Investigating the depositional environment of sedimentary fan features on Mars using orbital stratigraphy

Abstract:

A sedimentary fan outcrop records the environment in which the deposit formed and can provide key information as to past conditions on the surface of Mars. Examining the stratigraphy of fan outcrop using orbital data, it is possible to constrain whether these landforms formed in the presence of standing water (and thus broadly constrain the amount of water that was present on the surface) by classifying them as one of two end members: deltas or alluvial/fluvial fans. Deltas are formed subaqueously in a standing body of water, while alluvial fans are formed subaerially. The two end members can be distinguished using orbital outcrop models (from high-resolution images overlain on digital elevation models), which allow us to study their stratigraphy; deltas are characterised by a clinoform shape with slope breaks identified based on the dips of exposed strata, while alluvial fans lack such slope breaks. Each delta indicates where there was once a standing body of water and provides further constraints on the location and frequency of water reservoirs recorded in the sedimentary rock record on Mars.

Two example fans include a fan at the mouth of Tyras Vallis and another in Hargraves crater. Preliminary results show an obvious slope break can be identified within the fan in Hargraves, while no such slope break can be found in the Tyras Vallis fan. This leads us to the interpretation that the Hargraves fan is a delta and was deposited in a subaqueous environment while the Tyras fan is consistent with the subaerial deposition of an alluvial fan. Another key finding is that the fan features have undergone a significant amount of erosion since formation, which is important to consider when interpreting the modern geomorphology of these landforms. We are working to expand this catalog of fan features, and will further study and categorize ~15 sedimentary fans across the surface of Mars.