Effective role of petrographic evidences (microfacies), organic geochemistry and trace fossils in determination of depositional environment (an example from Pabdeh Formation, Zagros Basin, SW Iran)


- early Oligocene in age) was investigated in Zagros Basin (SW Iran). Main goal of this research was elucidating depositional environment of the formation. This research was performed based on field observations and measurements, thin section petrography, trace fossils studies and OM (TOC% and kerogen type) measurements. Utilizing this multidisiplinary approach, seven microfacies assemblages were recognized. Pabdeh Formation in studied area comprises alternation of thin to Thic limestones and dark to light gray shale beds, rich in planktonic microfauna. These facies consist of mudstone, wackstone, packstone and shale beds. Based on systematic study of the ichnofossils, five ichnogenera were recognized in two separate assemblages. The first assemblage with relatively high ichnodiversity (Arenicolites, Chondrites, Planolites and probably Neonetrites) occurs in upper unit of Pabdeh Formation. This assemblage represents the Cruzian ichnofacies, formed in a relatively high-energy condition. The second assemblage with lower ichnodiversity (Zoophycos and Chondrites) is found in middle and lower units of the formation and represents the Zoophycos ichnofacies wich were formed in a low energy and deeper condition. Vertical stacking of these facies suggests a shallowing-upward trend of the facies deposited in a distally steepend carbonate ramp, wherein the depositional setting evolved from an outer ramp with pelagic and turbidite facies (Microfacies 4B and 4C) and reducing condition to oxygen depleted waters (TOC>2.5%) that gradually changed into middle ramp where marine currents (e.g. turbidity currents) had important roles in sediment reworking and redeposition.