Depositional history of Jahrum (Zagros) and Ziyarat (Alborz) Formations based on distribution of benthic foraminifera

Foraminiferal assemblages of Paleocene to Late Eocene in carbonate rocks of the Jahrum Formation in Gisakan Mountain, 10 Km east of the Borazjan (Zagros Basin), and the Ziyarat Formation with Upper Paleocene to Middle Eocene carbonate sequences in Alborz were studied. Based on distribution and abundance of large benthic foraminifera, ancient sedimentary environment reconstructed. Gradual decreases of the sea level and hence the distribution of large benthic foraminifera resulted in deposition of six sedimentary facies in the inner, middle and outer carbonate ramp system in the Jahrum Formation. These facies, from deep to shallow parts of the basin, include argillaceous carbonates containing planktonic foraminifera that were deposited in a hemipelagic to pelagic environment. In the Jahrum Formation, planktonic mudstone and operculina wackestone-packstone belong to outer ramp; discocyclina /nummulites packstone to wackestone occure in middle to outer ramp; nummulites orbitolites wackestone to packstone occur in middle to inner ramp and finally orbitolites/bioclast packstone is belong to inner ramp. Microfacies analysis in Ziyarat Formation led to recognition of 11 microfacies that are related to 3 sub-environments such as: inner ramp, middle ramp and outer ramp. Outer ramp consists of radiolar sponge spicule wackestone and benthic foraminifera packstone; four microfacies are belong to middle ramp. These are: nummulites packstone, red algae nummulites packstone, discocyclina nummulite wackestone and nummulite discocyclina wackestone to packstone. Inner ramp consist of five microfacies of alveolina nummulite packstone, miliolide wackestone, intraclast ooid packstone to grainstone, dolomicrite and evaporates. Based on benthic foraminifer distribution and abundances, and comparison of sedimentary model, it can be concluded that Jahrum and Ziyarat Formations were deposited in a similar sedimentary conditions in a homocline platform carbonate ramp system in both Zagros and Alborz Mountain ranges.