BIOSTRATIGRAPHY AND PALEOGEOGRAPHIC AFFINITIES OF EARLY SILURIAN CHITINOZOANS FROM CENTRAL SAUDI-ARABIA

Palynological investigations carried out on subsurface material have d emonstrated the occurrence of rich and well diversified Llandovery chi tinozoan assemblages in the Qusaiba and Sharawra members of the Qaliba h Formation in Saudi Arabia. Both core and cuttings samples from wells located in the central part of Saudi Arabia have been investigated. T he biozonation and stratigraphic correlations proposed here are exclus ively based on core material. Eight local chitinozoan biozones are dis tinguished within the investigated Llandovery subsurface strata. Becau se of significant contamination due to caving in the cuttings samples these are only used to control the extension of the biozones in the in tervals devoid of core samples. Several species of the most abundantly recorded chitinozoans in the Early Silurian of Saudi Arabia are typic al components of contemporaneous chitinozoan assemblages from the subs urface of Libya, Algeria and to a lesser extent from outcrops in north western Spain (e.g. Plectochitina paraguayensis, Conochitina edjelensi s, C. alargada, C. elongata, Pterochitina deichai, Belonechitina parav itrea). In addition to these typical North Gondwanan species, a few fa irly ubiquitous forms are helpful for inter-palaeocontinental correlat ions (e.g. Spinachitina S. agilis, S. maennili). The relationship exis ting between North Gondwanan chitinozoan assemblages and the Baltic on es are quantified for Aeronian time by calculating their respective co efficient of similarity. A great similarity exists between North Gondw ana and Saudi Arabia chitinozoan populations from the Middle Llandover y. On the other hand, moderate to fairly low similarities are noticed with the contemporaneous chitinozoan populations from Baltica. Such re sults are consistent with the existence of a wide mid-European Rheic O cean implying a remote latitudinal position between Northern Gondwana and Baltica and consequently contrasting climatic conditions prevailin g on both margins of this ocean. So far, no typical Afro-Brazilian chi tinozoan species (9.8. Pogonochitina djalmai) have been recorded in th e Saudi assemblages. Although data on the Llandovery chitinozoan assem blages from Asia are still limited, some affinities, at the specific l evel, between the Saudi and the Chinese material are observed.