PLANAR DEFORMATION FEATURES IN QUARTZ GRAINS FROM A DEEPLY BURIED, CANDIDATE IMPACT STRUCTURE, CENTRAL NEW YORK

Previous geophysical study defined a ~ 3.5-km diameter candidate impact structure (Bear Swamp) in the Finger Lakes area of central New York. This annular structural feature is buried at an average depth of ~ 1.2 km. This feature appears to have a central peak and an apparent crater moat. Previous bore-hole data and samples indicated a structure-filling sequence consisting of lithic arenite and carbonate breccia overlain by finely laminated, presumed post-impact sedimentary unit. The apparent target strata were part of the Queenston Formation, an Upper Ordovician clastic wedge deposit consisting mainly of lower coastal plain sediments. Erosional truncation of the apparent crater rim and abrupt termination of the laminated, presumed post-impact filling unit indicate that the candidate impact structure formed approximately coeval with the basal Silurian transgression. According to previous work, the age of the local post-Tippecanoe transgression is at or near the global Ordovician-Silurian boundary. Even though previous geophysical and bore-hole evidence supported this candidate structure as being of impact origin, evidence such as measurable PDFs in quartz was not reported until now. We have examined several thin sections consisting of impact breccia matrix, which is mainly lithic arenite consisting of quartz, feldspars, mica, and lithic fragments (e.g., clastics, carbonates, and chert). These thin sections were made from a short drill-core segment extracted from a well that penetrated the apparent crater moat adjacent to the structure’s apparent central uplift. Quartz grains bearing PDFs are rare in the thin sections examined, occurring at a rate of about 4-5 per slide. The PDFs are both decorated and fresh and there are 1-3 sets per grain. Quartz with PDFs typically displays one or more PFs, more than one internal domain, at least some undulose extinction, and a slight grey-brown discoloration in plane light. PDFs are mainly low polar angles. Omega (ω) or {10-13}, a plane common in many quartz grains with PDFs from sedimentary targets, is likely the most common crystallographic orientation. We thank Chesapeake Energy, Oklahoma City, for samples and background information.