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  • Print publication year: 1992
  • Online publication date: February 2010

2 - The Warm Mode: early Cambrian to late Ordovician

Summary

The first Warm Mode began at the termination of the late Precambrian glaciation, probably in the earliest Cambrian; definite Cambrian glacial deposits are not known. Over the approximate 100 m.y. of the early Palaeozoic Warm Mode (∼560 to ∼458 Ma) continents were mostly dispersed over the low-latitude zones and glacial deposits are lacking. This Warm Mode eventually terminated with the initiation of glaciers in North Africa in the late Ordovician (Caradocian). Major problems exist in trying to explain, first, the occurrence of Plate Cambrian to Precambrian glaciation at low latitudes, and second, the termination of the previous glaciation and the beginning of this early Palaeozoic Warm Mode.

Age and distribution of the latest Precambrian glaciation

Past research has established only a poor framework for dating and distribution of late Precambrian glacial deposits, and as a result the synchroneity versus diachronism of these deposits is not resolved. When the geochronological dates are assembled (Fig. 2.1) it is apparent that glaciation extended over at least 230 m.y. (∼800 to ∼570 Ma). These data, taken from Hambrey and Harland (1981), are of variable quality and several, included on the diagram, are based on stratigraphic estimates. Moreover, age data are limited to Africa, Asia, Australia and Europe.

The shape of the histogram in Fig. 2.1 is not obviously polymodal but it is not suitable to consider the results as a normal distribution of data about a mean value because, when considered in detail, continents have individual dating patterns. Africa for example, contributes all pre-800 Ma dates and, together with Asia, all dates younger than about 610 Ma.