Coastal mountains in the northeastern Gulf of Alaska expose continuous, along-strike sections over many tens of kilometers through the 5-km-thick infill (Yakataga Formation) of a glacially influenced active margin basin. The basin has been thrust and uplifted as a result of continuing compression between the underlying Pacific and North American plates. The Yakataga Formation is the best exposed and most complete late Cenozoic record of cool temperate and glacially influenced marine sedimentation in the world. Glacial marine sedimentation began during the late Miocene and is recorded in lowermost Yakataga strata exposed at Yakataga Reef by the abrupt arrival in a deep basin of turbidites and chaotically bedded debris flows. Debris flows, as much as 19 m thick are composed largely of glacial debris brought down to sea level by tidewater glaciers. A depositional setting characterized by a narrow shelf terminating in a steep slope and deep water, and subject to frequent downslope mass flow events is indicated. Overlying Yakataga strata exposed at Icy Bay are characterized by interbedded turbidites, diamictites, and shallow marine sandstones; these facies probably record progradation of a continental slope by mass flow processes in response to high rates of sediment supply from glaciers draining rapidly uplifting (1 to 10 m/yr) and eroding coastal mountains, and earthquake activity. Seismic and outcrop data show that the slope experienced multiple episodes of syndepositional compressional folding, resulting in a pronounced structural influence on sedimentation style. Uppermost Yakataga strata exposed on Middleton Island are dominated by thick “rain-out” diamictites resulting from iceberg transport of coarser debris and the deposition of muds from suspended sediment plumes in an outer shelf setting. Graded gravel facies record the infilling of submarine channels similar to the valleys that traverse the modern Gulf of Alaska shelf; coquinas indicate episodic sediment starvation. Boulder pavements record repeated surge-like ice advances to the outer continental shelf. By this time (late Pliocene to Pleistocene) a low-relief subsiding shelf was established in the Gulf of Alaska on which a high-resolution record of sea-level change, tectonism, and glaciation was preserved; deposition rates may have been as high as 10 m/ky. The single most important influence on sedimentation in the late Miocene to Pleistocene Gulf of Alaska, especially in allowing the preservation of a thick active margin basin fill in a compressional tectonic setting, has been, and continues to be, the abundant production of meltwater from temperate tidewater glaciers and associated sediment from rapidly uplifting (10 m/ky) coastal mountains.

Abstract DNAG Transect A-3. Part of GSA’s DNAG Continent-Ocean Transect Series, this transect contains all or most of the following: free-air gravity and magnetic anomaly profiles, heat flow measurements, geologic cross section with no vertical exaggeration, multi-channel seismic reflection profiles, tectonic kindred cross section with vertical exaggeration, geologic map, stratigraphic diagram, and an index map. All transects are on a scale of 1:500,000.