SEDIMENTOLOGY IN PLAN VIEW: HOW TERRESTRIAL ANALOGS CAN INFORM AND CAUTION REMOTE INTERPRETATIONS OF ANCIENT FLUVIAL AND ALLUVIAL ENVIRONMENTS ON MARS

The Aeolis Dorsa of Mars are many sinuous ridges, interpreted to be ancient (>3 Ga) fluvial and alluvial deposits that experienced induration, burial, exhumation, and topographic inversion. These interpretations are based on plan-view satellite images and topographic data. However, the rarity of plan-view data of similar deposits on Earth adds some uncertainty to these interpretations. To improve the specificity of plan-view interpretations, we examine the geomorphology of ancient and modern terrestrial deposits. With corresponding sedimentology provided by previous investigators, we correlate the plan-view morphologies of terrestrial analogs to specific architectural elements and sedimentary environments. The plan-view attributes derived from these terrestrial analogs are then used to interpret the hydrology, sedimentology, and topography involved in the formation of the Aeolis Dorsa.

Our examination shows that on Earth point bars of ancient and modern meandering fluvial environments have ridges and swales with level to slightly decreasing elevations from the bend interior to the exterior. In contrast, laterally aggrading channel beds would likely show increasing elevations, although terrestrial examples exposed in plan view are unknown to us. Level, slightly decreasing, and increasing elevation trends are found within the Aeolis Dorsa. Each type implies a meandering environment, which in turn implies a regular hydrologic regime, constant sediment supply, and stable channel banks.

Fluvial channel fills have flat, nondescript upper surfaces and form in different fluvial environments with different width to thickness ratios. Without observations in cross-section to provide thickness data or discernable additional units (e.g., point bars) to support interpretations, the formation conditions of channel fills remain equivocal.

To our knowledge, the rock record lacks plan-view exposure of ancient debris-flow-dominated alluvial fans. Modern debris-flow fans exhibit paired levees and lobes, which imply infrequent hydrology, weathered sediment, and distinct topographies (e.g., elevated depositional slopes, and escarpments). From our examination, interpretations regarding sheetflood, incised-channel, or traction deposits on alluvial fans remain ambiguous.