INTERROGATING CLIMATE RESPONSE MODELS OF TERRACE FORMATION IN THE RIO GRANDE RIFT USING 36CL AND 14C DATING

Understanding of Rio Grande incision history since the end of basin filling (c. 780 ka) remains hampered by poor age control. Robust correlations among Rio Grande terrace sequences in southern and northern New Mexico are lacking and important process-related questions about terrace formation cannot be addressed. We present terrace chronology from the central New Mexico Socorro Basin, based on 36Cl surface exposure and 14C ages, and correlations to adjacent rift basins, and test climate response models of terrace formation. Five distinct, post-Santa Fe Group alloformations exist in the Socorro Basin and were mapped with photogrammetric methods, soil characterization and stratigraphic descriptions. These exhibit strath and tread heights <70 m above the valley floor and are 5 to >30 m thick. Their fills generally have coarse gravel to cobble straths overlain by fine to pebbly sand and local thin silt and clay tops. Treads contain calcic soils mantled by desert pavement. Alluvial fan terraces and associated geomorphic surfaces grade to former valley levels defined by axial terrace treads. Carbon-14 ages from detrital charcoal above and below a buried tributary terrace tread show that the most recent aggradation event persisted until at least 3 ka during the transition from glacial to modern climate conditions. Well data show widespread valley fill ~30 m thick that began aggrading after local glacial retreat (c. 14 ka). Aggradation during this transition was likely due to hillslope destabilization, increased sediment yield, decreased runoff and reduced stream competence. Preliminary 36Cl ages imply similar controls on earlier terrace fills (~58, 90, and 174 ka) and suggest that net incision occurred during glacial expansions, when increased runoff favored downcutting and bedload mobilization. Our preliminary terrace chronology supports existing climate response models of arid environments (i.e. Gile et al.,1981 and Bull, 1990) and links tributary responses to the axial Rio Grande system throughout the central Rio Grande rift.