Evolution of sedimentary environment and sequence stratigraphy of the Sarvak Formation in Aleshtar area (north of Lorestan)

The Sarvak Formation is mainly composed of limestone and is one of the most important rock reservoirs in the Zagros sedimentary basin. This formation has been studied extensively in many parts of the Zagros; however the information about various aspects of this formation in northern Lorestan is limited. Due to the discovery of a pycnodont fish tooth plate from Aleshtar area, and because of scarce data on Cretaceous deposits in this region, we conducted this research.
Material &

In order to study the age, sedimentary environment and sequence stratigraphy of the Sarvak Formation, field and microscopic studies of 100 thin sections from an outcrop near Aleshtar (north of Lorestan) has been carried out.
Discussion of Results &

The Sarvak Formation in the studied section is Cenomanian in age and is 835 meters thick. It is bounded by Garu and Surgah formations at the base and top respectively. It is mainly composed of thick, medium, and thin bedded limestone rich in chert nodules, bioturbation and rudists. The studied outcrops can be subdivided into four units litho-stratigraphically. The first unit is 200 meters of thick, grey limestone with benthic foraminifera and echinoids. Second unit is 320 meters of mainly grey, medium bedded limestone with reddish intercalation of shale, containing benthic foraminifera, rudist debris and bioturbation. 180 meters of thick bedded, grey limestone with rudist and other bivalve debris forms the third unit. The last litho-stratigraphic unit consists of 135 meters of medium to thin bedded, grey limestone with interlayers of dolomitic limestone and rudist/bivalve debris and bioturbation. Benthic foraminifera are less abundant here.
Our study led to the identification of 12 microfacies and 3 facies-belts, which belong to tidal flat, lagoon, and open marine environments. These microfacies are differentiated based on abundance and diversity of skeletal and non skeletal remains and rock texture. These facies, ordered from shallower to deeper environments, include: bioclastic stromatolite wackestone to bindstone, and pisoid rudstone in tidal-flat facies belt; bioclast peloid intraclast grainstone, bioclast green algae wackestone to packstone, peloid ooid grainstone to packstone, bioclast echinoderm benthic foraminifera wackestone, rudist benthic foraminifera wackestone, orbitolina miliolid wackestone to packstone, bioclast miliolid wackestone, and benthic foraminifera rudist floatstone to rudstone in lagoon facies belt; and rudist floatstone to rudstone, and bioclast oligosteginid wackestone within open marine facies belt.
According to type and gradual changes of microfacies in lower parts of the Sarvak Formation, and compared to abrupt changes of microfacies and presence of extensive rudists in the upper parts of this formation, lower and upper parts of the Sarvak Formation are deposited in two different sedimentary environments. Presence of micrite in most of microfacies, formation of sedimentary textures such as wackestone to packstone and absence of traceable reefs on the ground, shows that the carbonate sediments of the Sarvak Formation in the lower parts in the studied section were deposited in a ramp carbonate platform. Widespread distribution of rudists in the field, abundance of oligosteginids, and abrupt changes of microfacies along the section; like alteration of pisoid and stromatolite facies to oligosteginids in a short interval, indicates that the upper parts of the studied section were deposited in a carbonate shelf.
Three third-order depositional sequences were determined based on the deepening and shallowing trend of facies. The transgressive systems tract consists mainly of microfacies belonging to the lagoon and open marine sedimentary environments, which contain the benthic foraminifera, echinoids and oligosteginids. The highstand systems tract consists mainly of microfacies belonging to the lagoon with green algae and the benthic foraminifera, and also to the microfacies belonging to the tidal flat, which contain stromatolite and pisoid. The maximum progression level in sediment sequences 1 and 2 was determined by microfacies rich in echinoid and rudist debris. In sedimentary sequence 3 it is identified with oligosteginid rich microfacies.