Microfacies, geochemical characters and possible mechanism of rhythmic deposition of the Pabdeh Formation in SE Ilam (SW Iran)

Rhythmical alternations between limestone and marls characterize the Pabdeh Formation, southwestern Iran. Three intervals of these rhythmites were studied using sedimentary, petrography and geochemical parameters analysis, to unravel the possible mechanisms responsible for the origin of these rhythmites. The microfacies analysis reflects calm deepwater sedimentation that were interrupted by sporadic traction currents from shallowmarine. The geochemical analysis indicates the primary difference between limestones and marls in interval 1. However, for the intervals 2 and 3, there is no significant difference in the geochemical results between intercalated lithologies. The isotope data (δ18O and δ13C) for limestones and marl rhythms compared with Eocene marine calcite show post depositional alteration. Limestone–marl alternations in the interval 1, therefore, originated from the cyclic changes in siliciclastic input by shallowwater derived currents (probably turbidity currents). However, for the intervals 2 and 3, although the field observations (such as extensive lateral continuity of individual beds and sharp contact between different lithologies) and some of the sedimentological parameters such as existence of the microfossils with similar preservation quality, suggest the primary rhythm as a major mechanism, the geochemical data do not strongly support this conclusion.