Magnetostratigraphy in the Trans-Pecos volcanic field: preliminary results from the Eocene-Oligocene Vieja group
— Margaret M. Testarmata and Wulf A. Gose

The Vieja Group, which accumulated on the western edge of the Trans-Pecos volcanic field, consists of 600 m of volcaniclastic sediments subdivided by several interbedded extrusive units (fig. 1). Radiometric dates (McDowell, 1978), diagnostic mammal fossils (Wilson and others, 1968; Wilson, 1978), and geologic setting suggest that the Vieja Group represents a continuous record of deposition from 40 to 30 m.y.b.p., thereby making it ideal for a magnetostratigraphic study of latest Eocene through early Oligocene.

Initial results of this study have been published in another guidebook (Testarmata and Gose, 1979). Since then, we have more than doubled the number of samples in order to fill in data gaps and to extend the section for a firmer correlation.