Early-Middle Miocene Planktic Foraminifera from the Quilon Formation, Kerala

Praeorbulina glomerosa (Blow), Praeorbulina sicana (de Stefani), Globigerinoides diminutus Bolli are recorded for the first time from the Quilon Formation in a subsurface section at Quilon. These planktic foraminifera are considered as diagnostic forms of the Early-Middle Miocene. Two biostratigraphic subdivisions were established based on the detailed qualitative analysis of planktic foraminiferal assemblages. The co-occurrence of Gs. diminutus and Gs. alttapertura Bolli and the absence of Praeorbulina Olsson in the assemblages of the lower portion of the section suggest this part to be referable to Globigerinatella insueta Zone of Bolli (1966), which is of late Early Miocene age. The assemblages from the upper part of the section are characterised by the presence of Praeorbulina glomerosa, P. Sicana and Gs. diminutus, representing early Middle Miocene Praeorbulina glomerosa Zone of Srinivasan (1989). The Praeorbulina glomerosa Zone (=Zone N8) lies within the European Langhian Stage, whereas the Globigerinatella insueta Zone (=Zone N7) corresponds to the upper part of the Burdigalian Stage. Thus, the stratigraphic age of the Quilon Formation is late Early Miocene to the earliest Middle Miocene (upper Burdigalian to lower Langhian) and not Burdigalian as previously inferred by many workers.