Abstract

The chief expression of Ordovician clastic sediments in Pennsylvania is the Martinsburg group which forms a broad band from the east-central to south-central border. In central Pennsylvania is an isolated area of Reedsville shale. The Bald Eagle conglomerate and sandstone and the Juniata red beds, younger than the Martinsburg and Reedsville, have a wide distribution in central and south-central Pennsylvania. Fossils, age of subjacent and superjacent beds, and other data delimit the Ordovician clastics. The Martinsburg consists of a lower, shaly part (slaty in east) whose age ranges from early Trenton through Eden. Above, the shale may pass into sandstone of early Maysville (Pulaski) age. Locally, the Jonestown red beds, Eden and younger, are continental equivalents of the marine Martinsburg with which they intergrade. Structures, stratigraphy, and paleontology indicate that the marine Martinsburg is twofold: shale below sandstone, not threefold: shale, sandstone, shale, as sometimes stated. The Reedsville is less arenaceous than the Martinsburg and ranges from late Trenton into early Maysville. The Bald Eagle and Juniata are of Maysville-Richmond age. The Martinsburg has a maximum thickness between 3000 and 4000 feet, the Reedsville about 1500, the Bald Eagle and Juniata each range from zero to 1000 or 1500 feet. Fossils are fairly abundant in the Martinsburg and Reedsville but absent from the other units. Metamorphism has altered the shaly, lower Martinsburg in the east into commercial slate. Local, basic, igneous intrusives cut the Martinsburg. The Taconic Disturbance affected the Martinsburg from the Susquehanna Valley eastward, but not the other Ordovician clastic beds. This tectonic interval can therefore be dated post-Pulaski but pre-Richmond. Since it did not come at the close of the Ordovician, it may not be precisely the chronologic correlate of the Taconic Disturbance of New York. The Martinsburg and Reedsville together represent the offlap phase of the Cambro-Ordovician Sedimentary cycle, but the record of terminal events is lost due to Taconic folding and succeeding erosion. The Bald Eagle and Juniata are stratigraphically, structurally, and in provenance distinct from the normal cyclic sequence.

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