The biostratigraphic potential of Precambrian stromatolites

Abstract

The morphology of stromatolites is controlled partly by the biological composition of algal mats and partly by the direct action of environmental factors. Both trapping of detrital grains and precipitation of carbonate in algal mats are important processes in stromatolite formation. Precambrian stromatolites were probably not subject to the environmental restrictions of Recent stromatolites. Stromatolites can be classified on the basis of a number of morphological features, some of which relate directly to the biological characteristics of algal mats (lamina shape and microstructure) and some of which have at least partial environmental control (e.g., column shape, branching style). The taxonomic significance of characters may vary in different taxa. A binomial (but not biological) form nomenclature is applied, using “groups” (analogous to genera) and “forms” (analogous to species). The biostratigraphic potential of Precambrian stromatolites, theoretically possible from what is now understood of the controls of Recent stromatolite morphology, can only be tested empirically by determining the time ranges of defined taxa. This relies heavily on other means of dating and correlation. Initial results of stromatolite biostratigraphy in Australia proved encouraging and appeared to confirm the applicability of the biostratigraphic scheme worked by Russian geologists. However, none of the stromatolite assemblages described from Australia is identical to any from the U.S.S.R.: none contains all of the elements described from the Russian assemblages, and most of the common elements are comparable only at group level. New data suggest that the time ranges of several groups are longer than previously accepted, and caution is therefore urged in using group level identifications alone as a basis of correlation. Identity of forms is a better criterion for correlation, but most forms have a limited geographic distribution. Further systematic description and identification of stromatolites from rock sequences of all ages on all continents are needed to establish the actual time ranges of taxa and assemblages of taxa, and hence their biostratigraphic applicability. Stromatolite assemblages presently known from various Proterozoic basins of Australia are discussed. Pre-Riphean assemblages include groups previously known only from the Riphean, as well as groups not found in the younger rocks. Partial equivalents of each of the subdivisions of the Riphean occur in a number of basins, but their stromatolite assemblages are not yet well known. Late Riphean equivalents have the best documented assemblages and the groups present are generally consistent with their time ranges determined in the U.S.S.R. However, the Burra Group (Adelaidean) of South Australia lacks typical Late Riphean stromatolites and was originally correlated with the Middle Riphean. New radiometric and stromatolite data suggest that deposition of the whole Adelaidean sequence commenced during the Late Riphean.