Sequence stratigraphy and sea level history of the upper Paleocene strata in the Kopet-Dagh basin, northeastern Iran

Abstract

The intracontinental Kopet-Dagh basin formed after the Middle Triassic orogeny in northeastern Iran and southwestern Turkmenistan. This underexplored basin could provide significant petroleum reserves in the 21st century. The upper Paleocene Chehel-Kaman Formation is exposed along northwest-southeast-trending folds and is composed of carbonate strata and minor amounts of siliciclastic and evaporite beds. Six stratigraphic sections in the central and eastern parts of the basin have been used to divide the upper Paleocene (Thanetian) carbonate supersequence into four major carbonate lithofacies, each having multiple subfacies. These lithofacies represent open-marine, shoal, semirestricted lagoon, upper intertidal, and tidal-flat subenvironments that formed on a shallow carbonate ramp. In addition, there are two siliciclastic lithofacies consisting of calcareous shale (marl) and calcite-cemented sandstone. The upper Paleocene interval consists of three depositional sequences (DS1, DS2, and DS3), bounded by type 2 (within the top of the underlying Pestehleigh Formation), type 2, and type 1 sequence boundaries, respectively. Sea level changes during the Thanetian in the Kopet-Dagh basin are similar to global changes proposed by Haq et al. (1988), with differences related to local and regional geological events. Upper Paleocene strata were deposited in about 4 m.y., within the range of second-order cycles. Each depositional sequence was developed as a third-order cycle composed of several shallowing-upward parasequences (fourth- to fifth-order cycles). We estimate that sea level fluctuations in the study area were between 5 and 11 m during development of parasequences. (Begin page 840)