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Curiosity meets the Gediz Vallis Ridge: a remnant of a late-stage debris-flow dominated fan

  • Le Mouélic, Stéphane
  • Fedo, Christopher
  • Caravaca, Gwénaël
  • Lamb, Michael P.
  • Grant, John A.
  • Gasnault, Olivier
  • Grotzinger, John
  • Stack, Kathryn
  • Gupta, Sanjeev
  • Lewis, Kevin W.
  • Rubin, David M.
  • Rapin, William
  • Fraeman, Abigail
  • Banham, Steven
Publication Date
Dec 11, 2023
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Recent studies using HiRISE and CTX imagery show that fan-shaped deposits are widespread across Mars and likely formed during the Hesperian and Amazonian periods. Remote imagery and topographic analysis suggest that they may have been produced by both fluvial and debris flow processes and are generally distinct from deltaic deposits such as those in Jezero. Curiosity-based observations of the deeply eroded deposits of the Gediz Vallis Ridge (GVR), lying at the base of Mt. Sharp, now provide our first cross-sectional observation of a Martian fan, revealing episodic deposition by debris flows and reworking by fluvial processes. The fan was deposited on the Stimson formation, a pediment-mantling lithified aeolian deposit. Above the Stimson is a discontinuous fine-grained (≤ gravel size) basal unit with decimeter-scale bedding that is parallel to top of the Stimson. The unit reaches a maximum observable thickness of 15m and is unconformably onlapped by stratified gravel and boulder deposits. Discontinuous layering within these deposits ranges in thickness from 0.25 to 3 m and exhibits a mean slope of 7% oriented parallel to the ridge axis, much gentler than the slope of the basal contact. At progressively higher elevations, five depositional packages are exposed. Each package is composed two facies whose frequency and lateral extent differ: 1) a light toned typically matrix-supported coarse facies with scattered erosionally resistant dark toned gravel and boulders, and 2) a dark-toned typically clast-supported gravel and boulder facies. Grainsize analysis of Chemcam data indicates similar median grainsizes (170-190 mm) in light- and dark toned facies with isolated 1-3m boulders scattered throughout. The light-toned strata appear to be debris flow deposits. The dark toned strata are interpreted to be fluvial, probably derived from surface runoff that concentrated dark stones from the light toned unit into channels and sheetflood deposits while destroying the weaker light toned sediment during transport. The GVR is an erosional remnant of a fan that may have built 10 km downslope, likely in the late Hesperian or early Amazonian. Stratigraphic differences in the five packages suggest significant changes in the magnitude and frequency of surface runoff, but with a supply of meter scale boulders throughout fan evolution.

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