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Short-term variations in accommodation and sediment 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 up of six such sequences, which have average durations of 0.8 million years 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 deposition. 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 fluvio-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 and wave-dominated deposition, retrogradation of parasequences, and shoreface bypassing onto the shelf. When the accommodation and supply factors were in equilibrium, sequences containing both 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. 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 readjustment between sediment supply and accommodation. Continuous readjustments between these factors result in long-term equilibrium conditions. |
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