Sequence Stratigraphy and Composition of Late Quaternary Shelf-Margin Deltas, High Island Area, Offshore Texas

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1993

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High-resolution seismic profiles and foundation borings from the northwestern Gulf of Mexico reveal the physical attributes of several late Quaternary depositional sequences that were deposited by wave-modified, river-dominated shelf-margin deltas during successive periods of lowered sea level. Each progressively younger sequence is thinner, and overall they exhibit a systematic decrease in the abundance and concentration of sand, which is attributed to a shift in the axes of trunk streams and greater structural influence through time. Results of the study show that (1) contemporaneous structural deformation controlled the thickness of each sequence, the oblique directions of delta progradation at the shelf margin, and the axes of major fluvial channels; (2) a soil zone capping the oldest sequence is a regressive surface of subaerial exposure that was later preserved during marine transgression; (3) the downlap surfaces are not true surfaces but zones of parallel reflections that become progressively higher and younger in the direction of progradation; (4) the downlap zones are composed of marine muds that do not contain high concentrations of shell debris as would be expected in condensed sections; (5) evidence of submarine erosion and reworking of the delta surface during transgression (ravinement surface) is not widely observed probably because rapid subsidence coupled with rapid eustatic sea-level rise quickly submerged the delta plain below the depth of effective wave reworking; (6) no evidence exists that incised valleys or submarine canyons formed along the paleoshelf margin, even though moderately large rivers were present and sea-level curves indicate several periods of rapid sea level fall; and (7) boundaries of these high-frequency type 1 eustatic sequences are flooding surfaces that occupy the same stratigraphic position as boundaries separating parasequences.

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