QUATERNARY GLACIOPLUVIAL TERRACES OF THE RIO GRANDE, NORTH CENTRAL NEW MEXICO: PRELIMINARY TL CONSTRAINING AGES AND CORRELATIONS

Deposits of four Quaternary alluvial terraces are preserved discontinuously for >150 km along the Rio Grande through the Albuquerque valley, New Mexico. Mapping, subsurface data, and isotopic and thermoluminescence (TL) ages support correlations of terrace deposits on both sides of the Rio and downstream. Terrace 4 surface deposits lie 110 m above the Rio Grande flood plain (+110 m RGf) and contain Bandelier ashes, 1.6 Ma and 1.2 Ma. Terrace 3 deposits are +90 m RGf and contain 665 ka Lava Creek B ash. Terrace 2 deposits are +44 m RGf, contain interbedded basalt (156 ka Ar/Ar), and are overlain by basalt (110 ka, 98 ka Ar/Ar). First terrace cobble gravel in overlapping terrace deposits is +21 m RGf and is correlated with the latest pluvial stage of aquifer recharge (25 to 15 ka ¹⁴C). From 75 to 80 km south of Albuquerque, remnants of terrace 4 (> +106 m RGf), terrace 3 (>+80 m RGf), terrace 2 (+45 to +38 m RGf), and terrace 1 (+21 to +18 m RGf) are preserved. TL age estimates of fine sand beds confirm correlation of terrace 2 deposits across the Rio and for 20 km downstream. Older TL age estimates (based on high water-content through time) range between 85 ka and 124 ka. OSL ages are about 70 ka, but appear to be saturated and too young. Based on our thesis of glaciopluvial climate control of erosion/terrace deposition, we favor a history of ground-water saturation of these deposits. Thus, we conclude the older ages are closer but still minimal estimates of the age of deposition. The ages quoted above are interpreted as minima in any event, due to possible incomplete zeroing, partial resetting during sampling, or other factors.