Anatomy of carbonate mounds from the Middle Anisian of Nakhlak (Central Iran): architecture and age of a subtidal microbial-bioclastic carbonate factory

The Anisian succession of Nakhlak (in Central Iran) is characterized by a siliciclastic succession with minor carbonate units, with massive carbonate mounds up to 50 m thick in its upper part. The mounds, constrained in age to the late Bithynian (Ismidicus Zone) by ammonoids and conodonts, are characterized by a flat top and a lateral pinch-out marked by clinostratified slopes (about 15A degrees in dip). Stratigraphic and microfacies analyses document an inner part of the mound characterized by massive microbial carbonates with open-space structures (stromatactis) filled with fine-grained internal sediments and marine cements. Isolated sponges (up to 5 cm), serpulids and bryozoans are present, which grew on the calcimicrobial limestone. A narrow bioclastic margin (mainly with crinoids and brachiopods) produces most of the slope facies (consisting of bioclastic grainstone and packstone, with intraclasts from the inner part of the mounds) which interfinger basinward with volcaniclastic sandstones. The demise of carbonate productivity is marked on the top of the carbonate mounds by a condensed surface, rich in ammonoids, glaucony grains, and articulated crinoids, documenting a rapid drowning. Paleolatitude data support deposition in a tropical setting, and sedimentological constraints indicate deposition close to the fair-weather wave base, within the photic zone. The late Bithynian Nakhlak carbonate mounds developed before the appearance (documented since the Pelsonian in different parts of the world) of scleractinians which, despite the favorable environmental conditions, are absent at Nakhlak. The Nakhlak mounds thus represent one of the last occurrences of the microbial factories (which developed after the Permo-Triassic extinction event and persisted for most of the Middle Triassic, but with a gradually increasing role played by scleractinians) before the first appearance of the Mesozoic corals

Bio-chronostratigraphic calibration of the Upper Carnian-Lower Norian magnetostratigraphic scale at Pizzo Mondello (Sicani Mountains, Sicily).

Pizzo Mondello section is known since 15 years because of the continuous Late Triassic pelagic record of great significance for the establishment of an integrated chronostratigraphy of the Late Triassic (Gullo et al. 1996; Muttoni et al. 2001, 2004). During the last 4 years, Pizzo Mondello section has been studied in detail to provide a new and high resolution integrated bio-chronostratigraphy for the calibration of the magnetostratigraphy and chemostratigraphy proposed by Muttoni et al. (2001, 2004), and now it is one of the GSSP candidates for the definition of the base of the Norian. The lowest 143 m of the Cherty Limestone, straddling the C/N boundary have been studied in detail. The preliminary data of the ongoing research have been presented in all the meetings of the STS from Albuquerque 2007 and here we summarize the final results. The key correlation to the standard marine Triassic Scale is provided by the ammonoids. They are relatively rare, however the available collections document the Upper Carnian Discotropites plinii and Gonionotites italicus Subzones, from meter 15 to meter 80 from the base of the section. The following 15 meters are poor in ammonoids, while higher up the lower part of the Lower Norian Guembelites jandianus Zone is documented by Dimorphites cf. n. sp.1 of Krystyn, 1980. Conodonts are very abundant and have a great potential as practical tool for global correlations. The abundance of specimens at Pizzo Mondello gave the opportunity to point out clear relationships among the five most widespread Upper Camian/Lower Norian conodont genera (Paragondolella, Carnepigondolella, Metapolygnathus, Epigondolella and Norigondolella) and to identify trends of the generic turnovers (Mazza et al. 2010). The two biomarkers so far proposed as possible marker events for the GSSP were the FAD of E. quadrata (sample FNP88A) and the FAD of M. communisti (sample NA35). However, the FAD of E. quadrata occurs within the Gonionotites italicus Subzone, while the FAD of M. communisti is on its top. Halobiids are extremely common in the Cherty Limestone and they have also a great potential for large scale correlations. Nine species of Halobia have been recognized: Halobia carnica, H. lenticularis, H. simplex, H. superba, H. cf. rugosa, H. radiata, H. austriaca, H. styriaca and H. mediterranea. The best possible marker for the base of the Norian is the first occurrence of Halobia austriaca, that is recorded in the middle of the interval between the record of the Gonionotites italicus Subzone and the Guembelites jandianus Zone. Radiolarians were found in few samples but with high diversity assemblages. In the upper Gonionotites italicus Subzone to the Guembelites jandianus Zone there is an overlap of species previously considered Late Carnian with species usually regarded as Early Norian. About 4 m above the FAD of E. quadrata, in the Gonionotites italicus Subzone, the first assemblage with Capnuchosphaera deweveri Kozur & Mostler, Capnuchosphaera tricornis De Wever, Kahlerosphaera norica Kozur & Mock and Xiphothecaella longa Kozur & Mock, usually referred to Early Norian, occurs. These integrated bio-chronostratigraphic studies lead to identify some possible GSSP marker events especially on conodonts and halobiids, which occur in the upper part of magnetozone PM 4n, within PM 4r and in the lower part of PM 5n. Possibly the most suitable magnetostratigraphic event to recognize the basal Norian is the base of magnetozone PM 5n, as already suggested by Krystyn et al. 2002 and Muttoni et al. 2004

