LARAMIDE BASIN OR LARAMIDE UPLIFT? USING SUBSURFACE DATA TO INTERPRET THE HISTORY OF THE ESPAÑOLA BASIN, RIO GRANDE RIFT, NEW MEXICO

The southern Española basin of the Rio Grande rift has been variously interpreted as being a Laramide basin or an inversion of a Laramide uplift. The lithostratigraphy of the Yates #2 La Mesa well near Santa Fe, New Mexico, is central to the reinterpretation of the subsurface geology and tectonic history of the present basin. Petrographic analysis of 46 thin sections from well cuttings, as well as examination of these cuttings and geophysical logs has helped to resolve this controversy.

462 m of limestone and clastic-sedimentary rocks overlying Precambrian basement rock were previously interpreted as Eocene basin fill, including thick lacustrine limestone. Petrographic examination of limestone cuttings reveals many fragments with fossils characteristic of Late Paleozoic marine facies. Along with the absence of redbeds typical of the local Eocene section and the presence of carbonaceous fragments in the cuttings, the Paleozoic fossils require reassignment of this interval to the Pennsylvanian section.

The Paleozoic rocks are overlain by 625 m of volcanic and volcaniclastic rocks previously assigned to the Oligocene Espinaso Formation. Petrographic study shows that only the lower 307 m belong to the latitic Espinaso Formation. The remaining 318 m of the volcanic interval are interbedded mafic lava flows and arkosic sedimentary strata. The mafic lava flows correlate to ~26 Ma basalts and basanites seen in nearby outcrops and the sedimentary layers resemble overlying rift-basin fill.

This study supports interpretation of the Española basin forming by inversion of a Laramide uplift. Oligocene volcaniclastic rocks buried what is now the basin floor before or during early rift-basin subsidence. The well cuttings yield no evidence of Eocene sedimentary deposits. By no later than 26 Ma basalts were erupted onto alluvial surfaces that were already accumulating rift basin fill from the subsidence that was underway.