CORRELATION OF MAGNETOTELLURIC RESISTIVITY SOUNDINGS AND REGIONAL BOREHOLE GEOPHYSICAL ROCK PROPERTIES TO STRATIGRAPHY OF THE NORTHERN RIO GRANDE RIFT, NEW MEXICO

Geophysical interpretations of magnetotelluric (MT) soundings, calibrated by sparse borehole geophysical data from deep wildcat petroleum exploration wells along the flanks of the Rio Grande Rift and in the Albuquerque and Española basins, significantly improve geologic constraints for mapping subsurface stratigraphy and lithology in the Rio Grande Rift near Santa Fe and Los Alamos, New Mexico. To distinguish between various stratigraphic units, regional borehole resistivity data were used to characterize the typical resistivities encountered in the Rio Grande rift. Digitized induction logs were initially smoothed with a 100' moving average to minimize the effects of thin layers unresolvable with MT soundings. These averaged resistivities were then used to test the lowest thickness detectable at a given stratigraphic horizon with MT soundings given the averaged resistivities likely to be encountered above and below the horizon. In the Española basin, MT soundings, located south of Los Alamos across the Pajarito fault and west of Santa Fe across the Cerros del Rio volcanic field provide significant new geologic interpretations of buried Cenozoic sedimentary and igneous rocks, Santa Fe Group, Espinaso Formation, Galisteo Formation, and for older Mesozoic and Paleozoic sedimentary rocks. Borehole resistivity data for Santa Fe Group and the Galisteo Formations below the water table were moderately conductive (about 5-50 ohm-m), while Tertiary igneous rocks were resistive (greater than about 100 ohm-m). Resistivity data for the Mancos Shale indicate that it is a strong (2-10 ohm-m) electrical conductor. Where the Mancos occurs in the upper kilometer of crust it may function as local hydrogeologic basement. Permian rocks generally had both conductive and resistive signatures, while Pennsylvanian carbonates and Precambrian crystalline rocks were resistive (greater than about 100 ohm-m). Conductive Mesozoic rocks appear to be down-dropped about 1 km across the Pajarito fault accompanied by about 1 km thickening of Tertiary sediments. Conductive Mesozoic rocks appear to be down-dropped beneath the Cerros del Rio volcanic-field near the 1200' well by about 700 m accompanied by about 600 m thickening of Tertiary sediments.