Chesapeake Bay impact crater

From Academic Kids

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The Chesapeake Bay Impact Crater was formed by the impact of an extraterrestrial bolide that hit about 35.5 million years ago, in the late Eocene Epoch. It is one of the best-preserved 'wet-target' or marine impact craters, and the largest impact crater in the USA. Continued slumping of sediments over the rubble of the crater have helped shape Chesapeake Bay.

During the warm late Eocene, sea levels were high, and the Tidewater region of Virginia lay in the coastal shallows. The shore of eastern North America, about where Richmond, Virginia is today, was covered with dense tropical rainforest, and the waters of the gently sloping continental shelf were rich with marine life that was depositing dense layers of lime from their microscopic shells.

The bolide impacted at a speed of many kilometers per second, punching a deep hole through the sediments and into the granite continental basement rock. The bolide itself was completely vaporised, with the basement rock being fractured to depths of 8 kilometers, and a 'peak ring' being raised around it. The deep crater, 38 km across, is surrounded by a flat-floored terrace-like ring trough with an outer edge of collapsed blocks forming ring faults. The entire circular crater is about 85 km in diameter and 1.3 km deep, an area twice the size of Rhode Island, and nearly as deep as the Grand Canyon.

The surrounding region suffered massive devastation. USGS scientist David Powars, one of the impact crater's discoverers, has described the immediate aftermath: "Within minutes, millions of tons of water, sediment, and shattered rock were cast high into the atmosphere for hundreds of miles along the East Coast." An enormous seismic tsunami engulfed the land and possibly even overtopped the Blue Ridge Mountains. The sedimentary walls of the crater progressively slumped in, widened the crater, and formed a layer of huge blocks on the floor of the ring-like trough. The slump blocks were then covered with the rubble or 'breccia'. The entire bolide event, from initial impact to the termination of breccia deposition lasted only a few hours or days. In the perspective of geological time, the 1.2 km-thick breccia is an instantaneous deposit. The crater was then buried by additional sedimentary beds, which have accumulated during the ensuing 35 million years.

Until 1983, no one suspected the existence of such a crater buried 300–500 meters beneath the lower part of Chesapeake Bay and its surrounding peninsulas. The first hint was a 20 centimeter-thick layer of ejecta that turned up in a drilling core taken off Atlantic City, New Jersey, far to the north. The layer contained the fused glass beads called tektites and shocked quartz grains that are unmistakable signs of a bolide impact.

In 1993 oil exploration revealed the extent of the crater.

The continual slumping of the rubble within the crater has affected the flow of the rivers and shaped Chesapeake Bay. The impact crater created a long-lasting topographic depression, which helped predetermine the course of local rivers and the eventual location of Chesapeake Bay. Most important for present-day inhabitants of the area, the impact disrupted aquifers. The present freshwater aquifers lie above a deep salty brine, making the entire lower Chesapeake Bay area susceptible to groundwater contamination.

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