Latest Chesterian (Carboniferous) initiation of Gondwanan glaciation recorded in facies stacking patterns and brachiopod paleocommunities of the Antler foreland basin, Idaho

Stacking patterns of facies and brachiopod communities of Latest Chesterian (Middle Carboniferous) units of east-central Idaho record an increase in the frequency and amplitude of sea level changes. Water depth, as inferred from lithology and paleoecology, provides the basis for quantifying sea level fluctuations. The Surrett Canyon Formation is dominated by thick sequences of subtidal carbonates. The Arco Hills Formation consists of thin parasequences of mixed carbonate and siliciclastic lithologies, representing sea level changes up to 50 m in amplitude, bounded by flooding surfaces. Overlying the Arco Hills, the Bluebird Mountain Formation (very fine-grained quartz arenite, dolomite, and unfossiliferous limestone) records a major lowstand just below the mid-Carboniferous boundary. Increased amplitude and frequency of sea level changes in the Antler foreland basin initiated at the C. naviculus/A. unicornis conodont zone boundary which occurs at the base of the Arco Hills Formation. These are interpreted as 4th-order glacio-eustatic sea level changes related to the onset of Gondwana glaciation comparable to those seen in the Illinois Basin, Alaska, and elsewhere.