Upper Proterozoic (1800-570 Ma) stratigraphy: A survey of lithostratigraphic, paleontological, radiochronological and magnetic correlations

Abstract

Classical stratigraphy, which was conceived in relation to the Phanerozoic, can be extended into the Precambrian, particularly the upper part. Stromatolite biostratigraphy provides natural subdivisions of the Upper Proterozoic, as indicated by studies on the Riphean of the U.S.S.R. The correlation techniques are identical to those used in the Phanerozoic. Imperfections in the biostratigraphic record, however, have encouraged the development of lithostratigraphic, radiochronological and paleomagnetic techniques. Rocks of volcanic or volcano-sedimentary origin and rocks that indicate extreme climatic conditions (glaciogenic rocks, red beds, evaporites) are widespread and easily identified; they are accordingly useful for lithostratigraphic correlation. Tillites have for a long time been considered as useful in stratigraphic correlation, but recent work has shown that many such glacial deposits are in fact slightly diachronous; this is especially true of the widespread glaciogenic deposits of the uppermost Proterozoic. Stromatolites are probably the most useful biostratigraphic tools. Progress has been made in separating the influence of the environment and that of the communities of algae and bacteria on the development of the stromatolitic structure. Biostratigraphic subdivisions based on biologically-controlled structures such as the nature of microlamination have been found to be useful in large scale correlations; in some cases, such correlations have been verified by geochronology. While such correlations are still somewhat crude (100–350 Ma subdivisions) they are probably more reliable than those based on microbiotas extracted from siliceous layers and/or shales for the greater part of Upper Proterozoic time. The possibility of dating argillaceous sedimentary rocks largely depends on the care taken in selection of the samples. The samples should have the necessary mineralogy and should not contain inherited 40Ar or 37Sr. Good results have been obtained on glauconites rick in K2O and particularly on illite and smectite, using the RbSr method. The age of a magnetized rock from which the ancient pole position has been deduced can be obtained from the apparent polar-wandering curve, but there are still many problems with regard to the meaningful application of these techniques. On the other hand, magnetostratigraphy involving documentation of the Earth's polarity inversions, is a field of study which holds much promise for the subdivision and correlation of Precambrian rock successions. Geochronology is the most precise of the four methods of correlation considered. It also provides a framework on which to hang the results of lithostratigraphy and magnetostratigraphy.