HIGH-FREQUENCY SYNTECTONIC DEFORMATION IN A DISTAL FORELAND SETTING, POWDER RIVER BASIN, WYOMING

High-frequency tectonic movement in a distal foreland setting of Cenomanian age is documented in the Powder River Basin, Wyoming. A number of regionally correlatable bentonite layers within the Belle Fourche Member of the Frontier Formation, integrated with a newly established allostratigraphic framework of the unit allow determination of at least two pulses of uplift and subsidence. These structures had amplitudes of tens of meters and wavelengths of tens of kilometers and occurred after the deposition of the Clay Spur bentonite dated at 97.17 ± 0.69Ma and before a bentonite dated at 95.78 ± 0.69Ma, constituting a period of roughly 1.4 m.y. The findings suggest syndepositional, parasequence-scale base level changes of largely tectonic origin with frequencies of hundreds of thousands of years. This has important geodynamic and sequence stratigraphic implications for the origin of high-frequency stratigraphic discontinuities, that are historically interpreted to be purely eustatic in origin.