STRUCTURAL AND STRATIGRAPHIC STUDIES OF MESOPROTEROZOIC STRATA IN THE SHEWAG LAKE QUADRANGLE, IDAHO AND MONTANA

The newly mapped Shewag Lake quadrangle in the Beaverhead Range of east central Idaho (supported by USGS EdMap) contains a >5.5 km succession of Mesoproterozoic “Big White” subarkosic arenites. The Beaverhead Divide Fault (a northwest striking thrust-fault which runs along the base of the Beaverhead Range) passes southwest of the Shewag quadrangle. The quadrangle consists of a northeast-dipping homocline cut by minor, north-striking normal faults, and at least one larger, northwest trending thrust-fault. The Carmen Creek anticline is exposed in the southeast corner of the quadrangle, exposing the stratigraphically deepest strata (Gunsight Formation).

In the Allan Mountain quadrangle to the northwest, Stewart et al., (2009) correlates the Big White quartzite unit with the two lower members of the Mt. Shields Formation of the Missoula Group. In contrast to progradation and retrogradation of sediment types found in the Missoula Group near Missoula, the Beaverhead Big White unit is homogenous, subaerial, and interpreted as periodic sheet flood events within an alluvial apron. It contains abundant planar and trough crossbeds. Fining-upward cycles of 75 cm to 1.5 m are common. Paleocurrent measurements (n= 47) taken from 3-dimesional dunes of the Sheep Creek quartzite strata plot between 340 NW and 90 E, indicating a unimodal, north north-east flow direction. The Beaverhead quartzites can be interpreted as vast alluvial apron deposits which drained in a northward direction, towards the regressive and transgressive Missoula Group lake to the north.

U-Pb ages on detrital zircon populations from the Carmen Creek quartzite and the Fourth of July Creek quartzite are similar to populations in other Missoula Group strata, and have unimodal peaks at 1730 Ma. Smaller syn-Belt (1400-1470 Ma) and Archean populations are also present.