توزیع آمونیت ها و ارتباط آنها با تغییرات سطح تراز دریا در سازند های سرچشمه و سنگانه در حوضه کپه داغ در شمال شرق ایران

دسته چینه شناسی و فسیل شناسی
گروه سازمان زمین شناسی و اکتشافات معدنی کشور
مکان برگزاری بیست و چهارمین گردهمایی علوم زمین
نویسنده سید ناصر رئیس السادات
تاريخ برگزاری ۰۹ اسفند ۱۳۸۴

چکیده:

حوضه رسوبى کپه داغ در شمال شرق ایران و جنوب ترکمنستان قرار دارد. در این تحقیق ارتباط بین توزیع آمونیت ها و تغییرات سطح تراز دریا در سازند هاى سرچشمه و سنگانه بحث شده است. براى هر مقطع چینه شناسى اندازه گیرى شده سنگ شناسى، اولین ظهور جنس ها یا گونه هاى آمونیتى، پارا سکانس ها، تغییرات نسبى سطح تراز دریا و تعدا نمونه هاى جمع آورى شده در مقابل همدیگر قرار گرفته و بررسى شده است. این مطالعه نشان میدهد تعداد و تنوع گونه ها به بالاترین حد خود در بارمین پسین و آپتین پیشین مى رسد و در آپتین میانى و پسین کاهش یافته است. پاراسکانس ها نشان مى دهد در بارمین پسین و آپتین پیشین سطح تراز دریا بیشتر از آپتین میانى و پسین دستخوش تغییر بوده است.

 

Ammonite distributions and its relation with the sea-level changes in the Sarcheshmeh and Sanganeh Formations at the <?xml:namespace prefix = st۱ ns = "urn:schemas-microsoft-com:office:smarttags" />Kopet۱:PlaceName> Dagh۱:PlaceName> Basin۱:PlaceType> in north east of Iran۱:place>۱:country-region>

 

Seyed Naser Raisossadat, Geology Department, Faculty of Sciences, Birjand University, P.O.Box ۷۹/ ۶۱۵, Birjand, Iran

E-mail: snaser_rais@yahoo.co.uk, snreaisosadat@birjand.ac.ir

 

Abstract:

The Kopet Dagh sedimentary basin is situated in the north-east of Iran۱:country-region> and the south of Turkmenistan۱:place>۱:country-region>. The relationship between ammonite distributions and the sea-level changes has been discussed in the Sarcheshmeh and Sanganeh Formations. Lithological log, the first appearance of ammonite genera or species, parasequence boundaries (PSB), relative sea-level changes and ammonite taxa numbers have been integrated in measured sections. The number and diversity of ammonite genera and species reached its highest in the Late Barremian and Early Aptian, and decreased in Mid and Late Aptian times. Parasequences (and sequences) show that in the Late Barremian and Early Aptian sea-level fluctuated more than in Mid and Late Aptian times

Introduction

The Kopet Dagh sedimentary basin is situated in the north-east of Iran and the south of Turkmenistan. The Iranian part of the Kopet Dagh basin is geographically located between 54o 00′ and 61o 14′ east longitude and 36o 00′ and 38o 16′ north latitude.

The Kopet Dagh basin formed as an intracontinental basin in north-east Iran, after the closure of the Hercynian Ocean following the Early Kimmerian orogeny (Berberian & King, 1981). From the Jurassic through to the Eocene, relatively continuous sedimentation is recorded by five major transgressive- regressive sequences in the eastern Kopet Dagh (Afshar-Harb, 1979,1983). Subsidence started in the Kopet Dagh basin in the late Middle Jurassic (Afshar-Harb, 1979; Seyed-Emami and Alavi-Naini, 1990; Seyed-Emami et al., 1994,1996). Fault-controlled subsidence of the basin from Jurassic to Oligocene times has resulted in up to 10 kilometres of sediment being deposited (Berberian and King, 1981).

Jurassic-Cretaceous ammonites were reported by Seyed-Emami (1980), Seyed-Emami and Aryai (1981), Seyed-Emami et al. (1984, 1994, 1996) and Immel et al. (1997). Moreover Sarchasmeh and Sanganeh Formation have been studied on lithostratigraphy, biostratigraphy and paleontology (Raisossadat 1999, 2000, 2001, 2002, 2003a,b; 2004a,b; Raisossadat and Moussavi-Harami 1993; 2000).

