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A part of the Unalaska Formation of Drewes, Fraser, Snyder, and Barnett was reexamined in 1976 and a new suite of fossils collected. This Tertiary formation is exposed over about 70 percent of Unalaska Island and is 900-16,700 meters thick. It consists mainly of volcanic breccia, flows, tuff, and intercalated sedimentary rocks 1 cm to more than 100 m thick. A 126-m-thick sequence of early Miocene(?) sandstone and conglomerate, all thermally altered, was measured and examined in detail. This sequence, herein designated the Dutch Harbor Member, shows evidence of deposition by turbidity currents and debris flows. ,The stratigraphic position of the Dutch Harbor Member within the Unalaska Formation is not well known, but it may be near the top of the formation. Fossils collected within the member include the bivalves Chlamys and Pododesmus of Miocene age. INTRODUCTION Unalaska Island, near the east end of the insular Aleutian arc, is perched atop the mostly submerged Aleutian ridge. This ridge separates the abyssal northern Pacific Ocean from the Bering Sea (fig. 1). About 70 percent of the island is covered by Tertiary volcanic and sedimentary rocks named the Unalaska Formation (Drewes and others, 1961). The thickness of the Unalaska Formation has not been determined, but on the basis of topographic data it is at least 900 m thick. From calculations on the regional dip, Drewes, Fraser, Snyder, and Barnett (1961) figure that it could be as thick as 16,000 m; they suspect that the actual thickness is of the order of 5,00010,000 m. During August 1976, we examined the Unalaska Formation and collected a new suite of fossils in the vicinity of the original collection site reported by Drewes, Fraser, Snyder, and Barnett (1961). The study area extends about 1-4 km southeast of the village of Bi B2 CONTRIBUTIONS TO STRATIGRAPHY

Stratigraphy and depositional environment of the Dutch Harbor Member of the Unalaska Formation, Unalaska Island, Alaska