ABSTRACT

Deposition of carbonate sediment of the Rierdon Formation (Callovian) in southwestern Montana was strongly influenced by the presence of Belt Island, a northeast-southwest-trending tectonic high that persisted in the region throughout the deposition of the Jurassic System. Stratigraphic sections chosen for this study extend along the southern flank of Belt Island, from the Boulder batholith eastward to the Bridger Range, and are located within a few meters to 40 kilometers of the zero-edge of Rierdon rocks. In this area, the Rierdon Formation unconformably overlies the Upper Permian Phosphoria Formation, and is unconformably overlain by the Upper Jurassic Swift Formation.

Peloidal grainstone is the dominant lithofacies in the lower part of the Rierdon Formation and occurs mainly in the western part of the study area. Quartz and chert sand is moderately abundant in these rocks, and chert pebbles are locally profuse. The terrigenous detritus was derived from adjacent sources on Belt Island where rocks of the Phosphoria Formation were exposed. Climbing ripples, ripple-drift cross-lamination and low angle planar sets of cross strata (some with reactivation surfaces and others with bimodal orientation of foresets) indicate strong tidal influences upon the deposition of these sediments. Bio-wackestones and micrite are scarce in the Rierdon Formation but are present in the western part of the study area where they overlie peloidal grainstones. Algal stromatolites characterize several horizons of this lithofacies. Oolitic grainstones are the major lithofacies in the Rierdon Formation. In many sections in the western part of the study area, these rocks comprise the uppermost unit of the formation, and also occur as interbeds in the peloidal grainstone sequence of the lowermost Rierdon. Some sections (especially to the east along the southern margin of Belt Island) are comprised entirely of oolitic grainstone. Although the oolitic horizons generally lack current structures, ripple bedding, low-angle planar sets of cross strata, and evidence of cut and fill are present in some sections.

The facies mosaic of the Rierdon Formation suggests the existence of a shallow peritidal shelf on the southern flank of Belt Island during Rierdon time. Peloidal grainstones with sedimentary structures indicative of tidal influence were deposited on the inner shelf during the transgression of Callovian seas over the margins of Belt Island. Small transitory oolite bars migrated across this shelf. Siliciclastic detritus was transported to the inner shelf from sources to the north on Belt Island. Shallow lagoons existed on the shelf and were characterized by the deposition of lime mud and scattered skeletal grains and the formation of algal stromatolites. Oolite shoals developed near the outer edge of the shelf and migrated landward over the muds of the lagoons and the peloidal sands of the inner shelf. In some localities, presumably near low-lying islands, shoaling conditions persisted throughout deposition of the Rierdon, so that oolitic grainstone was the only lithofacies deposited.