QUATERNARY CLIMATE-CYCLE CONTROL OF INSET ALLUVIAL TERRACE DEPOSITS IN THE RIO GRANDE DRAINAGE, NORTHERN NEW MEXICO

Quaternary alluvial terrace deposits are discontinuous along drainage paths of the Rio Grande drainage system and are inset into Tertiary rift-fill deposits of the Albuquerque basin. Previously, unique lithologic features in each terrace deposit seemed to preclude correlation of terrace segments at similar elevations. New subsurface data and age controls for terraces west of the Rio Grande permit revision of a genetic model and correlation. Highest terrace gravel, 320 ft above the Rio Grande flood plain (RGf), is early Quaternary in age. Tercero Alto terrace gravel and sand deposits, 295 ft above RGf, locally are 200 ft thick. Segundo Alto terrace deposits, 145 ft above RGf, include basal gravel, a series of coarse sand-to-silt sequences, capped by 25 ft of sand and gravel; total thickness is 200 ft. Interbedded basalt, dated 156.2 ka, indicates a terrace age of late ¹⁸O stage 6. Primero Alto terrace sand and gravel, 70 ft above RGf, extends laterally beneath the RGf; cobble-boulder gravel is present from 40 ft to 65 ft below Holocene alluvium of the RGf. Primero deposits are correlated with late ¹⁸O stage 2, the time of high river discharge and high local precipitation that produced ground-water recharge rates 6x the present rate. All terrace deposits consist of cobble basal gravel and overlying channel-fill deposits of interbedded sand and granule-pebble gravel. In tributary embayments or in leeward settings, fine sand-silt-clay strata predominate. No sedimentary facies is unique to any one terrace deposit, leading us to correlate these allostratigraphic units with the forcing glacial-climate hemicycle. Each cycle began with cooling, increased effective precipitation and river discharge, and deep erosional entrenchment for a few thousand years, followed by thousands of years of aggradation of coarse bed load, 135-200 ft thick, supplied by slope erosion. 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.