GEOMORPHOLOGY AND CONTEXT OF ANCIENT BEDFORMS ON MARS

The surface of Mars hosts numerous occurrences of sedimentary rocks, some of which formed by aeolian or fluvial processes in an earlier era. Sedimentary landforms such as dunes and ripples are often eroded and buried, thereby making the identification of ancient bedforms challenging without field observations. However, certain isolated landforms retained what are perceived as their gross geomorphic structures and original plan view morphologies. These features meet the morphologic criteria for dunes and ripples within 25 cm/pix HiRISE images, yet appear degraded and eroded. The objective of this project is to characterize the spatial distribution, origin, and geologic context of these preserved ancient bedforms on Mars.

Paleobedform candidates were identified using criteria that include: 1) features recognized as a component of a coherent bedform (characteristic wavelength, branching and merging crest patterns), and 2) having been rendered dormant in the distant past and showing signs of lithification. Using these criteria, 258 locations hosting sedimentary landforms were characterized and evaluated using HiRISE data.

Some of the most compelling sites include organized groups of large (3-40 m-tall), crescentic features (200-600 m wavelengths) within Apollinaris Sulci and Melas Chasma, as has been previously reported. These metrics along with superposed craters, boulders and fractures led to the confirmation that these were partially eroded barchan dunes with a high degree of lithification. New regions hosting candidate paleobedforms were also found, some of which have yet to be documented (e.g. Li crater, Meridiani, Noctis Labyrinthus). Crest-to-crest wavelengths (10-100 m), heights, and morphologies suggest some are lower-order bedforms of (ancient) megaripples or transverse aeolian ridges. More enigmatic paleobedform candidates occur concentrated along the steep Valles Marineris wall slopes. These intermediate-sized, arcuate landforms are heavily cratered, but typically arranged perpendicular to the local gradient, perhaps formed by wall slope winds and slope creep. Most paleobedforms appear to be aeolian in origin, however, Lethe Vallis and Holden crater sites show features consistent with fluvial processes. Potential landscape evolution scenarios will be discussed at the conference.