Paleomagnetism and 40Ar/39Ar ages of ignimbrites, Mogollon-Datil volcanic field, southwestern New Mexico

Abstract

This report presents 40Ar/39Ar sanidine ages and paleomagnetite data for 36 ignimbrites and associated lavas in the Eocene-Oligocene Mogollon-Datil volcanic field of southwestern NM.40Ar/39Ar age spectra from the ignimbrites yield plateau ages that range from 36.2 to 24.3 Ma and show within-sample and within-unit 1s precision of +/- 0.5% or better. These ages agree closely with independently established stratigraphic order and indicate that Mogollon-Datil ignimbrite activity was highly episodic, being confined to four brief (< 2.6 Ma) eruptive intervals separated by 1.5-3 Ma long hiatuses.Ignimbrite outflow sheets show stable paleomagnetic remanence directions that are uniform over most of their areal extents, including facies ranging from thick, densely welded proximal ignimbrites to unwelded distal fringes as thin as 1.5 m.Used in concert with lithologic and stratigraphic position data, 40Ar/39Ar ages and paleomagnetic directions allow accurate long-range ignimbrite correlations that provide reliable ties between previously established subregional stratigraphic sequences. These correlations have helped to establish an integrated time-stratigraphic framework for the Mogollon-Datil volcanic field.

Geology of the Caballo Mountains, New Mexico

“…Based on similar radiometric dates, stratigraphic position, and lithologic characteristics, McIntosh et al (1991) suggest that sev eral of the Bell Top tuffs are outflow sheets that may be correlat ed with caldera sources in distant parts of southwestern New Mexico. In particular, they believe that ash-flow tuff 5 is the dis tal part of the Kneeling Nun ash-flow tuff sheet whose source was the Emory cauldron, located in the Black Range, 50 km (31 mi) west of the Caballo Mountains.…”

“…Withington cauldrons, which had major eruptions between 36.2 and 27.4 Ma (Fig. 111; Elston et al, 1975;Seager et al, 1976;Seager, 1981;McIntosh et al, 1991). At about the same time as eruption of the Organ and Dona Ana cauldrons, flow-banded rhyolite domes, dated at 35.4 Ma, were emplaced along the north-trending Cedar Hills vent zone Mack et al, 19986).…”

“…Its source caul dron, the Emory cauldron in the Black Range, was described by Elston et al (1975) and Abitz (1986). An 40 Ar / 39 Ar age of 34.9 Ma obtained from the Kneeling Nun Tuff corresponds almost exact ly with the 34.8 Ma age of tuff 5 of the Bell Top Formation (McIntosh et al, 1990(McIntosh et al, , 1991(McIntosh et al, , 1992.…”

“…A wide array of rock types make up the forma tion regionally, including basaltic flows and flow-banded rhyo lite in addition to ash-flow tuffs and volcaniclastic sedimentary rocks. In the Sierra de las Uvas, where the most complete section is exposed, the following members (with radiometric ages) are present in ascending order: tuff 2; basalt; tuff 3, 35.7 ± 0.2 Ma; tuff 4, 35.0 ± 0.4 Ma; tuff 5, 34.8 Ma; lower sedimentary unit; tuff 6, 33.6 Ma; upper sedimentary unit; and tuff 7, 28.6 ± 0.8 Ma (McIntosh et al, 1991). Tuff 5 and younger members are general ly present in the Caballo Mountains.…”

“…The predominant date of 27.4 Ma suggests that most of the tephra in the Thurman Formation was derived from the 27.4 Ma Mount Withington caldera (McIntosh et al, 1986(McIntosh et al, , 1991, located 80 km (50 mi) north of the Caballo Mountains. Consistent with this interpretation are two lines of sedimentological evidence described by Kieling (1993).…”