Response of larger foraminifera to mixed carbonate-siliciclastic environments: an example from the Oligocene-Miocene sequence of Kutch, India

The late Oligocene-early Miocene mixed carbonate-siliciclastic deposits of Kutch developed on the stable shelf of a peri-cratonic rift basin. It hosts a number of larger foraminiferal genera. The depositional environment of the sequence is interpreted and carbonate microfacies analysis has been carried out. The environmental and microfacies distributions of different genera and their response to energy conditions and terrigenous influx are examined. It is interpreted that the sequence was deposited in a wide range of environments, including back-reef lagoon, shallow subtidal seas, intertidal, swamps, tidal channels and supratidal. Larger foraminifera are recorded from the lagoon and shallow subtidal part of the sequence. Heterostegina and Spiroclypeus are found to occur in lagoon while Miogypsina, Lepidocyclina and Operculina occur in both lagoon and shallow subtidal environments. Archaias and Sorites prefer subtidal conditions. The distribution of different genera in relation to carbonate microfacies varies. Miogypsina, Archaias and Sorites occur in more microfacies than Heterostegina, Spiroclypeus, Operculina and Lepidocyclina. Larger foraminifera also differ in their susceptibility to clastic influx in the environment. Spiroclypeus and Sorites prefer a carbonate-rich environment while Miogypsina is most tolerant to terrigenous input. The genera Heterostegina and Spiroclypeus are found to be restricted to low energy environments but Miogypsina, Lepidocyclina, Operculina, Archaias and Sorites can tolerate agitated water conditions. It is interpreted that Heterostegina and Spiroclypeus are extreme K-strategists while Miogypsina tends toward r-mode opportunists. Between the two ends of the spectrum lie Lepidocyclina and Operculina. The life history of Archaias and Sorites is possibly comparable to Miogypsina.