Responses of benthic foraminifera to environmental variability: A case from the Middle Jurassic of the Kachchh Basin (Western India)

At the western margin of the Indian plate, the Jurassic sedimentary succession of the Kachchh Basin provides well developed exposures for fauna-based studies. Based on a quantitative analyses of 67 samples spanning Middle Bathonian-Late Callovian interval, the paleoenvironment of the Jumara section (the depocenter of the Kachchh basin), is inferred. Four benthic foraminiferal assemblages are recognized by both Clustering and NMDS ordination methods. These assemblages vary in biotic traits such as life-habit and diversity as well as in abiotic traits such as sediment type, nutrient availability, and oxygen level. The Bathonian Spirillina polygyrata assemblage that dominates an outer neritic oligotrophic setting, has a preference for calcareous substrates. In the earliest Callovian, the Epistomina mosquensis assemblage replaced the latter, as oxic conditions decreased and terrigenous influx increased. Two successive and less diverse assemblages had a preference for non-calcareous substrates and dominated the mid-Early-Late Callovian landscape. These are the oxic Lenticulina subalata assemblage (inner to middle neritic oligotrophic setting) in the mid Early to mid-Middle Callovian and the dysoxic Reophax metensis assemblage (mesotrophic to eutrophic middle neritic setting) in the late-Middle to Late Callovian. Linear regression models suggested that sea level, oxygenation, and sediment type are the main abiotic factors controlling the distribution of the fauna. Moreover, taxa with specific biotic traits such as shell composition (calcareous vs agglutinated), and selection strategy (r vs k strategy) occupied different environmental settings. Furthermore, diversity and epifaunal/infaunal ratio had a cyclic pattern, comparable to those of third order sea-level fluctuations.