Sequence Stratigraphy of Qom Formation in SouthCentral, Central Iran Basin: Chronosratigraphic Significance of Sequences in Age Determination of Members
The mainly marine Oligo-Miocene Qom Formation overlies and underlies the Lower Red and Upper Red Formations respectively. The formation has been divided into eleven members. The unnamed member and a, c1, c3, f and g members (mainly carbonats), b, c4 and e members (mainly sandstone and shale/marl) and c2 and b members (mainly evaporates) have been recognized. Poor fossil data and absence of index fossils in parts of Qom Formation have caused uncertainity in age determination of the formation and its members. The age of these successions could acurately be determined using sequence stratigraphic concepts.
The Qom Formation, in south central Central Iran Basin, include 7 depositional sequences with both type 1 and 2 sequence boundaries. Sequences 1-3 (Chattian), 4-5 (Aquitanian) and 6-7 (Burdigalian), related to global relative sea-level changes, have been recognized. Deposition of continental facies of the Lower Red Formation, most certainly, occurred during Late Rupelian lowstand. Sea-level lowstand in Central Iran Basin during deposition of c2 and d members and at the upper boundary of the Qom Formation are correlated to Late Chattian, Late Aquitanian and Late Burdigalian sea-level lowstands. Correlation of Qom Formation sequences with those of global sea-level cycles indicate that global sea-level changes were more important than local/regional tectonics during their formation.