Sedimentology of the Swift Formation (Jurassic) in the Little Rocky Mountains of Montana

Abstract

The marine strata of the Swift Formation (Upper Callovian-Oxfordian) are widely distributed and well exposed in the Little Rocky Mountains of north-central Montana. The contact between the Swift and the underlying marine Rierdon Formation is sharp, whereas the upper contact with the non-marine Morrison Formation is gradational. The Swift Formation is about 30 m to 50 m thick and is divided into two members: a lower shale and an upper sandstone. Detailed sedimentological analysis defined six facies; three in each member. The shale member contains a conglomerate facies (Facies A), a shale-siltstone facies (Facies B), and a bioclastic limestone facies (Facies C). The facies of the sandstone member comprise a sandstone­-siltstone-shale facies (Facies D), a cross-bedded sandstone facies (Facies E), and a limestone facies (Facies F). The Swift Formation forms a coarsening-upward sequence from mud to sand-silt-mud intercalations to sand, which has been interpreted by other people as a progradational sequence across a shelf. The Rierdon-Swift contact is a disconformity spanning three ammonite zones. The whole section of the Swift Formation is considered to be a shallow marine shelf deposit that formed in the course of a transgressive-regressive episode during Late Callovian-Oxfordian time. Facies A was produced by the reworking of sediment by waves in a near­shore setting during the early stage of the transgressive sea. Facies B was deposited from suspension in relatively deep, open, marine waters during the maximum expansion of the Oxfordian sea. Facies C was formed by the winnowing effect of frequent storm-generated waves, reworking the muddy platform deposits of Facies B. Facies D and E form a continuous regressive sequence that was deposited in a storm-dominated, lower shoreface environment. Facies F was deposited in a shallow, relatively protected setting. The depositional model proposed for the Swift Formation in the study area is one of a shifting pattern of sedimentation in a shallow marine setting, where inner shelf deposits passed transitionally into lower shoreface deposits; these, in turn, gave way to middle-to-upper shoreface sediments. The sea-level changes during the deposition of the Swift Formation were as a result of mainly local and regional tectonism; eustatic factors, if any, were minor