The Tethyan Seaway Iranian Plate Oligo-Miocene deposits (the Qom Formation): distribution of Rupelian (Early Oligocene) and evaporate deposits as evidences for timing and trending of opening and closure of the Tethyan Seaway

摘要

The aim of this research is to shed light on the distribution of Rupelian (Early Oligocene) and evaporate deposits as evidences for timing and trending of opening and closure of the Tethyan Seaway. To accomplish these goals, four sets of data were scrutinized: (1) two stratigraphic sections (Ghohroud and Vidoja) of the Qom Formation in the southern and southwestern Kashan (to determine the age of their lower deposits), (2) results of the study of more than 100 stratigraphic sections of the Qom Formation, (3) Geological maps of Iran Sheets and (4) more than 30 geological maps of different parts of Iran. Consideration of four sets of data led to the following conclusions. Deposition of the Qom Formation (with Rupelian–Burdigalian range) took place in three NW–SE-trending basins: Sanandaj–Sirjan (fore-arc basin), Urumieh–Dokhtar magmatic arc (intra-arc basin) and Central Iran (back-arc basin). Marine conditions in the low latitudes of these three basins started in the Rupelian. Rupelian deposits are vastly distributed in the intra-arc and fore-arc basins but in the back-arc basin Rupelian deposits are deposited only in a few places in close proximity of the magmatic arc. It is remarkable that mostly all of the intra-arc Rupelian deposits-bearing sections are underlain by Eocene volcanic rocks. Latitudinally, the Rupelian deposits of the Qom Formatiom are deposited below 34°2′N, the Chattian deposits continue to 35°N, but above 35°N the Qom Formation is deposited during the Miocene, based on previous age dating; therefore, transgression of the Tethyan Seaway on the Iranian Plate started from the southeast and continued northwestward gradually. The last true Nummulites spp. became extinct at the top of Late Rupelian and all of the in situ Nummulites-bearing layers of the Qom Formation are Rupelian in age. According to the presence of Nummulites spp. (Nummulites fichteli, Nummulites vascus, Nummulites sp.), the lower 185 m of the Ghohroud section and whole deposits of the Vidoja section are attributed to the Rupelian. Evaporate deposits of the Qom Formation are deposited in a rather small area of Central Iran back-arc basin and are deposited totally in the Early Miocene (Aquitanian–Burdigalian). This indicates that due to the compressive tectonic regime in the Central Iran back-arc basin, the gates to the open ocean became restricted in the Early Miocene gradually. During the Aquitanain and Burdigalian, restricted marine conditions prevailed and episodic precipitation of evaporate deposits occurred. In the Qom area near the depocentre of the Qom basin, evaporates were precipitated episodically. Deposition of evaporate deposits took place along the concave side of an assumptive curved line crossing from SE of Anarak, Nain, Navab Anticline (SE of Kashan), Shourab (SE of Qom), immediate S, SW and W of Qom, N of Howz Soltan Lake, SW of Tehran, SW, W and SE of Varamin, NE of Garmsar and NW, N and NE of Semnan. Distribution of the Qom Formation, evaporate deposits and Rupelian deposits are illustrated on new maps shown herein as figures.