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Abstract: Formation of High-Frequency Sequences and Their Bounding Surfaces in a Paleogene Supply-Dominated System, Texas Gulf Coast, U.S.A.

L. D. Meckel, III, W. E. Galloway

Short-term variations in accommodation and supply on the continental shelf cause imbalances within longer term dynamic equilibrium conditions. These imbalances result in the deposition of high-frequency sequences. The Eocene Yegua Formation

of the Texas Gulf Coast is made of six such sequences, which have average durations of 0.8 million yr or less. They formed in response to distinct variations in the rate and quantity of sediment supply, increases and decreases in the average grain size carried into the basin, relative sea level position, and the capacity of basinal energy regimes to transport and rework sediments.

Each sequence may be characterized by the mix of particular regime conditions, which were dominant during its de-position. If the rate of sediment supply and the average grain size were more significant factors than the rate of relative sea level change and the capacity for basinal transport, then the sequence was supply dominated and was characterized by deltaic deposition, progradation of parasequences, and river mouth bypassing onto the shelf and slope. If, instead, relative sea level rise and basinal transport were the dominant factors, the sequence was accommodation-dominated and was characterized by estuarine deposition, retrogradation of parasequences, and shoreface bypassing onto the shelf. When the accommodation and supply factors were in equilibrium, sequences containing bot fluvial and estuarine deposits characterized by aggradational parasequence stacking were deposited.

No two consecutive sequences in the Yegua were dominated by the same mix of regime conditions. Therefore, the sequence boundaries are important indicators of changes in regime conditions. Significantly, all of the boundaries are highly correlatable, well-constrained regional marine flooding surfaces. Flooding of the shelf thus apparently either causes or occurs as a result of a fundamental readjustment in the dynamic interplay between sediment supply and accommodation.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994