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Abstract

The Lower Mississippian Mission Canyon Formation and stratigraphic equivalents in Montana and Idaho were deposited on a progradational carbonate ramp that developed on the foreland side of the Antler foredeep. Shallow subtidal and peritidal lithofacies were deposited in ramp-interior settings across most of Montana. The ramp to basin transition in westernmost Montana was a relatively narrow belt of stacked skeletal grainstone banks. Farther west, skeletal grainstone banks prograded over and interfingered with outer ramp/slope cherty limestones. In east-central Idaho, coeval lower slope and basinal strata consisted of silty to argillaceous, spicular limestones, spiculites, and spicular calcareous siltstones/fine-grained sandstones.

In individual outcrops, stacked parasequence sets are the most prominent sequence stratigraphic units. Lateral lithofacies relationships across the deformed ramp to basin transition were reconstructed using regional biostratigraphic and lithostratigraphic correlations of measured sections. Depositional sequences and system tracts were identified from characteristics of bounding surfaces, stacking patterns of parasequences and parasequence sets, and lateral lithofacies relationships.

The reconstructed ramp to basin transect illustrates a progressive upward change in third-order sequence boundary type during evolution of the Mission Canyon platform. Type 2 sequence boundaries formed early in platform development, whereas type 1 sequence boundaries dominated later platform development. Associated ramp-margin wedges thicken and are composed of progressively larger proportions of peritidal lithofacies upward. Compared to correlative surfaces on global onlap-offlap curves, third-order sequence boundaries that formed early in Mission Canyon platform development appear subdued, whereas those that formed later appear enhanced. Combined with subsidence analysis, these relationships suggest that gradually waning flexural subsidence and falling second-order eustatic sea level permitted higher order eustatic sea levels to fall progressively farther basinward as the Mission Canyon platform evolved. This long-term decrease in accommodation profoundly influenced progradation of the Mission Canyon platform and was a major factor in maintaining a ramplike profile across the platform to basin transition.

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