Sedimentary environment and depositional sequences of the Qom Basin (north-eastern margin of the Tethyan seaway)

Abstract

This research attempted to determine the depositional sequences of the Qom Formation in the Urumieh-Dokhtar arc (Ghamsar section) and Esfahan-Sirjan fore-arc (Abadeh section) sub-basins in Iran, using microfacies and microtaphofacies analyses. The authors also investigated connections between the Qom Basin and the Zagros and Paratethys basins during the Oligocene. In this regard a total of eight microfacies, two terrigenous facies, and five microtaphofacies were identified on the basis of 269 samples from the Ghamsar section and 93 samples from the Abadeh section. The studied microfacies, terrigenous facies, and microtaphofacies were deposited on a homoclinal carbonate ramp. This carbonate platform can be divided into inner, middle and outer ramp environments. On the basis of the distribution of microfacies and sequence stratigraphy studies, five third-order depositional sequences and one incomplete depositional sequence were identified in the Ghamsar section and three third-order depositional sequences in the Abadeh section. According to the distribution of microtaphofacies and palaeobathymetric studies based on Amphistegina, the energy, and depth of the Qom sea in the Ghamsar section were greater than those evidenced in the Abadeh section. The results of local fault activity in the different sub-basins of the studied sections indicate a lesser effect of global sea-level changes in the Paratethys basin. On the basis of the formation of depositional sequences in these sub-basins; and differences in the number of depositional sequences; intense local fault activity is indicated during the Chattian Age (especially in the Urumieh-Dokhtar arc sub-basin). Regional sea-level fluctuations of the south Tethyan Seaway and the Paratethys Basin controlled sea-level changes in the Chattian Age. The depositional basins of the Tethyan seaway (southern Tethyan seaway, Paratethys Basin and Qom Basin) probably were related during the Burdigalian to Langhian and the early Serravallian. The results show that, the effect of sea-level changes of the Zagros Sea on the formation of depositional sequences in the Esfahan-Sirjan fore-arc sub-basin was significant.