Age, facies, and geometry of the Sandbian/Katian (Upper Ordovician) pelmatozoan-bryozoan-receptaculitid reefs of the Vasalemma Formation, northern Estonia

摘要

The Vasalemma Formation (early Katian, Late Ordovician) of northern Estonia consists of a succession of biodetrital grainstones up to 15 m thick with numerous intercalated reef bodies, which reach diameters of more than 50 m. Four dominant facies types are distinguished within the reef core limestones: (1) a bryozoan framestone—bindstone, (2) an echinoderm bindstone, (3) a receptaculitidbryozoan-microbial framestone, and (4) a tabulate bafflestone. A linking theme between the different reef-core limestones is the presence of clotted microbial bindstone, which in some places contains spicules. Except for the tabulate bafflestone, all facies types occur in the youngest and oldest intervals of reef growth. Generally, a tendency can be observed with a dominance of echinodermframestone low in the formation and at the base of individual reefs, towards a more receptaculitid dominated facies at the top of the formation. The reefs developed in a narrow, ca. 20-km-long and max. 5-km-wide band on a shallow NE-SW-directed platform in the central part of the North Estonian Confacies Belt. Reef growth can be constrained toward the latest Keila age, representing the rising limb and the peak interval of the Guttenberg Isotopic Carbon Excursion (GICE). Reef termination falls within a second-order sea-level lowstand, the Frognerkilen Lowstand Event, which led to partial subaerial exposure of the reefs. The dead reefs subsequently and rapidly drowned during the Nakkholm Drowning Event at the Oandu/Rakvere Stage. This timing is nearly equivalent to a phase of enhanced reef development elsewhere in Baltica and probably is related to locally increased nutrient availability during the GICE interval.