On the age of rocks and Precambrian time scales

Abstract

Specific type regions are needed to adequately define Precambrian time intervals whose limits are established–relative to temporally and stratigraphically ordered sequences of intrusive and stratified rocks–through the use of field relationships, fossil content, paleomagnetic characteristics, and relevant isotopic age determinations. Other Precambrian rocks are then correlated with the standard by establishing the synchroneity of geologic events as recorded by isotopic, paleomagnetic, paleontologic, tectonic, stratigraphic, and lithologic attributes of the rocks. A time-stratigraphic system comparable to that for the Phanerozoic cannot be established for the Precambrian, as individual beds of stratified sequences cannot be correlated with the same precision. The [North American] “Code of Stratigraphic Nomenclature” reflects neither this fundamental difference between Precambrian and Phanerozoic rocks nor the distinction between a general temporal ordering of both intrusive and stratified rocks and a stratigraphic ordering alone. These distinctions are basic to understanding the different requirements for establishing geologic age categories for Precambrian and Phanerozoic rocks and are manifested in the need to redefine time-rock, time-stratigraphic, magnetostratigraphic, and magnetochronologic concepts in the Code.