Cyclic Platform Dolomites of Devonian Jefferson Formation, Montana and Idaho

S. L. Dorobek

Preliminary field study indicates that the Devonian Jefferson Formation in southwestern Montana and adjacent parts of Idaho consists of cyclic sequences of shallow marine platformal dolomites that grade basinward into slope sediments deposited on a steepened carbonate ramp. Individual shallowing-upward, platformal cycles are 25 to < 1 m (82 to 3 ft) thick and, from top to bottom, consist of: solution-collapse breccia caps; cryptalgal dolomudstone; rare ooid dolograinstone; thin-bedded Amphipora dolowackestone; sucrosic dolostones with abundant lenticular to domal stromatoporoids; thin-bedded, fine-grained, shaly dolostones with closely spaced hardgrounds that grade upward into burrow-homogenized, irregularly bedded dolostones. Thinner cycles (< 5 m or 16 ft thick) c ntain fewer basal lithologies and typically consist only of cryptalgal dolomudstone with breccia caps. The 1 to 25-m thick cycles comprise larger scale sequences (30-60 m or 100-200 ft thick), in which smaller scale cycles become progressively thinner toward the top of large-scale sequences. These shallowing-upward carbonate cycles probably formed in response to glacio-eustatic sea level fluctuations. Current estimates for the time span represented by the Jefferson Formation (9 m.y.), divided by the number of shallowing-upward cycles, indicate that each cycle represents an average time span of 0.6 to 1.0 m.y. This time span suggests that either: (1) average sedimentation rates were unrealistically slow during deposition of each cycle (< 0.1-3 cm/1,000 years); (2) breccia caps represen long periods of subaerial exposure (> 0.5 to about 0.2 m.y.); or (3) the Jefferson Formation was deposited during a much shorter time span than previously thought.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.