42nd Lunar and Planetary Science Conference (2011) 1433.pdf

ANCIENT MARTIAN LAKESTANDS IN IANI CHAOS AND THEIR RELATIONSHIP TO ARES VALLIS OUTFLOW CHANNELS. L. Guallini 1, M. S. Gilmore 2, T. N. Harrison 3, L. Marinangeli 1, 1IRSPS, Università d’Annunzio, Pescara, Italy, [email protected] ; 2Wesleyan University, Middletown, CT, USA, 3MSSS.

Introduction: Iani Chaos is a ~30,000 km 2 region (Fig. 1b) have been distinguished with varying severity that lies at the mouth of and is presumably the source of modification (primarily by erosion and fracturing) for the Hesperian age Ares Vallis outflow channel sys- starting from a common terrain (Noachian highlands). tem [e.g., 1, 2]. Mapping of Ares Vallis reveals multi- 1) Mesas (Ms) : broad and flat plateaus bordered by ple episodes of erosion [3, 4], likely linked to multiple wide and deep extensional fracturing and faulting sys- discharge events in the Iani Chaos aquifer system. tems (grabens). 2) Mesas & Knobs (MK) : broad pla- Multiple discharge and recharge events have been pro- teaus bordered by wide and deep extensional fracturing posed for chaos terrains, and Iani in particular, based and faulting systems spaced out and/or overlapped by on geophysical grounds [e.g., 5, 6]. Interior Layered mounds. 3) Knobs (Kb) : groups of dome-shaped hills Deposits (ILDs) have been partially mapped and de- spaced out by marked valleys (tectonic and/or ero- scribed in Iani Chaos [7, 8], and are found to be com- sional) and continuous to the Ms and MK terrains. 4) prised of sulfates [9, 10, 11] consistent with their for- Knobby Terrain (Ky) : small and diffuse mounds show- mation by evaporation of water. A goal of this study is ing a smooth morphology and defining a regional to relate the evolution of Iani Chaos to the Ares Vallis rough topography. It’s often a site for ILD deposits. outflow channels by 1) identifying and mapping fluvial 5) Hummocky Terrain (Hy) : unit characterized by an features in Iani and by 2) comparing topographic base- irregular hummocky topography where light-toned levels of morphologic units and ILD deposits to Ares ILDs are visible on eroded surfaces. Vallis channels. ILD Units. At visible-light wavelenghts, these ter- Methodology: The region has been analyzed using rains are clearly distinguished by a marked light-toned high-resolution visible images acquired by cameras on albedo and their morphology. At CTX and HiRISE the ESA Mars Express (HRSC orthoimages, 12.5 and resolution they usually show a fractured/faulted 25.0 m/pix) and on the NASA Mars Reconnaissance (Fig.1d, red lines) and polygonal texture down to the Orbiter (CTX, 6.0 m/pix and HiRISE, 0.25 m/pix). limit of resolution and a meter scale “stair-stepped” Topographic basemaps are from MGS MOLA 128 layering. The main dome-shaped bodies fill the basins pix/degree (460 m grid spacing) and from HRSC made by the collapsed chaotic terrains (Fig. 1c, 1d), DEMs Da4 level (where available, 75 m and 125 m occupying the valleys between knobby and hummocky grid spacing). The datasets have been processed using mounds. ILDs also overlap the mounds or are them- the USGS Integrated Software for Imagers and Spec- selves eroded into mounds after deposition. The ILDs trometers (ISIS 3) and georeferenced into ArcGIS 9.x are mostly modified by eolian processes (to form yar- software environment. CTX and HiRISE raster images dangs) and craters are rare or absent on their surface, are overlaid onto HRSC DTMs in ArcScene. indicating that these materials are eroded rapidly. Geomorphology and Stratigraph y: Regional For the first time, we have mapped potential fluvial geomorphologic units has been mapped at HRSC and features (channels, streamlined islands, terraces) on the CTX resolution and are defined by depositional tex- surface of the ILDs. The channels form a semi con- tures and/or erosional patterns and albedo (Fig. 1). tinuous system from central Iani, northward to Ares Ares Outflow Channels. Five main erosional sur- Vallis (Fig. 1b). In northernmost Iani, ILD channels faces have been defined in Ares Vallis on the basis of are of similar orientation and elevation to channels at their altitude and relative stratigraphic relationships the mouth of the Ares Vallis; we interepret the ILD and (Fig. 1a, S5-S1 from youngest to oldest), and are in Ares Vallis channels to have once been a continuous agreement with previous studies [3, 4]. The channel floodplain later interrupted by local chaos formation. floors appear regionally flat but include several outflow Topography: A detailed statistical topographic and erosional morphologies as streamlined islands, grooved morphometric analysis has been developed using terrains, terraces and cataracts. Their shape and topog- HRSC DEM elevations. raphy indicates dominant SW to NE flow direction. Chaotic Terrains. We observe a general progres- Possible evidence of glacial features, superimposed on sive decrease of mean elevation from the Ms and MK the fluvial morphologies (i.e. pitted texture, kettle- terrains (average of -2600 m) to the Kb and Ky mor- holes), are also observed. phologies (average of -3400 m). This trend is consis- Chaotic Terrains. These units are characterized by tent with all or some of the following: greater initial kilometer scale irregular block morphologies including collapse of the original surface with an increase of the both Noachian bedrock and ILD deposits. Five units fracturing and/or of the erosional degree of terrains . A 42nd Lunar and Planetary Science Conference (2011) 1433.pdf

