Description of peritidal environments in the Plattin Group (Middle Ordovician), Missouri

The Plattin, Group (Middle Ordovician) of eastern Missouri contains, at least two horizons of conspicuously light-colored limestones which show similarities to Recent tidal flat carbonates accumulating in the Bahamas described by Hardie (1977) and his co-workers. One of these light-colored limestones occurs in the Beckett Formation and appears to be a local development. The other light-colored limestone, the Victory Member of the Hager Formation, is widespread in eastern Missouri and western Illinois. Both of these limestones are only 1 to 3 m thick in the areas examined here, often consist of one layer sharply set off from adjacent layers and weather to a distinctive white color. There are composed of ostracodal micrite or pelmicrite. These layers are interpreted to have been deposited in channel-fed, tidal flat ponds based on comparison with similar deposits on Bahamian tidal flats described by Hardie and Ginsburg (1977). The white limestones are underlain by a series of thin (3 to 10 cm) layers of several types of carbonates: wavy bedded pelletal layers with thin interlayers of dolomite, thin micrite or pelletal beds often containing sand-sized fossil debris, sometimes, with interbedded biocalcarenite layers and cross-bedded calcarenite layers. The fossil debris consists of remains of crinoids and other echinoderms, ostracodes, bryozoans, brachiopods, the coralTetradium and, less commonly calcareous algae. Small burrows, 1 to 2 mm wide, vertical or horizontal and dolomite — or calcite-filled, are present. On the basis of comparison with sediments accumulating on the Bahamian Andros Island tidal flats as described by Hardie and Ginsburg (1977), the lower Hager beds were deposited in various parts of the tidal flat including marshes, channels, and supratidal beach-ridges. Both the Beckett and Hager white beds sequences are embedded in much thicker sequences of typical Plattin wormburrowed (“fucoidal”), fossiliferous limestones which are interpreted to have been formed in a shallow subtidal setting. At the time of formation of these beds the area was located south of the Equator in the trade wind belt along the eastern flank of Ozark Island which provided essentially no sediments. Lack of evaporites in the sections suggests the climate was humid which is consistent with such a wind pattern. The sequences of intertidal and supratidal environments represented by these beds were initiated by a lowering of sea level which continued through the time of the formation of the layers underlying the white layers. Some of the subenvironments possibly originated by lateral shifting of environments as a result of accumulation of sediments. A slight rise of sea level after this submerged more of the intertidal area and allowed ponds to expand. This rise continued causing shallow seas to inundate the intertidal area permitting the younger Plattin (“fucoidal”) subtidal beds to accumulate. Examination of about 25 stratigraphic sections in the outcrop area shows that the upper white bed sequence (Hager Formation) represents a widespread event while the lower white bed sequence of the Beckett is restricted to a local area.