BEDROCK CONTROL ON MARINE SEDIMENT DEPOSITION AND PRESERVATION IN THE EASTERNMOST PIEDMONT: RESULTS FROM 1:24,000-SCALE GEOLOGIC MAPPING IN THE PETERSBURG, VA AREA

Recent 1:24,000-scale geologic mapping in the Petersburg, Virginia area completed by the Virginia Department of Mines, Minerals and Energy (DMME), funded by the National Park Service and STATEMAP, has revealed numerous marine sedimentary deposits that have previously only been documented east of the fall zone. These marine deposits sit unconformably on top of fresh to saprolitic Petersburg Granite and are unconformably underneath younger fluvial-estuarine and nearshore marine deposits. The upper surface of the granite in this area undulates in a manner that is generally consistent with the overlying topography of the Eastern Piedmont but there is some variability in the depth to bedrock. The buried marine units appear to be constrained laterally by the geometry of topographic lows in the surface of the granite where it seems to form a depression. These depressions within the Petersburg Granite provided adequate accommodation space for deposition and perhaps enhanced preservation of sediments during subsequent transgressions. Many of the buried marine sediments documented in the Petersburg, VA area do not crop out at the surface and were discovered using DMME’s trailer-mounted drill, explaining why they were not recognized by previous workers mapping outcrops along drainages. Sedimentary marine deposits are direct evidence of past marine transgressions and their occurrence can help constrain the overall magnitude of flooding associated with respective transgressions. Further analysis of these preserved marine sediments will provide critical details regarding their depositional age and will further refine the environment of deposition. Ultimately, the presence and documentation of these marine sediments could help future workers correlate specific marine transgressions to their respective scarps and help us better understand the magnitude of flooding associated with specific marine transgressions, further refining our understanding of the landward extent of transgressive flooding in the recent geologic past.