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Large-Scale Marine Slides as Analogies for the Heart Mountain Fault and South Fork Fault, Rocky Mountain Foreland, Wyoming, U.S.A


Clarey, Timothy L., Delta College, University Center, MI


Petroleum exploration efforts in the Bighorn Basin have been complicated by the shal­low, Eocene-age Heart Mountain Fault (HMF) and South Fork Fault (SFF). The origin of these two faults has been debated for over 100 years. The total extent of the HMF/SFF system is obscured by later volcanic deposits, alluvium, and recent erosion, further hindering its understanding and exploration efforts underneath.

A new conceptual model for the HMF/SFF resulted from a comparison to recent studies of South Kona Landslide, Hawaii and Fish Creek Slide, Alaska. Although marine features, the South Kona and Fish Creek Slides have similarities with the non-marine HMF/SFF system: 1) all appear to have formed rapidly and/or catastrophically, 2) all are of similar size (>3000 km?), 3) all involve block transport ranging from 40-80 km, 4) all contain blocks of similar thicknesses (0.2-1 km), 5) all involve transport along very flat surfaces (<2º), 6) all have nearly flat detachment horizons, and 7) all have a compressional component, involving thrusting, duplexes, and/or triangle zones.

Development of the Eocene-age, Absaroka volcanic field west of the Bighorn Basin apparently caused disorganized sliding of HMF blocks to the southeast with some blocks moving 50 km over the former land surface. The comparison to marine slides supports the tectonic denudation model whereby individual blocks broke free in short succession. The smaller, thrust-dominated, SFF formed slightly later as large-scale slumping continued to expand the toe southeast. The two faults are viewed as a two-tiered, nearly simultaneous, extensional system, overriding deeper, earlier-formed, foreland structural traps.