PHYSICAL STRATIGRAPHY ALONG the CURIOSITY TRAVERSE and the TRANSITION to MOUNT SHARP. K. W. Lewis1, W. E. Dietrich2, L. A. Edgar3, J

46th Lunar and Planetary Science Conference (2015) 2698.pdf

PHYSICAL STRATIGRAPHY ALONG THE CURIOSITY TRAVERSE AND THE TRANSITION TO MOUNT SHARP. K. W. Lewis1, W. E. Dietrich2, L. A. Edgar3, J. P. Grotzinger4, S. Gupta5, L. C. Kah6, N. Mangold7, D. M. Rubin8, K. M. Stack-Morgan9, R. M. E. Williams10, and the MSL science team. 1Johns Hopkins University, Dept. of Earth and Planetary Sciences, Baltimore, MD 21218 ([email protected]), 2UC Berkeley, Berke- ley, CA, 3USGS, Flagstaff, AZ, 4California Institute of Technology, Pasadena, CA, 5Imperial College, London, UK, 6University of Tennessee, Knoxville, TN, 7Université de Nantes, Nantes, France, 8UC Santa Cruz, Santa Cruz, CA, 9Jet Propulsion Laboratory, Pasadena, CA, 10Planetary Science Institute, Tucson, AZ.

Introduction: Over the first 800 sols of its mis- tionships between these sedimentary systems is critical sion, the Curiosity rover traversed several kilometers to determining the mechanisms and timing of deposi- across the floor of Gale crater from its landing site on tion and erosion within Gale crater. In particular, the Aeolis Palus toward its destination at Aeolis Mons lower strata of Mount Sharp are most promising for (Mount Sharp). This initial phase of the mission cul- determining whether Gale crater experienced long- minated with the recent arrival at the base of Mount term lacustrine phases early in its history. Here we Sharp, represented by an outcrop informally known as describe a combination of orbital and rover-based the Pahrump Hills. Over this route, the rover explored mapping, integrating stereo images and topography several distinct geologic units representing the transi- from the HiRISE camera with those from Curiosity’s tion zone between sediments of the Peace Vallis fan Navcam and Mastcam instruments. Outcrop and bed and Mount Sharp. From its landing site, the rover has geometries are used to identify the location of this geo- climbed nearly 50 meters in elevation to date, with the logical transition, as well as to constrain physical strat- Pahrump Hills outcrop nearly 70 meters above the igraphic relationships and depositional mechanisms. lowest point in the traverse, at Yellowknife Bay. The Bedding Geometry: From orbit, the lower strata of natural topography along the route has provided an Mount Sharp have been observed to dip consistently opportunity to explore the nature of this stratigraphic away from the center of the mound [1]. In the vicinity transition in three dimensions. Understanding the rela- of the planned Curiosity ascent, strata have a consistent dip of ~4 degrees to the northwest where they can be measured, [1,2]. Extrapolation suggests the layers of the Lower Unit of Mount Sharp might have extended hundreds of meters above the current topography of the areas traversed by the rover, but did not completely fill the crater. This trend is not yet observed from the ground, suggesting a change in bedding attitude between Pahrump and the lowermost strata measurable from orbit, at the Hematite Ridge of [3]. From the surface, Curiosity has encountered a di- verse array of bedded sedimentary rocks ranging from fluvial conglomerates to lacustrine mudstones [4-6]. Since the Cooperstown waypoint (Fig. 2), the rover has imaged recurring outcrops of consistently south- dipping crossbedded rocks. These occur primarily in a geomorphic terrain type mapped as the Striated Unit from orbit. Figure 2 shows a number of instances of south-dipping beds exposed along the traverse, typical- ly inclined at 5-15 degrees from horizontal. This pat- tern indicates a sediment transport direction from the north, possibly originating from the crater rim. How- ever, the elevation of the striated unit (of order 1 meter vertical thickness) increases to the south at a slope of Figure 1: Projected extent of strata within the lower ~1 degree, implying an uphill flow direction (aeolian) formation of Mount Sharp above Curiosity's cur- or an aggradation-dominated environment (deltaic). rent location at Pahrump, which dip at roughly 4 Interpretations: Currently, Curiosity is poised at degrees to the northwest. Curiosity is currently the boundary between the sedimentary units of Aeolis located near minimum of the cross-section shown. Palus and those of Mount Sharp. Given the measured Regional topography from MOLA. differences in bedding geometry between these re- 46th Lunar and Planetary Science Conference (2015) 2698.pdf

gions, the nature of the transition will provide clear by Curiosity to date are not correlative with the in- information regarding the formation of and evolution clined layers found higher on Mt. Sharp, and shown in of Mount Sharp. Plausible endmember scenarios in- Figure 1. We explore the current observations in sup- clude an onlap relationship of crater floor units onto port of these multiple hypotheses arising from the di- the base of Mount Sharp (as inferred from orbital map- verse geologic units in the transitional region at the ping [2,7]), or a smooth transition with more complex base of Mount Sharp. New geologic units exposed interfingering, as suggested by some ground-based near this boundary include repetitive thin-bedded rocks observations [8]. In either scenario, the units observed exposed at Hidden Valley (Fig. 3). These beds are

Km relatively flat-lying compared to other units observed

0120.5 4

Shaler in the area, and may represent a distal fluvial or lacus- Yellowknife Bay o o Mean dip direction 4 MSL Traverse trine environment. Further work is needed to deter- mine the significance of the cm-scale repetitive layer-

Darwin ing observed at Hidden Valley, and any potential rela- tion to annual or other periodic climate variations.

Detailed analysis of the section now being explored at

o Cooperstown

7 Pahrump Hills, inferred to be the lowermost exposed Kylie

o Dingo Gap

2 portion of the orbitally-defined Mount Sharp Lower Kimberley 6oo Unit, will further constrain the nature of this basal transition. References: [1] Kite, E.S. et al., (2013) Geology

41 (5), 543-546 [2] Le Deit, L. et al., (2013) JGR Hidden Valley o 14 Planets 118 (12), 2439-2473. [3] Fraeman, A. A. et al., (2013) Geology 41 (10) 1103-1106. [4] Grotzinger, Figure 2: Average bedding orientations observed at J.P. et al., Science 343 (6169). [5] Williams, R.M.E. et several major waypoints along the rover traverse. al, Science 340 (6136) 1068-1072. [6] Edgar, L.A. et al Since Cooperstown, layers have exhibited consistent (2014) LPS XLV, Abstract #1648. [7] Anderson, R. B. southward dips even as terrain increases in eleva- and Bell, J. F. (2010) Mars 5 (76-128). [8] Stack, K. tion to the south. M. et al., (2015) LPS XLVI.

Figure 3: Mastcam view of repetitive, flat-lying layering observed near the base of Mount Sharp at Hidden Valley on Sol 710. This and other transitional units exposed in the area provide information regarding the relationship between Mount Sharp and surrounding units on the floor of Gale crater.