Abstract

Three Lower Jurassic volcanic sequences are present in the Deerfield, Hartford, and Newark basins of eastern North America. Volcanics from the Deerfield and Newark basins display similar magnetic directions that also agree with magnetizations from associated Lower Jurassic intrusives. The intermediate volcanic sequence, including the Deerfield basalt, the Holyoke basalt, and the base of the Second Watchung basalt, yields anomalous directions that differ by more than 30° from the nominal Early Jurassic direction. These anomalous directions record a short-lived geomagnetic secular variation event that provides a chronostratigraphic marker. The simplest interpretation of the paleomagnetic and geochemical data is a one-to-one correspondence between successive volcanic units of the Hartford and Newark basins, a correlation that agrees with a previous scheme based upon geochemical inferences. Our data suggest that initiation of the second eruptive phase was synchronous over the 300 km from the Deerfield Basin to the Newark Basin. Reevaluation of the widely quoted N1 Early Jurassic paleomagnetic pole for North America indicates that the 72 virtual geomagnetic poles from which it was originally derived are not all independent observations. The N1 reference pole probably represents 7 independent samples of the geomagnetic field, each of which averages at best a small amount of geomagnetic secular variation. As a consequence, the N1 paleomagnetic pole may not represent the time-averaged paleofield.

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