MAGNETOASTROCHRONOLOGY OF THE TRIASSIC-JURASSIC BOUNDARY INTERVAL AND THE TEMPO OF CAMP VOLCANISM

An extended interval of strong normal polarity bias interval or even a superchron have often appeared in the earliest Jurassic in compilations of paleomagnetic data and seem to coincide with the CAMP large igneous province. In the Newark continental rift basin, the last Triassic geomagnetic polarity reversal (the younger end of subchron E23r, which at ~10 k.y. appears to the shortest Late Triassic polarity interval) occurs only ~40 k.y. prior to the Tr-J event level, which is followed within ~40 k.y. by the first CAMP lava. The succeeding normal polarity chron E24n extends over the entire 1000 m thickness of CAMP lavas and interbedded sediments, an interval of about 600 k.y. in duration in this section, but the top of the magnetozone was not found in the abbreviated section overlying the lavas. In the nearby Hartford basin there are more than 2000 m of cyclical sediments of the lower Portland Fm. overlying the CAMP igneous extrusive zone. We have established that the homologous lavas and interbedded sediments are also of uniformly normal polarity (chron H24n) and what we believe are the first reverse polarity intervals of the Jurassic occur about 1000 m above the youngest CAMP lavas. Although unusually thick, magnetozone H24n (correlative with E24n) is approximately 1.6 m.y. long according to cycle stratigraphy. This duration is not unusually long in the context of the Late Triassic geomagnetic polarity time scale, which has several polarity intervals that are at least as long (e.g., chrons E11r and E6n). The magnetocyclostratigraphic data thus show that indications of an early Jurassic long normal interval are most probably due to the propagation of dating errors on CAMP lavas, which are almost universally of normal polarity.