Ooids and oncoids in a Middle Cambrian sandstones from Närke, Sweden

In the lowermost part of the Middle Cambrian in Narke, a thin sandstone bed between the lower Cambrian Lingulid Sandstone and the Oelandicus Shale represents the initial stage of a marine transgression. The bed contains several layers with white to grey ooids, and small oncoids, mainly of phosphatic composition. The layers are cemented by calcite or phosphate. This investigation examines the development of the coated grains with respect to mineralogy, chemical composition, ultrastructures and size distribution. Three kinds of ooids could be distinguished: clay ooids, phosphatic ooids and calcite ooids. The first kind appears in the bottom of the sequence and is composed of a kaolinite type mineral and Fe-chamosite. Phosphatic ooids occur in all layers and their composition varies from almost pure francolite to mixtures of francolite with charnosite or calcite. The calcite seems to be the result of diagenetic replacement, in many cases primary phos- phatic ooids have been completely converted to secondary calcite ooids. The oncoids occur only in the lower part of the section. They have a nucleus of clay or cemented quartz grains, and the cortex is composed of phosphate.