Duncan Canal

Basaltic flows in the Duncan Canal area of the central SE Alaska panhandle are both subaerial and submarine. Pahoehoe and aa lava flows overlie glacial till at Kupreanof Island. Quaternary basaltic lava flows up to 10 m thick form several broad peninsulas and scattered outcrops on the shores of Sumner Strait at the southern end of Kupreanof Island (Brew et al., 1985). On a peninsula in Kah Sheets Bay a basaltic flow depositionally overlies till of the youngest glaciation, and youthful-looking flows are exposed along a broad 12-km-wide front westward toward Douglas Bay. Two probable vents are found to the north within Mesozoic and Paleozoic rocks at the northern end of the Castle Islands in the Duncan Canal fault zone, and at Indian Point the flows contain inclusions of picrite. At High Castle Island, the basaltic flows are columnar-jointed and overlie planar to cross-bedded coarse sandstone and poorly sorted volcaniclastic conglomerate.