ASSIGNING SYSTEMS TRACTS TO NON-MARINE FACIES ASSOCIATED WITH THE CONIACIAN TO SANTONIAN, PLEASANT CREEK I AND II SEQUENCES - UPPER CRETACEOUS, NORTH CAROLINA COASTAL PLAIN, USA
As a first iteration, strata were divided into marine and non-marine facies. Calcareous nannofossil zones, Sr-isotope ages, gamma logs, sedimentologic criteria, and published data helped establish sequence boundaries and a nomenclature for shelf successions. These were correlated with global sea level curves. Within each sequence, systems tracts and maximum flooding surfaces were demarcated. Pleasant Creek I (PCI) and Pleasant Creek II (PCII) sequences were defined in downdip cores (KB, Dixon, Etown). PCI is Coniacian to Santonian in age and includes calcareous nannofossil zones CC14, CC15 and CC16. Overlying PCII is Santonian, and includes CC16 and CC17.
For PCI/PCII, KB and Dixon contain no non-marine facies. Updip, Smith and Hope have thick non-marine facies (<125 m), with no marine units at Hope. At Smith, marine to non-marine facies are complexly interstratified, providing keys to the stratigraphic interpretation. Non-marine facies thin southward to Etown, where an 8 m interval forms the highstand systems tract (HST) of PCII; this outcrops on the Cape Fear River.
For the second iteration, process-based graphic logs, based on standardized texture and independent from composition, were produced for a detailed facies analysis within PCI/PCII. Non-marine facies were divided into channel belts and floodplains; paleosol, backswamp, lacustrine, levee, crevasse splay, bayfill, and other specific fluvial to marine facies were identified. Upstream trends in stacking (patterns in facies, grain size, composition, fabric, sorting, and gamma-ray logs) were compared with downstream stacking trends. This evaluation was instrumental in assigning systems tracts to the updip non-marine intervals and helped confirm or modify the sequence stratigraphic framework in the associated marine units.