Absence of clay diagenesis in Cretaceous-Tertiary marine shales, Campos Basin, Brazil

In these shales which underlie the continental shelf off SE Brazil, mixed-layer illite/smectite (I/S) has remained randomly interstratified to depths of 3500 m and = or <100 degrees C, in contrast to the typical pattern of shale diagenesis in, for example, the Gulf Coast area. Clay minerals in samples from a Gulf Coast well off the Texas shore are I/S (montmorillonite-type), discrete illite, chlorite and minor kaolinite, whereas those from the Campos basin are kaolinite, clay-size biotite and I/S (nontronite-type). Kaolinite is abundant in the Campos basin well; and the variation in its abundance with depth seems to reflect variations in sea-level stands. The original composition of the I/S (nontronite-type) was probably the main factor controlling the lack of illitization in the shales of the Campos formation.