Sequence Stratigraphy of Middle Albian to Cenomanian sediments (Sarvak Formation) in Tang-e-Chenarbashi (southeast Ilam) and role of fossil fauna for recognition and interpretation of the depositional sequences

Middle Albian to Cenomanian strata of the Sarvak Formation indicate pelagic facies in Tang-e-Chenarbashi stratigraphic section. These strata deposited in five sequences of the third order and nine sequences of the fourth order. These sequences were determinated in a succession including six microfacies mudstone, sponge spicule packstone, peloidal packstone, radiolarian packstone, Oligosteginid, planktonic foraminifera wackestone - packstone, benthic foraminifera echinoidal wackestone - packstone. Study of the third order sequences show that in some of them exist paraconformity which related to sequence boundary of the fourth order sequences. Investigating these sequence boundaries and recognition the sequences were done based on planktonic foraminifera biozones. In this study, separating the fourth order sequences is generally based on fossil fauna. In the fourth order sequences, maximum flooding surfaces (mfs) were recognized based on maximum diversity and abundance of planktonic taxa and decreasing of benthic foraminifera. The Sarvak Formation, one of the four formations of the Bangestan Group in Zagros basin, generally includes two facies, deep facies in the Lurestan province and shallow facies in the Khuzestan and Fars provinces. The Sarvak Formation deposited during Middle Albian to Early Turonian which the two main orogenic phases, Austrian (Albian / Cenomanian) and Sub-Hercynian (Cenomanian / Turonian) occurred. The warmest climate and the highest sea level took place along the Earth history in Albian to Cenomanian, and this could be a good and unique objective to investigation the effects of many interesting phenomena due to the amazing and unrepeated events. In addition, depositions of this time span could be included interesting depositional sequences. The aim of this research is recognition of depositional sequences of the Sarvak sediments by using of fossils. One stratigraphic section with 712 meters thickness was selected for studying of deep facies of the Sarvak Formation in Lurestan province. Totally, 148 thin sections were studied for recognizing microfacies characteristics and paleoenvironment based on Flugel (2010) and Wilson (1975). Study of sequence stratigraphy of the Sarvak Formation in the examined section was carried out based on Hunt and Tucker (1992, 1995) by using microfacies with fossil content. A critical and detail biostratigraphic study with relay on planktonic foraminifera and microfacies is used here is based on Daneshian et al.(2011, 2013). On the basis of the mentioned references, age determination clears to us a Middle Albian to Cenomanian age for the Sarvak Formation in the selected section. Also, 6 microfacies including mudstone, sponge spicule packstone, peloidal packstone, radiolarian packstone, Oligosteginid, planktonic foraminiferal wackestone - packstone, benthic foraminiferal echinoidal wackestone – packstone, were determinated. Study of depositional sequences in the pelagic and hemipelagic sediments is complicated and confusing. In fact, lack of exposures and also similarity and monotonous between the facies constructing the depositional sequences, make it difficult to differentiate systems tracts. Geological field studies do not play a critical role in sequence stratigraphic studies of the Sarvak deposits in Tang-e-chenarbashi section. thus, we focused on microfacies data to find the system tracts. The microfacies show that the strata deposited in five third-order sequences. The problem displayed when we faced to some paraconformity existed in the third order sequences. The paraconformities were recognized by biostratigraphic studies. Since these paraconformities are belong to the fourth order sequences, we persuade to elicit the fourth order sequences for the studied section, but when we know this fact the cause of fourth and lower order sequences is Milankovitch cycles, we should draw our attention to fossils than microfacies. The purpose of this paper is not Cyclostratigraphy but is to clear role of fossils in separating depositional sequences. In this research two functions of fossil data are considered: 1) dating the depositional sequences using global biozones, and 2) investigation of system tracts where microfacies are ineffective tools. In this study the abundance of four main fossil groups including planktonic foraminifera, benthic foraminifera, oligosteginids and echinoid fragments illustrated against lithostratigraphic column, sea level changes and investigated sequences. Many features such as amount of planktonic foraminifera, broken fossil fragments, dramatically the increase or decrease in r-selected or k-selected taxa and many other similar to these, coincide with ts surface or mfs which help us to demonstrate the forth-order depositional sequences. This study tries to establish relation between classic usage of fossils just for age determining and use it as main allochem that is able to reflect many small scale events which tracking them in the deposit is impossible.
Generally, five third-order sequences in a succession including six microfacies were recognized. Biostratigraphic data helped us to know the age of depositional sequences to attribute them to correct order with more confidence. Also, on the basis of extracted biozones, some paraconformities have been identified.