The geomagnetic polarity timescale for the Triassic: linkage to stage boundary definitions

Abstract

AbstractStudies of Triassic magnetostratigraphy began in the 1960s, with focus on poorly fossilferous nonmarine red-beds. Construction of the Triassic geomagnetic polarity timescale was not consolidated until the 1990s, when access to magnetometers of sufficient sensitivity became widely available to measure specimens from marine successions. The biostratigraphically-calibrated magnetostratigraphy for the Lower Triassic is currently largely based on ammonoid zonations from Boreal successions. Exceptions are the Permian–Triassic and Olenekian–Anisian boundaries, which have more extensive magnetostratigraphic studies calibrated by conodont zonations. Extensive magnetostratigraphic studies of nonmarine Lower Triassic successions allow a validation and cross-calibration of the marine-based ages into some nonmarine successions. The Middle Triassic magnetostratigraphic timescale is strongly age-constrained by conodont and ammonoid zonations from multiple Tethyan carbonate successions, the conclusions of which are supported by detailed work on several nonmarine Anisian successions. The mid Carnian is the only extensive interval in the Triassic in which biostratigraphic-based age calibration of the magnetostratigraphy is not well resolved. Problems remain with the Norian and early Rhaetian in properly constraining the magnetostratigraphic correlation between the well-validated nonmarine successions, such as the Newark Supergroup, and the marine-section-based polarity timescale. The highest time-resolution available from magnetozone correlations should be about 20–30 ka, with an average magnetozone duration of c. 240 ka, for the Lower and Middle Triassic, and about twice this for the Upper Triassic.