DEPOSITIONAL ENVIRONMENTS AND STRATIGRAPHIC HIERARCHY OF UPPER CRETACEOUS TUSCALOOSA MARINE SHALE IN CENTRAL U.S. GULF COAST – A CORE OBSERVATION

Lithology, sedimentary structures, and stratigraphic stacking patterns of Upper Cretaceous Tuscaloosa Marine Shale in 3 cores in SW Mississippi and Louisiana are used to establish the stratigraphic hierarchy to assess the heterogeneity. The cores are 73, 66, and 54 m thick and 10s of km apart. 5 well-laminated lithofacies are clay – f-silt and m-silt mudrocks, m-c siltstone, and vf-f and gravelly f-m sandstones. Laminae are 0.1 mm-10 cm thick, mostly 0.5-3 mm. 5 orders of stratigraphic hierarchy include lamina, lamina set (LS), bed (B), and parasequence (PS) and parasequence set. 5 types of LS are 0.5-30 cm thick and include: LS1 – wave-enhanced sediment gravity flow deposits, LS2 – tempestite, LS3 – turbidite, LS4 – current deposits, LS5 – pelagic clay-f-silt mudrocks. LS1-LS3 consist of light gray, massive, normal graded, or cross-laminated m siltstone to sandstone and common gravelly lags with erosional bases. LS4 is gray, parallel-laminated m siltstone to sandstone with flat sharp bases. LS5 is dark gray and parallel laminated with sharp or gradational bases. LS1-LS4 are interpreted as current deposits; LS5 as suspensional pelagic deposits, all on the outer shelf. LS stacks into 3 types of beds, 1-30 cm thick: B1 consists mainly of light gray LS1-LS4 with <30% of LS5; B2 of dark gray interlaminated LS1-LS5 with 30-60% of LS5; B3 of dark gray LS5 with <40% LS4. Beds stack to form bipartite or tripartite upward-fining or coarsening, 10-100 cm-thick PS. The former dominates, indicating upward-decreasing current energy and current deposition. Finally, stacking of PS defines PS sets that are grouped into systems tracts. An overall upward-fining-to-coarsening trend suggests a deepening-to-shallowing trend, corresponding to late and maximum transgression and early regression. The hierarchical stratigraphic framework provides a context for detailed reservoir characterization using specific attributes and stratigraphic correlation. Comparing to the well in the underlying lowstand incised valley in the south, the well in the north has thicker and coarser tempestites, skeletal lags, and macrofossil debris and intense bioturbation. It is outside of the valley and more up-dip and closer to clastic sources. The stratigraphic variability is controlled by sea-level change and shelf topography.