The record of ground zero in the Chesapeake Bay impact crater—Suevites and related rocks
A. Wittmann, W.U. Reimold, R.T. Schmitt, L. Hecht, T. Kenkmann, 2009. "The record of ground zero in the Chesapeake Bay impact crater—Suevites and related rocks", The ICDP-USGS Deep Drilling Project in the Chesapeake Bay impact structure: Results from the Eyreville Core Holes, Gregory S. Gohn, Christian Koeberl, Kenneth G. Miller, Wolf Uwe Reimold
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The International Continental Scientific Drilling Program (ICDP)–U.S. Geological Survey (USGS) Eyreville boreholes through the annular moat of the Chesapeake Bay crater recovered polymict impact breccias and associated rocks from the depth range of 1397–1551 m. These rocks record cratering processes before burial beneath resurge deposits. Quantitative analyses of clast sizes, matrix contents, and distribution of impact melt reveal a shock metamorphic gradient in these impactites. The reason for the low estimated quantity of impact melt in the crater (~10 km3) remains elusive. Possible causes may relate to increased excavation efficiency due to a high ratio of water column and sedimentary target to depth of excavation, an oblique impact, or a buried melt sheet at depth. A plausible petrogenetic scenario consists of a lower block-rich section that slumped from an outer region of the transient cavity into the annular moat ~1.5 min after impact. This blocky debris was mixed with the remains of the excavation flow, which contained a pod of melt entrained in ground-surge debris on top. Subsequently, melt-rich suevites were emplaced that record interaction of the expanding ejecta plume with fallback material related to the evolving central uplift. A clast-rich impact melt rock that likely shed off the central uplift covers these suevites. Incipient collapse of the ejecta plume is recorded in the uppermost subunit, but the arrival of resurge flow terminated its continuous deposition ~6–8 min after impact. Limited thermal annealing allowed preservation of glassy melt and high-pressure polymorphs. Mild hydrothermal overprint in the central crater was likely driven by the structural uplift of ~100 °C warmer basement rocks.