INTERFFLAG SANDSTONE LAMINAE AS A NEW CLUE TO THE HABITAT AND PRESERVATION OF EDIACARAN VENDOBIONTS

Interflag sandstone laminae is a new name for sedimentary structures previously called “shims”, and informally as “microbial mat sandwiches”, from Ediacaran fossil localities of Nilpena, South Australia. However, interflag sandstone laminae are not unique to the Ediacaran Period, nor indicative of marine environments. Interflag sandstone laminae from Eocene (Wasatch Formation, Colorado) and Pennsylvanian (Mansfield Formation, Indiana) 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 the interflag sandstone laminae were eolian, whereas intervening flagstones were deposited by fluvial traction currents. Other evidence of exposure to wind include microbial earth textures, shallow cracking structures, root traces, and insect trackways. In contrast, 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. Other Ediacaran examples of interflag sandstone laminae have been discovered in the Dabis Formation of Namibia and Arumbera and Central Mount Stuart Formations of central Australia, and also are an indication that associated vendobionts were non-marine. Quartzose flagstones of demonstrable marine origin, with fossil stenohaline organisms such as brachiopods and trilobites, have also been examined, but lack interflag sandstone laminae. Interflag sandstone laminae are evidence of alternating flood and wind, and unlike many sedimentary structures, are specific to fluvial environments.