The evolution of the Lower Cretaceous carbonate ramp in sequence stratigraphy framework: A case study from the Tirgan Formation, Kopet-Dagh Basin, NE Iran

Abstract

Lower Cretaceous strata in Kopet-Dagh Basin are mostly composed of medium- to thick-bedded limestones with various skeletal (bivalves, foraminifera, algae, echinoderms, brachiopods) and non-skeletal (peloids, ooids, and intraclasts) grains. In this study, two stratigraphic sections of Chaman-Bid and Sorkh-Ghaleh with thicknesses of 247 and 190m respectively in NE Iran, have been studied in order to determination of the sedimentary environment and sequence stratigraphy. Based on the field observations, petrographic studies, textural and facies characteristics, 16 carbonate microfacies have been identified. Facies analysis led to identifying the tidal-flat, lagoonal, shoal, and open-marine environments with low - to high-energy conditions. Because of the wide lateral distribution of microfacies and the absence of distinct paleo bathymetric changes, the depositional system likely represents a westward-deepening homoclinal ramp. Four third-order depositional sequences can be distinguished in each of the two stratigraphic sections during the
Baremian-Aptian. Transgressive system tracts (TST) show deepening-upward trends, in which shallow-water (tidal flat and lagoonal) facies are overlain by deeper-water (shoal and open-marine) facies. Highstand systems tracts (HST) show shallowing-upward trends in which deep-water facies are overlain by shallow facies. All sequence boundaries in the study sections are SB2 type. According to the study conducted from the west of the basin to the east, the thickness of carbonate deposits decreased. During the Aptian, the basin moved to open marine settings. The occurrence of such changes reflects the tectonic events and global sea-level rise during the deposition of the Tirgan Formation in the Kopet-Dagh Basin.