| Because of its implications for habitability, understanding the longevity and role of standing water in Martian geomorphology is the primary focus of the U.S. and European Mars exploration programs. This dissertation addresses four related issues: (1) the relevance of particular landform assemblages to the occurrence of Martian paleolakes; the age, longevity, and maturity of these landforms; and their implications for paleoclimate; (2) the development of a large mid-latitude paleolake in the southern highlands, which appears to have overflowed catastrophically to carve Ma'adim Vallis; (3) the origin and development of fretted terrain along the highland/lowland boundary, which some investigators have suggested may include shorelines; and (4) the hydrology of Martian watersheds that supplied the enclosed basins. Most known putative deltas record a relatively brief period of aggradation without subsequent erosion controlled by a lower base level. Most of these deposits occur at the ends of V-shaped, entrenched lower reaches of valley networks, and some late-stage basin overflows are evident. These observations suggest a late episode of valley network incision that was intense but short-lived relative to the more general Noachian landscape degradation. Ma'adim Vallis, one of the largest valleys in the Martian highlands, appears to have originated somewhat earlier by catastrophic overflow of a large paleolake located south of the valley heads. Support for the paleolake overflow hypothesis comes from the morphology and morphometry of the valley, its tributaries, and the contributing Eridania basin to the south. At Aeolis Mensae along the dichotomy boundary, we show that fretted terrain formed exclusively in a >2 km thick, late Noachian (∼3.7 Ga) sedimentary deposit that overlies the base of an older cratered slope. New orbital imaging reveals 21 late-stage channels within valley networks, which we use to estimate formative discharges and to evaluate water supply mechanisms. We find that channel width and associated formative discharge are comparable to terrestrial valley networks of similar area and relief. In four large valleys with few tributaries, wider channels may represent large subsurface outflows or paleolake overflows, as these four channels originate at breached basin divides and/or near source regions for the catastrophic outflow channels. |
