Correlation of and Facies Changes in the Carbonaceous, Calcareous, and Dolomitic Formations of the Precambrian Belt-Purcell Supergroup

Abstract

Two thick carbonate-bearing sequences are exposed in most eastern Belt-Purcell sections, whereas only one is exposed in western sections. Stratigraphic evidence suggests that the lower eastern sequence (Fort Steele, Altyn and Waterton, and Newland Formations) intertongues with or grades westward into carbonate-poor but carbonaceous strata (Aldridge and Prichard Formations). The upper sequence in the east (Siyeh and Helena Formations) is correlated with the western sequence (Kitchener and parts of Kitchener-Siyeh of British Columbia, Wallace of northern Idaho and western Montana, and “Newland” of the Philipsburg and Missoula areas, Montana) and forms the only laterally continuous carbonate-bearing sequence in the entire Belt-Purcell Supergroup. Based on these correlations, a fourfold division of the Precambrian Belt-Purcell Supergroup is recognized. In the United States these divisions are referred to as the Pre-Ravalli sequence and the Ravalli, Piegan, and Missoula Groups. Formally the classification is identical to previous classifications, but the informal term “Pre-Ravalli sequence” emphasizes the importance of the lowest Belt-Purcell subdivision. These four subdivisions are recognized throughout most of the Belt-Purcell terrain, dividing the succession into two cycles. The lower cycle comprises the Pre-Ravalli sequence, whose finer-grained fraction is characterized by black to gray, carbonaceous and/or carbonate-bearing, pyritic and commonly very evenly and finely laminated (“varved”) strata and the Ravalli Group, whose finer-grained fraction is typified by reddish to purplish (hematitic) or greenish rocks that contain abundant mud cracks and intraformational mud-chip conglomerates and are less evenly and less finely laminated. The upper cycle is composed of the Piegan and Missoula Groups, which are lithologically similar to the Pre-Ravalli sequence and the Ravalli Group, respectively. A third carbonate-bearing sequence, the Shepard Formation, is less extensive and generally much thinner than the Piegan Group. Its regional variations and correlatives are little known. Stratigraphic evidence suggests that the Shepard may be a tongue of the uppermost Piegan Group of northern Idaho and westernmost Montana (and its equivalent in the Purcell Supergroup of adjacent British Columbia) that interdigitates with the lowermost Missoula Group (and its Purcell equivalent) throughout much of the Belt-Purcell terrain. Facies belts within the Pre-Ravalli sequence (and equivalent Purcell strata) trend northwest, with shallower water strata toward the northeast. Facies belts within the Piegan Group (and equivalent Purcell strata) have a similar trend but a symmetrical disposition owing to a deeper water, calcite-bearing facies in the center of the depositional trough, flanked on both sides by shallower water, dolomite-bearing strata. The principal differences between Pre-Ravalli, Piegan, and Shepard deposition are attributed to variations in the average water depth in the Belt-Purcell basin. Although present Belt-Purcell Stratigraphic classification is confusing, only one important proposals is made for its revision—the Belt Supergroup is defined to exclude strata equivalent to the Winder-mere Group of Canada. Thus the Belt and Purcell Supergroups are to be regarded as precise stratigraphic equivalents.