Late Carnian-Early Norian ammonoids from the GSSP candidate section Pizzo Mondello (Sicani Moutains, Sicily)

A small collection of ammonoids from the Upper Triassic Scillato Formation at Pizzo Mondello (Agrigento, Sicily) is studied. These specimens were collected within the framework of a project intended to provide integrated high-resolution bio-chronostratigraphic support for the Upper Carnian-Norian magnetostratigraphic scale defined at this site, which is located in an historical area from which G.G. Gemmellaro collected and monographed Upper Triassic ammonoids at the beginning of the 20th century. The specimens, which were collected utilizing the bed-by-bed sampling method, represent the first collection of Upper Triassic ammonoids described from western Sicily since Gemmellaro's time. Several levels of the Pizzo Mondello section yielded ammonoids, but very few beds have provided more than one specimen. This scarcity of specimens has resulted in a complex taxonomie analysis because Gemmellaro, who described 166 new species, did not explain the variability of many of his taxa. Sixteen taxa belonging to eleven genera are described. They include Placites sp. ind., Discotropites plinii (Mojsisovics), Anatropites sp. ind., Microtropites cf. paronai, Metathisbites cf. affinis, Hyattites cf. praefloridus, Projuvavites boehmi (Gemmellaro), Projuvavites inflatus (Gemmellaro), Gonionotites cf. italicus, Gonionotites aff. recuperoi, Pregriesbachites n. gen., P. bukowskii (Gemmellaro), Dimorphites noricus n. sp., Dimorphites selectus Mojsisovics, Dimorphites sp. and Discophyllites insignis. Among the new taxa, Dimorphites noricus n. sp. formalizes the nomen nudum "Dimorphites n. sp. 1", which has been quoted in the literature for several years as the index ammonoid for the lowest subzone of the Norian stage. The small collection documents the Discotropites plinii and Gonionotites italicus subzones of the uppermost Carnian Spinosus Zone as well as the Dimorphites noricus and D. selectus subzones of the Jandianus Zone, the first zone of the Lower Norian. This chronostratigraphic classification not only confidently ties the Pizzo Mondello succession to the Tethyan chronostratigraphic scale, but it is also crucial for the calibration of the Halobia and conodont bioevents identified in the section. Also discussed is the chronostratigraphic ammonoid-based correlation of the Pizzo Mondello section with Feuerkogel (Austria), Jomsom (Nepal), West Union Canyon (Nevada, USA), Black Bear Ridge (British Columbia, Canada) and the Yana Okhotskaya River (Siberia, Russia). These are the most complete ammonoid-bearing sections in the world that span the Carnian/Norian boundary, and they exhibit a discrete distribution of ammonoid-bearing beds across the boundary. Ammonoids will not provide the primary marker event for the definition of the GSSP of the Norian, but they are crucial for the selection of the most significant events based on other groups