Curiosity meets the Gediz Vallis Ridge: Remnants of a late-stage debris-flow dominated fan in Gale Crater

Recent studies using HiRISE and CTX imagery show that fan-shaped deposits are widespread especially across the walls of craters on Mars and likely formed during the Hesperian and Amazonian periods. Remote imagery and topographic analyses 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 rover-based observations of the deeply eroded deposits of the Gediz Vallis Ridge (GVR), lying at the base of Aeolis Mons (Mt. Sharp), now provide our first cross-sectional observation of a steep, central peak-based Martian fan, revealing episodic deposition by debris flows, and reworking by fluvial processes, and likely fluvial sheetfloods. The fan remnants today rest on top of the Stimson formation, a lithified aeolian sandstone that mantles the Greenheugh pediment. Above the Stimson is a discontinuous, fine-grained (≤ gravel size), Basal unit with decimeter-scale bedding that is concordant with the top of the Stimson. The unit reaches a maximum observable thickness of 15 m 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, which is much gentler than the slope of the basal contact. At progressively higher elevations, five depositional packages are exposed. Each package is composed of two facies whose frequency and lateral extent differ: 1) a light-toned, boulder-rich coarse facies, typically matrix-supported, with scattered erosionally resistant dark-toned gravel and boulders, and 2) a dark-toned gravel and boulder facies, typically clast-supported. Both facies have median grain sizes of about 180 mm and host 1–3 m boulders scattered throughout. The light-toned strata are interpreted as debris flow deposits. The dark-toned strata are interpreted to be fluvial, probably derived from surface runoff that concentrated dark clasts from the light-toned unit into channels and sheetflood deposits while disaggregating the weaker light-toned sediment during transport. The gentle slopes of these deposits indicate that the depocenter back-stepped up the steep underlying pediment surface, and probably formed the core of fan that may have extended as much as 10 km downslope, likely in the late Hesperian or early Amazonian. Most of the fan may have been composed of the light-toned sediment highly erodible by wind, leading to a near complete removal of the fan deposit after the cessation of fan construction. Stratigraphic differences in the five packages suggest significant changes in the magnitude and frequency of surface runoff and sediment supply likely reflecting variable climate conditions. Reconstructed fan sediment volume estimates indicate that at least many tens of meters (and likely much more) of precipitation-generating runoff from Mt. Sharp were needed to build the fan, and this involved thousands of runoff events. This variability supports proposals based on orbiter data that Martian fans are characterized by mixtures of debris flow and fluvial deposits and that fan development was episodic, rather than a single pulse. The GVR fan records one of the last significant hydrogeomorphic phases in Gale and likely the youngest observed in situ by Curiosity .