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ABSTRACT: The LaSalle Arch and Its Effect on Wilcox Sequence Stratigraphy

Paul N. Lawless, George F. Hart

The LaSalle Arch is a southerly trending anticline separating the Louisiana and Mississippi interior salt basins. The structural trend of the Arch is suspended along basement paleo-highs. The paleo-high beneath the Nebo-Hemphill field, as seen on reflection seismic data, is the nose of an Ouachitan thrust fault that was partially rifted during the opening of the Gulf of Mexico. The western limb of the Arch formed owing to differential subsidence expanding the stratigraphic

section toward the southwest. The eastern limb of the Arch formed because of regional tilting to the east after deposition of the Claibornian Sparta Formation. Uplift of the LaSalle Arch occurred during the Late Cretaceous Period and is seen as a truncational unconformity within the Tayloran Demopolis Formation.

The Wilcox and Midway groups of central Louisiana have been subdivided into three genetic sequences. They are T1 (the Midway), T2 (the Holly Springs), and T3 (the Carrizo). A genetic sequence is bounded above and below by condensed sections and represents a progradation into the basin followed by transgression. Each sequence represents a potential major migration route for the crude oil.

The high stand systems tract of T1 and T3 were deposited during falling sea level resulting in similar homogenous sheetlike sand bodies. The paleo-highs subtly controlled the location of depositional environments, but did not prevent progradation to the southwest. The highstand systems tract of T2 was deposited during rising sea level resulting in heterogenous sediment dominated deltaic deposits that are very different from T1 and T3. Although the stratigraphic section expands to the southwest, giving the impression that the interval thins over the Arch, the LaSalle Arch did not control the location of depositional environments within T2.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990