Workshop on Early Mars (1997) 3073.pdf

FLUVIAL AND LACUSTRINE DEGRADATION OF LARGE MARTIAN BASINS DURING THE NOACHIAN. T. J. Parker, Jet Propulsion Laboratory, California Institute of Technology, (Mail Stop 183-501, 4800 Oak Grove Dr., Pasadena, CA 91109. E-mail: [email protected]).

Introduction occupy deep graben and collapse depressions in the Studies of potential sites of Martian paleolakes Martian crust, and appear to have been fed by have taken two approaches. One group of investiga- groundwater rather than overland flow [9,10]. The tors has focused on the planet's heavily cratered, deposits are on the order of kilometers thick and relatively ancient southern highlands as the most appear finely laminated in high resolution Viking likely place to preserve evidence of a former wet Orbiter images, and so probably indicate the pres- climate. This is because the valley networks, while ence of lakes for long periods of geologic time. they may not always indicate atmospheric precipita- These layered deposits and two notable examples of tion, nevertheless require significant discharges for highland basins have been the focus, for a number of sustained periods of time. The other group of inves- years, of investigations of future lander missions to tigators has focused on the northern lowlands as a Mars that might search for evidence of fossil organic likely site for paleolakes or even a paleo-ocean. The materials [e.g., 11]. The first of these, the 135 km primary sources of influx into this basin are the cir- crater Gusev (15° S lat., 184° lon.), lies at the mouth cum-Chryse outflow channels and several smaller of the large, tributary-fed channel Ma'adim Vallis outflow channels and valley networks west and south and contains the remnants of deltaic or alluvial fan of the Elysium volcanic complex. These channels deposits and possible lake sediments [12]. The other are large enough to have formed large lakes, or even site is an unnamed, 200 km diameter depression oceans, in the northern plains. along the Parana-Loire valley system in Margaritifer Sinus (23° S lat., 13° lon.). This basin contains a Early Noachian Basin Origins peculiar, hummocky deposit that has been inter- Tectonism is probably the most important proc- preted as the eroded remnants of lake sediments by ess on Earth for producing closed depressions on the Goldspiel and Squyres [13]. More recently, Argyre continents, and is clearly responsible for mainte- Planitia and Shiaparelli B Crater have been added to nance of the ocean basins through geologic time. On the list of basins being studied as possible landing Mars, however, tectonism appears limited to rela- sites for their exobiology potential (by this author tively small amounts of regional extension, com- and by Ken Edgett, respectively). pression, and is dominated by vertical motion largely “Perched” chaos deposits, similar to those in due to crustal loading of the two major volcanic Gusev and Margaritifer Sinus, can be seen in other provinces - Tharsis and Elysium [1-4]. large, shallow basins in the Phaethontis region of Impact craters and large impact basins, and Mars at 35° S latitude, 177° longitude (Atlantis broad topographic depressions between them, are Chaos) and nearby at 30° S latitude, 170° longitude clearly more important sites for potential lake basins (Gorgonum Chaos), and in the very degraded Schia- on Mars, though they were likely more important on parelli B Basin at 4° S latitude, 358° longitude. On Earth as well prior to 3.5 Ga. The global topo- Earth, eroded lake deposits are among the more graphic dichotomy separating the heavily cratered common host terranes for badlands topography, southern highlands from the sparsely cratered north- which bears some resemblance to the Martian de- ern lowland plains is an ancient feature, due either to posits, though fluvial dissection is more pronounced. tectonism or to the formation of a single, or several The largest highland basins, Argyre (900 km overlapping giant impact basins in the northern interior diameter) and Hellas (1500 km interior di- plains [5-7]. ameter) appear to contain massive accumulations of Highland Basins layered sediments that are now being exposed In the southern highlands, several basins on the through eolian deflation. Those in Argyre may be on order of several tens to a few hundred kilometers the order of hundreds of meters or more thick [14], across and parts of Valles Marineris have been found whereas those in Hellas appear to have been as much to contain remnants of former alluvial or lacustrine as a few kilometers thick [15]. Both basins received sedimentary deposits. Many of these basins were discharges from radial valley networks and outflow simply in the paths of short-term catastrophic floods, channels. Cassini Crater (24° N lat., 328° lon.) ap- and thus may not have contained lakes for very long pears to contain similar, though less extensive lay- [8]. The thick Valles Marineris layered deposits ered deposits [e.g., 16]. Oddly, however, it lacks Workshop on Early Mars (1997) 3073.pdf

