Facies analysis and sequence stratigraphy of the evaporite bearing Sachun Formation at the type locality, South East Zagros Basin, Iran

Abstract

The Paleocene-Early Eocene Sachun Formation across the interior Fars in the SE of the Zagros Basin (Iran) consists of carbonates, evaporites and siliciclastic sediments. Lithostratigraphic evidence shows that the Sachun Formation can be divided into three units: lower evaporite (unit 1), middle limestone and marl (unit 2), and upper evaporite (unit 3). The lower and upper evaporate units are represented by gypsum-anhydrite layers that are interbedded with thin-bedded dolomite and red marl layers. In the evaporites of the Sachun Formation, brecciated gypsum, nodular, nodular-banded gypsum and laminated-banded gypsum with chicken-wire fabrics are identified. These associations indicate a sabkha or shallow-water setting, such as a lagoon. Petrographic studies led to the identification of twelve carbonate facies types that have been formed in four different types of depositional settings, across a ramp with a gentle dip, including; supratidal sabkha, shallow subtidal lagoons, barrier and shallow open-marine. The mixed carbonate and siliciclastic lithofacies are composed of red and gray marl which both are interbedded with carbonate and evaporite deposits. Three depositional sequences (A, B and C) are recognized from deepening to shallowing trends in the depositional facies, changes in cycle stacking patterns and sequence boundary features. Sequences A and C are evaporite-dominated shallowing-upward cycles and sequence B consists of carbonate-dominated shallowing-upward cycles.