PETROGRAPHY AND PROVENANCE OF THE NORTHERNMOST SHAWANGUNK FORMATION; HUDSON VALLEY, NEW YORK

The Late Silurian Shawangunk Fm, a white quartz pebble conglomerate with a quartz-rich matrix, is the oldest unit to unconformably overly Ordovician flysch strata in eastern Pennsylvania, New Jersey, and New York State. The unit was deposited as a result of synorogenic to postorogenic sedimentation in the Taconic foreland basin. The Shawangunk Fm thins northward in the Hudson Valley and stratigraphically pinches out approximately 3 km north of Rosendale NY. South of Rosendale, the Shawangunk Fm forms prominent ridges in the northern and central segments of the Appalachian foreland fold-thrust belt. Unlike the characteristically white outcrops of the Shawangunk Fm to the south, the northernmost exposures are gray to dark gray sequences of interbedded quartz pebble conglomerate and coarse-grained sandstone that weather red-brown. These observations are supported by the results of a preliminary petrographic analysis of three samples collected from exposures north of Rosendale that indicate that the Shawangunk Fm at this latitude contains a much higher ratio of matrix to polycrystalline quartzite pebbles than the commonly clast-supported rocks to the south. U-Pb Laser Ablation - Inductively Coupled Plasma - Mass Spectrometry geochronology of detrital zircon was carried out on two of the samples collected north of Rosendale. Sample HV22 was collected from one of the northernmost exposures of the Shawangunk Fm along the Wallkill Valley rail trail in the vicinity of Fourth Lake. Sample HV21 was collected along the same rail trail near Fifth (Williams) Lake, ~1.3 km south of HV22. Both samples have a large ~1.5-0.95 Ga detrital zircon population that we interpreted as derived primarily from (recycled?) Grenville orogen and Granite-Rhyolite province. The samples also show evidence for zircon as young as ~465 Ma, indicating input from the Taconic orogen to the east. Thus, our U-Pb detrital zircon data are consistent with those from the Shawangunk Fm. to the south, but along-strike differences in composition exist, suggesting that potential differences in provenance may exist despite the similar detrital zircon populations.