NOACHIAN BASIN DEGRADATION: T. J. Parker

large inward-draining channels, so influx of water would have ended earlier, before the rims could be could only have been through the subsurface or eroded to the extent that occurred in Argyre, Hellas through precipitation over the large catchment area and the fretted terrains. within the basin’s rim. This basin exhibits well- (IV) During the Amazonian, the planet's atmos- developed terraces that may be paleoshorelines, that pheric pressure declined, and the climate changed are cut into older layered mesas, crater rims, and gradually to its modern very cold, very dry condition. fluidized ejecta ramparts. Eolian deflation of the Argyre and Hellas interior sediments was probably initially intense during the Basin Degradation early Amazonian, but fell with the steady drop in The degree of degradation of large highland ba- pressure and temperatures to the present day. For sins appears to be directly related to the size of the smaller sediment-filled basins, deflation may have basin and the lay of valley networks relative to the nearly completely removed these deposits (as in basin and regional topography. Cassini (and other, “perched” chaotic materials above) somewhat earlier similar-size basins) retains its ancient rim morphol- (no crater counts have been conducted to verify this ogy, but shows moderate erosion of interior stuctures as yet). and deposits of extensive flat-lying, layered sedi- The above scenario has important implications ments. The rim of Argyre was found to include regarding the question of exobiology on Mars, par- many mountains that are flat-topped erosional rem- ticularly since long periods of geologic time with nants of the surrounding Noachian highlands surface pluvial conditions are indicated. [17]. Scattered knobby remnants of a very degraded outer rim to Argyre have now been identified several References: [1] Banerdt, W. B. et al. (1982) JGR hundred kilometers southeast and nothwest of 87, 9723-9733. [2] Plescia, J. B. (1991) JGR 96, Charitum Montes. Prior to late Noachian, Argyre 8883-8895. [3] Tanaka, K. L. et al. (1991) JGR 96, may have been similar to the nearby, but very de- 5617-5633. [4] Watters, T. R., (1993) JGR 98, graded Ladon Basin. The modern, "fresh" appear- 17049-17060. [5] McGill, G. E., and S. W. Squyres, ance of Argyre is due primarily to erosional (1991) Icarus 93, 386-393. [6] McGill, G. E., and A. "enhancement" of the basin rim. Hellas’ rim is M. Dimitriou (1990) JGR 95, 12595-12605. [7] similarly eroded. The observation is made that the Breuer, D. T. et al. (1993). Planet. and Space Sci. rims of these two highland basins are analagous to 41, 269-283. [8] DeHon, R. A. (1992) Earth, Moon the extensive fretted terrains along the low- and Planets 56, 95-122. [9] Nedell, S. S. (1987) land/upland boundary. It is proposed that these ba- Icarus 70, 409-441. [10] Komatsu, G. et al. (1993) sin margins developed in the same way, through JGR 98, 11105-11121. [11] Landheim, R. et al. basin broadening by lakeshore erosion during the (1993) LPS XXIV, 845-846. [12] Schneeberger, D. late Noachian and early Hesperian [18]. M. (1989) LPS XX, 964-965. [13] Goldspiel, J. M., Conclusions and S. W. Squyres (1991) Icarus 89, 392-410. [14] The following scenario for Mars' climate his- Parker, T. J., and D. S. Gorsline (1993) Am. Geo- tory, originally proposed to explain Argyre’s mor- phys. Union Spring Meeting, 1p. [15] Moore, J. M., phology, is being further developed to explain the and K. S. Edgett (1993) GRL 20, 1599-1602. [16] apparently direct correlation between the state of Albin, E. F. (1997) LPS XXVIII, 2p. [17] Parker, T. basin degradation and basin size. J. (1996) LPS XXVII, 1003-1004. [18] Parker, T. J. (I) The early Noachian was warm and wet, with et al. (1989) Icarus 82, 111-145. atmospheric precipitation and surface runoff re- sponsible for the early, nearly complete destruction of the rims of large impact (and tectonic) basins. (II) The late Noachian saw a change from this warm/wet climate to a drier climate that allowed surface water, in the form of channels and lakes, but in which atmospheric precipitation was very limited (so truly advanced dendritic valley networks could no longer form). (III) Lakes may have existed intermittently in the largest highland basins from the Noachian through the early Amazonian, a span of more than two billion years. For smaller basins, this period