New Mexico Geological Society Annual Spring Meeting — Abstracts

The definition, extent, and significance of the Neogene Cochiti Formation (CF) has beenasource of frustration for anyone interested in the stratigraphy of the northem Albuquerque basin, Santo Domingo basin, and southern Jemez Mountains. The problems leadingto these ambiguities include: (1) In defining the CF, Bailey et at (1968, USGS Bull. 1274-P) did not designate a type section, and referred to a type area where strata are poorly exposed and have only limited accessibility on Indian reservations. (2) Although Baileyet at (1968) defined the CF as sediinent derived from penecontemporaneous erosion of Keres Group (KG) volcanics in the Jemez Mountains, most of the strata in the type area post-date KG volcanics. (3) The classic map of Smith et al. (1970, USGS 1-571) does not represent the CF as it was defined by, Bailey et al. (1968). (4) The CF was not included in the KG by Baileyet al. (1969), but was by Gardner et at (1986; J. Geophys. Res.; 91:1763). (5) Although the CF is correlative to the Ceja Mbr. of the Santa Fe Fm. (Manley, 1978; USGS GQ-1446), no one has defined criteria for assignment of strata to the CF or Santa Fe Fm. (or Group) where the units are interstratified. Tedford (1982; NMGS Guidebook, 33:273) assigned the Ceja Mbr. to the CF. (6) Gardner et al. (1986) and Goff et al. (1991; NMBMMR Geol. Map 69) extended the CF into the Jemez Mountains to include coarse sedimentary strata interbedded with KG volcanic rocks; this assignment was not made in the definition of the CF by Bailey etat. (1968). (7) Gardner et al. (1986) and Goff et al. (1991) included within the CF, vent breccias, and flowbreccias that Bailey et al.(1968) explicitly included within the Paliza Canyon Fm. (PCF) of the KG. Bailey et al. (1968) also imply that coarse sedimentary breccias, assigned to the CF by these later workers, were intended to be included in the PCF. (8) Outcrops of interbedded lava flows and volcaniclastic breccias mapped as PCF by Smith et al. (1970) were mapped as CF by Goff et al. (1991) highlighting, the problem of separately representing, even at 1:24,000 scale, interbedded volcanic and sedimentary units assigned to different formations.

We are currently working on volcaniclastic facies of the KG and hope to formally resolve these problems at a later date. Our preliminary proposal is as follows: (1) CF be defined as the volcanic sands and gravels that overlie KG volcanic rocks and extend southward from the Jemez Mountains (thus preserving most of what Smith et al. [1970] mapped as CF). This removes the penecontemporaneity of CF sedimentation and KG volcanism in the Bailey et al. (1968) definition, but age cannot be part of lithostratigraphic definition. The proposed redefinition would leave the CF out of the KG but correlative to the Santa Fe Fm. (or Group) and to the Puye Fm. (2) Volcaniclastic sediments interbedded with KG volcanic rocks are assigned to the appropriate KG formation (i.e., Paliza Canyon Fm.; Canovas Canyon Rhyolite; Peralta Tuff Member of the Bearhead Rhyolite) based on dominant interbedded volcanic rock, type. This definition preserves most of the map representation of Smith et al. (1970) and is consistent with the terminology of Bailey et al. (1968). It is also Consistent with the typical assignment of interbedded volcanic and volcaniclastic-sedimentary strata to the same, rather than different, lithostratigraphic units (e.g. Rubio Peak Fin. and Datil Group of NM; Conejos and San Juan Fms. of CO). At a future time it may be appropriate to assign member status to volcanic and sedimentary units within the PCF. (3) The peralta Tuff Member of the Bearhead Rhyolite grades upward into the CF in lower Peralta Canyon; we propose that the contact be placed at the top of the highest tuff encountered in any section. (4) Only sediment of entirely volcanic composition should be assigned to the CF. Where volcaniclastic and nonvolcaniclastic sediment is mixed to the south and east (e.g., under Santa Ana Mesa). strata of mixed composition should be assigned to the Santa Fe Fm. (or Group), including the Ceja Mbr.