Browsing by Author "Banham, Steven G."

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    Burial and Exhumation of Sedimentary Rocks Revealed by the Base Stimson Erosional Unconformity, Gale Crater, Mars
    (Wiley, 2022) Watkins, Jessica A.; Grotzinger, John P.; Stein, Nathan T.; Banham, Steven G.; Gupta, Sanjeev; Rubin, David M.; Morgan, Kathryn Stack; Edgett, Kenneth S.; Frydenvang, Jens; Siebach, Kirsten L.; Lamb, Michael P.; Sumner, Dawn Y.; Lewis, Kevin W.
    Sedimentary rocks record the ancient climate of Mars through changes between subaqueous and eolian depositional environments, recognized by their stratal geometries and suites of sedimentary structures. Orbiter- and rover-image-based geologic mapping show a dynamic evolution of the 5-km-thick sedimentary sequence exposed along the flanks of Aeolis Mons (informally, Mt. Sharp) in Gale crater, Mars, by deposition of subaqueous strata followed by exhumation via eolian erosion and then deposition of overlying, onlapping strata of inferred eolian origin. This interpretation suggests that a significant unconformity should occur at the base of the onlapping strata, thus predicting lateral variations in elevation along the contact between the underlying Mt. Sharp group and overlying Stimson formation. Curiosity rover and high-resolution orbital image data quantify paleotopographic variability associated with the contact; ∼140 m of net elevation change and a slope closely aligned with the modern topography is expressed along the regional contact. These results support the interpretation of an erosional unconformity between these strata and that it was likely formed as a result of eolian erosion within the crater, indicative of a transition from wet to dry climate and providing insight into the stratigraphic context, geologic history, and habitability within Gale crater.
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    Extraformational sediment recycling on Mars
    (The Geological Society of America, 2020) Edgett, Kenneth S.; Banham, Steven G.; Bennett, Kristen A.; Edgar, Lauren A.; Edwards, Christopher S.; Fairén, Alberto G.; Fedo, Christopher M.; Fey, Deirdra M.; Garvin, James B.; Grotzinger, John P.; Gupta, Sanjeev; Henderson, Marie J.; House, Christopher H.; Mangold, Nicolas; McLennan, Scott M.; Newsom, Horton E.; Rowland, Scott K.; Siebach, Kirsten L.; Thompson, Lucy; VanBommel, Scott J.; Wiens, Roger C.; Williams, Rebecca M.E.; Yingst, R. Aileen
    Extraformational sediment recycling (old sedimentary rock to new sedimentary rock) is a fundamental aspect of Earth’s geological record; tectonism exposes sedimentary rock, whereupon it is weathered and eroded to form new sediment that later becomes lithified. On Mars, tectonism has been minor, but two decades of orbiter instrument–based studies show that some sedimentary rocks previously buried to depths of kilometers have been exposed, by erosion, at the surface. Four locations in Gale crater, explored using the National Aeronautics and Space Administration’s Curiosity rover, exhibit sedimentary lithoclasts in sedimentary rock: At Marias Pass, they are mudstone fragments in sandstone derived from strata below an erosional unconformity; at Bimbe, they are pebble-sized sandstone and, possibly, laminated, intraclast-bearing, chemical (calcium sulfate) sediment fragments in conglomerates; at Cooperstown, they are pebble-sized fragments of sandstone within coarse sandstone; at Dingo Gap, they are cobble-sized, stratified sandstone fragments in conglomerate derived from an immediately underlying sandstone. Mars orbiter images show lithified sediment fans at the termini of canyons that incise sedimentary rock in Gale crater; these, too, consist of recycled, extraformational sediment. The recycled sediments in Gale crater are compositionally immature, indicating the dominance of physical weathering processes during the second known cycle. The observations at Marias Pass indicate that sediment eroded and removed from craters such as Gale crater during the Martian Hesperian Period could have been recycled to form new rock elsewhere. Our results permit prediction that lithified deltaic sediments at the Perseverance (landing in 2021) and Rosalind Franklin (landing in 2023) rover field sites could contain extraformational recycled sediment.
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    Grain Size Measurements of the Eolian Stimson Formation, Gale Crater, Mars and Implications for Sand Provenance and Paleoatmospheric Conditions
    (Wiley, 2024) Preston, Sarah L.; Siebach, Kirsten L.; Lapôtre, Mathieu G. A.; Banham, Steven G.
    The Stimson formation is a late-infilling eolian sandstone in Gale crater, Mars that formed from sand accumulation in a dune field analogous to the modern active Bagnold dune field, enabling a unique opportunity to compare the past to the present dune fields on Mars. Previous work suggested that the Stimson has a coarser grain-size distribution than the active Bagnold dunes based on three images of the Stimson. We analyze grain size in the Naukluft and Emerson plateaus of the Stimson by observing 115 images throughout the formation to classify textures and quantitatively measuring grains in eight representative individual images. Results indicate that the Stimson has a primary grain size mode at <200 μm. In addition, more than 50% of the observed Stimson rock targets display a coarser grain population with a long-tailed distribution including grains ∼600–1200 μm. The primary grain size mode is similar to that observed in the Bagnold dunes, but the coarse grain size mode was neither observed in the Bagnold dunes nor in ripples adjacent to the dune field. Models for saltation mechanics indicate that the favored grain size for eolian transport on Mars, ∼100–200 μm, is independent of atmospheric density, though atmospheric density affects the wind speeds at which grains can be transported by winds. We conclude that the source of the Stimson dunes was more proximal and coarser than the source of the Bagnold dunes and that the paleoatmosphere was likely not significantly denser than the modern Martian atmosphere.
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    Ice? Salt? Pressure? Sediment deformation structures as evidence of late-stage shallow groundwater in Gale crater, Mars
    (Geological Society of America, 2024) Banham, Steven G.; Roberts, Amelie L.; Gupta, Sanjeev; Davis, Joel M.; Thompson, Lucy M.; Rubin, David M.; Paar, Gerhard; Siebach, Kirsten L.; Dietrich, William E.; Fraeman, Abigail A.; Vasavada, Ashwin R.
    Persistence of near-surface water during the late evolution of Gale crater, Mars, would have been fundamental for maintaining a habitable environment. Sedimentation in aqueous conditions is evident during the early stages of crater infilling, where accumulation of lower Mount Sharp group strata is characterized by fluviolacustrine sedimentary rocks. The basal unit of the Siccar Point group—the Stimson formation—which unconformably overlies the Mount Sharp group and represents conditions postdating the exhumation of Aeolis Mons, is characterized by accumulation of aeolian strata under arid conditions. Water was largely absent near the surface during its deposition. At the Feòrachas outcrop, discovery of soft sediment deformation structures in aeolian Stimson strata challenges the notion that Gale crater was devoid of water during its later depositional phase. We identified deformed wind-rippled and vertically laminated sandstones, hosted within erosion-resistant ridges forming boxwork patterns. Broadly, these structures are diagnostic of water (as liquid or as ice) in the shallow subsurface. Comparison with Earth analogues suggests formation by subsurface fluid escape, freeze-thaw processes, or evaporite deformation. Regardless of the mechanism, these structures signify the presence of water at or near the surface much later than previously documented and may extend the habitability window in Gale crater.