EUSTATIC VARIATIONS WITHIN A MISSISSIPPIAN-PENNSYLVANIAN SUBTIDAL MIXED CARBONATE-SILICICLASTIC SUCCESSION, BEAVERHEAD MOUNTAINS, IDAHO-MONTANA

An ~600 m thick succession of Uppermost Mississippian-Middle Pennsylvanian interbedded carbonate and siliciclastics was mapped, measured, described and collected for conodonts during an EDMAP project in the summer-fall of 2006. These rocks include a rich marine biota (bryozoans, crinoids, phylloid algae, brachiopods, corals, and fusulinids among others) indicative of a normal marine deposition spanning from deep subtidal to peritidal environments. The conodont collections indicate almost the entire Chesterian-lower part of the Desmoinesian stages is preserved. The succession contains a robust record of eustatic variations juxtaposing subtidal carbonates with nearshore to eolian siliciclastics. Greater than 50 meter-scale shallowing-upward cycles (parasequences) range from 2 to 16 m thick. The parasequences are grouped into parasequence sets and sequences that range from 50 to 80 m thick; and each is comprised of 5-10 parasequences. Given the duration of the succession (Latest Mississippian-Middle Pennsylvanian) 318 Ma to ~308 Ma the parasequences are interpreted to record 100-200 k.y. (4th-order) eustatic variations on a tectonically quiescent passive margin. The parasequence sets and sequences range from 1-2 Ma each. The juxtaposition of the subtidal facies with eolian facies indicates high-frequency sea level fluctuations during the Morrowan-Atokan were moderate- to high-amplitude (> 30 m) likely produced by the waxing and waning of Gondwanan glaciers. However, the stacking patterns of the parasequences and their attendant facies do not, as yet, indicate a short-lived non-glacial interval in the Atokan (between C2 and C3) as suggested by Fielding et al. (2006).