Thoughts on the early Paleozoic tectonic setting of the Pennsylvania piedmont

Blackmer, Gale C., Pennsylvania Geological Survey, 3240 Schoolhouse Rd., Middletown, PA 17057.

Some commonly-held assumptions shape current models of Early Paleozoic paleogeography of the central Appalachian Piedmont.  Among these are: 1) The lateral distribution of units reflects their original lateral distribution (a possible exception is the Wissahickon Formation); 2) The Setters Quartzite and Cockeysville Marble are regionally continuous platform deposits; 3) The presence of ocean-floor metabasalts implies that some units were deposited on oceanic crust.  The resulting model is Grenvillian basement with a platform-type cover sequence adjacent to pelitic schists representing ocean floor deposits.  One alternate version (Faill, 1997) makes the basement/cover assemblage a microcontinent.  In this version, the Octoraro and Peters Creek sediments accumulated in the sea northwest of the microcontinent, and the Wissahickon sediments were deposited in the ocean basin southeast of the microcontinent and later thrust over the other units.

New mapping in Chester County, Pennsylvania introduces some additional observations: 1) Locally, marble is surrounded by schist and is not everywhere associated with basement gneiss; 2) Quartzite and marble have interlayers of highly (50-75%) microcline-rich rock (evidence of trachytic volcanism involving continental crust?); 3) The Wissahickon Formation in Chester County hosts both ocean-floor and continental-initial-rift metabasalts (Smith and Barnes, 1994; Smith, pers. com.).  In addition, we can alter one of our previous assumptions to recognize that thrusting can laterally distribute units originally in vertical succession.

Incorporating these observations, the picture for Pennsylvania looks like this: quartzose clastic rocks irregularly overlying basement, succeeded by marble and/or pelitic schist containing discontinuous lenses of marble and metabasalt with geochemistry evolving from continental rift to ocean floor, tracing the progression of rifting into drift.  This is more like the sediments found in modern and ancient continental rift basins (e.g., Gulf of Suez, Ocoee Supergroup) than those found on continental margins.

Poster paper presented at 38th Annual Meeting of the Northeastern Section Geological Society of America, Halifax, Nova Scotia, March 28, 2003.

Reference:

Blackmer, G. C., 2003, Thoughts on the early Paleozoic tectonic setting of the Pennsylvania Piedmont: Geological Society of America Abstracts with Programs, v. 34, no. 1, p. 30.