Carbonate Sequence Stratigraphy of a Back‐Arc Basin: A Case Study of the Qom Formation in the Kashan Area, Central Iran

Abstract

Abstract: The Qom Formation comprises Oligo‐Miocene deposits from a marine succession distributed in the Central Basin of Iran. It is composed of five members designated as A‐F. Little previous work exists on the sequence stratigraphy. Based on an integrated study of sequence stratigraphy with outcrop data, wells and regional seismic profiles, the Qom Formation is interpreted as a carbonate succession deposited in a mid‐Tertiary back‐arc basin. There are two second‐order sequences (designated as SS1 and SS2) and five third‐order sequences (designated as S1‐S5). Five distinct systems tracts including transgressive, highstand, forced regressive, slope margin and lowstand have been recognized. The relationship between the sequences and lithologic sub‐units has been collated and defined (S1 to S5 individually corresponding to A‐C1, C2‐C4, D‐E, the lower and upper portions of F); a relative sea level change curve and the sequence stratigraphic framework have been established and described in detail. The coincidence of relative sea level change between that of the determined back‐arc basin and the world indicates that the sedimentary cycles of the Qom Formation are mainly controlled by eustatic cycles. The variable combination of the systems tracts and special tectonic‐depositional setting causally underpin multiple sequence stratigraphic framework styles seen in the carbonates of the back‐arc basin revealing: (1) a continental margin basin that developed some form of barrier, characterized by the development of multiple cycles of carbonate‐evaporites; (2) a flat carbonate ramp, which occurred on the southern shelf formed by the lack of clastic supply from nearby magmatic islands plus mixed siliciclastics and carbonates that occurred on the northern shelf due to a sufficient clastics supply from the land; and (3) a forced regressive stratigraphic stacking pattern that occured on the southern shelf and in basin lows due to the uplifting of the southern shelf. Thick and widespread aggradational framework limestone usually occurs in the initial sequences (S1 and S3) of the supersequence, which led to preferential oil reservoir deposition but a lack of source and cap rocks, whereas the retrogradational and progradational framework limestone usually occurs in the later sequences (S2 and S4‐S5) of the supersequence, which results in two perfect sets of source, reservoir and cap rock assemblies, so that the limestone in sub‐member C2‐C4 and the F‐Member can be predicted as important objects for oil exploration.