Sedimentary facies and depositional environment of the Aitamir Formation in the Gorgan Plain: A wave-dominated shoreface model

Abstract

Abstract: The siliciclastic sediments of the Aitamir Formation (Albian–Cenomanian) at the Oghchi locality of the Gorgan Plain consist of a thick succession of the black-grey shales and thin to very thick-bedded sandstone, deposited in a shoreface to offshore depositional system. Four facies associations including offshore, offshore transition, lower and mid-upper shoreface associations are identified in this succession. Offshore facies association consisting of a thick package of the fissile dark shales with the abundance of the framboidal pyrites displays deposition below the SWWB and dysoxic condition. The offshore transition facies association includes the heterolithic sediments of the shale and very thin-bedded sandstones/siltstones. The lower and mid-upper associations are characterized by an increase in abundance and the thickness of the sandstone layers and display a generally thickening and coarsening up-ward trend of a classical shoreface depositional system. The lateral correlation of the studied section and central sections of the Aitamir Formation of the Kopet-Dagh Basin indicates an increase in subsidence effect in the creation of the accommodation space and a decrease in the sediment supply of the N-NW parts of the basin. The presence of a thick package of dark shales with abundant framboidal and chamber filling pyrites of the Aitamir Formation suggests these sediments as a prone candidate for hydrocarbon exploration.Keywords: Aitamir, Kopet-Dagh, shoreface, tectonic, hydrocarbon Introduction: The Kopet-Dagh Basin, located in the NE Iran and Turkmenistan, was formed as an intracontinental Basin due to the southeastern extension of the South Caspian Basin by Neotethyan back-arc rifting after the closure of the Palaeotethys and the early Cimmerian Orogeny (Middle Triassic) (Stampfli et al., 1991). A continuous sedimentary package (up to seven kilometers), including five transgressive-regressive super-sequences from the Jurassic to Miocene were deposited in the Eastern part of the sedimentary basin (Moussavi-Harami and Berner 1992) that is mainly controlled by NW-SW running major faults. The Cretaceous succession in the Kopet-Dagh Basin is divided into nine formations, composed mainly of sandstones, conglomerates, mudstones, limestones and dolomites with minor amounts of evaporates (Afshar-Harb 1979). The considerable lateral changes of the sedimentary packages and the thickness of the Aitamir Formation in the Kopet-Dagh basin are related to the tectonic and eustatic sea-level changes that are the main controlling factors on the sediment supply and available accommodation space. The objectives of this paper are: (1) to describe lithofacies/facies associations and the role of wave-induced currents on the sediment distribution as the main autogenic controlling factor in the Aitamir Formation and (2) to use sedimentological data to interpret the depositional environment of the Aitamir Formation. Representing a more detailed depositional model for the studied succession could provide valuable insights into basin evolution of Kopet-Dagh Basin during Albian–Cenomanian. Material &Methods:This study focused on one measured section in the western Kopet-Dagh Basin in the northwest of Iran. The geographical coordinates of the Oghchi section is 37° 45´ 00̋ N and 55° 25´ 33̋ E (about 35 km northeast from Gonbad-Kavous City). Lithology, macro-fossils, grain-size, bedding geometry, bedding contact, stacking pattern, and sedimentary structures were recorded in detail. Lithofacies were classified/entitled following Miall (2006) and petrofacies according to Folk’s (1980) classification. One hundred and forty thin sections were examined to identify fine-scale physical characteristics (mineralogical composition and fossil contents). Discussion of Results & Conclusions:The Aitamir Formation in the studied section is about 1630 m thick and subdivided into two lower and upper parts. The lower part of includes medium- to the thick-bedded sheet-like glauconitic sandstones and thick packages of dark grey shales (up to 80 m) and thick packages sandstone constitute the uppermost part of this unit. The upper part of the Aitamir Formation mainly consists of a thick package of dark grey shales intercalated with the thin-bedded sandstones. Based on sedimentological features, four facies associations are recognized in the studied succession of the Aitamir Formation as follows: offshore (FA), offshore transition (FB), lower shoreface (FC), and mid-upper shoreface (FD). The offshore facies association consists of a thick package of dark grey fissile shales intercalated with the thin-bedded, very fine-grained sandstones. The offshore facies association includes abundant framboidal and chamber filling pyrites displaying a dysoxic condition in the deepest part of the sedimentary environment suggesting these sediments as a prone candidate for hydrocarbon exploration. The offshore transition facies association includes a thick package of grey shales with several thin to the medium-bedded sheet-like sandstones, displayed by Sm, St, Sp., Shl. The lower shoreface facies association consists of an alternation of dark grey shales and thin- to the medium-bedded sheet-like sandstones with sharp lower and upper boundaries. Sm, Shl., Sp, and HCS are the most common facies in this facies association. The mid-upper shoreface facies association includes amalgamated, sheet-like, medium- to thick-bedded sandstones with large scale St, Sp, and Sm, Shl. The sedimentary structures such as HCS, Shl, and sheet like geometry of the sandstone layers with the sharp and/or erosional boundaries as well as thickening and coarsening upward stacking pattern of the sediments of the Aitamir Formation display a wave-dominated shoreface depositional system. The structural morphology and normal faults direction of the Kopet-Dagh Basin are the allogenic and large scale elements that were directly controlled the sedimentary processes and hydrodynamic regime and type of the lithofacies/facies associations (indirectly) of the Aitamir depositional system. In this context, the Kopet-Dagh basin in the Barremian–Aptian time was located in the northern part of the Neo-Tethys Ocean and was characterized by the normal faults with E-SE to W-NW direction. The formation of the rift-related faults led to the development of the elongated, open coast morphology of the Kopet-Dagh Basin, where wave-generated currents increased as a sedimentary mechanism of the sediment distribution. In a general view, the petrography and field observations, facies associations relationship, and vertical trend of the studied successions suggest Aitamir sediments in the Kopet-Dagh basin were deposited in a wave-dominated shoreface environment with mid-upper shoreface, lower shoreface, offshore transition, and offshore zones. These sediments were deposited in a shallow marine depositional setting, characterized by gradationally vertical changes in the facies associations and abundant storm deposits. This depositional system was influenced by storm waves, suggested by the abundant storm-induced facies such HCS, graded-bedded sandstone with erosional lower surface and Shl. The presence of a thick package of the dark shales with abundant framboidal and chamber filling pyrites of the Aitamir Formation suggests these sediments as a prone candidate for hydrocarbon exploration.

Keywords

• aitamir
• kopet-dagh
• shoreface
• tectonic
• hydrocarbon