New data on sequence stratigraphy of the Sarvak Formation in Malekshahi city, (Ilam province) Zagros basin, Iran

Abstract

Abstract Two surface sections of the Sarvak Formation (late Albian-late Turonian) named Tange-e Bawlek and Shahnakhjir were studied., These sections are about 24 km apart, located in the flanks of the Kabirkuh and Shahnakhjir anticlines in Ilam province, in the western part of the Zagros Fold Thrust Belt (ZFTB). The Sarvak Formation is 845 m and 412 m thick in these sections respectively. The lower contact with the Garau Formation is erosional with karst features and abundant glauconite at the first section, whereas it isn't exposed at the last section. Field evidences and laboratory examinations on thin sections of collected samples revealed five 3rd order depositional sequences in the Sarvak Formation of which, the four lower sequences were deposited in a carbonate platform, whereas the last one was deposited in a foreland basin. The lower contact of the first and last sequences are type І, whereas rest of the boundaries are type II. In compare with the Arabian Plate, lower boundary of the topmost sequence is assumed to coincide with the boundary between AP8 and AP9. Sequences 1 and 2 predominantly comprising neritic facies, whereas sequences 3 and 4 are composed of pelagic facies. Although the topmost sequence basically varies in biota content, lithology and depositional setting from the rest of the formation. Overwhelming shale lithofacies in contrast to the other part of the formation along with unconformity in lower contact of this sequence, strongly support to consider this part as separate lithostratigraphic unit. Hence we suggest this part to be considered as a potential new formation after further complementary studies.