A PROVENANCE ANALYSIS OF THE SHAWANGUNK FORMATION IN EASTERN PENNSYLVANIA AND WESTERN NEW JERSEY

The Shawangunk Formation is a classic clastic unit that is well exposed through New York, New Jersey, and Pennsylvania due to the erosional resistance of its predominantly coarse quartzose lithology. The unit was deposited in association with Taconic Orogeny tectonic uplift as sediments were shed westward (paleo-northward) from upland sources into foreland basin settings from Ordovician into Silurian time. The transport of Shawangunk sands and gravels into lowland settings has been interpreted to be via high energy braided fluvial systems. In the field area for this study in eastern Pennsylvania and western New Jersey, deposition fined upward as the basin matured with decreased slope gradients through the Silurian, as the Shawangunk environment gradually transitioned into overlying Bloomsburg red bed strata.

Although much work has been done on the unit’s depositional environment, the character of its source materials remains poorly known. Here we present clast lithology, grain size, grain shape, bedform, and grain roundedness data from conglomeratic facies in the Shawangunk Formation toward understanding its provenance in eastern Pennsylvania and western New Jersey. Thin section analysis of a variety of quartz, chert, and lithic clasts is compared with candidate bedrock exposures now outcropping to the southeast of the Shawangunk study sites.

Preliminary data from field exposures show consistency in gravel clast character through measured sections where quartz dominates the clast abundance with a minor chert and lithic component. The sampled conglomerates lack carbonate lithologies, clasts with macroscopic metamorphic foliation, and large pieces of the underlying Ordovician Martinsburg Formation. We compare the quartz clast lithologies to vein quartz in metamorphic basement rocks and Cambrian to Ordovician strata. We also compare quartz clasts to samples of gravel clasts in the Cambrian Hardyston Formation and chert clasts to samples taken from the Cambrian Allentown Formation.

Our examination of possible hinterland source rocks will add to our understanding of the full environmental setting of the Shawangunk Formation and augment published data on the unit’s detrital zircon population.