EPISODIC PLIOCENE AND PLEISTOCENE DRAINAGE INTEGRATION ALONG THE RIO GRANDE THROUGH NEW MEXICO

Drainage through the Rio Grande tectonic rift zone has evolved over the last several million years from multiple closed basins, to interconnected basin systems, to a fully integrated system that drains from Colorado to the Gulf of Mexico. The evolution has been affected to variable degrees by tectonic subsidence/uplift, by episodic dams formed by basalt and ash-flow eruptions, and by sediment-supply factors along different segments of the rift zone. We compare the sedimentation/incision history of the Santa Fe-Socorro reach of the Rio Grande with the Socorro-El Paso reach, and with the headwaters-Taos reach, to show that each drainage segment evolved independently until a through-flowing connection was established between pairs of them. The Santa Fe-Socorro reach was a closed-basin system through the middle Pliocene, when it connected southward after about 3.4 Ma (Mack and others, 1993). Sedimentation in the lower, closed basins of the Socorro-El Paso reach (Lake Cabeza de Vaca of Gile and others, 1981) was fed by material incised from the Miocene-lower Pliocene closed-basin deposits along the Santa Fe-Socorro reach. Shortly before about 750 ka (Bishop ash), the integrated Santa Fe-El Paso reach was captured by drainage that connected to the Gulf of Mexico (Gile and others, 1981; Mack and others, 1993). The headwaters-Taos reach remained isolated from the Rio Grande until erosion cut through the basalt dam of the Taos volcanic plateau after about 690 ka (Wells and others, 1987). We interpret the sedimentologic and chronologic record of the last 5 million years to indicate that progressive change toward cooler, wetter climate has been more significant than tectonics in driving drainage integration for the region.