds. (1) The impact excavated a roughly circular crater, twice the size of Rhode Island (6400 km2) and nearly as deep as the Grand Canyon (1.3 km deep). Differentiation is aided, however, by noting the vertical stacking order of the units. These two profiles are based on 12-channel, 6-fold, digital seismic data that were collected by a 15-in3 airgun and a 393.6-ft long hydrophone streamer. ?85 km in diameter. Appendix 1B lists the altitudes of the tops of the stratigraphic units used in this report. pp. 119136., Emry, S. R., and B. Miller. Woods Hole, MA 02543 17401745, doi:10.1126/science.1158708. ]: Geological Society of America Abstracts with Programs, v. 25, A-378. Depth equivalence for the deeper, unconsolidated Coastal Plain sediments from 0.2 second twt to 0.9 second twt is about 262-328 ft. The Chesapeake Bay Impact Crater | U.S. Geological Survey - USGS.gov Griscom D. L., Akiyoshi A., Homae T., Kondo K., Yamanaka C., Ueno T., Ikeya M., Affatigato M., Schue A. There are also several schools of thought about the specific type of impactor that created the crater -- an asteroid, a comet, or a fragment of a larger NEO that may have also been responsible for the Toms Canyon impact in New Jersey and possibly the Popigai crater in Siberia, Russia. Similar to the Aquia, the Marlboro Clay is present north and northwest of the crater but is too thin to resolve in the seismic sections. The crater was then buried by additional sedimentary beds, which accumulated during the following 35 million years. These anomalies help explain the distribution of saline water in the Virginia Coastal Plain aquifers and need to be fully considered in any revisions of the conceptual hydrogeologic framework and existing ground-water-flow models of the aquifer system. The upper Cenomanian beds (100 to 200 ft thick) are found in all three coreholes and appear to have the most consistent distribution of the three units. The Chesapeake Bay Impact Crater: An Educational Investigation for The sediment layers surrounding the crater, on the other hand, were already partly consolidated, and so the mushy breccia compacted much more rapidly under its subsequent sediment load than the surrounding strata. The distribution of crater-fill materials i n the CBIS is related to the morphology. Earth impacts are a very infrequent event, but having a 100-year impact event, doesnt mean it will be another hundred years until it happens again. These lower Miocene beds contain distinctive marine bioclastic sands that were deposited along the outer rim and western side of the annular trough. Transcript Show Transcript The impact crater created a long-lasting topographic depression which helped predetermine the course of local rivers and the eventual location of the Chesapeake Bay. Differentiation from the underlying Delmarva beds is discussed in the previous section. (3) Synimpact depositional processes, including ejecta fallback, massive crater-wall failure, water-column collapse, and tsunami backwash, filled the crater with a porous breccia lens, 6001200 m thick, at a phenomenal rate of 1200 m/hr. 275-285., http://palaios.geoscienceworld.org/content/18/3/275.abstract, Elbra T., Kontny A., Pesonen L. J. The glauconitic sand unit is a loose, fine to very coarse glauconitic quartz sand and appears to be restricted to the more downdip eastern side of the area. ; Family Resources Helpful information from our experts, partners and other trusted resources. Development and refinement of a pollen zonation for these deposits (Brenner, 1963; Robbins and others, 1975) have given more recent investigations a basis for subdividing the sequences into units of temporal and possibly genetic significance (Reinhardt and others, 1980; Meng and Harsh, 1988; Powars and others, 1992). In fact, as late as 18,000 years ago, the bay region was dry land; the last great ice sheet was at its maximum over North America, and sea level was about 200 m lower than at present. Pollen is the only biostratigraphic indicator found consistently in these continental fluvial-deltaic deposits. Differentiation of the marine Chickahominy strata from the overlying Delmarva beds, which have a similar seismic signature, was primarily based on correlation with the coreholes. Below are multimedia items associated with this project. Lithostratigraphic, biostratigraphic, and isotopic data derived from several