MICRO-CONTINENTAL MARGIN DEPOSITS OFF EARLY PALEOZOIC LAURENTIA (CENTRAL APPALACHIANS)

The Peach Bottom Area in the Piedmont of south-central Pennsylvania and north-central Maryland is centered on the distinctive Peach Bottom Slate/Quartzite belt. The Peach Bottom Slate is black with 1.5% insoluble carbon and 5 to 20% chloritoid. In contrast, the conformably overlying Cardiff Quartzite indicates deposition in a high-energy environment. Their age and provenance are unknown. Talc and blackwall chlorite occur at the upper contact of the Cardiff Quartzite. Thick (4 to 6 km), siliciclastic (fine- to coarse-grained) metasedimentary sequences lay both southeast (Peters Creek Formation) and northwest (Scott Creek Formation, new name) of the Slate/Quartzite belt. Both formations exhibit cross beds, troughs, and upward-fining cycles that suggest deposition in a subaerial, fluvial environment. The lower Paleozoic Laurentian carbonate shelf to the northwest suggests a southeastern provenance in Iapetus. Sediment accumulated on a micro-continental margin in the Octoraro basin, a part of Iapetus blocked off by micro-continents. The Baltimore Mafic Complex and the Sykesville Formation (an ophiolitic mélange) are two components of an island arc system that was obduced onto the Peters Creek rocks and microcontinents sometime during the Paleozoic.

The Delta Duplex separates the Peters Creek and Scott Creek Formations. It includes the Slate/Quartzite belt and a fragment of the Sykesville Formation. The dominant foliation (S₁) parallels bedding (S₀) across the entire area; both dip moderately steeply (65 degrees) to the southeast across the entire area. The predominance of chlorite over biotite throughout all the metasediments indicates a widespread chlorite grade, greenschist facies metamorphism of indeterminate age. The absence of nappe structures suggests that the Taconic orogeny did not affect these rocks.

The proposed history begins with opening of Iapetus late in the Neoproterozoic. Blockage by microcontinents early in the Paleozoic formed the Octoraro basin, which filled with sediment from the microcontinents. Deep burial during the middle(?) Paleozoic produced bed-parallel foliation under chlorite-grade greenschist metamorphism. Alleghanian décollement tectonism created the steep dip of bedding/foliation, the Delta Duplex, and obduction of the island arc system.