Upper Cretaceous Siliciclastic Depositional Sequences and Their Relationship to Potential Aquifers and Confining Units, Examples from the North Carolina Coastal Plain

Siliciclastic sediments in a 323 m Craven County core provide a record of Cenomanian-Maastrichtian base-level lowering. Through application of wireline log analysis, facies analysis, process sedimentology, and hydraulic head, sequences are identified. ⁸⁷Sr/⁸⁶Sr dates and calcareous nannofossil zonation provide a time framework for sequences and help establish correlation to the regional stratigraphic framework.

Cretaceous sequences vary from fluvial to marginal marine to exclusively marine. In marine sequences, the transgressive systems tract (TST) overlies sequence boundaries characterized by: 1) erosional features, 2)weathering profiles, 3) or faunal and floral gaps. In the middle to outer shelf, the TST consists of fine to coarse, retrogradational, fairly well-sorted, thin siliciclastic sand that is commonly glauconitic. TST parasequences are thin (3 m) and each younger parasequence contains finer sand.

The overlying highstand systems tract (HST) represents a series of aggradational and progradational parasequences that first thicken and then thin upward as accommodation moves laterally. Coarser sediments at the top of the HST usually have better porosity and permeability than sediments in the early, more aggradational part of the HST. Gamma-ray log analysis demonstrates that parasequences in the lower HST have greater clay content that gradually diminishes upward in the late HST. Condensed sections usually exhibit the highest gamma ray response within the sequence and are confining units.

Resistivity measurements through a complete marine depositional sequence indicate that suitable aquifer zones are contained within the thinner TST and the upper parts of the thicker HST. In some cases, hydraulic connectivity exists between the HST of an older sequence and the overlying TST of a younger sequence. In sequences where fluvial, fluvial-lacustrine, and deltaic sediments are common, the lower siliciclastic channel sands and distributary mouth bar complexes have the greatest porosity. The gamma-ray log indicates that these sands have little clay and are suitable aquifers.