[meteorite-list] A Western Crater Field? - Flathead_Lake

[Paul H.]

In “A Western Crater Field?” at
http://six.pairlist.net/pipermail/meteorite-list/2012-May/085463.html
E.P. Grondine wrote:

“Hi all -
A while back, I related to you the Assiniboine memories 
of the Holocene Start Impact Event (HSIE):

… URLs to unrelated previous posts omitted…

“I think that there is data to suspect that the lake 
the Assinboine remembered waz Flathead Lake:

http://en.wikipedia.org/wiki/Flathead_Lake

If you look at the current west-north-west exit of 
the lake in this image:
http://en.wikipedia.org/wiki/File:Flathead_lake.jpg
you will see what appear to be several astroblemes.

The exit of this lake prior to their formation appears 
to have been south down a river which would have 
formed a rather perfect animal migration path. 
Whether they are or not will require proper geological 
field examination.”

The structural and Quaternary geology of Flathead Lake
has been studied in great detailed and the results of these 
studies have been published in a series of peer-reviewed 
journal papers and other scientific publications. This 
research included detailed field study, including geologic 
mapping, of the Flathead Lake area; high-resolution 
gravity surveys of the Flathead Lake area; 270 km of 
single channel, 3.5 kHz reflection seismic reflection data 
of sediment underlying Flathead Lake; a set of eight, 
5 to 11.5 m long, piston cores taken from the bottom of 
Flathead Lake; and additional low-frequency seismic 
data gathered by the United States Geological Survey.

All of this data and research clearly demonstrates a
lack of any evidence for extraterrestrial impact having
been associated with the formation of Flathead Lake and 
soundly refutes any such hypothesis. This research 
demonstrates that Flathead Lake occupies the actively 
subsiding southern end of a rift valley known as the Rocky 
Mountain Trench. The seismic data shows five different
periods of increased tectonic activity and associated 
subsidence have occurred during the last 15,000 years.
The core and seismic data show that acoustically-stratified, 
undisturbed Pleistocene glacial lake and Holocene sediment 
underlies the bottom of Lake Flathead. The cores and 
seismic data demonstrates that the oldest of these sediments
date to about 14,475±150 cal yr BP, the maximum age of 
the oldest varves. This is when the main Flathead Lake 
basin was at least partially deglaciated and lake sediments
could start accumulating within the basin now occupied
by Flathead Lake. Field studies of the area around Flathead
Lake show that the southern shore of this lake is formed 
by a recessional moraine of glacial origin, which acts as 
dam. The north shore of Flathead Lake consists of younger
recessional glacial moraines. Although volcanic ash beds 
are preserved in lake and glacial sediments, there is a 
complete absence of any disturbance or event beds, 
which can be interpreted as being related to any sort of
extraterrestrial impact since the deglaciation of the area
between 13,000 to 15,000 years.

References about Flathead Lake include: 

Edwards, J., 2006, Evidence for Glacial Outburst Floods 
along the Lower Flathead River: Results from Geologic 
Mapping, Geomorphologic Analysis, and a Gravity Survey
near Polson, Montana. unpublished MSc thesis, University 
of Montana, Missoula, Montana.
http://etd.lib.umt.edu/theses/available/etd-02282007-160250/

Hoffmann, M. H., 2005, Sedimentary record of glacial 
dynamics, lake level fluctuations, and tectonics:
Late Pleistocene-Holocene structural and stratigraphic 
analysis of the Flathead Lake basin and the Mission 
Valley, Montana, USA. Unpublished PhD. dissertation,
University of Montana, Missoula, Montana.
http://gradworks.umi.com/32/05/3205772.html

Hofmann, M. H., and M. S. Hendrix, 2010, Depositional 
processes and the inferred history of ice-margin 
retreat associated with the deglaciation of the 
Cordilleran Ice Sheet: The sedimentary record 
from Flathead Lake, northwest Montana, USA
Sedimentary Geology. vol. 223, no. 1-2, pp. 61–74
http://www.sciencedirect.com/science/article/pii/S0037073809002322

Hofmann, M. H., M. S. Hendrix, J. N. Moore, and M. Sperazza, 
2006a, Late Pleistocene and Holocene depositional history 
of sediments in Flathead Lake, Montana: evidence
from high-resolution seismic reflection interpretation. 
Sedimentary Geology. vol. 184, no. 1, pp. 111–131.
http://www.sciencedirect.com/science/article/pii/S0037073805003325

Hofmann, M. H., M. S. Hendrix, J. N. Moore, and M. Sperazza, 
2006b, Neotectonic evolution and fault geometry change 
along a major extensional fault system in the Mission and 
Flathead Valleys, NW-Montana. Journal of Strucutral Geology. 
vol. 28, no. 7, pp. 1244–1260.
http://www.sciencedirect.com/science/article/pii/S0191814106000885

LaFave, J. I., L. N. Smith, and T. W. Patton, 2004, 
Ground-water resources of the Flathead Lake Area: 
Flathead, Lake, and parts of Missoula and Sanders counties.
Montana Ground-Water assessment Atlas no. 2, The 
Montana Bureau of Mines and Geology, Butte, Montana.
http://www.mbmg.mtech.edu/mbmgcat/public/ListCitation.asp?pub_id=10310&

Smith, L. N., 2001. Hydrogeologic framework of the southern 
part of the Flathead Lake Area, Flathead, Lake, Missoula, and 
Sanders Counties, Montana. Montana Bureau of Mines and 
Geology, Montana Ground-Water Assessment Atlas No.2, 
Part B, Map 10.

Smith, L. N., 2004. Late Pleistocene stratigraphy and implications 
for deglaciation and subglacial processes of the Flathead 
Lobe of the Cordilleran Ice Sheet, Flathead Valley, Montana, 
USA. Sedimentary Geology, vol. 165, No. 3-4, pp. 295–332.
http://www.sciencedirect.com/science/article/pii/S0037073803003427
http://gsa.confex.com/gsa/2004RM/finalprogram/abstract_72168.htm

Wold, R. J., 1982. Seismic reflection study of Flathead Lake, 
Montana. Miscellaneous field studies map no. MF-1433, 
scale 1:117,647. U.S. Geological Survey, Reston, Virginia

Best wishes,

Paul H.