Postdiagenetic Clay Mineral Environmental Relationships in the Gulf Coast Eocene

Abstract An extensive survey of clay mineral relationships in the subsurface Wilcox formation (Eocene) has shown progressive diagenetic conversions with depth. Montmorillonite, a common constituent of Wilcox outcrop material, becomes loss evident below 3000 ft and is not normally found in an unmixed state below the 9000—10,000 ft overburden level. At depths between 3000 and 14,000 ft, montmorillonite lattices are commonly interpersed with illite components, the frequency of which increases with depth to a virtual elimination of montmorillonite swelling characteristics below 14,000 ft. Chlorite is present at all stratigraphic levels including surface exposures. It appears to be a more dominant constituent at depth; however, observed increases in basal intensities in samples that had been more deeply buried may result from more perfect crystal development rather than quantitative differences. The diagenetic conversion of montmorillonite to illite and possibly to chlorite has resulted in a distribution of the last two minerals that is related to estimated depositional environments as reconstructed from micro-paleontological criteria in at least one well in southern Louisiana. It is inferred, therefore, that different chemical characteristics in the ancient Wilcox seas are responsible for the distribution coincidence even though the mineral groups defining the distribution were not necessarily indigenous to the ancient Wilcox sea. Correlations between clay minerals and environment are not particularly noted in the investigation of analogous environmental situations in Recent sediments, and it is thus concluded that the environment indicators which appear as minerals in lithified sediments such as the Wilcox are present in freshly deposited sediments in the form of submineralic chemical constituents such as absorbed ions, molecular groups or subcrystalline lattice configurations which are not detected by the ordinary clay mineral identification procedures. The change to mineralic form is aided by the natural “bombing” resulting from normal increases in temperature and pressure due to burial. The process is apparently a continuum through the entire subsurface residence of the sediment and the results are seemingly in accordance with phase equilibria.