Depositional Topography and Major Marine Environments, Late Cretaceous, Wyoming

TLDR: In this article, the authors use electric-log resistivity patterns that result from variations in bentonite content of marine shale and siltstone reveals the presence of inclined time-stratigraphic units within the post-Turonian, marine Late Cretaceous section of Wyoming.

Abstract: Detailed correlation of electric-log resistivity patterns that result from variations in bentonite content of marine shale and siltstone reveals the presence of inclined time-stratigraphic units within the post-Turonian, marine Late Cretaceous section of Wyoming. Similar units have been recognized in rocks of the Permo-Pennsylvanian of Texas, the Devonian of Alberta, and the Mississippian of Illinois, and are attributed to submarine depositional topography. This interpretation, applied to the Late Cretaceous section of Wyoming, divides the epicontinental marine section into three major environments: shelf, slope, and basin. The inclined time-stratigraphic units were deposited on the slope, whereas thinner, more flat-lying units were deposited on the shelf and in the basin Three examples demonstrate the wide areal distribution and the almost continuous presence of significant submarine topography in at least part of Wyoming during post-Turonian Late Cretaceous time. The first example, from the lower Cody Shale of the southeastern Big Horn basin, is used to introduce the concepts involved and methods of correlation and mapping. The second example, from the Niobrara Formation and the lower Pierre Shale of the southeastern Powder River basin, is used to illustrate the complexities that can result from multiple sequences of progradation. The third example, the regressive part of the Lewis Shale of the Washakie and Red Desert basins, includes thick sandstone bodies. For each example, the areas assigned to the shelf, slope, and basin environments are determined from isopach maps of time-stratigraphic units within each section. This type of analysis is useful in solving problems of paleogeography and paleoecology. In the Upper Cretaceous of Wyoming, analysis indicates previously unrecognized complexities in the basin-filling process and the resulting sequence of units. The areal distribution of the three environments, the lithologic characteristics of the rocks, and the volume of sediment deposited in each environment are functions of the balance between rate of subsidence and sediment supply. The lower Pierre Shale and the Niobrara Formation of the Powder River basin are examples of the "wide-shelf" configuration, in which subsidence is relatively slow, progradation is rapid, and sand is largely confined to the shelf. The regressive Lewis Shale of the Washakie and Red Desert basins illustrates the complex relations between a "narrow shelf"-delta sequence (Lewis Shale) and a barrier island-lagoon sequence (Fox Hills Sandstone and Lance Formation). The submarine topography within a basin at a particular time in the sequence of deposition (the relief between the shelf edge and the basin, and the water depth in the basin environment) can be estimated from the present thickness of slope deposits. These estimates require correction for compaction and for water depth at the outer edge of the shelf. Such estimates indicate that at times the depth of water in areas of active sedimentation exceeded 2,000 ft. The Upper Cretaceous section of the Western Interior long has been considered a "textbook example" of shallow-water sedimentation. It has been studied by hundreds of geologists with varied interests and from different backgrounds, including the petroleum industry, the Geological Survey, and academic institutions. The section abounds with excellent time-stratigraphic correlations, and well control is more than adequate for detailed study in many areas. The fact that the relations described here could be overlooked under conditions such as these shows that they may have been missed in stratigraphic sections in other areas.