THE DEMISE STAGE OF RUDIST BEARING MISHRIF FORMATION (LATE CENOMANIAN – EARLY TURONIAN), SOUTHERN IRAQ

Abstract

The Cretaceous carbonates are of unique significance in the stratigraphic succession of the Middle East, because much of the discovered prolific carbonate reservoir facies are in the rudist bearing formations. The Late Cretaceous carbonates of the Mishrif Formation are one of these prolific reservoir facies, which extends along the southern parts of Iraq and the Arabian Gulf. Five subsurface sections were selected from wells of three fields in south and southeastern parts of Iraq, including West Qurna 21, 57 and 215, Tuba 4 and Nasiriyah 2, to determine the radiation and the demise of the rudists in the Mishrif Formation. This study showed that the rudists of the Mishrif Formation were affected in two stages of demise. The first (Fd) took place during the early Late Cenomanian, recorded by a drastic decline in the abundant occurrence of the recumbent rudists (Caprinidae and Ichthysarcolitidae), which together represent Cenomanian species of the Mishrif Formation. Consequently, the followed Cenomanian – Turonian transition was not only a significant crisis in the abundance of the rudists, but also the benthic foraminifera were affected. The second demise of the rudists was recorded by the disappearance of elevator Hippuritidae and Radiolitidae; represented by these species: Durania araudi, Sauvagesia sharpie, Praeradiolites echennesis, Praeradiolites sarladensis and benthic foraminifera, specially Miliolids, with abundance of planktonic foraminifera of the Khasib Formation, this demise took place during the late Early Turonian.