Lower Ordovician ooids from northern Öland, Sweden

In the Lower Ordovician of N. Öland, Sweden, several beds contain ooids. Five localities have been examined, Hälludden, Hagudden, Byrum, Horns Udde, and Gillberga. The chemical and mineralogical composition of the ooids differ between beds and between localities. At Hälludden and Horns Udde, the ooids are mainly composed of francolite, with nuclei of glauconite or skeletal grains, whereas at Hagudden both phosphatic and chamositic ooids occur. At Byrum, several beds of chamositic ooids have been discovered, but also one thin bed containing both hematitic and geothitic ooids. At Gillberga, only scattered phosphatic ooids have been found. Most ooids contain two or more mineral fractions, including calcite. This paper describes their mineralogy, ultrastructure, size variation and distribution in the sections. The origin of the ooids is also discussed. It is suggested that the francolite ooids are primary, and that the ferriferous ooids might have a polygenetic origin.