EARLY TEMPORAL DEVELOPMENT OF SEVIER FORELAND BASIN, WYOMING: THE TRANSITION FROM CRATONIC MARGIN TO A FULLY PARTITIONED FORELAND BASIN

During the medial Jurassic North American convergence with the ancestral Farallon Plate, led to the initiation of a broadly defined Sevier foreland foldbelt and associated foreland basin. In Wyoming, biostratigraphy, facies correlations, sediment isopach characteristics, and site-specific FT, U-Pb and K-Ar dates on pyroclastic units have constrained a general picture of this basin development. Of particular importance in this evolution is the recognition of foreland basin partitioning into a migrating foredeep, forebulge, and back bulge.

In this study, two positions of relative stasis in forebulge development during medial- and late- Jurassic have been recognized from sediment isopach data. The Sevier forebulge in central Wyoming appears to have undergone little oscillation of position from Bajocian to Oxfordian, whereas a migration of several hundred kilometers easterly occurred during the late Oxfordian to Kimmerdigian. Detailed micro-scale facies analysis and regional teprachronology have been applied to provide additional evidence for this basin partitioning and evolution.

In central Wyoming, deposition of medial and late Jurassic sediments occurred in dominantly shallow marine environments represented by the highly cyclic nature of the Sundance Sea stratigraphic record which thins easterly. This thinning is interpreted to occur primarily over the forebulge, with structurally higher positions of the forebulge being expressed through peritidal and terrestrial facies, including sub-tidal thrombolites, storm-induced shallow-water tempestites, salt crystal casts in supratidal facies, paleosols, aeolianites, ventifacts, karst and sabkha fabrics.

Numerous outcrops containing multiple horizons of tuffs have been identified within these marine, transitional and terrestrial sediments. Interestingly, the two stages of forebulge stasis appear to coincide with pulses of arc-related volcanism to the west, indicated by primarily tuffs, bentonites and bentonitic mudstones preserved in these source-medial to source-distal sediments. This pyroclastic record has served as a facies-independent temporal control on the sedimentary record, and in turn constrains the tectonic significance of the facies mosaic found within these evolving foreland basin elements.