PROTEROZOIC AND TERTIARY ROCKS OF THE COYOTE CREEK 7.5' QUADRANGLE SOUTHWESTERN MONTANA

New information from the Coyote Creek 7.5' Quadrangle southwestern Montana consists of geologic mapping, U-Pb ages of detrital zircons, and discovery of a fossil beaver jaw. Mesoproterozic feldspathic quartzites and Paleoproterozoic gneiss and schist comprise the western and northern 3/5ths of the quadrangle. They are separated from Tertiary basin fill that comprises the southeastern 2/5ths of the quadrangle by the basin-defining Meriwether-Lewis normal fault.

Detrital zircons from the five definable quartzite units were age dated. Three units are similar to Lemhi Group rocks with sparse zircon grains as old as 2650 Ma, a strong unimodal age-peak at 1720 to 1740 Ma, and sparse grains as young as 1400 Ma. Two units are comparable to Missoula Group rocks with a slightly older and less unimodal set of zircon age-populations, with age-peaks from 1750 to 1800 Ma and smaller peaks ranging from 2400 to 2800 Ma. Stratigraphic relationships imply alternating zircon sources. Correlation among the quartzites remains uncertain. The small area of schist and gneiss on the southwest corner of the quadrangle at the mouth of Bloody Dick Creek has a strong unimodal zircon age peak of 1800 Ma and sparse grains as old as 2750 Ma. It is in transpressive contact with quartzites to the northeast and normal fault contact with quartzites to the west. Quartzite previously mapped as thrust eastward on a thrust fault in the Bloody Dick Creek fault zone is now mapped as down-dropped to the west along a normal fault. The previously reported Cambrian age of quartzites in the northeast part of the quadrangle has not been corroborated.

Tertiary rocks in which a beaver jaw of Arikareean age was found consist of late Oligocene to early Miocene interbedded volcanic ash, mudstone, tuffaceous shale, siltstone, and conglomerate. The northeast dipping Coyote Creek normal fault extends southeast to northwest across the quadrangle down-dropping both Proterozoic and Tertiary rock to the northeast.