Paleomagnetism and oxide mineralogy of Upper Triassic to Lower Jurassic red beds and basalts in the Newark Basin

Paleomagnetic and petrographic results from late Carnian to Sinemurian red beds in the Newark Basin suggest that significant components of chemical remanence (CRM) were acquired within a few million years or less after deposition. The red beds are devoid of magnetite and contain several forms of clearly authigenic hematite. Hematite grains of indisputable detrital origin are rare. Following removal of Cenozoic components by thermal demagnetization, 56% of the 606 outcrop and subsurface red-bed samples showed normal polarity, 30% were reversed, and 14% were intermediate. The average remanence direction of the normal red beds (D = 7°, I = 26°) was in close agreement with the average direction of the intercalated basalts (D = 7°, I = 27°) and with published Late Triassic/Early Jurassic paleopole data. The magnetostratigraphic pattern shown by the red-bed and basalt samples is: late Carnian normal, Norian to late Rhaetian mixed polarity, and late Rhaetian to Sinemurian normal. Within the mixed polarity interval is a pattern of thick (as much as 200 m), alternating normal and reversed zones, in some places enclosing numerous thin (0.5 to 20 m), “anomalous” zones of opposite or intermediate polarity. The anomalous zones are associated with lithologic boundaries and are discontinuous along strike; they cannot represent penecontemporaneous records of the geomagnetic field at the time of deposition. Boundaries between the thick polarity zones within the mixed polarity interval, however, are not associated with lithologic boundaries and can be traced along strike for as much as 52 km; these features suggest relatively rapid CRM acquisition. The normal polarity of the late Rhaetian to Sinemurian red beds agrees well with the normal polarity of correlative basaltic rocks throughout eastern North America and Morocco. Similar correlations can be made with the Norian to late Rhaetian mixed polarity interval. An upper limit of a few million years is indicated for acquisition of CRM sufficient to establish polarity in the red beds; the actual period was probably much shorter. We conclude that the gross magnetostratigraphic pattern of the Newark Basin strata may be useful for purposes of correlation. The application of the red-bed data to tectonic reconstructions, however, requires caution. The data do not show the expected stratigraphic record of progressive plate movement, and the average remanence direction of the reversed samples (D = 191°, I = −1°) is anomalously shallow.