Biostratigraphy and Paleoecology of Lower Carboniferous Strata in Hutk Section (Kerman) Based on Conodonts

Introdaction
Upper Devonian-Lower Carboniferous rocks are widely exposed throughout Kerman Province (Central-East Iran Microplate). They are mainly represented by limestones, sandstones and interbedded shales, of the Bahram and Hutk Formations. The regional geology and stratigraphy of the Kerman area have been described by Huckriede et al. (1962) and Wendt et al. (2002). The Hutk section is located about 35 km north of Kerman in the Khajeh Mountain, where the Precambrian to Jurassic rocks are exposed. The study area is located in the southwestern part of the geological map of Zarand (Vahdati Daneshmand et al., 1995), and structurally belongs to the southwestern part of the Central-East Iran Microplate. This paper is the first study on stratigraphy and biostratigraphy of the Hutk Formation in the Hutk section, providing evidence of a Tournaisian age of the Formation. Nineteen samples (3-4 kg each) were collected from the Hutk Formation in Hutk section, and processed for conodonts. Of these, twelve samples yielded more than 101 conodont elements. In general, the preservation of the conodont elements is good, although a few specimens being broken or incomplete. The identified conodont fauna consists of thirteen species and subspecies of seven genera including: Gnathodus, Protognathodus, Polygnathus, Mehlina, Bispathodus, Clydagnathus, Siphonodella. In the present paper, we follow the standard conodont zonations of the Early Carboniferous, established by Sandberg et al. (1978). The conodont associations found in the 12 productive conodont samples from the Hutk section allows us to subdivide the deposits into three conodont biozones: the duplicata Zone, Upper part of Upper duplicata-Lower crenulata zones and anchoralis-latus Zone
duplicata Zone: Neopolygnathus communis, Polygnathus inornatus, Polygnathus sp. B, Polygnathus longiposticus, Mehlina sp., Clydagnathus cavusformis occur in this interval associated with the Bispathodus aculeatus aculeatus and Bispathodus stabilis. According to Molloy et al., (1997) Polygnathus longiposticus ranges from within the Lower duplicata Zone in to the Lower Crenulata Zone. Co-occurrence of this species with Clydagnathus cavusformis, which ranges from Middle expansa-Upper duplicata zones, defines the interval.
Upper part of Upper duplicata-Lower crenulata zones: An undifferentiated Upper part of Upper duplicata-Lower crenulata zones interval is discriminated from samples A12 to A16 by co-occurences of Neopolygnathus dentatus and Siphonodella obsoleta. Its lower limit is recognized by the first occurrence Siphonodella obsoleta, which ranges from the Upper part of Upper duplicata to the isostica-U. crenulata zones (Sandberg et al., 1978). The upper limit is identified by the last occurrence of Neopolygnathus dentatus, which ranges from the Latest marginifera to the Lower crenulata zones (Barskov et al., 1991; Ji & Ziegler, 1993).
Barren interval: There is a five-meter barren zone intervenes between the lowest sample attributable to anchoralis-latus Zone and the uppermost bed attributable to Upper part of Upper duplicata-Lower crenulata zones which do not yield any conodont specimen, so the age of this bed has been recognized on the basis of stratigraphic position as Upper crenulata-typicus zones.
anchoralis-latus Zone: The assosiation of Gnathodus pseudosemiglaber and Neopolygnathus communis can be seen here. Its lower boundary is defined by FAD of Gnathodus pseudosemiglaber that ranges from anchoralis–latus Zone to texana Zone (Lane et al., 1980). Co-occurences of this species with Neopolygnathus communis which extinct at anchoralis-latus Zone defines the interval.
Biofacies and Paleoecology: During the Early Carboniferous, the lateral distribution of conodonts across the shallow carbonate shelf to the open sea and basinal environments follows a similar pattern as during the Late Devonian time (Schonlaub & Kreutzer, 1993). The basic concept of conodont biofacies introduced by Sandberg (1976) and later completed by others (e.g. Sandberg & Ziegler 1979; Sandberg et al., 1988; Sandberg & Dreesen, 1984, 1987; Pohler and Barnes, 1990 and Savoy & Haris, 1993). The Paleoenvironment and paleoecology were interpreted from field observation and conodont assemblages. Shallow water conodont biofacies (icriodid-polygnathid) in Lower Famennian deposits of the studied section indicates an inner shelf to foreshore depositional environment for this part of section (Ahmadi et al., 2012). This is similar to most Iranian sequences of Central Iran such as Hojedk (Gholamalian & Kebriaei, 2008), Dalmeh (Hairapetian & Yazdi, 2003) and Chahriseh (Gholamalian, 2007). The middle-late Famennian deposits are characterized by clastic sediments which indicates a considerable sea level fall and marine regression at the Latest Devonian time. This situation is different from other sequences of Central Iran. Middle and late Famennian in some sections (eg. Ghale-Kalaghu and Howz-e-Dorah 1, 2) shows predominance an inner shelf environment (Bahrami et al., 2011) whereas in Kale-e-Sardar section (Eastern Tabas) deep marine sediment were deposited (Gholamalian et al., 2009). The duplicata Zone is characterized by 8 conodont taxa. Species of Polygnathus is most abundant followed by those of Clydagnathus and a low number of Bispathodus+Mehlina. The polygnathid biofacies and occurrences of Clydagnathus represent an inner shelf to mid-continental shelf environment. The upper beds (Upper part of Upper duplicata-Lower crenulata zones, anchoralis latus Zone) is characterized by the appearances of Gnathodus and Siphonodella. These genera have low abundance so the statical analysis is impossible but the presence of these genera represents a sea level rise and establishment of deeper environment during the late Tournaisian. From the Early Carboniferous sequences of Hutk section, north of Kerman, thirteen species and subspecies of conodonts were identified. The age of Hutk Formation has been determined as Tournaisian based on its fauna. Three biozones were recognized on the basis of vertical distribution of these taxa along the stratigraphic column. These strata lie unconformably over the Middle-Upper Famennian terrigenous sediments and covered disconformably by dolostones of Jamal Formation.