A DETAILED LOOK AT THE MISSISSIPPIAN MAUCH CHUNK FORMATION, CENTRAL CLINTON COUNTY, PENNSYLVANIA

Two geologic mapping projects in Clinton County, near Lock Haven, Pa, yielded surprising new insights into lithologic characteristics and depositional history of the Mississippian Mauch Chunk Formation (Mmc). The first project, was conducted in the late 1990s in the Tangascootack Creek watershed, located in the Howard NW and Farrandsville quadrangles. Though focused on coal-bearing Pennsylvanian strata, it also included 50 field stations in the Mmc and 5 core holes, all of which penetrated the upper Mmc. The second project, conducted from 2017-2019 in the Farrandsville quadrangle, included over 450 field stations and a 500’ core hole containing the entire Mmc.

The 400’-thick Mmc can be separated into three informal members. The lower member consists of three parts: the basal 60’ is red beds with deeply weathered paleosols; the middle is 40’ of thin-bedded, crossbedded, micaceous, noncalcareous to locally calcareous gray sandstone; and the top is 50’ of red claystone with siltstone interbeds, paleosols, and several marine fossils near the top. The middle member is the Loyalhanna. This unit is a calcareous sandstone with high-angle crossbedding and contains scattered, well-rounded grains, some of which are frosted. The Loyalhanna is exceptionally thick at 175’ and contains unexpected components, including bioturbation and horizontal laminations in the bottom half, and rare ripup clasts of clay and limestone near the top. The upper member consists of up to 160’ of noncalcareous to calcareous sandstone, siltstone, and olive or red claystone. The sandstone is conglomeratic in places, containing mainly a variety of quartz pebbles, along with some chert and lithic pebbles. The upper member is usually capped by non-bedded claystone, some of which is a high-alumina flint clay.

Erosion during the long period of subaerial exposure at the Miss-Penn unconformity generally beveled the surface of the Mmc progressively deeper to the north and lasted into the Middle Pennsylvanian in this region. This is observed in the study area. At the northern border the upper member and most of the Loyalhanna have been removed. At the same time, less intensive, local erosion further sculpted the top of the Mmc forming up to 70’ of relief. Flint clay developed on paleotopographic highs while sand and gravel of the Pottsville Formation filled the lows.