Interflag sandstone laminae, a novel sedimentary structure, with implications for Ediacaran paleoenvironments

摘要

Interflag sandstone laminae are thin, silty to sandy, layers between thicker sandstone beds, and a new name for sedimentary structures recently called "shims", and "microbial mat sandwiches", from Ediacaran fossil localities of Nilpena, South Australia. They have been regarded as structures unique to Precambrian marine environments, but both the age and environmental associations are reinterpreted in this study. Interflag sandstone laminae from Eocene (Wasatch Formation, Colorado, U.S.A.) and Pennsylvanian (Mansfield Formation, Indiana, U.S.A.) fluvial levees and scroll bars were studied in the field, and by means of petrographic thin sections and granulometry. Climbing translatent ripples and distinct grain size distributions are evidence that interflag sandstone laminae were eolian, whereas intervening flagstones were deposited by fluvial traction currents. Other evidences of exposure to wind include microbial earth textures, shallow cracking structures, zibars, setulfs, root traces, and insect trackways. Other evidences of flagstone deposition in traction currents include intraformational claystone breccias, oscillation and current ripples, and microbial mat textures. Similarly distinct beds can be seen in modern sandy river levees, such as the Murchison River of Western Australia and Green River of Utah (U.S.A.). These observations reveal that interflag laminae are created by exposure and wind-drift, but flagstones are produced by floods. Quartzose flagstones of demonstrable marine origin, with fossil brachiopods and trilobites, have also been examined, but lack interflag sandstone laminae. Interflag sandstone laminae are evidence of alternating flood and wind, only known from fluvial environments, and are further support for the idea that Ediacaran vendobionts from South and central Australia, and Namibia lived on land. (C) 2018 Elsevier B.V. All rights reserved.