Western limit of Taconic allochthons in Pennsylvania

Middle and Upper Ordovician pelitic distal turbidite rocks of the Great Valley in Pennsylvania southwestward from Carlisle are autochthonous and compose the Martinsburg Formation. From Carlisle northeastward to the Lehigh River the Martinsburg Formation is supplanted by lithic units including coarse graywackes, limestone conglomerates, maroon and green mudstones, and cherts that are older than the Martinsburg. These constitute Taconic allochthons introduced as lithified masses from the east by gravity gliding and actual thrusting. Detailed mapping between the Susquehanna River and Carlisle reveals several distinct allochthons and an associated wildflysch of gravitational slide origin. Here, distinctive structures of Taconic, early Alleghanian, and late Alleghanian deformations are recognizable in allochthonous rocks, whereas only a single early Alleghanian deformation significantly affects autochthonous rocks west of Carlisle. Locally, Mesozoic normal faulting also affects the rocks. Taconic-type allochthons in the Great Valley of Pennsylvania are restricted to within and in advance of the Lebanon Valley nappe, which is the farthest traveled among the Reading Prong system of nappes. Emplacement of allochthons into the envelope of the Lebanon Valley nappe was essentially classical Taconic in time and style. Continuing Taconic deformation redeposited some of the Taconic-type material from the envelope into the bathymetrically deepest part of the Martinsburg depositional basin situated in advance of the Lebanon Valley nappe. Regionally, these nappes, which involve shelf-facies rocks cored by continental basement, demonstrate increasing displacement progressively southeast of the type Taconic area. © 1978 Geological Society of America.