Sedimentology and Stratigraphy Observed at Vera Rubin Ridge by the Mars Science Laboratory Curiosity Rover

49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 1704.pdf

SEDIMENTOLOGY AND STRATIGRAPHY OBSERVED AT VERA RUBIN RIDGE BY THE MARS SCIENCE LABORATORY CURIOSITY ROVER. L. A. Edgar1, A. A. Fraeman2, S. Gupta3, C. M. Fedo4, J. P. Grotzinger5, K. M. Stack2, K. A. Bennett6, V. Z. Sun2, S. G. Banham3, N. T. Stein5, K. S. Edgett7, D. M. Rubin8, J. Van Beek7, 1USGS Astrogeology Science Center, (2255 N. Gemini Drive, Flagstaff, AZ 86001, [email protected]), 2Jet Propulsion Laboratory, Pasadena CA, 3Imperial College, London, 4University of Tennessee, Knoxville TN 5California Institute of Technology, Pasadena CA, 6Northern Arizona University, Flagstaff AZ, 7Malin Space Sci- ence Systems, San Diego CA, 8University of California, Santa Cruz.

Introduction: The Mars Science Laboratory Curi- Chemistry Camera (ChemCam) instrument. These in- osity rover has been exploring a prominent geomorphic struments provide images at spatial resolutions ranging and spectrally distinctive feature known as Vera Rubin from several cm/pixel down to tens of µms/pixel, ena- Ridge, searching for changes in composition and depo- bling studies of sedimentary texture and structure. sitional environment recorded in strata exposed on the northwest slope of Aeolis Mons (informally known as Mt. Sharp). Prior to landing in Gale crater, Vera Rubin Ridge was identified as a target of interest due to its associated hematite signature in orbital spectroscopic data [1, 2]. Curiosity’s ground-based investigation of the topographic ridge began with a close-approach for imaging starting on Sol 1726, and subsequent ascent starting around Sol 1800. Several key regions on the ridge were identified based on High Resolution Imag- ing Science Experiment (HiRISE) data as waypoints for more in-depth investigations. Details of the Vera Rubin Ridge campaign are described by [3]. Here we review the distinct sedimentary facies from Sols 1726- 1910, covering more than 100 m of stratigraphic thick- ness over a lateral distance of more than 800 m. Background: Based on geologic mapping using data from HiRISE and the Compact Reconnaissance Fig. 1: White line shows Curiosity’s traverse path Imaging Spectrometer for Mars (CRISM), the ridge along Vera Rubin Ridge. Dashed lines indicate approx- was subdivided into three units, known as the lower, imate distinctions between the lower, middle, and up- middle and upper ridge units [3, 4] (Fig. 1). As ob- per ridge units as defined in orbital data [3, 4]. served in HiRISE orbital images, the lower ridge unit is defined as gently sloping, bright, fractured bedrock Stratigraphy and Sedimentology: The base of with clear stratification and relatively few craters; the Vera Rubin Ridge is characterized by a fine-grained middle unit is distinguished as a topographic bench; facies exhibiting extensive planar lamination. Individ- and the upper unit is marked by darker, more heavily ual laminae can be traced laterally for several meters, cratered bedrock without an obvious expression of and are generally ~0.6-0.7 mm thick. Outcrops are stratification in HiRISE images [3, 4]. Collectively, the crosscut by abundant fine fractures and curvi-planar strata are part of the Vera Rubin Ridge member of the calcium sulfate veins (Fig. 2A), which in some places Murray formation [5]. The Murray formation is a di- obscure primary sedimentary structures. Thin planar verse package of sedimentary facies, more than 250 m laminations are interpreted as the result of fallout from thick, interpreted to consist predominantly of mud- suspension in a lacustrine setting. Possible cross- stones deposited in a lacustrine setting [5, 6]. Over the bedding is observed in a few locations (Fig. 2B), which past hundred sols, Curiosity has had the opportunity to may indicate minor fluvial or eolian processes. The investigate the sedimentary facies of the Vera Rubin rocks that make up the lower ridge are conformable Ridge member. with the underlying Sutton Island member of the Mur- Datasets and Methods: The stratigraphy and sed- ray formation, with no obvious breaks in the strati- imentary structures of Vera Rubin Ridge were docu- graphic record. mented by the Mast Cameras (Mastcam), Navigation The geomorphic expression that defines the middle Cameras (Navcam), Mars Hand Lens Imager (MAHLI) ridge unit in HiRISE images is not as obvious from and Remote Micro Imager (RMI) subsystem of the rover ground data. Resistant outcrops observed around 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 1704.pdf

