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. 7 Mangold , 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, Berkeley, CA, 3USGS, Flagstaff, AZ, 4California Institute of Technology, Pasadena, CA, 5Imperial College, London, UK, 6 University of Tennessee, Knoxville, TN, 7Université de Nantes, Nantes, France, 8UC Santa Cruz, Santa Cruz, CA, 9 Jet Propulsion Laboratory, Pasadena, CA, 10Planetary Science Institute, Tucson, AZ. Introduction: Over the first 800 sols of its mission, the Curiosity rover traversed several kilometers across the floor of Gale crater from its landing site on Aeolis Palus toward its destination at Aeolis Mons (Mount Sharp). This initial phase of the mission culminated with the recent arrival at the base of Mount Sharp, represented by an outcrop informally known as the Pahrump Hills. Over this route, the rover explored several distinct geologic units representing the transition zone between sediments of the Peace Vallis fan and Mount Sharp. From its landing site, the rover has climbed nearly 50 meters in elevation to date, with the Pahrump Hills outcrop nearly 70 meters above the lowest point in the traverse, at Yellowknife Bay. The natural topography along the route has provided an opportunity to explore the nature of this stratigraphic transition in three dimensions. Understanding the rela- Figure 1: Projected extent of strata within the lower formation of Mount Sharp above Curiosity's current location at Pahrump, which dip at roughly 4 degrees to the northwest. Curiosity is currently located near minimum of the cross-section shown. Regional topography from MOLA. tionships between these sedimentary systems is critical to determining the mechanisms and timing of deposition and erosion within Gale crater. In particular, the lower strata of Mount Sharp are most promising for determining whether Gale crater experienced longterm lacustrine phases early in its history. Here we describe a combination of orbital and rover-based mapping, integrating stereo images and topography from the HiRISE camera with those from Curiosity’s Navcam and Mastcam instruments. Outcrop and bed geometries are used to identify the location of this geological transition, as well as to constrain physical stratigraphic relationships and depositional mechanisms. Bedding Geometry: From orbit, the lower strata of Mount Sharp have been observed to dip consistently away from the center of the mound [1]. In the vicinity 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 diverse 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 southdipping 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, typically inclined at 5-15 degrees from horizontal. This pattern indicates a sediment transport direction from the north, possibly originating from the crater rim. However, the elevation of the striated unit (of order 1 meter vertical thickness) increases to the south at a slope of ~1 degree, implying an uphill flow direction (aeolian) or an aggradation-dominated environment (deltaic). Interpretations: Currently, Curiosity is poised at the boundary between the sedimentary units of Aeolis Palus and those of Mount Sharp. Given the measured differences in bedding geometry between these re- 46th Lunar and Planetary Science Conference (2015) gions, the nature of the transition will provide clear information regarding the formation of and evolution of Mount Sharp. Plausible endmember scenarios include an onlap relationship of crater floor units onto the base of Mount Sharp (as inferred from orbital mapping [2,7]), or a smooth transition with more complex interfingering, as suggested by some ground-based observations [8]. In either scenario, the units observed 0 4 Km 2 1 Yellowknife Bay Mean dip direction Shaler 4 o o 0.5 MSL Traverse Darwin 7 o Kylie 2 o o 14 o Hidden Valley o 6 Kimberley Cooperstown Dingo Gap Figure 2: Average bedding orientations observed at several major waypoints along the rover traverse. Since Cooperstown, layers have exhibited consistent southward dips even as terrain increases in elevation to the south. 2698.pdf by Curiosity to date are not correlative with the inclined layers found higher on Mt. Sharp, and shown in Figure 1. We explore the current observations in support of these multiple hypotheses arising from the diverse geologic units in the transitional region at the base of Mount Sharp. New geologic units exposed near this boundary include repetitive thin-bedded rocks exposed at Hidden Valley (Fig. 3). These beds are relatively flat-lying compared to other units observed in the area, and may represent a distal fluvial or lacustrine environment. Further work is needed to determine the significance of the cm-scale repetitive layering observed at Hidden Valley, and any potential relation to annual or other periodic climate variations. Detailed analysis of the section now being explored at Pahrump Hills, inferred to be the lowermost exposed portion of the orbitally-defined Mount Sharp Lower 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 Planets 118 (12), 2439-2473. [3] Fraeman, A. A. et al., (2013) Geology 41 (10) 1103-1106. [4] Grotzinger, J.P. et al., Science 343 (6169). [5] Williams, R.M.E. et al, Science 340 (6136) 1068-1072. [6] Edgar, L.A. et al (2014) LPS XLV, Abstract #1648. [7] Anderson, R. B. and Bell, J. F. (2010) Mars 5 (76-128). [8] Stack, K. 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.