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Evidence is presented that the ejecta blanket of the 35.5-Myr-old Chesapeake Bay crater is still extant and covers ~5,000 km2 of the U.S. mid Atlantic Coastal Plain. (Part 3 of 3)

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  1. 1. The Case for Interpreting the ~5,000 km2 "Upland Deposits" of the U.S. Mid-Atlantic Coastal Plane as Chesapeake Bay Crater Ejecta Part III David L. Griscom impact Glass research international San Carlos, Sonora, M éxico Slightly modified and lengthened from talk presented at the: Penrose Conference “Late Eocene Earth,” Monte C ò n e ro, Italy, October 6, 2007
  2. 2. Response of Eustatic Sea Level Due to Polar-Ice-Cap Formation: A Major Constraint on Coastal-Plane Deposition Rates Chesapeake Bay Impact (35.5 Ma) Triassic Jurassic Cretaceous Eocene Miocene Holocene 155 m 65 m 0 m -140 m Oligocene 250 65 24 5 Time (Millions of Years) Arctic Ice Sheets Antarctic Ice Sheets Global deep-sea drill-core  18 O studies by Zachos et al., Science 292 , 686 (2001) Sea Level (meters above present) The highest point of the Calvert Formation (that I am aware of) is 78 m above sea level. Therefore, a high stand >78 m would have been needed to deposit Calvert I . 33.7 CALVERT I It follows that, Calvert I was more likely deposited earlier than ~34 Ma. - - - - - - - (Hallam, 1984) ~2 My 600 400 200 0 -200
  3. 3. Why? It’s harder than quartz! 500 μ m  3 cm  -> 6 mm External flange Red-brown material penetrates sandstone to uniform depth Thin section viewed under crossed polars reveals multiply fractured quartz grains instantly indurated by the matrix. There are no relative rotations of the fragments!
  4. 4. Red-Brown Materials: Energy Dispersive Analysis <ul><li>EDX scan was recorded for a quartz-free spot in the red-brown matrix. </li></ul>Quartz Clasts Matrix of Nearly Pure Iron Oxide! Fe Fe P Si Al O C (Data compliments of J. Quick, USGS ) SEM Fe keV This looks like a melt-matrix breccia!
  5. 5. Missing Spall Internal Fractures with No External Expression Dark Stains Contiguous with the Flanges Penetrate Solid Rock!! External Flanges Join Rocks Together A New Type of Impactite Endemic to the Chesapeake Bay Crater All of these features can have been made by shock waves – and probably in no other way .
  6. 6. Model for Shock-Induced Iron-Oxide Melt Sheets Penetrating Sandstone Cobbles  Transmitted Pulse  First Reflection Vacuum  Distance I II III IV Second Reflection Unstained Rock Spall Probable fossil record of multiple reverberations Water (not shown elsewhere) Shock Front (Pressure Pulse)  Iron oxide melt sheet overtakes rock Reflected Pulse (Rarefaction) Pressure Pulse Pushes Fe-Oxide Melt into Inter-Granular Spaces Opened by the Rarefaction Pulse
  7. 7. Conclusions The “upland deposits” of the U.S. Mid-Atlantic Coastal Plain… <ul><li>were created 35.5 million years ago by shock waves passing through wet siliciclastic sediments ( including Devonian-source quartzite gravels ) in the target area of the Chesapeake Bay impactor (Koeberl et al., 1996). </li></ul><ul><li>The gravel member of the “upland deposits” is here imputed to interference-zone ejecta (Melosh, 1989) from the Chesapeake Bay crater . </li></ul><ul><li>The extreme cobble-size gradient reported by Schlee (1957) is thus reasonably ascribed to atmospheric size sorting (Shultz and Gault, 1979). </li></ul><ul><li>Schlee’s (1957) alluvial-fan statistics for the “upland gravels” plausibly indicate that the lower-Cretaceous target rocks included alluvial sediments . </li></ul><ul><li>Iron oxyhydroxides precipitated in aquifers of the target area were concentrated and melted by impact shock waves. These ~1-cm-thick melts penetrated, indurated, and finally welded target gravels into irregular masses  1 m (Schlee, 1957), interpretable as “spall plates” (Melosh, 1989). </li></ul>
  8. 8. Further Conclusions The clay terraces underlying “upland deposits” of the U.S. Mid-Atlantic Coastal Plain therefore date from the Late Eocene. <ul><li>The microfossils contained therein represent mostly shallow-water species, many of which likely suffered extinctions consequent to the extreme regressions of the early Oligocene (Hallam, 1984). </li></ul><ul><li>Any lingering doubts about the veracity of these conclusions can be resolved by Ar-Ar dating of the hard ferric-oxyhydroxide materials associated with the Chesapeake Bay crater impactites elucidated here. </li></ul><ul><li>The evidence presented here for the “upland deposits” and the Bacons Castle fm. comprising the ejecta blanket of the Chesapeake Bay crater is merely the “tip of the iceberg.” </li></ul>They contain ~1% K.
  9. 9. Epilogue Otherwise, the following are figures from the manuscript submitted for the Penrose Conference proceedings and were not part of the original lecture. The viewer should understand that the original lecture was extensively animated and that these helpful animations cannot play on SlideShare.
  10. 10. Interference Zone Circumferential Topographic Low: Rivers diverted during early post-impact regressions Present Sea Level Late Eocene Sea Level Haynesville Corehole Langley Corehole Jurassic Lower-Cretaceous Poorly Lithified Non-Marine Siliciclastic Sediments Upper-Cretaceous Marine Paleogene Clays and Marls Granite and Metasedimentary Basement Trough Plausible Cross Section of Young Chesapeake Bay Crater <ul><li>Note its inevitable influence on rivers in early post-impact times. </li></ul>
  11. 11. A B Chickahominy River Pamunkey River Mattaponi River N 20 km 38° 37° N 77° 76°W Chesapeake Bay Crater: Possible Secondary Impact Chain and Relict Circumferential Course of the York River Lunar Crater Copernicus: Chain of Secondary Craters York River Possible Secondary Crater Chain Possible Paleo Channel of the York River