FORMATION OF SINUOUS RIDGES BY INVERSION OF RIVER-CHANNEL BELTS NEAR GREEN RIVER, UTAH, USA, WITH IMPLICATIONS FOR MARS (Invited Presentation)

Sinuous ridges are important landforms on the surface of Mars that show promise for quantifying ancient martian surface hydrology. Morphological similarity of these ridges to river channels in planform led to a hypothesis that ridges are topographically inverted river channels, or “inverted channels”, formed due to an erosion-resistant channel-filling material that preserved a snapshot of channel geometry in inverted relief. An alternative deposit-inversion hypothesis proposes that ridges represent exhumed river-channel belts, with geometries that reflect the lateral migration and vertical aggradation of rivers over significant geologic time, rather than the original channel geometry. To investigate these hypotheses we studied well-known sinuous ridges within the Cretaceous Cedar Mountain Formation near Green River, Utah, USA. The ridges extend for hundreds of meters, are up to 120 meters wide, and are capped by sandstone bodies 3-10 meters thick that contain dune- and bar-scale inclined stratification. We interpret these caps as eroded remnants of channel belts that record paleo-river migration and aggradation, rather than channel fills that would preserve the original channel geometry. Caprocks overlie mudstones and thinner sandstone beds that are interpreted as floodplain deposits, and in cases additional channel-belt sandstones also exist lower in the ridge stratigraphy. Ridge-forming sandstone bodies also have been narrowed during exhumation by cliff retreat and bisected by fluvial erosion. Using a large compilation of channel-belt geometries on Earth and our measurements of ridges in Utah, we propose that caprock thickness is the most reliable indicator of paleo-channel geometry, and can be used to reconstruct river depth and discharge. In contrast, channel lateral migration and caprock erosion during exhumation make ridge breadth an uncertain proxy for channel width. An example in Aeolis Dorsa, Mars, illustrates that river discharge estimates based on caprock width may differ significantly from estimates based on caprock thickness. Overall, our study suggests that sinuous ridges are not inverted channel fills, but rather reflect exhumation of a thick stratigraphic package of stacked channel belts and overbank deposits formed from depositional rivers over significant geologic time.