The Chama-El Rio Member of the Tesuque Formation, Espanola Basin, New Mexico
— Leslie M. Ekas, Raymond V. Ingersoll, W. Scott Baldridge, and Muhammad Shafiqullah

The Chama—El Rito is the lowest member of the Tesuque Formation on the north and west sides of the Espanola Basin (Galusha and Blick, 1971; Manley, 1979; May, 1979; Tedford, 1981). The Tesuque Formation and the underlying Abiquiu Formation mark the earliest deposition of sediments in the Espanola Basin of the Rio Grande rift (Galusha and Blick, 1971). The Chama—El Rito Member is a thick sequence (440-460 m; May, 1980) of arkosic and volcaniclastic sandstones and mudstones with interbedded volcanic-rich conglomerates. It includes sparse airfall tuffs and limestones to the southeast and volcanic vents with associated debris-flow deposits (May, 1980). The sediments were deposited in alluvial channels on a broad plain at the distal edge of an alluvial fan. They are relatively undisturbed, showing faulting with minor offset and minor eastward tilting. In general, they are poorly lithified and eroded to a badlands topography. Since the sediments are relatively flat-lying, the best exposures are in areas of rapid erosion.

Hayden (1869) first applied the name "Santa Fe Marls" (now called the Santa Fe Group). Spiegel and Baldwin (1963) defined the Tesuque Formation in the southern part of the type area of the Santa Fe Group. Galusha and Blick (1971) further subdivided the Santa Fe Group into two formations (Tesuque and Chamita), and subdivided these formations into members. The Chama—El Rito and Ojo Caliente Members in the western part of the basin are laterally equivalent to the Nambé, Skull Ridge, and Pojoaque Members in the Tesuque Formation to the east. Kelley's (1978) map of the Espanola Basin delineates the extent of the Chama—El Rito Member. May (1980) mapped the Chama—EI Rito and surrounding units in detail and described the lithology.

This paper summarizes stratigraphic, sedimentologic, and petrographic data from a recent study of the Chama—El Rito (Ekas, 1984), and presents new radiometric and chemical data on volcanic clasts of intermediate composition and interbedded basaltic-vent deposits. This information provides new constraints on the paleotectonic and paleoenvironmental development of the Espanola Basin in particular, and the northern Rio Grande rift in general.