44th Lunar and Planetary Science Conference (2013) 1617.pdf
CURIOSITY’S MASTCAM IMAGES REVEAL CONGLOMERATE OUTCROPS WITH WATER- TRANSPORTED PEBBLES. R. M. E. Williams1, W. E. Dietrich2, J. P. Grotzinger3, S. Gupta4, M. C. Malin5, M. C. Palucis2, D. Rubin,6 K. Stack3, D. Y. Sumner7, R. A. Yingst1, J. C. Bridges8, W.Goetz9, A. Koefoed10, J. K. Jen- sen10, M. B. Madsen10 S. P. Schwenzer11, R. G. Deen12, O. Pariser12, and the MSL Science Team. 1Planetary Science Institute, Tucson, AZ, [email protected], 2 University of California, Berkeley, 3Caltech, Pasadena, CA, 4Imperial College, London, UK, 5Malin Space Science Systems, San Diego, CA, 6USGS, Santa Cruz, CA, 7University of Cali- fornia, Davis, 8University of Leicester, UK, 9Max-Planck-Institut für Sonnensystemforschung, , Katlenburg-Lindau, Germany, 10Niels Bohr Institute, University of Copenhagen, DK, 11Open University, UK, 12JPL, Pasadena, CA.
Introduction: Rock fragment morphology reflects indurated deposit contains light to dark-colored, the origin, transport, deposition and modification histo- rounded to sub-rounded pebbles (5-30 mm in diameter) ry of a clast. In terrestrial studies, documenting the that appear to be arranged in clast-to-clast contact. dimensions, shape, texture and fabric of rock fragments Link is a weakly stratified and ungraded conglom- is a standard technique used in field studies to help erate. Grains in the rock are pebbles (5-20 mm diame- determine the depositional history [e.g., 1]. ter) and very coarse sands. At the base of the outcrop, Prior observations of rock fragments (constrained pebble piles have accumulated, presumably a reflection here to diameters between 4-256 mm) at Mars rover of clasts that eroded from the conglomerate. The peb- and lander sites show a dominance of clasts with sharp ble piles display a range of colors, are tabular to edges and corners [2-4]. The relative roundness of equant, and range from subrounded to well rounded. these clasts was most often characterized as angular or The Hottah outcrop contains multi-colored, round- subangular/subrounded. Collectively, the particles ed pebbles (5-30 mm diameter) and sand (Figure 1). reflected a single lithology of volcanic origin with min- Hottah pebbles have a mean relative roundness of imal modification by ballistic impacts and aeolian pro- ~0.38, an order of magnitude greater than measured at cesses [2-4]. Intermediate roundness values for a por- Pathfinder [3]. It is a thinly laminated, moderately tion of the particles at the Viking 1 and Pathfinder sorted outcrop with pockets of tightly packed pebbles. landing sites, located in circum-Chryse outflow chan- Elongate clasts are aligned parallel to stratification, and nels, are consistent with high-energy, short duration in several examples, stacked, elongated clasts show abrasion associated with catastrophic flooding [2, 4]. imbrication. In places, there are alternating layers of New observations from Curiosity’s Mast cameras pebbles with intervening, finer-grained, recessive lay- (Mastcam [5]) show rock fragments with morphology ers. Like Link, Hottah has piles of loose pebbles adja- that is distinctly different from previous observations. cent to the outcrop with similar attributes to the peb- These particles contrast with the highly angular rock bles in the rock. fragments present at other martian landing sites, and Loose Rounded Pebbles. Patches of unconsolidat- reflect high-energy sediment transport. In outcrops, the ed rounded pebbles are located intermittently along stratification and arrangement of pebbles suggests Curiosity’s route over an elevation range of 15 m (Fig- aqueous bedload transport. These rounded pebbles ure 2). Loose pebbles range in size from 2 to 30 mm provide the first direct observation of an ancient mar- (median size ~8 mm). They vary in shape (ranging tian stream bed, and the first conglomerate on another from flat to equidimensional) and color, two attributes planet. that suggest heterogeneous lithologies are present. Observations: Rounded pebbles are present in Most areas show no alignment of elongated clasts, but discrete, unconsolidated deposits and bound in