Stratigraphic analysis of the Yates No. 2 La Mesa Well and implications for southern Espanola Basin tectonic history

The Neogene Espanola Basin of the Rio Grande rift has been alternatively interpreted as overlapping an Eocene Laramide basin or as representing inversion of a Laramide uplift. The stratigraphy of the rocks penetrated by the Yates #2 La Mesa well near Santa Fe, New Mexico, is essential to the interpretation of the subsurface geology and to resolving this controversial aspect of the tectonic history of the basin. Petrographic analysis of 46 thin sections from well cuttings, as well as general examination of cuttings and geophysical logs, addresses these problems. Limestone and clastic sedimentary rocks overlying Precambrian granite were previously interpreted as Tertiary basin fill, but petrographic examination of limestone cuttings reveals many fragments with diagnostic late Paleozoic marine fossils. Along with the absence of typical local Eocene red beds, the Paleozoic fossils require reassignment of this 462-m-thick (1,516-ft-thick) interval to the Pennsylvanian section. Volcanic and volcaniclastic rocks overlying the Paleozoic strata were previously assigned to the Oligocene Espinaso Formation. Only the lower 307 m (1,007 ft) of this succession consist of latitic detritus characteristic of the Espinaso Formation. Mafic lava flows and arkosic sedimentary strata compose the upper 318 (1,043 ft) m of the volcanic interval. The mafic lava flows correlate to upper Oligocene‐lower Miocene basalts and basanites seen in nearby outcrops, and the sedimentary layers resemble overlying rift-basin fill. The results of this study support formation of the Espanola Basin by inversion of a Laramide uplift. The well cuttings provide no evidence for Eocene syn-Laramide basinfill sedimentary deposits. Oligocene volcaniclastic rocks buried the uplift that was denuded of all Phanerozoic cover strata except for a partial Paleozoic section. Basalts erupted onto an alluvial surface that was already accumulating rift-basin fill.