LATE CRETACEOUS BASE-LEVEL LOWERING, OUTER COASTAL PLAIN, NORTH CAROLINA

Sequence stratigraphic analysis combined with strontium isotope stratigraphy (SIS) performed on the Upper Cretaceous in a 1010 ft continuous core in Onslow County, NC provides a record of base-level lowering. Twenty-six 86Sr/87Sr ratios and corresponding dates determined on foraminifers, ostracods, and mollusks used with lithology and gamma ray log analysis, suggest seven Santonian through Maastrichtian depositional sequences. These sequences are correlated to those identified in the Kure Beach core, New Hanover County, and help establish a preliminary Late Cretaceous sequence stratigraphic framework for the outer coastal plain of North Carolina. The oldest sequence, which occurs between the TD and 881.5 ft contains little shell material. A single 86Sr/87Sr date of 86.1 Ma determined from just below the upper boundary indicates a Santonian age. The next sequence occurs between 881.5 and 790.8 ft, and provides four 86Sr/87Sr dates ranging between 85.0 to 84.0 Ma; this is also a Santonian age. These two sequences correlate to two unnamed sequences in the Kure Beach core. Between 790.8 and 627.0 ft. Sr ratios from the next sequence produce dates between 81.5 to 77.6 Ma and indicate and early to middle Campanian age. The large range in 86Sr/87Sr dates and their vertical distribution suggest this sequence corresponds to Tar Heel I and Tar Heel II in the Kure Beach core; the sequence boundary between the sequences is indistinct. 86Sr/87Sr dates suggest the Maastrichtian-Campanian boundary occurs between 579.4 and 563.35 ft as a 2 Ma gap in dates occurs. Below the boundary, three dates range from 70.7 to 70.4 Ma, whereas above the boundary, four dates range from 68.6 to 66.6 Ma. The older dates indicate this sequence corresponds to the late Campanian Donoho Creek II sequence in the Kure Beach core hole. 86Sr/87Sr dates from the overlying sequence (563.35 – 422.4 ft) suggest correlation to the Peedee I sequence in the Kure Beach core hole. Between 422.4 and 274.8 ft three 86Sr/87Sr dates range between 66.3 to 65.7 Ma and indicate correlation to the Peedee II sequence in the Kure Beach core hole. SEM analysis of the sample splits that produced anomalous dates identified the impact of diagenesis on some of the 86Sr/87Sr ratios.