Sequence biostratigraphy and paleoenvironmental reconstruction of the Oligocene-early Miocene deposits of the Zagros Basin (Dehdasht area, South West Iran)

Abstract

The Asmari succession is a predominantly carbonate-lithostratigraphic unit corresponding to an Oligocene-Lower Miocene shallow and homoclinal ramp system that developed in the southwest of the Zagros Basin. The sedimentation occurs within a transition from a mixed carbonate–siliciclastic to carbonate-dominated depositional environment. Carbonate production in this system is dominated by diverse benthic foraminifera, coralline algae, mollusks, bryozoans, echinoids, and corals. Based on fossil content and texture, 16 different facies types have been distinguished that accumulated in three interpreted paleoenvironments: The euphotic inner ramp is characterized by two facies zones: an inner and an outer shallow water zone. The inner shallow water zone is characterized by the occurrence of imperforated foraminifera together with mollusks in restricted parts (foramol assemblage). The outer shallow water zone is dominated by wackestones-packstones with benthic foraminifera and coralline red algae, minor mollusks, bryozoans, echinoids, and small epiphytic biota (LB-foralgal assemblage). The higher-energy parts on platform margin are represented by ooids, scattered corals, and robust foraminifera-rich grainstones (coralgal assemblage). The middle ramp is characterized by small coral patch reefs that developed in mesophotic conditions, and larger foraminiferal packstone facies often associated with coralline red algal debris (LB-foralgal assemblages) characterize the sediments of the deeper oligophotic zone, whereas the outer ramp facies–aphotic zone–are represented by bryozoans, mollusks, and echinoid association (bryomol assemblage). This facies passes basinward into wackestone, packstone, and rarely mudstone with pelagic and small benthic foraminifera, sponge spicule, and echinoid fragments (echinofor assemblages). Changes in the depositional facies and cycle stacking patterns indicate four transgressive–regressive sea-level cycles from the bottom to the top of the section. These depositional sequences have been separated by type 2 sequence boundaries. Sea-level fluctuations that cause changes in light penetration which may produce shifts from aphotic to oligophotic and euphotic conditions are an important factor in skeletal production and spatial distribution of carbonate factories. The presence of non-skeletal grains (ooids and ploeids), the occurrence of Porites patch reefs, and the existence of LBF such as Heterostegina, Operculina, Amphistegina, Archaias, and Lepidocyclina in the larger foraminifera associations together with Lithoporella mellobesioides and Spirolithon in coralline red algae assemblage all suggest that carbonate sedimentation took place in tropical waters under oligotrophic to slightly mesotrophic conditions.