A novel population of composite mounds: their initiation, growth and demise. San Emiliano Formation, Cantabrian Mountains, Spain

Abstract

Purpose: Mounds from the Pennsylvanian aged San Emiliano Formation (Cantabrian Mountains, Spain) are commonly well exposed. These mounds range from 2 to 50 m in height and are observed to be primary geological features. The mounds are described and classified and the factors and controls of mound nucleation, growth and demise have been established. Methods: Microfacies analysis of 177 thin sections has revealed the composition of the mounds and surrounding carbonates. Results: Composite mounds, exhibiting characteristic components of both Cluster mounds and Agglutinated Microbial mounds are described. The mounds are skeletal-microbial/pack-wackestones. Peloidal, homogenous and clotted micrites are the main sedimentological constituents of the mounds. Microfossils are dominant with Donezella, Claracrusta, Rothpletzella and Girvanella being common. Small foraminifera, bryozoans, corals and algae are all present within the mounds, but are more common within off-mound carbonates. Conclusions: The formation of the mounds was controlled by a dynamic relationship between Donezellacean algae, and microscopic encrusters, a bio-mechanism not observed in mud mounds elsewhere. Fluctuating environmental conditions lead to the alternate dominance between the two groups, resulting in accretion and stabilisation of carbonate muds. These mounds are compositionally different to their Pennsylvanian counterparts.