PETROFACIES AND SEQUENCE STRATIGRAPHY OF THE QOM FORMATION (LATE OLIGOCENE-EARLY MIOCENE?), NORTH OF NAIN, SOUTHERN TREND OF CENTRAL IRANIAN BASIN

摘要

The Qom Formation (Late Oligocene-Early Miocene?) to the north of Nain along the southern trend of the central-Iranian Basin represents a carbonate-marl depositional system. Upward, the predominant microfacies are sandstone (litharenite); reworked lithobioclastic grainstone, bioclastic packstone to wackestone with the larger benthic foraminifera and mollusks; bioclastic packstone to wackestone with the smaller foraminifera; bryozoa grainstone; and peloid and miliolid wackestone. Microfacies analysis and fades interpretations suggest that the dominant environments were marginal to restricted open marine, open marine to restricted marine and bar; and open marine to lagoon and nearshore. Three depositional sequences were also recognized. These sequences are characterized by the retrograding and aggrading stacking patterns of transgressive and highstand system tracts, respectively. A marginal prograding stacking pattern of lowstand system tracts was also recognized at the base. Sequence boundaries were characterized by either an abrupt change in lithology or by superposition of transgressive beds upon aggrading ones. Maximum flooding surfaces were coincident with deepening and shallowing in each depositional sequence.