Long cores through the Wilcox Group, Gulf of Mexico, show process variability across different time scales

Spatial and temporal changes in facies and depositional processes are frequently reported in shallow-marine deltas and estuaries. However, such process variations are less well documented across (and at the time scale of) transgressive-regressive cycles. We use two continuous long cores to describe the transgressive-regressive cycle that is the fundamental building block for the lower and middle Wilcox Group, Gulf of Mexico, and from there, interpret the preferred changes of river, wave, and tide process and signals within the stratigraphic sequences selected in this 7.5-m.y. succession. This fluvial to shallow-water Wilcox Group is of current interest because it forms the topsets of the Gulf of Mexico continental-margin clinothems. Vogelsang 1 Frieda core (2240 ft thick [682.8 m]) shows 14 transgressive-regressive cycles, and the Letco TOH-2A Settlemyre core (1730 ft thick [527.3 m]) is divided into 11 such cycles, each ranging from 60 to 352 ft (18.3-107.3 m) thick, and with an estimated duration of X300-680 k.y. A complete transgressive-regressive cycle, in core, passes upward from wave-or river-dominated regressive delta-front deposits (30-247 ft thick [9.1-75.3 m]) to fluvial or fluvial-tidal channel (sometimes estuarine) deposits (26-108 ft thick [7.9-32.9 m]), and then to wave-or tide-dominated transgressive deposits (9-97 ft thick [2.7-29.6 m]). Each cycle exhibits differing facies at different levels because the proportions of regressive, fluvial, and transgressive intervals vary according to the location on the cross-shelf transect. The entire Vogelsang 1 Frieda core is 48% wave-dominated deposits, 40% river-dominated deposits, and 12% tide-dominated deposits, whereas the entire Letco TOH-2A Settlemyre core is 69% wave dominated deposits, 24% river-dominated deposits, and 7% tide-dominated deposits. The dominant process in transgressive-regressive cycles (<100 k.y., up to several hundreds of feet thick) is critical for hydrocarbon production and exploration because it defines the type of shoreline and controls the shoreline morphology. We demonstrate the frequent variations of dominant processes across the different time scales and provide a template for fundamental fluvial transgressive-regressive intervals from 25 top set cycles. This is helpful for constructing the essential stratigraphic framework as the basis for internal reservoir architecture.

Automated summary

This study focuses on the spatial and temporal changes of facies and depositional processes in the shallow-marine deltas and estuaries, specifically on the transgressive-regressive cycles. Two long cores were used to describe the cycles and understand the changes in river, wave, and tide process signals. The dominant process in these cycles has significant implications for hydrocarbon production and exploration.