Despite the studies on the Kopet Dagh basin quoted above, in many fields research has yet to be undertaken. For instance there is no study on sea-level changes and its effects on macro-fauna distribution. This paper discusses the relationship between ammonite distributions in the Kopet Dagh Basin and the sea-level changes that are indicated by sequence stratigraphical analysis. The analysis is a preliminary one and further field research is required to test the provisional conclusions.

Ten stratigraphical sections have been measured and sampled. About seven hundred specimens were collected. Most of the sections include the whole of the Sarcheshmeh and Sanganeh Formations. For some sections only parts of the sequence could be measured, because of weathering, faulting and poor exposure.

 

The faunal response to sea-level change

Several authors have studied Early Cretaceous faunas. In particular, Rawson (1993) examined the influence of sea-level changes on the migration and evolution of Early Cretaceous taxa. Based on palaeobiogeographical evidence he showed that migration of families to a new realm was related to sea-level rise, while sea-level fall led to provincialism and speciation, even when seaway connections remained open.

Reboult and Atrops (1997) studied quantitative variations in the Valanginian ammonites faunas within limestone-marl cycles and within parasequence sets in the Vocontian Basin, Southern France. They concluded that the abundance of ammonite taxa from the base to the top in some parasequences is different and shows a rhythmic variation. Hoedemaeker (1995) also discussed the relationship between ammonites and 2nd and 3rd order sea-level fluctuation in the lowest Cretaceous of south-east Spain. He found that ammonite fauna diversity reached its minimum in sea-level falls through the Berriasian to Barremian.

 

Relationship between ammonites and sea-level changes in the Kopet Dagh Basin

Takal Kuh area

Three stratigraphical sections were measured in the Takal Kuh area. As a page limitation, just the Takal Kuh section 1 has been explained here (Figure 1). In this section heteromorph genera, including Argvethites, Heteroceras, Imerites, Martelites and Paraimerites, and the planispiral form Barremites appear in the lower part of the Sarcheshmeh Formation (Upper Barremian). Here the first appearance of every genus or species is in the shaly limestone, marl or marly limestone of the transgressive and highstand systems tracts, during an inferred sea-level rise (Figure 6.1????).

In the middle part of the Sarcheshmeh Formation (uppermost Upper Barremian-Lower Aptian) the planispiral ammonites Turkmeniceras and Deshayesites and the heteromorph form Ancyloceras occur. Turkmeniceras and Ancyloceras first appear in the shaly limestones and marls, which mark transgressive and highstand systems tracts. Deshayesites species also often appear first in shaly limestone, shales, limy marls and marls. But in some cases, like samples 48 and 82, their first appearance is in limestones, which means they also occur in lowstand systems tracts.

With the beginning of the Aptian, the number and diversity of heteromorphs falls. It is not clear if this is because of ecologic conditions or sea-level changes. However there is no gap or pause in deposition at the Barremian-Aptian boundary. In the middle part of the section the number of ammonite taxa falls for about the next 150 metres. Afterward in the middle and upper part of the formation new genera including Phylloceras, Phyllopachyceras, Aconeceras, Cheloniceras and Melchiorites appear, and the diversity of ammonite taxa increases upwards. The parasequences are thinner in this part of the formation. If we assume an equal time interval for each parasequence, the depositional rate was slower and relative sea-level changes were more frequent in the upper part of the Sarcheshmeh Formation.

In the Sanganeh Formation the lithology changes mainly to shales and they are less calcareous than the Sarcheshmeh Formation shales. In contrast the number of sandy and fossiliferous limestones decreases. The Sanganeh Formation was probably deposited in deeper and quieter environments compared with the underlying and overlying Sarcheshmeh and Aitamir Formations. It is in this formation that Pseudosaynella, Tonohamites and Australiceras first appear. The first two genera occur in shale beds, which are taken as highstand systems tracts, but the third one occurs with two Deshayesites species in a sandy fossiliferous limestone bed (lowstand systems tract). The numbers of collected specimens also increase in transgressive and highstand systems tracts as whole.

