Zuni Sequence in Williston Basin--Evidence for Mesozoic Paleotectonism

Abstract

The Zuni sequence in the Williston basin is a large-scale lithogenetic package bounded by interregional unconformities. Within the sequence, three major subdivisions are separated by unconformities or marker beds and correspond with chronostratigraphic units: (1) Middle and Upper Jurassic, (2) Lower Cretaceous, and (3) Upper Cretaceous and Paleocene. The basin has clear expression in the Jurassic subdivision, poor expression in the Lower Cretaceous, and good expression in the Upper Cretaceous. Jurassic units are thick in the basin center, thin in central Montana and northeastern Wyoming, and thick in southern Montana and western South Dakota. Jurassic marine carbonates are found along the western basin margin and marine sandstones mark the southern and eastern margins. Lower Cretaceous rocks display a regional thinning from west to east with little expression of the basin center and margins. Lower Cretaceous marine and nonmarine sandstones form blanket deposits. Upper Cretaceous units preserved below the Paleocene show a clearly defined depocenter at basin center. Upper Cretaceous shales characterize the depocenter; facies patterns of marine sandstones on the west and south and carbonates on the east correspond with paleotectonic elements distributed around the basin margin . A series of seven marginal paleotectonic elements surround the basin center on the west, south, and east in the United States. Five more marginal elements have been described in Canada. Occurrences of oil in the Jurassic and Lower Cretaceous and of natural gas in the Upper Cretaceous are broadly related to the pattern of marginal paleotectonic elements.