Concretionary cementation of a Scottish Middle Jurassic sandstone by hot, Paleocene fluids: a clumped isotope study

Abstract

This study focuses on new clumped isotope data from concretionary calcite cements in the Middle Jurassic Valtos Sandstone Formation (Great Estuarine Group) of the Inner Hebrides. Clumped isotopes show that concretion cementation began at 45 ± 6°C increasing to temperatures in excess of 70°C before cooling slightly to 57 ± 7°C at the concretion margin. In the early stages of cementation, calculated δ18OFLUID values were c. −12‰ VSMOW, identical to an estimate of Paleocene Hebridean meteoric water based on hydrothermal reactions close to Paleocene igneous centres. These δ18OFLUID values imply that concretion cementation started in the Paleocene probably during the earliest stages of phreato-magmatic effusive igneous activity. As the concretion grew, temperature changes were accompanied by progressively evolving δ18OFLUID compositions up to +2.1 ± 1.1‰ VMOW. These evolving δ18OFLUID compositions were caused by isotope exchange reactions between18O-rich lithologies and hot basinal fluids migrating upward along faults. This fluid flow was driven by progressive crustal loading from the thickening Paleocene lava pile, which also caused sandstone compaction. Published radiometric dates that constrain the emplacement time of the Skye Lava Group, and its subsequent rapid erosion, suggest that concretion formation and final compaction was completed in no more than 2.6 myr, far more rapidly than modelled previously. Initial concretion growth that predates development of volcanic topography shows that the strongly negative compositions of Hebridean Paleocene meteoric water are mainly of latitudinal rather than orographic origin.