Morphological changes in planktonic foraminifera in the Garau Formation and its relation to oceanic anoxic events

Deposition of organic rich black shale during Valanginian–Turonian, nearly all over the world, has marked oceanic anoxic events (OAEs). In Lorestan zone, black shale, marl, thin bedded argillaceous limestone and fissile limestone of Garau Formation, with bitumen, are deposited in correspondence to oceanic anoxic events. These complicate events cause frequent periods of radiation, variety, evolution and extinction in planktonic foraminifera. Planktonic foraminifera experienced their first major adaptive radiation and evolution nearly with the OAES onset. The most important character is the elongation of last whorl chambers and occurrence of species with twin chambers in last whorl in planktonic foraminifera that has been interpreted as an adaptation to low oxygen levels. The study of the Garau Formation (Early Aptian-Early Cenomanian in age) in subsurface section recorded widespread morphological changes in planktonic foraminifera in correspondence to deposition of organic rich argillaceous limestone and some marl intervals.