برش میغان در 5 کیلومتری روستای میغان و 20 کیلومتری شمالشرق شاهرود، در زون ساختاری البرز شرقی قرار دارد. در این برش، توالی نسبتاً کاملی از نهشتههای پالئوزوئیک (سیلورین؟- پرمین) وجود دارد که نهشتههای دونین بالایی- کربونیفر زیرین بهمنظور مطالعۀ زیستچینهنگاری نهشتههای گذر دونین- کربونیفر مطالعه شدند؛ به این منظور، 45 نمونۀ سنگی بهطور سیستماتیک از 91 متر توالی مرز برداشت و با مطالعههای آزمایشگاهی، تعداد 280 عنصر کنودونتی شامل 23 گونه متعلق به 5 جنس شناسایی شدند. باتوجهبه کنودونتهای بهدستآمده از این توالی، شش بایوزون زیر تعیین شدند:
Bispathodus aculeatus aculeatus Zone, Bispathodus costatus Zone, Bispathodus ultimus Zone, praesulcata Zone, ckl, sulcata Zone.
مقایسۀ زیستچینهنگاری و سنگچینهنگاری برش مطالعهشده با توالی سنگچینهای و بایوزونهای استاندارد جهانی ارائهشده برای گذر دونین- کربونیفر به شناسایی ناپیوستگی فرسایشی در این مرز و نبود بایوزون کنودونتی kockeli Zone از آخرین بایوزونهای فامنین پسین منجر شد. همچنین بررسیهای صحرایی و آزمایشگاهی به شناسایی افقی از شیل تیره و در ادامه، یک افق ماسهسنگی در مرز دونین-کربونیفر منجر شد که با حادثۀ زﻳﺴﺘﻲ ﻫﻨﮕﻨﺒﺮگ قابلانطباق است. این حادثه ﻛـﻪ ﺑـﺎ اﻧﻘـﺮاض ﺷـﺪﻳﺪ و ﻳـﺎ میزان کم ﻇﻬـﻮر ﺗﺎﻛﺴـﺎ ﻣﺸـﺨﺺ ﻣـﻲﺷﻮد، ﺑﻴﺸﺘﺮﻳﻦ ﺗـأﺛﻴﺮ را روی ﻓﻮﻧـﺎی آمونوئیدها، ﺗﺮﻳﻠﻮﺑﻴـﺖﻫـﺎ، براکیوپودها و ﻛﻨﻮدونتها در برش مطالعهشده داﺷـﺘﻪ است.
عنوان مقاله [English]
Biostratigraphy of the Upper Devonian-Lower Carboniferous deposits in Mighan section, northeast Shahrood, Eastern Alborz
The Devonian/Carboniferous (DC) transition is characterized by several transgressive/regressive cycles which led to a widespread ocean anoxia known as the Hangenberg Black Shale Event (HBS), close to the D/C boundary a major sea-level fall (Hangenberg Sandstone, HSS), can be recognized in many sections around the world. Both events known as the Hangenberg Crises. In order to examine the D/C transition, the Mighan section, in Eastern Alborz was selected and studied. The outcrop section is located 25 Kms NE Shahrood city. Frothy five conodont samples (4-5 kg) were systematically taken from 91 ms of the D/C transition interval. Although the conodonts show low frequency but high diversity exhibit important zonal index taxa of the widely applied conodont standard zonation. Twenty-three conodont species belong to five genera were identified and let to discrimination of six following zonal boundaries: Bispathodus aculeatus aculeatus zone, Bispathodus costatus zone, Bispathodus ultimus zone, praesulcata zone, CKl interregnum, sulcate zone. At the D/C transition in Mighan section a black shale and sandstone units are observable that corresponds to the Hangenberg Crisis, that highly affected trilobite, ammonoid, brachiopod and conodont faunas. Kockeli Zone at the latest Famennian is missing at the studied profile due to the major sea level regression.
The study of DC boundary, the mechanism and reasons for the reduction of different fauna, has been an important issue for many paleontologists in the world and in Iran as well.
