Supplemental road log: Tejana Mesa (El Porticito) to US-60 via Zuni Salt Lake Maar
— Orin J. Anderson, Spencer G. Lucas, and William A. Cobban

Tejana Mesa is capped by a late Tertiary basalt flow that has yielded a K-Ar date of 6.7 ± 0.2 Ma (Dethier et al. 1986). The main vent for this flow lies 0.2 mi north of NM-60 I, as indicated by the dramatic increase in thickness of the flow. The vent area, called El Porticito, lies on a N25° to 35°E trending chain of vents, as much as 10 mi long. New ⁴⁰Ar/³⁹Ar data indicate an age of 7.92 ± 0.20 Ma for the intrusive rock at El Porticito (McIntosh and Cather, this volume). Thus the Tejana Mesa vent complex may represent multiple episodes of eruption in late Miocene time.

At the latitude of Tejana Mesa (35°25'N), two important changes affect topography and landform. First, here, near the southern margin of the Colorado Plateau, we are also at the southern limit of continuous exposure of Cretaceous and older Mesozoic rocks. Second, from this latitude southward, Neogene volcaniclastic sediments (Fence Lake Formation) and pre-volcanic sediments of late middle Eocene age (Baca Formation) comprise most of the surface rock. In addition, the area southward is characterized by late Oligocene and Miocene basalt and andesite flows, by ignimbrites of Oligocene age, and volcaniclastic sedimentary rocks of late Eocene age.