Part3GriscomPenroseConferenceLecture
Transcript of Part3GriscomPenroseConferenceLecture
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