29 Sedimentology and Stratigraphy

Tuesday, 13 March 2001
1:50 PM - 3:30 PM

SEDIMENTARY TRENDS IN THE LOWER AND UPPER BALD EAGLE SANDSTONE, MILROY, PA

Paper number: 29-2
Program: Technical Sessions
Time: 1:50 PM – 2:10 PM

Authors

Abstract

Petrographic and petrologic studies of the Upper Ordovician, Bald Eagle Sandstone at Milroy, Pennsylvania have been the focal point of the study. The 510 foot-thick formation contains the lower (Centennial School) and upper (Lost Run) members, with the boundary between these at 192 feet at this particular outcrop. The lower member contains generally fine-grained sandstone with interbedded shale and siltstone, while the upper member consists of coarser grained sandstone and interbedded conglomerate. We sampled at ten-foot intervals from the lower 480 feet of section and performed 300-grain point counts for these samples. The overall section shows a gradual coarsening upward with quartz being the main constituent. Grains are well-rounded suggesting significant transport.

Sandstone from this section ranges from quartz arenite to sublitharenite. Using Dickinson plots, all samples plot as quartzose recycled orogen. Undulatory quartz is most abundant (typically >50%) while nonundulatory and polycrystalline quartz are also main constituents (~25%). In general, rock becomes less quartzose and more lithic-rich in the lower 120 feet of the section. The 0 to 70 foot interval contains mature sandstone, comprised of 87-93% quartz. Between 70 to 120 foot, quartz content ranges between 82-89%. A significant lithic-rich interval falls between 120 and 230 feet in the section with lithic concentrations of 12-17%, and quartz concentrations as low as 76%. This interval encompasses the lower/upper member boundary, with no significant change in lithology at the boundary. Above 230 feet in the section, the sandstone is more quartzose approaching 90% quartz. It is presently unclear if this mid-Bald Eagle lithic pulse is related to uplift in the source area, climatic influences, or increases in sedimentary accommodation space. However, it is a lithostratigraphic signature that can be investigated at other exposures of the Bald Eagle.