Biomagnetostratigraphy of the Vikinghogda Formation, Svalbard (Arctic Norway), and the geomagnetic polarity timescale for the Lower Triassic

Abstract

A bio-magnetostratigraphy for the Lower Triassic is constructed, using the ammonoid biostratigraphy from arctic Boreal successions. Combined thermal and alternating field demagnetisation determines the Triassic magnetic field polarity in 86% of specimens, with 36% showing linear trajectory line-fits and the remainder great circle trends towards the characteristic magnetisation. Mean pole directions for the Deltadalen (=50°, φ=159°, dp/dm=3.9°/5.1°), Lusitaniadalen (=56°, φ=163°, dp/dm=4.4°/5.4°) and Vendomdalen (=57°, φ=143°, dp/dm=4.4°/5.4°) members fall close to the European Lower Triassic apparent polar wander path. Mean directions for two of these member-means pass the reversal test. The remanence is predominantly carried by magnetite. The polarity stratigraphy, when integrated with the ammonoid and meager conodont data is similar to that determined from successions in the Sverdrup Basin (Canada). The Permian-Triassic boundary post-dates a pronounced palynofloral turnover, and pre-dates a short duration reverse magnetozone (LT1n.1r). In the correlated Shangsi section (in S. China) LT1n.1r occurs after the FAD of H. parvus, but in the arctic is within the Otoceras boreale Zone. The late Griesbachian to early Smithian is mostly reverse polarity, with three normal polarity intervals, overlain by mid and late Smithian normal polarity. The Spathian contains four reverse polarity intervals, the oldest one within the early Spathian with the remainder in the late Spathian. Transition into the Anisian is within the uppermost reverse magnetozone, a feature documented in other sections of this age. The polarity pattern is correlated to other marine sections, indicating the robustness of the bio-magnetostratigraphic composite and its utility in calibrating Lower Triassic time.