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

Response of Eustatic Sea Level Due to Polar-Ice-Cap Formation:Response of Eustatic Sea Level Due to Polar-Ice-Cap Formation:A Major Constraint on Coastal-Plane Deposition RatesA Major Constraint on Coastal-Plane Deposition Rates

Chesapeake Bay Impact (35.5 Ma)

Tri

assi

c

Jura

ssic

Cre

tace

ou

s

Eo

cen

e

Mio

cen

e

Ho

loce

ne

155 m

65 m 0 m

-140 mO

lig

oce

ne

250 65 24 5Time (Millions of Years)

Arctic Ice Sheets

Antarctic Ice SheetsGlobal deep-sea drill-core δ18O studies by Zachos et al., Science 292, 686 (2001)

Sea

Lev

el(m

eter

s ab

ove

pre

sen

t)

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 Iwas more likely deposited earlier than ~34 Ma.

-

-

-

-

-

-

- (Hallam, 1984)

~2 My

600

400

200

0

-200

Why?

It’s harder than quartz!

500 μm

← 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!

Red-Brown Materials: Energy Dispersive AnalysisRed-Brown Materials: Energy Dispersive Analysis

EDX scan was recorded for a quartz-free spot in the red-brown matrix.

QuartzClasts

Matrixof Nearly Pure Iron

Oxide!

Fe

Fe

PSiAlOC

(Data compliments of J. Quick, USGS)

SEM

Fe

keV

This looks like a melt-matrix breccia!

MissingSpall

Internal Fractureswith No ExternalExpression

Dark Stains Contiguous

with the FlangesPenetrate

Solid Rock!!

External Flanges Join Rocks Together

A New Type of ImpactiteA New Type of Impactite Endemic to the Chesapeake Bay CraterEndemic to the Chesapeake Bay Crater

All of these features can have been made by shock waves – and probably in no other way.

Model for Shock-Induced Iron-Oxide Melt Sheets Penetrating Model for Shock-Induced Iron-Oxide Melt Sheets Penetrating Sandstone CobblesSandstone Cobbles

⇒

TransmittedPulse⇒

First Reflection

Vac

uum

⇐

Distance

I

II

III

IV Second Reflection

Unstained Rock

Spall

Probable fossil record of multiplereverberations

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

ConclusionsConclusionsThe “upland deposits” of the U.S. Mid-Atlantic Coastal Plain…

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).

The gravel member of the “upland deposits” is here imputed to interference-zone ejecta (Melosh, 1989) from the Chesapeake Bay crater .

The extreme cobble-size gradient reported by Schlee (1957) is thus reasonably ascribed to atmospheric size sorting (Shultz and Gault, 1979).

Schlee’s (1957) alluvial-fan statistics for the “upland gravels” plausibly indicate that the lower-Cretaceous target rocks included alluvial sediments.

Iron oxyhydroxides precipitated in aquifers of the target area were concentrated and melted by impact shock waves. These ~1-cm-thick meltspenetrated, indurated, and finally welded target gravels into irregular masses ≤1 m (Schlee, 1957), interpretable as “spall plates” (Melosh, 1989).

Further ConclusionsFurther ConclusionsThe clay terraces underlying “upland deposits” of the U.S. Mid-Atlantic Coastal Plain thereforedate from the Late Eocene.

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).

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.

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.”

They contain ~1% K.

EpilogueEpilogue

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.

Inte

rfer

ence

Zon

e

CircumferentialTopographic Low:

Rivers diverted during early post-

impact regressions PresentSea Level

Late EoceneSea 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

-200 -100 0 100 200-1000

-800

-600

-400

-200

0

200

400

600

Ele

vatio

n (

m)

Distance (km)

Plausible Cross Section of Young Chesapeake Bay CraterPlausible Cross Section of Young Chesapeake Bay Crater

Note its inevitable influence on rivers in early post-impact times.

A B

Chickahominy River

Pamunkey River

Mattaponi River

N

20 km

38°

37°N

77° 76°W

Chesapeake Bay Crater: Possible Secondary Impact ChainChesapeake Bay Crater: Possible Secondary Impact Chainand Relict Circumferential Course of the York Riverand Relict Circumferential Course of the York River

Lunar CraterCopernicus:Chain ofSecondaryCraters

York River

Possible SecondaryCrater Chain

Possible PaleoChannel of the

York River