Sedimentary History of the Jahrum Formation, Based on Large Benthic Foraminifera (Upper Eocene), Bushehr Area, SW Iran

Abstract

In order to determine the depositional environment of the carbonate sequences in the Jahrum Formation, Upper Eocene foraminiferal assemblages of this formation in the Mount Gisakan located 10 km east of the Borazjan City (southwest Iran) and adjacent fields, were studied. Based on the foraminiferal distribution and abundances, the paleoenvironment and the sedimentary basin were reconstructed.The sediments were gradually deposited during a mega-regressive cycle. The Gradual decrease in the sea level and hence vast spread of large benthic foraminifers resulted in deposition of six sedimentary facies along the inner-middle to outer ramp system. These facies, respectively from deep to shallow parts of the basin, include argillaceous carbonates containing planktonic foraminifera that were deposited in a hemipelagic to pelagic environment (which is the transition zone from the Pabdeh to Jahrum Formation). Carbonate facies of Operculina wackestone to packstone related to the outer ramp, Discocyclina /Nummulites wackestone to packstone facies deposited in the middle/outer ramp environment, Nummulites wackestone facies occurred in the middle ramp environment, Nummulites Orbitolites wackestone to packstone belong to the middle to inner ramp, and finally the Orbitolites/milliolide packstone were deposited in the inner ramp environment.