ANALYSIS OF CLAY DEPOSITS IN AND AROUND LADON BASIN AND LADON VALLES. C. M. Weitz1, J. L. Bishop2, and J. A. Grant, 1Planetary Science Institute, 1700 E Fort Lowell, Tucson, AZ 85719, USA (weitz@psi.edu), 2SETI Institute, Carl Sagan Center, 189 Bernardo Ave., Mountain View, CA 94043, USA, 3Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, 6th at Independence SW, Washington, DC, 20560.

Introduction: We have identified, mapped, and

analyzed light-toned deposits, many of which are clay-bearing, in and around Ladon basin and Ladon Valles in Margaritifer Terra (Figure 1). The study region has clays that likely formed from multiple aqueous proc-esses, including fluvial, lacustrine, hydrothermal, and in situ alteration. CRISM analysis of the light-toned deposits indicates the presence of Fe/Mg-phyllosilicates. Light-toned layered outcrops in Holden and Eberswalde craters, near the mouth of Ladon Valles, inside Ladon basin, and in several of the small upland basins west of Ladon are all characterized by broadly similar morphology and expression [1-8], sug-gesting that their sedimentary depositional settings were perhaps similar. Some of the phyllosilicate-bearing sediments may be sourced from weathered upland rocks later transported into lower-lying areas whereas others may be the result of alteration after the deposits were emplaced [4]. Although the origin of the clays in the deposits could be from different processes, the clays almost certainly reflect past environments characterized by prolonged chemical weathering in-volving water.

Figure 1. THEMIS mosaic showing the Ladon basin and Ladon Valles study region. Type 1 are light-toned layered deposits along the western uplands (magenta). Type 2 are light-toned layered deposits within Ladon basin and Ladon Valles (yellow). Type 3 are light- to medium-toned units that do not show fine-scale layer-ing (blue). Inset is the topography of Mars from MOLA data in color with the location of Ladon basin marked by the black square.

We divide the deposits into three types: (1) Light-toned layered deposits in the western uplands outside Ladon basin; (2) Light-toned layered deposits in Ladon Valles and southern Ladon basin; and (3) Light- to medium-toned deposits that lack fine-scale layering.

Type 1: Light-toned layered deposits in the western uplands outside Ladon basin: The light-toned layered sediments we have identified along the western uplands of Ladon basin are associated with valley networks that eroded Noachian and Early Hes-perian geologic units and deposited these sediments within small basins, likely similar to the valley net-works that deposited the delta in the larger Eberswalde basin. Valleys sourcing many of these deposits head along an ancient ridge to the west forming one of the eroded rings of the ancient Holden impact basin [9,10] that likely exposes rocks weathered during an early wetter period of the Noachian [11]. One deposit in-cludes an inverted channel that lies in a shallow valley that may have been blocked by topography associated with Cardona crater ejecta. CRISM spectra from the deposit are consistent with nontronite-type clays as well as additional clay signatures that appear to be saponite, although the phyllosilicate signatures are weak in these deposits. Drainage from the ridge into Arda Valles and deposition of the layered sediments likely continued until outlets were established to the east, thereby enabling incision of the deposits and drainage onto the lower-lying floor of Ladon basin. Another smaller valley network intersected a N-S trending Holden secondary crater chain, depositing sediments within the crater chain during the Hesperian after the Holden impact [12].

Type 2: Light-toned layered deposits in Ladon Valles and southern Ladon basin: The utility of combined CRISM and HiRISE analyses is demon-strated using an example of the deposits located at the mouth of Ladon Valles (Fig. 2). Most CRISM spectra show features consistent with multiple types of OH-bearing materials, like Fe/Mg-rich smectites, and HiRISE images indicate numerous beds with variable lithologies, including color and brightness variations. Spectra from several of the brightest upper beds in these deposits exhibit a 2.3 µm absorption but no hy-dration band at 1.9 µm (Fig. 2), which could reflect dehydration from high temperatures that drove out the water in the clays [13].

1929.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)

Strike and dips for bedding planes within these de-posits, calculated from HiRISE-derived DTMs, are shallow and between 1-4°. Although the general ap-pearance of the deposits suggests they are light-toned in nature, HiRISE images reveal numerous very bright beds interspersed with medium-toned and even darker beds, suggesting different source materials for the sed-iments over time. The brightest upper beds can be traced over 65 km in distance from Ladon Valles into southern Ladon basin, consistent with a lacustrine or perhaps distal alluvial fan setting.

Type 3: Light- to medium-toned deposits that lack fine-scale layering: Clays within Ladon basin can be associated with medium-toned fractured materials that exhibit few or no layering. The deposits in Ladon basin tend to be adjacent to fractures and could have resulted from fluids emitted from the fractures [14]. Additional clays without layering are also found in older terrain exposed along the floor of Ladon Valles (Fig. 2, l and m spectra) and could be mixed-layer smectite/chlorite produced by alteration from water that once flowed through Ladon Valles. Finally, light-toned deposits are found in several impact craters along the western uplands of Ladon basin. These crater floor deposits could be altered materials from when

water ponded inside the craters, as these craters all have small valleys that intersect them. Unlike the Type 1 and Type 2 deposits, the absence of layering suggests either limited deposition of sediments or alteration of pre-existing materials to explain the Type 3 deposits. References: Pondrelli, M., et al. (2005), J. Geophys. Res., 110, 2004JE002335; [2] Pondrelli, M. A. et al. (2008), Ica-rus, 197, 429-451, doi:10.1016/j.icarus.2008.05.018. [3] Grant, J. A. et al. (2008), Geology, 36, 195-198, doi: 10.1130/G24340A. [4] Milliken, R. E., and D. L. Bish (2010), Philosophical Magazine, 1478-6443, DOI: 10.1080/14786430903575132. [5] Rice, M. S., et al. (2011), Geophys. Res. Letts., 38, doi:10.1029/2011GL048149. [6] Rice, M.S., et al. (2013) MARS 8, 15-57, doi:10.1555/mars.2013.0002. [7] Weitz, C. M., and J. L. Bishop (2012), Lunar Planet. Sci. Conf., XXXXIII, Abstract 1243. [8] Weitz, C.M., et al. (2013) Lunar Planet. Sci. Conf. 44th, Abstract 2081. [9] Schultz, P. H., et al. (1982), J. Geo-phys. Res., 87, 9803-9820. [10] Grant, J. A. (1987), in Ad-vances in planetary geology, NASA Technical Memorandum 89871, p. 1-268. [11] Bibring, J.-P., et al. (2006), Science 312, 400-404, DOI: 10.1126/science.1122659. [12] Irwin, R.P. and J. A. Grant (2013) Geologic Map of MTM -15027, -20027, -25027, -25032 Quads, Margaritifer Terra region of Mars. [13] Morris, R.V., et al. (2010) LPSC 41, Abstract 2156. [14] Thomas R.J. et al. (2017), J. Geophys. Res., 122(3), doi:10.1002/2016JE005183.

Figure 2. (a) HiRISE mosaic of clays in Ladon Valles. CRISM spectral parameters are overlain in color with green indicating the presence of phyllosilicates (D2300). The clays are associated with light-toned layered deposits and also medium-toned altered material along the floor of Ladon Valles. Letters indicate the CRISM spectra with the same color (right), with color circles taken from image FRT0000B306 and colored squares taken from image FRT00008076. Spectra from the upper bright materials (b,c,d,e) exhibit a 2.3 µm absorption but no hydration band at 1.9 µm, which could reflect dehydration from high temperatures that drove out the water in the clays [15].

1929.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)