Cretaceous ostracoda of the super family Cytheracea from Iraq, their biostratigraphy and correlation with adjacent regions

[摘要] The cretaceous ostracods of Iraq are poorly known, so the aim of the project was to study their taxonomy and to discover their value in correlation between wells in Iraq and between Iraq and neighbouring countries. The Cenomanian-Santonian ostracods of the superfamily Cytheracea were studied in detail from five wells in central and southern Iraq (East Baghdad Well-3, South Rumaila Well-104, Safawi Well-1, Ghalaisan Well-1, and Kifl Well-2). 5 families, 6 subfamilies, 28 genera, 4 subgenera and 74 species were discovered. Two new genera, Iraqicythereis and Archeocosta (the latter published by Al-Bashir and Keen in 1984 during the preparation of this thesis) and one subgenus Peloriops (Hemipeloriops) are proposed; of the 74 species described 58 are new. Four ostracod biozones and three subzones are proposed for the areas studied on the basis of the Cytheracea ostracods; these are arranged from the base to the top as follows: Veeniacythereis streblolophata - Veeniacythereis maghrebensis - Total Range Zone (A-1); Dumontina? mdaouerensis - Partial Range Zone (A-2); Veeniacythereis ibnalhaithami - Partial Range Zone (A-3); Peloriops (Peloriops) alrazii - Partial Range Zone (A-4). The Veeniacythereis streblolophata - Veeniacythereis maghrebensis - Total Range Zone is subdivided into three subzones: A - Glenocythere bahrreinensis - Total Range Subzone (A-1a), B - Soudanella? alkhansai - Assemblage Subzone (A-1b), and C Cythereis alfarazdaki - Assemblage Subzone (A-1c). The ages of the zones and subzones have been determined from a study of the diagnostic ostracod species and stratigraphic position. A Cenomanian age is assigned to A-1 zone; the A-1a and A-1b subzones are of Lower Cenomanian age; a Lower Turonian age is suggested for the A-2 zone; Upper Turonian-Coniacian for the A-3 zone; and a Santonian age for the A-4 zone. The ostracod zones and subzones have been recognised in each of the wells studied. The age of the formations studied has been determined on the basis of the ostracod fauna and stratigraphic position. The upper part of the Mauddud Formation, and the Ahmadi Formation are assigned to the Lower Cenomanian; the Rumaila Formation is placed in the Upper Cenomanian; the Mishrif Formation is placed in the Upper Cenomanian to Lower Turonian; the Kifl Formation is placed in the Lower Turonian; the Khasib Formation is placed in the Upper Turonian-Lower Coniacian; the Tanuma Formation is placed in the Upper Coniacian, although it may include strata of Santonian age in some localities; and the Sadi Formation is assigned to the Santonian. The Ahmadi/Rumaila contact in Ghalaisan Well-1 is placed at the top of the Soudanella? alkhansal subzone. The lower part of the Rumaila Formation in East Baghdad Well-3 is contemporaneous with the Ahmadi Formation on the basis of the occurrence of the Glenocythere bahreinensis Subzone, and is considered to be of Lower Cenomanian age. In Kifl Well-2 the Maotsi Formation is probably of Upper Cenomanian age and the Mahilban and Fahad Formations may be of Lower Cenomanian age. The base of the Khasib Formation is marked by the appearance of many new ostracod species and genera such as Acanthocythereis, Brachycythere, Buntonia, Paracytheridea, Protobuntonia and Phymacythereis. This indicates a major faunal break between the Mishrif/Kifl Formation below and the Khasib Formation above. This faunal break is marked by an unconformity referred to as the Aruma-Wasi unconformity which is recognised throughout the Arabian Gulf. The ostracod biozones of Iraq have been correlated with corresponding zones in other Middle Eastern and North African countries. Problems of variation in the size of some ostracods have been studied. There is a considerable range in size of Peloriops (Peloriops) ulosa, Rehacythereis arabica, Brachycythere basrahaensis sp. nov., Archeocosta alkhazwinii, Veeniacythereis maghrebensis, and Veeniacythereis ibnalhaithami sp. nov. The size range appears to be continuous, and the smaller specimens are considered to be adult. This size variation appears to exclude precocious sexual dimorphism as well as the possibility of larger and smaller forms belonging to two closely related species. It is difficult to determine whether the size variation is due to environmental or genetic causes. Intra-specific variation in ornamentation of the following species has sbeen studied: Iraqicythereis Kadisiya gen. et sp. nov., Veeniacythereis ibnalhaithami sp. nov., Cythereis? ibnyunusi sp. nov., Peloriops (Peloriops) sphaerommata, Peloriops (Hemipeloriops) djabirbnhaiyani sp. nov., and Metacytheropteron berbericus. This variation may be continuous or discontinuous. In the first case it is difficult to recognise distinct morphotypes, but in the second case it is possible to separate them into distinct morphotypes. The morphotypes appear to represent a case of stable polymorphism. The variation is believed to be genetically controlled. The environments of deposition have been determined by using the characteristic ostracod species and other fauna (e.g. foraminifera), and type of sediment. The Ahmadi Formation was deposited in an open, shallow, neritic environment with restrictions in some areas. The Rumaila Formation is distinguished by its low species diversity and poor fauna, but this is considered to be due to preservation rather than environmental conditions. Generally the Rumaila Formation represents deeper water conditions than those of the Ahmadi Formation, becoming shallower with restrictions from the sea in some areas. The Mishrif Formation indicates open shallow water neritic conditions followed by a shallowing phase associated with oscillations of sea level, or with an influx of fresh water. The Kifl Formation represents shallow water conditions partly connected with the open sea, accompanied with the development of lagoonal areas. The Khasib and the Tanuma Formations indicate open shallow marine conditions (infra-neritic), but in some areas (in East Baghdad Well-3 and Kifl Well-2) coastal conditions are suggested, perhaps with restricted access to the open sea because of the presence of a poor ostracod fauna dominated by Ovocytheridea.

[发布机构] University:University of Glasgow;Department:School of Geographical and Earth Sciences

[关键词] QE Geology