FORMATION OF HIGH-FREQUENCY SEQUENCES AND THEIR BOUNDING SURFACES - CASE-STUDY OF THE EOCENE YEGUA FORMATION, TEXAS GULF-COAST, USA

Short-term variations in accommodation and sediment supply on the cont inental shelf cause imbalances within longer-term dynamic equilibrium conditions. These imbalances result in the deposition of high-frequenc y sequences. The Eocene Yegua Formation of the Texas Gulf Coast is mad e up of six such sequences, which have average durations of 0.8 millio n 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 s ediments. Each sequence may be characterized by the mix of particular regime conditions which were dominant during its deposition. If the ra te of sediment supply and the average grain size were more significant factors than the rate of relative sea-level change and the capacity f or basinal transport, then the sequence was supply dominated and was c haracterized by fluvio-deltaic deposition, progradation of parasequenc es, and river mouth bypassing onto the shelf and slope. If, instead, r elative sea-level rise and basinal transport were the dominant factors , the sequence was accommodation dominated and was characterized by es tuarine and wave-dominated deposition, retrogradation of parasequences , and shoreface bypassing onto the shelf. When the accommodation and s upply factors were in equilibrium, sequences containing both fluvial a nd estuarine deposits characterized by aggradational parasequence stac king were deposited. No two consecutive sequences in the Yegua were do minated by the same mix of regime conditions. Therefore, the sequence boundaries are important indicators of changes in regime conditions. A ll of the boundaries are highly correlatable, well-constrained regiona l marine flooding surfaces. Flooding of the shelf thus apparently eith er causes or occurs as a result of a readjustment between sediment sup ply and accommodation. Continuous readjustments between these factors result in long-term equilibrium conditions.