Abstract

The Blacktail anticline in northwestern Wyoming was interpreted by Pierce and Nelson (1973) to be a pop-up anticline which rose into a pull-apart chasm created during an early stage of movement of the Heart Mountain detachment fault (HMD). The Crandall Conglomerate was thought to be the sedimentary fill of that chasm. Alternatively, Hauge (1990) suggested that the channel formed by normal erosional processes and that the Blacktail structure is a late Laramide thrust-fold localized along this channel. Based upon new field observations, we interpret the Blacktail structure to be a northeast-verging, early Laramide thrust-fold, similar to nearby structures. All Crandall Conglomerate exposures in the HMD footwall are always just northeast of the crest of the thrust-fold, a geometry which suggests that a canyon incised in a plateau surface was initially localized along fractured rock on the crest of the thrust-fold. As the canyon eroded vertically, it departed with depth from the southwest-inclined thrust-fold. Crandall Conglomerate deposited in this canyon and the adjacent thrust-fold were then decapitated by movement of the HMD. This history negates the need for an early period of HMD movement.

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