Dolomitization and diagenesis of the lower Muschelkalk, Northeast Spain

Abstract

The Lower Muschelkalk crops out in the Catalan Coastal Ranges (the Catalan Basin) and eastern Iberian Ranges (the Valencia-Cuenca Basin) and occurs extensively in the subsurface of the Ebro Basin. The Lower Muschelkalk forms a carbonate platform that was deposited on red Buntsandstein siliciclastics and locally-developed evaporitic and calcareous shales. The Middle Muschelkalk rests with a sharp contact on the underlying Lower Muschelkalk and in contrast to it, exhibits marked regional thickness and facies variations. The Triassic of northeast Spain generally subsided during the Mesozoic and was uplifted during the early Paleogene. Tertiary subsidence was important in the Ebro Basin and the Neogene basins of the Catalan Coastal Ranges. The Lower Muschelkalk has a maximum thickness of about l(X)m in the Catalan Basin and southeastern Valencia-Cuenca Basin, but thins towards the northwest before pinching out completely. The lower part of the Lower Muschelkalk consists of peri tidal carbonates (El Brull Unit) passing upwards into bioclastic lagoonal sediments (Olesa Unit) and lagoonal to oolitic sand-belt deposits (Vilella Baixa Unit) interpreted as broadly transgressive sediments deposited on a homoclinal ramp which form a Transgressive Systems Tract (TST). The TST is overlain by locally-developed shallow subtidal to intertidal deposits and by intertidal to supra tidal dolomicrites (Lower Member and Upper Member of the Colldejou Unit). This aggradational to retrogradational package constitute a Highstand Systems Tract (HST). The Olesa Unit and Vilella Baixa Unit pinch out towards the northwest of the Valencia-Cuenca Basin and the Ebro Basin and the succession becomes dominated by peri tidal sediments. Lateral facies changes in the Olesa Unit and Vilella Baixa Unit indicate a downramp transition from northeast to southwest of the Catalan Basin. Paleokarstic surfaces are locally important in the Vilella Baixa Unit and the Lower Member of the Colldejou Unit and the underlying sediments appear to have been subjected to meteoric stabilization with early aragonite dissolution and replacement. These horizons have locally resisted later dolomitization. Subsurface paleokarsts are locally important in the Catalan Basin and may be related to Paleogene uplift and intraformational dissolution. Sulphates and evidence of former sulphates are common in the Lower Muschelkalk. Most sulphates were syndepositional but others were related to shallow-burial fabric-replacive dolomitization. Sulphate dissolution occurred in two phases; during burial diagenesis related to hydro thermal fluids; and in association with uplift into meteoric phreatic and vadose environments when dolomites suffered calcitization. Three types of dolomite are recognised in the Lower Muschelkalk: peritidal dolomite occurring in the Upper Member of the Colldejou Unit and in the peritidal facies of the El Brull Unit; fabric-replacive dolomite occurring in the TST; and baroque dolomite as a late cement and locally mosaic-forming phase. Trace element and isotope geochemistry indicates consistently greater depletion in strontium and (^18)O in the fabric-replacive dolomite than in the peritidal dolomite and differing trends in iron and manganese. These features are consistent with downwards-migrating fluids acting on contrasting precursors. The fabric-replacive dolomite and peritidal dolomite are considered to have formed as a result of Middle Muschelkalk evaporite-related brines migrating downwards through relatively homogeneous, unstable aragonite-rich peritidal sediments of the Colldejou Unit into more heterogeneous, partially stabilised calcitic limestones of the TST. Sequence stratigraphy allows easy comparison with other similar brine-reflux models of dolomitization. The baroque dolomite is geochemically distinct with depleted δ(^18)O values and high iron and manganese contents indicating precipitation at elevated temperatures from reducing fluids associated with local hydrothermal mineralization. Calcitization has locally affected the Lower Muschelkalk dolomites producing a wide variety of fabrics. Geochemistry and petrography indicate that dedolomitic calcite formed as a result of uplift- related dissolution of sulphates by near-surface, soil-influenced meteoric-derived groundwaters.