Keele Research Repository

Carbonate mud mounds from the Pennsylvanian aged San Emiliano Formation (Cantabrian Mountains, Spain) are commonly well exposed. The mounds range from 2 to 50 m in height and were observed to be primary geological features. Microfacies, ultrafacies, palaeontological and geochemical studies have revealed the composition of the mounds and surrounding carbonates.The factors and controls of mound nucleation, growth and demise have been established. 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. The mounds show evidence of deposition within shallow water environments. The formation of the mounds was controlled by a dynamic relationship between Donezellacean algae, and microscopic encrusters. Fluctuating environmental conditions lead to the alternate dominance between the two groups, resulting in accretion and stabilisation of carbonate muds. Off-mound carbonate sediments generally show more evidence for deeper, and in some cases, higher energy environment of deposition. These carbonates are generally packstones, and two distinct types occur: a Donezella dominated carbonate with oncoids, and a cyclical micrite-marl sequence which is dominated by ‘phylloid’ algae, bryozoans and corals. Comparable mounds from the Lois-Cigüera Formation, of the local Bernesga Valley, were deposited in a deeper environment. A relationship between Donezella, Claracrusta, Rothpletzella and Girvanella was not observed within the mounds of the Lois-Cigüera Formation. These mounds are compositionally different to their San Emiliano counter-parts. The San Emiliano Fm. was deposited on the sub-aerial to submarine transition on a tilted foreland basin. Carbonate and mound deposition took place within a basin during periods of heightened tectonic activity. Carbonates of the San Emiliano Fm. were originally aragonitic. Three new fossil morphotypes are presented.