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The "lower Tuscaloosa fairway" can be understood best as a boundary phenomenon. Sands of this trend were deposited along the demarcation between two distinct tectonic elements. Also, these sands were superposed upon one of the greatest sedimentary breaks of northern coastal areas, i.e., the angular unconformity which separates Comanche and Gulf (Cretaceous) Series.
Basic crustal differences produced a marked correspondence between Mesozoic tectonic and physiographic features. The boundary between the Mesozoic shelf and the central Gulf subbasin constituted a structurally controlled shelf break which remained constant in position through the major part of the Mesozoic Era. The shelf was an exceptionally broad feature where sea
level changes effected migration of depositional environments. Such migrations were evidently cyclic in nature, i.e., they comprise a repetitive series of transgressive-inundative-regressive sequences.
Retreat of Mesozoic shorelines, and inception of active erosion of former depositional areas, was commonly anticipated by the gulfward migration of lithotopes. An example of this is seen in the upward increase in grain size and clastic ratio in strata below the sub-Gulfian unconformity. Thus, in some areas of east Texas, the Buda Limestone is succeeded upward by the "Maness Shale," which is then succeeded by sands of the upper member of the "South Tyler Formation." The sub-Gulfian unconformity constitutes regionally the most useful means for determining the Gulf and Comanche Series boundary. Accordingly, strata deposited immediately before culmination of the South Tyler regression are assigned to the Washita Group. Strata deposited immediately after the beginning of Woodbine-Tuscaloos transgression are assigned to the Woodbine Stage. Consequently, some lower sands along the lower Tuscaloosa fairway could be latest Comanche regressive deposits.
Fluviatile deposits have been described in the lower Tuscaloosa Formation near the inland edge of the Toledo Bend flexure. The presence of continental facies, within such a short distance of the Mesozoic shelf break, indicates that the shoreline previously had migrated almost completely across the Mesozoic shelf by the end of Washita deposition. The fluviatile deposits grade upward into finer grained strata of the middle Tuscaloosa Formation, thereby suggesting increasing distance of transport and deepening of waters. The upward change from the fine-grained middle Tuscaloosa to the coarse-grained strata of the upper Tuscaloosa suggests the gulfward spread of shallow-water conditions. Accordingly, the lower, middle, and upper Tuscaloosa divisions are interpreted to represent, respectiv ly, the transgressive, inundative, and regressive phases of the Woodbine-Tuscaloosa depositional cycle.
An interesting phenomenon of inland areas was the opening of the Mississippi embayment. During this event, Gulf-related deposition spread inland across a series of basin-concentric, Paleozoic elements inland from the Ouachita deformed belt. These elements included a linear trend of fore-basins and a trend of large foreland uplifts. The embayment first extended northward across the Black Warrior basin, then across a saddlelike feature between the Nashville dome and the Pascola arch (i.e., the buried southeastern extension of the Ozark uplift). Coarse clastics were supplied to the Tuscaloosa by Paleozoic formations, such as the Fort Payne Chert, which cropped out on nearby structures. The embayment axis has since migrated progressively westward to its present position near the course of the Mississippi River.
Indicated reserves along the lower Tuscaloosa fairway seem impressive. Furthermore, the fairway opens up whole new types of Mesozoic reservoirs, those deposited along the Mesozoic slope. Since this trend defines the gulfward edge of the Mesozoic shelf, it permits the study of a complete suite of shelf facies and the definition of a new frame of reference for stratigraphic and environmental studies.
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