Fluvial Sinuous Ridges of the Morrison Formation, USA: Meandering, Scarp Retreat, and Implications for Mars

Abstract

Sinuous ridges have been interpreted as evidence for ancient rivers on Mars, but relating ridge geometry to paleo‐hydraulics remains uncertain. Three analog ridge systems from the Morrison Formation, Utah, are composed of sandstone caprocks, up to 50 m wide and 8 m thick, atop mudstone flanks. Ridge caprocks have narrowed significantly compared to sandstone bodies preserved in outcrop, consistent with a new ridge‐erosion model that can be used to estimate original sandstone‐body extent. Ridge networks represent caprocks intersecting at distinct stratigraphic levels, rather than a preserved channel network. Caprocks are interpreted as amalgamated channel belts, rather than inverted channels, with dune and bar cross stratification that was used to reconstruct paleo‐channel dimensions. Curvilinear features on ridge tops are outcropping lateral accretion sets (LAS) from point bars and indicate meandering. We found that caprock thickness scales with paleo‐channel depth and LAS curvature scales with paleo‐channel width. Application of these relations to a ridge in Aeolis Dorsa, Mars, yielded consistent water discharge estimates (310–1,800 m³/s). In contrast, using ridge width or ridge radius of curvature as paleo‐channel proxies overestimated discharge by a factor of 30–500. In addition, the ridge‐erosion model suggests that scarp retreat may be less efficient on Mars, resulting in taller and wider ridges, with more intact caprocks. Altogether, our results support the hypothesis that ridges are exhumed channel belts and floodplain deposits implying long‐lived fluvial activity recorded within a depositional basin.

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