New Mexico Geological Society Annual Spring Meeting — Abstracts

The 2350 m (7710’) Yates #2 La Mesa well, west of Santa Fe, is surrounded by a considerable amount of controversy because of different interpretations of formations present in the well. Petrographic analysis of 46 thin sections of cuttings is underway to resolve these controversies, and their implications for the tectonic history of the southern Española basin.

Below the Tesuque Formation are 625 m (3966'-6018' in the well) of volcanic and volcaniclastic rocks. The upper 318 m (3966'-5012') are mafic rocks rich in olivine and clinopyroxene. These rocks lithologically correlate to basanite and basalt lava flows of the Cieneguilla "limburgite" that crop out near La Cienega. The great quantity of vesicular tachylite and sideromelane glass (some replaced by zeolite) implies proximity to a vent. The lower 307 m (5012'-6018') of volcanic material resembles the latitic lava flows and sedimentary deposits of the Espinaso Formation.

Above the Precambrian basement at 2297 m (7534') and below the volcanic interval the cuttings are a mixture of limestone and clastic-sedimentary fragments. Many of the limestone fragments contain fossils characteristic of Paleozoic marine facies. Some fossils present are crinoids, brachiopods, bryzoans, benthic foraminifera, and bivalves. The 462 m (6018'-7534') of sedimentary rocks between the Espinaso Formation and basement rocks are, therefore, interpreted as Upper Paleozoic, in contrast to previous published interpretations of Eocene lacustrine limestone in this position.

These results are consistent with Cather's (1992) hypothesis that the Laramide Pajarito uplift occupied the area of the current Española basin. There is little if any Eocene sediment in the La Mesa well and the concept of an Eocene "La Mesa limestone" is inconsistent with the diagnostic fossils in the cuttings. Volcanic rocks buried the denuded Laramide uplift before or during early rift-basin subsidence.

We acknowledge Steve Cather, New Mexico Bureau of Geology and Mineral Resources for providing the thin sections and encouraging this study.