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Witmer, J.W., 2009

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Bibliographic Reference

Witmer, J.W., 2009, Neogene deposition, provenance, and exhumation along a tectonically active, glaciated continental margin, Yakataga and Redwood Formations, southern Alaska syntaxis: West Layfayette, Indiana, Purdue University, M.S. thesis, 330 p.

Abstract

The southern Alaskan syntaxis is characterized by high coastal topography, extremely high rates of erosion and exhumation, and tidewater glaciation. The eastern margin of the syntaxis is defined by strike-slip tectonics and a wide continental shelf with a distinct shelf-slope break. The western margin of the syntaxis is defined by convergence and an onshore thrust belt that trends offshore into the Aleutian trench. More than 5 km of Neogene sedimentary strata are exposed onshore across southeastern Alaska and ~7 km are documented in offshore wells. New stratigraphic, U-Th/He thermochronologic, and U-Pb geochronologic data from onshore Neogene strata, known as the Yakataga and Redwood Formations, provide a record of sediment transport, burial, and exhumation along this tectonically active, glaciated continental margin. Stratigraphic analyses record facies associations that represent two regional glacial-fed depositional systems. Along the eastern margin of the syntaxis, fan deltas and glaciomarine slope aprons were the dominant depositional systems. The western margin of the syntaxis consisted of a narrow shelf that fed sediment to submarine fan systems; these fans transported sediment southwestward into the Aleutian subduction zone. Sediments along the western margin were subsequently incorporated into the accretionary prism. U-Pb detrital zircon geochronologic data shows that the primary sources of Neogene strata were the Coast Plutonic Complex (~190-50 Ma), Chitina arc (175-135 Ma), Sanak-Baranof plutonic belt (~61-48 Ma), and isolated near-trench intrusions (~42-29 Ma) emplaced in the Wrangellia composite terrane and Chugach terranes. These plutons were eroded and recycled into Paleogene sedimentary strata that were incorporated into the thrust belt in the western margin. Exhumation and erosion of the thrust belt redeposited these sediments into Neogene depositional systems. U-Th/He thermochronologic results from apatites and zircons in clasts of conglomerate suggest that in the syntaxial corner, strata were not buried more deeply than ~2 km. In the central part of the thrust belt along the western margin, strata were buried to depths of 2-5 km. Our westernmost sample, part of the exhumed accretionary prism, was buried to depths of 5.0-7.5 km. An integration of our stratigraphic, geochronologic, and thermochronologic data document a record of erosion and exhumation in the eastern margin of the syntaxis and fairly continuous sedimentation and deeper burial along the western margin. The depositional, burial, and exhumation framework that is established for Neogene strata along the southern Alaska syntaxis provides a proxy for understanding crustal processes in a glaciated syntaxial corner.

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