Covariance of C- and O-isotopes with magnetic susceptibility as a result of burial diagenesis of sandstones and carbonates: an example from the Lower Devonian La Vid Group,Cantabrian Zone,NW Spain

The burial diagenesis of sandstones, limestones, and dolostones of the Lower Devonian La Vid Group in the Cantabrian Zone (NW Spain) reveal a covariance of carbon and oxygen isotope values with magnetic susceptibility. Also, strontium isotopes, and to a minor degree Fe, follow this trend. The main carriers of the magnetic susceptibility appear to be diagenetic Fe-carbonates, i.e., siderite, ferroan dolomite, and ankerite, which occur as cements in primary and secondary voids, as well as in fractures. In some layers, especially at the top of the succession there occurs additionally secondary Fe-chlorite and pyrite. The Fe-carbonates were formed during upward migration of a reducing, iron-bearing, petroliferous fluid that was depleted in ¹³C and carried radiogenic Sr. Similar geochemical covariance and/or correlations can be expected in other sedimentary successions affected by the migration of petroliferous formation fluids.