Abstract

A glacial origin for the Mineral Fork Tillite in the Big Cottonwood Canyon area of the Wasatch Range in Utah is reaffirmed. The formation consists of two members: a lower sequence of thick, massive diamictites, with minor lensoid beds of shale-siltstone, sandstone, and conglomerate, and an upper sequence of more abundant shale-siltstone and siltstone that displays prominent bedding. The lower member is interpreted to be a sedimentary pile of tills and outwash deposited by multiple advances of continental glaciers. A grooved, polished, and striated surface marks the basal contact with the Big Cottonwood Formation. The upper member is considered to be of glaciomarine origin, mainly sediments released from icebergs and/or a floating ice shelf, along with rock flour washed from marine-based glacier margins. One major continental to marine cycle of environmental change is indicated as a result of slow but steady subsidence along a continental margin. Evidence cited here strengthens the case for a major glacial event during late Precambrian (Eocambrian) time in western North America.

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