PETROFACIES AND PROVENANCE OF A LATE CRETACEOUS SUTURE ZONE THRUST-TOP BASIN, CANTWELL BASIN, CENTRAL ALASKA RANGE

The lower Cantwell Formation, in the Cantwell basin, provides an excel lent opportunity to study the petrofacies and provenance of siliciclas tic deposits from an ancient thrust-top basin. The Late Cretaceous Can twell basin formed on a series of thrust sheets along a suture zone be tween the allochthonous Wrangellia composite terrane and the North Ame rican continental margin (i.e.,, the Yukon-Tanana terrane), Analysis o f the lower Cantwell Formation provides an understanding of sediment t ransport in the interior of an ancient suture zone. Sandstone and cong lomerate compositional data indicate two main source areas for the low er Cantwell Formation: (1) uplifted thrust sheets, located south of th e basin, associated with development of the suture zone, and (2) a reg ional anticlinorium, located north of the basin, along the North Ameri can continental margin. In the northwestern Cantwell basin, petrofacie s, southward paleocurrent directions, and alluvial lithofacies record reactivation of a regional anticlinorium located in the metamorphic Yu kon-Tanana terrane, Lithic types in this part of the basin are dominat ed by quartz mica tectonite, mica schist, metamorphic chert, and other quartz-rich metamorphic and sedimentary varieties (Q(48)F(3)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 i n sandstone detrital modes, northward paleocurrent indicators, and pro gressively rotated strata record syndepositional uplift of local thrus t 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 C antwell basin may be typical of thrust-top basins that form along sutu re zones. The two main petrofacies of the lower Cantwell Formation con sist of a petrofacies rich in sedimentary and volcanic detritus associ ated with uplifted oceanic racks along areas of the basin proximal to the suture zone, and a petrofacies characterized by quartz-rich metamo rphic and sedimentary detritus along areas of the basin proximal to th e deformed continental margin. Compositional data from this study, com bined with previous petrologic studies of similar basins, show that sa ndstones from suture zone thrust-tap basins are more feldspathic than sandstones from ''typical'' foreland basins, These compositional diffe rences reflect a complex assemblage of accreted terranes, which includ e relict volcanic arcs, batholiths, oceanic sedimentary deposits, and ophiolitic melanges, that are accreted to continental margins along su ture zones in collisional-accretionary orogenic belts.