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

Authors
  • Wiens, Roger C1
  • Edgett, Kenneth S2
  • Stack, Kathryn M3
  • Dietrich, William E4
  • Bryk, Alexander B4
  • Mangold, Nicolas5
  • Bedford, Candice6
  • Gasda, Patrick1
  • Fairen, Alberto7, 8
  • Thompson, Lucy9
  • Johnson, Jeff10
  • Gasnault, Olivier11, 12
  • Clegg, Sam1
  • Cousin, Agnes11, 12
  • Forni, Olivier11, 12
  • Frydenvang, Jens13
  • Lanza, Nina1
  • Maurice, Sylvestre11, 12
  • Newsom, Horton14
  • Ollila, Ann1
  • And 3 more
  • 1 Los Alamos National Laboratory, Los Alamos, NM, USA.
  • 2 Malin Space Science Systems, San Diego, CA, USA. , (Mali)
  • 3 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA.
  • 4 Department of Earth and Planetary Science, University of California-Berkeley, Berkeley, CA, USA.
  • 5 Laboratoire de Planétologie et Géodynamique, UMR 6112 CNRS, Université Nantes, Université d'Angers, Nantes, France. , (France)
  • 6 Lunar and Planetary Institute, Houston, TX, USA.
  • 7 Centro de Astrobiologia (CSIC-INTA), Madrid, Spain. , (Spain)
  • 8 Department of Astronomy, Cornell University, Ithaca, NY, USA.
  • 9 Planetary and Space Science Centre, University of New Brunswick, Fredericton, New Brunswick, Canada. , (Canada)
  • 10 Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA.
  • 11 Université de Toulouse, UPS-OMP, Toulouse, France. , (France)
  • 12 Institut de Recherche en Astrophysique et Planéetologie, CNRS, UMR 5277, Toulouse, France. , (France)
  • 13 University of Copenhagen, Copenhagen, Denmark. , (Denmark)
  • 14 Institute of Meteoritics, University of New Mexico, Albuquerque, NM, USA. , (Mexico)
  • 15 Earth, Environmental, and Planetary Sciences, Rice University, Houston, TX, USA.
  • 16 Department of Earth Sciences, Dartmouth College, Hannover, NH, USA.
Type
Published Article
Journal
Icarus
Publisher
Elsevier BV
Publication Date
Nov 01, 2020
Volume
350
Pages
113897–113897
Identifiers
DOI: 10.1016/j.icarus.2020.113897
PMID: 32606479
Source
Medline
Keywords
Language
English
License
Unknown

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.

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