The first rover-based investigation of a pediment and a mixed alluvial and debris flow fan on Mars

Evidence of paleo-rivers, fans, deltas, lakes, and channel networks across Mars has forced much debate about what climate conditions would support such surface water driven features. Pediments, gently sloping erosional surfaces of low relief developed in bedrock at the base of mountains, have also been identified on Mars. On Earth, these erosional landforms, created by overland flow and shallow channelized runoff, are typically capped by fluvial sediments, and thus in exceptionally arid regions, pediments are interpreted to record past wet periods. From satellite imagery of Mars, fan deposits, primarily derived from canyons cut into crater walls, have been documented over a wide region. The Curiosity Rover has provided the first opportunity for a close inspection on Mars of a pediment and the depositional remnant of a fan. In this dissertation, I document the Greenheugh pediment (Chapter 1) and Gediz Vallis Ridge (fan deposit) (Chapter 2) in Gale Crater, exploiting the observational capability of the Curiosity rover. In Chapter 1, I show that the pediment was likely cut by wind erosion, not fluvial processes. The pediment was capped by an eolian deposit (Stimson sandstone) that mantled the lower footslopes of the 5 km tall Mt. Sharp that occupies the center of Gale Crater. This burial terminated active wind erosion, preserving the pediment surface (as an angular unconformity). Groundwater was present during Stimson deposition (as evidenced by soft sediment deformation), contributing to lithification and diagenesis. Post lithification, wind erosion first cut canyons in the northern most footslopes (north of Vera Rubin ridge). Gravels were deposited in these canyons, likely due to runoff from Mt. Sharp and boulder-rich fluvial and debris flow deposits built a >70m thick sequence (Gediz Vallis Ridge) on the southern Greenheugh pediment. Continued wind erosion left elevated patches of gravel on the northern footslopes, but exposure age dating showed erosion ceased by 1 Ga (but possibly much earlier). Continued erosion to the south led to emergence of Vera Rubin ridge, retreat of the Greenheugh pediment, and the formation of Glen Torridon valley. Hence, this footslope environment of Mt. Sharp records climate-driven cycles of wind erosion, aeolian deposition (and groundwater activity), surface runoff and sediment deposition, followed by further significant wind erosion that declined to present very slow rates. This cyclicity likely occurred during the late Hesperian and possibly into the Amazonian. In Chapter 2, I review recent studies using HiRISE and CTX imagery that 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 found in Jezero Crater. 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 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, 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 boulder-rich coarse facies with scattered erosionally-resistant dark-toned gravel and boulders, and 2) a dark-toned typically clast-supported gravel and boulder facies. Both facies have median grain sizes of about 180 mm and host 1-3 m 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 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 fan deposits back-stepped up the steep underlying pediment surface, and probably formed the core of fan that may have built 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 at 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. This variability supports satellite-based proposals 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 This is the first close in-situ examination of a pediment and of the stratigraphy of a mixed alluvial and debris fan on the surface of Mars. It may also be last opportunity for decades. Consequently, considerable effort was made to thoroughly characterize and document these features in these two chapters such that others may draw their own conclusion in the future.

Advisor: Dietrich, William E