New observations on Cretaceous carbonate-siliceous successions in Sahlabad province (Sefid Abeh basin), Eastern Iran
|دسته||چینه شناسی و فسیل شناسی|
|گروه||سازمان زمین شناسی و اکتشافات معدنی کشور|
|مکان برگزاری||بیست و چهارمین گردهمایی علوم زمین|
|نویسنده||Seyed Ahmad BABAZADEH|
|تاريخ برگزاری||۰۹ اسفند ۱۳۸۴|
The Sahlabad province is located within Sistan ocean in eastern <?xml:namespace prefix = st۱ ns = "urn:schemas-microsoft-com:office:smarttags" />Iran۱:place>۱:country-region>. This province is known for its variety of deposits from Cretaceous to Eocene times. This study is focused on the radiolarian and planktic foraminiferal assemblages during Cretaceous age. Two distinct sections (sections A & B at Kuh-e-Atashkadeh) are reported from ultrabasic rocks, interbedded planktic limestone and red radiolarian siliceous sedimentary rocks. The studied radiolarian assemblages in section A and section B yielded Turonian and Late Aptian ages, respectively. The various species of planktic foraminifera have been used to produce a biozonation through the Turonian to Campanian. Four biozones are recognized from the Upper Turonian۱:place> to the Lower Campanian in the section A and five biozones are distinguished in the section B. The identification of specimens is based on microscopic observations of thin sections. The definitions of the biozones are given following the works of Caron (۱۹۸۵), Sliter (۱۹۸۹), Premoli Silva and Sliter (۱۹۹۴) and Robaszynski and Caron (۱۹۹۵). The Aptian radiolarian age is the oldest obtained from the red argillaceous chert all this province and has important implications for the rifting/openinig age of Sistan suture zone.
The Sahlabad province (formerly called Sefid Abeh basin, Tirrul et al, 1983) is situated in Sistan ocean between two blocks: Lut and Afghan. The Sistan ocean belongs to Cainozoic Alpide suture of Sengor et al. (1988), is subdivided into two provinces: Gazik and Sahlabad provinces in north-central part of Sistan ocean (Babazadeh, 2003). The main lithofacies components in the studied comprise gray limestone, red chert and resedimented facies, which are distributed at eastern part of Sahlabad region. The ultrabasic rocks constitute the basement rocks of this region.
The study area is comprised between 59° 56’ to 59° 58’ E longitude and 32° 14’ to 32° 17’ N latitude (pic-1). The purpose of this paper is to described lithofacies and radiolarian and planctic foraminiferal assemblages of the Early to Late Cretaceous deep marine carbonate-siliceous successions in this area. The planktonic foraminifera in the Upper Cretaceous sections of Eastern Iran were rarely studied in previous works (Stِcklin et al., 1972; Tirrul et al., 1983; Fauvelet and Eftekhar-nezhad, 1990). The material is composed of 55 samples which all have been examined in thin sections and SEM.
DISCUSSIONS AND RESULTS
The section A, is only made of Member I, mainly composed of sedimentary and volcano-sedimentary lithologies. The lower part of sequence consists of the succession of gray limestone with radiolaria and planktonic foraminifera and green tuff. It passes upward to the thin bedded limestones and radiolarian cherts with the intercalated layer of volcano-sedimentary rocks. The volcano-sedimentary rocks consists of green vitric tuffs and radiolarian tuffs. The upper part consists of gray to cream fine-grained limestones without volcano-sedimentary rocks. The real thickness of this section reaches about 64 meters. This sedimentary unit (section A) conformably overlies the tuff and basalt and is faulted by the basaltic rocks.
The section B is composed of both Members I and II. At the base of Member I, there are bedded green tuff with underlying breccia and intercalations of red radiolarian cherts. This sequence continues with gray limestones containing radiolaria and planktic foraminifera, therefore the upper part of Member I is dominated by the bedded limestone such as in the section A. The thickness of Member I is 85m. The top of the sequence is unconformably overlain by a conglomeratic horizon passing upward to a debris flow deposit (debrite) assigned to Member II. This member consists of mixed lithologies ranging from sandy mudstones to boulder-sized clasts to a bouldery mass containing mud. This deposit is an “olistostrome”. In the mudstone planktonic foraminifera are found. The thickness of Member II attains 115m. The lower and upper contacts of the section are faulted.
Four biozones are recognized from the Upper Turonian to the Lower Campanian in the section A and five biozones are distinguished in the section B.
Biozone A: Marginotruncana sigali Zone, (Definition: Interval zone between the last occurrence (LO) of Helvetoglobotruncana helvetica (supposed) and the first occurrence (FO) of Dicarinella concavata).
Age: Late Turonian
Biozone B: Dicarinella concavata Zone, (Definition: Interval zone between the FO of Dicrainella concavata and the FO of Dicarinella asymetrica).
Age: Latest Turonian to Early Santonian
Biozone C: Dicarinella asymetrica Zone, (Definition: the range of the nominal species is LO of D. asymetrica.
Biozone D: Globotruncanita elevata Zone, (Definition: Zone defined by the FO of Globotruncanita elevata).
Age: Early Campanian
Biozone E: Globotruncana ventricosa Zone
As the succession is truncated by an olistostrome this biozone is lacking.
Biozone F: Globotruncanita calcarata Zone, (Definition: it is defined as total range zone of Globotruncanita calcarata).
Age: early Late Campanian
Pic-2: Cretaceous planktonic foraminifera from Kuh-e-Atashkadeh
The radiolarian assemblage in the section A is commonly characterized by Crucella cf. C. cachensis, C. aff. C. messinae, Patellula helios, Dictyomitra montisserei etc. this assemblage is attributed to Turonian. In the section B, the fauna consists of the following radiolarian association: Cryptamphorella sphaerica, C. conara, Pseudodictyomitra lodogaensis, Saitoum sp. etc. this fauna is dated Late Aptian.
Pic-3: Cretaceous radiolarian from Kuh-e-Atashkadeh
The succession of gray pelagic limestones and red radiolarian cherts upon the ophiolite unit show that a normal marine sedimentation without any terrigenous input occurred during these periods. The absence of the Globotruncana ventricosa biozone supported by the sedimentary indicates that tectonic movements occurred within the sedimentary basin without emergence. The radiolarian assemblages were confirmed from deep marine basin and can be related to Tethyan radiolarites (Babazadeh & De Wever, 2004).
Support for this research was provided by Djahad of University, Birjand, I thank for this support. I wish to thank to Prof. F. Robaszynski for helpful comments on determination of a few of planktic foraminifera species.
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