Facies and depositional environment of the Upper Cambrian of the central Alborz, members 3 and 4 of the Mila Formation (Deh-Molla Formation), with emphasis on their ribbon rocks

The Late Cambrian was associated with tremendous changes in world oceans and lands including the deposition of specific facies in different parts of the world known as “ribbon rocks” due to their thinly bedded undulatory banded nature. In this study, the Upper Cambrian succession of the central Alborz (members 3 and 4 of the Mila Formation) is sedimentologically investigated with an emphasis on its upper part’s ribbon rocks. The study tries to give an understanding of facies analysis and depositional environment of this succession. In this regard, the strata are divided into nine lithostratigraphic units and 12 microfacies representing three subenvironments of supra- inter-and sub-tidal. The results show that the microfacies were deposited in an epeiric platform with marked changes in the facies distribution and platform physiography in time so that the evolution of this platform can be divided into three stages. During the first stage, dominantly tidal flat facies were deposited in broad belts without significant effects from disturbing agents such as storms. In the second stage, the platform was drowned due to a rapid transgression where deep-marine glauconiterich green shale was deposited on former platform facies. In the third stage, the environment was strongly affected by disturbing factors such as storms and tides where severe facies changes occurred, represented by the deposition of ribbon rocks. The ribbon rocks of the Deh-Molla Formation show a spectrum of subtidal to intertidal-supratidal depositional settings.