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Constructional Subcomponents of a Shelf-edge Delta, Northeast Gulf of Mexico
Johan Sydow (1), Harry H. Roberts (2), Arnold H. Bouma (1), Robert Winn (3)
A dense grid of high-resolution seismic profiles is used to map the constructional subcomponents of a sea-ward-thickening, progradational, clinoform wedge, a late Pleistocene shelf-edge delta situated on the outer Mississippi-Alabama shelf. The clinoform wedge is subdivided into clinoform sets that are bounded by internal truncation surfaces of local extent and their downdip equivalent surface. These truncation surfaces have high-angle toplap terminations below and downlap terminations above and are referred to as internal downlap/toplap surfaces. The downdip equivalent surfaces have downlap terminations above, and are referred to as internal downlap surfaces. In strike-oriented profiles, the clinoforms within a clinoform set have bi-directional downlap, diverging away from a progradational center. Clinoform sets are interpreted as representing the progradational product of a delta lobe (or sublobe) constructional event, and the bounding surfaces. These sets are the result of hiatal to erosional periods during the intervening destructional phase. The alternating constructional/destructional phases are controlled by delta lobe switching processes. The progradational centers are a result of advancing depositional loci related to prograding distributary mouths. Mapping of clinoform sets and their progradational centers reveals the development history of the delta. Relative stacking of clinoform sets and the progressively lower elevations of top-of-clinoforms indicate overall progradation from the northeast to the southwest, forced by eustatic sea level fall. The steep portion (4°-5°) of the clinoforms at a control boring site in Main Pass Lease Area, Block 303, corresponds to the upper, sand-dominated half of the deltaic section, and the low-angle clinoforms correspond, to the mud-dominated, lower portion. The prevalence of steep clinoforms throughout the shelf-edge region and lack of a well-developed prodelta platform, suggest rapid progradation of a sand-rich deltaic system.
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