REEVALUATION OF A DEVONIAN PALEOCHANNEL DEPOSIT, CENTRAL LEMHI MOUNTAIN RANGE, IDAHO

An unnamed Devonian unit near the head of Spring Mountain Canyon in the Lemhi Mountain Range of Idaho has previously been interpreted as a paleovalley fill comprised of sandstone, conglomerate, and shale. This interpretation was based primarily on a distinctive lens-shaped outcrop. However, further investigation suggests that the apparent channel shape is actually the result of a series of tear faults in steeply dipping strata along the eastward limb of a plunging anticline that now forms part of a cirque, and that the unit is comprised mostly of sandy dolomite. Evidence for a tectonic interpretation includes a planar lower contact with the Laketown Dolomite that shows a stair-step offset pattern and a vertical lateral contact with the Laketown at the southern end of the outcrop. Additionally, the unit is not restricted to the apparent channel exposure, but is consistently found stratigraphically between the Laketown Dolomite and the Jefferson Formation throughout Spring Mountain Canyon.