Episodic Aqueous Conditions Punctuated dominantly Aeolian Deposition within the Layered Sulphate-bearing Unit, Gale crater (Mars)
Résumé
Aeolis Mons (informally known as Mount Sharp) in Gale crater (Mars) shows a major stratigraphic transition from mudstone-rich strata (with subordinate sandstones) recording deposition in lacustrine to fluvial settings into a sulphate-bearing unit (the Layered Sulphate-bearing unit (LSu)). The lower section of the LSu is dominated by stacked, cross-bedded facies with variable diagenetic overprint, that likely records a purely dry aeolian dune environment. Higher up in the section, the aeolian cross-bed succession shows interstratified lenses of a different facies. Here, we describe the sedimentology of one of these lenses, The Prow, to interpret depositional processes. The Prow is an ~18-m-long, ~0.5-1-m-thick lenticular sedimentary body. Its lowermost part comprises decimetre-scale cross-beds indicative of deposition from subaqueous dune migration. By contrast, the upper sections are dominated by cm-scale ripple structures. Lenticular ripple forms with convex upper surfaces are common. The crests generally show rounded symmetric profiles. Locally, symmetric vertical accretion over ripple crests is observed implying rapid sediment aggradation. The ripple cores comprise a sandstone and draping laminations are finer grained). The drapes are interpreted as mud drapes formedfrom suspension fallout onto ripple topography during low energy quiescent episodes. The symmetric form of many of the ripple structures with preservation of formsets is suggestive of formation by oscillatory flow by wave action. The presence of drapes of finer-grained material superimposed on coarser-grained ripples is interpreted to record episodes of higher-energy sand transport separated by recurrent intervals of low-energy conditions during which sand grains could not be mobilised. The presence of mudstone-draped wave and current ripple forms is strongly indicative of depositionfrom aqueous flows and moreover suggests the existence of a likely aerially small, shallow standing body of water in which the sediments were deposited. We infer that The Prow and by inference the other lenses observed may record the episodic presence of transient small standing bodies of water that infilled interdune areas or fluvial scours in anotherwise dominantly dry aeolian dune-dominated environment.