Coral reef diagenesis records pore-fluid evolution and paleohydrology of a siliciclastic basin margin succession (Eocene South Pyrenean foreland basin, northeastern Spain)

Abstract

An integrated field, petrographic, and geochemical study has determined the fluid-rock interaction history in part of an Eocene reef complex on the margins of the siliciclastic-dominated Catalan sector of the South Pyrenean foreland basin. Results show that at least four distinct fluid systems influenced the basin margin, and demonstrate the sensitivity of reef rocks as paleohydrological archives in siliciclastic environments. The earliest calcite cements precipitated from meteoric waters at shallow burial depths, and mineralogical stabilization of reef carbonate was mostly completed during this episode. Textural and isotope trends typical of paleo-exposure surfaces are lacking, and trace element results suggest that meteoric waters were transmitted laterally and/or upward into the reef via delta-lobe sandstone paleoaquifers. The distribution of late cements is highly variable and fracture-fed, demonstrating the importance of deformation in controlling fluid flow subsequent to reef lithification. Isotopic and elemental compositions of burial calcites, plus their paragenetic association with barite and dolomite, show that fluids of marine parentage were expelled from compacting basinal marls into the basin margin paleoaquifers. The Calders reef also records an episode of extrabasinal fluid circulation, probably associated with faulting of the basin margin. Ferroan saddle dolomite contains fluid inclusions with elevated homogenization temperatures, indicating thermal disequilibrium with the surrounding strata. Hydrothermal activity was possibly related to Neogene extensional tectonics. The final cement stage in the reef records the influence of telogenetic meteoric water, possibly reflecting the present-day basin margin hydrology.