GLACIOPLUVIAL CLIMATE-CYCLE CONTROL OF INSET ALLUVIAL TERRACE DEPOSITS IN THE RIO GRANDE DRAINAGE, NORTH CENTRAL NEW MEXICO

Quaternary alluvial terrace deposits are discontinuous along streams of the Rio Grande drainage system and are inset into Tertiary rift-fill deposits of the Albuquerque basin. New mapping, subsurface data, and consistent age controls permit revision of a genetic terrace model correlated with marine ¹⁸O glacial stages. Well known regional pluvial-lake records, including nearby Lake Estancia, and ages of ground-water recharge during the last glacial cycle further support the glaciopluvial correlation. Terrace deposits typically consist of basal cobble gravel and overlying channel-fill deposits of sand and granule-pebble gravel, 50-85 m thick. Sand-silt-clay strata predominate in leeward settings. Gravel-clast rock types and sediment types provide local lithologic continuity in these otherwise allostratigraphic units. The highest terrace gravel, 98 m above the Rio Grande flood plain (RGf), is early Quaternary in age (included Bandelier ash 1.6 Ma and 1.2 Ma). Tercero Alto terrace gravel and sand deposits, 90 m above RGf, are 640 ka, late stage 16 (Lava Creek B ash). Segundo Alto terrace deposits, 44 m above RGf, include basal gravel, overlying sand-silt sequences, capped by 7 m of sand and gravel. Interbedded basalt, dated 156 ka, and overlying basalt, dated 140 ka, closely constrain a terrace age of late stage 6. Primero Alto terrace sand and gravel, 21 m above RGf in overlapping terrace deposits, extends across the inner valley beneath the RGf. Cobble-boulder gravel is present 12-20 m below Holocene alluvium. Primero deposits are correlated with late stage 2, the time of high river discharge and high local precipitation that produced ground-water recharge rates 6x the present rate. The strong correlation of dated terrace deposits with ¹⁸O glacial stages indicates genetic linkage to the forcing glaciopluvial-climate hemicycle. Each cycle began with cooling, increased effective precipitation and river discharge, and deep river erosional entrenchment for a few thousand years, followed by thousands of years of aggradation of coarse bed load, supplied by slope erosion and small glaciers. Late-glacial warming, and vegetation and hillslope stabilization subsequently decreased gravel loads and discharge, leading to incision recorded by small, low terraces, and the inset modern, interglacial flood-plain alluvium.