Middle Paleozoic rocks change facies from east to west across central Idaho reflecting sedimentation in different environments of the Cordilleran geosynclinal complex and its eastern margin. A thin cratonic assemblage of Upper Devonian Jefferson Dolomite, punctuated by local diastems, unconformably oversteps Ordovician Kinnikinic Quartzite and Precambrian Belt strata in southwestern Montana and in the Beaverhead Range along the Idaho-Montana border. West of the southern Lemhi Range the importance of the Devonian regional unconformity diminishes and variable amounts of Upper Ordovician Fish Haven and Silurian Laketown dolomites are preserved below Jefferson Dolomite. The basal quartzite-carbonate succession of Kinnikinic Quartzite and Fish Haven, Laketown, and Jefferson do omites thickens markedly in the Lost River Range toward the west. This miogeosynclinal succession is replaced farther west in the Wood River region by Lower Ordovician to Silurian graptolitic shale, chert, and quartzite, the Phi Kappa-"Trail Creek" sequence of eugeosynclinal affinity. About 3,500 feet of interbedded shaly limestone, dolomite, shale, and quartzite of the Middle-Upper Ordovician Saturday Mountain Formation and the Silurian "Laketown Dolomite" are present in the Bayhorse region and appear to be transitional in facies between the time-equivalent quartzite-dolomite sequence on the east and the graptolitic shale-chert-quartzite sequence on the south.

Since newly measured sections imply approximately north-south facies boundaries and thickness trends, the graptolitic shale-chert-quartzite sequence in the Wood River region lies due north and on depositional trend with transitional facies in the Bayhorse region. This eastward invasion of the graptolitic shale-chert-quartzite sequence into the miogeosynclinal carbonate belt is attributed to thrusting as has been demonstrated for corresponding facies contrasts in Nevada.

Contrary to classical concepts of geosynclinal sedimentation, most of the middle Paleozoic detrital rocks on the west side of the miogeosynclinal carbonate belt and intertongued with the carbonates are of quartz sand and mature detrital fines apparently derived from eastern cratonic sources rather than from unknown orogenically active borderlands or volcanic island arcs on the west. In contrast, late Paleozoic orogenic movements in the eugeosynclinal belt caused uplifts from which a thick wedge of coarse detritus forming the Milligen and Wood River Formations spread eastward to intertongue and grade into finer clastics and carbonates of the "Brazer Limestone" near the craton margin.

Late Paleozoic orogenic activity in central Idaho represents a northern continuation of the Antler orogeny of Nevada. The Antler orogeny thus had extremely widespread and profound effects on tectono-stratigraphic patterns of the Cordilleran geosyncline: for the first time in the Paleozoic history of Idaho and Nevada it replaced the craton as a principal source for detrital sediment; in Idaho, the orogeny brought to a close the eugeosynclinal episode of essentially depositional continuity extending from at least early Ordovician to late Devonian.

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