Analysis of the smectite content of mixed-layer illite/smectite minerals in 4 wells in the Paris Basin indicates that the sediments have not experienced the same thermal history throughout their burial history. Reconstruction of the total burial depths and thermal maxima for the sediments in the different levels of the wells indicates that the present-day depth-temperature relations reflect the existence of the same continuous sequence of diagenetic change in 2 of the wells. An empirical relation giving smectite content as a function of maximum paleotemperature was derived. A strong break in the clay mineral compositions in the upper levels of 2 wells at a third site indicated an anomaly in the diagenesis process. This break in the sequence of clay diagenetic transformation can be traced to major subvertical faults which cross-cut the wells. These anomalies are attributed to the faults which are assumed to have acted as thermal drains towards the surface, locally increasing the temperature of the sediments for a major portion of the burial history when the faults were active. Closure of the fault system coincides with the return to a normal pattern in clay diagenesis for sediments in the upper portions of the wells. It is concluded that clays may be used to trace thermal histories of sediments in a sedimentary basin.