Abstract

The Late Jurassic Swift and Morrison Formations in western and central Montana make up an unconformity-bounded marine-nonmarine depositional sequence which records coastal onlap and subsequent progradation. This marine-nonmarine sequence overlies the J-4 erosional unconformity developed upon older Jurassic rocks wherever the Swift is thick, and upon late Paleozoic rocks where the Swift is thin. Thicker accumulations developed over paleotopographic lows. Ancient relief was on the order of 10’s of meters or less.

Oxfordian marine sedimentation in the Swift Formation was controlled in part by intrabasinal paleotopographic elements which were a result of pre- and perhaps syndepositional reactivation of basement structures. An elongate NE-oriented structurally positive area, the Belt Island Complex, extended through the central part of the study area. The southern margin of the Belt Island Complex coincides in part with the Proterozoic Willow Creek normal fault at the southern margin of the Central Montana Salient. Here, facies were strongly controlled by isolated emergent areas and intervening embayments. Basal allochthonous chert-pebble-cobble conglomerate derived from local sources was deposited in wave-dominated fan-delta and shoreface environments, and was succeeded by tidal-channel and bar-top deposition of stacked intervals of multi-story fining-upward, trough cross-bedded, bivalve-rich sandstone.

The northern margin of the Belt Island Complex in the vicinity of the Great Falls Tectonic Zone includes South Arch, a north-northwest-oriented structural high on the Sweet grass Arch. Facies data from around South Arch indicate low-relief paleotopographic control upon moderate to high-energy tide-dominated sedimentation. Initial sedimentation in estuarine paleovalleys is represented by the basal conglomerate unit which includes fluvial-transported pebble-cobble conglomerate, fine-grained siliciclastic tidal deposits, abundant oyster debris, and scattered plant debris. Completion of paleovalley filling and transgression over paleohigh areas is represented by a laterally extensive, multi-story sandstone body composed of tidal-channel and tidal-bar and platform lithofacies. Fluvial-transported conglomerate thins over paleohigh areas and is replaced by gravel derived in situ within the base of the upper sandstone body. These relationships indicate transgressive erosion with in situ gravel derivation atop paleohighs.

Farther west of South Arch, a deeper sub-basin was passively inundated and served as a major mud trap during initial Oxfordian sea level rise. Here, the lower Swift Formation consists of a local, thin basal conglomeratic lag and a 10–20 m-thick, upward-coarsening dark gray mudstone. The laterally extensive tide-dominated sandstone body overlies the mudstone sequence.

Throughout western and central Montana, the upper Swift sandstone body is capped by bioturbated fine-grained sandstone deposited on tidal flats, grading upward to green mudrocks of the lowermost Morrison Formation which were deposited in coastal-plain environments. Facies succession in the uppermost Swift Formation indicates progradation and a decrease in paleobathymetric irregularity related to basin filling.