Graywacke of Buchia Ridge and correlative Lower Cretaceous rocks in the Goodnews Bay and Bethel quadrangles, southwestern Alaska

Abstract

Lithologically distinct coeval strata of Lower Cretaceous age occur in four northeasttrending belts in the Goodnews Bay and Bethel quadrangles, southwestern Alaska. The belts are, from southeast to northwest, the Buchia Ridge belt, Ungalikthluk belt, Mount Gratia belt, and Eek Mountains belt. The belts are largely bounded by thrust faults. Rocks in the Mount Gratia and Eek Mountains belts are stongly deformed and infolded with older rocks, whereas the Ungalikthluk and Buchia Ridge belts are mildly deformed and (except for intrusive rocks of Tertiary age) include only rocks of Early Cretaceous age. The Buchia Ridge belt is composed mainly of graywacke and related rocks defined as the graywacke of Buchia Ridge. This formation is exposed in a wedge-shaped area of about 350 km2 in the Goodnews Bay A-2, A-3, B-2, and B-3 quadrangles. The beds of this unit are deformed into a southeast-dipping homocline in a thrust sheet that is underlain and overlain by highly deformed older volcanic and sedimentary strata of Jurassic and Early Cretaceous(?) age. The graywacke of Buchia Ridge is estimated to be about 5,000 m thick. The lower part of the formation is thick-bedded graywacke, siltstone, and conglomerate; the upper part shale, shaly siltstone, and thin-bedded graywacke and calcarenite. The lower part contains abundant Buchia of Valanginian age; the upper part sparse Inoceramus and Belemnitesof Hauterivian age. If the section is not repeated by faults or folds, it is the thickest, least deformed section of Lower Cretaceous sedimentary rocks known in southwestern Alaska. The Ungalikthluk belt consists of isolated synclinal outcrop areas of limestone, greenish limy grit, and conglomerate overlain by noncalcareous graywacke and grit. Although the total outcrop area is no more that 4 km2 , these erosional remnants form a distinct belt northwest of the Buchia Ridge belt. Locally, Buchia crassicolis is found in the limestone, indicating that the Ungalikthluk sequence is coeval with the graywacke of Buchia Ridge. The Mount Gratia belt, up to 25 km wide and extending more than 120 km northeast from the central part of the Goodnews Bay B-5 quadrangle, is bounded on the southeast and northwest sides by southeast-dipping reverse faults. Rocks within the belt are strongly folded and cut by many faults. 1 2 CONTRIBUTIONS TO STRATIGRAPHY Stratigraphy in the Mount Gratia belt is not definitely known because of structural complexity, but in general consists of an upper section of thickbedded graywacke, shale, and other sedimentary rocks and a lower section of multicolored tuff and other volcanic rocks. Locally, there are small areas of Permian and Triassic rocks that are probably faulted in. The occurrence of Buchia crassicolis and radiolarians identified as forms of Early Cretaceous (Valanginian) age within the Mount Oratia belt again indicates contemporaneity with rocks in the other belts. The Eek Mountains belt, 2-25 km in width, extends northeastward about 75 km from the Goodnews Bay C-6 quadrangle. The belt encompasses a large anticline consisting of older rocks flanked by Cretaceous rocks. The rocks in the belt are strongly folded and commonly overturned northwestward. The Lower Cretaceous (Valanginian) section in the Eek Mountains belt consists of graywacke, shale, argillite, and conglomerate at least 1,000 m thick. The tuff and other volcanogenic rocks of the Mount-Oratia belt have not been identified within the Eek Mountains sequence. Buchia crassicolis has been found in thin calcareous beds and in pebbly sandstone at several places within the Eek Mountains belt, thus substantiating an age coeval with rocks in the other three belts.