Abstract: Sequence Stratigraphic Implications of Quaternary Shelf-Margin Deltas, High Island Area, Northwestern Gulf of Mexico

Robert A. Morton, John R. Suter

High-resolution seismic reflection profiles and foundation borings from the High Island area of the Gulf of Mexico reveal the physical attributes of several wave-modified, river-dominated Quaternary shelf-margin deltas deposited during successive gladoeustatic lowstands. Three retrogradationally stacked depositional sequences can be distinguished by parasequence stacking patterns, stratal relationships on seismic profiles, and lithologic changes in borings. Each progressively younger deltaic sequence is thinner and exhibits a systematic decrease in the abundance and concentration of sand, attributed to a shift in the axis of trunk streams and greater structural influence through time. Contemporaneous structural deformation controlled the axes of major fluvial channels, the oblique dir ctions of deltaic progradation at the shelf margin, and the thickness of each sequence. Incised fluvial systems become deltaic distributaries that terminate landward of the shelf margin, which shows no evidence of submarine canyon formation. Apparent downlap surfaces that separate the deltaic sequences are actually zones of asymptotic reflections that become progressively higher and younger in the direction of progradation. These downlap zones are composed of marine muds without high concentrations of shell debris that would be expected in condensed sections. Rapid subsidence and eustatic rise quickly submerged the shelf-margin deltas below wave base, preserving some thick paleosols that developed on the aggrading delta plain during prolonged subaerial exposure. Transgressive ravinement f each sequence increases updip of the shelf margin and landward of major growth faults where subsidence was relatively low. These high-frequency sequences produced by the interaction of eustasy, sediment supply, salt tectonics, and growth faulting provide valuable analogs for source, reservoir, and seal facies in deltaic settings.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994