THE FORMATION OF INSITU CARBONATE-SILICICLASTIC SHELF DEPOSITS BY STORM PROCESSES: THE EARLY TRIASSIC DINWOODY FORMATION, SW MONTANA

The upper limestone member of the Dinwoody Formation in southwest Montana consists of mixed carbonate-siliciclastic shelf deposits of early Triassic age. The sediments were deposited in a large north-south-trending seaway that developed during the last major transgression of the craton prior to the Cordilleran Orogeny. Although the faunal turnover at the Permian-Triassic boundary has been extensively studied, the depositional systems of early Triassic sedimentary deposits in southwest Montana are poorly understood. To reconstruct these depositional systems, five sections of the upper Dinwoody Formation were measured and described within Beaverhead County, Montana. The upper limestone member of the Dinwoody Formation consists primarily of interbedded chocolate-brown grainstone and tan-colored siltstone. The carbonate component of the system is typically normally graded with tan siltstone as the cap to each graded sequence. The fossils in the carbonate component consist of disarticulated to fragmented brachiopod shells. In contrast, brachiopods are rare in the siliciclastic beds, except near the top of each graded sequence, where they are typically articulated and in the living position. The depositional system is interpreted as a storm-dominated, shallow-shelf environment where the carbonate component is produced by the insitu excavation of brachiopod shells from the siliciclastic shelf deposits during storms to form widespread graded beds of carbonate grainstone across the shelf. The graded sequence is capped by the fine-grained, suspended siliciclastic shelf sediment that accumulates as the storm energy dissipates.