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Part2GriscomPenroseConferenceLecture

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Evidence is given 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

Evidence is given 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


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  • 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 II 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. 50 km Atlantic Ocean Richmond, VA Washington, DC Chesapeake Bay Gravity map showing negative gravity anomaly coinciding with the inner basin of the Chesapeake Bay structure. The position of the outer rim is shown as the dashed curve. (After Koeberl et al., 1996). “ Upland deposits” of southern Maryland (studied by Schlee (1957)) (Unsorted siliciclastic sands, silts and striated gravels) Note that incision of the “upland deposits” is mainly the work of major streams and rivers from the west. There are virtually no tributaries cut into these uplands. Both the “upland deposits” and the Bacons Castle fm. appear to armor the underlying clay terraces against pluvial erosion. Fall Line Blue Ridge “ Upland Gravels” of eastern Virginia Bacons Castle fm. 115 km
  • 3. The Chesapeake Bay Impact Structure (Cross section from Koeberl et al., 1996 and Poag, 1997) Koeberl et al. and Poag: The Lower-Cretaceous target rocks are poorly-lithified, non-marine, mainly siliciclastic sediments ~500 m thick. Griscom: It seems possible that these target rocks included alluvial deposits rich in Devonian quartzite gravels – exactly matching the description of the “upland deposits”. 400-360 Ma (Devonian) : Quartzite sandstones deposited Time 250 Ma: Appalachian Mountains folded, uplifting anticlines of Devonian quartzites 140-100 Ma (Lower Cretaceous) : Alluvial fans crept seaward from the Appalachians 35.5 Ma: Impact!!! My View:
  • 4. Projectile Diameter: 6 km Projectile Density: 1500 kg/m 3 Impact Velocity: 30 km/s (Vertical Exaggeration ~330 ) X 33 Hypothetical Coastal Plane Contour Just Before Impact
  • 5. The “jetting” stage is well known in cratering physics …but this may represent the first ever appeal to jetting in an attempt to explain an actual geological feature on the face of the Earth. 15-cm Ball of Marine Chalk Found in an Upland Depression (Sliced Cross Section) Note “toasted” exterior and shattered (brecciated) interior. My Model for the Jetting-Phase: The Ocean – and the Soft Coastal Deposits – Are Chamfered at an Angle of ~0.06 o Present day slope of the base of the “upland deposits” Jetting-Induced Debris Flows
  • 6. “ Upland Deposits”  R -3 4,300 km 3 (Poag, 1997) It is known from explosion experiments that the thicknesses of ejecta blankets follow the -3 rd power of the radius R from the crater center. The normalization factor is determined from the total volume of ejecta, which in turn can be scaled from the known diameter of the crater. About Ten Minutes after Impact 0.06 ° Bacons Castle fm.
  • 7. H.J. Melosh, Impact Cratering – A Geologic Process (Oxford University Press, New York, 1989)  J.N. Head, H.J. Melosh, B.A. Ivanov Science 298 (2002) 1752  The fastest interference-zone ejecta can leave with ~0.5 times the speed v i of the impacting object! ~2.0 Impactor Diameters Interference-Zone Ejecta But let us drop back to the first few tenths of a second… ~0.02 v i
  • 8. But let us drop back to the first few tenths of a second…  GRANITE Vertical Exaggeration X 50 Distance (km) Chesapeake Bay Crater 10 min “ Effective” Interference Zone ( v  0.02 v i ) Frye (1986) found this granite “dropstone” in a mudstone bed 800 km due west of the crater center. 27-kg granite object found among the “upland gravels” 200 km northwest of the crater center (speaker’s front yard). Lower Limit of Interference Zone USGS
  • 9. CALVERT I Crystalline Basement: Granites, Metamorphics Marine Clays Cretaceous Non-Marine Siliciclastic Sand and Gravel Clay Confining Units “ Effective” Interference Zone Precipitated Ferric Oxyhydroxides Ground Water with Dissolved Fe ~212 km 3 Vertical Exaggeration X330 I propose that the “upland gravels” are interference-zone ejecta comprising pre-formed alluvial, mostly-quartzite gravels >2 mm that were subject to size sorting by atmospheric drag during ballistic flight. Tysons ~90 km 3 gravel >2 mm Washington Loam = base surge or silt?
  • 10. CALVERT I “ The Day After” About 500,000 Years Later 35.5 Ma Chickahominy Fm. CALVERT II “ Exmore Breccia” Glass ~35.0 Ma About Two Million Years Later Present The two Calverts are diachronous in anyone’s model. In my model Calverts I and II are diachronous across the time plane of just 2 millon years. In the canonical model Calvert II rests on a formation supposedly ~28 m.y. older! However, materials deposited on the crater floor ca. 8 Ma should have been removed by fluvial erosion during the low stands of the Quaternary. N.B. The fossils in Calverts I and II are neritic species. Lower Eocene clays ??? ??? ~8 Ma