Remagnetization in bituminous limestones of the Neoproterozoic Araras Group (Amazon craton): Hydrocarbon maturation, burial diagenesis, or both?

Abstract

Neoproterozoic carbonates of the Araras Group exhibit two distinct magnetic components across the same carbonate succession in a cross-section between the Amazon craton and the Paraguay fold belt. Pink dolostones of the Mirassol d'Oeste Formation carry a dual polarity, primary component, whereas black bituminous limestones of the Guia Formation yield a secondary postfolding component. Magnetic signatures of the Guia limestones, such as high anhysteretic remanence magnetization/saturation isothermal remanence magnetization ratios, high-frequency-dependent magnetic susceptibility and contradictory Lowrie-Fuller and Cisowski tests, are typical of remagnetized carbonates. Unblocking temperatures suggest that the stable high-temperature remanence is carried by both pyrrhotite and magnetite for which an authigenic origin is suggested by scanning electron microscope observations. The different magnetic properties noted between dolostones with or without bitumen and between dolostones and limestones in the same metamorphic conditions lead to the hypothesis that the amount of hydrocarbon as well as the lithology influence nucleation of authigenic magnetic minerals in these rocks. Presence of magnetite pseudoframboids and euhedral iron sulphide crystals occurring in fracture and voids are in favor of a chemical remanence (CRM). The presence of pyrrhotite as one of the main carriers of CRM in these rocks, and its association with bitumen in fractures is probably related to an epigenetic enrichment of sulfur due to hydrocarbon seepage. However, hydrocarbon maturation solely could not explain the differences of the magnetic mineralogy observed in the craton and the fold belt. Enhanced magnetite formation in the thrust and fold belt is interpreted to be the result of higher temperatures leading to stronger diagenesis of clay minerals. Copyright 2006 by the American Geophysical Union.