What is the Cochiti Formation?
Gary A. Smith and Alexis Lavine
A variety of Neogene strata have been assigned to the Cochiti Formation in and adjacent to the southern Jemez Mountains in north-central New Mexico. Considerable ambiguity now exists on what constitutes the Cochiti Formation. This problem originated with inconsistencies in the original definition by Bailey et al. (1969) and has been perpetuated by varying approaches to the regional stratigraphy by basin-fill stratigraphers and volcanic-field stratigraphers. We suggest that the name be retained but the formation redefined to include only sedimentary strata of entirely volcaniclastic composition that overlie Miocene Keres Group volcanic rocks and correlative volcaniclastic sediment south of the Jemez Mountains and are unconformably overlain, in many places, by a variety of Pliocene and lower Pleistocene gravels. If adopted, this redefmition would preserve most, although not all, of what was mapped as Cochiti Formation by Smith et al. (1970) while not including strata mapped as Cochiti Formation by Manley (1978) and Goff et al. (1990). Alternatively, the name could be suspended and new stratigraphic units defined in its place. Regardless of the approach taken, there is a distinct lithostratigraphic unit that can be mapped in both surface and subsurface, which will be useful in establishing the hydrostratigraphic framework of the northern Albuquerque Basin. The stratigraphic nomenclature problems reflected in the history of use of the name "Cochiti Formation" are not unique to the Jemez Mountains but are illustrative of the problems of effectively representing the common interfingering of volcanic and sedimentary strata in all volcanic provinces and the inherent biases of geologists who are either primarily volcanic stratigraphers or basin-fill stratigraphers.
- Smith, Gary A.; Lavine, Alexis, 1996, What is the Cochiti Formation?, in: The Jemez Mountains Region, Goff, Fraser; Kues, Barry S.; Rogers, Margaret Ann; McFadden, Les D.; Gardner, Jamie N., New Mexico Geological Society, Guidebook, 47th Field Conference, pp. 219-224.