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Bays and impacts (was RE: Theories on the extinction of dinosaurs)



> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Randy King
>
> Some time back, it was suggested that Hudson Bay could be the result of a
> meteorite impact.  Has that been disproven or was it never viable?

It has been suggested, specifically, that the semi-circular coastline
southeastern part of the Bay (just north of the mouth of James Bay) is an
erosional remnant of an Archean impact.  However, other than the curvature
of that part of the bay (and the similar curvature of the Belcher Islands
within) I don't know of any geological evidence for such an impact (i.e.,
melt breccias, etc.).

Incidentally, as a Chesapeake-region earth scientist, I want to point out
that the Chesapeake Bay is NOT an impact structure!!!

There is evidence that an asteroid impact occured in this region during the
Late Eocene.  However, the crater that was formed was filled in rather
quickly, because by the latest Eocene and Miocene (and up to the
Pleistocene) sedimentation was continous from the Appalachians eastward to
the edge of the continental shelf.

There is some evidence that the fractures from the cratering event influence
groundwater flow in the region, and may indeed influence the surface water
drainage.

However, the Chesapeake Bay is simply a flooded river system.  During the
glacial maxima sea level dropped significantly below current levels, so that
the shoreline moved out towards the edge of the Continental Shelf.  In
response the ancient "Chesapeake River" (with its tributaries the Potomac,
the Susquehanna, etc.) carved their channels and floodplains deeper into the
sediments of the Coastal Plain.  During interglacials (like the current
interglacial), glaciers melt, sea level rises, and what were once low
floodplains are submerged and become estuaries.

So, although the Eocene impact may have had some influence on the course of
the Chesapeake River, the impact is not responsible for the Chesapeake Bay.

Not that this has much to do with dinosaurs...

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-314-7843