LATE CRETACEOUS PALEOSEISMITES IN THE NORTHERN BIGHORN BASIN: A RESULT OF INITIAL LARAMIDE DEFORMATION

Paleoseismites record deformation synchronous with deposition, fold growth, and onset of major orogenic events. Late Cretaceous paleoseismites in the form of clastic dikes and convolute bedding are present in the Eagle Formation, along the eastern and western flanks of the Elk Basin anticline. Clastic dikes were injected upward and laterally across overlying strata, and along pre-existing, nearly vertical, near-surface joints. Geographically, clastic dikes are present only in the central portion of the anticline, while convoluted-bedding is present in the central and southern portions. Comparison of clastic-dike measurements with Laramide joint-set orientations for the Elk Basin demonstrate trends to be similar. Clastic dikes preferentially fill cross joints oriented normal to the axial trace of the anticline, while strike joints illustrate a dominant later Laramide joint set oriented subparallel to the axial trace. Paleoseismite formation is consistent with ~ M 5.5 earthquakes during earliest Laramide deformation associated with development of the Elk Basin thrust fault, whereas strike joints formed during subsequent Laramide deformation after burial and cementation of Late Cretaceous strata.