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

Abstract

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 morphology at Bradbury Rise, except for a single float block, Oscar (Sol 516). Conglomerates observed along a low, sinuous ridge at Bimbe consist of matrix and clasts with compositions similar to the Stimson formation, suggesting that stream beds likely existed nearly contemporaneously with the dunes that eventually formed the Stimson formation, or that they had the same source material. In either case, they represent a later pulse of fluvial activity relative to the lakes associated with the Murray formation. These three units may be local remnants of infilled impact craters (especially circular-shaped Brandberg), decayed buttes, patches of unconsolidated fluvial deposits, or residual mass-movement debris. Their incorporation of Stimson and Murray rocks, the lack of lithification, and appearance of being erosional remnants suggest that they record erosion and deposition events that post-date the exposure of the Stimson formation.

Extraformational sediment recycling on Mars

“…3H; behind boulder left of Tumba-Funda in Fig. 6D) resembles that of Stimson formation sandstones, suggesting the possibility that the pebble-bearing boulders at Bimbe could have come from an unidentified conglomeratic facies of the Stimson formation (Wiens et al, 2020). The Tumba-Funda and Balombo conglomerates are interpreted to consist of fluvial sediment (Supplement S5 [footnote 1]).…”

“…Occurring on the lower north slope of Aeolis Mons, Bimbe (Fig. 6) is an ~16,840 m 2 unconsolidated deposit of boulders, cobbles, and windblown sand that overlies mudstones of the Hartmann's Valley member (Gwizd et al, 2020) of the Murray formation (Wiens et al, 2020). The cobbles and boulders (Figs.…”

“…The cobbles and boulders (Figs. 6C and 6D) are mostly fragments of conglomerate and sandstone; some cobbles are locally derived mudstones (Wiens et al, 2020). Sedimentary rock fragments occur in two conglomeratic boulders examined using Curiosity's instruments: Tumba-Funda (Figs.…”

“…6D and 9). The bedrock from which these boulders were dislodged is not evident in the vicinity of Bimbe, nor was it identified in outcrops along the rover traverse (Wiens et al, 2020). The geochemical composition of a nearby conglomeratic boulder ( Fig.…”

Supplemental Material: Extraformational sediment recycling on Mars

“…MGS MOLA elevation map product (e.g., Smith et al, 2001). (Wiens et al, 2020); sandstone boulder in the foreground contains ChemCam and APXS targets Sonneblom and Zambesi (Wiens et al, 2020). Figure 15B.…”