out- in one location near Rocknest the long axes of the peb- crops along the 400 m route from Bradbury Landing to bles are strongly aligned parallel to each other. Rocknest that Curiosity traversed in the first 60 sols. Discussion: The qualitative roundness of these Rounded Pebbles in Outcrops. Indurated deposits pebbles is diagnostic of a sediment transport process of rounded pebbles and sand are best exposed in three involving abundant collisions to wear away irregular rock outcrops: Goulburn_scour, Link and Hottah. The- edges and produce a smooth surface [e.g., 6]. The clast se outcrops are located along the first 175 m traveled size (few tens of mm) requires a rapid flow speed to by the rover, and are within a ~3 m elevation range. move. The pebbles are too coarse to be mobilized by The tabular Goulburn_scour outcrop was exposed wind [7-9] and are best explained by water flow when the rover’s descent engine plumes removed un- transport. Further evidence in support of this interpre- consolidated sediment from the surface. Rock frag- tation includes local observations which indicate trac- ments are embedded in finer-grained sediment forming tional aqueous transport: planar lamination, clast- a fine-pebble conglomerate. This thin (~6 cm thick), supported fabric, and the presence of imbrication. In 44th Lunar and Planetary Science Conference (2013) 1617.pdf
addition, the cumulative size frequency of rock frag- Figure 3 (bottom right): Cumulative grain size dis- ments within the outcrops is comparable to sedimen- tribution plot for Link, Hottah, and Goulburn, and a tary deposits in terrestrial rivers [e.g., 10]. Using con- representative terrestrial fluvial deposit (Atacama De- ventional hydraulics [11], we determine the critical sert, Chile). Stream deposits tend to approximate a flow depth (0.08 – 0.8 m) and average velocity (14 – normal (Gaussian) distribution. 63 cm/s) to mobilize the observed grain size for a range of slopes (0.1%-1%). We interpret these rounded pebbles as sediment transported and deposited by vigorous fluvial flows. The qualitative roundness of the pebbles suggests a transport distance of >10 km, based on published flu- vial abrasion experiments that document the control of lithology on progressive diminution and rounding on sediment [12]. This is consistent with the location of the conglomerate ~14 km down slope from the Peace Vallis alluvial fan apex [13]. The pebbles likely sam- ple ancient crustal rocks from the Gale crater rim. Acknowledgement: The research described in this abstract was carried out in part by the Jet Propulsion Lab, California Institute of Technology, under contract with the National Aeronautics and Space Administra- tion. Copyright [2012]. All rights reserved. Govern- ment sponsorship acknowledged.
References: [1] Pettijohn F.J. (1975) Sedimentary Rocks. [2] Garvin, J.B. et al. (1981) Moon Planets, 24, 355. [3] Yingst, R.A. et al. (2007), JGR, 112, E06002. [4] Yingst, R.A. et al. (2008) JGR, 113, E12S41. [5] Malin M.C. et al. (2010) LPS XLI, Abstract #1123. [6] Krumbein, W.C. and Pettijohn, F.J. (1966) Manual of Sedimentary Petrography. [7] Greeley, R. et al. (1976) GRL, 3, 8, 417-418. [8] Greeley, R. et al. (1980) GRL, 7, 2, 121- 124. [9] Iversen, J.D., and White, B.R. (1982) Sedimentology, 29, 1, 111-119. [10] Bunte, K. and Abt, S.R. (2001) US Dept. Agric. Tech. Report RMRS-GTR-74, 428 pp. [11] Chow, V.T. (1959) Open- Channel Hydraulics. [12] Lewin, J. and Brewer, P. (2002) EPSL, 27, 145-164. [13] Palicus, M. et al. (2013) LPS XLIV this volume.
Figure 1 (top right): The Hottah outcrop was imaged by the 100-mm Mastcam on sol 39. A) Planar lami- nation (white arrow) and rounded pebble (~30 mm long axis) protruding from outcrop (black arrow). B) Black arrows mark couplets of alternating pebble (pro- truding) and finer-grained (recessive) layers. A clast- supported pebble cluster is within the white oval.
Figure 2: Distribution of con- glomerate outcrops (red dots) and relative percent ground cover of unconsolidated, rounded pebbles observed in Mastcam clast sur- veys (yellow dots) along Curiosi- ty’s route. Martian sol is marked by yellow number.