THE NORTHERN EXTENT OF THE MESOPROTEROZOIC LEMHI GROUP, IDAHO AND MONTANA, AND STRATIGRAPHIC AND STRUCTURAL RELATIONS WITH BELT SUPERGROUP STRATA

The newly revised Lemhi Group in Idaho and Montana is an immensely thick (>14,000 m) succession of mainly feldspathic quartzite deposited in a southwestern arm of the Mesoproterozoic Belt basin. Previous maps placed the boundary between the Lemhi Group and more typical Belt Supergroup strata near the Idaho-Montana border, and interpretations of the boundary’s character have varied widely, ranging from post-depositional structural juxtaposition of two separate basins to facies changes within a single basin. New mapping and published detrital zircon data suggest that most of the Lemhi Group is correlative with the much thinner (<3,500 m) Missoula Group of the upper Belt Supergroup, indicating that subsidence of the Lemhi subbasin was significantly faster than that of the main Belt basin during this time.

Lemhi Group strata extend northward into the Sapphire and West Pioneer Mountains of Montana, with the northern and eastern limits of these rocks defined by the Lewis and Clark Line and the Georgetown-Philipsburg thrust system, respectively. In the southern Sapphire Mountains, a thick quartzite succession correlated with the upper Lemhi Group gradationally overlies the Piegan Group of the Belt Supergroup. Farther north and east, this sandy Lemhi succession appears to grade laterally into the thinner and finer-grained Missoula Group. The spatial distribution of extensive Mesoproterozoic(?) breccia in the underlying Wallace Formation, at the top of the Piegan Group, is also roughly coincident with the northern and eastern boundaries of the Lemhi Group rocks. The Wallace breccia occurs in the middle of the Wallace Formation in some areas, and at the top of the Wallace in other areas where it is overlain by quartzite correlated with the Lemhi Group. The breccia may have formed along growth faults that produced the rapidly subsiding Lemhi subbasin in the southwestern part of the Belt basin.