MIOCENE-PLIOCENE MARINE SEDIMENT PRESERVATION IN THE FALL ZONE: RESULTS FROM 1:24,000 SCALE MAPPING NEAR PETERSBURG, VIRGINIA

Recent mapping near Petersburg, Virginia, funded by the National Park Service and STATEMAP has further refined the stratigraphy of the westernmost Virginia Coastal Plain sediments, which overlap crystalline rocks of the Piedmont. The contact between Coastal Plain sediments and underlying rock rises in elevation to the west, sediment cover thins to the west and relief on the underlying Petersburg Granite ranges up to 30 feet with varying thickness of saprolite above the granite body.

Data collected via power auger shows interfluves of stacked Miocene-Pliocene marine sediments that are laterally discontinuous and hidden, not outcropping at the surface, covered by younger Cold Harbor, Bacons Castle, or Pleistocene alloformations. The development of this patchwork of mapped sediments is attributed to either erosion/dissection via incision related to sea level drops or depositional environments that would prohibit their lateral extension. Detailed mapping at 1:24,000-scale shows the variable geometry and thickness of saprolite above the granite batholith creating depressions which we argue could be responsible for preserving Miocene-Pliocene marine units under younger fluvial-estuarine Coastal Plain units. The discovery of these hidden depressions through drilling and other borehole records indicates another possible cause for the laterally discontinuous nature of marine coastal plain sediments.