Subdivision of precambrian time: recommendations and suggestions by the subcommission on precambrian stratigraphy

Abstract The Subcommission on Precambrian Stratigraphy, consisting at the time of its last formal meeting in 1982 of 12 titular Members and 30 Correspondents, has been actively working toward an acceptable subdivision of Precambrian time for more than a decade. Comprehensive time-rock charts have been prepared for major Precambrian shields, and this composite record has been analysed for patterns and events common to several or to all. A geochronologic subdivision scheme has evolved that reflects these patterns of major events in Earth history, while still recognizing that those events are typically varied in intensity and timing from one region to another. Time unit boundaries have been selected so as to enclose or delimit principal cycles of sedimentation, orogeny, and magmatism, but the boundaries are defined in years, without specific reference to any bodies of rock. Several recommendations, developed sequentially at formal meetings held at intervals of 2–4 years and announced in geologic journals with calls for comment and criticism, are proposed: 1. (1) the recommendation to establish two major time units of Eon rank, Proterozoic and Archaean (Archean), separated by a time boundary at 2500 Ma, is reaffirmed; 2. (2) the provisional recommendation to subdivide the Proterozoic Eon into three units of Era rank, provisionally labelled Proterozoic I, II, and III, with time boundaries at 2500, 1600, 900, and (about) 570 Ma, respectively, is confirmed; and 3. (3) further subdivision of the Proterozoic into eight time units of Period rank, as yet unnamed, with time boundaries at 2500, 2100, 1800, 1600, 1400, 1200, 900, 700, and (about) 570 Ma, respectively, is provisionally recommended. The nature of the Subcommission's deliberations and the guidelines developed are described herein. Comment and criticism, both on the procedure followed and on the recommendations themselves are invited.