ABSTRACT

Anomalously thick shale intervals of limited extent within normally sandy facies of the Wilcox are evident on published regional electric-log structure sections through south Louisiana. These were investigated with a series of subregional stratigraphic electric-log sections and by mapping. The stratigraphic sections show that in places the shale cuts out nearly 1000 feet (300 meters) of adjacent interstratified sandstone-and-shale section and has lenticular, channellike cross-sectional geometry. The sections also indicate that the uppermost parts of the thick shale intervals are stratigraphically equivalent to, or at least contiguous with, a persistent subregional shale marker bed known informally as the "Big Shale," but are distinguishable from it in places by more uniform log character. A map of the anomalously thick and uniform shale indicates a south-trending, relatively straight channel with a single bifurcation in southern Avoyelles Parish. The branches probably continue downdip although they cannot be traced with confidence south of central St. Landry Parish because of the overall shalier Wilcox section and scarcity of adequately deep control wells. The channel shale is analogous to other early Cenozoic subsurface truncational shale masses that have been mostly interpreted as submarine canyon-channel fills. Stratigraphic trapping possibilities are evident from the truncation of bounding permeable units, but the possible continuation of the branches downdip suggests the potential for deep sandy equivalents and deep gas.