The Sierra del Cuera (Cantabrian Mountains, Asturias, Spain) exhibits an exceptionally well-preserved upper Bashkirian-lower Moscovian succession of a high-relief carbonate platform with steep adjacent slope. During the late Bashkirian, the platform margin prograded rapidly basinward, and during the early Moscovian, platform aggradation increased. Fusulinida distributions have been estimated in three platform study windows representative of the Bashkirian and Moscovian lithofacies.
Microbial boundstone accumulations at the platform margin and slope contain scarce fusulinoideans (particularly during the Moscovian aggradation, when water depth was relatively greater). Boundstones are characterized by tetrataxids, lasiodiscids, pseudoammodiscids, biseriamminids, endothyrids and palaeotextulariids. Grainstone units with sediments deposited above wave base are characterized by the predominance of Pseudoendothyra, Eostaffella (Bashkirian) or Profusulinella (Moscovian), Bradyina and palaeotextulariids. The open-marine facies, below effective wave-base but intermittently affected by storm currents, is enriched in endothyrids and palaeotextulariids in the Bashkirian part, and in Profusulinella, Eofusulina and Schubertella in the Moscovian part. The Moscovian facies, of lower energy and deeper water depth, contain some microbially precipitated micrite and have scarce Fusulinida, mostly represented by endothyrids, biseriamminids, and fusulinoideans (Eostaffella, Ozawainella and Profusulinella). Lagoonal open- to restricted-marine facies with beresellid algae contain Profusulinella, Schubertella and biseriamminids. Crinoidal packstones were deposited in moderate-energy settings close to the platform margin, below wave-base, and are characterized by Ozawainella, Eostaffella, palaeotextulariids and tetrataxids.
These analyses show that fusulinid foraminifera inhabited mostly shallow and open-marine environments. Their distribution was controlled by depth-related variables, water energy and open-marine vs. restricted-marine conditions. Fusulinoideans increase in the Moscovian strata, and show different trends in the upper Bashkirian and lower Moscovian platforms, possibly because of: a) changes in the paleoecology of Fusulinida at the species level from the Bashkirian to the Moscovian; and/or b) variations in the depositional environments and physicochemical characteristics of seawater from the Bashkirian progradational phase to the Moscovian aggradational phase. Pseudoendothyra, Profusulinella, Schubertella, bradyinids and palaeotextulariids showed an affinity for high-energy settings. Profusulinella and Schubertella, however, preferred the shallowest facies and also tolerated restricted marine conditions. Eostaffella’s distribution largely differs between the Bashkirian and Moscovian parts because it characterizes the high-energy grainstones of the upper Bashkirian and the open-marine, moderate- to low-energy environments of the lower Moscovian. Endothyrids were excluded from lagoonal environments with restricted circulation and abnormal salinity. Biseriamminids preferred open-marine, moderate- to low-energy settings with muddy substrates, but were not limited by water restriction. Greater tolerance to deeper, darker habitats was exhibited by lasiodiscids, pseudoammodiscids and tetrataxids, which occurred from the platform top down to 200 m depth on the Moscovian slope.