FINDING THE ELUSIVE CATSKILL DELTA. THREE UNDERGRADUATE RESEARCH PROJECTS IDENTIFY DEVONIAN MARGINAL MARINE STRATA ON OYARON HILL, EAST-CENTRAL NEW YORK

To date, relatively few marginal marine deposits have been recognized and described from the Devonian Catskill clastic wedge strata of New York. Isolated outcrops on Oyaron Hill (Oneonta, New York) expose shallow storm-influenced subtidal portions of the Gilboa Formation, as well as fluvial portions of the overlying Oneonta Formation. Historically, one large “sandstone and shale” outcrop midway up Oyaron Hill was thought to be part of the lower Oneonta Formation and fluvial in origin. However, three student research projects identify marginal marine lithofacies, bedding geometries, and fossils in this outcrop.

The outcrop contains 3 scour-based, meter-scale bedsets made of conglomeratic greenish-gray sandstone beds that truncate subjacent cm-scale gray heterolithic (lenticular to flaser) wave-rippled sandstone and mudshale strata. A few sandstones within the heterolithic strata form graded beds and hummocky cross-stratified beds (tempestites) that bear a low-diversity marine faunal assemblage featuring abundant small bivalves, as well as common spirifierid and rhynchonellid brachiopods. The lowermost sandstone bedset: 1) fills small (up to 1.25 m thick and 3 m wide) channels above the heterolithic strata, 2) bears abundant coarse plant/wood fragments in the basal mud chip conglomerate, 3) and contains prominent thin gray shale beds or drapes between cross-stratified sandstone beds. The middle sandstone bedset exhibits prominent medium-scale trough cross strata with few shale partings. The uppermost sandstone bedset displays both planar bedding and medium-scale trough cross-stratification, locally preserving isolated dune bedforms. A recent rock fall from the upper sandstone bedset revealed the first known marine fossils from the bedsets; a low diversity transported assemblage very similar to fauna of the heterolithic strata. Large concentrations of small convex-up shells line the upper surfaces of multiple sandstone bed partings, and a few larger shells are found within basal mud chip-rich sandstone. The shells are mostly disarticulated but unfragmented.

We interpret the sandstone bedsets as storm- and tide-influenced distal delta distributary channel and channel mouth bar deposits. The heterolithic strata were likely storm-influenced tidal flats adjacent to the delta distributaries.