The light-toned stratified sedimentary rock exposures in western Juventae Chasma, Mars, in context

Abstract

Juventae Chasma, an isolated depression northeast of the Valles Marineris trough system, hosts four mounds of light-toned, layered materials. These are erosional remnants interpreted to consist of sedimentary rocks. The geological setting of Juventae Chasma is a complex amalgamation of terrains and surfaces of different morphologies, ages, and structures. Most previous published studies assumed or interpreted that the light-toned mound-forming materials were deposited in the chasm after it opened or began to open. Here, we use detailed observations of the geologic setting, the nature of chasm wall rocks, and the distribution and nature of the contacts of the light-toned materials with adjacent geologic materials to argue that the light-toned mounds in Juventae Chasma are, more likely, remnants of light-toned materials that existed in the subsurface before the chasm opened up. The rock cut by the chasm was not a monolithic unit, (e.g., lava flows) and might have included both light-toned and dark-toned sedimentary rocks bearing a range of diagenetic properties. The light-toned materials were deposited (as clasts, precipitates, or both) within craters or depressions that were filled, buried, lithified, and then exposed when Juventae Chasma formed. Because the sedimentary rock types and stratal package properties differ from one mound to the next, the buried craters or depressions could have existed at different times and provide sedimentary records of different environments that existed at different times during the planet's early history (Noachian Period). Upon exposure, these materials, as well as chasm wall rocks and chaotic terrain blocks, underwent differential erosion as a function of rock physical properties to produce the geomorphic expressions observed today.

the rocks were present in the Martian subsurface before the opening of the chasms and were exposed by the events which formed and modified them (e.g., Malin 1976; Malin and Edgett, 2000; Catling et al., 2006; Montgomery and Gillespie, 2005); or</ce:para>

the rocks consist of materials deposited in the chasms by processes that occurred after the chasms opened up (e.g., McCauley 1978; Lucchitta, 2001; Bishop et al., 2009); or</ce:para>

the materials are a combination of both #1 and #2 - some light-toned rock units pre-date the opening of the chasms and some (which in some cases superpose pre-chasm light-toned rock units) post-date the opening of the Valles Marineris (e.g., Chapman et al., 2003).</ce:para>

</ce:para>Metz et al. (2010) considered the question of the stratigraphic relations between light-toned layered material in the chasms of the Valles Marineris and the darker-toned wall and concluded that the least ambiguous case in which light-toned material appears to be emergent from within a chasm wall is found in Juventae Chasma. In this case, their observation provided at least a limited case in support of the first model listed above. With this observation as a starting point, we report here on our exploration of this topic with regards to Juventae Chasma. Specifically, we used data acquired by cameras aboard orbiting spacecraft to assess the relative stratigraphic position of the four light-toned, stratified mounds in western Juventae Chasma (Fig. 2a) relative to the bedrock cut by the adjacent chasm walls and the blocks of "chaotic terrain" (terminology of Sharp et al., 1971) within the chasm. We argue that the light-toned, mound-forming rocks existed in the subsurface before the chasm formed. While this is not the commonly held viewpoint in the literature, it is important because their timing relative to chasm formation has important implications for timing of aqueous activity on Mars.</ce:para>