Early Silurian (Llandovery) Graptolite Biozonation of Saudi Arabia

H. Boukhamsin¹, M. Williams², and J. Zalasiewicz²

Abstract

Analysis of Silurian graptolite assemblages from over 1000 sample horizons in 132 cores (from 65 wells) through the Qusaiba Member, Qalibah Formation of Saudi Arabia, provides a refined graptolite biostratigraphy for the Arabian Plate comparable in its resolution to that from the British Isles and Czech Republic. Over 116 graptolite species characterize ten biozones from the earliest Rhuddanian ascensus-acuminatus biozone to the early Telychian guerichi biozone, and also provide subzonal resolution through several stratigraphic intervals. Graptolite biozonal boundaries and the Rhuddanian-Aeronian and Aeronian-Telychian Stage boundaries can be placed with high precision. Detailed palaeogeographical analysis of graptolite distribution, coupled with associated fauna and sedimentological data, suggests persistent depth-stratified assemblages across the Silurian palaeo-Arabian Plate. Shallow marine assemblages occupied the surface ‘mixed-layer’ of Silurian oceans, enabling them to colonize shallow shelf environments. These assemblages are low-diversity (one to two species), being characterized by species of Neodiplograptus, Neolagarograptus and Stimulograptus: often these graptolites are preserved in strata with extensive shelly fauna. Deeper marine graptolite assemblages are higher diversity (with up to 15 species per horizon), and are typified by helically coiled graptolites such as Spirograptus and Torquigraptus. Recognition of depth stratified graptolite assemblages provides an important tool for establishing palaeo-basin topography and water depth at the time of sedimentary deposition.

AAPG Search and Discovery Article #90188 ©GEO-2014, 11th Middle East Geosciences Conference and Exhibition, 10-12 March 2014, Manama, Bahrain