Studies of drill cores - Chesapeake Bay impact structure

The late Eocene Chesapeake Bay impact structure is among the largest and best preserved of the known impact craters on Earth. In 2004, a multidisciplinary and international drilling project has been accepted by ICDP (the International Continental Drilling Program) for this crater, involving an international research team (the PI of the present proposal is one of the PIs of the ICDP project). The Chesapeake Bay structure is unique among subaerial and submarine impact craters on Earth because: (1) it is a relatively young structure and, in comparison to other known impact structures of such size, very well-preserved; (2) its location on a passive continental margin has prevented tectonic or orogenic disruption or distortion that is typical of many large terrestrial craters; (3) its original location on a relatively deep continental shelf allowed marine deposition to resume immediately following the impact, which buried it rapidly and completely, thereby preventing subsequent erosion; (4) the upper part of the breccia section inside the crater was derived from resurge currents and impact-generated tsunami waves; (5) the breccia body contains a large volume of impact-generated brine; (6) the crater underlies a densely populated urban corridor, whose two million citizens are still affected by crater-related phenomena, such as freshwater availability. Previous studies have raised many questions that can only be answered by a deep drilling project. The ICDP project involves drilling an about 2.2 to 2.5-km-deep corehole near the central uplift of the structure (as defined from seismic and gravity data), to obtain as thick and undisturbed a post-impact succession as possible including a thick section of impactites (hopefully including impact melt rocks), and to reach basement to study the shock barometry in these rocks. This project is aimed at comprehensive, multidisciplinary analysis of the various impactite lithologies of the crater fill to be retrieved through this ICDP drilling project. The work that is covered by this proposal comprises one of the main scientific studies to be done on the impact-derived rocks (i.e., the hard- rock cores) within the international drilling project. It includes complete geochemical, mineralogical, and petrographic documentation and analysis of a complete set of drill core samples through the impactite section. The overall goal is to contribute to the understanding of the chemical and physical processes involved in impact breccia formation (i.e., transient cavity formation) and deposition within the crater and below.

The Chesapeake Bay impact structure (East Coast of the USA) is 35 Myr old and 85 km in diameter, and is one of the largest and best preserved impact structures on Earth. It is also associated with the North American tektite strewn field (one of only 4 such strewn fields on Earth). The structure was drilled in the central part at Eyreville in 2005-2006 during an ICDP-USGS drilling project. Three drill cores (Eyreville A, B, and C), intersecting the impact structure to a total depth of 1766 m, were recovered within the central zone of the structure in the deep crater moat. The study of the impactite section of the drill cores was the subject of the present FWF project. The crater fill comprises post-impact sediments, sediment clast breccias and sedimentary megablocks (the so-called Exmore breccia beds, interpreted as resurge breccias), a large granitic and a small amphibolitic megablock, gravelly sand, suevites and lithic impact breccias (1397-1551 m), and granites/pegmatites and mica schists. In the upper part (above 1474 m) of the section the suevite is mostly melt rich and contains two intervals (5.5 and 1 m thick) of impact melt rock. In the deeper parts of the section (below 1474 m) mostly suevitic polymict impact breccia alternates with large blocks of cataclas-tic gneiss. The drill core studies provided new insights into the structure and formation of the Chesapeake Bay impact structure. Information from the Eyreville drill core a wide range of data, from general (e.g., geologic column) to detailed observations (on, e.g., mineralogy, shock metamorphism). New important information regarding, e.g., overall shape and structure, formation mechanisms, shock metamorphic effects, and nature and volume of melt, were gained.