NON-TRADITIONAL MAPPING METHODS AND A PROCESS-BASED APPROACH TO ESTABLISH MEANINGFUL STRATOTYPES AND ALLO- UNITS FROM A SEQUENCE STRATIGRAPHY PERSPECTIVE: THE PLIO-PLEISTOCENE OF NORTH CAROLINA'S COASTAL PLAIN

Defining new alloformations for the Plio-Pleistocene of North Carolina’s Coastal Plain requires applying modern exploration methods that integrate outcrops, subsurface analysis, core and comprehensive landscape analysis to define a sequence stratigraphic framework. This region, the emerged updip sector of the Atlantic continental shelf, is a relict Plio-Pleistocene landscape characterized by stairstep topography - a series of terraces and scarps that step down to the coast and into drainages. Relief here is low, outcrops are few, and wetlands cover is extensive. Geomorphic features include nested incised valleys that separate shelf ramps and locally merge with prominent paleoshorelines. Deposits lack lithologic specificity because successive transgressive-regressive cycles include identical facies that repeat in the rock record and are difficult to date.

To date, the Plio-Pleistocene section in NC lacks formally defined allostratigraphic units, and stratotypes that are useful for establishing facies and interpreting the sequence stratigraphic framework. According to the North American Code of Stratigraphic Nomenclature (2009), an allostratigraphic unit is a mappable body of rock that is defined and identified on the bases of its bounding discontinuities, with lithology playing no role in its definition. An allostratigraphic approach can distinguish identically similar, but superposed, contiguous, or geographically separated, discontinuity-bounded deposits; it is compatible with sequence stratigraphy and facies analysis, and readily applies here.

A non-traditional method, called three-dimensional (3D) subsurface mapping, is useful for defining alloformations and establishing stratotypes. This method combines landscape analysis with targeted subsurface analyses along key transects. Maps of interpreted landforms (from high-resolution LiDAR elevation models) provide bases for choosing key transects. Stratigraphy is characterized beneath each landform type using a variety of subsurface methods. Confirmation core is required to define surfaces, facies and the sequence stratigraphic framework. If described using a process-based approach, the stratotype will be useful from multi-disciplinary perspectives. Examples are provided.