CLIMATE CHANGE OVER A GLACIAL-INTERGLACIAL CYCLE DURING THE MIDDLE PLEISTOCENE: A LONG TERM RECORD FROM THE VALLES CALDERA, NEW MEXICO

The Valles Caldera in the Jemez Mountains of northern New Mexico contains a thick sequence of lacustrine sediments and volcaniclastic sediments that date from the inception of the caldera (c.a. 1.25 Myr). Lakes formed in the caldera immediately after its formation and existed for some period of time before the caldera wall was breached. A major lake formed during the middle Pleistocene in the Valle Grande when a post-caldera eruption (c.a. 520 kyr) filled the drainage to San Diego Canyon. The deposits of this ancient lake were cored in May 2004 (GLAD 5 drilling project) and a total depth of 81 m of lacustrine mud and silts and gravels was recovered. Based on geomorphic relationships and typical sedimentation rates in caldera lakes, the record obtained from the Valles Caldera should span ~80,000 years. As such, it offers a unique opportunity to gain insight into the climatic response to a middle Pleistocene glacial cycle in southwestern North America.

Initial analyses show considerable down-core variability in sedimentary facies, magnetic susceptibility, and gamma-ray density. Pumice sands and gravels at the base of the core grade up into variably laminated and bioturbated diatom-rich silty muds. Turbidites in the lower section interrupt laminated mud sequences and in some sections, thick diatomites occur and are indicative of surface eutrophication in the lake. The mm-scale laminations found in most of the lacustrine mud present possibly represent varves. Higher in the core, thin sand lenses indicate episodes of enhanced runoff into the lake, and occasional dropstones are observed. In the middle sequence, a brecciated, diatom poor facies correlates with high magnetic susceptibility. The rapid vertical facies changes found within this core and intervals with well-developed mudcracks indicate multiple water level changes over the lake history that spans a glacial-interglacial cycle during the middle Pleistocene.