Facies changes, sedimentary environments and sequence stratigraphy of the lower Carboniferous deposits in the Kalmard area, Central Iran, Member D of Gachal formation

AbstractLower Carboniferous deposits of Kalmard area identified with informal Gachal formation which in different outcrops have different lithologic features. This formation is consists of four members A, B, C and D that predominantly consists of carbonate and evaporite rocks. In the Gachal section, the B, C, and D members are outcropped. Member B consist of 200 m massive to thick-bedded dolostone, C member 150 m white massive gypsum, and D member from 50 m carbonate deposits with intercalation of evaporite interlayers. The lower boundary is not exposed but the upper boundary with Khan Group is unconformable. In this paper, Member D, in the Godar-e-Gachal is investigated. Based on lithologic and microscopic studies, 20 carbonate microfacies are identified which belong to the tidal flat, open to the semi-restricted lagoon, bar/shoals and open marine sub-environments. Vertical changes of microfacies and depth change curve represent that the supratidal, intertidal and lagoonal microfacies are thicker than open marine microfacies. Member D of the Gachal formation were deposited in the homoclinal ramp that was situated in the southern margin of paleo-Tethys Ocean in the Visean–Serpukhovian? in the hot and dry climatic conditions that is comparable with the conditions of modern Persian Gulf homoclinal ramp. The Member D deposits of Gachal formation consist of three depositional sequences that differentiated with sequence boundary type 2. Relationships between Member D and C in the lower part and with Khan Group in the upper part are identified with sequence boundary type 1. The above-mentioned third depositional sequences shows the late Visean age that correlateable with the Kaskaskia IV supersequence. The boundary between Gachal formation and Khan Group is correlateable with falling of sea-level in the global scale in the late Visean–Serpukhovian? stages which represent a clear disconformity and erosional surface between Gachal formation and Khan stratigraphic Group.Keywords: Gachal formation; Kalmard area; Lower Carboniferous; Sequence stratigraphy  IntroductionCentral Iran is one of the basic tectonostratigraphic and complicated units in the geology of Iran that is located in the center of Iran and has a triangle shape. The Kalmard Block is a little part of Central Iran that has a northeastern trend and is located between the Kalmard Fault in the east and Naeini Fault in the west. The Lower Carboniferous rocks in the Kalmard area mainly consist of carbonate rocks and a unit C of Gachal formation that is composed of approximately 170 m gypsum and anhydrite that the name of Gachal formation derived from this unit (Aghanabati 1977). Unit D of the Gachal formation is composed of carbonate rocks such as limestone, dolomitic limestone with intercalations of gypsum and red paleosoils and collapse breccias that the complete section of this unit is seen in the Godar-e-Gachal section studied in this paper. With respect to complete Unit D in the southern part of Kalmard area and a necessary of the study of sequence stratigraphy of Tournaisian–Visean strata in this part of Central Iran, this stratigraphic section has been selected. Materials & MethodsFor identification of microfacies characters and analysis, environmental conditions, and sequence stratigraphy of unit D of Gachal formation, the Godar-e-Gachal section has been selected that has 50.4 m thickness. With respect to thickness and lateral facies change, carbonate-evaporate sedimentary cycles and key stratal sequence stratigraphy surfaces. Thirty-two rock samples have been collected and 100 microscopic thin sections have been prepared. Classification and studies of carbonate rocks were conducted based on Dunham (1962) and reconstruction of the depositional environment was based on Walther’s law of correlation of facies (Walther, 1984 in Middleton 1973). Also, vertical and lateral facies changes and comparison with recent sedimentary environments were examined by using the standard carbonate platform models (Wilson 1975; Flügel 1982; Carozzi 1989; Burchette & Wright 1992). Discussion of Results & ConclusionsThe identified depositional sequences of Member D of the Gachal formation have a lot of similarities with global depositional sequences in the Early Carboniferous (Sloss 1988). By studying Member D of Gachal formation, the following results have been obtained:The thickness of unit D changes from north to south (22 to 98 m). In the Godar-e-Gachal section the thickness of Member D is 50.4 m. With respect to carbonate rocks and intercalation of gypsum beds, it seems that this member was deposited in the south of Kalmard mixed carbonate –siliciclastic platform, under hot and dry climatic conditions. Based on microscopic and field studies, microfacies types are deposited in the tidal flat, lagoon, bar and open marine sub-environments in the Klamard homoclinal platform in the lower Carboniferous was located in the passive margin of the southern part of the Gondwanaland.  Member D of the Gachal formation consists of three depositional sequences that the first depositional sequence has sequence boundary type 2 in the lower part with gypsum of Member C, and the third depositional sequence has a sequence boundary type 1 in the upper part with the Khan formation which, indeed, is a disconformity and an erosional surface. Glacial events belong to the Milankovitch orbital forcing and tectonic processes, and are main factors for the formation of sedimentary cycles and depositional sequences of Member D of Gachal formation.