WIDESPREAD CLASTIC ROCK EXPOSURES IN THE MARTIAN SOUTHERN HIGHLANDS: CHARACTERISTICS AND CANDIDATE ORIGINS (Invited Presentation)

The cratered highlands in Mars’ southern hemisphere contain hundreds of flat, areally extensive (> 250 km²) bedrock plains in intercrater and intracrater basins. They exhibit high thermal inertia relative to surroundings and rough textures at <20 m scales, consistent with lithified materials. We conducted a global survey of the spectral and morphological properties of over 200 individual bedrock plains units in an effort to determine their petrogenetic origins. Though a significant fraction exhibit high crater retention and morphologies consistent with lava plains, the majority exhibit poor retention of craters smaller than 500 m diameter and erosional morphologies (yardangs, buttes) consistent with friable materials. Olivine enrichments compared to surrounding non-bedrock materials are common but typically below 5%; enrichments may be lag deposits or be primary in origin. Thin (meter-scale) bedding or banding is not apparent in exposed vertical sections. However, coarse layering suggestive of vertical variation in material properties can be observed in crater walls or through lateral differences in tone/texture across broad plains, which may indicate multiple depositional episodes. A few exposures contain ridge morphologies consistent with inverted channels and possible deltas/fans that connect with valleys incised into higher-standing units. With exceptions in Ladon basin and Eridania basin, hydrated minerals are not detected.

As described by [1] and references therein, known volcanic plains (e.g. Gusev crater plains) exhibit relatively low thermal inertia from orbit due to the development of thick regolith cover. The bedrock materials described above lack well-developed regolith, suggesting that the bedrock comminutes into fine, wind-mobilized particles. This implies that the bedrock itself is predominantly clastic and fine-grained [1]. Candidate petrogenetic origin(s) for the interpreted clastic units include pyroclastic, impact-related, or detrital sedimentary processes. Though airfall and aeolian transport processes cannot be ruled out, evidence for fluvial morphologies suggest a role of aqueous transport at least for some units. Regardless, the sedimentary rock volume and record is more extensive than previously recognized.

[1] https://doi.org/10.1002/2018GL077030