شاهد اولیه در مورد رادیولرهای ژوراسیک – کرتاسه زیرین در ناحیه آب بند ، زاگرس مرتفع ، جنوب غرب ایران
| دسته | چینه شناسی و فسیل شناسی |
|---|---|
| گروه | سازمان زمین شناسی و اکتشافات معدنی کشور |
| مکان برگزاری | بیست و ششمین گردهمایی علوم زمین |
| نویسنده | سید احمد بابازاده |
| تاريخ برگزاری | ۰۱ اسفند ۱۳۸۵ |
چکیده
ناحیه آب بند در زاگرس مرتفع (غرب ایران) قرار دارد. دو مقطع ((AA & CC در این ناحیه مورد مطالعه قرار گرفته است. رخساره چرت قرمز تا خاکستری شامل فونا رادیولر دار (۸۰% رادیو لر) و رخساره توربیدیت از کالکارنیت ، پل-بیو میکرو اسپاریت با ندول هایی از چرت رادیولر دار و آهک آرژیلیتی حاوی فرامینیفرا بنتیک تشکیل شده است. دو فو نای رادیو لر دار شامل
Holocryptocanium aff. barbui, Stichocapsa cf. robusta, Archaeodictyomitra aff. apiarium, Archaeodictyomitra sp. , Cryptamphorella sp. and etc.
در بخشهای زیرین مقطع AA و در طول مقطع CC مشاهده شده است.
فونای دریایی کف زی شا مل
Pseudocyclamina sp. , Textularids, miliolids می باشد
شاهد اولیه در مورد رادیولرهای ژوراسیک – کرتاسه زیرین در ناحیه آب بند ، زاگرس مرتفع ، جنوب غرب ایران
سید احمد بابازاده
دانشگاه پیام نور بیرجند ، بیرجند ، ایران
چکیده
ناحیه آب بند در زاگرس مرتفع (غرب ایران) قرار دارد. دو مقطع ((AA & CC در این ناحیه مورد مطالعه قرار گرفته است. رخساره چرت قرمز تا خاکستری شامل فونا رادیولر دار (80% رادیو لر) و رخساره توربیدیت از کالکارنیت ، پل-بیو میکرو اسپاریت با ندول هایی از چرت رادیولر دار و آهک آرژیلیتی حاوی فرامینیفرا بنتیک تشکیل شده است. دو فو نای رادیو لر دار شامل
Holocryptocanium aff. barbui, Stichocapsa cf. robusta, Archaeodictyomitra aff. apiarium, Archaeodictyomitra sp. , Cryptamphorella sp. and etc.
در بخشهای زیرین مقطع AA و در طول مقطع CC مشاهده شده است.
فونای دریایی کف زی شا مل
Pseudocyclamina sp. , Textularids, miliolids می باشد.
کلمات کلیدی: رخساره چرتی ، رخساره توربیدیتی ، ناحیه آب بند ، زاگرس مرتفع ، ایران
First evidence of Jurassic-Early Cretaceous Radiolarian fauna in Ab-Band area, High Zagros, South-west Iran
Seyed Ahmad BABAZADEH
BirjandPayamnoorUniversity, Birjand, Iran
Corresponding e-mail: ababazadeh2001@yahoo.fr
ABSTRACT
The Ab-Band area is located in the High Zagros (west Iran). Two sections (AA & CC) are examined in this area. The red to gray chert facies includes the radiolarian fauna (80% Radiolaria) and turbidite facies is composed by calcarenite, pel-biomicrosparite with nodular radiolarian chert and argillaceous limestone bearing benthic foraminifera. Two radiolarian fauna such as Holocryptocanium aff. barbui, Stichocapsa cf. robusta, Archaeodictyomitra aff. apiarium, Archaeodictyomitra sp., Cryptamphorella sp and etc. are occured at lower part of section AA and throughout of section CC. The benthic marine fauna include Pseudocyclamina sp., textularids, miliolids and echinoids.
Key words: Chert facies, Turbidite facies, Ab-Band area, High Zagros, Iran
INTRODUCTION
Iran is, one of the transit blocks in the Tethyan zone, a part of short sections of the Alpine orogenic belts located between the Arabian-African block (Gondwana margin) and the Asian plate (Eurasia margin).
On the basis of paleomagnetic data as well as paleotectonic and paleogeographic reconstruction, three major tectonic units are recognized in Iran: Turanian plate, Iranian plate and Arabian plate (Lensch et al. 1984) (pic-1). These units are separated from each other by ophiolitic complexes (Stöcklin 1977). These are subdivided into smaller elements on the basis of other criteria such as intensity of deformation, structural type, age, nature of sedimentary basin and magmatism. They are represented by Kopet Dagh, Southern Caspian sea, Zagros thrust, Zagros folded belt, Alborz mountain and Central Iran. The Zagros thrust belt extends for about 2000 km from southeastern Turkey through northern Syria and Iraq to western and southern Iran.
