Depositional environment, diagenesis, and geochemistry of Devonian Bahram formation carbonates, Eastern Iran

Abstract

The Middle-Upper Devonian Bahram Formation carbonates in eastern Iran were investigated in three stratigraphic sections including Cheshmeh Shir (NE Tabas), Kamar Kuh (S Kashmar), and Hozedorah (E Tabas), in order to define their depositional environment, diagenesis, and geochemistry of major and minor elements. Petrographic and microfacies pattern studies led to the recognition of five microfacies groups including shore face, tidal flat, lagoonal, shoal and open-marine facies. Evidences such as absence of calciturbidite deposits, reefal facies, slumping and sliding facies, gradual facies changes, absence of the special grains of carbonate shelf such as cortoid, oncoid, pisoid, and aggregate and abundant micrites indicate that the Bahram Formation carbonates deposited on a ramp type carbonate platform, shallower southward. The most important diagenetic processes which affected the Bahram Formation carbonates are micritization, cementation, dolomitization, compaction, dissolution, fracturing, and calcitic veins formation. Cementation, dissolution, and dolomitization are the main diagenetic processes that affected the original texture. The abundant presence of bioclasts such as ecinoid, ostracod, and brachiopod together with geochemical analysis results such as low strontium content, low average of strontium/sodium ratio, high average of strontium/manganese ratio and low sodium content suggest that the primary mineralogy of carbonates were mainly calcitic. Paleoclimate conditions at the time of deposition seem to have been similar to climate of recent temperate regions. Investigation of the Middle-Late Devonian geographic maps reveals that the studied region which is located 30°S had been a part of Gondwana land so that geochemical results confirm this fact (Scotese 2014). In addition, the diagenetic and geochemical changes are indicators of long term effect of meteoric diagenetic fluids on the Bahram carbonates. So, it seems that the effect of meteoric diagenesis in north of the basin is greater than the south of it.