ELONGATE MOUNDS: EVIDENCE FOR HYDRODYNAMIC INFLUENCE ON ORIGIN OF PUGET LOWLAND MIMA MOUNDS

For over a century, the Mima Mounds of the Puget Lowland have spurred geologic research. Eliciting a variety of hypotheses to explain their enigmatic origin, the Mima Mounds are composed of a sandy loam diamicton fashioned into elliptical domes covering prairies along recessional proglacial terraces (Washburn, 1988). Though early researchers (such as Dalquest and Scheffer, 1942) developed their models on the assumption that the mounds were nearly symmetrical, Tabbutt (2016) demonstrated using prairie scale LiDAR analysis that mounds are elongated parallel to the downslope gradient of the terraces. Such mound elongation is confirmed by recent field investigation of Mima, Rocky, and Violet Prairies where mounding takes a variety of forms. At Mima Prairie, some mounds can be elongated to the point where several mounds can be bridged into a train of several mounds. Furthermore, some mounds overlie a gravel pedestal that is elongated parallel to the mounds (Pope, In Review), yet the pedestal is compositionally distinct from the underlying coarse-bedded gravels. Conversely, the mounds of Rocky Prairie are generally more voluminous and symmetrical than at Mima Prairie except in lenticular channels where mounds along the periphery are highly elongated parallel the channel. The ubiquitous nature of mound elongation provides a constraint on the range of conjecture on mound formation. Following initial LiDAR mapping, Tabbutt (2016) noted that mound elongation generally does not conform to paleowind directions, while the asymmetry does not support a fossorial rodent or seismicity origin. Due to the evidence for down-gradient activity, Tabbutt (2016) suggested that ice could have been a contributing factor in mound elongation but acknowledged the lack of direct evidence for periglacial activity. Furthermore, clast lithology investigation by Pope et al. (2020) indicates that both the mound diamicton and the underlying coarse-bedded gravels were deposited during the two-phased late-glacial Tanwax flood (Goldstein and Pringle, 2020), leaving no time for models based on vegetation anchoring or ice. To the contrary, the presence of mound elongation most commonly along channels indicate a hydrodynamic influence on mound formation, which may be explained by the Tanwax flood.