PETROGRAPHY AND PROVENANCE OF MIDDLE PENNSYLVANIAN ALLEGHENY FORMATION AND CONEMAUGH GROUP SANDSTONES OF THE ALLEGHANIAN FORELAND BASIN, SOUTHWESTERN PENNSYLVANIA

The Middle Pennsylvanian Allegheny Fm. and overlying Conemaugh Gp. of southwestern Pennsylvania are foredeep deposits of the Alleghanian foreland basin system. Petrographic analysis of medium to coarse-grained sandstones in these units (minimum of 400 grains point counted for 49 samples) reveals unroofing of lower and middle Paleozoic sedimentary rocks from the Alleghanian fold and thrust belt. The Allegheny Fm. has an average total quartz (Q), feldspar (F), and lithic grain (L) percentage of 92.9:0.1:7. The Conemaugh Gp. is subdivided into the Glenshaw Fm. and the overlying Casselman Fm. The Glenshaw Fm. has an average % Q:F:L of 95:0:5, similar to the Allegheny Fm. Lithic grains in the Allegheny and Glenshaw Fms. are mostly foliated quartz-mica metamorphic types with a minor sedimentary population. The Casselman Fm. has a higher percentage of feldspar and lithic grains with an average % Q:F:L of 76:13:11. Plagioclase is the dominant feldspar in the Casselman and lithic grains include foliated quartz-mica, metasedimentary, and minor sedimentary types. The Casselman also contains a higher percentage of total mica grains (mostly muscovite) than the Allegheny and Glenshaw Fms.

The compositional change between the Allegheny-Glenshaw Fms. and the Casselman Fm. can be attributed to a change from marginal marine to non-marine depositional environments but we also emphasize the potential for progressive unroofing of mature quartz-rich sandstones into less mature sandstones (with more feldspar and lithic grains). This type of change in source rock composition is present in Lower Mississippian through Devonian rocks (foreland basin deposits of the Acadian orogeny) and in Lower Silurian through Ordovician rocks (foreland basin deposits of the Taconian orogeny), all of which were incorporated into the Alleghanian fold and thrust belt. Recent detrital zircon age data (e.g., Gray and Zeitler, 1997, Tectonics, v. 16; Thomas, et al. 2004; J. Geol., v. 112) reveal source rocks as young as Devonian age for Middle Pennsylvanian foreland basin sandstones. Our results, coupled with the detrital zircon data, show that Mississippian-Devonian strata in the fold and thrust belt (e.g., Pocono Fm.-Catskill Fm.) were progressively unroofed as sediment sources for the Alleghanian foreland basin during Middle Pennsylvanian time.