DETRITAL HIGH-VOLATILE BITUMINOUS TO SEMI-ANTHRACITE COAL IN MIDDLE MIOCENE COASTAL PLAIN SEDIMENTS, EASTERN SHORE CHESAPEAKE BAY, MARYLAND: EVIDENCE OF SUSQUEHANNA RIVER HEADWATERS ERODING THE VALLEY AND RIDGE PROVINCE DURING THE NEOGENE

The South Dover Bridge (SDB) core was drilled in 2007 to 699.3 ft (213.3 m) near Easton, MD, eastern shore Chesapeake Bay, as part of the Geology of Atlantic Watersheds Project (USGS OFR 2012-1218). Fourteen core samples were also examined for vitrinite reflectance (VR), part of a downhole Atlantic Coastal Plain (ACP) diagenesis/thermal-maturity study (Malinconico & Weems, 2010); five had appropriate vitrinite. SDB downhole VR increases only slightly from 0.27% (40.3 ft; 122.3 m) to 0.29% (699.3 ft; 213.1 m), consistent with VR time-temperature modeling (Sweeney & Burnham, 1990) of these sediments.

The most intriguing organic-petrographic SDB result, however, is unrelated to thermal maturity. The sample at 40.3 ft (middle Miocene Choptank Fm) was from a mm-scale medium-gray organic-rich layer in beige clay. The organic assemblage was dominated by medium- to high-rank (>0.7% Ro) particles of vitrinite and of tri-macerite pieces of coal, that is, particles containing woody plant matter (vitrinite) with embedded fossil charcoal and spores. The VR histogram shows 2 populations: a lower population with values <0.6% (n=17) used to determine maturity plus oxidized vitrinite not included in that mean, and a higher right-skewed population (range 0.7-2.8%; mean 1.33%; median 1.19%; n=100) indicating deposition of detrital high-volatile bituminous to semi-anthracite coal. The nearest source of coals of this rank upstream of SDB is in the Valley & Ridge/Alleghany Plateau of Pennsylvania, north of the Blue (Kittatinny) Mountain Structural Front (BMSF), particularly the Susquehanna River watershed which empties into Chesapeake Bay. Naeser et al. (AJS, 2016) documented, using detrital zircon fission-track ages and lithic detritus, that provenance of ACP sediments did not include material west & north of the Blue Ridge/BMSF, until late early Oligocene-middle early Miocene when the BMSF was breached westward by east-flowing rivers. The biostratigraphic age of the 40.3 ft sample is middle Miocene, dinoflagellate zone DN6 (OFR 2012-1218). While lack of reported similar high-rank coal detritus in older sediments does not indicate unavailability of a Pennsylvania coal source, the Miocene “event” layer at 40.3 ft supports Susquehanna drainage through the BMSF into Chesapeake Bay at that time.