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Geologic Division ---- Coastal and Marine Geology ---- Woods Hole Field Center

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Location of Chesapeake Bay

image of lower sea level many years ago 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.

effect of impact on surrounding rivers 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?

subsidence of land surface 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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