1991 Articles

Magnetostratigraphy and paleomagnetic poles from Late Triassic-earliest Jurassic strata of the Newark basin

Witte, William K.; Kent, Dennis V.; Olsen, Paul E.

The Newark basin contains a 7-km-thick sedimentary section, which spans approximately 25 m.y. of the Late Triassic and earliest Jurassic (middle Carnian to Hettangian). Previously paleomagnetic study of the Newark red beds has demonstrated that complete progressive thermal demagnetization can effectively isolate a high-temperature characteristic magnetization. In the lower Newark strata (middle Carnian to early Norian in age), this magnetization yielded a pole position at 54°N, 102°E, and in the upper Newark strata (Hettangian in age) a pole position at 55°N, 95°E. Results from 23 new sites in the middle and upper Norian red beds of the Passaic Formation, including 15 sites from the Jacksonwald region that yield a positive fold test, fill the temporal gap between our two prior studies and yield a pole position at 56°N, 95°E (A_95 = 4.4°). These results from the middle Newark confirm that North American apparent polar wander was very slow (~0.2°/m.y.) during the Late Triassic through earliest Jurassic. These new sites define reversed and normal polarity magnetozones that are stratigraphically consistent with and extend our previous results. The Newark reversed and normal polarity characteristic magnetizations form a correlatable pattern of 12 magnetozones that are stratigraphically coherent throughout the basin with respect to independent lithostratigraphic marker units that reflect synchronous, basinwide variations in water depth. Temporal calibration of the Newark magnetostratigraphy on the basis of biostratigraphy, radiometric age determinations, and Milankovich-driven cyclostratigraphy indicates that geomagnetic polarity was reversed 70% of the time and that the mean polarity duration was 2 m.y. or less during the Late Triassic and earliest Jurassic.