Microfossils associated with silicified Stratifera undata Komar 1966 from the late Proterozoic Bambuí Group, south-central Brazil

Abstract

Abstract Small, partly silicified, laterally linked stromatolites exhibiting a high degree of inheritance in the shape of successively stacked laminae from the lower Bambui Group (late Proterozoic), near Unai, Minas Gerais, south-central Brazil, are here described and classified as Stratifera undata Komar 1966. Rare, moderately preserved microfossils and stratiform concentrations of poorly preserved probable microfossils consist essentially of coccoidal forms, there being no convincing evidence of filamentous forms. If, in fact, coccoidal micro-organisms were the overwhelmingly dominant mat-formers, this represents an unusual situation among silicified microflorules from morphologically distinct Precambrian stromatolites yet is quite similar to that observed in another form of Stratifera ( S. biwabikensis from the Gunflint Iron-Formation), which, like the Unai stromatolites, also formed under permanently submerged conditions. S. undata is now known in Brazil from similar upper Proterozoic settings in two very widely separated localities, which suggests its potential use in regional stratigraphic correlation. As a result of this study, we recommend that greater attention be paid to the ‘simpler’ stromatolite morphologies, especially when micro fossiliferous, since such forms commonly comprise the basal portions of biostratigraphically significant stromatolites and may share a common microstructure with these same, more complex forms. Inasmuch as microstructure is generally acknowledged as the stromatolite property most closely controlled by biological factors, study of silicified microflorules within simpler forms may permit inferences regarding not only the microbial communities responsible for more complex stromatolites having the same microstructure but also possible biological reasons for the succession of distinct stromatolite assemblages observed in upper Precambrian rocks.