SILICICLASTIC SHELF AND ESTUARINE FACIES OF THE MARINE JURASSIC SWIFT FORMATION, MONTANA, U.S.A

This study investigates the facies and depositional environments in the Upper Jurassic Swift Formation of central Montana, which is depositionally down dip of the Sundance Formation of Wyoming. The lowermost portion of the Swift consists of offshore facies and is primarily homogenous siltstone to mudstone lacking trace fossils or sedimentary structures. The offshore facies is highly fossiliferous, with abundant belemnites, Gryphaea, bivalves, rarer ammonites, echinoids, and ichthyosaur bones. It also often contains large concretions (7–55 cm) that occasionally contain echinoids, bivalves, belemnites, and Gryphaea. The offshore facies is overlain by very thin-bedded, very fine to fine-grained shoreface sandstone. This facies contains abundant glauconite, current and wave ripples, and planar lamination. Within this facies are densely packed shell beds, composed of whole valves of bivalves. The shoreface facies are overlain by tidal bar and tidal sand flat facies of the uppermost Swift. The tidal bar facies is a very thin to thin-bedded, very fine to medium-grained sandstone with pervasive large and small-scale trough cross stratification, mud intraclasts, planar lamination, and sigmoidal cross stratification. Trace fossils include Ophiomorpha, Diplocriterion, and Skolithos and bioturbation locally obscures sedimentary structures. The tidal sand flat facies is very thin to thin-bedded, very fine to fine-grained sandstone with current ripples that may be climbing and trace fossils are similar to the tidal bar. Overall, the facies of the Swift match well with those found in the Redwater Shale and Windy Hill Sandstone (Sundance Formation) of Wyoming, but there is a new facies separating the Swift from the Sundance. At or near the top of the Swift on the east flank of the Little Belt Mountains is a conglomerate facies consisting of clast-supported, 1–7 cm, well-rounded pebbles of chert and quartzite which likely indicates an unconformity.