Post by Dr. Ryan B. Anderson, Astrogeology Science Center, United States Geological Survey
One of the most environmentally diagnostic features of sedimentary rocks is cross-bedding, which occurs when sediment is transported by wind, water, or volcanic processes, resulting in horizontal strata composed of inclined beds. The geometry of cross-bedded sedimentary deposits provides information about the depositional setting and post-depositional history of the rocks, making the identification, measurement, and interpretation of cross-beds particularly interesting on Mars, where past conditions are of great scientific interest. This post describes cross-bedding in Upper Aeolis Mons in Gale crater (Image 1).
Image 1: Example of complex bedding patterns in upper Aeolis Mons, interpreted to be large-scale aeolian cross beds. Image is an inset of HiRISE observation PSP_001620_1750.
One location on Mars where numerous examples of cross-bedding have been observed is Gale crater, a ~154 km diameter crater containing a ~5 km tall mountain of layered rock called Aeolis Mons (informally dubbed Mt. Sharp). The Curiosity rover landed in the northwestern crater floor in 2012 and has observed numerous outcrops of cross-stratified sandstone, providing evidence for an ancient fluvial system with sediment transport toward the southeast and occasional aeolian deposits (Vasavada et al., 2014; Grotzinger et al., 2015; Stack et al., 2016; Edgar et al., 2017; Rice et al., 2017; Banham et al., 2018). Complex patterns interpreted as cross-bedding were observed on erosional “benches” in the upper unit of Aeolis Mons (Anderson and Bell, 2010), well beyond where the rover will be able to access, and we have recently completed a detailed study of these patterns to test this hypothesis (Anderson et al., 2018) (Image 1).
Using high resolution topography, we traced the bedding in upper Aeolis Mons, measured its orientation, and then compared our measurements with the results of computer models of cross-bedding. We found that the complex patterns do indeed reflect non-planar bedding and that our measurements are consistent with large-scale (hundreds of meters) dunes. Several locations with near-concentric bedding patterns likely indicate that sediment deposition was more rapid than bedform migration (Image 2).
Image 2: Left: Traced of bedding (red) and apparent truncations (cyan). Upper right: Simulated bedforms and simulated horizontal section. Lower right: The simulated distribution of bedding oreintations is similar to the measured distribution (blue rose plot). Figure adapted from Anderson et al. (2018).
Our measurements provide an important constraint on the formation and evolution of Aeolis Mons. Although the areas studied occur at the top of a modern-day mountain, large dune fields tend to accumulate in sediment sinks, typically in topographic lows. This constraint, combined with known unconformities in Aeolis Mons, suggests a formation scenario for Aeolis Mons involving multiple rounds of significant infilling and erosion of the crater.
Further Reading
Anderson, R.B., Bell III, J.F., 2010. Geologic mapping and characterization of Gale Crater and implications for its potential as a Mars Science Laboratory landing site. Mars J. 5, 76–128. doi:10.1555/mars.2010.0004
Vasavada, A.R., Grotzinger, J.P., Arvidson, R.E., Calef, F.J., Crisp, J.A., Gupta, S., Hurowitz, J., Mangold, N., Maurice, S., Schmidt, M.E., et al., 2014. Overview of the Mars Science Laboratory mission: Bradbury Landing to Yellowknife Bay and beyond. J. Geophys. Res. Planets 119, 1134–1161. doi:10.1002/2014JE004622
Grotzinger, J.P., Gupta, S., Malin, M.C., Rubin, D.M., Schieber, J., Siebach, K., Sumner, D.Y., Stack, K.M., Vasavada, A.R., Arvidson, R.E., et al., 2015. Deposition, exhumation, and paleoclimate of an ancient lake deposit, Gale crater, Mars. Science 350, aac7575–aac7575. doi:10.1126/science.aac7575
Stack, K.M., Edwards, C.S., Grotzinger, J.P., Gupta, S., Sumner, D.Y., Calef, F.J., Edgar, L.A., Edgett, K.S., Fraeman, A.A., Jacob, S.R., et al., 2016. Comparing orbiter and rover image-based mapping of an ancient sedimentary environment, Aeolis Palus, Gale crater, Mars. Icarus 280, 3–21. doi:10.1016/j.icarus.2016.02.024
Edgar, L.A., Gupta, S., Rubin, D.M., Lewis, K.W., Kocurek, G.A., Anderson, R.B., Bell, J.F., Dromart, G., Edgett, K.S., Grotzinger, J.P., et al., 2017. Shaler: In situ analysis of a fluvial sedimentary deposit on Mars. Sedimentology. doi:10.1111/sed.12370
Rice, M.S., Gupta, S., Treiman, A.H., Stack, K.M., Calef, F., Edgar, L.A., Grotzinger, J., Lanza, N., Le Deit, L., Lasue, J., et al., 2017. Geologic overview of the Mars Science Laboratory rover mission at the Kimberley, Gale crater, Mars: Overview of MSL at the Kimberley. J. Geophys. Res. Planets 122, 2–20. doi:10.1002/2016JE005200
Anderson, R.B., Edgar, L.A., Rubin, D.M., Lewis, K.W., Newman, C., 2018. Complex bedding geometry in the upper portion of Aeolis Mons, Gale crater, Mars. Icarus 314, 246–264. doi:10.1016/j.icarus.2018.06.009
Banham, S.G., Gupta, S., Rubin, D.M., Watkins, J.A., Sumner, D.Y., Edgett, K.S., Grotzinger, J.P., Lewis, K.W., Edgar, L.A., Stack-Morgan, K.M., et al., 2018. Ancient Martian aeolian processes and palaeomorphology reconstructed from the Stimson formation on the lower slope of Aeolis Mons, Gale crater, Mars. Sedimentology. doi:10.1111/sed.12469
Planetary Geomorphology Image of the Month 