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Did the Chesapeake Bay bolide affect the location of Chesapeake Bay
itself? The answer is a complicated one. We know that the bay is
nowhere near 35 million years old, which is when the bolide struck. In
fact, as late as 18,000 years ago, the bay region was dry land; the
last great ice sheet was at its maximum over North America, and sea
level was ~200 m lower than at present. This exposed the yellow area
shown in the figure, which now is the bay bottom and continental shelf.
With sea level this low, the major east coast rivers had to cut narrow
valleys across the region all the way to the shelf edge, where they
dumped their sediment loads into deep water. About 10,000 years ago,
however, the ice sheets began to melt rapidly, causing sea level to
rise and flood the shelf and the coastal river valleys. The flooded
valleys became the major modern estuaries, like Delaware Bay and
Chesapeake Bay. But notice that the rivers of the Chesapeake region
converged at a location directly over the buried crater. Is that merely
coincidence? Let's look at some field data.
Here is a map showing the location of three successive buried ice-age channels
of the ancient Susquehanna River (formed from 450,000 years ago to 20,000
years ago). Note that each channel changes course significantly just after
it crosses the rim of the buried crater. This river diversion, combined
with seismic evidence that the post impact units sag and thicken over the
crater, indicates that the ground surface over the crater remained lower
than the areas outside the crater for 35 million years. Why should it do
that?
The two sets of different-colored arrows in this figure represent different
components of subsidence of the land surface. The black arrows represent
subsidence due to loading during the past 35 million years since the impact.
The blue arrow represents subsidence due to compaction of the breccia.
Remember that the breccia is 1.2 km thick, and was deposited as a water-saturated,
sandy, rubble-bearing slurry (like a Jello fruit salad before it jells).
The sediment layers surrounding it were already partly consolidated, so
the mushy breccia would compact much more rapidly under its subsequent
sediment load than the surrounding strata. This produced a subsidence
differential, causing the land surface over the breccia to remain lower
than the land surface outside the crater. Therefore, the river valleys
converged over the crater and were located in those particular places when rising
sea level flooded them. In short, the impact crater created a long-lasting
topographic depression, which helped predetermine the eventual location
of Chesapeake Bay.
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