MIMALIKE MOUNDS ARE COMMONLY ASSOCIATED WITH TERRACES: IMPLICATIONS FOR CONJECTURE AND CLASSIFICATION

For over a century, Mima mounds have evaded geologic understanding. At their type locality in Washington’s Puget Lowland, Mima mounds are dome-like ellipsoids of locally thickened diamicton that mantles proglacial terraces. Similar mounds, termed “Mimalike mounds” by Washburn (1988), have since been found across the United States and beyond in a plenitude of geologic conditions. Though Mimalike mounds are characterized by a well-drained loam overlying a relatively impermeable substrate, Washburn (1988, pp. 7, 48) explained that “The term ‘Mima mound’ has subsequently come to be used in a much wider sense for occurrences in other, sometimes distant regions, but still implying an assumption of similar origin. ...The basic question is the extent to which widely separated but similar mounds are really of comparable origin.” Despite continued research over the subsequent three decades, no form of classification has been proposed to distinguish variant mound forms and their origins. Even so, further study has found repeated cases of Mimalike mounds associated with Pleistocene terraces. Ranging from the proglacial terraces of the Puget Lowland and strath terraces of Winsconsian Missoula floods to the Merced fluvial terraces or uplifted Pleistocene platforms in Arizona, the association of Mimalike mounds with Pleistocene terraces raises a number of questions: 1) Why are mound-bearing terraces primarily Pleistocene in age? Were there certain unique conditions (e.g., glacial) that were not present during the Holocene, and, if so, why is there a paucity of Pliocene mound-bearing terraces? 2) Is there a difference between terraces produced from high-energy events (e.g., Missoula floods strath terraces) and long-term processes (e.g., marine terraces)? 3) Is mound formation related to the process(es) that produced the terraces, or are the terraces merely non-causal hosts? Because mound morphology can be influenced by substrate characteristics, terraces have the potential to provide a more unifying classification than one solely based on mound morphology or composition. By examining the origin of the terraces and their relationship to the associated mounds, terraces may constrain the range of conjecture on Mimalike mounds and lead to a better understanding of mound formation.