INFLUENCE OF PALEOTOPOGRAPHY AND ACTIVE BASEMENT STRUCTURES ON DEPOSITIONAL PATTERNS, MIDDLE TO LATE JURASSIC, BIGHORN BASIN, WYOMING

Incipient paleotopography and syndepositional tectonic activity influenced the distribution of sedimentary facies across local Jurassic basins and margins in present-day northern Wyoming. The Middle–Upper Jurassic Ellis Group and Upper Jurassic Morrison Formation are characterized by irregular internal facies distribution, abrupt lateral facies changes, and local unconformities resulting from variable erosional relief and active basement structures. Local basement structure activity may be associated Intermontane belt tectonics in Pacific Northwest which has been recognized as the source of some detrital sediments in the Ellis Group (Fuentes et al., 2009).

Effects of incipient paleotopography and syndepositional tectonic activity are recorded by the variable thicknesses of units and distribution of depositional facies. Topographic relief influenced distribution of facies ranging from saline basins depositing localized gypsum beds, carbonate storm and shoal grainstone deposits, shallow-water microbial mats, and concentrated dinosaur footprint trace fossils. The Ellis Group was deposited during a series transgressive-regressive episodes related to inundation of a foreland basin in the Western Interior. Transgressive episodes in present-day Wyoming were dominated by carbonate and evaporite deposition related to variable ocean chemistry and accommodation space. Regressive periods are distinguished by progradational supratidal redbeds and detrital clastics and trace elements. Likewise, facies distribution in the Morrison is influenced by syndepositional topographic relief, such as embayments, paleohighs, and active basement structures. For example, bentonite thickness distribution in the Morrison may also correlate to syndepositional topographic lows, such as filling of lakes with volcanic ash.