significant subsidence of the chaos is marked by clear interleaved with collapsed basins. Lakes are a surface steps between the northern chaotic terrains and the manifestation of the recharging aquifer. Layering in Ares Vallis surfaces. An exception to this progressive evaporites suggests cyclic wetting and drying. 3) New subsidence trend is related to the Hk unit. The pres- depressurization of confined aquifer, due to hydraulic ence of ILD remnants on its surface, its relative tex- overpressurization or melting during new warmer con- tural uniformity (fractures are rare), and the absence of ditions. Tectonic subsidence and fluvial erosion of exposed knobs or mesas and its relatively high eleva- chaos (Kb? to Ky terrains) and ILDs (Ares Vallis S2 to tion (average of -2400 m) are possible evidence of an S3 channels). 4) Lake(s) phase and aquifer recharge: extended depositional body (ancient ILDs?) occupying deposition of younger ILD units in central and northern a wide and deep basin in central Iani. Iani on top of chaotic terrains and older ILDs. 5) Third ILD Units and Ares Outflows. A relative strati- confined aquifer depressurization phase. Tectonic sub- graphic relationship between outflow surfaces in Ares sidence and outflows, erosion of chaos and ILDs (Ares Vallis, the ILD bodies and the ILD channels can be Vallis S4 to S5 channel origin and subsequent down- derived from HRSC DEMs. In particular, the peak dropping of NW and N(e) Iani). frequency of the histogram of the elevations of ILD We have, for the first time, identified likely fluvial channels in northern Iani (-3000m) is comparable with features within Iani Chaos contiguous with several younger Ares outflows peak elevations (S5 and S4 sur- Ares Vallis outflow channels. These systems erode faces, -3200m to -2700m), while the central region (- ILDs and control their elevation. We map at least two 2700m) is comparable to the S3 and S2 elevation peaks major episodes of ILD deposition that we suggest oc- (-2800m to -2500m). Secondary peaks outline a possi- curred in lakes that were the surface manifestation of a ble reworking of central surfaces by S4 outflows. The recharging aquifer recharge within Iani. These pre- average peak elevations of southern ILDs (-2880 m to - liminary results are consistent with chaos and hydro- 2300 m) are the highest in the region, comparable to logical modeling [e.g., 5, 6]. elevations of the mesa surfaces and the oldest Ares References: [1] Carr M. H. and Clow G. D. (1981) surface S1 (-2500m to -1700m). Icarus 48 , 91-117. [2] Grant J. A. and Parker T. J. (2002) Discussion and Conclusions: The altitudes of JGR 107 , 5066, doi: 10.1029/2001JE001678. [3] Pacifici, channels mapped on top of the ILDs bodies is the first A., Komatsu G. and Pondrelli M. (2009) Icarus 202 , 60-77. clear evidence of a possible fluvial system within Iani [4] Warner N., Gupta S., Muller J-P., Kim J-R., Lin S-Y. linked to the Ares Vallis outflow system . We propose (2009) EPSL 288 , 58-69. [5] Carr M. H. (1979) JGR 84 , the following stratigraphic scheme: 1) Confined aquifer 2995-3007. [6] Andrews-Hanna J. C. and Phillips R. J. depressuring phase: initial fracturing and tectonic sub- (2007) JGR 112 , E08001. [7] T. Harrison (2008) M.S. The- sidence of the pristine Noachian materials (Ms to Kb sis, Wesleyan University. [8] Sefton-Nash E. (2010) LPSC terrains) and subsequent outflow erosion of the bed- 41 st , Abstract #1957. [9] Glotch T. D. and Rogers A. D. rock (Ares Vallis S1 channel origin). 2) Lake(s) phase: (2007) JGR 112 , E06001. [10] Noe Dobrea E. Z., Poulet F., evaporitic deposition of older ILDs units (unit Hy ) on Malin M. C. (2008) Icarus 193 , 516-534. [11] Gilmore M. S. top and between chaotic terrains. Thicker deposits are et al. (2010) LPSC 41 st , Abstract # 2374.