Abstract

Rhythmical alternations between limestone and marls characterize the Pabdeh Formation (Paleocene–Oligocene), in Zagros basin, south–west Iran. Using petrography (microfacies analysis and SEM investigation) and geochemical parameters (elemental, XRD and stable 13C and 18O isotopes) analysis, three intervals of limestone/marl alternations in one exposed section were studied to unravel the possible mechanisms responsible for the origin of these rhythmites. Microfacies study shows alternation of carbonate microfacies (mudstone and wackestone) with marl lithofacies. The microfacies analysis reflects calm deep–water sedimentation that was interrupted by sporadic traction currents from shallow–marine. Evidence such as mixed broken and oriented shallow–marine bioclasts, detrital quartz grains with planktonic foraminifera and micritic matrix imply the traction currents, responsible for detrital transport from shallow–marine to deep–marine. The content of TiO2, SiO2 and Al2O3 (obtained from the element composition analysis) shows the difference between limestones and marls in interval 1, each following an individual trend line indicating a bimodal chemical composition and varying delivery mechanisms. However, for the intervals 2 and 3, there is no significant difference in the trend lines. The oxygen isotope signature of samples is between –5.68 to –1.01‰ and the carbon isotope signature is between –3.53 to +0.73‰. 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 shallow– water derived currents (probably turbidity currents; sensu Mohseni et al., 2011). 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 petrography parameters such as existence of the microfossils with similar preservation quality suggest the primary rhythm as a major mechanism, whereas the geochemical data do not strongly support this conclusion.