In Late Devonian and Mississippian times, well-bedded, shallow- to deep-water marine, flysch-like mudstone, siltstone, sandstone, conglomerate, and minor impure limestone were deposited in a subsiding, elongate, structural foreland basin (exogeosyncline) on the continental shelf (Cordilleran miogeocline)

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east of the Antler orogenic continental margin. The terrigenous detritus was derived from a rising cordillera composed of Devonian and older oceanic rocks that during the Antler orogeny was first 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 shown by chert detritus, derived from the allochthon, in Lower and Upper Mississippian deposits. In Late Mississippian time, clastic sediments filled the foreland basin and spread eastward across the carbonate shelf onto the craton. Volume of detritus diminished in latest Miss ssippian to Early Pennsylvanian time, as evidenced by widespread carbonate deposition in the Pennsylvanian.

Locally in the early Late Devonian and regionally in the late Late Devonian and Mississippian, western detritus came as a steady influx of mostly clay, silt, and sand debris, and as an irregular influx of conglomeratic debris. Many units exhibit turbidite features--graded bedding, convolute laminae, and sole marks. Some fine-clastic units contain indigenous ichnofossils and displaced fossil invertebrates, fish remains, and terrestrial plant debris; few limestones contain indigenous fossil invertebrates. The sequence and thickness of rocks vary areally, indicating irregular bottom relief which produced a complex system of sediment traps and environments.

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