Hippuritid rudist bivalves from Late Cretaceous deposits in southwest of Bajestan, Khorasan Razavi Province

This region is located in south west of Khorasan Razavi Province, between Ferdows and Bajestan cities, which is coordinated between E 58 00׳ to 58 30׳ longitude and N 34 30׳ to 35 00׳ latitude. The Mazar and Bahabad sections have been selected in order to study of Rudist fauna in the Cretaceous deposits in the southwest of Bajestan city. Rudist fauna in the Late Cretaceous deposits in east of Iran have been considered mainly in the Eastern Iran Zone, but there is no significant publication about Lut Block in Central Iran so far, except the preliminary report of this study (Rezaie Soolgani et al., 2015). Some of the noticeable previous works in the other parts of eastern Iran have been done by Vredenburg (1909), Kuhn (1932), Vogel (1971) and Khazaei et al. (2009, 2014). In order to identify and analyze the collected specimens from both sections, detailed field works for measuring and recognition of rudist-bearing units have been done and the total number of 200 samples have been collected. Also, according to the field data, stratigraphic column of the studied sections have been drown. Based on the shape, size and position of internal structures, such as ligament, pillars, myophors and teeth, samples have been identified at the level of genus and species. Finally, 21 specimens from Mazar section and 4 specimens from Bahabad section belonging to 2 genera and 5 species of Hippuritidae family have been recognized. Mazar section, with 117.5 m thickness, is composed of 9 rock units, including alternating fossiliferous marl and limestone beds with bivalve and rudist shells, limestone and rudist-bearing limestone beds. Bahabad section, with 98 m thickness, consists of 4 main rock units which is composed of marl and limestone beds with bivalve, echinoderm and rudist bioclasts. This section starts with an erosional contact at the base of the succession.
Hippuritidae family derived from Caportinidae during the Turonian time and shows some features, such as the presence of pillars in right valve and perforates channels in left valve. This family along with Radiolitidae family have occurred during the Late Santonian to Middle-Late Campanian time, at the third stage of rudist's boom (Cestari & Sartorio, 1995).
One of the most important structural characteristics on the cross section of right valves is the angle between the structural elements (Moro & Cosovic, 2004) that were measured and mentioned in the biometric tables. These parameters consist of the angle between the ligamental ridge (L), first pillar (p1) and second pillar (p2). It is noteworthy that there is no possibility to measure the myocardinal axis, because of the poor preservation of teeth in most cases, the interior angles are measured based on the ligamental ridge.
According to the results of systematic identification of specimens and the biostratigraphic range proposed by many researchers, the Late Coniacian to Early Campanian age is suggested for the rudist-bearing units of these two sections.
The results of strontium stable isotope analyses which have been carried out on numerous indicator species in large areas of the Tethyan and Caribbean realms (Steuber, 2012) indicate calibrated numerical age that is different from the previously published works. The consequence of the present investigation on two species of genus Vaccinites, reveals the Late Turonian to Late Santonian age for these sequences. Hippurites family is one of the main Rudist families known in this region that includes five species as below: Hippurites canaliculatus, Vaccinates cornuvaccinum, Vaccinites galloprovincialis, Vaccinites chaperi and Vaccinites sulcatus. In addition, based on stratigraphical range of these species, Late Turonian–Santonian age is suggested for these successions.