Palynology of the Bryn Mawr Formation (Miocene): insights on the age and genesis of middle atlantic margin fluvial deposits

Abstract

The ages of fluvial deposits at the head of Chesapeake Bay, thought to be the updip, chronostratigraphic equivalents of a well-dated late Oligocene to Quaternary marine sequence in the Salisbury Embayment, are poorly known. We present data regarding a new occurrence of a palynoflora recovered from the Bryn Mawr Formation in Cecil County, Maryland. The floral assemblage for the Bryn Mawr Formation includes at least 40 taxa at the generic level where Quercus, Cupuliferae, Ilex, Carya, Taxodium, and Pinus are important elements. Most of the taxa identified from the Bryn Mawr Formation palynoflora are extant and occur within the modern middle Atlantic Coastal Plain; however, several important taxa such as Alangium, Engelhardia, Sciadopitys, Tricolporopollenites sp., and Cupuliferoidaepollenites sp. are at present either extinct or exotic to the middle Atlantic Coastal Plain. Comparison of the Bryn Mawr Formation palynoflora to well-dated marine deposits of the Salisbury Embayment suggests a late middle to early late Miocene age (late Serravallian-early Tortonian) for Bryn Mawr Formation phase-2 deposition, supporting previously proposed genetic links to the marine deposits of the Choptank Formation in the subsurface of the Delmarva Peninsula. Relative abundances of common, extant taxa such as Quercus, Carya, Pinus, and NAP (the total non-arboreal pollen) vary considerably throughout late Cenozoic deposits of the middle Atlantic Coastal Plain. We present data for common, extant taxa in a ternary diagram to define discrete palynofacies that discriminate among middle Miocene, late Miocene, Pliocene, and Pleistocene palynofloras. These results show that relative abunances of common, easily identifiable extant pollen may be as diagnostic as exotic taxa in assigning ages to middle Atlantic Coastal Plain deposits. The Bryn Mawr Formation palynoflora, like other middle to late Miocene palynofloras of the middle Atlantic Coastal Plain, suggests terrestrial climatic cooling. In the larger temporal scale of climate change throughout the Cenozoic, this change in Miocene climate is one of three major events that may have had a significant impact on the rate of sediment yield from the Appalachians as recorded in the volume of offshore basin sediments.