STRATIGRAPHIC EVOLUTION OF THE PALEOZOIC STIKINE ASSEMBLAGE IN THE STIKINE AND ISKUT RIVERS AREA, NORTHWESTERN BRITISH-COLUMBIA

The Stikine assemblage, the "basement" of Stikinia, extends 500 km along the western flank of the Intermontane Belt, east of younger Coast Belt plutons. Four different stratigraphic successions are characteristic of Lower to Middle Devonian, Carboniferous and Permian rocks in the Stikine and Iskut rivers area. West of Forrest Kerr Creek are penetratively deformed Lower to Middle Devonian island-arc volcaniclastic rocks, coralline limestone, and felsic tuff. Fringing carbonate buildups in an arc setting are best illustrated in the sequence at Round Lake where Lower Carboniferous mafic-dominated, bimodal submarine volcanic rocks grade upward into two distinctive coarse echinoderm limestone units and medial siliceous siltstone and limestone conglomerate. Conodont colour alteration indices for Lower Carboniferous rocks near Newmont Lake indicate an anomalously low-temperature thermal history. Upper Carboniferous - Permian polymictic volcanic conglomerate and Lower Permian limestone overlie these strata there. The Scud River sequence is distinguished by subgreenschist- to greenschist-grade Carboniferous(?) volcanic and sedimentary rocks overlain by a structurally thickened package (greater than 1000 m) of Lower Permian limestone. Local calcalkaline pyroclastic rocks interfinger with limestone near the top of the Scud River sequence. Basinal, shelf, and shallow-water carbonate facies developed in the Early Permian, giving way to calcalkaline volcanism in Late Permian followed by deposition of deep-water chert and argillite. The tectonic setting during the Devonian and Carboniferous is comparable with modern Pacific volcanic arcs and atolls, but there is no modern analogue for the shelf-carbonate accumulation during the Early Permian which characterizes the Stikine assemblage and permits Cordilleran-scale correlations. Permian fusulinid and coral species have very close affinity to those of the McCloud Limestone of the eastern Klamath Mountains, California. Other geologic events common to both Stikinia and the Eastern Klamath terrane are Devonian limestone breccia deposition, Lower Permian limestone accumulation with McCloud faunal affinity, Carboniferous and Permian calcalkaline volcanism, and Upper Permian tuffaceous limestone. Stratigraphic differences include the absence of quartz detritus in Devonian strata and lack of thick Upper Permian volcanic rocks in the Stikine River area.