Cyclic Platform Dolomites and Platform-to-Basin Transition of Jefferson Formation (Frasnian), Southwest Montana and East-Central Idaho

S. L. Dorobek

The Jefferson Formation (Frasnian) in southwestern Montana consists of cyclic sequences of shallow marine platformal dolomites that grade westward into slope/basinal facies in east-central Idaho. Regional sedimentologic characteristics of slope facies in Idaho indicate that the Jefferson platform resembled a distally steepened ramp. Slope facies consist of slope laminites with local small scale slumps and slope breccias. Shallow water platform-derived clasts are lacking in the slope breccias.

Individual shallowing upward platform cycles are 25 m to < 1 m thick and consists of, in descending order: local solution-collapse breccia caps; cryptalgal dolomudstone; rare ooid dolograinstone; thin-bedded Amphipora dolowackestone; coarsely crystalline dolostones with abundant lenticular to domal stromatoporoids; and basal thin-bedded, fine-grained, shaly dolostones with closely spaced hardgrounds that grade upward into burrow-homogenized, irregularly bedded dolostones.

Sections that occur closer to paleostrandline positions contain thinner cycles (< 5 m thick), which have fewer basal, subtidal facies and typically consist only of cryptalgal dolomudstone with breccia caps. More basinward sections contain thicker individual cycles with thicker subtidal sequences. Also, basinward sections have greater numbers of cycles than more shoreward sections, suggesting that thin cycles deposited on basinward parts of the platform may not have any sedimentary record on shallower parts of the Jefferson platform.

Most Jefferson cycles probably were formed during 20,000- to 100,000-year periods, which are very similar to periods for sea level oscillations caused by perturbations in the earth's orbit (i.e., Milankovitch cycles). Rare, very thick cycles may record high amplitude sea level oscillations, long cycle periods, increased subsidence rates during cycle deposition, or some type of harmonic amplification of the mechanisms responsible for generating shorter period cycles.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.