DOSECC

Valles Caldera

Hole VC-3 (Valles Caldera #3) was cored using DOSECC's CS-500 drilling rig through a buried lacustrine section in Valle Grande, Valles Caldera, New Mexico, from May 15 to 22, 2004. The project was a collaboration among the US Geological Survey, Los Alamos National Laboratory, the University of New Mexico, the Valles Caldera National reserve, the University of Minnesota, and DOSECC.

Final depth was 82.1 m (269.3 ft) and the stratigraphy is as follows: About 1 m of soil; 3.3 m of terrace gravel; 71.9 m of lacustrine beds and related rocks; 5.9 m of volcaniclastic silt, sand, gravel, and clay. Most of the lacustrine section was successfully cored with the hydraulic piston corer producing an excellent non-deformed sample.

Valles caldera formed 1.25 Ma and has a complicated lacustrine record. Although research on the core has only begun, it is believed that the lake sequence penetrated by VC-3 was formed when a postcaldera rhyolite dome and flow complex (South Mountain Rhyolite, 0.52 Myr) dammed the drainage of the East Fork of the Jemez River, creating a lake in Valle Grande. Most of the lake deposits consist of laminated, diatomaceous silty clay.

If the laminations reflect varves, the lake sequence could represent between 50 to 100 kyr of deposition. The core also contains charcoal fragments (believed to be too old to date), ma y diatomite layers and diatom species, considerable authegenic calcite and siderite, pollen, drop stones (mostly rhyolites), and bioturbated zones. There are several disturbed zones (soft sediment deformation), a few turbiditic horizons, and several thin layers of glass-rich sand and silt. Whether or not these last layers are tephras from outside the caldera remains to be seen.

Fraser Goff, Private geologic consultant as of July 1, 2004 Los Alamos, NM 87545

Project Summary: VallesCalderaOnePager.pdf