FACIES ANALYSIS AND BASIN ARCHITECTURE OF THE UPPER PART OF THE MURRAY FORMATION, GALE CRATER, MARS. C. Fedo1 (Earth & Planetary Sciences, Univ Tennessee, Knoxville, TN 37996, cfedo@utk.edu), J. Grotzinger2, S. Gupta3, N.T. Stein2, J. Watkins2, S. Banham3, K.S. Edgett4, M. Minitti5, J. Schieber6, K. Siebach7, K. Stack-Morgan8, H. Newsom9, K.W. Lewis10, C. House11, A.R. Vasavada8 1University of Tennessee, Knoxville, TN, 2California Institute of Technology, Pasadena, CA, 3Imperial College, London, 4Malin Space Science Systems, San Diego, CA, 5Planetary Sciences Institute, Tucson, AZ, 6University of Indiana, Bloomington, IN, 7StonyBrook University, Stony Brook, NY, 8JPL, Pasadena, CA, 9University of New Mexico, Albuquerque, NM, 10Johns Hopkins University, Baltimore, MD, 11PSU, State College, PA

Introduction: The Mars Science Laboratory mis-sion to Gale crater, using the Curiosity rover, provides the opportunity to directly test hypotheses regarding past climate through understanding the past environ-ments recorded by sedimentary facies. Gale crater is located on fluvially dissected, cratered, highlands crust, along the Martian topographic dichotomy boundary. The crater formed at ~3.8-3.6 Ga [1]. The interior plains of northern Gale crater (Aeolis Palus) contain remnants of eroded alluvial fan deposits, and sedimentary deposits that form a large mound (Aeolis Mons, informally known as Mt. Sharp) cored by a cen-tral peak; the sediments were deposited, buried, lithi-fied, and exhumed and eroded before 3.3-3.1 Ga [1].

Having discovered evidence for ancient stream and thin (m-thick) lake deposits within its landing region [1], Curiosity recently explored the basal strata of Gale crater’s central mountain, Aeolis Mons (informally known as Mt. Sharp). Rocks exposed on the lower slopes of Mt. Sharp include hundreds of meters of stra-ta containing evidence of hydrated minerals. Previous work has demonstrated a succession of sedimentary rock types deposited dominantly in river-delta settings (Bradbury group), and interfingering/overlying con-temporaneous/younger lake settings (Murray for-mation, Mt. Sharp group) [1]. The Murray formation is on the order of 200 meters thick, composed domi-nantly of mudstones, and it mostly records lacustrine deposition in which hematite-phyllosilicate and mag-netite-silica mineralogic facies record primary chemi-cal sedimentation [2] and diagenetic overprints [3]. The Stimson formation unconformably overlies the Murray formation [4].

Here we present recent observtions from an ~105 meter thick interval of the Murray formation, studied between sols 1350-1565, discuss preliminary interpre-tations of the facies and stratigraphy, and place this in the broader context of the Mt. Sharp stratigraphic suc-cession. This pairs with chemostratigraphic work on the same interval [5].

Facies: A summary stratigraphic column shows all

previous rocks studied, in addition to the interval re-ported here, between the Naukluft Plateau and the

“Precipice” location (Fig. 1). The Murray formation is subdivided into four recognizeable facies.

Figure 1. Stratigraphy to-date compiled for rocks encountered

during ascent fo Mt. Sharp. New strata discussed here span from -4438 to -4330 in elevation. Solid dots = drill holes.

Finely laminated gray-colored rocks interpreted to

be of lacustrine origin that typify the basal Murray

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formation are overlain by a thick (~25 m) interval of cross-bedded siltstone to very fine-grained sandstone. The cross-bedding occurs as meter scale troughs, with angle of repose foresets. Sediment grain size is below the resolution (~64 µm/pixel) of the routine MAHLI imges. Decimeter-scale cross stratification also is asso-ciated with the larger-scale concave-up bodies that extend for >10 m across the outcrop, such as at “Baynes Mountain”. The Oudam drill hole indicates the presence of the most hematite measured so far dur-ing the mission, associated with Ca-sulfate [6, 7]. Cross stratification at this scale and grain size is con-sistent with bedload sediment transport in an aeolian setting, particularly curved-crested dunes [8], although other environments may produce deposits of this type.

