Abstract: Samples from initially smectite-rich Tertiary continental volcaniclastic sediment from the Deer Lodge and Big Hole basins of southwestern Montana show a general decrease in illite/smectite (I/S) expandability with increasing burial depth. The mineralogical trends in cuttings from seven wells are interrupted by discontinuities in which I/S expandability abruptly decreases by 30 to 80%. These discontinuities coincide with stratigraphic unconformities in four wells in which the stratigraphy is known. Core samples show a wide range of I/S expandabilities over short stratigraphic intervals, possibly due to composition, porosity, and permeability variations. Sericite coexists with I/S in the deep core samples. A core sample from 7958 ft (2425 m) contains an R3-ordered I/S having a nearly ideal 3:1 illite : smectite ratio, similar to the mineral tarasovite. The structure of this I/S is dominated by stacks of four 2:1-layer fundamental illite particles and small proportions of thicker particles randomly interstratified among the four-layer particles.