Burial Diagenesis of the Eocene Sobrarbe Delta (Ainsa Basin, Spain) Inferred From Dolomitic Concretions

Little attention has been focused on the burial diagenesis of deltas deposited on active foreland-basin margins, where tectonics is likely to strongly impact fluid–rock interactions. A petrographic, geochemical, and microthermometric study of several fractured dolomite concretions and enclosing prodelta marls provides insights into the evolution of burial diagenesis in the Eocene Sobrarbe deltaic complex (Ainsa Basin, Spain), and more generally, on the paleohydrology of the South Pyrenean foreland basin. Shallow burial diagenesis was controlled by microbial activity in marine-derived porewaters. Microbial sulfate reduction was first responsible for the formation of pyrite and early calcite, followed by the growth of dolomite concretions during methanogenesis. Subsequent diagenesis was limited to temperatures and depth of less than approximately 75°C and 2 km, respectively. Diagenesis was recorded in porous bioturbation traces and septarian fractures found inside dolomite concretions, as well as in tectonic shear fractures. Neomorphic tabular barite, found only in the bioturbation traces, is interpreted to have formed early in marine-derived porewaters. Septarian fractures were then filled by Fe-rich calcite and centimeter-size celestine. Stable isotopes indicate that calcite probably formed in meteoric-derived waters coming from the overlying fluvial delta plain. The sulfur isotope composition of celestine is compatible with precipitation in waters of mixed parentage, but the exact origin of dissolved sulfate remains poorly constrained. In tectonic fractures, celestine precipitated coevally with calcite displaying evidence of strong fluid–rock interaction. Dissolved sulfate may have migrated to the fractures during active tectonics from the late Eocene to the Oligocene. The paragenesis and the proposed paleohydrologic model are similar to those previously described for other deltaic systems deposited in active foreland basins, including the South Pyrenean foreland basin. These features point to common diagenetic processes in syntectonic foreland-basin deltas, involving both meteoric and marine fluid sources. Similar to passive margin settings, early diagenesis appears to be controlled mainly by relative variations of sea level, whereas during further burial, the development of permeable tectonic fractures is likely to facilitate the influx of basinal or continental waters into fine slope deposits, impacting the diagenetic record. These results emphasize the importance of fracture development in the fluid-flow regime of syntectonic foreland-basin deltas. They demonstrate the necessity to take this parameter into account in fluid-flow modeling of foreland-basin margins.