High-resolution morphologic and spectral characteristics of Crater-exposed Bedrock on Mars: Insights into the petrogenesis, stratigraphy and geologic history of the Martian crust

Rocks form under a variety of geologic settings and conditions, thus the mineral composition, texture, structures and stratigraphic relationships of exposed rocks provide geologists a means to access information about the past geologic and climatic history. Typically, tectonic events (e.g., orogenic) and erosional processes expose sections of older terrestrial rocks at the surface. On Mars, a lack of complex tectonics and lower erosion rates make these tectonic exposures virtually non-existent. Impacts, however, generate localized displacements and structural uplift of target rocks and exposes them within the crater rim, walls, terraces and central structural uplifts. Imagery from the High Resolution Imaging Science Experiment (HiRISE) of this Crater-Exposed Bedrock (CEB) reveals unprecedented meter to decameter textural and structural detail [1]. Our initial work, based on previous efforts [1-3], has revealed that not all craters are well exposed due to impact melt coatings and ongoing degradation, infilling, and mantling of crater rims, floors and walls. Thus, making a database (DB) of craters with good exposures is an essential step towards understanding the spatial and temporal distribution of CEB textures, structures and compositions. When complete, the DB will aid our ability to make inferences regarding the petrogenesis, evolution and geologic history of the upper crust at regional and potentially global scales. Our CEB DB will be used to focus on spectral units that specifically correlate with CEB textures and stratigraphic relationships. Our preliminary results suggest that CEB can be classified into three textural categories, 1) Megabreccias (MB), 2) Intact layered Stratigraphy (IS), and 3) a massive textured Fractured Bedrock (FB), with each of these classifications being informative with respect to a specific geologic setting or possible set of histories (e.g., late-heavy bombardment, cyclical volcanism and sedimentation). Preliminary spectral analyses indicate that some aqueous alteration phases correlate with CEB, which may either pre- or post-date the exposing crater’s formation [4]. Some of these lithologies may even represent some of the oldest rock exposures on the surface, and thus record early conditions on Mars [3]. Therefore, our resulting CEB database will also be useful in our companion study by Skok et al. [5] to identify the oldest exposures of crustal materials. Here we present some of our early results showing that trends in the spatial distributions CEB can indeed be used not only as a window into subsurface composition, but also as a probe of regional and global geologic history. [1] Tornabene L.L. et al. (2010) LPSC XLI, 1737. [2] Grant J.A. et al. (2008) Geology, 36, 195-198. [3] McEwen A.S. et al. (2008) AGU, P43D-03. [4] Marzo, G.A. et al. (2010) Icarus, 208, 667-683. [5] Skok J.R. et al. (2010) LPSC XLI, 1926.