Sequence Stratigraphy of a Continental Margin Subjected to Low-Energy and Low-Sediment-Supply Environmental Boundary Conditions: Late Pleistocene–Holocene Deposition Offshore Alabama, U.S.A.
Louis R. Bartek, Brian S. Cabote, Tonja Young, William Schroeder, 2004. "Sequence Stratigraphy of a Continental Margin Subjected to Low-Energy and Low-Sediment-Supply Environmental Boundary Conditions: Late Pleistocene–Holocene Deposition Offshore Alabama, U.S.A.", Late Quaternary Stratigraphic Evolution of the Northern Gulf of Mexico Margin, John B. Anderson, Richard H. Fillon
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In 1994, The University of Alabama Seismic Stratigraphy Group collected approximately 2,125 km of high-resolution seismic profiles on the continental shelf offshore Alabama, Mississippi, and Florida. This research utilizes these data to characterize the stratigraphy and facies distribution that evolve as sea level fluctuates on a passive continental margin with low storm and wave energy, microtidal range, and low to moderate sediment supply. Emphasis is placed upon determining how changes in the rate of Holocene sea-level rise (fast, slow, punctuated) effect incised-valley fill and deposition. Stochastic models were generated from Q-mode factor analysis, ANOVA, and binomial Markov process analysis to identify quantifiable and statistically significant lateral seismic facies associations and vertical facies successions within a chronostratigraphic framework for the incised valleys and interfluves.
The oxygen isotope Stage 2 lowstand led to bifurcation of the Mobile–Tensaw river system and yielded two distinct incised valleys, an eastern incised valley and a western incised valley, which appear to have formed by headward knickpoint migration. The major incised valleys have geometries and orientations different from the paleo-shoreline, and contain both similar and dissimilar quantifiable seismic facies assemblages, which vary as a result of lateral variations in shelf gradient, sediment supply, and subsidence. Three parasequences and associated flooding surfaces and transgressive retrogradation of depositional systems within the incised valleys are a product of the punctuated relative sea level-rise of the late Pleistocene–Holocene. Presence of a thicker lowstand component and a fluvially dominated lowstand delta at the shelf edge suggests that the western incised valley received the majority of sediment input.
The research presented in this paper shows that the facies architecture of the Mobile incised-valley system has many counterparts throughout geologic history, and supports the validity of using modern or recent systems as analogs to understand the architecture of ancient systems. Given the similarity of the Mobile incised-valley system to ancient systems, it is clear that it is important to quantify facies distribution within the modern system, to develop stochastic models of spatial variability of facies. The stochastic models can be used to assign probability of encountering a particular facies during oil and gas exploration and production or during attempts to remediate contamination in an aquifer of an incised-valley system.
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The northern Gulf of Mexico margin encompasses a variety of depositional settings characterized by different drainage basin size, physiography, fluvial morphology, climatic setting, and structural and diapiric activity. This, plus the abundance of long sediment cores and platform borings from oil industry activities, make it an unparalleled natural laboratory for sedimentological and stratigraphic studies and for testing sequence stratigraphic concepts. This volume contains twelve papers describing results from high-resolution stratigraphic studies of late Quaternary strata of the northern Gulf of Mexico, from the mouth of the Apalachicola River to the Rio Grande. These papers focus on fluvial response to climate and base-level change, variations in delta growth and evolution across the shelf, lowstand delta-fan evolution, the evolution of transgressive deposits on the shelf, the preservation of these deposits. The robust chronostratigraphic frameworks developed for the different study areas allows comparison of stratal geometries produced by contemporaneous depositional systems operating under identical eustatic conditions. This volume will appeal to sedimentologists and stratigraphers interested in source to sink issues, such as how various forcing mechanisms influence strata formation on continental margins.