Coated grains in Lower Viruan limestones in Västergötland, central Sweden

The transition from Lower (Oelandian) to Middle (Viruan) Ordovician at Hällekis and Gullhögen in Västergötland is associated with breaks marked by discontinuity surfaces. At the base of the Viru series, various kinds of coated grains have been observed: ooids of chamosite, phosphate and limonite, pisoids, micro-oncoids, and quartz grains with a black phosphatic film. At Gullhögen, ooids having mostly a well preserved chamosite lamination occur, but many have been affected by oxidation so that the chamosite is now partly replaced by quartz and hematite. In many of the grains previously described as hematite ooids the lamination is irregularly wavy, resembling oncoids; these are here termed micro-oncoids. At Hällckis, chamosite and phosphate ooids occur together in a thin black layer of phosphorite just above the break at the boundary between the Lower and Middle Ordovician. The phosphorite is terminated by a second discontinuity surface, covered by a thin oolitic layer which contains poorly sorted chamosite ooids. Also, these ooids have been diagenetically altered in various ways, but they have not been oxidized as those at Gullhögen. The ooids and pisoids in the grey clay-rich calcarenite, up to about 0.2 m above the boundary, have a rather complex mineralogical composition. The nucleus consists of a phosphatic shell fragment, stained yellow by goethite, but the cortex is chamositic.