Structure, Timing, and Kinematics of the Early Eocene South Fork Slide, Northwest Wyoming, USA

Abstract

The South Fork slide (SFS) is exposed over an area of 800 km2 in northwest Wyoming, and its evolution is connected in space and time to the adjacent and overlying 48.87 Ma Heart Mountain slide (HMS). The youngest rocks deformed as part of the SFS are the Eocene Willwood Formation sands and mudstones, which have a U/Pb detrital zircon youngest depositional age (TuffZirc calculation) of 51.80 +1.04/−1.08 Ma as well as a spectrum of older zircons that were eroded from the Sevier Highlands to the west and the Archean Beartooth uplift to the north (n = 379 U-Pb zircon ages). The SFS is overlain by allochthonous Paleozoic carbonate and Eocene volcanic rocks of the HMS. The SFS deformation involves shallowly plunging thin-skinned folds that trend northeast-southwest, where the slip surface could have been the top of the Jurassic Gypsum Springs Formation. Detailed mapping reveals the absence of any cleavage or joints and a plethora of minor structures unreported in any thin-skinned belt as well as the fact that nearly a third of the exposed allochthonous sediments are overturned. Calcite in Sundance Formation limestones is mechanically twinned, and the resultant strain analysis reveals that the predetachment Sevier layer-parallel shortening strain axes (31/31 samples; n = 1231 twins) are now in a random orientation. Synemplacement calcite veins (320°, 90°; n = 2) record a horizontal shortening strain parallel to the veins. Neither calcite strain data set has any record of a strain overprint (low negative expected values). Rare slip indicators in the basal Sundance suggest northwest-southeast motion. Poorly constrained cross sections and palinspastic restorations indicate >25 km of shortening (and uncertain transport distances), and the above evidence suggests that the SFS formed as an Eocene landslide slightly earlier than the HMS. The SFS, then, represents the largest single allochthon (terrestrial or marine) that maintained its internal stratigraphy and did not disaggregate while in motion.