Recognition of Milankovitch cycles during the Oligocene–Early Miocene in the Zagros Basin, SW Iran: Implications for paleoclimate and sequence stratigraphy

Abstract

Milankovitch cycles during the Phanerozoic affected paleoclimate, the onset/termination of glacial/interglacial periods, sea-level changes, and thus the distribution of different facies in sedimentary basins. The effects of astronomical forcing have been recorded as astronomical frequency signals in sedimentary sequences through geological history. In this research, for the first time, the signal of Milankovitch cycles and orbital obliquity modulation cycles from the Oligocene–Early Miocene Asmari Formation at one of the oil fields in the Dezful Embayment of the Zagros Basin, Iran, have been identified, through wireline logs. Based on that, ten ~1.2 orbital obliquity modulation cycles have been recognized and correlated with global third-order sequences and δ18O events. The number of cycles during the Oligocene–Early Miocene in the Asmari Formation is equivalent to obliquity of the La2004 orbital solution. Additionally, the optimal sediment accumulation rates, depositional duration of this formation, and foraminiferal biozone durations have been calculated. The result of this study shows that sediment accumulation rates varied in the range 2.9 to 3.2 cm/kyr. The foraminiferal biozones are diachronous in this area. Based on the identified 405 kyr long-eccentricity cycles in the three studied wells, three astronomical time spans of 11.5 Myr, 9.42 Myr, and 9.02 Myr were obtained for the Asmari Formation in Wells A, B, and C, respectively.