Abstract

Dr G. E. Williams writes: In Dr D. H. Tarling's most interesting paper and the ensuing general discussion (Tarling 1974), the apparent contradiction of glacial varves—that is, of significant seasonal inequability of climate—near the late Precambrian equator seems to have escaped notice. From such climatic conflict may emerge a better understanding of the enigmatic late Precambrian glacial climate.

An outstanding feature of the late Precambrian glacial climate, as revealed by numerous sequences throughout the world, is its remarkable inequability. Evidence for marked climatic inequability of seasonal and longer periodicity in late Precambrian time includes:

(1) The characteristic association of tillites with dolomites and limestones, locally stromatolitic and oolitic, apparently of warm-water origin (e.g. Martin 1965, Dunn et al. 1971, Spencer 1971, Young 1972).

(2) The association of tillites in southern Africa (Martin 1965), South Australia (Dalgarno & Johnson 1965, Daily & Forbes 1969) and North America (Gabrielse 1967, Cloud 1971) with major sedimentary iron-formations. Since no evidence of contemporaneous volcanism is exhibited by the African and Australian sequences in question, the ultimate source of the abundant iron probably was ferruginous regoliths comparable to modern tropical laterites.

(3) The common occurrence of varves and varve-like laminae in argillites (for example, Martin 1965, Thomson 1969, Spencer 1971, Binda & Van Eden 1972), carbonate rocks (Dow & Gemuts 1969, Spencer 1971, Roberts et al. 1972) and iron-formations (Martin 1965, Daily & Forbes 1969), strongly suggesting the influence of seasonal inequability of climate in all sedimentary environments. Such climatic inequability is further emphasized

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