STRATIGRAPHY AND STRUCTURE OF THE HORSE PRAIRIE BASIN AREA OF SOUTHWESTERN MONTANA

Geologic mapping of the Coyote Creek 7.5 minute quadrangle in southwestern Montana along with detrital zircon dating of several quartzites and a gneiss has produced revisions to the currently accepted stratigraphy and structure of the central portion of the Horse Prairie basin and Bloody Dick Ck. areas, while confirming some earlier interpretations. The south and east part of the quadrangle is composed of Tertiary basin sediments while the north and west part consists of Mesoproterozoic Belt quartzites.

The southwestern part includes a fault-bounded block of the gneiss of Bloody Dick Creek, which includes schist with a Paleoproterozoic sedimentary protolith <1830 Ma. Some or all of the Bloody Dick Creek unit is thus Paleoproterozoic and unrelated to either the Archean gneiss of the Maiden Peak “Prong” that abuts the basin to the south or the Dillon granite gneiss to northeast.

The quartzites within the Coyote Creek quad generally lack sedimentary features. Definition of units based solely on grain size and/or color is unreliable. Detrital zircons from two fault-separated units suggest affinity with the Jahnke Lake member of the Apple Creek Fm. near the top of the Belt Supergroup. Detrital zircons from two other fault-separated units confirm their mapping as Swauger Fm. In the northeastern part of the quadrangle we map the Swauger Fm. thrust over the younger (Jahnke Lake?) Belt Supergroup quartzites of the Grasshopper thrust plate confirming earlier work that the Medicine Lodge thrust plate extends north of the Horse Prairie basin.

The Horse Prairie basin and Bloody Dick Ck. have been posited as the location of a regional-scale Mesoproterozoic left-lateral shear zone (the Great Divide Megashear) originating in Idaho and continuing northwestward into Washington, separating the Belt Supergroup from the Lemhi Gp. and overlying strata. There are no indications of left-lateral shearing anywhere within the quad and stratigraphic relationships do not imply a shear zone exists.