ABSTRACT

Marine and marginal marine Lower Triassic rocks are sporadically exposed in southwestern Montana and adjacent states. These rocks disconformably overlie Upper Permian marine rocks, and are unconformable with younger rocks of Mesozoic or Tertiary age. Triassic rocks here are assignable to four formations. These formations, from youngest to oldest, are Dinwoody, Woodside (Red Peak to the east), and Thaynes. After deposition of the Thaynes, the Triassic sea withdrew from the region. All Triassic units were progressively removed from north to south by postdepositional erosion during pre-Middle Jurassic and pre-Cretaceous uplifts in the northern part of the study area (Moritz, 1950, 1951).

This report is based on forty-two measured sections of Triassic rocks in Park (1), Gallatin (2), Madison (12), Beaverhead (17), and Silver Bow (2) counties in southwestern Montana; Lemhi (2), Clark (1) and Fremont (1) counties in east-central Idaho; and Park (1) and Teton (3) counties in northwestern Wyoming. Specific information on each measured section is provided in the Appendix.

Conodonts recovered from closely spaced carbonate samples collected from the measured sections define the initial Mesozoic marine transgression at the north and west limits of the Triassic depositional basin on the flanks of the North American craton. Information on the distribution, thickness, and lithology of stratigraphic units within a conodont-defined biostratigraphic framework establishes depositional environments and clarifies a pattern of transgressions and regressions during the Griesbachian, Dienerian, and Smithian stages of the Early Triassic.

Previous work on Triassic rocks in the southwestern Montana area created some misconceptions that are now firmly entrenched in geologic literature. These include: (1) the character and significance of the Permian-Triassic contact, (2) the position of the Meekoceras zone within the Triassic stratigraphic succession, (3) paleontologic and lithologic subdivisions of the Dinwoody Formation, and (4) occurrences of Thaynes directly overlying Dinwoody without intervening Woodside or the presence of the Meekoceras zone. Each of these concerns is described in more detail and resolved in the text that follows. Postdepositional structural dislocation of Triassic rocks and an evaluation of the petroleum potential of this region based on the conodont color alteration index (CAI) (Epstein and others, 1977) are also described.