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Taylor Blood, Alfred E. LaPointe, Clifton E. Cran, Mark F. Stanczyk
Regional stratigraphic studies conducted by the Minerals Management Service, Gulf of Mexico, Outer Continental Shelf Region, on the offshore Texas shelf have identified at least eight major and several minor unconformities within the Cenozoic sediments. The identified erosional features associated with these unconformities include structural truncation, valley incision, and submarine canyon formation, which are the characteristics of a Type I unconformity as defined by Vail et al. (1977). There is also evidence for onlap by the overlying beds. The major unconformities are observed on the upthrown and downthrown sides of regional growth fault systems which are roughly parallel to the Texas coast. Where data quality permits, the major unconformities are identified over most of the Texas shelf. The minor unconformities are observed in the expanded sections on the downthrown blocks of major growth faults only. Although these erosional surfaces may also be Type I unconformities, no evidence of erosion is observed on the upthrown sides of the major growth faults.
The unconformities of Oligocene and lower and middle Miocene age were initially interpreted from electric log correlations and subsequently verified on seismic data. The upper Miocene and younger unconformities were first recognized through seismic stratigraphy studies and then tied to electric log correlations. Most of the unconformities are observed near partially eroded diapiric ridges located along what was the outer shelf/upper slope area at the time of the erosion. A major upper Miocene unconformity is observed in the inner shelf/transitional environment of deposition in Matagorda and Mustang Island areas. This erosional surface was most likely formed by wave erosion during the subsequent transgression after deposition of the Discorbis "12" age sediments. There is also evidence of submarine scour by turbidites or massive slumping on this upper Miocene unconformity in the area of lower slope deposition. No evidence of erosion of outer shelf or upper slope sediments is apparent. Mapping in the lower Miocene trend delineated shale ridges plunging in a basinward direction perpendicular to regional strike. Electric log correlation of regional markers, which represent flooding surfaces, facilitated correlation of the unconformities on the seismic sections across these ridges. Occasionally very detailed seismic stratigraphy studies were necessary to establish which flooding surface should be used as the regional marker across large faults and ridges.
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