Petrographic Studies of Rocks from The Chesapeake Bay Impact Structure (USA): Implication for Moderate Shock Pressures in Sedimentary Breccias

K Amare

doi:10.4314/mejs.v2i2.57679

Abstract

Shock petrographic investigations were carried out on samples collected from drill cores from the Chesapeake Bay impact structure (USA). The late Eocene Chesapeake impact structure is, at 85 km diameter, currently the largest impact structure known in the United States, buried at shallow to moderate depths beneath continental margin sediments underneath southeastern Virginia. To better define the variety of the samples collected from the shallow drill cores and the shock degrees experienced by the target rocks and breccias in the Chesapeake impact crater, thin section analyses were conducted on more than 50 samples from the various zones of the impact structure. The study involves measurements of the orientations of planar deformation features (PDFs) using a universal stage attached to a petrographic microscope. The aim of this study is to determine the shock pressures of various clasts in the shallow breccia fill of the crater. As a result, we note that the overwhelming numbers of shocked grains, which are now present in the sedimentary breccia, are derived from the basement granitoids. Our studies involved samples from four shallow drill cores (Exmore, Windmill Point, Kiptopeke, and Newsport News).The breccia fill is termed the Exmore breccia, which is dominated by particulates of silt, shocked and unshocked granitic fragments, shale, clay, and free shocked quartz grains. The Kiptopke and Windmill Point samples contained rare fragments showing a variety of different shock effects, whereas the Newporte News samples, show several fragments and impact melt with the evidence shock metamorphism was noted. The most abundantly observed shock indicators are shock fracturing, indicative of shock pressures of less than about 10 GPa, as well as 1-2 sets of PDFs in quartz grains, which is indicative of moderate shock pressures of up to about 20 or 25 GPa. Key words: Chesapeake Bay Crater, PDFs, Shock pressure, Universal stage, Impact structure.

Highlights

Impact cratering is a rapid surface-modification process, which happens when a large meteoroid hits a planet or a satellite (e.g., Koeberl, 1998)
The best diagnostic indicators for shock metamorphism are features that can be studied by using the polarizing microscope. They include planar micro-deformation features; optical mosaicism; changes in refractive index; optical axis angle; isotropization and phase changes. These samples were taken from the following cores: Exmore, Windmill Point, Kiptopeke, and Newport News; all of these have penetrated into the Exmore breccia, but not into the deeper crater filling, which was recently intersected by the 2005/6 International Continental Scientific Drilling Program (ICDP)-U.S Geological Survey (USGS) project (Gohn et al, 2006, 2008, 2009)
The Chesapeake impact structure is a large complex crater of late Eocene age and 85 km diameter, which is hidden beneath the shallow waters of the coastal plain sediments

Summary

Introduction

Impact cratering is a rapid surface-modification process, which happens when a large meteoroid (asteroid or comet) hits a planet or a satellite (e.g., Koeberl, 1998) It is a unique geological process in which vast amounts of energy are released in a small area in a very short time (e.g., Grieve, 1990). The 85-km-wide crater includes the Virginia Coastal plain sediments, the southern part of the Chesapeake Bay, and a small part of the Atlantic Ocean (Fig.2) It includes an inner basin surrounded by a ring of raised basement rock, as well as a flat-floored terrace zone that is bounded along the outer rim by a zone of concentric faulting. Details are described by Gohn et al (2006, 2008, 2009)

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