GSA Connects 2021 in Portland, Oregon

Paper No. 14-3
Presentation Time: 8:30 AM


GUPTA, Sanjeev1, MANGOLD, Nicolas2, DROMART, Gilles3, GASNAULT, Olivier4, LE MOUELIC, Stephane2 and BELL III, James5, (1)Imperial College London, London, SW7 2AZ, United Kingdom, (2)Laboratoire De Planétologie Et Géodynamique, U. Nantes, Nantes, 44322, France, (3)Laboratoire de Géologie de Lyon, Université de Lyon, ENS Lyon, Lyon, 69364, France, (4)Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, UPS-CNRS-OMP, Toulouse, France, (5)School of Earth & Space Exploration, Arizona State University, Tempe, AZ 85287

A prominent sedimentary fan deposit at the western margin of Jezero crater has been inferred to be a river delta that built into an ancient lake basin during the Late Noachian-Early Hesperian epochs on Mars (~3.6-3.8 Ga). The fan is among the primary targets for NASA’s Mars 2020 Perseverance rover mission which is searching for evidence of ancient life in Jezero crater rocks. During the first months of the mission, high-resolution images obtained from the Mastcam-Z camera and from the Remote Micro-Imager (RMI) of the SuperCam instrument provided the first ground-based observations of the western fan. Here, we report its stratigraphy and sedimentology based on these long-distance observations. Observations of a prominent butte, informally named Kodiak, located ~1 km south of the western fan, show that its stratigraphy is characterized by distinct sedimentary geometries. The SE-facing wall of Kodiak exhibits distinct packages of inclined strata up to 10 m in thickness sandwiched between horizontally stratified packages of strata. From the geometric arrangement of these strata, we interpret the inclined beds to be foreset strata, with the underlying strata representing bottomsets. Foreset strata are overlain by horizontally stratified topset strata. We interpret the distinct tripartite geometry as representing deposition in steeply fronted Gilbert-type deltas that prograded into an ancient lake basin in Jezero crater. By contrast, the uppermost western fan strata are dominated by poorly-sorted, clast-supported conglomerates of rounded cobbles and boulders, which imply deposition from episodic high-energy fluvial floods. Our observations demonstrate a marked temporal transition in the energy regime of fluvial systems feeding the western fan, from sustained fluvial activity that built deltas prograding into the Jezero crater lake to one characterized by high-magnitude flood flows. Close-range investigation of these deposits by Perseverance will permit fine-scale quantitative reconstruction of this dramatic transition in the hydrologic system. Finally our results have important implications for Mars sample return strategies. The deltaic strata in particular finer-grained bottomset deposits are promising targets for the search for ancient biosignatures and organic matter. Secondly, boulders >1m in diameter present an opportunity to sample crustal rocks sourced from outside Jezero, which will provide invaluable constraints for understanding Mars’ earliest history.