Silurian

Abstract

The 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

Process regime change on a Silurian siliciclastic shelf: controlling influences on deposition of the Gray Sandstone Formation, Pembrokeshire, UK

“…Brachiopod faunas from high in the Coralliferous Formation indicate a lower Sheinwoodian riccartonensis Biozone age (Siveter 2000). To the east, carbonate-rich mudrocks and bioherms accumulated across the Midland Platform (Bassett et al 1992;Ratcliffe and Thomas 1999;Siveter 2000;Cherns et al 2006).…”

“…The Welsh Basin ultimately became oversupplied with sediment and shoaled in a diachronous manner (Allen 1985a), with the transition into the continental Old Red Sandstone (ORS) being of Ludlow age in southern Pembrokeshire (Walmsley and Bassett 1976;Hurst et al 1978;Hillier and Williams 2004) to mid-Přídolí in the east (Bassett et al 1992). Sediment was possibly locally sourced from the southern Avalonian hinterland Pretannia (Cope and Bassett 1987), or from uplift created by Laurentian/ Avalonian collision to the west (Soper and Woodcock 1990;Woodcock et al 1996).…”

Early Telychian changes in graptoloid diversity and sea level

“…Examination of the palaeogeographical maps of the Welsh Basin for the Llandovery (Bassett et al 1992) offers a clue to the possible cause of the basin's unusual oxicity record. The Welsh Basin during the Llandovery was semi-enclosed, with the Midland Platform to the east, Pretannia to the south and the Irish Sea Landmass to the west, this running subparallel to the passive margin of the Iapetus Ocean.…”

Depositional environment and sequence architecture of the Silurian Coralliferous Group, Southern Pembrokeshire, UK

“…The Coralliferous Group was deposited predominantly within shallow marine environments on the southern margin of an important early Palaeozoic depocentre, the Welsh Basin ( Figure 1; Bassett 1982;Siveter et al 1989). The basin lay on the northwestern margin of the Eastern Avalonia microplate (Bassett et al 1992). Figure 2.…”

“…It is uncertain whether the fluvial system was antecedent to the topography, or whether the route taken to the basin proper was of a tortuous nature with topographic steerage around tilted fault blocks and their associated relay ramps. As the study area is approximately 10-15 km south of the palaeogeographic shelf break (Bassett et al 1992), it seems likely that this significant basinward shift in sedimentary environments defines a Type 1 sequence boundary (Van Wagoner et al 1988), with sealevel dropping below the palaeogeographic offlap break (possibly being defined by the Benton Horst to the north). The fluvial and corresponding deep-water facies thus constitute a lowstand system tract (Van Wagoner et al 1988).…”