Reconstructing the ancestral (Plio-Pleistocene) Rio Grande in its active tectonic setting, southern Rio Grande rift, New Mexico, USA

Deposits of the ancestral Rio Grande (aRG) belonging to the Camp Rice Forma tion are preserved and exposed in the uplifted southern portion of the Robl edo Mountains horst of the southern Rio Grande rift. The sediments are date d palaeomagnetically to the Gauss chron (upper Pliocene). The lower part of the succession lies in a newly discovered palaeocanyon cut into underlying Eocene rocks whose margins are progressively onlapped by the upper part. D etailed sedimentological studies reveal the presence of numerous river chan nel and floodplain lithofacies, indicative of varied deposition in channel bar complexes of low-sinuosity, pebbly sandbed channels that traversed gene rally dryland floodplains and shifted in and out of the study area five tim es over the 1 Myr or so recorded by the succession. Notable discoveries in the deposits are: (1) complexes of initial avulsion breakout channels at th e base of major sandstone storeys; (2) common low-angle bedsets ascribed to deposition over low-angle dunes in active channels; (3) palaeocanyon flood plain environments with evidence of fluctuating near-surface water tables. Sand-body architecture is generally multistorey, with palaeocurrents indica tive of funnelling of initial avulsive and main fluvial discharge from the neighbouring Mesilla basin through a narrow topographic gap into the palaeo canyon and out over the study area. An avulsion node was evidently located at the stationary southern tip to the East Robledo fault during Gauss times , with aRG channels to the north flowing close to the fault and preventing fan progradation. Subsequent Matuyama growth of the fault caused (1) deposi tion to cease as the whole succession was uplifted in its footwall, (2) dev elopment of a thick petrocalcic horizon, and (3) fan progradation into the Mesilla basin. Parameters for the whole aRG fluvial system are estimated as : active single channels 2 m deep and 25 m wide; valley slope 0.24-0.065 de grees; maximum mean aggradation rate 0.05 mm year(-1); major channel belt a vulsion interval 200 ky; individual channel recurrence interval 100 ky; min imum bankfull mean flow velocity 1.54 m s(-1), minimum single-channel disch arge 77 m(3) s(-1), bed shear stress 22.3 N m(-2); and stream power 34.3 W m(-2).