DRIFTWOOD-TRANSPORTED MID-MIOCENE LITHIC ERRATICS IN THE CALVERT CLIFFS, MARYLAND COASTAL PLAIN

Rare lithic erratics (5-50mm)were collected over 25 years from the intermittently exposed mid-Miocene shallow marine upper Calvert Formation, S. Kidwell's informal units younging from Parkers Creek Bone Bed (PCBB, Shattuck "zone" #12, 25 % of samples), through her Glossus-Chione Interval (GCI, #13, 4%), and overlying Kenwood Beach Shell Bed (KBSB,#14, 71%), along 2 km of the Calvert Cliffs, western shore of Chesapeake Bay. Of ca. 220 samples collected since 1982, a subset of 191 (collected 1982-1992) was described by John Glaser (Md Geological Survey) in terms of roundness (ranging from angular to rounded)), as well as lithology and putative original provenance (ca. 50% Piedmont quartz, 22 % Coastal Plain phosphorite, lesser amounts of western Piedmont and Appalachian quartzites and sedimentary rocks). Only ca.3% of the pebbles (quartz diorite gneiss) can reasonably be ascribed to a particular source--the Port Deposit Gneiss, near the present mouth of the Susquehanna, ca. 120km NNE of the sampling locality. Distant northern or western sources can be ruled out, due to lack of mafics or fossiliferous Paleozoic sediments. Most or all samples were probably captured in roots of river-bank trees, liberated by bank erosion and transported into the shallow Calvert Sea, serving as independent proxy tracers for terrestrial vertebrate carcasses moved into the same sea via "bloat and float". Common carbonized wood in the sampled strata supports driftwood pebble transport; the long-term average delivery was only of the order one rock per square km seabed per year. Under long-term constant input by rivers, the dramatic difference between pebble density in GCI vs PCBB and KBSB, mirroring that of marine macrofossils, is understood in terms of depositional environments, GCI deposition being farther offshore, farther from riverine driftwood sources, with a less energetic bottom environment causing greater pebble dilution by fine sand and silt. However, no concentrated lag of pebbles was observed at the base of KBSB,as would be expected from Kidwell's proposed PP-3 disconformity at the unit base.