Abstract

The lower Cantwell Formation, in the Cantwell basin, provides an excellent opportunity to study the petrofacies and provenance of siliciclastic deposits from an ancient thrust-top basin. The Late Cretaceous Cantwell basin formed on a series of thrust sheets along a suture zone between the allochthonous Wrangellia composite terrane and the North American continental margin (i.e., the Yukon-Tanana terrane). Analysis of the lower Cantwell Formation provides an understanding of sediment transport in the interior of an ancient suture zone. Sandstone and conglomerate compositional data indicate two main source areas for the lower Cantwell Formation: (1) uplifted thrust sheets, located south of the basin, associated with development of the suture zone, and (2) a regional anticlinorium, located north of the basin, along the North American continental margin. In the northwestern Cantwell basin, petrofacies, southward paleocurrent directions, and alluvial lithofacies record reactivation of a regional anticlinorium located in the metamorphic Yukon-Tanana terrane. Lithic types in this part of the basin are dominated by quartz-mica tectonite, mica schist, metamorphic chert, and other quartz-rich metamorphic and sedimentary varieties (Q 48 F 2 L 50 ; Lv 0 Lm 54 Ls 46 ). In the northeastern and southern parts of the basin, sediment was derived from volcanic and sedimentary strata exposed in the hanging walls of thrust faults that define the southern margin of the basin. Source-diagnostic conglomerate clasts, spatial variations in sandstone detrital modes, northward paleocurrent indicators, and progressively rotated strata record syndepositional uplift of local thrust sheets along the suture zone. The dominant sandstone grain types in these parts of the basin are plagioclase grains and lithic fragments of argillite, radiolarian-bearing mudstone, and metabasalt (Q 44 F 28 L 28 ; Lv 2 Lm 23 Ls 75 ). Sandstone compositions documented for the Cantwell basin may be typical of thrust-top basins that form along suture zones. The two main petrofacies of the lower Cantwell Formation consist of a petrofacies rich in sedimentary and volcanic detritus associated with uplifted oceanic rocks along areas of the basin proximal to the suture zone, and a petrofacies characterized by quartz-rich metamorphic and sedimentary detritus along areas of the basin proximal to the deformed continental margin. Compositional data from this study, combined with previous petrologic studies of similar basins, show that sandstones from suture zone thrust-top basins are more feldspathic than sandstones from "typical" foreland basins. These compositional differences reflect a complex assemblage of accreted terranes, which include relict volcanic arcs, batholiths, oceanic sedimentary deposits, and ophiolitic melanges, that are accreted to continental margins along suture zones in collisional-accretionary orogenic belts.

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