--> An Early Permian, Postglacial, River-dominated Paleopolar Delta: Hyperpycnites and Shelf Channels at the Mackellar-Fairchild Transition, Turnabout Ridge, Central Transantarctic Mountains, Antarctica

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An Early Permian, Postglacial, River-dominated Paleopolar Delta: Hyperpycnites and Shelf Channels at the Mackellar-Fairchild Transition, Turnabout Ridge, Central Transantarctic Mountains, Antarctica

Abstract

Outcrops at Turnabout Ridge, located in the Beardmore Glacier Region of the Central Transantarctic Mountains, comprise Early Permian glacial deposits of the Pagoda Formation (Fm), and postglacial deposits of the Mackellar and Fairchild formations (fms). A collaborative sedimentological and ichnological study of the Mackellar and Fairchild fms identified seven facies that combine to form four environments of deposition: 1) prodeltaic shelf turbidites, 2) turbidite channels and levees, 3) sandy delta-front hyperpycnites, mouth bars, and subaqueous terminal distributary channels, and 4) a sand-dominated braidplain. Compensationally stacked turbidite channels and levees in the Mackellar Fm were likely the conduits that delivered sediment from the delta front to the prodeltaic shelf. Deltaic deposits of the Mackellar Fm are dominated by sedimentary structures (climbing ripples, low-angle planar lamination) and stratal geometries (foresets) indicative of recurring sediment-laden underflows, and lack evidence of tidal or wave modification, except wave ripples. We suggest that sediment laded underflows on the delta front fed the turbidite-channel system. Facies, sedimentary structures, and stratal architectures of the Turnabout Ridge delta are grossly similar to deltas of the North American Cretaceous Western Interior Seaway identified as river-dominated, including the Ferron and Panther Tongue deltas. Braided channels of the Fairchild Fm overlie the deltaic system, recording the ultimate advancement of braidplains into the basin. Thirty ichnogenera and six ichnocoenoses were identified from the Mackellar Fm. None of these trace fossils are known solely from freshwater systems, twelve are only known from marine settings, and eighteen are found across marine, brackish, and freshwater systems. Historically interpreted as either the deposits of a large glacial lake or inland sea, our ichnologic and sedimentologic observations are consistent with a river-dominated marine deltaic system that contained an ecosystem stressed by recurring freshwater and sediment input. Results suggest that sediment-laden glacial meltwater freshets from braided rivers of the Fairchild Fm likely prepped a marine basin, producing recurring hyperpycnal flows along the delta front. We suggest that strata at Turnabout Ridge were deposited in a proximal-axial position relative to a river-dominated delta along the shoreline of an Antarctic epeiric seaway during the Early Permian.