A New Twist on Mud Deposition—Mud Ripples in Experiment and Rock Record

Abstract

Recent flume experiments with muddy suspensions have demonstrated that low-energy settings are not a prerequisite for the accumulation of muds and shales. Flocculation produces deposition-prone aggregates even in freshwater, and these floccules form bedload ripples at flow velocities that would also produce ripples in sandy sediments. Floccule ripples are crosslaminated but have a much higher water content (80-90 % by volume) than sand ripples and are thus subject to substantial compaction. Because of this, we must expect that original cross-laminae become severely flattened and are difficult to recognize in the rock record. Nonetheless, a survey of ancient shales has shown that certain intrinsic features of cross-lamination, such as basal downlap and top truncation of laminae, are still recognizable and allow identification of current-deposited muds. The example of the Cambrian Eau Claire Formation is used to illustrate how this approach can change our perception of the depositional setting of certain mudstone successions.