ABSTRACT

The Dry Creek Valley is a northern extension of the Three Forks basin in southwestern Montana. It owes its origin to block-faulting along the Bndger Range front and to erosion by through-flowing steams in middle Eocene time. By late Eocene, the Dry Creek Valley stood amidst a mature topography carved by the through-flowing streams maintained by a humid climate.

Deposition of the Renova Formation began during middle Oligocene time (30.6 ± 1.2 m.y.b.p). Deposition was initiated by uplifts of the Bridger Range which blocked major drainages. Internal drainage and basin-filling was further promoted by a climatic shift toward arid conditions A succession of fine-grained limestones and tuffaceous mudstones were deposited in low energy lacustrine and floodplain environments.

By late Oligocene, the climate became humid enough to maintain through-flowing steams. An unknown amount of Renova strata was subsequently removed from the Dry Creek Valley This erosional period continued until the late Miocene.

Deposition of the Sixmile Creek Formation began in late Miocene time. Deposition was initiated by recurrent uplifts of the Bridger Range. A climatic shift toward aridity accompanied these tectonic movements. This period of aridity was much more severe than that of Oligocene time. Coarse angular detritus was transported by high energy ephemeral runoff and deposited in stream channel and alluvial fan environments. Deposition continued until middle Pliocene time. By the late Pliocene, the adjacent highlands were partially buried in their own debris A surface of low relief subsequently formed on the thick section of basin-fill.

With the onset of the humid Pleistocene climate and epeirogenic uplift, through-flowing streams began to remove large quantities of Sixmile Creek and Renova sediments During this period of erosion, the mature Eocene topography has been partially exhumed. The history of the dissection of the late Pliocene surface is intimately tied to the development of the present topography.