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

4 | June 2009 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 crosslaminae 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.