continuously cored test holes with high recovery rates provide the stratigraphic control for this investigation (pl. The lower 10 ft of the section is much more clayey and probably belongs to the Potapaco Member. Crater, marked by rocks . The outer rim conforms well to the transition zone that separates ground water of high salinity inside the outer rim from fresher water outside the outer rim. Similar to the St. Marys Formation, the Eastover is deep enough inside the crater to be resolved and is represented by long, thin, parallel marine reflectors. (2004) The Chesapeake Bay Crater: Geology and Geophysics of a Late Eocene Submarine Impact Structure, Springer-Verlag, Berlin, 2004.ISBN-10: 3540404414; ISBN-13: 978-3540404415; doi: 10.1017/S0016756805260430, http://www.springer.com/us/book/9783540404415, Poag C. W., Powars D. S., Mixon R. B. The structural depression of Cederstrom (1943), however, has been identified as the CBIC (Powars and others, 1993, 1994). 10). The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units of Southeastern Virginia, South of the James River Professional Paper 1622 Download the complete report as a PDF document(2.8 MB) Download a free copy of Adobe Acrobat Reader version 4.0 In ascending order, they are (1) upper Cenomanian beds consisting of marine and deltaic deposits, (2) a glauconitic sand unit, and (3) red-bed fluvial-deltaic deposits that include multiple paleosols. Subsequent southward progradation of sediments along this headland during interglacial sea-level high formed the lower Delmarva Peninsula and resulted in the southward temporal progression of the Susquehanna paleochannels. Descriptions of the location and geometry of the Chesapeake Bay impact crater are based on correlation of lithostratigraphic and biostratigraphic data from cores and well cuttings, borehole geophysical logs, and seismic-reflection data. Pliocene deposits dip radially away from the center of the impact structure over regions several miles in width, resulting in dips that differ from the typical eastward regional dip of Cenozoic strata. America's Largest Asteroid Impact Left a Trail of Destruction Across The Delmarva beds (lower Oligocene), first described and informally named by Powars and others (1992), are characterized by variable lithology and thickness and include strata that span the planktonic Foraminifera zones P18 through P21a (C.W. This information also is needed to more accurately model and evaluate the ground-water flow and the potential movement of salty water to well fields in the vicinity of the impact crater. Overlying bioclastic to shelly sand deposits of early Miocene age are considered a separate stratigraphic unit in this report. 3 and 4). The crater is now covered by Virginia's central to outer Coastal Plain sediments and the lower Chesapeake Bay. Inside the crater, the Delmarva beds are better sorted and much finer grained (clayey sands) than they are outside the crater, where they become a poorly sorted, intensely burrowed, clayey, fine to very coarse, glauconitic quartz sand. 3 A and B ) is typical across the region and contrasts with the flat multipoint resistivity and single-point resistance lines shown for the uniformly very clayey Chickahominy Formation (figs. "Studies of the Chesapeake Bay Impact Structure - The USGS-NASA Langley Corehole, Hampton, Virginia, and Related Coreholes and Geophysical Surveys." The Effects of the Chesapeake Bay Impact Crater on the Geological Framework and Correlation of Hydrogeologic Units of the Lower York-James Peninsula, Virginia U.S. Geological Survey , U.S. Department of the Interior (2008) Deep drilling into the Chesapeake Bay impact structure, Science, Volume 320, Issue 5884, pp. This transition zone separates saltier ground water inside the crater from fresher ground water present outside the outer rim. A line tracing of seismic-reflection data, including basement data down to 6.0 seconds two-way travel time, shows the seismic "fingerprint" of a bowl-shaped zone of intensely shocked basement rocks down to about 3.5 seconds two-way travel time (about 33,000 to 37,000 feet; 6.2 to 7 miles). Outside the outer rim and disruption boundary of the CBIC, the stratigraphic sequence beneath the lower York-James Peninsula consists of the following geological units: Lower Cretaceous Potomac Formation; upper Paleocene Aquia Formation; uppermost