The Margin of the Mishrif Platform, Rumaila Field, Southern Iraq
The Rumaila Field lies on the SW margin of an elongated (100×500km) NW to SE trending Cenomanian/Turononian shallow carbonate platform. The platform was surrounded by intra-platform basins on both sides. The thickness changes between the East and West margin suggest differences in platform geometry and composition. The lower part of the Mishrif Formation (mB) is composed of a progradational ramp margin grainstone shoal formed from echinoderm debris and rudist fragments. The southern extent of the carbonate platform margin might have been controlled by a regional structural lineament, whilst the western margin was controlled by progradation into the Najaf intrashelf basin. Over time, the mB platform became more aggradational and developed tidal channels and an extensive lagoon in the lee of the platform margin. Large, elongated shoals within in the lagoon might have formed as part of interplay between tidal currents and prevailing winds. A major karstified sequence boundary at the top of the mB suggests a significant drop in relative sea level. After renewed flooding with the deposition of mudstones containing planktonic foraminifera, shallow carbonates of the mA developed in a backstepped position. The mA is a very different carbonate platform from the underlying mB and is composed of coral boundstones leading up to intertidal and lacustrine mudstones. The mA is overlain by palaeosols and a major karst surface. The OAE2 is recorded in the lower part of the mA by a positive δ13C excursion which might have influenced the observed change in biota. The thickness of the Mishrif over the Rumaila Field is significantly less than on the east-side of the platform (more than 2 times). Part of the thickness change might be explained by the diachroneity of the base of the Mishrif across the platform, the more agradational character on the east-side, and differences in subsidence rates. Information from published material from surrounding fields suggests that deposition might have started earlier on the eastern-side of the platform.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017