Facies analysis and depositional environment study of the mixed carbonate-evaporite Asmari Formation (Oligo-Miocene) in the sequence stratigraphic framework, NW Zagros, Iran

摘要

The carbonate-evaporite mixed Asmari Formation was investigated on the three representative outcrops (these of Zarrin-abad, Abhar, and Darreh-shahr, corresponding to three sections) of the Lurestan Zone in NW Zagros, where the sedimentary facies of the Asmari Formation shows significant differences to those in other parts of Zagros. In the light of study results from the field observation and laboratory measurements, 16 facies have been recognized and differentiated, which are grouped into 5 facies associations, representing 6 sub-environments on a carbonate ramp. The sub-environments include peritidal area, lagoon, platform margin high-energy belts, middle ramp, outer ramp and evaporitic saltern. The evaporitic saltern points to such a condition, during which there occurred periodic change from carbonate to evaporitic settings. The alternation of carbonate with evaporitic settings appeared on a carbonate ramp connected with a hydrographically isolated intrashelf basin. The development of evaporite-dominated deposits of the Kalhur Member of Asmari Formation is related to the restriction of the basin during sea-level falls. This restriction led to succeeding evaporation of seawater and precipitation of evaporites. In times of sea-level rising, the basin was well reconnected to open ocean and thus the carbonate factory was reestablished over the basin. From sequence stratigraphic points of view, the formation is composed of two second-order regressive sequences, representing transitional conditions from deep-marine facies underlying Pabdeh Formation to shallow evaporitic facies overlying Gachsaran Formation. Each sequence is composed of a unit of evaporites at bottom displaying a falling-stage systems tract followed by thick carbonate strata, representing transgressive and highstand systems tracts. It is obvious that the major relative sea-level fall of the first second-order sequence was recorded by the facies development of the upper Kalhur gypsum of the Asmari Formation in the Zarrin-abad section, and there developed the paleokarst facies located at the sequence boundary in the Abhar, and Darreh-shahr sections.