Paleocene to lowermost Eocene Marlboro Clay; lower Eocene Nanjemoy Formation; middle Eocene Piney Point Formation; lower Oligocene Delmarva beds; upper Oligocene to lower Miocene (?) The studies of thecorepresentedinthis volume provide detailed information on the outer part of the crater, including the crystallinebasement, the overlyingimpact-modified andimpact-generated sediments (physical geology, paleontology, shocked minerals, and crystalline ejecta), and the upper Eocene to Quaternary postimpact sedimentary section (stratigraphy, paleontology, and paleoenvironments). The clayey beds commonly contain abundant Foraminifera and scattered shells. 3), the homogeneous lithology of the section here apparently causes the wide spacing of high-amplitude reflectors. The Chesapeake Bay impact crater is the largest in the United States, but geologists discovered it only in 1983. In the Jamestown corehole (see fig. The structure and geometry of the crater were determined by seismic profiling from ships in the bay, according to Gerald Johnson, Professor Emeritus of Geology at the College of William and Mary, who spoke during an August presentation at the Hampton History Museum about the Chesapeake Bay Crater impact and its effect on the geology of Hampton Roads. These cores are stored at the USGS core-storage areas in Reston and Herndon, Va., or at the VDEQ in Richmond, Va. Corehole names are derived from nearby geographic features and include (listed in the order drilled) Exmore, Dismal Swamp, Jenkins Bridge, Fentress, Kiptopeke, Newport News Park 2, Windmill Point, Airfield Pond, and Jamestown. B., Daniels D. L., and Shah A. K. (2009) Gravity investigations of the Chesapeake Bay impact structureGeological Society of America Special Papers, Volume 458, p. 181-193, doi:10.1130/2009.2458(09), http://specialpapers.gsapubs.org/content/458/181.abstract, Poag C. W. (1996) Structural outer rim of Chesapeake Bay impact crater - Seismic and borehole evidence. Within the study area, the Piney Point strata are overlain by shelly glauconitic clayey sands of the Old Church Formation and the Delmarva beds (Oligocene) and underlain by glauconitic clayey sands of the Nanjemoy Formation. The geometry and slope of the escarpment in some places can be characterized as a steep wall; other parts of the escarpment resemble stairs stepping down into the annular trough. The Pleistocene scarps were formed by fluvial, estuarine erosion (valley-facing scarps) and shoreline erosion (coast-facing scarps) caused by changes in sea level that occurred during the glacial-interglacial period. The basement rocks lining the crater cavity were melted, and the basement rocks in a region beneath and around the crater were faulted and fractured. 5), where the Old Church is interpreted to consist only of Oligocene beds. Geological Society of America Abstracts with Programs, Southeastern Section meeting, March 30-31, 1998., Koeberl C., Poag C. W., Reimold W. U., Brandt D. (1996) Impact origin of the Chesapeake Bay structure and source of the North American tektites. The Exmore tsunami-breccia's chaotic seismic signature sharply contrasts with the long, continuous, parallel, high-amplitude reflectors of the overlying Chickahominy Formation strata and is easily recognized. The CBIC was created approximately 35 Ma, when a comet or meteorite struck the inner continental shelf producing a complex impact crater, ejecting a large amount of debris, and generating a series of gigantic tsunamis that spread the debris over most of the U.S. Atlantic shelf (the coastline was then west of the present-day Fall Line; Poag, Powars, Poppe, and Mixon, 1994). This information is critical to revisions of the existing ground-water flow models that have been used to guide water-supply management decisions. These deposits are interpreted as representing stacked deposits of meandering streams, braided streams, and river- and wave-dominated delta-plain and delta-front facies (Glaser, 1969; Reinhardt and others, 1980; Owens and Gohn, 1985; Meng and Harsh, 1988). Poag (USGS, written commun., 1988 and 1993) reported these same beds contain Foraminifera indicating foram zone N18. data, 1990-92). The crater investigation shows that we need to be especially conservative of ground-water use in that area.
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chesapeake bay impact crater
