Origin of Aeolis Mensae, Mars, Fretted Terrain in a Thick Sedimentary Deposit

Sixth International Conference on Mars (2003) 3092.pdf

ORIGIN OF AEOLIS MENSAE, MARS, FRETTED TERRAIN IN A THICK SEDIMENTARY DEPOSIT. R. P. Irwin III1,2, T. R. Watters1, A. D. Howard2, T. A. Maxwell1, and J. R. Zimbelman1, 1Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, 4th St. and Independence Ave. SW, Washington DC 20560- 0315, [email protected], [email protected], [email protected], [email protected]. 2Dept. Environmental Sciences, University of Virginia, Charlottesville VA 22903, [email protected].

Introduction: The origin of fretted and knobby terrain on Mars has remained uncertain since the landforms were first described by Sharp in 1973 [1]. Subsequent studies have focused primarily on the fretted terrain in northern Ara- bia Terra, where investigators generally agree that ground ice has been important in modifying knobs and fretted valleys [2-5]. The initial processes isolating mesas from the high-standing terrain are less certain. Some fretted channels exhibit characteristics that suggest origin by fluvial erosion, despite their poorly devel- bounded by 0°N, 10°S, 130°E, and 150°E, 1185 km oped drainage networks [5]. Other proposed mecha- across. The fretted terrain mesa materials appear to be nisms include crustal extension and structural control emplaced at a higher topographic level than the termini of groundwater sapping [6]. of wide valleys. Gale crater (right-center) contains Situated near the martian equator at the crustal di- thick sediments, higher than the crater rim, that may be chotomy boundary, Aeolis Mensae (Fig. 1) provides a of similar origin to layers in the fretted unit. Locations pristine example of fretted terrain development without of Figs. 2 and 3 are indicated by arrows. the younger landforms attributed to ice. We examined an area bounded by 10°S, 0°N, 130°E, and 150°E, ad- Where thick sequences occur, a massive (or perhaps jacent to the cratered and dissected area described by layered at sub-meter resolution) deposit overlies the Irwin and Howard [7]. Here we present evidence for a thinner stair-stepped sequences. Mars Orbiter Camera compositional difference between the Aeolis Mensae (MOC) images suggest that knobs in Aeolis Mensae and the adjacent highland crust, and we discuss the include the same stratigraphy and landforms as sedi- interaction of fluvial valley networks with the dichot- mentary deposits in other regions on Mars [8] (Fig. 2). omy boundary in this region. Our observations indi- Relationships with highland valley networks: cate that Aeolis Mensae fretted terrain developed in a The dichotomy boundary in the study area consists of a thick sedimentary deposit. Sedimentary layers were steep scarp dropping into elongated open and closed emplaced and eroded as fluvial activity declined, with basins (Fig. 1). The density of valley networks on this minimal influence from highland valley networks. north-facing scarp is low relative to degraded crater Distinguishing highland bedrock from sedimen- rims in the adjacent highlands. The few valley net- tary deposits: The extensive degradation of Aeolis works crossing the boundary commonly steepen Mensae relative to the adjacent cratered highlands, abruptly near the scarp [7], consistent with a base-level where 20–50 m deep valley networks are preserved [7], reduction or southward advancement of the boundary suggests either that the mensae are composed of differ- scarp [7,9]. These valley networks do not appear to ent materials than the highland crust, or that geomor- continue into the multibasinal lowland knobby terrain. phic proceses were concentrated along the dichotomy The Aeolis Mensae materials appear to strati- boundary [1]. Malin and Edgett [8] describe criteria graphically overlie or “dam” highland valley networks for distinguishing sedimentary deposits on Mars from that drain toward the dichotomy boundary (Fig. 1). highland bedrock. These criteria include step-like ex- The mesas and knobs are not dissected by valley net- posures of horizontal sedimentary layers, fluting or works as are slopes in the adjacent highlands (Fig. 2). yardangs, and an absence of boulders in talus slopes. It is therefore likely that erosion of Aeolis Mensae oc- Fig. 1. South-facing Mars Orbiter Laser Altimeter curred in the absence of abundant precipitation, during perspective view of the study area in Aeolis Mensae, Sixth International Conference on Mars (2003) 3092.pdf

AEOLIS MENSAE FRETTED TERRAIN IN SEDIMENTARY DEPOSITS: R. P. Irwin et al.

a time when activity of highland mon origin, as fine-grained sedimentary valley networks was waning. deposits were stripped from low-standing Very limited fluvial activity con- settings and transported to their current tinued after material was stripped locations by wind. to form the knobby and fretted terrain. Crater counts [11,12] Fig. 2 (left). MOC image E05-01183 and the activity of highland (3 km across) of mesas in Aeolis Mensae. drainage basins can be used to Mass wasted deposits on the mesa slopes constrain the emplacement and (bottom) are free of large boulders even at erosion of the Aeolis Mensae to full resolution (5.8 m/pixel). Between the late Noachian/Hesperian pe- mesas a layered deposit has been partially riods. stripped in the upper part of the image. Possible emplacement and An indurated layer caps the mesa. erosional processes: Sedimen- Fig. 3 (right). Subframe of Mars Od- tary materials could have been yssey THEMIS daytime infrared image transported into the Aeolis Men- I00957001 (32 km across). Aeolis Men- sae region from the highlands by sae fretted terrain (top) formed in an fluvial activity, or they could etched sedimentary layer (center), which have originated from either the overlies cratered terrain and valley net- highlands or lowlands as an air- works. A higher-inertia layer is exposed fall deposit. For their present between knobs at the top of the image. In appearance and erosion, the me- MOC images, this high-inertia material sas must have disaggregated en- underlies a largely stripped, thin layer, tirely to grain sizes that are trans- which itself underlies the thick mesa- portable by wind, so emplace- forming unit. ment mechanisms should be consistent with this complete sorting References: [1] Sharp R. P. (1973) JGR, requirement. 78, 4073–4083. [2] Squyres S. W. (1978) A more durable cap rock out- Icarus, 34, 600–613. [3] Lucchita B. K. crops in many flat-topped mesas (1984) JGR, 89, B409–B418. [4] McGill (Fig. 2). Erosion of these knobs G. E. (2000) JGR, 105, 6945-6959. [5] may have been limited by the Carr M. H. (2001) JGR, 106, 23,751– backwasting or disaggregation of 23,593. [6] McGill G. E. and Dimitriou the capping unit. Transport of A. M. (1990) JGR, 95,12,595–12,605, disaggregated sedimentary mate- 1990. [7] Irwin R. P. and Howard A. D. rials into the lowlands could have (2002) JGR, 10.1029/2001JE001818. [8] been accomplished by wind, wa- Malin M. C. and Edgett K. S. (2000) Sci- ter, or ice. Yardangs, etched ter- ence, 290, 1927-1937. [9] Kochel R. C. rain, absence of continuous flow and Peake R. T. (1984) JGR, 89, C336– paths, and the temporal relation- C350. [10] Malin M. C. and Edgett K. S. ships with valley networks favor (2001) JGR, 106, 23,429–23,570. [11] wind as the transport medium Frey, H. V. et al. (1988) Proc. LPSC 18, [13]. 679–699. [12] Maxwell T. A. and McGill Regional and planetary cor- G. E. (1988) Proc. LPSC 18, 701–711. relations: Malin and Edgett [13] Irwin R. P. et al., Geology, submitted. [8,10] described sedimentary mantles covering substantial ar- eas of low-standing ground. These layered units showed no obvious relationship to highland valley networks. One possibility is that layered deposits in Terra Meridiani, Arabia Terra, Valles Marineris, and along the dichotomy boundary may share a com-