SEDIMENTOLOGIC STUDY OF MONONGAHELA FORMATION AND CORRELATION OF ITS SEQUENCES IN SOUTHWESTERN PENNSYLVANIA

Sedimentologic study of the beds of Monongahela formation was conducted by the group of students as a course project in Sedimentology. Students described lithology, sedimentary structures, and mineral content of units, measured beds and correlated sedimentary sequences in 0.5 mile long outcrop along Rt. 88 near Brownsville, PA.

Lithologic units described in the outcrop include Pittsburgh coal seam, located at the base of the exposed sequence at the elevation of Monongahela Floodplain, gray shale interbedded with sandstone, organic-rich black shale, siltstones, sandstones, limestones, and argillaceous shale. Beds of the Monongahela formation were interpreted as cyclothem, cyclic sedimentary sequences. The sequences were formed at the landward edge of the shelf under the influence of repeated transgressions and regressions during Pennsylvanian period. Massive seams of coal, such as Pittsburgh coal that reaches thickness of 16 feet, were formed in the tropical swamps in anaerobic conditions. Beds of organic-rich black shales were interpreted as buried soils, formed as a result of transgressions. Massive units of sandstone were probably deposited in a large fluvial system that flowed from the southeast to the northwest across Western Pennsylvania during the Pennsylvanian. Sedimentary structures observed in massive Pittsburgh sandstone were interpreted as soft sediment deformation probably related to slumping. Large ripple marks described in the topmost unit of sandstone document an active wave climate in the lagoonal or lake-type environment. Limestones of Monongahela formation are freshwater limestones, deposited during highstands in the lakes of alluvial plain. Argillaceous shales described in the top part of the sequence were formed in the coastal lagoonal environment.

Students taking course of Stratigraphy have continued investigation of local stratigraphy on 140 square miles study area. Rock units that became recently exposed along new highway revealed deltaic deposits and series of incised channels. New and previously available stratigraphic data is documented in composite stratigraphic column and used for paleogeographic reconstruction of the study area.