Chamita Formation


The Chamita Formation is a geologic formation in New Mexico. It preserves fossils dating back to the Neogene period.

Description

The formation is mostly coarse quartz sandstone with some coarse gravel. It varies greatly in thickness, likely due to extensive erosion.
There are two prominent tuffaceous zones used by Galusha and Blick for regional correlation. Each is about thick and they are separated by of tan sediments. Radiometric dating shows that the age of the Upper Chamita Tuffaceous Zone is 6.93 ±0.05 million years at its base and 6.78 ±0.03 million years at its top, essentially identical with the Peralta Tuff, which it also chemically resembles. This helps establish the absolute date for the late Hemphillian, since a fossil assemblage of that stage is found in the zone. The Lower Chamita Tuffaceous Zone has a radiometric age of 7.7 ±0.3 million years.
The base of the formation contains sediments resembling the underlying Ojo Caliente Member of the Tesuque Formation. This is interpreted as reworking of sediments in a changed sedimentation environment rather than interfingering.
Members of the formation include the Cejita and Cuarteles Members west of the modern Rio Grande River and the Pilar Mesa, Vallito, and Hernandez Members on both sides of the Rio Grande.

Fossils

The Round Mountain Quarry has produced numerous fossils from the Chamita Formation. Osbornoceros osborni was first located in the formation in the Lyden area.

History of investigation

The beds making up the unit were originally included by Bryan and McCann in 1937 in the Middle Red member of the Santa Fe Formation. This became the Tesuque Formation in 1956.
In 1971, Galusha and Blick split the uppermost beds from the Tesuque Formation as the Chamita Formation based on differences in lithology and fossil assemblage.

Footnotes