THE CLAY PETROLOGY OF SEDIMENTS

Studies of the Recent indicate that, at the most, somewhat less than half the clay minerals arc altered to any extent in a marine environment. Probably most of this alteration is in the form of cation adsorption or reconstitution of slightly weathered illites and chlorites to their original form. There appears to be little if any evidence that the detrital basic latt ice is being altered to a measurable xtent in Recent mar ine sediments. In the near-shore nv i ronments here is usual ly a coincidence of clay minera l suites and environments. A major change in the clay mineral composit ion of sed imentary rock occurs wi th in the iVIississippian. I l l ite is the dominant clay mineral of the pro-Upper Mississippian sediments. PesbLower Mississippian clay suites are more variable in compos i t ion; illite becomes less abtmdant and montmori l loni te and kaolinite more abundant. This change is best related to a change in regional tectonics. The clay minera ls seem to have no preferred lithologic associations, a l though owing to cpigenetie a l terat ions porous sandstones commonly have different clay mineral suites f rom those of adjacent shales and carbonate rocks. h i many instances clay mineral facies coincide with envi ronmenta l facies. As the clay mineral criteria for d ist inguishing any given type env i ronment are extremely variable, it is thought hat segregation of clay minera l suites by sort ing is usual ly mere effective than by diagcnesis. Expanded clay minerals appear to be partially contracted by the time they have been buried to 10,000-15,000 ft. It remains to be proved whether this is caused by chemical modif ication of the basic latt ice with bmfial or if the detr ital clay latt ice has the inherent abil ity to contract, wi thout chemical rearrangement, when buried to a sufficient depth