The paleomagnetism of red beds and basalts of the Hettangian Extrusive Zone, Newark Basin, New Jersey

Abstract

A high unblocking temperature component of magnetization residing in hematite was isolated in 11 outcrop sites and 4 boreholes from Hettangian sedimentary rocks interbedded with the igneous extrusives of the Newark basin. This normal polarity characteristic magnetization has a mean direction (declination = 6.3°, inclination = 12.9° after tilt correction) which is significantly shallower (by about 15°) than reported magnetization directions from the coeval basalts. Based on our demagnetization analysis of representative basalt samples, we attribute the difference between the sedimentary and basaltic directions to contamination of some of the basalt magnetizations by a steeper overprint. An intermediate unblocking temperature magnetization recovered from the extrusive zone sedimentary rocks, consistent with evidence elsewhere in the Newark basin for a pervasive hydrothermal and remagnetization event at about 175 Ma, is a likely cause of the basalt contamination. The characteristic magnetization from the Hettangian sedimentary rocks yields a paleopole (55.5°N, 94.6°E, dp = 5.2°, dm = 5.4°) which lies on the westbound, precusp portion of current paleomagnetic Euler pole apparent polar wander (APW) paths, in contrast with the Newark (and Hartford) basin basalt reference pole positions (such as the “Newark trend” N1 pole) which lie on the eastbound, postcusp portion of the APW path. A precusp pole position is more compatible with the relative age of the Hettangian extrusive zone pole with respect to the Sinemurian/Pliensbachian poles which define the cusp but implies large changes in the rate of APW in the Late Triassic and Early Jurassic.