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ABSTRACT

Silurian core samples from the central region of Saudi Arabia provide an excellent insight into the temporal evolution of acritarchs and other related forms. Based on palaeogeographical reconstructions, the area was situated in a temperate region on the eastern margin of Gondwana. The core samples have furnished the basis for site-to-site sequence correlations that made use of biostratigraphy. The composition of the microflora changes rapidly and distinctly, and the palynological composition of acritarchs and related forms reflect the nature of changes in the depositional environments immediately above the uppermost Ordovician. The uppermost Ordovician and lowermost Silurian interval is characterised by a low diversity of simple forms of acritarchs and some prasinophytes. The acritarchs recovery is marked by the appearance of innovative morphologies in the atavus-acinaces Zones above the “hot shales” interval, and distinct microfloral turnovers occurred later. Some similarities are evident between the Rhuddanian assemblages of Saudi Arabia and those of North America. By comparison with the acritarch biozonation established in the type areas of Great Britain, the precocious appearance of taxa such as Carminella, Leprotolypa gordonense, Visbysphaera, and others, occurs in this part of the northern Gondwanan margin. Characteristic Telychian species allow precise correlations with the Silurian sequences of southwestern Libya and Parnaiba Basin of Brazil. A readjustment of the age of the previously defined acritarch Biozones 6 and 7 (Le Hérissé et al., 1995) is proposed taking account of the details and stratigraphic precision revealed here by the study of additional samples.

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