زيست چينه‌نگاري، ريز رخساره، محيط رسوبي و چينه‌نگاري سكانسي سازند قم در ايران مركزي (آباده تا ساوه)

Biostratigraphy, microfacies, sedimentary environment an‎d sequence stratigraphy of the Qom Formation in Central Iran (Abadeh to Saveh)

زمين شناسي

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.

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