The Campanian paleoenvironment: Inferences based on benthic foraminifera from northeastern Iran

Abstract

The benthic foraminifera from the Campanian rocks (Globotruncanita elevata to Globotruncana aegyptiaca zones) exposed in the Kopeh-Dagh Basin (northeast Iran) are studied quantitatively to reconstruct the Campanian paleoenvironment. Based on unconstrained heriarchial clustering, Nonmetric Multidimensional Scaling (NMDS), and analysis of similarity (ANOSIM), three statistically significant benthic foraminiferal assemblages are noted, Paralabamina lunata (lower and upper), Haplophragmoides spp., and Gyroidinoides nitida. Based on benthic foraminiferal marker species, the following paleodepths are assigned - lower P. lunata: upper bathyal; upper P. lunata: upper-middle bathyal boundary; Haplophragmoides spp. and G. nitida: outer neritic. The P. lunata Assemblage can be further subdivided into a lower Coryphostoma incrassata-dominated interval, whereas the upper interval is P. lunata-dominated, and reflects a change in the quality of food; the upper experienced higher input of fresh phytodetritus. The overall P. lunata Assemblage reflects a stable bottom water environment. The Haplophragmoides spp. Assemblage is characterized by reduced paleoproductivity and environmental instability, whereas the G. nitida Assemblage is marked by increasing oligotrophy within a well-oxygenated bottom water and ventilated basinal conditions. The Partial Least Squares regression yielded significant correlations with % Planktic (positive; proxy for sea level) and % agglutinated species (negative; proxy for shallow depths and low organic-flux); the Reduced Major Axis regression models also yielded significant negative correlations with % agglutinated and % infaunal species (nutrient availability), suggesting that these factors were responsible for the inferred benthic foraminiferal distribution patterns. Two globally comparable shallowing trends are also noted - one in the middle of the R. calcarata Zone (= Haplophragmoides spp. Assemblage) and the other in the G. aegyptiaca Zone (= Gyroidinoides nitida Assemblage) that coincide with KCa6 and KCa7 sequence boundaries and lowstands, respectively. For the KCa6 event, the occurrence of Spiroplectammina (= Haplophragmoides spp. Assemblage) represents a shallow-water outer neritic fauna invasion event within a deeper upper bathyal setting (= Paralabamina lunata Assemblage) and for the KCa7 event, a similar shallowing from upper-middle bathyal boundary to outer neritic depths is noted (= G. nitida Assemblage), and attributed to eustasy and basinal paleohighs and lows.