REVISIONS OF MESOPROTEROZOIC AND CAMBRIAN STRATIGRAPHY: IMPLICATIONS FOR BELT BASIN PALEOGEOGRAPHY WITHIN THE BUTTE SOUTH 30’ X 60’ QUADRANGLE, SW MONTANA

The Montana Bureau of Mines and Geology recently completed geologic mapping in the Butte South 30’ x 60’ quadrangle. The diverse geology of the quadrangle includes Archean(?) and Paleoproterozoic crystalline basement rock; Mesoproterozoic through Cretaceous metasedimentary and sedimentary rock; Cretaceous and Tertiary intrusive and volcanic rock; and Cenozoic valley-fill deposits. The stratigraphy of the Mesoproterozoic Belt Supergroup and overlying Cambrian strata were a main focus of our research and mapping. Our work revises some of the Mesoproterozoic and Cambrian stratigraphy in the Highland and Pioneer Mountains and improves our understanding of the paleogeography of the Belt basin.

In the Highland Mountains, originally mapped by O’Neill and others (1996), we assign the uppermost Belt strata to the Ravalli Group rather than to the Missoula Group; the underlying calc-silicate-bearing interval to the Greyson Formation rather than to the Helena Formation; and the Newland, Table Mountain, and Moose Formations to facies of the LaHood Formation. These interpretations better explain the thin (<750 m) Belt section in the Highland Mountains and place the Precambrian-Cambrian unconformity at a stratigraphic level consistent with surrounding areas. The coarse clastic deposits of the LaHood Formation overlie crystalline basement and record deposition into the Belt basin along its uplifted, structurally controlled southern margin.

In the Pioneer Mountains, we interpret the Black Lion Conglomerate (Zen, 1988) as Mesoproterozoic rather than Cambrian age. The Black Lion bears a strong resemblance to the LaHood Formation and similarly rests on crystalline basement. We interpret it as a geographically restricted facies also deposited along a steep margin of the Belt basin. We therefore extend the southern margin of the Belt basin from the Highland Mountains west into the northern Pioneer Mountains. From there, the margin turns southward and extends as far south as the central Beaverhead Range, where Proterozoic conglomerate and quartzite overlie basement. West of the N-S-trending margin segment, the Grasshopper thrust system places fine-grained basin deposits correlated with the Lemhi Group of Montana and Idaho over the Belt-basin marginal facies.