Sinus Meridiani and Arabia Terra: Phyllosilicate and Sulfate Statigraphy
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Third Conference on Early Mars (2012) 7061.pdf SINUS MERIDIANI AND ARABIA TERRA: PHYLLOSILICATE AND SULFATE STATIGRAPHY. S. M. Wiseman1, R. E. Arvidson2, J. F. Mustard1, J. C. Andrews-Hanna3, 1Department of Geological Sciences, Brown University, Providence, RI, [email protected], 2Department of Earth and Planetary Sciences, Washigton University, Saint Louis, MO, 3Department of Geophysics, Colorado School of Mines, Golden, CO. Introduction: Exposures in Sinus Meridiani and the phyllosilicates, 2) fluvial erosion, and 3) formation Arabia Terra are well suited to examine the transition of sedimentary sulfate deposits. between Noachian and Hesperian aqueous processes. A laterally extensive, 100s of meters thick section of layered sedimentary rock [e.g., 1, 2] records prolonged aqueous activity. This stack of sedimentary rock is capped by the late Noachian/early Hesperian-aged [3] hematite-and sulfate- bearing unit explored by the Op- portunity rover and overlies phyllosilicate-bearing Noachian cratered terrain. Phyllosilicates also occur within the sedimentary rocks that underlie the sulfate- and hematite-bearing unit and are stratigraphically above phyllosilicates associated with the heavily cra- tered Noachian basement. In addition to mineralogic evidence for aqueous processes, there are deeply in- cised fluvial valleys along topographic gradients that indicate the past presence of flowing water [4]. Cross cutting and embayment relationships show that the channels postdate phyllosilicate formation in the cra- tered basement and predate the sulfate-and hematite- bearing unit. Inverted channels in the sedimentary rocks that are stratigraphically below the sulfate- and hematite-bearing unit suggest that fluvial activity over- lapped with formation of lower layers. Figure 2. upper) Zoom in of box outlined in Fig 1, middle) CRISM FRT 9105, A013 color composite and parameter map, location shown by black box, lower) FRT9105 on CTX DTM. Cross cutting and embayment relationships are clear. The phyllosilicates could be consistent with sub- surface alteration to form the Fe/Mg smectites fol- lowed by near surface alteration to form the Al phyllo- silicates [5, references within]; however, the Al phyl- losilicates are overlain in some places by Fe/Mg phyl- Figure 1. Units map of Sinus Meridiani overlain on MOLA. losilicates. Also, except for the 10s of meters of fluvial Ph = sulfate- and hematite-bearing unit explored by Oppor- incision, there is no evidence that the phyllosilicates tunity, ET1 = sedimentary etched terrain that occurs strati- formed at depth and were later exhumed. In addition graphically below Ph, and CT= cratered Noachian basement. to recording a mineralogic transition, a shift from net Noachian/Hesperian Contact: A well exposed erosion to deposition indicates changing hydrologic contact between Noachian and Hesperian-aged units conditions and is consistent with a shift toward ground occurs in southern Sinus Meridiani (Fig. 2). Both water dominated processes in the Hesperian. Fe/Mg smectites and Al-phyllosilicates are evident in Sedimentary Phyllosilicates: Fe/Mg phyllosili- layers in the Noachian-aged cratered terrain. These cates occur in the section of sedimentary rock that un- phyllosilicate-bearing rocks were incised by fluvial derlies the sulfate- and hematite-bearing unit. Alt- activity and subsequently embayed by the sulfate- and hough most of this unit exhibits a spectral signature hematite-bearing unit. This exposure indicates at least showing enhanced hydration [6,7], some areas exhibit 3 distinct episodes of aqueous activity: 1) formation of a distinctive phyllosilicate signature. The exposure in Third Conference on Early Mars (2012) 7061.pdf Fig 3 occurs near an eroded edge of the overlying sul- are associated with high thermal inertia deposits (Fig. fate-and hematite-bearing deposits. 