Paleosecular variation during the Kiaman Superchron: a 1150 year record from glacial varves of the Seaham Formation, New South Wales

Abstract

Glacial varves of the Seaham Formation at Abercrombie Quarry in eastern New South Wales, Australia, preserve a reverse-polarity characteristic remanence acquired at high paleolatitude during the early stages of the Kiaman Superchron, the longest known interval of constant geomagnetic polarity. Normal polarity overprinting, including the pervasive Cretaceous overprint commonly observed on the southeastern Australian seaboard, is present but does not obscure the primary magnetisation. Ferrimagnetic grains are in the pseudo-single domain size range, confirming the reliability of the varves as paleo-remanence recorders. Rip-up clasts maintain the original direction of the chararacteristic remanence despite their random orientation following redeposition, a form of conglomerate test that restricts the timing of remanence acquisition to within at most a few years of original sedimentation. Characteristic remanence directions from 11 sites spanning 1150 years of annual layers move through a simple, systematic cycle, resembling patterns of secular variation observed for the modern geomagnetic field. Mean virtual geomagnetic poles (VGPs) calculated for the sites trace part of a secular variation cycle around the Upper Carboniferous paleomagnetic pole for the region. The mean of site poles at this locality is at longitude 132.8°E, latitude 31.1°S, with A ₉₅ = 21.3°, although this pole does not span sufficient secular variation to represent a true paleomagnetic pole. The angular standard deviation of the site VGPs exceeds both the standard Model G phenomenological description of the modern field and the Time-Averaged Field Initiative results for comparable latitudes, and appears to agree with the original interpretation of non-suppressed paleosecular variation from late Kiaman, low latitude sites in France. Large scatter in the Abercrombie Quarry poles supports the interpretation that the primary (dipole) family dominates the geomagnetic field during a superchron, and that this condition was already established within the first 11 million years of its 52 million year extent.

Key Words:

• Kiaman Superchron
• paleosecular variation
• Seaham Formation
• varves
• geomagnetic field
• Carboniferous

Acknowledgements

We thank Mr Ian Eggleton for allowing access to the Abercrombie Quarry. We also thank two reviewers for their helpful suggestions. This paper is published with the permission of the Director, Geological Survey of New South Wales, Industry & Investment NSW.