Mechanisms of remanent magnetization acquisition in marl and limestone alternations. Case study: Upper Cretaceous (Chron 31–30), Sopelana, Basque Country

Abstract

The marl and limestone (M/L) alternations of the Cretaceous/Tertiary section in Sopelana in the Basque Country provide a good example of two chemical magnetizations which have been acquired at different times. A detailed study was conducted on a core which contained an unusual paleomagnetic record in the vicinity of Chron 30 R. The magnetic polarity switches far more rapidly than expected from the reversal time scale. A 10 cm sampling interval provided, on average, six samples in each M/L couplet that was identified from a curve of carbonate content. The lithostratigraphic NRM pattern is related to the lithology. The intervals (three to four M/L couplets) along which the contrast in the carbonate content between the limestone beds and the marly layers is low ( < 20%) carry the expected NRM. In contrast, the intervals (two to three M/L couplets) along which there is a large carbonate content contrast ( > 20%) are characterized, in normal polarity intervals, by the expected NRM in the limestone beds and a reverse or blurred NRM in the marly layers. The dominant magnetic carrier in this complex zone is diagenetic hematite, whereas titanomagnetite was identified below this interval. The characteristics of the detrital minerals indicate an erosion of a mature landscape (dominance of weathered ilmenite), probably related to deformation in the Pyrenees. Diagenetic hematite is characterized by two families. The first (microgranular size, only tenths of a micrometre) is present in all the samples and is interpreted as a very early chemical remanent magnetization (CRM) acquired in the oxic zone that probably extended over a few decimetres below the sediment surface. The second is restricted to the marly layers of intervals with large carbonate content contrasts in the M/L couplets. The grains in this family are larger (very fine, some micrometres in size) and carry a reverse or blurred polarity in a normal polarity zone. It is proposed that this chemical remagnetization was acquired during the circulation of oxygenated fluids. The timing for this process is at least 2 m.y. after deposition, and more likely > 7 m.y. The second diagenetic family grew at least 90 m below the seafloor, and its distribution shows that the marly layers in intervals with large contrasts in the carbonate content between the marls and the limestones were preferential drains along which circulation was favoured. This suggests that, although the porosity was still high (40–60%), the diagenetic evolution in the M/L alternations was already being expressed by different physical properties. Limestone beds and marly layers in intervals with low carbonate content contrasts in the M/L couplets were impermeable to fluid circulation and the early diagenetic CRM signal has been preserved. In contrast, marls in intervals with a large carbonate content contrast in the M/L couplets were sites of later diagenesis and their early CRM was overprinted by another magnetization.