ABSTRACT

A Middle Devonian Jefferson Formation channel deposit 0-87 m thick and 650 m wide is present in Spring Mountain Canyon, Lemhi Range The deposit fills a previously eroded channel in the Silurian Laketown Dolomite and consists of three units. The 14 m thick basal unit consists of thin to fissile beds of green, gray, and black shale and gray siltstone. It contains occasional fish fragments, rare internal molds of low conispiral gastropods, and macerated vascular plant fragments A 60 cm shale bed containing mud clasts up to 5 cm in diameter marks the unit's upper boundary. The middle unit is made up of interbedded shales and sandstones in the lower 6 m with the upper 3 7 m containing medium to thick bedded sandstone and conglomerate. The sandstones consist of very fine grained dolomite detritus and poorly sorted, subangular to subrounded quartz grains; show some poorly preserved bimodal crossbeds; and are commonly burrowed. Fish fragments, mostly Psephaspis sp, are common and highly abraided. Conglomerate beds contain subangular to subrounded dolomitic mud clasts up to 13.5 cm in diameter. Current directions appear to be NW-SE. The upper unit, 30 m thick, consists of interbedded sandstone and shale with occasional dolomite beds.

The channel represents tidal-flat deposition on a regional pre-Jefferson erosion surface with a general transgressive sequence. Coarse clastics were derived from tidal muds and accumulated in the channel during storms or high energy fluctuations in tidal currents. Fish remains represent both marine and fresh water forms including Psephaspis idahoensis, Psephaspis sp , Holonema haiti, Astrolepididae, and a Holocephalian.

Structural relationships indicate the rocks in the channel were deposited about 13 km to the west and thrust eastward to their present position.