Geomagnetic polarity in the early Cretaceous and Jurassic

Abstract

Knowledge of the polarity history of the Earth’s magnetic field during the Mesozoic stems primarily from oceanic (M-sequence) magnetic anomalies. Certain of these anomalies have been biostratigraphically dated from sediments immediately overlying basaltic basement at Deep Sea Drilling Project (D. S. D. P.) sites within the M-sequence. The biostratigraphic control is such that these age estimates are generally imprecise, and moreover the sediment ages represent only minimum ages for the basaltic basement itself. Magnetostratigraphy in sedimentary land sections provides improved age estimates for magnetozones, which can then be correlated with M-sequence anomalies. Magnetostratigraphy provides not only enhanced dating of the ocean floor and resolution of the behaviour of the geomagnetic field during the Cretaceous and Jurassic oceanic ‘quiet zones’, but also a world-wide correlation tool for sedimentary stratigraphy. The middle Cretaceous is characterized by a long period of normal polarity extending from the early Aptian to the early Campanian. Reversed magnetozones corresponding to oceanic anomalies M-0, M-1 and M-3 have been palaeontologically dated as early Aptian, middle Barremian and early Barremian respectively. Magnetozones corresponding to oceanic anomalies M-5 to M -15 are Hauterivian to Valanginian in age but have not been individually associated with stage boundaries, mainly due to poor biostratigraphic control in Italian pelagic limestone sections of this age. However, magnetozones corresponding to oceanic anomalies M -16 to M-22 have been individually correlated to Lower Berriasian and Tithonian palaeontological zones, and extrapolation of this correlation gives an age for anomaly M-25 close to the Oxfordian-Kimmeridgian boundary. The Jurassic 'quiet zone’ in the oceans is recognized in land sections as an extended period of predominantly normal polarity, and although poor biostratigraphic control in this interval limits our knowledge of its duration, we consider that it extended through the Oxfordian and Callovian stages. The Bathonian and older Jurassic stages are characterized by mixed polarity. A precise correspondence of magnetozones to ammonite zonations and stage boundaries is only available for the Aalenian to Pliensbachian interval, where the frequency of reversal was apparently very high.