CLAY MINERALOGY OF THE MORRISON FORMATION: UTILITY IN PALEOENVIRONMENTAL INTERPRETATIONS AND TEMPORAL CORRELATIONS

Clay-rich rocks dominate the strata of the Morrison Formation. Although clay minerals in a rock can be indicative of paleoenvironmental conditions, they also are controlled by the composition of the sediment source area and conditions of burial and diagenesis. In addition, the Morrison Formation in many areas had a large input of volcanic ash. The alteration of volcanic ash in various chemical environments can result in different types of clays.

As a result of: 1) differences in depositional environment across the Morrison Formation, 2) differences in the amount of volcanic ash input, and 3) differences in the composition of ground and pore waters the formation has been exposed to since deposition, the clay types of the Morrison Formation can be expected to vary from region to region, both vertically and laterally. This conclusion is supported by X-ray diffraction work.

Clay mineralogy alone therefore should not be relied upon to provide information about paleonvironmental conditions. In addition, since the compositional variations in clay mineralogy indicate more about depositional and/or diagenetic environments than they do about geologic time, clay mineralogy should also not be used for temporal correlation.