Brian S. Brister¹, William C. McIntosh¹

(1) New Mexico Bureau of Mines and Mineral Resources, New Mexico Tech, Socorro, NM

ABSTRACT: Application of high resolution 40Ar/39Ar geochronology to well cuttings: an example from the San Luis Basin, Colorado

Recent developments in the 40Ar/39Ar dating method, a variation of the K/Ar method, allow highly increased precision as well as dating of very young and very small samples. Because this is a ratio technique that is sample size independent, single mineral crystals may be tested by laser fusion. The technique is used to date potassium-bearing minerals in plutonic, volcanic, metamorphic, and sedimentary rocks. Sanidine, a K-feldspar commonly found in volcanic and volcaniclastic rocks, is particularly useful because it has high potassium content (>3%), is very homogenous, and is easy to separate and analyze.

In an experiment to test the applicability of 40Ar/39Ar geochronology to a petroleum exploration setting, sanidine crystals were extracted from well cuttings from six wells in the San Luis Basin, Colorado. Prior petrologic study had established the presence of a package of volcanic/volcaniclastic rocks in the basin. A collection methodology was devised to reduce the chance of selecting "caved" material. Dating of sanidine crystals confirmed some correlations made on the basis of petrology, but also contributed new correlations. Integrating well log correlation, petrology and geochronology, the volcanic package can be divided into discrete interbedded units of welded ash flow tuffs, volcaniclastic units, and lava flows. The project confirmed the 33-23 ma. age range of the seismic-resolvable volcanic package, a chronostratigraphic unit that separates Rio Grande rift syntectonic sediments from pre-rift stratigraphic units. The success of this "ideal" example suggests the technique may be applied in other basins where exploration efforts would benefit from geochronologic characterization.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado