Evidence for plunging river plume deposits in the Pahrump Hills member of the Murray formation, Gale crater, Mars

Abstract

Recent robotic missions to Mars have offered new insights into the extent, diversity and habitability of the Martian sedimentary rock record. Since the Curiosity rover landed in Gale crater in August 2012, the Mars Science Laboratory Science Team has explored the origins and habitability of ancient fluvial, deltaic, lacustrine and aeolian deposits preserved within the crater. This study describes the sedimentology of a ca 13 m thick succession named the Pahrump Hills member of the Murray formation, the first thick finegrained deposit discovered in situ on Mars. This work evaluates the depositional processes responsible for its formation and reconstructs its palaeoenvironmental setting. The Pahrump Hills succession can be subdivided into four distinct sedimentary facies: (i) thinly laminated mudstone; (ii) low-angle cross-stratified mudstone; (iii) cross-stratified sandstone; and (iv) thickly laminated mudstone-sandstone. The very fine grain size of the mudstone facies and abundant millimetre-scale and sub-millimetre-scale laminations exhibiting quasi-uniform thickness throughout the Pahrump Hills succession are most consistent with lacustrine deposition. Low-angle geometric discordances in the mudstone facies are interpreted as 'scour and drape' structures and suggest the action of currents, such as those associated with hyperpycnal river-generated plumes plunging into a lake. Observation of an overall upward coarsening in grain size and thickening of laminae throughout the Pahrump Hills succession is consistent with deposition from basinward progradation of a fluvial-deltaic system derived from the northern crater rim into the Gale crater lake. Palaeohydraulic modelling constrains the salinity of the ancient lake in Gale crater: assuming river sediment concentrations typical of floods on Earth, plunging river plumes and sedimentary 1768 structures like those observed at Pahrump Hills would have required lake densities near freshwater to form. The depositional model for the Pahrump Hills member presented here implies the presence of an ancient sustained, habitable freshwater lake in Gale crater for at least ca 10 3 to 10 7 Earth years.

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

“…The facies associations and observed southward transport direction predicted that the rover would eventually encounter deposits from a body of standing water. This was confirmed when the rover drove into Hidden Valley and the Pahrump Hills and the team discovered a thick succession of fine‐grained, well laminated rocks, interpreted to have formed through lacustrine deposition (Grotzinger et al, ; Rivera‐Hernández et al, ; Stack et al, ). Evidence for plunging river plumes at the Pahrump Hills favors a freshwater lacustrine origin based on paleohydraulic modeling (Stack et al, ).…”

“…To do so requires a detailed understanding of the depositional environments recorded in sedimentary rocks exposed at the surface in Gale crater, including characterization of the role and duration of ancient aqueous environments. Since landing in Gale (Figure ), the Curiosity rover team has been able to investigate a diverse array of siliciclastic sedimentary rocks, interpreted to have formed in fluvial, deltaic, lacustrine, and eolian environments (Banham et al, ; Edgar et al, ; Grotzinger et al, ; Grotzinger et al, ; Rice et al, ; Stack et al, ; Williams et al, ). Ancient lacustrine deposits are highly desirable targets in the search for habitable environments, due to their ability to concentrate and preserve organic matter (Farmer & DesMarais, ; Hays et al, ; Meyers & Ishiwatari, ; Summons et al, ).…”

Mineral‐Filled Fractures as Indicators of Multigenerational Fluid Flow in the Pahrump Hills Member of the Murray Formation, Gale Crater, Mars

“…Fine-grained mudstones and sandstones observed at Yellowknife Bay indicate suspension settling in a relatively low-energy lake environment . The Pahrump Hills field site contains approximately 12 m of sedimentary rock and is divided into several distinct sedimentary facies, including (1) finely laminated mudstone, (2) thickly laminated mudstone-sandstone, and (3) cross-stratified sandstone (Grotzinger et al, 2015;Stack et al, 2018). Together, these erosional windows through the surface of Aeolis Palus reveal a long history of deposition defined by interfingered fluvial, deltaic, and lacustrine environments.…”

“…Together, these erosional windows through the surface of Aeolis Palus reveal a long history of deposition defined by interfingered fluvial, deltaic, and lacustrine environments. Together, the facies investigated at Pahrump Hills are interpreted to represent subaqueous lacustrine deposition with episodic fluvial-deltaic progradation (Grotzinger et al, 2015;Stack et al, 2018). The Pahrump Hills field site contains approximately 12 m of sedimentary rock and is divided into several distinct sedimentary facies, including (1) finely laminated mudstone, (2) thickly laminated mudstone-sandstone, and (3) cross-stratified sandstone (Grotzinger et al, 2015;Stack et al, 2018).…”

Reevaluation of Perchlorate in Gale Crater Rocks Suggests Geologically Recent Perchlorate Addition

“…The geologic setting of bedrock deposition in Gale crater was not likely conducive to perchlorate precipitation. The stratigraphy observed in Gale crater includes mostly fluvial and deltaic sandstones and lacustrine mudstones, the latter of which indicate fresh to mildly saline lake waters (e.g., Grotzinger et al, , ; Stack et al, ). This setting differs from where perchlorate salts are found on Earth, in arid environments such as the Antarctic Dry Valleys and the Atacama (Ericksen, ; Kounaves et al, ).…”