BIOGENIC DEPOSITS IN UPPER CRETACEOUS TUSCALOOSA MARINE SHALE: IMPLICATIONS ON BIOGENIC, DEPOSITIONAL, AND BOTTOM WATER CONDITIONS ON THE CONTINENETAL SHELF

Microbial and algal deposits in deep-water shelfal siliciclastic shale, although rare, provide insights on the seafloor biogenic, sediment, and water conditions. They were documented in a 175-ft (35 m) thick core of the Upper Cretaceous Tuscaloosa Marine Shale (TMS) in Louisiana, Gulf of Mexico. The TMS is a maximum-transgressive deposit and classified into four lithofacies of very fine sands – coarse silts (LF1), medium - fine silt (LF2), fine silt – clay (LF3), and microbial-algal deposits (LF4). LF1 is characterized by a basal erosional surface, well-developed stratification, indicating tractional deposition by density currents. LF3 is characterized by a sharp or gradational base and sub-mm laminations, interpreted as pelagic deposit. LF2 is a transitional between LF1 and 3.

LF4 contains 5-40% particles of a possible biogenic origin and is 1-30 cm thick. The particles are embedded in very fine silt and clay with abundant disseminated forams. The LF is well laminated, calcareous, and lack of bioturbation. The laminae, however, are poorly defined and discontinuous. The particles are dark brown, elongate, mm-sub-mm thick, 0.1-5 cm long, wrinkly, and subhorizontal with a mm relief. In thin sections, they are opaque, amorphous, some calcite-replaced, and contain common and unevenly distributed framboidal pyrites. Some intervals are sub-mm clotted. The elongate particles are interpreted as algal plates forming mats; the clotted texture suggests microbial-influenced deposit; both types of biogenic deposits are interlaminated. LF4 occur in a 50-ft interval in the lower TMS, ~70 ft above the base of the core. The preservation of algal and microbial mats, presence of pyrite, and minimal bioturbation suggest biogenic activities on the seafloor, an anoxic/dysoxic bottom water, and a sediment-starved condition. Thus, the interval rich in biogenic sediments may be related to OAE2 in Late Cretaceous. Four cores landward of or with a similar location to this well do not have biogenic sediments, suggesting that the anoxic condition was restricted to the relatively deep shelf area. Further petrographic and elemental geochemical studies will provide more evidence on biogenic activities and environmental conditions.