40AR/39AR GEOCHRONOLOGY-ENHANCED CORRELATION OF OLIGOCENE IGNIMBRITES IN WELL BORES, SAN LUIS BASIN, COLORADO

The combination of well cutting petrology, geophysical log correlation and ⁴⁰Ar/³⁹Ar geochronology of individual crystals extracted from well cuttings demonstrates that regional Oligocene ignimbrites (ash-flow tuffs) are present in the subsurface of the Alamosa basin (northern San Luis Basin) in south central Colorado. On the basis of well cutting petrology and well log characteristics, prior work had demonstrated the presence of a volcanic stratigraphic sequence in the Alamosa basin containing stratigraphic units interpreted as welded ignimbrites, non-welded tuffs, volcaniclastic units, and lava flows. A sampling methodology was devised to extract sanidine and biotite crystals from well cuttings containing the welded tuffs, minimizing contamination problems typical of well cutting samples. Single-crystal laser fusion analyses yield populations of crystals with high precision dates having 2 s errors typically less than ±200,000 years. Identified ignimbrites include the 27.3 Ma Carpenter Ridge Tuff, 27.8 Ma Fish Canyon Tuff, and at least one tuff member of the 29 Ma lower Treasure Mountain group, all erupted from the calderas in the San Juan volcanic field, and the 32.9 Ma Gribbles Park Tuff, erupted from the Bonanza Caldera of the central Colorado volcanic field. The Gribbles Park Tuff is interlayered with Conejos Formation andesitic lavas and volcaniclastic units derived from the San Juan volcanic field. Geochronology for selected wells allows recognition of specific ignimbrites, and well log analyses allow correlation of these ignimbrites with units in undated wells. By this method, the Oligocene ignimbrites have been identified in nine deep wells that span the east-west width of the basin. This paper confirms a 33-23 Ma range of the seismic-resolvable volcanic package, a chronostratigraphic unit that separates Rio Grande rift syntectonic sediments from pre-rift stratigraphic units.