Turonian Rudist Facies from Abu Roash area, North Western Desert, Egypt

Five radiolitid rudist species are described from the Turonian sequence of Abu Roash area. They are recognized in three rudist biostromes that occur in two informal members of Abu Roash Formation; the Rudist- and the Actaeonella-bearing limestone-marl members. The three biostromes show autochthonous and parautochthonous fabrics and moderate to high packing potential. The first rudist biostrome at the base of the Rudist-bearing limestone-marl member (Middle Turonian) contains Durania gaensis, Praeradiolites ponsianus and Bournonia fourtaui. The second biostrome in the same member consists of Bournonia roashensis. The third biostrome that recognized in the Actaeonella-bearing limestone-marl member (Late Turonian) consists of Durania arnaudi. Rudist biostromes in the Rudist-bearing limestone-marl member were deposited on subtidal rudist shoals with moderate to high energy versus that of the Actaeonella-bearing limestone-marl member that deposited in low to moderate energy on deeper part of subtidal rudist shoals. The exposed Turonian succession at Abu Roash area could be divided into three depositional sequences bounded by three sequence boundaries (paleosols and angular unconformity). The first rudist biostrome in the Rudist-bearing limestone-marl member represents the lower part of the transgressive systems tract of the first depositional sequence. The deepening upward trend of the transgressive systems tract is due to increase of accommodation space in transgressive context during relative sea-level rise episode. On the other hand, the second rudist biostrome in the Rudist-bearing limestone-marl member and the third rudist biostrome in the Actaeonella-bearing limestone-marl member are in shallowing-upward set sequence forming the highstand systems tract of the first and third depositional sequences. This indicates that, the accommodation space was being filled more rapidly than was being created during the highstand stage. (C) 2011 Elsevier Ltd. All rights reserved.