ABSTRACT

The Jefferson Formation (Frasnian) in southwestern Montana consists of cyclic shallow marine platform deposits that grade westward into a very thick sequence of cyclic shallow water to deep ramp/slope/basinal facies in central Idaho. The Jefferson platform had a ramp-like profile throughout its history.

Individual shallowing-upward platform cycles in Montana are 10's of feet to <3 feet thick (10's m to <1 m thick). Inner platform localities contain thinner cycles (<16.4 ft thick; <5 m thick) with fewer basal, subtidal facies. Outer platform localities contain thicker individual cycles with thicker subtidal facies. Medial parts of the platform generally have greater numbers of cycles than on either the inner or outer platform. This probably results because: 1) thin cycles on the medial platform are recorded as extremely thin sequences or nondepositional surfaces on the inner platform, 2) basal Jefferson deposits on the inner ramp may be younger than deposits on downdip parts of the ramp, and 3) the outer platform may not have become shallow enough at various times for cycles to be easily recognized.

Many Jefferson cycles have periodicities which fall within the Milankovitch band. Very thick cycles may record long period sea level oscillations, increased local subsidence rates during cycle deposition, some type of harmonic amplification of the mechanisms responsible for generating shorter period cyclic sequences, or a combination of these factors. The Jefferson Formation may be an important analogue for other cyclic carbonate sequences which form on barrierless platforms. The Jefferson Formation in Montana also contains the stratigraphic record of a carbonate platform's response during transition of a passive margin into a convergent margin. Therefore, the Jefferson Formation provides an important example of cyclic sedimentation on a tectonically active platform, unlike many other ancient cyclic sequences which formed in relatively quiescent tectonic settings.

The timing and mechanisms of dolomitization of the Jefferson Formation as inferred from petrographic and geochemical data, indicates adiagenetic history that includes episodes of early, near surface dolomitization and calcite cementation, as well as later episodes of shallow to deep burial dolomitization. Petrographic and geochemical studies of Jefferson dolomites resulted in recognition of three major dolomite types. Jefferson dolomites show a progression from early, nonstoichiometric, euhedral replacive dolomites, to nearly stoichiometric, subhedral/anhedral dolomite mosaics. 5laO and S13C ratios decrease with increasing extent of dolomitization. Minor amounts of syndepositional dolomite probably precipitated from evaporitic brines generated in arid supratidal environments. However, pervasive dolomitization of the Jefferson Formation probably occurred during deposition of evaporitic units in the overlying Three Forks Formation.