5). Many of these areas contain dark basaltic sand, indicating aeolian activity. Phyllosilicates are present at the base of the section in Schiaparelli crater. Hy- drated sulfates detected in the crater shown in Fig. 5 occur on the crater floor and were exhumed. Figure 3. a) FRT 97FB. Sulfate- and hematite-bearing unit is purple and underlying sedimentary layers are blueish, b) Figure 5. left) THEMIS NIR mosaic with CRISM parame- Parameter map (R=sindex, G=BD2300, B=BD1900). Hy- ter map (R=D2100, G=D2400, B=D1900) overlain for part drated areas are blue and phyllosilicates green, c) Phyllosili- of the region in Fig 4. Blue indicates hydrated deposits (red cate spectra, black spcorresponds to the black arrow. arrows) and cyan indicates polyhydrated sulfates (yellow arrows), right) Zoom in of crater outlined with the white box. These phyllosilicate exposures are located at least Discussion: Exposures in Sinus Meridiani and 100km from phyllosilicates in the Noachian cratered terrain. Spectra from sedimentary deposits also have Arabia Terra are well suited to examine the transition different spectral shapes (Fig 3). It is likely that the between Noachian and Hesperian aqueous processes. phyllosilicates formed in situ, although possible that A contact between Noachian-aged phyllosilicate expo- the phyllosilicates were transported and redeposited. sures and the late-Noachian/early Hesperian sulfate- Hydrated sulfates in Arabia Terra: Hydro- and hematite-bearing unit indicate at least 3 episodes logic modeling relevant to late Noachian through Hes- of aqueous activity: 1) phyllosilicate formation, 2) perian conditions on Mars predicts that Arabia Terra fluvial erosion, 3) sedimentary sulfate deposition. In was a region of enhanced groundwater upwelling that addition to the mineralogic transition, a shift from net resulted in a shallow water table that intersected the erosion to deposition indicates changing hydrologic surface at topographic lows [8] (Fig 4). conditions consistent with a shift toward ground water dominated processes. Occurrence of late Noachi- an/early Hesperian hydrated sulfate exposures in Sinus Meridiani and Arabia Terra are consistent with ground water upwelling, as predicted by hydrologic modeling [8]. Although phyllosilicates in the Noachian-aged cratered terrain may have formed via subsurface altera- tion followed by near surface alteration [5], phyllosili- cates in the sedimentary unit underlying the sulfate- Figure 4. MOLA colorized map of Arabia Terra (20°S- and hematite-bearing unit are more likely to have 45°N, 30°W-60°E). Deposits shown in Fig 1 are outlined. formed in situ. Sinus Meridiani deposits record pro- Inset shows predicted evaporate thickness map from [8]. longed aqueous activity and changes in mineralogy An extensive exposure of mono- and poly-hydrated between Noachian and Hesperian units likely reflect sulfates occurs within a valley in northern Sinus Me- geochemical variations driven by hydrology. ridiani [9]. These deposits likely formed contempora- References: [1] B.M. Hynek et al. (2002), JGR, 107, neously with or after the sulfate- and hematite-bearing doi:10.1029/2002JE001891,[2] R.E. Arvidson et al (2003), unit [7]. Although much of Arabia Terra is covered JGR, 108, doi:10.1029/2002JE001982, [3] R.E. Arvidson et with dust that obscures the spectral signature of under- al. (2006) JGR,111. [4] B.M. Hynek, R.J. Phillips (2001), lying deposits, erosional windows are present in some Geology, 29, 407-410, [5] B.L. Ehlmann et al. (2011), Na- ture, 479, [6] F. Poulet, et al. (2008), Icarus, 195, 106-130. locations. In the region between Sinus Meridiani and [7] Wiseman et al. (2010), JGR, 115, doi:10.1029/2009JE Schiaparelli crater (and within Schiaparelli), hydrated 003354, [8] Andrews-Hanna et al (2010) JGR, 115, doi:10. materials are exposed in local topographic lows and 1029/2009JE 003485, [9] J.L. Griffes et al. (2007) JGR, 112. .