EVIDENCE FOR LATE HESPERIAN FLUVIAL ACTIVITY IN NIRGAL VALLIS ON MARS

Geologic mapping in southern Margaritifer Terra helps constrain the timing, duration and relative importance of aqueous processes in shaping the Martian surface. The Noachian-Hesperian age Uzboi-Ladon-Morava (ULM) outflow system extends from Argyre to the northern plains, dominating the regional drainage. Uzboi Vallis is ~400 km long and is the southernmost segment of the ULM system. Evidence of Uzboi’s probable southern outlet from Argyre was destroyed by craters Bond and Hale and the mid to Late Hesperian Holden crater blocks Uzboi’s northern end. The formation of Holden created an enclosed basin in Uzboi that flooded and formed a paleolake. Nirgal Vallis is Uzboi’s largest tributary, yet it was uncertain whether Nirgal contributed to the relatively short-lived lake in Uzboi or if fluvial activity in Nirgal terminated prior to the filling and draining of the Uzboi basin.

This study investigates the deposit on the floor of Uzboi Vallis at the confluence of Nirgal Vallis using MOLA and HiRISE data. The estimated volume of the deposit is significantly smaller than the total volume of material removed by Nirgal Vallis. The deposit consists of a fairly symmetric fan-shaped landform (~40 km³) at the mouth of Nirgal that is stratigraphically on top of a larger mound of material (~200 km³) that is offset downstream toward Holden. The proximal fan surface consists of light-toned material that incorporates meter-scale blocks and lacks obvious layering whereas craters near the distal fan margin (~15 km from Nirgal) expose fine-grained, light toned, horizontal layers.

The net difference in volume between the deposit on the floor of Uzboi relative to the volume of material eroded from Nirgal Vallis suggests that 1) most of the material transported in Nirgal debouched into Uzboi when there was active flow through the system, thereby resulting in much of the sediment being transported downstream, and 2) the majority of incision in Nirgal likely pre-dates deposition into Lake Uzboi. The nature of the fan-shaped deposit at the mouth of Nirgal Vallis, however, is consistent with deposition into standing water in Lake Uzboi. If correct, this implies that late fluvial activity in Nirgal was concurrent with Lake Uzboi and (or) was related to water draining out of Uzboi as water in the lake drained northward into Holden.