44th Lunar and Planetary Science Conference (2013) 1617.pdf

CURIOSITY’S MASTCAM IMAGES REVEAL OUTCROPS WITH WATER- TRANSPORTED . 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 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 coarse . At the base of the outcrop, Prior observations of rock fragments (constrained piles have accumulated, presumably a reflection here to diameters between 4-256 mm) at 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 outcrop contains multi-colored, round- subangular/subrounded. Collectively, the particles ed pebbles (5-30 mm diameter) and (Figure 1). reflected a single 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 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 ’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 landing sites, and Loose Rounded Pebbles. Patches of unconsolidat- reflect high-energy 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 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 the long axes of the peb- crops along the 400 m route from 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 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 dis- ments within the outcrops is comparable to sedimen- tribution plot for Link, Hottah, and , 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 apex [13]. The pebbles likely sam- ple ancient crustal rocks from the 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] , 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 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.