A thick sandy and fossiliferous limestone occurs in the upper part of the Sanganeh Formation at the Takal Kuh sections (sample number 82 in Takal Kuh section 1), containing big Deshayesites and Australiceras. It may represent a fall in sea-level at the Takal Kuh area. This layer cannot be traced in other parts of the basin.

 

Other sections

In the other measured sections including Takal Kuh 2, Amand, Tirgan and Sanganeh sections relatinship between ammonites and sea-level changes has been examined. In some sections numbers of ammonite specimens are not enough to permit testing the relationship between ammonites and sea-level changes. However, parasequences can be identified and the appearance and presence of ammonite genera or species more or less is reflected by sea-level changes.

 

Discussion and Conclusion

The effect of relative sea-level changes can be recognised in the Sarcheshmeh and Sanganeh Formations throughout the Kopet Dagh Basin. A significant sea-level rise is marked by the change from the thick-bedded limestones of the Tirgan Formation to the marly and shaly limestone beds of the Sarcheshmeh Formation. With this event the Late Barremian heteromorphs such as Martelites and Heteroceras and planispiral forms such as Turkmeniceras invaded the basin. During the Early Aptian sea-level fluctuated and with some water influxes new genera or species entered the basin. The most important genus is Deshayesites with its numerous species.

As the thickness of the formations changes from one measured section to another, the thickness and numbers of parasequences also changes.  In particular, in the Takal Kuh area the Sarcheshmeh Formation is thicker than in other areas and there are more parasequences. Based on regional geology the Takal Kuh fault was active during the Cretaceous. It appears that the number of parasequences and sea level fluctuations in this area is partially related to the activity of this fault. If the Takal Kuh sections are correlated with sections to the east and south-east of this area, some beds pinch out and it may be assumed these layers and their respective parasequences had a regional tectonic origin.

When deposition of the Sanganeh Formation commenced in the western part of the basin (Takal Kuh area) during Early Aptian times, smooth-shelled genera such as Aconeceras, Melchiorites and Pseudosaynella entered the area. But the number of new genera and species is less than in the Sarcheshmeh Formation. These forms failed to penetrate further east, where the Sarcheshmeh Formation was still being deposited in the Tirgan and Sanganeh sections and no ammonites are known.

In the Sarcheshmeh and Sanganeh Formations about 45 parasequences have been recorded in the Takal Kuh area. These parasequences were deposited during the Late Barremian to Early Aptian. If we assume 5.5 million years for this interval (Gradstein et al., 1999) and complete preservation and recognition of all parasequences, then the average duration of a parasequence was 80000 years.

Over the basin as a whole parasequences are thicker in the lower part of the Sarcheshmeh Formation than in the upper part. The sandy and fossiliferous limestone beds of the uppermost part of the Sarcheshmeh Formation may represent an important fall in relative sea-level before significant transgression occurred over the basin to deposit the Sanganeh Formation.

The trend of relative sea level rise from the Sarcheshmeh Formation to the Sanganeh Formation can be traced in all measured sections. It is indicated by the predominance of shales in the latter formation. The shales are dark grey to black or green-grey, and were probably deposited on an intermittently anoxic sea floor. Marly limestone, marl, and limestone beds still occur but they are thinner in comparison with similar layers in the Sarcheshmeh Formation. The lithology of these harder beds gradually changes toward the eastern part of the basin, passing into silty shales and siltstones. This may relate to epirogenic activity in the lands surrounded the basin or reflect the influx of cool water due to sea level rise and a greater connection to the northerly basins.

Overall, sea level rose during the Late Barremian to Albian across the Kopet Dagh Basin. It appeared to reach a local peak in the Late Aptian to Early Albian, during deposition of the Sanganeh Formation. A preliminary correlation between biozones, numbers of genera and species, and third order relative sea-level changes in the basin have been done. The number and diversity of ammonite genera and species reached its highest in the Late Barremian and Early Aptian, and decreased in Mid and Late Aptian times. Parasequences (and sequences) show that in the Late Barremian and Early Aptian sea-level fluctuated more than in Mid and Late Aptian times. Correlation of third order sea-level changes in the Kopet Dagh Basin with Haq et al.'s (1988) curve suggests both differences and similarities. If the Haq et al. chart is accepted as a world wide trend in sea-level changes, then similar trends in the Kopet Dagh curve could reflect global effects and differences could reflect local effects.

 

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