CHALLENGES RECOGNIZING AEOLIAN MEGARIPPLE ACTIVITY IN SEDIMENTARY ROCK RECORDS, EARTH AND MARS (Invited Presentation)

Aeolian sands commonly are well-sorted, but wind acting on poorly sorted materials can produce megaripples: bedforms with crests covered by very coarse grains that move only in creep (driven by impacts from finer, saltating grains). On Earth, megaripples are rarely recognized in the sedimentary rock record, partly because they are relatively minor components in many aeolian settings, are commonly associated with surface deflation, and in many terrestrial climates aqueous sediment transport mechanisms compete with wind to transport sediments toward basins. On Mars, however, megaripple deposits might be preserved more commonly in the sedimentary rock record because (1) the martian surface has been arid for much of its history, so should be relatively more favorable for aeolian deposition than on Earth, and (2) over the course of this aridity, impact cratering and physical weathering have continuously provided poorly-sorted sediments to be worked by wind. Consistent with (1-2), megaripples are abundant at the surface, recognized at most lander/rover sites, commonly far from dune systems. Consequently, distinguishing in the sedimentary rock record between the mixed grain size deposits of aeolian megaripples vs. subaqueously transported sediments is important for correctly evaluating outcrops revealing the role of water in the ancient martian past. This presentation highlights some of Jim Zimbelman's contributions to understanding megaripples on Earth and Mars, including fieldwork around the world helping to reveal the diverse characteristics of active megaripples, and their ancient preserved signatures in outcrop; both approaches enable a more informed assessment of the martian sedimentary record during Mars rover operations. As confirmed by wind tunnel experiments, megaripple-transported materials commonly exhibit a sharply defined maximum grain size. This textural feature, in the absence of rare, whole-bedform preservation in the rock record, can raise the possibility on Earth or Mars that horizons of well-sorted coarse surface lags preserved in candidate aeolian strata are products of ancient megaripple activity. This work relates also to Jim's contributions evaluating martian Transverse Aeolian Ridges (TARs, the most common martian aeolian bedform seen from orbit), for which an origin related to megaripple development has been a persistent hypothesis.