Silurian

Abstract

AbstractThe history of establishment of the Silurian System (and hence of the Silurian Period) and its principal divisions has been reviewed by Cocks et al. (1971) and Holland (1984, 1989a). The System itself, three of the four Series into which it is divided, and the Stages within these three, are all based upon localities in Britain. The names of all the divisions shown in Fig. I have been ratified by the International Union of Geological Sciences. The two major divisions, Lower Silurian and Upper Silurian, remain informal, but they are useful when degree of precision in correlation does not allow reference at the level of Series (Holland 1989a). The boundary stratotypes are all described in Holland & Bassett (1989). Graptolite Biozones have proved especially useful for international correlation in the Silurian and the sequence of Biozones used in Britain is shown in Fig. 1. Correlation of the Biozones wth the global Standard Series and Stages is shown approximately, but is not known with exactitude in all cases.Geodetically the position of the British Isles in the Silurian remained astride the 25°S latitude, a position it had reached in the later Ordovician (Briden et al. 1973); movementwas confined to rotation. During this time the south magnetic pole wandered, in terms of present geography, from near to southwest Africa, to the Horn of Africa in early Silurian times, then back along the equator to west Africa/Brazil at the end of the Silurian.The Llandovery Epoch was chracterized by marine transgressions. Thus in