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In Late Devonian and Mississippian times, as much as 4500 m (15,000 ft) of well-bedded neritic and bathyal marine, flyschlike mudstone, siltstone, sandstone, conglomerate, and subordinate impure limestone was deposited in a subsiding, elongate, foreland basin (exogeosynclinal trough) on the continental shelf (Cordilleran miogeocline) east of the Antler orogenic belt and west of the cratonic platform. The term Antler flysch is applied to these Upper Devonian and Mississippian, and related Lower Pennsylvanian, homotaxial deposits that filled the Antler foreland basin in the western United States. The major source of siliceous flysch sediments in the foreland trough was terrigenous detritus derived from a rising cordillera to the west composed of strongly deformed Devonian and older oceanic rocks that during the Antler Orogeny was deformed and subsequently obducted eastward onto the outer carbonate shelf as the Roberts Mountains Allochthon. Significant amounts of westerly derived detritus in Upper Devonian deposits reflect early Antler orogenic activity along the continental margin. Recurring uplift of the cordillera followed Antler obduction, as indicated by the presence of chert and quartzite detritus derived from the allochthon in Mississippian and Pennsylvanian deposits of the foreland basin.

Continued orogenic compressive stress that was directed continentward during the Mississippian and Pennsylvanian resulted in a general eastward shift in sites of thick sedimentation. Near the end of Antler flysch deposition in Late Mississippian time, clastic sediments filled the foreland trough and spread eastward across the carbonate shelf onto the craton. Retarded subsidence of the foreland basin and significant decrease in volume of detritus from the reduced cordillera in latest Mississippian time are evidenced by widespread carbonate deposition in the Pennsylvanian.

Most of the flysch sediment within the foreland basin was deposited in a relatively deep-water trough by sediment gravity flows originating in relatively shallow water. Proximal and distal turbidites, debris-flow deposits, and hemipelagic deposits are recognized in the Antler flysch; these facies and their associations in the flysch trough indicate a complex system of submarine slope-fan-basin floor environments.

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