The Case for Interpreting the ~5,000 km2 "Upland Deposits" of the U.S. Mid-Atlantic Coastal Plane as Chesapeake ...
Geology of the State of Virginia   Silurian and Devonian Sandstones The Chesapeake Bay Structure Southern Maryland Piedmon...
Geology of U.S. Mid-Atlantic Coastal Plain   Sandstones: Silurian Washington, DC Upper Chesapeake Bay Silurian and Devonia...
The “Upland Deposits” of Southern Maryland: A 1957 Field Study/Review Article by John Schlee <ul><li>According to Schlee  ...
Soft coastal plane sediments “ Upland Deposits” of Southern Maryland Currently Accepted Emplacement Mechanism (Hack, 1955;...
“ Upland Deposits” of Southern Maryland  A Contemporary Schematic Cross Section N.B. “Upland deposits” are not present her...
The “Upland Deposits”   Data, Analyses, and Quotes from John Schlee (1957) Upland Gravels <ul><li>Anomalously large boulde...
The “Upland Deposits”   Gravel-Size Data and Analyses of John Schlee (1957) -6.0 -5.5 -4.5 -4.0 -5.0 -3.5 -3.0 -2.5 -2.0 W...
128 mm The “Upland Deposits”   Gravel-Size Data and Analyses of John Schlee (1957) Direction of the Center  of the Chesape...
The “Upland Deposits” of Southern Maryland  Model of Hack (1955) and Schlee (1957):  Deposition by the Potomac River ~10 t...
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Part1GriscomPenroseConferenceLecture

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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 (Part 1 of 3)

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Part1GriscomPenroseConferenceLecture

