SOFT-SEDIMENT DEFORMATION FEATURES IN THE DEVONIAN-MISSISSIPPIAN TRANSITION “HAYSTACKS” SANDSTONE, NORTHEASTERN PENNSYLVANIA: A PALEOSEISMITE?

The “Haystacks” sandstone occurs in the upper portion of the Devonian-Mississippian Huntley Mountain Formation (HMF), in the easternmost expression of the Bernice-Mahoopany and Noxen synclines just north of the Allegheny Structural Front, northeastern Pennsylvania. The Haystacks have undergone a distinctive disturbance not recorded in the bounding HMF rocks and represent a local time-stratigraphic marker. The HMF is a ~200 m thick, dominantly nonmarine clastic succession composed primarily of fining upward cycles of fluvial sandstones with minor siltstone and shale. The type locality for the Haystack sandstone occurs in and along a portion of Loyalsock Creek in the northwestern-half of the Laporte 7.5-minute quadrangle, Sullivan County, PA. The Haystacks are distinct lithologically and petrographically from the enveloping cross-bedded, planar to subhorizontal, fluvial sandstones of the upper HMF. The Haystacks are highly indurated and massively bedded with no primary relict internal sedimentary structures but 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. The contact between the underlying HMF and the Haystacks is characterized by a sharp, planar bottom surface. The upper contact with the overlying HMF is poorly exposed, but the Haystacks have a hummocky upper surface with as much as two meters of relief that decreases with distance from the type-locality. Petrographically, the Haystacks are distinct from the HMF in that they are typically angular to subangular monocrystalline quartz, which is super-cemented with virtually no clay matrix. The known outcrops of Haystack sandstone suggest deposition as a thin, laterally persistent sheet of sediment that tapers along its edges. The outcrops are interpreted to a large, ovoid lens running approximately 30 km NW-SE and 55 km NE-SW with an approximate present-volume of Haystack-type rocks of at least one cubic kilometer.