KHODAEE, M., University of Teacher's Training, Tehran, Iran; * S. FEIZNIA, University of Tehran, Tehran, Iran

ABSTRACT: Sedimentary Environment and Diagenetic Phenomena of Tirgan Formation (Hauterivian-Lower Aptian) Northeast Iran, Kopeh-Dagh Basin and Proposing the Facies Model

The Kpoeh Dagh Basin in northeast Iran was shaped in Middle Triasic. Sedimentation has been continued from Jurassic to Miocene. The Tirgan Formation belongs to Hauterivian-Aptian. In the type locality in Tirgan region, this section is mostly composed of carbonates. Toward south and southeast, the facies of the lower part of this formation has changed from carbonate to fluvial and deltaic silicilastics. In parts of Kopeh Dagh Basin, this formation is the reservoir rocks of gas.

About 840 vertically oriented carbonate, sandstone and shale samples were collected from four stratigraphic sections of Tirgan formation in central and southeast parts of the basin. All of the samples were studied with petrographic microscope. Some of the samples were analyzed by X-ray diffraction. Detailed petrographic investigation using the microfacies approach had led to the recognition of about 13 microfacies. Petrographic studies, curves of frequency and clasticity of the components, sedimentary structures, graphic logs and vertical succession and reciprocal relationships of the microfacies in the different sections led to the recognition of three shallowing upward megasequences. Each of the megasequences consists of smaller sequences with special sedimentary environment.

The shallowing upward magesequence in turn converted into a horizontal depositional model. The model consists of a shallow open marine carbonate platform with the following environments from offshore to onshore:

1. Lower slope of subtidal environment or basinal environment with micrite and biomicrite microfacies.

2. Upper slope of subtidal environment with oncomicrite and oomicrite microfacies.

3. Oolitic bank of intertidal environment with oosparite microfacies.

4. Lagoonal environment with biopelmicrite and (nonfossiliferous) micrite microfacies.

5. Supratidal environment with stromatolite microfacies in Tirgan section. In Hamam-Galeh and Zavein sections, a deltaic complex periodically prograded over the south-southwest shelf of the basin, substituting the supratidal and shallow marine environments.

Consequently, three distinct environments were juxtaposed: delta system, carbonate platform and basin. The depositional environments were belt-like, extending parallel to the general strike of the sedimentary basin (northwest-southeast). The supratidal environment was located in south-southwest and subtidal environment in north-northeast part of the basin.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.