Basis for chronostratigraphic classification of the Precambrian

Abstract

It is urged that the same stratigraphic principles and the same basis for chronostratigraphic classification be used in the Precambrian as in other parts of the earth's stratigraphic column, although relative emphasis on different methods of age determination and time-correlation may reasonably vary, depending on their applicability in different parts of the column. The definition of the boundaries of each chronostratigraphic unit should lie in the rock strata themselves. The standard time-scope of each unit should be delimited by an upper and a lower type boundary point (boundary-stratotype), each in a specifically designated and identified stratigraphic section, chosen at a position to give the unit the greatest significance and to allow the best use of criteria of age and time-correlation in extending the boundary geographically as widely as possible away from the type at as nearly an isochronous position as possible. Schemes for dividing Precambrian time on the basis of clustering of radiometric age dates, or arbitrarily by equal time intervals in hundreds of millions of years, are commendable and may be very useful for current mapping, for indicating general age relationships, or for other purposes; but there are cautions to be considered, and such schemes should not be allowed to replace or impede efforts at classification procedures tied more closely to the rocks themselves. Regional or worldwide classification of the Precambrian should desirably rest on a firm foundation of local chronostratigraphy, starting preferably in areas where thick, relatively continuous, relatively undeformed, and relatively unmetamorphosed sequences of Precambrian strata are present. The Precambrian—Cambrian boundary should not be defined by such generalizations as “at the lowest occurrence of organized fossils” or “at the base of shelly fossils” which give boundaries that are continuously subject to change and obviously will vary in age from place to place. Rather, the definition should refer to a boundary-stratotype, which desirably may coincide in the type area with these or any other criteria that will help in the correlation of the boundary as an isochronous horizon worldwide, but will still preserve for it a stable standard. The term “Archeozoic”, although not replacing the much more widely used “Precambrian”, still may be revived usefully as a term significant of the stage of life development, paralleling Cenozoic, Mesozoic, and Paleozoic, and including all strata older than Cambrian, since we cannot now deny the possible existence of life as far back in time as the age of the oldest known rocks.