Controls on carbonate deposition and microbialite formation in distal alluvial systems (Campanian, Montalbán subbasin, NE Spain)

The middle-upper Campanian Allueva Formation was deposited in the compressional intramountain Montalbán subbasin (central Iberian Ranges, NE Spain). This i.e. formation consists of alluvial terrigenous deposits, with local dominance of carbonates and microbialites, which are the main focus of this study. Three sfacies associations are differentiated: i) bioclastic facies association including limestone levels with accumulation of gastropods and charophytes, along with polygenetic carbonate breccias; ii) microbialite facies association dominated by irregular limestone beds rich in oncoids; iii) terrigenous facies association including metric levels of reddish-brown mudstone with intercalations of sandstones and conglomerates. These facies deposited in a low water level and short residence lacustrine-palustrine environments, in the distal areas of an alluvial system. Isotopic data (δ13C and δ18O) fall in the range of freshwater carbonates and support this interpretation. The most common microbialites are oncoids, which have been classified into six types based on their morphology, size, and lamination. Cyclic patterns in the lamination of oncoids have been related to changing wet and dry conditions. Our results suggest that the alluvial setting was mainly fed by meteoric waters, with a sub-humid climate gradually transitioning to more arid conditions.