Primary results of sedimetological research on the upper Jurassic to lower Cretaceous carbonate rocks in NW Zagros Mountains, Iran

The upper Jurassic-lower Cretaceous carbonate extensively distributed in Zagros Mountains (southern Iran) is correlated to hydrocarbon reservoir rocks, however has not been studied with sedimentological aspects. The studied section of about 1000 m thick exposed in Kuh-d-Yaghma (Aligdaz Province) mainly consists of shallow marine facies exhibiting sedimentary structures, such as paleosols, paleokarsts, biostromes, and stromatolites. The section was subdivided into nine units based on the results of observation of outcrops and thin sections. Depositional ages were estimated by fossil occurrences and strontium stable isotope.
Units 1 and 2 represent subaerial-meteoric diagenetic structures (paleosols. paleokarsts, and banded cements) and dolomite of a mixed-water origin. Originally, the dolostone was highly-permeable coarse-grained sediment, in which dolomitization selectively subjected. These diagenetic processes may have been associated with global sea-level low or a regional tectonic event during Kimmeridgian, and formed lithified substrate, which is suitable for sedentary organisms. Unit 3 abundantly yields potential reef-building organisms, such as stromatoporoids, corals, and calcareous algae. However, the dominant constituents are broken uniserial branching stromatoporoids, their constructions should be regarded as biostromes and did not form reef framework. Fossil association and strontium isotopic ratio indicate that this unit was deposited in Tithonian. Units 4～7 are alternations of two deeper and two shallower facies. The deeper units (units 4 and 6) mainly consists of micritic limestone with biofacies characterized by ostracodes, bryozoans, and sponge spicules. The shallower units (units 5 and 7) represents stromatolites, oncoids, and ooids with rich assemblage of calcareous algae. Jurassic/Cretaceous boundary was placed in unit 5. First appearance of orbitolinid foraminifers at the base of unit 7 was interpreted to correspond to the base of Barremian. Units 8 and 9 consist of four upward-shallowing sequences. The base of each cycle consists of thinly bedded limestone containing brachiopod shell, and change into thickly bedded and massive limestone with shallow marine stromatolies and fauna, such as corals and rudists. Gradual decrease in thickness of the sequence indicates that the platform was in progradation due to accumulation of the carbonate deposits.