A model of early calcite cementation in alluvial fans: Evidence from the Burdigalian sandstones and limestones of the Vallès-Penedès half-graben (NE Spain)

The Valles-Penedes half-graben developed during a Neogene extensive period as part of the Catalan Coastal Ranges in the northwestern edge of the Valencia Trough. The Neogene deposits of the Valles-Penedes half-graben consist of, from base to top, three lithostratigraphic complexes: i) a lower continental complex of Aquitanian?–early Langhian age; ii) a continental to marine complex with reefal carbonate platforms of Langhian age and; iii) an upper continental complex of middle Serravallian–Tortonian age. This study focuses on the calcite cements of the lower continental complex consisting of red beds (mudstones, sandstones and conglomerates) and lacustrine limestones deposited in alluvial fan environments. The studied materials are cemented by calcite precipitated from meteoric waters that circulated through the sediments during the early diagenesis at shallow burial depths. The calcite cement was studied by means of petrographic, cathodoluminescence, microprobe and δ18O and δ13C isotopic analysis. The petrographic and geochemical results show that these cements vary according to the sedimentary environment: (i) sandstones deposited in proximal alluvial fan environments cemented by calcite with low contents of Mg, Fe, Mn, Sr and Na, δ13C values of − 7,4‰PDB and δ18O values from − 6.5 to − 6.2‰PDB; (ii) sandstones sedimented in medium to distal alluvial fan environments cemented by Mn-rich and Mg, Fe, Sr and Na-poor calcite with δ13C values from − 7.9 to − 6.9‰PDB and δ18O values from − 10.5 to − 8.6‰PDB; and (iii) lacustrine limestones of distal alluvial fan environments cemented by Fe-rich calcite with variable contents of Mn and low contents of Mg, Sr and Na. The distribution of these calcite cements allows us to propose a model of fluid circulation and early calcite cementation within an alluvial fan indicating spatial and temporal variations in chemical composition of meteoric water during migration from proximal to distal alluvial fan environments. Residence time of waters, Eh–pH conditions, and the degree of sediment–water interactions, influenced by the mineral composition of the host sediments, are the most important parameters controlling variations in calcite cement composition.