Intercalibration of Boreal and Tethyan time scales: the magnetobiostratigraphy of the Middle Triassic and the latest Early Triassic from Spitsbergen, Arctic Norway

Abstract

An integrated biomagnetostratigraphic study of the latest Early Triassic to the upper parts of the Middle Triassic, at Milne Edwardsfjellet in central Spitsbergen, Svalbard, allows a detailed correlation of Boreal and Tethyan biostratigraphies. The biostratigraphy consists of ammonoid and palynomorph zonations, supported by conodonts, through some 234 m of succession in two adjacent sections. The magnetostratigraphy consists of 10 substantive normal—reverse polarity chrons, defined by sampling at 150 stratigraphic levels. The magnetization is carried by magnetite and an unidentified magnetic sulphide, and is difficult to fully separate from a strong present‐day‐like magnetization. The biomagnetostratigraphy from the late Olenekian (Vendomdalen Member) is supplemented by data from nearby Vikinghøgda. The early and middle Anisian has a high sedimentation rate, comprising over half the ca. 140‐m thickness of the Botneheia Formation, whereas the late Anisian and lower Ladinian is condensed into about 20 m. The two latest Boreal Ladinian ammonoid zones are absent as a result of erosional truncation below the Tschermakfjellet Formation. Correlation with Tethyan biomagnetostratigraphies shows the traditional base of the Boreal Anisian (base of the Grambergia taimyrensis Zone) precedes the base of the Anisian (using definitions based on the Dęsli Caira section in Romania). The Boreal upper Anisian Gymnotoceras rotelliforme and Frechites nevadanus ammonoid zones correlate with most of the Tethyan Pelsonian and Illyrian substages. The base Ladinian defined in the Tethyan global boundary stratotype and point (GSSP) is closely equivalent to the traditional base of the Boreal Ladinian at the Intornites oleshkoi Zone. The latest Olenekian—early Anisian magnetic polarity time scale is refined using the Spitsbergen data.

Palaeomagnetism and magnetostratigraphy of the Permian and Triassic of Spitsbergen: a review of progress and challenges

“…Palaeomagnetic data from the Triassic of Spitsbergen Hounslow, Hu et al 2008;Hounslow, Peters et al 2008) shows a greater proportion of specimens with linear-trajectory principlecomponent fitted lines than the Permian data (25% at Milne Edwardsfjellet, 36% at Vikinghøgda and 60% at Dalsnuten). This apparent difference from the Permian data of Nawrocki & Grabowski (2000) probably relates in part to differing demagnetization schemes, and magnetometers, as much as to differing intrinsic magnetic properties.…”

“…We suspect that magnetic sulphides (possibly greigite) may make a contribution to the magnetization of some of the Permo-Triassic sediments (e.g., blocking temperatures near to 300°C in some specimens [Nawrocki 1999]), although positive proof of this is currently lacking. It is clear from low-temperature measurements that pyrrhotite is not present (Nawrocki 1999;Hounslow, Hu et al 2008;Hounslow, Peters et al 2008). In some Permian samples goethite appears to make a minor contribution to the natural remanence (Nawrocki 1999).…”

“…Some 15-30% of specimens from Triassic rocks in central Spitsbergen only contain this secondary overprint magnetization, with the remainder containing evidence of a Triassic magnetization, found either by conventional principle component line fitting or great circle trajectories (Fig. 6;Hounslow, Hu et al 2008;Hounslow, Peters et al 2008). There is evidence for a lithological preference for the remagnetization, with early diagenetic calcite concretions from the Lower Triassic (Deltadalen, Lusitaniadalen and Vendomdalen members) displaying the largest proportion (ca.…”

“…7), it is probable that this component is a composite: a steeply down-directed remagnetization, with the addition of a small component of Triassic reverse. On the assumption that any secondary magnetization process will have affected both primary and ; (b) Hounslow, Hu, and Hounslow, Hu et al (2008); and (c) Hounslow, Hu et al (2008) and Hounslow, Peters et al (2008), but are presented in a different format. ᭣ reverse and normal polarity samples equally, it seems likely that the overprint directions from Triassic normal polarity specimens may be equally composite in nature (Hounslow, Peters et al 2008).…”

“…Line-fits: conventional principle component analysis derived from linear trajectory fitted-line to demagnetization data. Great circle fits: polarity interpreted according to great circle (remagnetization circle) trends (primary data from Hounslow, Hu et al 2008;Hounslow, Peters et al 2008; reformatted for this figure). N is the number of specimen data for each of the lithological categories.…”