Geophysics of the Rio Grande Basins
San Luis Basin Geophysics
Geophysical work in the San Luis Basin began in 2003 with a new helicopter magnetic survey over the Town of Taos and vicinity. Since then, acquisition of new aeromagnetic data has moved north, along with geologic mapping efforts. Presently, the southern two-thirds of San Luis Basin and an area over Great Sand Dunes National Park are covered by detailed aeromagnetic surveys.
Goals of the geophysics task in the San Luis basin are similar to those of basins to the south: Gravity, aeromagnetic, and ground-based electromagnetic geophysics are integrated with geologic mapping to (1) model the thickness of the basin-fill aquifers (2) locate concealed basin-bounding and intra-basin faults, (3) determine the lateral and vertical extents of buried igneous rocks, and (4) support geologic mapping efforts.
What's Unique about the San Luis Basin
In the southern San Luis Basin, a large basaltic plateau (Taos Plateau) covers most of the valley. Volcanism occurred late in rift history, so the early sedimentary record of the rift is buried and a primary focus of our geophysical efforts.
In the central San Luis basin, older volcanic rocks that predate the rift are preserved in the San Luis Hills. Studies here are focused on understanding where the rift goes through the area to be able to predict whether aquifers are composed of sediments, volcanic rocks, or both.
In the northern San Luis Basin (San Luis Valley of Colorado) a thick sequence of clay and interbedded sand and clay was deposited by a Pleistocene lake over a period of about 3 million years. This clay sequence, locally known as the "blue clay" is used as an important regulatory division between unconfined and confined aquifers in the San Luis Valley. Geophysical work specific to mapping the subsurface extent of this clay and faults that might cut it in the vicinity of the Great Sand Dunes National Park and Preserve started in 2006-2007 with support from National Park Service.
A detailed aeromagnetic survey centered over the Town of Taos, New Mexico, show a complicated structural picture that is buried from view. Structural patterns intersect at a wetlands area known as Buffalo Pasture.
Gravity and aeromagnetic surveys over the Taos Plateau volcanic field help unravel a buried view of the Rio Grande rift and the volcanic rocks that cover it.
Integrated geophysical/geologic models of the area of the San Luis Basin that straddles the Colorado/New Mexico border show the subsurface configuration of the rift and give clues about its evolution and the hydrogeologic framework.
New geophysical data acquisition in the Great Sand Dunes and vicinity in 2006-2009 include: a helicopter magnetic survey, more than 100 transient EM soundings, and gravity data to start filling gaps in coverage. The geophysical data address three inter-related topics: intrabasin faults, the extent and depth to clays, and structure of the rift basin.