ABSTRACT

The "Lower Tuscaloosa Fairway" can be understood best as a boundary phenomenon. Sands of this trend were deposited along the demarcation between two fundamentally distinct tectonic elements. Also, these sands were superimposed 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, as follows:

1. The central Gulf subbasin is underlain by oceanic crust. During the Mesozoic Era, it was physiographically expressed by depths corresponding to abyssal plain, continental rise and continental slope.
2. In some of its inland areas, the Mesozoic shelf is underlain by thick continental crust of the North American hedreocration. In the more gulfward areas, it is underlain by the modified continental crust of the Ouachita deformed belt. During the Mesozoic Era, the Mesozoic shelf was expressed topographically by continental shelf and low coastal plain features.

Thus, the boundary between the Mesozoic shelf and the central Gulf subbasin constituted a structurally controlled shelfbreak which remained essentially constant in position through the major part of the Mesozoic Era. This position can be approximated by a number of criteria: 1) the gulfward edge of the Stuart City (Comanchean) reef complex, 2) the inland limit of diapirs related to the coastal salt-dome basin, and 3) the inland edge of down-to-the-Gulf fault systems related to Cenozoic outbuilding of the continental shelf and slope. The most precise criterion maybe provided by the boundary between the normal pressured (landward) and the geopressured (gulfward), which extends along the lower Tuscaloosa fairway. This pressure demarcation is obviously related to differences in sedimentary loading. Contrasting thicknesses across the boundary reflect differences in topographic relief (water depths) and structural strength under loading. Both are related to fundamental crustal differences.

The Mesozoic shelf was an exceptionally broad feature. Mesozoic facies patterns show that coarse clastics reached the shelfbreak only when one, or both, of the following conditions prevailed: the rates of clastic influx exceeded those of subsidence, so that shallow-water conditions spread progressively gulfward, or the sea level dropped eustatically, so that clastic-laden rivers extended far across the Mesozoic shelf to dump their loads at, or near the tectonically controlled shelfbreak. Additional regional study is necessary before the relative importance of each condition is known in relation to the presence of sands along the lower Tuscaloosa fairway.

Fluctuations of relative sea level may have been caused either by eustatic variations or by vertical movements of the land area. Whichever the cause, sea level changes effected migration of depositional environments, either landward or gulfward across the gentle seaward slope of the Mesozoic shelf. The direction of migration, of course, depended on the direction of sea level change. Such migrations were evidently cyclic in nature, that is, they comprise a repetitive series of transgressive-inundative-regressive sequences.

Retreat of Mesozoic shorelines, and inception of active erosion of former depositional areas, was often anticipated by the gulfward migration of lithotopes. A prime 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". The Maness Shale is succeeded by sands of the upper member of the "South Tyler Formation" (Anderson, 1979). The sub-Gulfian unconformity constitutes the regionally most useful means for determining the Gulf/Comanche Series boundary. Accordingly, strata deposited immediately before culmination of the South Tyler regression are most logically assigned to the Washita Group. Strata deposited immediately after the beginning of Woodbine-Tuscaloosa transgression are most logically assigned to the Woodbine. Consequently, some of the lower sands encountered along the lower Tuscaloosa fairway could be latest Comanche regressive deposits.

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Whether of South Tyler or early Woodbine age, sands along the fairway were obviously deposited under a regime which differed markedly from that which geologists have come to associate with strata of similar age to the north. Obviously, a greater familiarity with phenomena of the continental slope and with depositional principles governing the distribution of such reservoir rock are in order (eg., Siemers, 1978; Foss, 1979).

Fluviatile deposits have been described (Karges, 1962; Berg and Cook, 1968) in the lower Tuscaloosa Formation near the inland edge of the Toledo Bend flexure (Anderson, 1979). The presence of continental facies, within such a short distance of the Mesozoic shelfbreak, indicates that the shoreline had previously 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. Indeed, the earlier name for the middle Tuscaloosa was "marine Tuscaloosa" (Moore 1962, fig. 4). 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, respectively, the transgressive, inundative and regressive phases of the Woodbine-Tuscaloosa depositional cycle.

The Mesozoic shelf, largely vacated by Washita seas, was not topographically featureless. Certain structures, such as the uplifts, stood with some relief above the general surface. The Sabine uplift, for example, was not entirely inundated by Gulfian seas until after the beginning of Tokio (Austin) deposition (Anderson, 1979, p 1.9B). Incomplete mapping indicates a marked effect of a number of positive features on the distribution of Woodbine-Tuscaloosa strata across the northern coastal region.

An interesting feature of these inland areas is the fact that Woodbine-Tuscaloosa shales wedge out and the sands become much more massive toward their upstructure limits onto positive features. This is true of the Woodbine in the East Texas field area on the western flank of the Sabine uplift, and of the Tuscaloosa in the Big Creek-Delhi field area on the southern flank of the Monroe uplift. This phenomenon may be explainable on the basis of reworking and better sorting due to greater relief. It is interpreted here to also reflect the emergence of the positive structures, where the emergent features served as local clastic source areas. This concept seems substantiated by irregularities in the trend of the line marking the gulfward facies change from Woodbine-Tuscaloosa sands into "Pepper Shale." In general, this line trends diagonally across the northern coastal region. However, it shows a marked gulfward offset in the vicinity of the Sabine uplift. The WNW trend of the line reflects the fact that the principal clastic source lay in the vicinity of the Appalachian uplands. The gulfward offset in the line evidently reflects an important local source in the emergent Sabine uplift.

Another 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 lying inland from the Ouachita deformed belt. These elements included a linear trend of forebasins and a trend of large foreland uplifts. According to Marcher and Stearns (1962), and Stearns and Marcher (1962), 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 of the time. 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 a whole new type 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. The northern coastal region is the prime example. This discrete areal unit is defined on the north by the inland limit of Gulf-related deposits, on the south by the gulfward edge of the Mesozoic shelf, on the west by the San Marcos arch, and on the east, by the Appalachian front and its buried extension. The well control supplied during development of the lower Tuscaloosa fairway has provided the means for a scientific breakthrough in the geology of the Gulf basin province.