Carbonate petrology of algal limestones (Lois-Ciguera Formation, Upper Carboniferous, León, Spain)

The Lois-Ciguera Formation is a unit of alternating limestones and terrigenous sediments of Lower to Upper Moscovian age in the Cantabrian Mountains of northern Spain. The proportion of limestones is fairly high, 30 to 50% of the total thickness. In the eastern part of the Lois-Ciguera Synclinorium, the formation consists almost exclusively of limestones. One section (LSW) is selected to serve as a model for the depositional and diagenetic textures of the limestones of the entire formation. More than 80% of the limestones appear to be algal-bound. Description and subdivision of these algal boundstones was possible by a modification of the classification scheme of Dunham (1962). The algal boundstones are classified as algal-bound lime mudstones, algal-bound lime wackestones and algal-bound lime packstones. Algal-bound lime wackestones and algal-bound lime packstones appear to be the most important. The first are thought to have been formed on the floor of a quiet lagoon by precipitation of algal micrite in the hairy masses of non-calcareous Algae (pseudostromata bioherms). Among the algal-bound lime packstones, three groups can be distinguished: (1) those formed by intergrowth of calcareous Algae (calcareous Alga bioherms), (2) those representing carbonate sand from littoral or lagoonal settings invaded and bound or agglutinated or entrapped by non-calcareous Algae, (3) those intermediate between groups (1) and (2). The bioherms of calcareous Algae are thought to have formed at a depth ranging between low tide level and ca. 12 m in an environment of variable turbulence. Neomorphism of algal-bound micrite is distinct from neomorphism in mechanically deposited micrite because of the interaction of pore-filling calcite in the originally porous algal micrite sustained by an organic framework. Several generations of pore-filling calcite can be distinguished. Complete filling of the pores with calcite may have occurred during an epidiagenetic interphase during syndiagenesis. There are indications that dolomitization was syndiagenetic. Both the capillary action/evapo-transpiration theory and the theory of a refluxing hypersaline brine may provide explanations which fit the conditions of formation of the LSW dolomitic limestones (dolomite content of 5 volume percent or more). The low dolomite content of 5 volume percent or less of the LSW limestones is explained by neomorphism of the originally high-magnesium algal micrite during cementation. Calcitized dolomite crystals and diagenetic silica are commonly observed together in the LSW limestones. It is shown that silicification is the cause of calcitization of the dolomite crystals. The origin of the diagenetic silica is ascribed to the ability of living algal mats to hold considerable concentrations of silica in solution in their interstitial waters. The silica is precipitated during early burial of the algal-bound sediment and goes into solution again during cementation of the limestones. Reprecipitation of the silica occurs after sharp-edged fracturing. Several phenomena of carbonate solution are described. Void creating solution is confined to limestones supported by an algal framework. At present all original pores and voids in the LSW limestones are filled with calcite and the porosity is low. The sequence of diagenetic changes has been analyzed and summarized separately for LSW limestones with an epidiagenetic interphase during syndiagenesis and those lacking an epidiagenetic interphase.