[meteorite-list] Help with new "Holocene" crater

From: Paul H. <oxytropidoceras_at_meteoritecentral.com>
Date: Fri, 16 Dec 2011 6:58:49 -0600
Message-ID: <20111216075849.L1BXC.405662.imail_at_eastrmwml303>

In ?Help with new Holocene crater, please? at
http://six.pairlist.net/pipermail/meteorite-list/2011-December/081869.html
Ed asked,

"The following structure has been proposed as a crater from
the Holocene Start Impacts:

http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1504.pdf

I do not play a geologist on television, nor am I one in real life,
but if the "floor" of this crater dates from 10,900 BCE, then
that would be the crater floor, and not infill sediments, and
thus the impact itself would have had to have been well after
10,900 BCE?"

The article stated,

"The minimum age was established using data from
a ~7 metre core taken in the central trough, which
almost reached the basement, as defined by seismic
data. Calibrated 14C ages of shells in the sediments can
be extrapolated to give an estimate of the age of the
base of the sedimentary sequence of ~12,900 cal BP,
if no hiatus or older sediments were preserved
between the base of the core and the bedrock. This is
taken to be the youngest possible age of the impact."

The "floor" of the crater was not dated to ~12,900 cal BP.
It is estimated age of the oldest sediments that covers the
crater that dates to that period. This age only represents
the latest period of sedimentation occurred in the area.
Without addition information, it is entirely possible that
either older sediments existed and have been eroded;
there was a period of nondeposition before the accumulation
of the sediments covering it; or a combination of both.
 All that can be said is the crater is older than ~12,900
cal BP and younger than Ordovician. The morphology of
this crater looks likes it has been significantly modified
by erosion, which suggests that parts of the crater and
possibly older sediments have been removed by erosion.

Thie ~12,900 cal BP date corresponds to the point in time
that the deglaciation of this part of Canada occurred. This
would have been the when the ice sheet that covered and
scoured this region melted back to expose the bottom of
the Gulf of St. Lawrence this part of the Canada. My
interpretation of this date is that it represents the point
in time when the Laurentide Ice Sheet melted back
enough to locally expose the bottom of the Gulf of St.
Lawrence and first allow sediments to accumulate on the
eroded and scoured surface of this crater. Prior to that
time, it was likely covered and scoured by Laurentide
Ice Sheet. Thus, this crater likely predates the last
glacial maximum by a unknown period of time much
like the Charity Shoals crater. From what I can see, this
is definitely not a ?Holocene? crater and certainly
predates the Younger Dryas by a significant period of
time.

Best wishes,

Paul H.
Received on Fri 16 Dec 2011 07:58:49 AM PST