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Sedimentology of shell-rich deposits (coquinas) in the glaciomarine upper Cenozoic Yakataga Formation, Middleton Island, Alaska

¹ Glaciated Basin Research Group, Department of Geology, University of Toronto, 1265 Military Trail, Scarborough, Ontario, Canada M1C 1A4
² Department of Geological Sciences, University of Texas at Austin, P.O. Box 7909, Austin, Texas 78712

The Yakataga Formation in southern Alaska contains the longest and most accessible record of late Cenozoic glaciation in the world; glacially influenced marine sedimentation began in the late Miocene (6 Ma) and continues in the area to the present day. Upper Yakataga Formation sediments (1.25 km thick) are exposed on Middleton Island, Gulf of Alaska. Facies are dominated by massive glaciomarine diamicts that record deposition in an outer shelf setting of terrigenous mud and ice-rafted debris released from expanded temperate glaciers around the gulf margin. Diamicts contain intraformational coquina beds as much as 1 m thick, composed of cemented bioclastic debris and ice-rafted boulders, that can be traced several kilometers along strike. Analysis of foraminifera contained within diamicts and coquinas suggests that coquinas record intervals of relative sea-level change, bottom winnowing and nondeposition of mud allowing molluscan communities to develop. Coquinas show graded beds produced by storms and wave-rippled, commonly "armored" upper surfaces composed of densely packed Chlamys valves. These bioclastic deposits are of considerable significance with regard to the origin of mixed glacial and carbonate successions in the rock record. Analogous depositional settings can be identified from the Permian of Australia.

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