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Home , Aromatum Chaos, Ravi Vallis

Lunar and Planetary Science XXXV (2004) 1299.pdf

RAVI VALLIS, – PALEOFLOOD ORIGIN AND GENESIS OF SECONDARY CHAOS ZONES. Neil M. Coleman (U.S. NRC, Washington, DC 20555; [email protected])

Introduction: is a large outflow chan- initial outflow from . The Ravi V. nel in the region. It emerges from flooding is interpreted as a continuous event that may Aromatum Chaos (Fig. 1), a large area of collapsed have lasted many weeks or months. It consisted of wide ground centered at 43.0W, 1.2S [1]. The channel overland flow of high discharge at the time of groundwa-

incised terrain of age (unit Npl2 of the plateau ter breakout, followed by prolonged, waning flow that sequence) [2]. TES spectra indicate that basalts and incised the deepest parts of the channel. Komar’s [12, andesites or weathered basalts dominate the Martian 10] analytical approach was used to estimate flow surface [3]. Fig. 2 shows longitudinal grooves parallel velocities for quasi-uniform turbulent flow. to the flow, consistent with fluvial erosion of layered Early overland flow. Using MOLA data, I estimate rocks and similar to erosional features in basalts of the the energy slope for the initial overland flow in two Channeled Scabland [4]. At its eastern terminus, Ravi ways: (1) using pre-flood terrain slopes (profile A–A in V. is truncated by a fault scarp at the western margin of Fig. 1), and (2) using changes in elevation along the . One researcher [5] concluded that southern margin of the channel to define lateral and

CO2 processes formed Ravi V. (and other outflow vertical limits of fluvial erosion (Fig. 4). For western

channels). However, the CO2 hypothesis lacks merit reaches of Ravi V., initial outflows would have had steep compared with an aqueous flood origin [6-9]. energy slopes of ~0.4–1 × 10-2. These values are approx- Secondary Chaotes: MOLA pass 1956 (Fig. 3) imate because subsidence and other secondary processes shows a cross-section of Ravi V. where the channel is may have affected the elevations in this region, espe- >600 m deep and 24 km wide. Further east the channel cially near faults, chaotes, and impact craters. I infer the splits into two smaller channels. MOLA pass 2786 (not influence of faults in the evolution of Aromatum Chaos shown) shows that the northern branch is deeper. because the chaos’ southern margin is remarkably linear Secondary chaos zones (Iamuna* & Oxia* Chaotes - over ~80 km. I used terrestrial Manning’s n values of IAU provisional names) formed along this northern 0.3–0.5 to estimate values of the Chézy friction coeffi- branch (Fig. 1). The location of these chaotes suggests cient over a range of discharge conditions. Flow depths that the flow lasted long enough to thin the cryosphere of 10–50 m were arbitrarily chosen to represent a range by incision and melting, permitting secondary breakout of shallow but wide (~23–26 km) overland flows. of groundwater along the channel. These flows, like the Actual flow depths depended on the roughness of the outflows from Aromatum Chaos, eventually reached pre-flood topography. I estimate mean velocities of 4–28 via Simud-. m s-1 and discharges of 1–36 × 106 m3 s-1 for the initial Floodwater Sources: Confined groundwater was flows. Estimates of flow power range from 500–50,000 the apparent source for the initial outflows. If this were W m-2. These results suggest that extensive cavitation the only source the flow could not have been sustained and macroturbulence would likely have occurred in these because confined aquifers, once released, tend to flows [13]. Fig. 4 plots elevations of the southern depressurize rapidly. The unconfined dewatering of an margin of Ravi V., along with thalweg elevations. Ravi aquifer takes much longer. In addition, the presence of V. was incised more deeply (~1200 m) near its outflow an ice-covered lake in ancestral Ganges would from Aromatum Chaos than in its eastern reaches (~700 have provided a substantial reservoir to recharge the m) where energy slopes were much reduced. aquifer source for both Ravi V. [7] and Shalbatana V. Late-stage flows. It is unlikely that floodwaters ever [7, 10], permitting outflows over an extended period. If filled Ravi V. to the full depth now observed (Fig. 3). the flows were concurrent, then the flooding occurred in The hydrograph for flows released from a confined mid- to upper- because Shalbatana V. incised aquifer should display initial rapid release of water, ridged plains material of lower Hesperian age [11]. resulting in broad scabland erosion. Subsequent drain- Paleoflood Analysis: To analyze the paleoflood age from an unconfined aquifer, augmented by recharge that formed Ravi V., I acquired THEMIS images and from an ice-covered lake in ancestral MOLA profiles to define the width and depth of fluvial [7], would produce a long recession of flood stage. erosion. I theorize that no channel existed before the These protracted flows of reduced discharge would have Lunar and Planetary Science XXXV (2004) 1299.pdf

concentrated erosion in the deepest parts of Ravi V. Hydrographs for Icelandic jökulhlaups, and theorized for Pleistocene Missoula paleofloods, are different, indicating relatively rapid cessation of flows [4]. Thalweg elevations (Fig. 4) show that energy slopes for the final flows in Ravi V. were small compared to the earliest flows. The thalweg also shows that gradients apparently reverse in some reaches, indicating post- fluvial changes in elevation. References: [1] USGS (1981) Margaritifer Sinus NW Quad., Misc. Invest. Series Map I-1381. [2] Witbeck N. et al. (1991) USGS Misc. Invest. Series Map I-2010. [3] Wyatt M. and McSween H. (2002) Nature 417, 263-266. [4] Baker V. and Nummedal D. (1978) The Channeled Scabland, NASA. [5] Hoffman N. (2001) LPS XXXII, Abstract #1257. [6] Stewart S. and Nimmo F. (2002) JGR 107, 10.1029/2000JE001465. [7] Coleman N. (2003) JGR 108, 10.1029/2002JE001940. [8] Urquhart M. and Gulick V. (2003) GRL 30, 10.1029/2002GL016158. [9] Coleman N. et al. (2003) 6th Intl. Conf. on Mars, Abstract #3071. [10] Carr Disclaimer: This article was prepared by an employee of M. (1996) , Oxford Univ. Press. [11] Scott D. the U.S. NRC on his own time apart from regular duties. NRC and Tanaka K. (1986) USGS Geologic Invest. Series Map I- has neither approved nor disapproved its technical content. 1802-A. [12] Komar P. (1979) Icarus 37, 156-181. [13] Baker V. (1979) JGR 84, 7985-7993.