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Crystalline-Rock Ejecta and Shocked Crystalline-Rock Ejecta and Shocked Minerals of the Chesapeake Bay Impact Structure, USGS-NASA Langley Core, Hampton, Virginia, with Supplemental Constraints on the Age of Impact By J. Wright Horton, Jr., and Glen A. Izett Chapter E of Studies of the Chesapeake Bay Impact Structure— The USGS-NASA Langley Corehole, Hampton, Virginia, and Related Coreholes and Geophysical Surveys Edited by J. Wright Horton, Jr., David S. Powars, and Gregory S. Gohn Prepared in cooperation with the Hampton Roads Planning District Commission, Virginia Department of Environmental Quality, and National Aeronautics and Space Administration Langley Research Center Professional Paper 1688 U.S. Department of the Interior U.S. Geological Survey iii Contents Abstract. E1 Introduction . .1 Purpose and Scope . .1 Study Area and Geologic Setting of the Chesapeake Bay Impact Structure. .2 General Criteria for Shock Metamorphism. .3 Previous Evidence of Impact Metamorphism near Chesapeake Bay. .4 Crystalline-Rock Ejecta and Shocked Minerals in the USGS-NASA Langley Core . .4 Methods. .4 Results. .5 Shocked Quartz Grains . .5 Impact-Derived Clasts of Crystalline Rock. .5 Cataclastic Fabrics and Planar Deformation Features . .5 Clast Composition. 10 Discussion . 15 Implications for the Chesapeake Bay Impact Event . 15 Crystalline Terrane beneath the Coastal Plain . 16 Age of Impact Metamorphism based on Argon Dating of Tektites. 16 Background. 16 Argon Dating Methods . 17 Results of Argon Geochronology . 17 Age Constraints for the Chesapeake Bay Impact . 17 Conclusions. 18 Acknowledgments . 19 References Cited . 19 Glossary. 25 Appendix E1. Descriptions of Matrix and Clast Samples from the Exmore Beds and Crater Unit B in the USGS-NASA Langley Core . 27 Figures E1. Regional map showing the location of the Chesapeake Bay impact structure, the USGS-NASA Langley corehole at Hampton, Va., and some other coreholes in southeastern Virginia. E2 E2. Photomicrographs of selected individual quartz grains from matrix samples of the Exmore beds in the USGS-NASA Langley core showing multiple sets of shock-induced planar deformation features . .6 E3. Stratigraphic column of part of the USGS-NASA Langley corehole showing positions of selected lithic clasts of crystalline rock in the Exmore beds and crater unit B . .7 E4. Photographs of selected lithic clasts of crystalline rock from the Exmore beds and from the upper part of crater unit B in the USGS-NASA Langley core. .8 E5. Photomicrographs showing cataclastic microfabrics in lithic clasts of crystalline rock from the Exmore beds in the USGS-NASA Langley core. .9 iv E6. Photomicrographs of quartz showing intersecting sets of shock-induced planar deformation features in lithic clasts of crystalline rock from the Exmore beds in the USGS-NASA Langley core . 11 E7. Photomicrograph showing the porphyritic texture in felsite (rhyolite) sample NL790.9 from the Exmore beds in the USGS-NASA Langley core. 12 E8. Photomicrographs showing spherulitic devitrification texture in felsite (NL812.55) from the Exmore beds in the USGS-NASA Langley core . 13 E9. Graph showing rare-earth-element (REE) abundances normalized to chondrite abundances (rock/chondrite) for two samples from the USGS-NASA Langley core . 14 Tables E1. Features of selected crystalline-rock clasts from the USGS-NASA Langley core that are interpreted to be possible impact ejecta. E12 E2. Results of X-ray fluorescence spectrometry analyses of two samples from the USGS-NASA Langley core. 13 E3. Results of long-count instrumental-neutron-activation analyses (INAA) of two samples from the USGS-NASA Langley core . 14 E4. Results of individual chemical analyses of two samples from the USGS-NASA Langley core . 14 E5. Total-fusion 40Ar/39Ar ages of North American tektites from Washington County, Ga., and Lee County, Tex.. 18 Crystalline-Rock Ejecta and Shocked Minerals of the Chesapeake Bay Impact Structure, USGS-NASA Langley Core, Hampton, Virginia, with Supplemental Constraints on the Age of Impact By J. Wright Horton, Jr.,1 and Glen A. Izett2 Abstract to the number of sites in the structure where their presence is confirmed. Most of the clasts of crystalline rock that are in and just The USGS-NASA Langley corehole at Hampton, Va., was below the Exmore beds are rounded, detrital, and typical of drilled in 2000 as the first in a series of new coreholes drilled in coastal plain sediments. However, a few have angular shapes the late Eocene Chesapeake Bay impact structure to gain a com­ and consist of cataclastically deformed felsite having aphanitic- prehensive understanding of its three-dimensional character. porphyritic to aphanitic texture and peraluminous rhyolite com­ This understanding is important for assessing ground-water position. Three of these clasts contain quartz grains that display resources in the region, as well as for learning about marine two sets of planar deformation features of shock-metamorphic impacts on Earth. We studied crystalline-rock ejecta and shock- origin. Shock-metamorphosed quartz is an integral part of the metamorphosed minerals from the Langley core to determine cataclastic fabric in these three clasts, indicating that both the what they reveal about the geology of crystalline rocks beneath fabric and the shocked quartz were produced by the same high- the Atlantic Coastal Plain and how those rocks were affected by energy impact event. Some felsite clasts have spherulitic tex­ the impact. tures that may be features either of an impact melt or of preim­ An unusual polymict diamicton, informally called the pact volcanic rocks. Exmore beds (upper Eocene), is 33.8 meters (m; 110.9 feet (ft)) A weighted-mean total-fusion 40Ar/39Ar age of 35.3±0.1 thick and lies at a depth of 269.4 to 235.65 m (884.0 to Ma (±lσ) for 19 analyses of 4 North American tektites records 773.12 ft) in the core. This matrix-supported sedimentary the age of the late Eocene Chesapeake Bay impact event. deposit contains clasts of Tertiary and Cretaceous sediment (ranging up to boulder size) and sparse pebbles of crystalline rock. The matrix consists of muddy sand that contains abundant quartz grains and minor glauconite and potassium feldspar. Introduction Significantly, the sandy matrix of the Exmore beds con­ tains sparse quartz grains (0.1 to 0.3 millimeter (0.004 to Purpose and Scope 0.012 inch) in diameter) that contain multiple sets of intersect­ ing planar deformation features formerly referred to as shock The USGS-NASA Langley corehole at Hampton, Va., was lamellae. As many as five different sets have been observed in drilled in 2000 as the first in a series of new coreholes drilled in some quartz grains. Planar deformation features also occur in the late Eocene Chesapeake Bay impact structure to gain a com­ quartz grains in reworked crystalline-rock clasts in the Exmore prehensive understanding of its three-dimensional structure and beds. Such grains are clearly of shock-metamorphic origin. The stratigraphic framework and its influence on regional ground­ presence of these features indicates that the quartz grains have water resources. We studied crystalline-rock ejecta and shock- experienced pressures greater than 6 gigapascals (GPa) and metamorphosed minerals from the Langley core to determine strain rates greater than 106/second. Thus, the shock-metamor­ what they reveal.
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