VARIABILITY OF LITHIC BOUNDARIES OF THE PALEOCENE-EOCENE THERMAL MAXIMUM ACROSS THE MARYLAND AND VIRGINIA COASTAL PLAIN: FROM RAPID BURIAL TO EROSIONAL TRUNCATION

Interbedded to wispy, kaolinite-dominated clays and silts of the Marlboro Clay represent the depositional response to the unique environment of the Paleocene-Eocene Thermal Maximum (PETM) across the Maryland and Virginia Coastal Plain shelf. Two new cores (Foxburrow 1, 2) located in MD, provide a more complete section of the updip Marlboro Clay where it is truncated at a burrowed contact by the Nanjemoy Formation (lower Eocene). In comparison, the Marlboro Clay in the MCBR cores, located 1.5 farther north, is truncated by Pleistocene fluvial deposits. Detailed analysis of 8-m-thick sections from these new cores reveals up to 270 stacked, 1-12 cm thick (one is 30 cm), fining-upward packages of silt to clay with numerous clay laminae (9-30/cm). Downdip the Marlboro Clay is thicker (up to 14 m) and consist of wisps of silt and faint laminae that appear more massive. On the uplifted sides of some syn- to early post-depositional faults the Marlboro Clay, in both updip and downdip areas, is completely eroded. The lower contact of the Marlboro Clay is described across the region (and in Foxburrow cores) as burrowed and sharp or gradational or with a transition zone. However we infer a sharp contact with rapid burial that trapped the burrowing organisms. This burrowing mixes the Marlboro Clay with the shelly glauconitic, muddy sands of the Aquia Formation (upper Paleocene). The Marlboro Clay is overlain by the Nanjemoy at a sharp, burrowed unconformity with burrows down as much as ~8 m into the Marlboro Clay. The basal Nanjemoy generally has a transgressive lag deposit of poorly sorted muddy micaceous, carbonaceous, fine to coarse glauconitic-quartz sand with fine phosphate pebbles, small mollusks and Turritella and locally includes small rip clasts of the Marlboro Clay, shark teeth, wood fragments, or a shelly cemented bed right on top of the Marlboro Clay. This lag is also found where the Marlboro has been completely removed by erosion. In a few cores the basal Nanjemoy is much finer-grained, consisting of a muddy very fine to fine sand. Dinoflagellate data indicate that one or more zones are locally missing where the Nanjemoy Formation overlies the Marlboro Clay. In summary, the base of the Marlboro Clay in this region is complicated by bioturbation and the upper extent is variably truncated and not complete.