Haematite pigmentation events and palaeomagnetic recording: implications from the Pilbara Print Stone, Western Australia

Abstract

SUMMARY Haematite pigment is a common constituent of sedimentary rocks, but its contribution to the natural remanent magnetization of rocks is poorly understood. Here, we describe magnetic properties of two distinct pigment types that produce a characteristic decorative ‘print stone’ foundinthe ∼2.5GaMount McRae ShaleFormation,Hamersley Province,WesternAustralia. Distinct magnetic remanence directions observed in the Print Stone can be correlated to each pigment type. By comparison with the Australian apparent polar wander path, the remanence carried by uniformly distributed pigment can be dated to ∼15–25 Ma, while two age options, the Mesoproterozoic (∼1.5 Ga) or the middle Carboniferous (∼320–310 Ma), are permissible for the remanence carried by the pigment responsible for the distinctive ‘newsprint’ pattern. Magnetic properties and demagnetization characteristics of the different pigment types overlap significantly, and thus are not predictive of the dominant remanence carrier. Magnetic characteristics of the uniformly distributed pigment vary significantly on short spatial scales. Strong local control on pigment formation raises the possibility that a primary remanent magnetization may survive locally in pockets within sedimentary red bed formations.