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

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.