THE TRIASSIC/JURASSIC BOUNDARY, COLORADO PLATEAU AREA, USA: MAGNETOSTRATIGRAPHIC CORRELATION OF THE MOENAVE FORMATION WITH THE STRATA FROM ST. AUDRIE'S BAY, UK, MOROCCO, AND NEWARK/HARTFORD BASINS

A composite magnetic polarity record for the uppermost Triassic/ lowermost Jurassic Moenave Formation, based on data from four sections in Utah and Arizona, provides a framework to tie Triassic-Jurassic sedimentation in this area to the marine St. Audrie’s Bay section, UK, and to the Atlantic rift history, including extrusive igneous rocks, preserved in Morocco and in the Newark/Hartford basins. Triassic-Jurassic fossils, including Eubrontes tracks, the conchostracans Euestheria and Bulbilimnadia, and palynomorphs, are found in the middle to upper Moenave Formation (Whitmore Point Member). Similar fossils are found in Triassic-Jurassic strata of the Newark/Hartford basins (Passaic/New Haven formations). The Moenave polarity record is characterized by mostly normal polarity, interrupted by three reverse polarity magnetozones. On the basis of available paleontologic data, we interpret the oldest reverse polarity magnetozone, M1R, found in the Dinosaur Canyon Member, to correlate with the Late Triassic magnetozones SA5n.2r or SA5n.3r of the St. Audrie’s Bay record, the oldest magnetozone in sedimentary rocks in Morocco, and with E23r of the Newark Basin. The younger two reverse magnetozones, M2r and M3r, found in the Whitmore Point Member, are correlated to the Early Jurassic magnetozones H24r and H25r of the Hartford record. Magnetostratigraphic correlations support placement of the Triassic-Jurassic boundary in the middle to upper Moenave Formation (low in the Whitmore Point Member), the Lias Group of St. Audrie’s Bay section, and in the CAMP extrusive zone in the Morocco and the Newark/Hartford records. The T-J boundary interval lies within a long normal polarity magnetozone of ~ 1.6 Ma, based on the estimated duration of magnetozone E24n/H24n, Newark Basin. Rich fossil assemblages preserved in the lower Whitmore Point Member of the Moenave Formation are typical of uppermost Triassic strata elsewhere and are therefore not Jurassic fossils, as previously interpreted. Our new chronostratigraphic interpretation of the events that straddle the T-J boundary support the initiation of CAMP volcanism, carbon isotope fluctuations, terrestrial mass extinction, and initiation of subsequent flora and faunal radiation during the latest Triassic, rather than at the T-J boundary.