New biostratigraphic information from the western part of the Hamburg klippe, Pennsylvania, and its significance for interpreting the depositional and tectonic history of the klippe

Abstract

Biostratigraphic investigation of the graptolite and conodont faunas of the western part of the Hamburg klippe of eastern Pennsylvania permits a revised stratigraphic framework and new conclusions regarding its likely tectonic setting. Graptolite and conodont data reveal an almost complete Lower to lower Upper Ordovician zonal section. No Cambrian strata appear to be present, with the possible exception of the uppermost part. During Early to early Middle Ordovician time, medium- to fine-grained siliciclastics and minor carbonate sediments were deposited in a lower slope and rise setting. These sediments were consolidated and incorporated as olistoliths in an olistostrome, possibly as a trench-fill complex, during the Middle Ordovician (Darriwilian 3/4). This olistostrome, which contains large Lower and lower Middle Ordovician fragments within a matrix of shales, siltstones, and sandstones of Da 3/4 age, is herein named the Shellsville Member of the Dauphin Formation. Turbidites, here assigned to the Nyes Road Member of the Dauphin Formation, were also deposited during Da 3/4 time. These rocks interfinger with red beds of the here-named Manada Hill Member of the Dauphin Formation. The red shales, cherts, and associated rocks of the Manada Hill Member are pelagic deposits that range in age from at least early Arenig through middle Llanvirn time. These allochthonous rocks were emplaced as a gravity-generated klippe into the Martinsburg foreland basin during late Climacograptus bicornis or early Dicranograptus clingani time. These three members compose the Dauphin Formation (new) in the western part of the klippe area. Prior to emplacement of the allochthon, syntectonic flysch and scattered wildflysch of the Martinsburg Formation were deposited. Some graptolite faunas from the Martinsburg Formation, where contiguous with the klippe, may be slightly older than those known from areas farther from the klippe. This could indicate an earlier start of deposition in the Martinsburg foreland basin in advance of the allochthon. The klippe occupied a large space in the Martinsburg foreland basin and it diverted deposition in this area until it was finally covered by late Martinsburg age sediment. The Dauphin Formation is now structurally interleaved and folded with the Martinsburg Formation as a result of late Taconian and later Alleghenian tectonism.