Abstract: 

Paleoseismites, in the form of clastic dikes and sills and convolute bedding record syndepositional tectonism prior to lithification, fold growth, and onset of major orogenic events. The Elk Basin anticline, located in the northern portion of the Bighorn Basin, contains paleoseismites in Campanian strata along its eastern (forelimb) and western (back limb) margins. In the shallow-marine Telegraph Creek and Claggett formations, convolute bedding typically involves individual sandstone beds with vertical to overturned strata on either side of a nearly vertical vent, whereas the nonmarine Eagle Formation contains planar clastic dikes and sills, derived from liquefied sand-source beds, and convoluted bedding involving distorted laminae. Planar clastic dikes and sills were injected upward and laterally across overlying strata, and along pre-existing, nearly vertical, near-surface joints. Geographically, clastic dikes and sills are present only in the central portion of the anticline, while convoluted-bedding is present in the central and southern portions. Comparison of 145 clastic-dike measurements with 61 previously reported 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 Campanian strata.

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