Genesis of phosphatic hardgrounds in the Lower Ordovician of northern Öland, Sweden

The Lower Ordovician limestone sequence at Hälludden, northern Öland, contains phosphatic hardgrounds and phosphatic constituents such as clasts, envelopes, ooids and moulds. The phosphate accumulated in a shallow epicontinental sea with extremely low relief, and is thus difficult to explain genetically by any actualistic model, e.g. the upwelling model. An alternative model, proposed for Cambrian phosphate deposits (which accumulated under similar conditions) in northern Queensland, Australia, is applicable also to the sequence at Hälludder. The model involves the deposition of blankets of organic material on semi-emergent to emergent pavements. Diagenetic processes in the organic material resulted in the release and precipitation of phosphate. Based on this model for phosphogenesis, it is possible to infer a cyclic sequence of events which could have produced the phosphatic surfaces at Hälludden. These cycles reflect small fluctuations in sea level, with conditions ranging from submergent to semi-emergent or emergent.