WHAT DOES “SURFICIAL” MEAN IN THE VIRGINIA COASTAL PLAIN?

The unconsolidated to semi-consolidated Coastal Plain geologic province of Virginia (CP) presents challenges when determining what is a surficial map unit due to its transitional nature, complex stratigraphy, and dynamic surface processes during the Quaternary period. In other geologic provinces throughout the state, loosely compacted and mostly unconsolidated sediments overlying bedrock are mapped as surficial deposits. The 1:500,000-scale 1993 Geologic Map of Virginia was digitized in 2003, with surficial and bedrock units remaining in a single feature class. In 2021, the GIS data was updated with recent geologic information and upgraded to a GeMS geodatabase with funding assistance from the USGS STATEMAP Program. Simplified colluvial and alluvial deposits were delineated as polygons using a 10 m LiDAR derived hillshade and digitized at a scale of 1:24,000. These polygons were added to a mapunitoverlaypolys feature class (MUOP) designed to overlay bedrock and CP sediment polygons assigned to a mapunitpolys feature class (MUOP), creating the first two-layer geologic map for Virginia.

While the layering exercise is straightforward in four of the five geologic provinces, this relationship becomes more complicated in the CP. For example, the Fall Zone of Virginia contains discontinuous and relatively thin cappings of CP sediments that unconformably overlie crystalline bedrock, and the underlying bedrock beneath these capping can be discerned with some confidence. These CP sediments become less dissected and more laterally continuous to the east of the Fall Zone and the underlying bedrock is not exposed. The discontinuous CP deposits in the fall zone are assigned to MUOP and the continuous deposits to the east of the fall zone are assigned to MUP. The issue with this approach is that these thin deposits, now in the “surficial” layer, thicken to the east and become what should be considered “bedrock” units of the Virginia CP. This functional layering is more consistent with bedrock mapping, but the unconsolidated nature of CP sediments makes them challenging to classify as such particularly in the shallow subsurface. Is it possible to have a systematic approach to how we classify surficial deposits in the Virginia CP and how can we communicate this information within the established GeMS schema?