David L. GriscomimpactGlass research internationalSan Carlos, Sonora, México

Slightly modified and lengthened from talk presented at the:Penrose Conference “Late Eocene Earth,” Monte Cònero, Italy, October 6, 2007

Geology of the State of VirginiaGeology of the State of Virginia

Silurian and DevonianSandstones

The Chesapeake Bay Structure

SouthernMaryland

Piedmont Coastal PlainFall Line

50 km

50 mi

This and other similar maps taken from K. Frye, Roadside Geology of Virginia (Missoula Press, 1986)

Blue Ridge

Geology of U.S. Mid-Atlantic Coastal Geology of U.S. Mid-Atlantic Coastal PlainPlain

Sandstones:Silurian

Washington, DC

Upper Chesapeake Bay

Silurian and DevonianSandstones

Hypothetical Anticlines ~200 Million Years Ago, Now Eroded

Atla

ntic

Oce

an

Blu

e R

idge

Fall Line

Sand, Graveland Clay (Soft)

350 km

AncientMetamorphicRocks (Hard)

, Devonian

“Upland Deposits”

The “Upland Deposits” of Southern The “Upland Deposits” of Southern Maryland:Maryland:

A 1957 Field Study/Review Article by John SchleeA 1957 Field Study/Review Article by John SchleeAccording to Schlee (Bull. Geological Society of America 68, 1371, 1957):

Geology: A “sheetlike deposit” ~9 m deep dipping southeastward from Washington, DC, covering ~1,600 km2 of southern Maryland

● 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”

• Lithology: (1) an upper “loam member” (~90% quartz silt) ~8 m thick (2) a lower “gravel member” (mostly quartzite) ~1 m thick

Loam90% Quartz

~1 cm

9 m

~1 m

“Upland Gravels”

“Peanut Brittle-Like”

Soft coastal plane sediments

““Upland Deposits” of Southern MarylandUpland Deposits” of Southern MarylandCurrently Accepted Emplacement Mechanism (Hack, 1955; Schlee, Currently Accepted Emplacement Mechanism (Hack, 1955; Schlee,

1957)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

• Geologists presently believe this process to have taken place within the past 10 million years

• This scheme models the 1-m deep “gravel member” only.

— 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.

• This model violates uniformitarian geology. We do not see such things happening today.

““Upland Deposits” of Southern Upland Deposits” of Southern Maryland Maryland

A Contemporary Schematic Cross SectionA 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

The base of “upland deposits” dips at ~1 m/km for ~100 km

NW SE

Crater 120 km

Cretaceous

Miocene (?)Potomac River

Definitely olderthan the crater

Currentlyregarded 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)

-240 -220 -200 -180 -160 -140 -120 -100

-100

-50

0

50

100

150

Ele

vatio

n (m

eter

s)

Distance along Contiguous Sections (km)

Upland Deposits

Idealized ChesapeakeBay Crater

Ejecta Blanket

The “Upland Deposits”The “Upland Deposits” Data, Analyses, and Quotes from John Schlee (1957)Data, Analyses, and Quotes from John Schlee (1957)

Up

lan

d G

rave

ls

Anomalously large boulders up to

~4 m3 are found among the “upland deposits” in many locations.

Schlee’s cumulative frequency distribution of sorting coefficients of the upland gravels is “suggestive of an alluvial fan deposit.”

Log2(Size in mm)

4 8 16 32 64 128 256 mm

“The loam member was not investigated, but structures and textures of the gravels were studied at 98 localities.”

The “Upland Deposits”The “Upland Deposits” Gravel-Size Data and Analyses of John Schlee (1957)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, DCN.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.

128 mm

The “Upland Deposits”The “Upland Deposits” Gravel-Size Data and Analyses of John Schlee (1957)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

0 10 20 30 40 50 602

3

4

5

6

Ancient Potomac River (?)

Cobble Size Reduction Rates

Rhine River Mur River

Mo

da

l Gra

vel S

ize

(P

hi U

nits

)

Distance (km)

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!

The “Upland Deposits” of Southern The “Upland Deposits” of Southern Maryland Maryland

Model of Hack (1955) and Schlee (1957): Model of Hack (1955) and Schlee (1957): Deposition by the Potomac River ~10 to ~3 Million Years Deposition by the Potomac River ~10 to ~3 Million Years

AgoAgo 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!

Was

hing

ton,

DC

Extra

pola

ted

Sourc

e

Problem: The “upland deposits” are far larger than the region studied by Schlee.

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”.

25 km

They extend far southward…

Laterally migrating channelgrowing ~1 m deeper perkilometer of sideways displacement