ABSTRACT

The Lower Mississippian Mission Canyon Formation and its stratigraphic equivalents in southwestern Montana and east-central Idaho were deposited on a ramp-like carbonate platform that graded into an actively subsiding foredeep. Platform facies of the Mission Canyon Formation in southwestern Montana consist of several 130-330 ft (40-100 m) thick, shallowing-upward parasequences. Carbonate sand facies or bioturbated skeletal wackestones form the basal facies of all platform parasequences. These subtidal deposits grade upwards into progressively finer grained, intertidal to supratidal facies. Facies which cap parasequences in central and southwestern Montana show evidence of subaerial exposure and include such features as cryptalgalaminites, desiccation features, evaporite pseudomorphs, and vadose diagenetic fabrics. Subaerial exposure surfaces at the tops of Mission Canyon platform parasequences represent relatively brief periods of time, when the aggraded platform was exposed during relative sea level falls. Mechanisms responsible for platform drowning and subsequent generation of each parasequence are not fully understood at present.

Parasequences also have been recognized in equivalent slope and basinal facies in western Montana and east-central Idaho. Bases of lower slope parasequences consist of horizontal-laminated, pyritic, calcareous siltstones and silty limestones. Overlying lithologies become progressively less pyritic, more calcareous, more bioturbated, and more skeletal upwards. Basal facies of upper slope parasequences consist of horizontal-laminated, cherty mudstones. Overlying beds become progressively thicker and more irregular bedded, more bioturbated, and more skeletal upwards.

Lateral facies variation within Mission Canyon parasequences can be assessed only through regional correlation of shallowing-upward rock units observed at widely separated stratigraphic sections. Accurate correlations can be made between stratigraphic sections which lack abundant biostratigraphic sample control through the use of sequence stratigraphy. This adaptation of sequence-stratigraphic concepts is an essential step in large-scale basin analysis and may prove useful for reconstructing other ancient, tectonically shortened, sedimentary sequences.