Cambrian paleomagnetism of the Llano Uplift, Texas

Abstract

Late Cambrian sandstones and limestones sampled from various members of the Riley and Wilberns formations of the Llano uplift show a progression of paleomagnetic pole positions as a function of age. The members, ages, and poles are the following for the Riley Formation: the Hickory Sandstone, Lower Dresbachian, 34°N, 145°E; the Cap Mountain Limestone, Dresbachian, 33°N, 140°E; the Lion Mountain Limestone, Upper Dresbachian, 24°N, 146°E. For, the Wilberns Formation they are the following: the Welge Sandstone/Morgan Creek Limestone, Lower and Middle Franconian, 24°N, 151°E; and the Point peak Shale, Upper Franconian, 6°N, 159°E. These poles are based on thermal, chemical, and alternating field demagnetizations and on vector analysis. Most directions are interpreted to be of reversed polarity, but the Cap Mountain Limestone also yielded normal polarity directions; all directions resided in hematite with blocking temperatures up to 680°C. Almost all Cambrian poles appear to fall in a broad streak between the equator at about 155°E (e.g., the poles from the Tapeats Sandstone and from the Late Cambrian Point Peak Shale of the Wilberns Formation) and about 60°N, 90°E, where Cambrian poles have been obtained by Elston and Bressler (1977) and French et al. (1977). Although, at the present time, partial to complete remagnetization or nondipole behavior of the geomagnetic field are adequate ad hoc hypotheses to explain some of the data, it is suggested that the simplest and most unifying hypothesis to explain all data involves a Cambrian loop of apparent polar wander with respect to North America. This loop occurs before the middle Late Cambrian, with the poles from the Llano uplift falling on the return track.