Origin and composition of three heterolithic boulder- and cobble-bearing deposits overlying the Murray and Stimson formations, Gale Crater, Mars

• Published in: Icarus
• Main Authors: Wiens, Roger C., Edgett, Kenneth S., Stack, K., Dietrich, William E., Bryk, Alexander B., Mangold, Nicolas, Bedford, Candice, Gasda, Patrick, Fairén, Alberto G., Thompson, L., Johnson, Jeff, Gasnault, Olivier, Clegg, Sam, Cousin, Agnes, Forni, Olivier, Frydenvang, J., Lanza, Nina, Maurice, Sylvestre, Newsom, H., Payré, Valerie, Rivera-Hernandez, Frances, Vasavada, Ashwin R.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Academic Press 2020
• Subjects: Bathurst Inlet; Hidden Valley; Pyramid; The Landing; Yellowknife; Yellowknife Bay
• Online Access: http://hdl.handle.net/10261/234109; https://doi.org/10.1016/j.icarus.2020.113897

Heterolithic, boulder-containing, pebble-strewn surfaces occur along the lower slopes of Aeolis Mons (“Mt. Sharp”) in Gale crater, Mars. They were observed in HiRISE images acquired from orbit prior to the landing of the Curiosity rover. The rover was used to investigate three of these units named Blackfoot, Brandberg, and Bimbe between sols 1099 and 1410. These unconsolidated units overlie the lower Murray formation that forms the base of Mt. Sharp, and consist of pebbles, cobbles and boulders. Blackfoot also overlies portions of the Stimson formation, which consists of eolian sandstone that is understood to significantly postdate the dominantly lacustrine deposition of the Murray formation. Blackfoot is elliptical in shape (62 × 26 m), while Brandberg is nearly circular (50 × 55 m), and Bimbe is irregular in shape, covering about ten times the area of the other two. The largest boulders are 1.5–2.5 m in size and are interpreted to be sandstones. As seen from orbit, some boulders are light-toned and others are dark-toned. Rover-based observations show that both have the same gray appearance from the ground and their apparently different albedos in orbital observations result from relatively flat sky-facing surfaces. Chemical observations show that two clasts of fine sandstone at Bimbe have similar compositions and morphologies to nine ChemCam targets observed early in the mission, near Yellowknife Bay, including the Bathurst Inlet outcrop, and to at least one target (Pyramid Hills, Sol 692) and possibly a cap rock unit just north of Hidden Valley, locations that are several kilometers apart in distance and tens of meters in elevation. These findings may suggest the earlier existence of draping strata, like the Stimson formation, that would have overlain the current surface from Bimbe to Yellowknife Bay. Compositionally these extinct strata could be related to the Siccar Point group to which the Stimson formation belongs. Dark, massive sandstone blocks at Bimbe are chemically distinct from blocks of similar ...