چكيده لاتين
In this research, biostratigraphy, microfacies, sedimentary environments, and sequence stratigraphy of the Qom Formation in the Central Iran (Abadeh to Saveh) were studied. In order to achieve these objectives, three outcrops were chosen in Fars (Abadeh), Isfahan (Ghamsar) and Markazi (Saveh) provinces.
The Qom Formation in the Abadeh (with 164 m thickness), Ghamsar (with 313 m thickness), and Saveh (with 435 m thickness) sections consist of limestones and shales. The Qom Formation overlies the Lower Red Formation in the Abadeh and Saveh sections and in the Ghamsar section overlies the Eocene volcanic rocks by non-conformity. In the studied sections, the Qom Formation is covered by the Recent alluvium. Four lithological units in the Abadeh and Ghamsar and six lithological units in the Saveh sections were identified based on the field observations, the lithological and macroscopic characteristics.
Based on biostratigraphy studies, 25 genera and 32 species of benthic foraminifera in the Abadeh section, 24 genera and 34 species in the Ghamsar section, 28 genera, and 33 species in the Saveh section were identified. Based on the vertical distribution of benthic foraminifera, the Qom Formation in the Abadeh and Ghamsar sections was deposited during the late Rupelian to Chattian ages, but in the Saveh section was deposited during the Aquitanian to Burdigalian ages.
Based on the field observation and microscopic studies of 488 thin-sections and 42 soft samples (shale), 8 microfacies belonging to lagoon, bar and open marine environments, 4 terrigenous facies belonging to lagoon, open marine and river environments and 5 microtaphofacies belonging to low and high-energy environments, were recognized in the studied sections. The deposits of the Qom Formation in the studied sections indicated that they were formed in the homoclinal ramp carbonate. This carbonate ramp in the Abadeh section can be divided into two sub-environments that consist of the inner and middle ramp and three sub-environments that consist of the inner, middle and outer ramp in the Ghamsar and Saveh sections.
Based on paleoecology studies in these sections, the salinity conditions were mostly normal (30-40 psu) to medium (40–50 psu), meso-oligophotic light conditions and meso-oligotrophic nutrient conditions. The Qom Formation was deposited in 18 to 25°C with seagrass substrate in the study areas.
Based on sequence stratigraphy studied, three third-order depositional sequences in the Abadeh sction, five depositional third-order sequences along with one incomplete depositional sequence in the Ghamsar section and three third-order sequences along with two incomplete depositional sequence were recognized in the Saveh section.
Correlation of the depositional sequences between the three basins of the northern margin of the Neotethys (Qom basin) and the southern margin of the Neotethys basin (Asmari basin) and the Paratethys basin indicates that local faults were more effective than global sea-level changes on the formation of depositional sequences in the Qom sub-basins (back-arc, arc and fore-arc sub-basins). These findings indicated that the effect of local faults on the number of depositional sequences in the Urumieh-Dokhtar arc sub-basin was higher than other sedimentary sub-basins. In all the sedimentary sub-basins of the Qom Formation, the activity of local faults during the Chattian age was more than the Rupelian age. In addition, on the basis of correlation of depositional sequences, it can be concluded that the effect of sea-level changes of Zagros Sea on the formation of depositional sequences in the Esfahan-Sirjan fore-arc sub-basin was significant.
