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AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain

Devonian Miospore Stratigraphy and Palaeogeography of the Northern Margin of Western Gondwana

Pierre Breuer1; Philippe Steemans2; Merrell Miller1

(1) Geological Technical Services Division, Saudi Aramco, Dhahran, Saudi Arabia.

(2) Paléobotanique-Paléopalynologie-Micropaléontologie, University of Liège, Liège, Belgium.

Well-preserved Devonian miospore assemblages from Saudi Arabia and North Africa allow a good correlation of the studied sections and the establishment of a biozonation for the northern margin of Western Gondwana. More than 200 miospore species, including many new species endemic to Western Gondwana, have been identified in 16 sections. Although the standard Devonian miospore zonations established in Euramerica are commonly used in most palynological studies, they are not always easily applicable to Western Gondwanan localities because of the endemic nature of the assemblages. Therefore, a new regional biozonation based on Gondwanan miospore species has been established. It consists of nine assemblage zones, eight interval zones and two acme zones, from the late Pragian to the early Frasnian. A biozonation based on the first downhole occurrence of species is also developed for oil exploration. This provisional downward biozonation consists of eight interval zones. Although it seems relatively reliable by comparison with the previously defined upward biozonation, it needs to be further tested. The review of the miospore assemblages from the literature has allowed evaluation the provincialism of assemblages on a worldwide scale for the Emsian-Givetian interval. Coefficient of similarity has been calculated between palynofloras from northern and southern Euramerica and eastern, southwestern and northwestern Gondwana. The resulting low values correspond to low to moderate similarity of miospore assemblages. The provincialism may be explained by a latitudinal climatic gradient as no significant palaeogeographic barrier is known during this time. Despite a certain degree of provincialism, floristic interchanges existed. Saudi Arabia and North Africa constituted an intermediate warm temperate region and shared taxa mainly from more arid Euramerican localities in the north, and cooler Southwestern Gondwanan localities in higher latitudes. It seems that a progressive homogenization of the vegetation took place in Middle Devonian as the standard Euramerican biozones are more easily recognized in Givetian than in Eifelian and Emsian. This transition from provincialism to cosmopolitanism during the Devonian is not only shown by palynofloras but also by the palaeogeographic distribution of many other fossil groups. It is likely due to a decrease of the latitudinal climatic gradient in Middle Devonian.