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Friday, 16 December 2011

Help with new "Holocene" crater

Help with new "Holocene" crater

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.

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