PETROLOGY OF THE “HAYSTACKS” SANDSTONE, NORTHEASTERN PENNSYLVANIA: A UNIQUE DEPOSITION SEQUENCE IN THE DEVONIAN-MISSISSIPPIAN TRANSITION

The “Haystacks” sandstone is a unique and poorly understood depositional sequence in the Devonian-Mississippian transition of Upper Huntley Mountain Formation (HMF). The Haystacks are a thin, but laterally persistent sheet of sediment enveloped by fluvial sandstones of the HMF. 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 remains unclear. The Haystacks crop out in the easternmost expression of the Bernice-Mahoopany and Noxen synclines, just north of the Allegheny Structural Front. At the type locality, the Haystacks are characterized by a sharp, planar bottom contact and a hummocky upper surface with as much as two meters of relief that decreases with distance from the type-locality. 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. The most distinctive petrographic feature of Haystack-type rocks are Boehm lamellae in a large percentage of the grains. Few grains having Boehm lamellae have been recognized in the bounding sandstones of the HMF. The Haystacks have undergone a distinctive disturbance not recorded in the bounding HMF rocks and represent a local time-stratigraphic marker. The nature of the unit implies that it had unique history, either in terms of the source of the sediment, diagenetic history, or both. Its enigmatic depositional history is still open to debate: Paleoseismite? Bolide impact? Tsunami? Subaerial debris flow?