LATE TRIASSIC MAGNETOSTRATIGRAPHIC CORRELATIONS FROM MARINE TO NONMARINE: ARE THEY RELIABLE?

The magnetostratigraphy of the Upper Triassic portion of the Newark Supergroup in the Newark basin of New Jersey has provided a standard to which recent workers have attempted to correlate magnetostratigraphic sections of Upper Triassic marine strata in Tethys (Turkey, Sicily, Austria and Slovenia). These correlations are fraught with problems, mostly because the marine sections contain fewer magnetochrons (about 50%) than does the presumed age-equivalent interval of the Newark. Furthermore, these correlations have abandoned the only well documented biostratigraphic datum in the Newark that allows a correlation to marine strata-the Carnian-Norian boundary, which is at about the base of the Passaic Formation based on nonmarine-marine cross correlation of palynostratigraphy and vertebrate biostratigraphy. Thus, the proposed Tethys-Newark magnetostratigraphic correlations lack an independent biostratigraphic datum by which to correlate, so they are of questionable reliability. Furthermore, the Tethys marine sections are thin (30-140 m of limestone represent late Carnian and most or all of Norian time) in comparison to the more than 4-km-thick Newark section. Therefore, it is not surprising that the Tethys marine sections yield a magnetostratigraphic record that does not directly correspond, in both reversal frequency and pattern, to the Newark section. Also, the Tethys sections are largely in condensed strata (Hallstatt limestone) or calciturbidites, and their age is determined by conodonts, which are readily reworked. Recently proposed magnetostratigraphic correlations of the Tethys-Newark records have been used to advocate major changes in the Triassic timescale (such as a 7 million year long Rhaetian, or an ~ 230 Ma Norian base). However, these conclusions are questionable because of the unreliability of the magnetostratigraphic correlations upon which they are based.