STABLE-CARBON ISOTOPE COMPOSITION OF ORGANIC MATTER FROM EARLY ANGIOSPERMS PRESENT IN THE ARUNDEL FORMATION (POTOMAC GROUP), MARYLAND, USA

The organic-rich Arundel Formation is one of oldest sedimentary deposits in the Maryland Coastal Plain. This rock unit has become a classic locality for the study of angiosperm origins, due to the abundant presence of plant remains of early angiosperms. A wealth of palynological studies addressing the evolution of early angiosperms places the Arundel Formation in the Aptian/Albian, a time period characterized by the occurrence of carbon-isotope excursions in marine black shales.

Samples were collected from a suitable locality of the Arundel Formation in eastern Maryland to determine the carbon-isotope composition of organic matter and plant remains of early angiosperms. The selected locality corresponds to a continuous, 30-m outcrop in the formerly known United Clay mine. Exposed rocks consist of dark, plastic, massive to diffusely-laminated claystones and mudstones, exhibiting fossil wood and leaves, and nodules of iron carbonate. The fossil leaves are small (< 2cm), mostly incomplete, but well-preserved. Fossil wood varies in size from 3 mm to 5 cm and is typically fragile. The studied fine-grained rocks of the Arundel Formation are interpreted to represent flood-plain sediments, with local swamp deposits. The organic-rich mudstones from the selected locality span the Aptian and possible the Lower Albian as indicated by a previous pollen study on rocks from this locality. About 25 samples of observable fossil leaves and 10 samples of visible fossil wood were collected for isotopic determinations. Preliminary carbon-isotope values from organic matter isolates fall within a small range, varying from about –22 to –23 per mil. These isotopic values are similar to those reported from other localities containing continental organic matter accumulated during the early Cretaceous. Additional isotopic determinations will provide information to test the possibility of global isotopic excursions recorded in both continental and marine organic matter.