The pre-Pleistocene Phanerozoic time-scale — a review

Abstract

The data in The Phanerozoic Time-scale (Geological Society, 1964) are reviewed, together with data relevant to the Phanerozoic time-scale published up to mid-1968. Quaternary problems are not considered, while the Tertiary is considered to be on a comparatively well-established basis. The use of glauconite ages to establish any part of the time-scale prior to the Oligocene is not to be recommended, as early glauconites can provide anomalously high apparent ages in addition to the occurrence of low ages. No criteria for distinguishing anomalously high or low glauconite ages have yet been established. The definition of inter-system and within-system boundaries in the Mesozoic provides an acute problem, to which there is at present no solution. Reconsideration of the Cretaceous data shows reasons for preferring 95 m.y. as the age of the base of the Cenomanian, but neither of the boundaries of the Jurassic are clearly defined. New data suggest that there should be an increase in the accepted age of the base of the Triassic to 235 ± 5 m.y., but this is in part dependent upon a firm knowledge of the decay rate of ⁸⁷Rb. The current six per cent range in commonly-used values of this constant is now of over-riding importance in causing uncertainty throughout the Palaeozoic scale; Rb-Sr determinations now provide the majority of critical Palaeozoic ages. Even without this uncertainty, increases in the age of the Westphalian, the base of the Carboniferous (to 360 m.y.) and the base of the Devonian (to ?405 m.y.) seem to be indicated by new data (using the 4·7 × 10¹⁰ y half-life for ⁸⁷Rb: if 5·0 × 10¹⁰ y proves correct, or any other figure higher than 4·7, then there will be correspondingly greater increases in the age of these boundaries). The Devonian data are particularly tantalizing: imprecision in stratigraphical correlations is allied to spreads in radiometric ages from individual complexes to cause considerable uncertainty. There are still no useful Silurian data, nor is any part of the Ordovician other than the Caradocian dated at all. The Caradocian data can be questioned and, if a half-life of ⁸⁷Rb > 4·7 × 10¹⁰ y. is adopted, would need reconsideration. No part of the Cambrian is satisfactorily dated, but its base (as far as it can be defined palaeontologically) appears to be now controlled by North American data as being at not more than 570 ± 10 m.y. (4·7 × 10¹⁰ y ⁸⁷Rb half-life), confirming the most recent estimates.