The next higher stratigraphic interval, which is ~30 m thick, returns to dominantly finely laminated, pur-ple-colored mudstone through very fine-grained sand-stone. These rocks resemble the lowermost Murray formation, except in color and presence of distinct dia-genetic features, including patches of cm-scale concre-tions. Additionally, there are irregular cracks with ta-pered terminae that are restricted to the interiors of fracture-defined polygons, which are filled with light-toned Ca-sulfate based on ChemCam data. Composi-tionally, these rocks differ significantly from the cross-stratified facies in that they contain decreased amounts of hematite and have significant amounts (20-25%) of phyllosilicate based on the Marimba and Quela drill holes [9]. The similarity with the lowermost Murray, and particularly, the presence of finely laminated mud-stone points to suspension fallout of detrital material in a lake environment.

The uppermost unit thus far includes ~50 meters at the top of the composite section, designated as “hetero-lithic mudstone-sandstone.” The rocks are exposed in a region of subdued topography, where outcrops are small and mostly exposed as slabs and plates of broken bedrock, with some blocks and slabs having been ro-tated from their original orientation. This makes facies assessment difficult, and particularly the recognition of important stratigraphic bounding surfaces, so the rocks are grouped according to what attributes are observed at smaller scale, including: dark red, finely laminated, fine-grained mudstone, cm-scale ripple cross-laminated siltstone or very fine sandstone, dm-scale cross-stratified siltstone and very fine grained sand-stone; massive intervals marked by extensive devel-opment of concretions that disrupt faint relicts of pri-mary lamination. In one case, a MAHLI image showed clear evidence for a sandstone with fine-to-medium grain size, but all the rest were very fine grained. In particular, cm-scale ripple cross-lamination is abun-dant, whereas it was observed at one locality in the

lowermost Murray formation [1]. This stratigraphic interval also shows distinctive, small-scale polygonal fractures, that resemble desiccation cracks [10]. Com-positionally, one drill sample of the facies (Sebina) provides evidence for very similar composition to Quela and Marimba; again, phyllosilicate dominated, with subordinate hematite and Ca-sulfate. Although this facies is difficult to assess, the potential presence of mud cracks associated with contemporaneous gyp-sum precipitation raises the possibility of a lacustrine environment dominated by suspension fallout with less common traction deposition, perhaps along a desiccat-ing lake.

Basin Architecture: The broad facies arrangement

of the sedimentary fill of Gale crater beginning at the landing site is consistent with progradation of terrestri-al deposits from the crater margin to a lake that occu-pied crater [1]. Overall, the facies types and architec-ture are consistent with an overfilled lake basin [11]. In such basins, precipitation and sediment yield ex-ceeds the rate of evaporation leading to perennial standing water fed by active fluvial systems and rela-tively poorly defined parasequences.

Above the Naukluft Plateau (Fig. 1), the upper Murray formation facies could represent a change in the expected environments relative to that of an open hydrologic system. The presence of nearly 30 m of large-scale trough cross stratified sandstone of possible aeolian orgin suggests climate aridification. In addi-tional support of subaerial exposure, is the potential presence of mud cracks, which form from the desicca-tion of fine-grained sediments, and coeval gypsum infilling. The association of these facies is more similar to underfilled lake basins, where evaporation exceeds precipitation [11]. A possible record of climate change is not particularly surprising given the stratigraphic thickness (and presumed duration of sedimentation) of the sedimentary succession.

References: [1] Grotzinger, J.P., et al. (2015) Sci-ence, 350, aac7575. [2] Hurowitz, J.A., et al., (2016) 47th LPSC Abstract #1751. [3] Rampe, E.B., et al. (2106) 47th LPSC Abstract #2543. [4] Banham, S. et al., (2017) 48th LPSC. [5] Milliken, R.E., et al. (2017) 48th LPSC. [6] Rampe, E.B., et al. (2017) 48th LPSC. [7] Vaniman, D.T., et al. (2017) 48th LPSC. [8] Brookfield, M.E., Silvestro, S. (2010) Facies Models 4, GAC GEOtext6, 139-166. [9] Bristow, T.F., et al. (2017) 48th LPSC. [10] Stein, N.T. et al. (2017) 48th LPSC. [11] Bohacs, K.M., et al (2000) AAPG Studies in Geology 46, 3-34.

1689.pdfLunar and Planetary Science XLVIII (2017)