Facies and stratigraphy of the basal Ediacaran cap carbonate, Naukluft Mountains, Namibia

Abstract

The termination of the Marinoan Snowball Earth glaciation constitutes a dramatic interval of climate change. This field study seeks to investigate this climate transition by characterizing the basal-Ediacaran cap carbonate succession of the Tsabisis Formation overlying the Blässkranz Formation glacial deposits exposed in the Naukluft Mountains of Namibia by using sedimentologic and stratigraphic field observations supplemented by chemostratigraphic analyses. Measured sections and lateral bed/unit tracing delineate stratigraphic stacking patterns and regional facies changes. In downdip areas Marinoan diamictite transitions upward into dolostone intermixed with sandstone and extrabasinal clasts that is gradually overlain by fine grained laminated dolostone. Updip localities show the diamictite is overlain by intercalated sandstones, gravels, and shales before an abrupt change to laminated dolostone of the cap carbonate. A succession of stromatolites, which become strongly elongate upward, prograde into the laminated dolostone in the updip localities. The stromatolites are overlain by laminated dolostone that grades upward into rhythmite with intercalations of shale. Near the top of the cap, rhythmites may be reworked into tabular intraclast conglomerate, locally intercalated with hummocky cross stratified sandstone, which passes upward into the shale and limestone members of the Tsabisis Formation. The Tsabisis Formation is overlain by the Noab Formation, dominated by sedimentary breccia, laminated dolostone, and sandstone. The lateral and vertical distribution of facies indicate a retreat of the shoreline and glacially sourced siliciclastics near the base of the cap carbonate, a shallowing succession to fair-weather wave base at the top of the stromatolite facies, and a second shallowing succession to storm wave base near the top of the cap carbonate. Maximum flooding occurred soon after the initiation of carbonate deposition and two sequence boundaries mark higher stratigraphic levels within the cap carbonate. The depositional patterns of the Tsabisis Formation cap carbonate resemble that of other Precambrian mixed carbonate-siliciclastic platforms and shows multiple base-level fluctuations that could be attributed to a range of mechanisms. Some of the base-level changes may have been directly coupled to events driven by deglaciation but distinguishing these from conventional controls on accommodation space in the rock record is challenging and multiple depositional models are feasible.

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