Grain Size Variations in the Murray Formation: Stratigraphic Evidence for Changing Depositional Environments in Gale Crater, Mars

Abstract

The lowermost exposure of the Murray formation in Gale crater, Mars, was interpreted as sediment deposited in an ancient lake based on data collected by the Curiosity rover. Constraining the stratigraphic extent and duration of this environment has important implications for the paleohydrology of Gale. Insights into early Martian environments and paleofluid flow velocity can be obtained from grain size in rocks. Visual inspection of grain size is not always available for rocks investigated at field sites on Mars due to limited image coverage. But grain sizes can also be estimated from the Gini Index Mean Score, a grain‐size proxy that uses point‐to‐point chemical variations in ChemCam Laser Induced Breakdown Spectroscopy data. New Gini Index Mean Score results indicate that the Murray formation is dominated by mudstones with grains smaller than the spatial resolution of all rover cameras. Intervals of fine to coarse sandstone also are present, some of which are verified using observations of grain size and sedimentary structures in associated images. Overall, results demonstrate that most of the Murray consists of mudstone, suggesting settling of grains from suspension in low energy depositional environments such as lakes. Some of the mudstones contain desiccation cracks indicating periods of drying with a lowering of lake water level. However, beds and lenses of cross‐bedded sandstones are common at specific intervals, suggesting episodes of fluvial and possibly eolian deposition. The persistence of lacustrine deposits interspersed with fluvial deposits suggests that liquid water was sustained on the Martian surface for tens of thousands to millions of years.

The Chemostratigraphy of the Murray Formation and Role of Diagenesis at Vera Rubin Ridge in Gale Crater, Mars, as Observed by the ChemCam Instrument

“…ChemCam provides an observation footprint with an equivalent diameter between 350 and 550 μm (Maurice et al, 2012, 2016). This footprint is large compared to the very fine sand or smaller grain sizes that comprise most of the Murray formation (Fedo et al, 2019; Mangold et al, 2019; Rivera‐Hernández et al, 2019, 2020) and implies that even compositions deduced from individual ChemCam observation points may be representative of the bulk rock composition. However, the ChemCam footprint is small compared to the approximately 1.6‐cm diameter footprint of the APXS instrument (Campbell et al, 2012; Gellert & Clark, 2015), and importantly, it enables finer‐scale differentiation between points on bedrock versus points on veins and other diagenetic features (see, e.g., L'Haridon et al, 2018; Nachon et al, 2014, 2017).…”

A Lacustrine Paleoenvironment Recorded at Vera RubinRidge, Gale Crater: Overview of the Sedimentology and Stratigraphy Observed by the Mars ScienceLaboratory Curiosity Rover

“…The Murray formation is a succession of sedimentary rocks that consist predominantly of mudstones that are interpreted to have been deposited in a lake or marginal lake setting (e.g., Fedo et al, 2018Fedo et al, , 2019Grotzinger et al, 2015;Rivera-Hernández et al, 2019, 2020Stack et al, 2019). The lowest exposed stratigraphic member of the Murray formation is the Pahrump Hills member (Figure 2), which is defined by millimeter-to centimeter-scale laminated mudstone to very fine sandstone, with decimeter-to meter-scale scour-and-drape structures.…”

Extraformational sediment recycling on Mars

“…14B). The fine-grained muds and sands of the Murray formation (e.g., Rivera-Hernández et al, 2020) were deposited during the first known cycle (1 in Fig. 14B).…”

“…2D). It is exposed on the lowermost north slope of Aeolis Mons and consists mainly of lacustrine mudstones and fine sandstones (Grotzinger et al, 2015;Fraeman et al, 2016;Rampe et al, 2017;Gwizd et al, 2020;Rivera-Hernández et al, 2020;. An additional ~4.5 km section of stratified rock, unexplored by Curiosity, lies above the Murray formation and above the elevation range in Figure 2D (Milliken et al, 2010;Grotzinger et al, 2015).…”

“…Because it is not traceable northward into the lowlands, the fluvial system could have terminated at the fretted terrain boundary and deposited a sedimentary fan or delta (Fig. 15B); such deposits are common at valley termini at the dichotomy boundary elsewhere in the region (Di Achille and Hynek, 2010;Rivera-Hernández and Palucis, 2019). We found no unequivocal pre-Gale crater volcanic landforms observable in the vicinity of the target area-no flows, edifices, or exposed dikes-although we note that Churchill (2018) proposed that one landform, located immediately northwest of Gale crater (brown in Fig.…”