ABSTRACT This study focuses on the western Alborz region, specifically the Chopoghlu section near the village of Chopoghlu, located about 90 km southeast of Zanjan. We investigated the Bayandor Formation and the Lower Dolomite Member of the Soltanieh Formation, both dating to the Late Precambrian. The results show a remarkable variety of stromatolite structures in these units. The Bayandor Formation is 605 m thick and is mainly composed of shale with interbedded dolomite. The Lower Dolomite Member of the Soltanieh Formation has a thickness of about 100 m. Field investigations and petrographic analysis of thin sections identified five stromatolite types: stratiform, crinkly, columnar, pustular, and oncoidal. Stratiform stromatolites are the most common and typically formed in upper intertidal to supratidal settings. Crinkly stromatolites occur mainly in the Bayandor Formation and indicate shallow subtidal environments influenced by tides and wave activity. Pustular stromatolites represent upper intertidal conditions. Columnar stromatolites are rare and are found mostly in upper intertidal zones and occasionally in lower intertidal settings. Oncoidal stromatolites are scattered throughout the section and are associated with supratidal to intratidal environments. Rock-Eval pyrolysis of the Bayandor Formation and the Lower Dolomite Member of the Soltanieh Formation indicates very low Total Organic Carbon (TOC) contents, ranging from 0.02 to 0.06 wt% and 0.02 to 0.03 wt%, respectively. Free hydrocarbons (S1) and remaining hydrocarbons (S2) values are minimal, reflecting limited hydrocarbon generation potential. Hydrogen Indices (HI) range from 20 to 243 mg HC/g TOC, indicating predominance of type III and mixed II-III kerogens, which are primarily gas-prone. Tmax values between 371-438°C reveal thermal immaturity for most samples, with only minor samples approaching the oil window. Low TOC, HI, and Tmax suggest deposition under shallow, low-energy, or periodically oxidising conditions that restricted organic matter preservation. Comparisons with global Precambrian formations show that Chopoghlu dolomites are significantly poorer in organic matter.
Pilehkouhi et al. (Sun,) studied this question.