The studied area, the Ab-Band radiolarite is located in the internal part of Zagros fold-thrust belt and belongs to Neyriz ophiolite. This ophiolite is considered to be an allochthonous fragments of Tethyan oceanic crust and mantle and is exposed in the southwestern part of the Sanandaj-Sirjan metamorphic belt (Stocklin 1974). It consists of abyssal facies of the Neo-Tethys ocean basin containing radiolarian cherts, and turbiditic facies bearing Jurassic oolitic and fossiliferous limestone.
The Ab-Band area is limited by 54º 15’ to 54º 30’E and 28º 45’ to 29º N (Pic-2). The aim of this paper is to describe lithofacies and radiolarian assemblages of the Jurassic to Lower Cretaceous marine carbonate-siliceous successions in this area.
DISCUSSIONS AND RESULTS
In the studied area, the main lithofacies comprise with red argillaceous chert, radiolarian chert, calcareous chert, calcarenite, pel-biomicrosparite and argillaceous limestone. Two units (chert and turbidite units) were recognized in this area. The chert unit was repeated three times throughout the examined sections. The calciturbidite (turbidite unit) is shown in central part of section AA. The Jurassic-Lower Cretaceous benthic foraminifera are found in the calciturbidite. The chert unit is characterized by the thin bedded red radiolarian chert and argillaceous chert. The contact between chert unit and calciturbidite unit is sharp. Iron and Manganese oxides are abundant in the chert unit. Two sections are examined in this area and the total thickness is 105m. The lower part of section is truncated by fault and the top of section is unconformably covered by Maastrichtian Tarbur Formation (pic-3).
These sections consist of pelagic facies containing Radiolaria refers to basin domain. The previous works were general and only indicated the presence of Radiolaria in thin section. This report presents a detailed study of lithostratigraphy and radiolarian fauna. The Maastrichtian Tarbur Formation, with a basal polymict pebble-conglomerate, unconformably on laps the Ab-Band radiolarite and the base of radiolarite is truncated by fault.
Among the microfossils encountered, the most interesting are Radiolaria. Other microfossils such as fragments of molluscan tests, echinoid spins have no chronological significance but are interesting as facies indicators. The benthic foraminifera are long range.
The layers S3 and S8 in the section AA, radiolarian chert yields the following fauna Archaeodictyomitra aff. apiarium, Parvicingula sp. and Hiscocapsa cf. asseni.
In the layer S14 at section CC, the red radiolarian chert contains Stichocapsa cf. robusta Cryptamphorella sp.
The layer S5in the section AA, pel-bio microsparite contains the benthic foraminifera such as Pseudocyclamina sp. and Textularids.
Two radiolarian assemblages were dating Late Jurassic to Early Cretaceous.
The depth of formation of radiolarites is questionable. The radiolarian chert of Neyriz area, south-west of Iran, could be similar to east Iranian radiolarites because both of them were associated with planktic foraminifera (Keshani 1997; Babazadeh & De Wever 2004 ) whereas the Ab-Band radiolarite was found with the calciturbidite bearing benthic foraminifera. The limestones in calcitubidite are oolitic and contain a poor microfauna indicating a low depth of deposition. The probable conclusion which may be drawn from this contradictory fact, the benthic foraminifera don’t belong to the autochthonous fauna, but are allochthonous (reworked fauna) and replaced by the turbidity currents.
If this hypothesis is correct, then there are two sedimentary cycles: the autochthonous phase for radiolarian chert and the allochthonous phase for calciturbidite.
REFERENCES
BABAZADEH S. A., DE WEVER P. 2004.Radiolarian Cretaceous age of Soulabest radiolarites in ophiolite suite of eastern Iran. Bulletin Société Géologique France, t. 175, No. 2, 121-129.
KESHANI, F. 1997. Biostratigraphy studies of Radiolaria in the Neyriz area, Fars Province, Iran. Geosciences, Scientific Quarterly Journal, Geology Survey of Iran, No. 21 & 22, 2-31.
LENSCH G., SCHMIDT K., DAVOUDZADEH M. 1984.Introduction to the geology of Iran. Neues Jahrbuch fur Geologie und Paläontologie, 155-164.
STÖCKLIN J. 1974.Possible ancient continental margin in Iran. In: C. A. Burk & C. L. Drake, Eds., The geology of continental margins. - Berlin, Springer, 873-887.
STÖCKLIN J. 1977. Structural correlation of the Alpine ranges between Iran and Central Asia. - Mém. H. S. Soc. géol. Fr., 8, 333-353.
Explanation of figures
Pic. 1: Modified sketch map of Iran showing the major tectonic units.
Pic. 2: Stratigraphic columns of Ab-Band area.