THE HAYSTACKS SANDSTONE: A PROPOSED DEVONIAN-MISSISSIPPIAN IMPACT EJECTA

The “Haystacks” sandstone is a unique and poorly understood depositional sequence in the Devonian-Mississippian transition of Upper Huntley Mountain Formation (HMF), NE Pennsylvania. The Haystacks are a thin, but laterally persistent sheet of sediment enveloped by fluvial sandstones of the HMF. The Haystacks are massively bedded with no primary relict internal sedimentary structures but also show fine, anastamosing, vein-like structures that are likely dewatering features. The lower portion of the unit has spaced, vertical features that have been interpreted to be pressure solution surfaces. Haystack-type rocks away from the type-locality may show small, subvertical sand-dikes that have downward tapering apophyses with thin lobate tops or ringed tops, which may be interpreted as clastic dikes. In thin section, Haystack-type rocks show no preferred grain orientation and most grains appear to be randomly oriented. Grains are typically angular to subangular monocrystalline quartz, although polycrystalline quartz is not uncommon. The grains are quartz super-cemented with virtually no clay matrix. They are distinct lithologically and petrographically from the under and over-lying cross-bedded, planar to subhorizontal, fluvial sandstones of the upper HMF. While, the bounding sandstone units of the HMF are clearly fluvial in origin, the genesis of the Haystacks has remained unclear. Based on our petrographic study of the “Haystacks”, we propose that it may represent a preserved remnant of an ejecta blanket from a previously unrecognized bolide impact. Eighty-two sets of PDFs were found in ~ 30 quartz grains from a series of seven oriented thin-sections using a four-axis universal stage. Determination of the crystallographic orientation of the PDFs was determined by measuring the angular relationship between the c-axis of each individual grain and the PDFs, then plotting them on the new stereographic projection template of Ferrière et al. (2009). Individual grains contained between one and five sets of PDFs. The most commonly indexed orientations of PDFs were 10‾13, 11‾22, and 10‾14; with every recognized PDF orientation observed except for the 51‾61. Based on studies relating PDF orientations to shock barometry (e.g. Grieve and Robertson, 1976), these orientations would suggest shock pressures of 12 – 20 GPa.