چينه نگاري سكانسي نهشته هاي پرمين (ابرسكانس آبزاروكاي مياني) در ناحيه گدوك، البرز مركزي

Sequence Stratigraphy of Permian Deposits (Middle Absaroka Supersequence) in Gaduk Area, Central Alborz, Iran

Permian deposits, including Doroud and Ruteh Formations, have extensive outcrops in Central Alborz. The Gaduk Section, east of Firuzkuh city provides a good example from these exposures. The siliciclastic and carbonate deposits of Permian in this section, about 348 meters thick, overlie the Lower Carboniferous limestones of Mobarak Formation and are unconformably overlain by Lower Triassic carbonate rocks of Elikah Formation. Detailed description of geometry, fossil content, sedimentary structures, and stratal surfaces in the field along with lithological characteristics, textural properties, and microfossi I components of the rocks in the lab led to identification of 4 terrigenous and 13 carbonate facies. Using major characteristics of the facies, diversity of fossil content especially forarninifra and green algae, and eodiagenetic siliceous cement; these facies were formed in the 7 depositional sequences. Depositional environment of the facies were determined as a continental shelf in the Paleotethyan passive margin (North of Gondwanaland). This environment shows the characteristics of a siliciclastic shelf for Lower Permian deposits and a carbonate ramp for the Middle Permian rocks. Comparing with sequences from other parts of the world, the studied sequences were classified in second and third order types. Relative sea level fall in the Late Permian led to development of cherty - lateritic unconformity in the Permo-Triassic boundary. Cherty nodule bearing strata can be related to maximum sea level fall as an indicator for type one sequence boundary determination.