CLAY MINERAL EVOLUTION IN THE ILLINOIS BASIN AND ITS CAUSES

Samples from deep wells of the Palaeozoic Illinois Basin show the evol ution of the illite-smectite (I-S) mineral transformation under low te mperatures for very long periods of time. Curve decomposition of the X RD traces was used to identify and follow the evolution of the differe nt clay minerals. All samples showed the presence of a large amount of illite (fine and coarse grained micaceous material) and a much smalle r quantity of I-S. In a vertical section the R=1, I-S diffraction peak initially changes with depth indicating change from 80 to 95% illite content but it changes little with further depth. The I-S intensity de creases with depth compared with the illite peaks. These relations ind icate that the clays in the Palaeozoic sequence of the Illinois Basin evolve continuously in the deep well. It appears that the final stages of I-S mineral transformation can reach a compositional stagnation (s mectite content) as the I-S phase is dissolved to produce illite. Usin g a kinetic model to describe the smectite to illite clay transformati on, it is deduced that the Illinois basin had a longer period of depos ition than indicated by present stratigraphy and that almost 1.5 km of sediments deposited in the Mesozoic were eroded in later Tertiary tim es. These conclusions are confirmed by published data on the maturatio n of organic matter.