40Ar/39Ar geochronology of basaltic rocks and constraints on late Cenozoic stratigraphy and landscape development in the Red Hill-Quemado area, New Mexico
William C. McIntosh and Steven M. Cather
Basaltic samples from the Red Hill-Quemado volcanic field of New Mexico and the White Mountains field of Arizona were dated by the 40Ar/39Ar incremental heating method to aid mapping and interpretation of the upper Tertiary sedimentary and volcanic rocks. The samples included 15 basaltic lavas, one volcanic bomb, and two shallow intrusive units. Step heating of groundmass concentrates from 17 of the samples yielded relatively flat age spectra from which accurate plateau or isochron ages could be determined. These results indicate two intervals of eruptive activity: 7.9-5.2 Ma and 2.5 Ma-71 ka. Vents for the late Miocene lavas define a narrow ENE-trending zone subparallel to the Jemez lineament. Vents for the late Pliocene-Pleistocene lavas occupy a broad N-S trending zone, within which 40Ar/39Ar results indicate south-tonorth migration of volcanic activity. Late Pleistocene centers, including Red Hill (71 ka) and Zuni Salt Lake (86-114 ka), are confined to the northern end of the zone of Plio-Pleistocene activity; it is likely that future eruptions will occur in this area. Based in part on the ages reported here, several episodes of middle to late Cenozoic erosion and sedimentation can be discerned. The Red Hill-Quemado area aggraded until the end of Mogollon-Datil volcanism, about 24-26 Ma. The ensuing degradational regime continued until <14.5 Ma, when aggradation of the Fence Lake Formation began, and continued until about 7 Ma, although cutting and filling of local insets within the Fence Lake may have continued until about 5 Ma. Local late Miocene basaltic volcanism accompanied the waning stages of Fence Lake sedimentation. Post-Fence Lake erosion continued until 3-4 Ma, when aggradation of the Quemado Formation began. The end of Quemado deposition appears to have been diachronous, possibly ending about 4 Ma near Springerville, Arizona, about 2.5 Ma near Cow Springs Draw in western New Mexico, and <1 Ma near Red Hill, New Mexico. Late Pliocene-Pleistocene volcanism occurred during and after Quemado sedimentation. A regime of fluvial degradation has prevailed in most areas since the end of Quemado deposition.
- McIntosh, William C.; Cather, Steven M., 1994, 40Ar/39Ar geochronology of basaltic rocks and constraints on late Cenozoic stratigraphy and landscape development in the Red Hill-Quemado area, New Mexico, in: Mogollon Slope, west-central New Mexico, Chamberlin, Richard M.; Kues, Barry S.; Cather, Steven M.; Barker, James B.; McIntosh, William C., New Mexico Geological Society, Guidebook, 45th Field Conference, pp. 209-224.