Palaeoecology and paleoenvironment of Parvadeh Formation (Middle Jurassic) based on Scleractinian corals in southwest of Tabas

IntroductionIn northwest of Tabas (NE Iran), the type section of Parvadeh Formation with 46 meters in thickness was measured which its lower part consists of 7 meters of conglomerate and the upper parts include a succession of dense and rarely pisolitic dark gray limestones which has fixed key position between sandstones of Hojedk Formation and marls of Baghamshah Formation. Previous studies on Jurassic corals of Central Iran were done systematically by Flügel (1966) on Scleractinian corals of Shotori Mountains (east of Iran) within Shemshak, Badamu, Baghamshah, Esfandiar and Ghaleh-dokhtar Formations. In these studies, Chomatoseris iranensis were introduced for the first time from Ghaleh-dokhtar Formation (Bathonian-Callovian). Moreover, Pandey and Fürsich (2003) studied 50 genera and 92 species of Scleractinian corals of Toarcian-Kimmeridgian ages in east of Iran. Also, Khaksar (2010) introduced some of the Jurassic corals in Ghaleh-dokhtar Formation in Tabas region.
In this research, coral specimens of carbonate units of Parvadeh Formation at Mazinou and Kamar-Mahdi sections in southwest of Tabas city were studied. In the aim of paleoecological and paleoenvironmental studies of Parvadeh Formation in these two sections, analysis of paleoenvironment with the aid of fossil records evidences have been carried out.
Materials and methodsIn order to study paleoenvironment and paleoecology of corals in studied sections, more than 263 coral specimens and some rock samples of the studied sections were collected. Among these samples, totally more than 100 thin sections were made of diverse coral specimens. Before preparation of thin sections, samples were immersed in cold water for 10 days. Then, after labeling and preparing descriptive specifications of each sample, general characters of each specimen such as external morphology including general coral shape, length, maximum diameter, existence or lacking transversal or longitudinal linear bands, theca preservation, rejuvenation and columella were noted. Then transversal and longitudinal thin sections were made from coral specimens. The size of thin sections depend on coral size which varies from 70x25 to 90x70 mm. Internal structures and microstructures of coral specimens were studied by Nikon stereomicroscope SMZ 1000 in Department of Researches and Studies of Paleontology and Geochemistry of Exploration Directorate of National Iranian Oil Company.
DiscussionCorals of Parvadeh Formation could be found in carbonate units of both sections, especially in the middle parts. Different factors could affect the coral's life such as sea level changes, substrate morphology, sediment supply, salinity and temperature. According to the morphological evidences, e.g. corallite arrangement, type of colony and thickness of dissepiments, paleo-environment of the studied successions could be reconstructed (Badpa et al., 2011). Systematic study of corals in Mazinou and Kamar-Mahdi sections led to identification of 15 genera and 22 species as follows:Solenocoenia sexradiata, Heliocoenia gracilis, Stylina micrommata, Pseudocoenia cf. slovenica, Montlivaltia caryophyllata, Montlivaltia cornutiformis, Coenothecazolleriana, Thecosmilia langi, Epistreptophyllum cornutiforme, Epistreptophyllum commune, Epistreptophyllum sp1, Epistreptophyllum sp2, aff. Calamophylliopsis flabellum, Thamnasteria concinna, Thamnasteria mattensis, Dimorphastraearenevieri, Actinastreapentagonalis, Isastrea aff. browni, Microsolena amorpha, Microsolena ornata, Microsolena verdati, Chomatoseris iranensis.
Abundance of identified genera and species varies from Mazinou to Kamar-Mahdi area. Differences between species in these two sections could be resulted due to distinction of some environmental factors especially ineffective factors such as water energy which causes changes in the amount of food and insoluble sediments that controls water clarity. Consequently, species with higher potential of adaptation will survive and be found.
Scleractinian corals have an important role in reconstruction of paleo-environment of Middle Jurassic in southwest of Tabas. These communities prefer shallow warm water, euphotic zones, normal salinity and oligotrophic conditions of carbonate platforms. Based on the mentioned morphological evidences, the suggested paleo-environment of the studied sequences could be considered as a part of back-reef, reef crest and reef front (shallow area) of a carbonate platform in Mazinou and Kamar-Mahdi sections.