Megascopic processes reflected in the microscopic realm: sedimentary and biotic dynamics of the Middle Ordovician “orthoceratite limestone” at Kinnekulle, Sweden

The Middle Ordovician (Dapingian–middle Darriwilian) “orthoceratite limestone” is documented in its traditional type area at Kinnekulle in the province of Västergötland in southern Sweden. Detailed field studies combined with systematic qualitative and quantitative analyses of carbonate microfacies at high stratigraphic resolution show that this suite of cool-water carbonate rocks is more variable than is suggested by its overall homogeneous macroscopic appearance. Long-term changes in carbonate textures and fossil grain assemblages, together with pervasive rhythmic/cyclic patterns, suggest a strong influence from sea level on microfacies characteristics. Assessment of the results in light of regional facies patterns indicates that the cool-water “orthoceratite limestone” behaved much like “model” siliciclastic sedimentary systems, in that carbonate texture varied with depositional depth as particle size of siliclastics does. Carbonate texture thus appears to reflect absolute depth well, whereas grain assemblages record high-frequency cycles of changes in both sea level and substrate conditions. A relative sea level curve compiled from the collective data shows excellent agreement with previously published curves based on different proxies. The most important factor for the long-term establishment and regional dominance of the “orthoceratite limestone” throughout much of the Early and Middle Ordovician appears to have been a limited terrigenous sediment input to the Baltoscandian paleobasin. Hence, much of the regional facies zonation may reflect distance from weathering sources rather than bathymetric conditions.