Magnetostratigraphy of a late oligocene volcanic sequence, durango, sierra madre occidental, northern México

Abstract

Paleomagnetic studies were carried out on a Late Oligocene volcanic sequence exposed in the Sierra Madre Occidental, west of Durango City, Mexico. Volcanic stratigraphy identified five reversals (six polarity periods) of the Earth's magnetic field as well as paleosecular variations recorded in the flows. Characteristic directions of remanent magnetization were obtained at seventeen sites. Emplacement of sorne units was very closely spaced in time, and other units may be composites of severa! different flows. An ignimbrite not previously recognized in geologic mapping and geochronological studies was identified below the oldest unit by means of paleomagnetic criteria, as its polarity was opposite to the unit on top.
Comparisons between the paleomagnetic polarity sequences of two sections, sampled sorne 30 km apart, shows a different polarity for the units below the Santa María Member of the Río Chico Formation, oldest unit correlatable in both areas. This substantiates previous lithologic correlations and suggests that the pre-Santa María sequence in the two areas was produced by different volcanic events.