Aeronian and early Telychian chitinozoans from Central Saudi Arabia

Abstract

Chitinozoan occurrences from three shallow boreholes penetrating the lower Silurian Qusaiba Member in central Saudi Arabia have been calibrated against graptolitebased age control. The age of the graptolite-bearing portion of the Qusaiba Member in the Qusaiba Depression is Aeronian and assigned to the Lituigraptus convolutus Biozone. Four chitinozoan assemblages were recognized from convolutus Biozone strata. The fifth assemblage, characterized by a new species of Linochitina, was from above the highest graptolite-bearing level in the Qusaiba Depression borehole. The Qusaiba Member in the Baq’a area ranges in age from the convolutus Biozone to the early Telychian middle part of the turriculatus Biozone sl. The Baq’a Qusaiba succession is significant in that the Angochitina hemeri and A. macclurei biozone boundary is identified in core for the first time. Angochitina hemeri Paris and Al-Hajri 1995 occurs with graptolites indicating the ?convolutus Biozone to just below the First Appearance Datum (FAD) of Spirograptus guerichi Biozone graptolites. The FAD of A. macclurei Paris and Al-Hajri 1995 defines the biozone base and occurs five feet below the FAD of guerichi Biozone graptolites indicating a revised level at, or near, the Aeronian-Telychian boundary. The Last Appearance Datum of A. macclurei is just below the highest recognized occurrence of turriculatus Biozone sl graptolites. A younger Telychian chitinozoan-bearing interval without graptolites occurs in the upper part of the Qusaiba Member. The macclurei Biozone is here succeeded by Tanuchitina obtusa (Taugourdeau and de Jekhowsky 1960) and is followed by Euconochitina silurica (Taugourdeau 1963). They are considered to be Telychian based on the absence of Margachitina margaritana (Eisenack 1937) in the succession. In addition to calibration of chitinozoan ranges with those of graptolites, five new local subzones were recognized.