Quantitative data on the carbonate constituents of the early Viruan (‘Middle Ordovician’) Segerstad Limestone of Sweden are given, based on modal ana lyses of thin sections. The grain size of the sediment transgressed the boundary between carbonate mud mixed with skeletal sand (micritic calcarenite), and skeletal sand mixed with carbonate mud (calcarenitic calcilutite). Increase in grain size is largely due to the increase in the abundance of echinoderm particles.
The term ostraconzass is introduced for the total mass of the skeletal material produced by the organisms in a given area at a given time. The composition of the skeletal sand is completely different from that of the macrofossils in the same beds. The originally aragonitic grains constitute only one to ten per cent of the skeletal sand. This supports suggestions that during the early Palaeozoic the aragonite-calcite ratio of the ostracomass was low. Attention is drawn to some observations which indicate that the aragonite-calcite ratio of skeletal sand in the modern temperate marine shallow-water environment may be considerably lower than in tropical and subtropical seas.