LITHOEVENTS - OOLITES IN EVENT STRATIGRAPHY

The ultimate cause for widespread oolite formation has proved to be enigmatic. A linkage to major extinctions or oceanographic-climatic factors has, however, been inferred. Such a scenario is supported from the Silurian carbonate platform succession of Gotland, Sweden. Here, oolites are exclusively linked to narrow stratigraphic intervals of major extinction events, although suitable conditions for oolite formation persisted throughout the Wenlock-Ludlow interval. Physical controls alone seem to fail in explaining suppressed skeletal carbonate production [SCP] and contemporaneous oolite deposition. A majority of ancient oolites are transgressive in origin, including at least one of the two oolites on Gotland. However, apart from a brief lag-phase, rapid sea-level rise alone, can with the present knowledge be ruled out as a cause for preventing re-establishment of SCP following platform exposure. Ecological controls, such as suppression of SCP at times of siliciclastic deposition, can similarly be ruled out as an ultimate cause, simply because many oolites are not genetically associated with siliciclastics. Hence, additional (co-occurring?) controls must be considered. On a local scale, strong loss of CO₂ from warm, shallow marine waters has shown to be of importance in promoting ooid formation. As indicated by global fluctuations in carbonate production/preservation during past extinction events [largely because of sea-level change], associated CO₂-fluxes were inferably important also on a global scale, and may together with physical/ecological controls be responsible for widespread oolite formation. Examples are given from the Silurian of Gotland, as well as from other areas and time periods. Although the precise mechanisms are still vague, it is suggested that oolites represent 'lithoevents', linking a specific sedimentary system closer to event stratigraphy. In the context of carbonate platforms as ecosystems this would be the lithological analogue to low-diversity disaster faunas. It is suggested that a re-evaluation of major Phanerozoic oolites to some extent will demonstrate their potential origin as extinction-related sediments.