Relative Ages of Fossil Caches, Synchroneity of Major Lithology Change, and Formational Age, as Determined by Magnetostratigraphic Correlation

Abstract

During deposition of the Lower and Middle Triassic Moenkopi Formation, a history of the reversals of the geomagnetic field was recorded. Twelve polarity reversals have been documented from a study of about 20 sections; this work established the existence of a network of polarity reversals across the 500-km depositional basin. This established network was used to test the relative synchroneity of the numerous vertebrate occurrences and the synchroneity of a major lithologic change from mudstone to sandstone. Magnetic polarity reversed in close proximity to the lithologic change. Seven magnetostratigraphic sections containing vertebrate localities were investigated near this boundary. Samples were stepwise thermally demagnetized to 690C. The polarity boundary was found in all sections, just as predicted by the existing polarity framework and consistently a short distance below the lithologic change. This result indicates that the onset of deposition of a predominantly sandstone lithology (the Holbrook member) was a rapid and therefore relatively synchronous event over much of this part of the basin. The seven fossil horizons were found not be synchronous with respect to the polarity change. However, it appears that their existences may have been grouped into three separate time intervals. The entire Moenkopi Formation polarity sequence can bemore » matched to those in other Early Triassic formations having good marine fossil control, particularly the Canadian Arctic stratotype sections and the south China Permo-Triassic boundary sections. The correlation indicates that deposition of the Moenkopi Formation began in the late Griesbachian, continuing relatively uninterrupted except for short hiatuses into the early Middle Traissic.« less