Sol 1829 (at an elevation of ~-4185 m) contain con- A. A. et al. (2018), this meeting. [4] Stack, K. M. et al. formable fine-grained, thinly laminated, parallel- (2017) LPS XLVIII, Abstract #1889. [5] Fedo C. M. et stratified bedrock. A higher abundance of diagenetic al. (2018), this meeting. [6] Grotzinger J. P. et al. features such as nodules or concretions observed in (2015) Science, 350(6257). [7] Bennett K. A. et al. MAHLI data may attest to the different character of the (2018), this meeting. bedrock [7] and this might influence the geomorphic expression of the unit as observed in HiRISE data. Fine laminae are persistent but less-well defined, alt- hough still consistent with lacustrine sedimentation. The geomorphic change from the middle to upper ridge units does not correspond to a sharp geologic boundary on the ground, but is marked by a change in erosional resistance, as observed at locations known as VRR Regions 6 and 7. At Region 6, the upper unit is characterized by a tan, well-laminated facies that grades upwards into a purple-hued facies that main- tains a blocky outcrop expression despite the continued presence of fine laminae. An additional fine-grained gray facies was first observed at this location, and is under investigation at the time of writing. At isolated outcrops, decimeter to meter-scale inclined strata that dip in multiple directions hint at possible eolian or subaqueous transport (but lack the characteristic pin- stripe appearance of eolian strata observed elsewhere) (Fig. 2C). However, most outcrops explored by Curi- osity within the upper ridge show continuous, fine- grained, thinly laminated, parallel stratification (Fig. 2D). Notable color variations in the upper ridge are currently under investigation and may correspond to variations in hematite content [3], either tied to a facies change or diagenetic process, or both. Summary: The strata exposed in Vera Rubin Ridge record deposition in a dominantly low-energy lacustrine environment. This is consistent with prior observations of the Murray formation [6], and the stra- ta in the ridge appear to be a continuation of the Mur- ray formation with no significant observable gaps in the stratigraphic record. Minor outcrops of low-angle stratification suggest possible interuption by eolian or subaqueous transport processes in what was otherwise a continuous record of lacustrine sedimentation. The lower, middle, and upper ridge units defined in orbital data have less pronounced expressions on the ground, but are likely tied to differences in erosional resistance as a function of subtle variations in grain size, compac- tion, or cement. Collectively, the rocks exposed at Vera Rubin Ridge provide additional evidence for a Fig. 2: Sedimentary structures observed at Vera Rubin long-lived lacustrine environment in Gale crater, in Ridge. A) Fine-grained planar laminated facies cross- 6 excess of 10 years via comparison to terrestrial rec- cut by abundant calcium sulfate veins near the base of ords of sedimentation scaled for tectonic and climatic the ridge. B) Possible cross-bedding (white arrow). C) differences. Inclined strata in the vicinity of Region 6. D) Typical References: [1] Fraeman A. A. et al. (2013) Geol- fine-grained, thinly laminated, parallel stratification ogy, 40(10), 1103-1106. [2] Fraeman A. A. et al. observed in MAHLI enhanced color image. (2016) JGR Planets, 121(9), 1713-1736. [3] Fraeman