DC boundary is one of the most important boundaries on the Earth, in or shortly before which, an important biological event called Hangenberg event, occurred. The Hangenberg event is one of the major events in the Phanerozoic, and affected more than 20% of marine invertebrate families and 45% of the general population (Simakov 1993; Sepkoski 1996). The Hangenberg event has been identified on epicontinental basins and continental margins in the world. Black shale and Sandstone deposits are very important in identifying this event in different climatic and oceanographic studies. Kaiser (2005) considered that the Hangenberg event is a polyphase crisis, and different groups of fossils from different climates affect, including various rock-diverse changes, and black shales is the main extinction phase of the Hangenberg crisis. These black shales (HBS) occurred during a vast and short-term marine flooding surface within costatus-kockeli interregnum. Successively, a vast eustatic sea-level fall at the end of the Devonian led to deposition of HSS (Becker 1993a).
DC boundary is defined via the first appearance Datum (FAD) of the basal Carboniferous conodont Siphonodella sulcata, from Global Stratotype Section and point (GSSP) located in La Serre Trench E’ section, Montag Norie, France (Paproth et al. 1991). However, the decision to define the DC boundary based on the evolutionary species of Siphonodella has some limitations such as: Detection of morphotype Si. sulcata is very hard because Holottape of Si. Sulcata is the interstitial state of Si. sulcata and Si. duplicate (Huddle 1934) and between the Si. praesulcata and Si. sulcata, there are many morphotypes whose exact diagnosis depends on the long-standing personal taste (Kaiser and Coradini 2008). In order to solve this, Corradini et al. (2011) by revising Siphonodellid in the type section and other global sections, divided Siphonodellids into seven groups based on the Platform's shapes, Basal cavity and features of Pseudokeel. The base of Carboniferous system is also defined by the FAD of Protognathodus kuheni (Corradini et al. 2011) and according to new global zonatin of Corradini et al. (2016) and Spalleta et al. (2017) is defined by the FAD of Protognathodus kockelio.
The main purposes of this study are to summarize a detailed stratigraphy below and above the D/C boundary in the Mighan section, on the base of conodonts according to new global zonation.
Material and Methods
During field work, 91m of the Upper Devonian–-Lower Carboniferous succession at Mighan section has been investigate and about 45 conodont samples (3-4kg each) were collected from the Mighan section. The samples were processed with diluted acetic acid (20%). The conodonts were extracted from residues by hand picking and are stored at the University of Isfahan, I.R. Iran and also State Museum of Natural History Stuttgart, Germany.
Discussion of Results and Conclusions
The Mighan area is located about 20 km northeast of Shahrud city which is located near the Mighan village. Geographical coordinates of the base and top of this studied section are: N: 36° 38′ 38″, E: 54° 57′ 55″ base and N: 36° 38′ 39″, E: 54° 56′ 55″ top. This section includes late Devonian uppermost sediments of Khoshyeilagh Formation (74.47 m thick) and lowermost Carboniferous of Mobarak Formation (16.53 m thick), which subdivided into six lithological units (units A to F).
The conodont zonation scheme proposed by Corradini et al. (2016) and Spalleta et al. (2017) for the Upper Devonian and Lower Carboniferous strata were utilized for lower part of the Mighan section in this paper. According to these conodont zonations Protognathodus kuheni is considered as the base of the Carbonoferous. However, due to the lack of Protognathodus in the Mighan section, we used conodont zonations of Kaiser et al. (2009) to define DC boundary based on the appearance of Siphonodella praesulcata and Siphonodella sulcata. Totally five bio-intervals have been discriminated in the Mighan section: Bi. aculeatus aculeatus Zone, Bi. costatusZone, Bi. ultimus Zone,The praesulcta Zone, The Costatus-kockeli interregnum (CKI), andthe sulcata Zone.
Field works and lab examinations led to identification of a dark and organic-rich shaley horizon at the D/C transition which corresponds to the costatus-kockeli interregnum (CKI) conodont biozone and are indicative of Hangenberg Crisis. Transgression of seawater and expansion of dark shales caused the organisms to wipe out and following regression formed HSS in the Mighan section.
These dark shales are representative of the dominance of dysoxic to anoxic conditions accompanied by low rate of deposition and considerable decline of organisms. The migration of anoxic waters during the transgression of sea water, which is evidenced through the occurrence and increase of conodont species such as Bispathodid, gave rise to the losing habitats, invasion and competition to occupy the habitat and extensive decline of benthic organisms. The reduction of brachiopods, trilobites and ammonoids in the dark shales and topmost beds at the Mighan section are potential documents for this event.
Despite intensive sampling in this study, no latest Famennian kockeli Zone was recognized at the studied section implying the presence of disconformity and discontinuity of deposition. Therefore, the basal limestone immediately above the sandstones at the Mighan section is comparable to Early Tournaisian Si. sulcata Zone.