Iron-carbonate concretions with inverse magnetic fabrics; a record of environmental changes in the middle Eocene marine marls of the Southern Pyrenees?

Abstract

This paper deals with the detailed analyses of magnetic fabrics accompanied by stable isotopic composition and microscopic observations in centimetric and metric scale authigenic carbonate concretions embedded in the Eocene flysch deposits of the Southwestern Pyrenees south of the cleavage front. Sampling was focused in the carbonatic concretions (in both metric and centimetric scale), in the marls lateral and nearby to the concretions and in the distant marls from the concretions. The inverse magnetic fabrics (kmax axes cluster perpendicular to bedding plane) detected in the concretions constitute a fast methodology to uncover their authigenic origin. The subfabric analyses indicate that magnetite is present in all four types of samples with normal magnetic fabric (kmin axes perpendicular to bedding). Paramagnetic fabric (low temperature anisotropy of magnetic susceptibility when magnetic susceptibility increases ~3.8 times the one at room temperature) parallels the room temperature magnetic fabric. The microscope observations reveal that iron-enriched dolomite is the main carrier of the inverse fabric in the dolomitic concretions at room temperature. The stable isotopic composition reveals minor differences between the Eocene flysch and the carbonate</p><p>concretions and with previous data, suggesting a marine pore water origin due to bacterial activity during burial