An intense terminal epoch of widespread fluvial activity on early Mars: 2. Increased runoff and paleolake development

Abstract

To explain the much higher denudation rates and valley network development on early Mars (>∼3.6 Gyr ago), most investigators have invoked either steady state warm/wet (Earthlike) or cold/dry (modern Mars) end‐member paleoclimates. Here we discuss evidence that highland gradation was prolonged, but generally slow and possibly ephemeral during the Noachian Period, and that the immature valley networks entrenched during a brief terminal epoch of more erosive fluvial activity in the late Noachian to early Hesperian. Observational support for this interpretation includes (1) late‐stage breaching of some enclosed basins that had previously been extensively modified, but only by internal erosion and deposition; (2) deposition of pristine deltas and fans during a late stage of contributing valley entrenchment; (3) a brief, erosive response to base level decline (which was imparted as fretted terrain developed by a suite of processes unrelated to surface runoff) in fluvial valleys that crosscut the highland‐lowland boundary scarp; and (4) width/contributing area relationships of interior channels within valley networks, which record significant late‐stage runoff production with no evidence of recovery to lower‐flow conditions. This erosion appears to have ended abruptly, as depositional landforms generally were not entrenched with declining base level in crater lakes. A possible planetwide synchronicity and common cause to the late‐stage fluvial activity are possible but remain uncertain. This increased activity of valley networks is offered as a possible explanation for diverse features of highland drainage basins, which were previously cited to support competing warm, wet and cold, dry paleoclimate scenarios.

Stratigraphic analysis of the distributary fan in Eberswalde crater using stereo imagery

“…The value of L is unconstrained given the limited knowledge of the depositional environment. However, for typical terrestrial lacustrine loss rates of 1 -10 m/yr [Tyler et al, 1997;Irwin et al, 2005], we estimate a minimum formation time of several hundred years. This is slightly longer, but in general agreement with the previous estimate of Jerolmack et al [2004], who used a sediment transport model to estimate a minimum formation time for the delta of 50 years.…”

Formation of a terraced fan deposit in Coprates Catena, Mars

“…1) in order to investigate the role that water may have played in its formation. Only about a dozen of these terraced fan deposits have been identified on Mars (Ori et al, 2000;Cabrol and Grin, 2001;Malin and Edgett, 2003;Irwin et al, 2005), although their small sizes make them difficult to identify with the current orbital data sets. Moore and Howard (2005) have identified several dozen alluvial fans in highland craters.…”

“…Moore and Howard (2005) have identified several dozen alluvial fans in highland craters. The few fan deposits that do not have terraces and are not classified as alluvial fans in highland craters (e.g., Eberswalde crater, Nili Fossae) have been interpreted as deltas in standing bodies of water (Malin and Edgett, 2003;Moore et al, 2003;Fassett and Head, 2005;Bhattacharya et al, 2005;Irwin et al, 2005). The surfaces of some of these fan deposits are now marked by inverted terrain (including crosscutting ridges representing former channel fills), more irregular fan shapes that reflect channel avulsion and discrete lobe extension, and shallower slopes relative to those of terraced fans.…”

“…Yellow arrow shows location of terraced fan. (Malin and Edgett, 2003;Irwin et al, 2005). The scarcity of positive-relief deposits could indicate that valleys formed primarily by corrosion with minimal transport of clastic sediment.…”

Characterization of fluvial activity in Parana Valles using different age-dating techniques

“…Milton, 1973;Schultz and Ingerson, 1973;Sharp and Malin, 1975;Carr and Clow, 1981;Mars Channel Working Group, 1983;Carr, 1996) are the best evidence that liquid water may have been stable on the surface at one time and that the past climate may have been warmer than today (Carr, 1981;Gulick, 2001;Craddock and Howard, 2002). Because they are located primarily in Noachian age terrain (>3.7 Ga) the general assumption has been that they formed sometime during the end of the Noachian Period and possibly through the beginning of the Hesperian Period (Tanaka, 1986;Scott and Tanaka, 1986;Greeley and Guest, 1987;Carr, 1996;Hartmann and Neukum, 2001;Irwin et al, 2005;Fassett and Head, 2008). However, some studies suggest that valley network formation and prolonged fluvial activity extended into the Late Hesperian (Mangold et al, 2004;Quantin et al, 2005;Ansan and Mangold, 2006;Bouley et al, 2009) and possibly even the Amazonian (Scott and Dohm, 1992;Scott et al, 1995), although these latter studies focused on valley networks that occur in discrete, isolated areas, such as the flanks of volcanoes.…”