Using surface-based sequence stratigraphy to determine the stratigraphic designation and age of the Upper Cretaceous Harris Delta System in the subsurface of the east Texas Basin

Abstract

The age and stratigraphic assignment of the Late Cretaceous Harris Delta System in the southern East Texas Basin has always been confusing (Figure 1). These sandstone-prone strata (Figure 1) are typically assigned to the Woodbine Group (Oliver, 1971; Turner and Conger, 1981; Hentz et al., 2014). However, they overlie organic-rich mudstones, coeval to strata mapped as the Lower Eagle Ford Formation (Figure 1) along the outcrop belt (Donovan et al., 2015 & 2019). A grid of regional well-log cross sections across the East Texas Basin revealed, that the classic low resistivity marker that defines the base of the Harris Delta System in the southern portions of the East Texas Basin, can be correlated into the base of the Upper Eagle Ford Formation along the outcrop belt. Furthermore, recent research on shallow boreholes adjacent to the outcrop belt revealed that the mudstone-prone strata at the base of the Upper Eagle Ford Formation, which are interpreted to be coeval to the Harris Delta, contain the positive carbon isotope (δC13) excursion associated with the Cenomanian/Turonian Boundary Event (CTBE). Thus based on these correlations and data, the Harris Delta System can be assigned to the Upper Eagle Ford Formation of Donovan et al. (2015), and interpreted as Latest Cenomanian to Early Turonian in age.