Depositional environment of the Barut Formation at the Barut Aghaji section, northwestern Soltanieh Mountains, western Alborz
Abstract The Lower Cambrian Barut Formation is one of the most important lithostratigraphic units in reconstructing the paleogeography of northern Iran during the early Cambrian time. Herein, we present detailed facies analysis data of this formation in the Barut Aghaji Section in southwest Zanjan t...
| Main Authors: | , , |
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| Format: | Article |
| Language: | fas |
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University of Isfahan
2023-01-01
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| Series: | Journal of Stratigraphy and Sedimentology Researches |
| Subjects: | |
| Online Access: | https://jssr.ui.ac.ir/article_27264_c9a7c236e76a83ec95345cc2c8bdb67b.pdf |
| Summary: | Abstract
The Lower Cambrian Barut Formation is one of the most important lithostratigraphic units in reconstructing the paleogeography of northern Iran during the early Cambrian time. Herein, we present detailed facies analysis data of this formation in the Barut Aghaji Section in southwest Zanjan to analyze the palaeoenvironment. The Barut Formation in this section consists of about 500 m thick mixed siliciclastics and carbonate rocks and its upper and lower contacts with both Soltanieh and Zaigun formations are gradational. Seven recognized lithofacies/microfacies include: red, fine- to medium-grained arkosic sandstone containing physical and biogenic tidal indicators (F1), red to green, finely laminated mudrock containing tidal biogenic structures (F2), dolomitic stromatolite boundstone (F3), dolomitic mudstone with evidence of former evaporites (F4), dolomitic thrombolite boundstone (F5), carbonate breccia/conglomerate (F6), and dolomitic oncoid wackestone (F7). These facies are arranged in shallowing-up cycles consisting of mixed flat sandstones overlain by mud flat mudrocks and supratidal carbonates. Evidence of evaporite minerals in the Barut Formation along with other contemporaneous early Cambrian evaporite basins in the Middle East, India and Pakistan, indicates a dry and warm palaeo climate during the deposition of the sediments.
Keywords: Early Cambrian, Barut Formation, Barut Aghaji Section, Tidal flat, Evaporite basins.
Introduction
The Late Neoproterozoic to Early Cambrian is one of the most important time intervals in the history of Earth. Extreme climate changes, major tectonic events such as the formation of the East African Orogens, oxygenation events, change of seawater composition and rising nutrient input, as well as the emergence and diversification of multicellular life, are among the most important events of this time (Zhang et al. 2014). A large part of the reconstruction of these important events has been obtained from the study of sedimentary rocks.
In the palaeogeographic reconstructions of Late Neoproterozoic–Early Cambrian, Iran is placed along the northeastern margin of Gondwana, between the Arabia and India plates. The oldest sedimentary rocks in Iran are siliciclastics with subordinate carbonate rocks known as Kahar, Bayndor, Soltanieh, Barut and Zaigun formations, which range in age from Ediacaran to lower Cambrian (Stocklin et al. 1964; Horton et al. 2008). Researchers believed that this sedimentary succession was deposited mainly in a shallow marine environment.
The Barut Formation in the northwest of the Soltanieh Mountains, 18 Km southwest of Zanjan and northwest of Barut Aghaji village, consists of about 500 m of purple mudrocks and carbonates. The lower and upper boundary of this formation, with the Soltanieh and Zaigun formations, respectively, is conformable and gradational. The depositional ages assigned to the Barut Formation range from Late Neoproterozoic (e.g., Horton et al. 2008) to early Cambrian (Salehi et al. 1998; Devaere et al. 2018). Despite the importance of the Barut Formation in the reconstruction of the palaeogeography of Iran during the Ediacaran–Early Cambrian, this formation has received little sedimentological study. For the first time, this study examines the lithology and palaeodepositional conditions of the Barut Formation in the Barut Aghaji section (Type section). Our findings provide new data on the palaeogeographical conditions of this area in northern Iran during the Early Cambrian time.
Material & Methods
In this study, detailed field investigations were carried out to prepare a stratigraphic section, sampling and recognizing lithofacies. The main lithofacies were recognized using lithology, geometry, the nature of bed boundaries, and sedimentary structures during field studies according to Stow (2005). A total of 100 siliciclastic and carbonate samples were collected at regularly spaced intervals. After preparing thin sections, petrographic studies were performed on thin sections according to the classification of Folk (1974), Tucker (2009), and Dunham (1962). Lithofacies were interpreted according to Stow (2005), Flügel (2010), Einsele (2000), Nichols (2009), and Davis (2012), and then were classified into facies associations that reflect the palaeoenvironments.
Discussion of Results & Conclusions
Seven lithofacies have been identified in the studied section, which can be grouped into two main facies associations: (1) Siliciclastic-dominated facies association and (2) Carbonate-dominated facies association. Siliciclastic-dominated facies association includes fine- to medium-grained red arkosic sandstone containing tidal bedding with flaser and lenticular laminations, herringbone cross stratifications, mud drapes on cross stratifications and ripples, and Microbially Induced Sedimentary Structures (MISS) (F1) and red to green, finely laminated mudrock containing MISS (F2). Carbonate-dominated facies association includes dolomitic stromatolite boundstone (F3), dolomitic mudstone with evidence of former evaporites (F4), dolomitic thrombolite boundstone (F5), carbonate breccia/conglomerate (F6), and dolomitic oncoid wackestone (F7). These facies contain several physical and biological tidal criteria and are arranged as shallowing upward cycles. Each cycle starts with mixed flat sandstones overlain by mud flat mudrocks and supratidal carbonates that are subjected to subaerial exposure. The results of this study indicate that tidal flat sedimentation conditions prevailed in the study area throughout the early Cambrian, from the Upper Dolomite Member of the Soltanieh Formation to the Zaigun Formation. In addition, evidence of evaporite minerals in the Barut Formation along with other contemporaneous Early Cambrian evaporite basins in the Middle East, India and Pakistan, indicates dry and warm palaeoclimate during the deposition of sediments. More information on palaeotectonic would help us to establish a greater degree of accuracy on this matter. |
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| ISSN: | 2008-7888 2423-8007 |