DISTINGUISHING ANCIENT SEDIMENTARY AND IGNEOUS ROCK UNITS ON MARS, FROM ORBIT

Numerous Mars observations made from orbit over the last decade have shown that broad (>1 km), rock-dominated outcrops are common in the Noachian highlands; determining the origins of these outcrops is central to interpreting the surface processes that operated on early Mars and resurfaced the Noachian crust. Clastic sedimentary units occurring as flat-lying plains can be difficult to distinguish from volcanic plains units on Mars from orbit due to similarity in composition, limited exposures of vertical stratigraphy, and degradation/mantling processes. However, using a combination of infrared and visible data sets, it is possible to distinguish between these origins in some cases. In an effort to develop criteria to aid in distinguishing sedimentary and igneous units, we examined the compositional, thermophysical, and textural properties of a number of bedrock exposures of likely sedimentary and volcanic origins, determined using high-resolution (meter to decameter per pixel) morphologic observations (e.g. inverted fluvial channels, lava flow morphologies, embayment relationships, fine-scale layering, or preserved dune forms). Whereas both sedimentary and volcanic units are typically basaltic in composition, sedimentary units tend to be the same composition as surrounding regolith, and do not exhibit olivine enrichments. In contrast, likely volcanic units tend to show compositional differences from surrounding regolith, and are commonly enriched in olivine and/or pyroxene compared to the regolith. Morphologies for both types of units can be highly variable, but for the volcanic units, rugged textures with meter-to-decameter scale relief are commonly observed. Though olivine enrichment could potentially occur via aeolian transport and sorting, we do not observe a gradual transition in olivine abundance between low thermal inertia regions (potential sediment sources) and high thermal inertia (rock-dominated) olivine-bearing units. Furthermore, strong compositional sorting at ~10-100 km scales is not commonly observed in many modern dune fields, suggesting that aeolian sorting likely does not play a major role in compositional segregation at these scales. These observations contribute to ongoing efforts to interpret the volcanic history and climatic conditions that affected early Mars.