New Mexico Geological Society Annual Spring Meeting — Abstracts

The Valles caldera in the Jemez Mountains contains a thick sequence of lacustrine sediments and hydromagmatic deposits that date from the inception of the caldera (c.a. 1.5 Myr.). Lakes formed in the caldera immediately after its formation existed for some period of time before the caldera wall was breached. Another lake formed during the mid-Pleistocene in the Valle Grande when a post-caldera eruption (c.a. 520 kyr) filled the drainage to the San Diego Canyon.

To determine their paleoclimatic significance, the deposits of the ancient lake were cored in May 2004 (GLAD 5). Hole VC-3 achieved a total depth of 81 m recovering a complete section of ~75 m of volcaniclastic lacustrine mud and silts and gravels. Preliminary analyses show considerable down-core variability in magnetic susceptibility, gamma–ray density and sedimentary facies. Pumice sands and gravels at the core, thin sand lenses indicate periods of enhanced runoff into the lake, and occasional dropstones are observed. In the middle sequence, a brecciated, diatom poor facies correlates with well-developed mudcracks indicate multiple lake level changes over the mid-Pleistocene lake history that probably span tens of thousands of years. Future work on the core will include pollen, diatom stable isotope and further geochemical and geophysical analyses.