Silica-rich septarian concretions in biogenic silica-poor sediments: A marker of hydrothermal activity at fossil hyper-extended rifted margins (Err nappe, Switzerland)

Silica-rich septarian concretions occur within Pliensbachian-Oxfordian syn-rift to early post-rift sediments of the fossil distal Adriatic rifted margin today exposed in the Err nappe (Central Alps). The concretions, ranging in size from few centimeters to about 2 m, consist of partially to completely siliceous spheroidal bodies developed within fine-grained sedimentary rock (mud to fine sand) with no or very scarce content in biogenic silica such as radiolarian tests. These concretions are characterized by a network of septarian cracks filled with chalcedony, quartz and locally, saddle dolomite. Some concretions are in their original position, whereas others are fragmentary and preserved in coarse-grained turbiditic beds, implying that concretionary growth had occurred prior to significant burial. Fluid inclusion temperatures (~150 °C), the positive δ18O values for original fluids (up to +7‰ SMOW), and the high 87Sr/86Sr values of dolomite point to a deep circulation of seawater into fault damage zones developed in granitic basement rocks. In their ascent, these hot, silica-rich fluids crossed compact sediments giving rise to quartz-dolomite veins. Lastly, when reaching the uncompacted soft sediments close to the seafloor, silica (quartz and chalcedony) locally precipitated in sediment pores and/or fissures leading to the formation of the siliceous septarian concretions. The anomalous quartz-rich mineralogy of the concretion body and of the authigenic crack-filling minerals, the occurrence in clastic sediments from deep-water distal rifted margins as well as high formation temperatures represent a new discovery enlarging the spectrum of septarian concretion types and the environment where they may grow. They provide a possible record of the subbottom products of hydrothermal venting typifying hyper-extended rifted margins where extensional detachment faults exhume crustal rocks to the seafloor and lead to an intense fluid-rock interaction and silica enrichment in the fluids.