The boundary between Cambrian and Precambrian

Generally, the Olenellus -bearing beds and associated fauna have been regarded by Russian geologists as occurring in the upper one-third of a continuous sedimentary sequence which was unfossiliferous below and which rested on crystalline basement. Presumably early fossil groups began producing carbonate skeletal material at different times and migrated so as to arrive in various areas earlier or later than elsewhere. Hence, paleontological identification of the Lower Cambrian boundary requires knowledge, now lacking, of vertical distribution of the skeleton-producing organisms, of the conditions of their emergence, and of related sedimentary facies. Previously used physical criteria for the recognition of the Cambrian-Precambrian boundary are unsatisfactory. It is suggested that this boundary be determined by absolute dating. The author reviews recent findings on geologic age, absolute age, and faunal content of many formations in the Siberian Platform and adjacent regions that bear on this problem. — C. G. Tillman.