  1. 1. The Case for Interpreting the ~5,000 km2 &quot;Upland Deposits&quot; of the U.S. Mid-Atlantic Coastal Plane as Chesapeake Bay Crater Ejecta Part I 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. Geology of the State of Virginia Silurian and Devonian Sandstones The Chesapeake Bay Structure Southern Maryland Piedmont Coastal Plain Fall Line 50 km 50 mi This and other similar maps taken from K. Frye, Roadside Geology of Virginia (Missoula Press, 1986) Blue Ridge
  3. 3. Geology of U.S. Mid-Atlantic Coastal Plain Sandstones: Silurian Washington, DC Upper Chesapeake Bay Silurian and Devonian Sandstones Hypothetical Anticlines ~200 Million Years Ago, Now Eroded Atlantic Ocean Blue Ridge Fall Line Sand, Gravel and Clay (Soft) 350 km Ancient Metamorphic Rocks (Hard) , Devonian “ Upland Deposits”
  4. 4. The “Upland Deposits” of Southern Maryland: A 1957 Field Study/Review Article by John Schlee <ul><li>According to Schlee ( Bull. Geological Society of America 68 , 1371, 1957): </li></ul><ul><li>Geology: </li></ul><ul><li>A “sheetlike deposit” ~9 m deep dipping southeastward from Washington, DC , covering ~1,600 km 2 of southern Maryland </li></ul>● A Petrological Oddity within the “Upland Gravels”: “ Secondarily introduced iron oxide locally cements the sand and gravel along definite zones and in large irregular masses up to 3 feet across” <ul><li>Lithology: </li></ul><ul><li>(1) an upper “loam member” (~90% quartz silt) ~8 m thick </li></ul>(2) a lower “ gravel member ” (mostly quartzite) ~1 m thick Loam 90% Quartz ~1 cm 9 m ~1 m “ Upland Gravels” “ Peanut Brittle-Like”
  5. 5. Soft coastal plane sediments “ Upland Deposits” of Southern Maryland Currently Accepted Emplacement Mechanism (Hack, 1955; Schlee, 1957) Silurian & Devonian Sandstone Outcrops Blue Ridge Water Gap Fall Line Mouth of Potomac River Erosion ~65 km ~130 km ~65 km 128 mm Cobbles <ul><li>Geologists presently believe this process to have taken place within the past 10 million </li></ul><ul><li>years </li></ul><ul><li>This scheme models the 1-m deep “gravel member” only. </li></ul>— 37 years before the discovery of the crater ! Shenandoah R. Potomac River “ Ancestral” Potomac River Transport Without Deposition Deposition Without Erosion … and that the 75-year controversy regarding origins had been resolved in 1957. <ul><li>This model violates uniformitarian geology. </li></ul>We do not see such things happening today.
  6. 6. “ Upland Deposits” of Southern Maryland A Contemporary Schematic Cross Section N.B. “Upland deposits” are not present here in this section. However, they do occur at similar elevations ~25 km west (at Tysons Corners, Virginia). Adapted from USGS (2000) Eocene Paleocene Miocene <ul><li>The base of “upland deposits” dips at ~1 m/km for ~100 km </li></ul>NW SE Crater 120 km Cretaceous Miocene (?) Potomac River Definitely older than the crater Currently regarded as younger than the crater Meters 150 – 100 – 50 – 0 – 0 – -50 – -100 – … and it exhibits no hint of ancient shorelines | | | | | | | | 0 20 40 60 80 100 120 140 Distance (Kilometers) Upland Deposits Idealized Chesapeake Bay Crater Ejecta Blanket
  7. 7. The “Upland Deposits” Data, Analyses, and Quotes from John Schlee (1957) Upland Gravels <ul><li>Anomalously large boulders up to ~4 m 3 are found among the “upland deposits” in many locations. </li></ul><ul><li>Schlee’s cumulative frequency distribution of sorting coefficients of the upland gravels is “suggestive of an alluvial fan deposit.” </li></ul>Log 2 (Size in mm) 4 8 16 32 64 128 256 mm <ul><li>“ The loam member was not investigated, but structures and textures of the gravels were studied at 98 localities.” </li></ul>
  8. 8. The “Upland Deposits” Gravel-Size Data and Analyses of John Schlee (1957) -6.0 -5.5 -4.5 -4.0 -5.0 -3.5 -3.0 -2.5 -2.0 Washington, DC N.B. Schlee’s gravel-size contours are labeled by the negative log to the base 2 of the observed modal sizes (phi units). Noting that the contours were more or less equally spaced in phi units, Schlee (1957) perceived a possible exponential progression. To test this notion, he took four additional sets of gravel-size data along four approximately-linear paths running generally southeasterly of the U.S. Capitol.
  9. 9. 128 mm The “Upland Deposits” Gravel-Size Data and Analyses of John Schlee (1957) Direction of the Center of the Chesapeake Bay Crater Mean Direction of Apparent Dip of the Gravel Exposures Washington, DC 32 mm 16 mm 8 mm 4 mm My Interpretation: Atmospheric size sorting of ejecta in flight. * -6.0 -5.5 -4.5 -4.0 -5.0 -3.5 -3.0 -2.5 -2.0 - - - - - Extrapolates to 128-mm cobbles at ~15 km northwest of Washington, DC – where most rocks this size are petrologically different from the upland gravels ! *Schultz, Gault (1979) 64 mm + Factor of 15!
  10. 10. The “Upland Deposits” of Southern Maryland Model of Hack (1955) and Schlee (1957): Deposition by the Potomac River ~10 to ~3 Million Years Ago <ul><li>Problem: Schlee’s cobble-size gradient extrapolates to a source region ~15 km NW of Washington, DC ( ), but no major outcrops of quartzite are found there! The nearest potential source of the Devonian quartzites in the “upland deposits” is ~60 km to the northwest ! </li></ul>Washington, DC Extrapolated Source <ul><li>Problem: The “upland deposits” are far larger than the region studied by Schlee. </li></ul><ul><li>Problem: The cobble-size gradients are much too large for a river unable to cut a deep channel in the “soft easily eroded Coastal Plain sediments” that underlie the “upland deposits”. </li></ul>25 km They extend far southward… Laterally migrating channel growing ~1 m deeper per kilometer of sideways displacement Hypothetical Courses of Ancestral Potomac River
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