Upper Silurian conodonts from the Ockerkalk limestone of southeastern Sardinia (Italy)

Upper Silurian, Lower and Upper Devonian conodonts are discovered for the first time from rare calcareous beds of the Serra S'llixi Group in the Pala Manna area, in connection with the nucleus of the "Sarrabus syncline" (southeastern Sardinia) where the youngest rocks of the Palaeozoic sequence of the Genn'Argiolas Unit crop out.
 Consequently, clastic rocks of the conformably overlying Pala Manna Formation, also affected by the main Hercynian folding fase, could be considered Famennian—Lower Carboniferous in age. It also seems reasonable that the same rocks could be compared with the more or less coeval "Culm—type" clastic sediments of the Villasalto Conglomerate, in the nearby Gerrei region.

The Silurian biostratigraphy, lithostratigraphy, and facies of Central Iran including the Kashmar (Boghu Mountains), Tabas (Derenjal Mountains, Ozbak-Kuh), Anarak (Pol-e Khavand) and Kerman regions is reviewed and updated. The current state of knowledge of the Silurian in the Zagros Basin, Alborz, Kopet-Dagh and Talysh regions, as well as in a few areas scattered across the Sabzevar Zone, and the Sanandaj-Sirjan terranes is also reviewed. Silurian volcanism in various parts of Iran is briefly discussed. The end of the Ordovician coincided with a widespread regression across Iran synchronous with the Hirnantian glaciation, and only in the Zagros Basin is there a continuous Ordovician-Silurian transition represented by graptolitic black shales of the Sarchahan Formation. In the Central-East Iranian Platform marine sedimentation re-commenced in the early to mid Aeronian. By the Sheinwoodian, carbonate platform depositional environments were established along its north-eastern margin. In other parts of Iran (e.g., Kopet-Dagh and the Sabzevar Zone), siliciclastic sedimentation continued probably into the late Silurian. The Silurian conodont and brachiopod biostratigraphy of Central Iran is significantly updated facilitating a precise correlation with the Standard Global Chronostratigraphic Scale, as well as with key Silurian sections in other parts of Iran. The Silurian lithostratigraphy is considerably revised and two new lithostratigraphical units, namely the Boghu and Dahaneh-Kalut formations, are introduced.

Late Silurian (Late Ludlovian) conodonts from the Kildrummie formation, south of Rockley, New South Wales

Conodonts are one of the stratigraphically most important fossil groups in the Silurian Period. We examine the regional diversity dynamics of the Llandovery condonts on the Upper Yangtze Platform. The data set of 41 species from seven sections is compiled from the Geobiodiversity Database. Four measures of taxonomic richness based on zonal counting are used to demonstrate the conodont diversity change. The CONOP (Constrained optimization) program is used to build up a more precise composite sequence, which provides the data for comparative analysis of diversity change. Conodont richness keeps growing from the Ozarkodina aff. hassi Zone through the Ozarkodina obesa Zone to the Ozarkodina guizhouensis Zone and reaches a peak at the fourth zone, the Pterospathodus eopennatus Zone. This significant growth was followed by a rapid decrease, which probably represents an extinction in the mid-Telychian. This extinction event can also be observed in other fossil groups such as graptolites and chitinozoans based on recent studies.

Analyses of 87Sr/86Sr in Silurian conodonts recovered from localities in North America and Europe representing 13 of the 14 defined Silurian conodont zones provide a high-resolution record of seawater chemistry for the Silurian Period. These data, which are characterized by little or no scatter, depict several high-frequency cycles superimposed on a gradual longer term rise in 87Sr/86Sr for the Silurian. High-frequency cycles have a duration of about one conodont zone, and many correlate with sequence boundaries recognized around the world. These data provide a much higher resolution image of secular changes in 87Sr/86Sr during the Silurian and may require a rethinking of models of strontium isotope flux in marine basins.

The conodont biostratigraphy for the Upper Ordovician – Upper Silurian carbonate shelf (Irene Bay and Allen Bay formations) and interfingering basinal (Cape Phillips Formation) facies is established for parts of Devon and Ellesmere islands, central Canadian Arctic Islands. Revisions to the interpreted regional stratigraphic relationships and correlations are based on the stratigraphic distribution of the 51 conodont species representing 32 genera, identified from over 5000 well-preserved conodonts recovered from 101 productive samples in nine stratigraphic sections. The six zones recognized are, in ascending order, Amorphognathus ordovicicus Local-Range Zone, Aspelundia fluegeli Interval Zone, Pterospathodus celloni Local-Range Zone, Pt. pennatus procerus Local-Range Zone, Kockelella patula Local-Range Zone, and K. variabilis variabilis – Ozarkodina confluens Concurrent-Range Zone. These provided a more precise dating of the members and formations and, in particular, the range of hiatuses within this stratigraphic succession. The pattern of regional stratigraphy, facies changes, and hiatuses is interpreted as primarily related to the effects of glacioeustasy associated with the terminal Ordovician glaciation and smaller Early Silurian glacial phases, the backstepping of the Silurian shelf margin, and the geodynamic effects of the collision with Laurentia by Baltica to the east and Pearya to the north. Conodont colour alteration index values (CAI 1–6.5) from the nine sections complement earlier graptolite reflectance data in providing regional thermal maturation data of value in hydrocarbon exploration assessments.

Integrated Silurian conodont and carbonate carbon isotope stratigraphy of the east-central Appalachian Basin

The Lewis Ponds carbonate and volcanic‐hosted Zn–Pb–Cu–Ag–Au–rich massive sulfide deposits are located near the western margin of the Hill End Trough in the eastern Lachlan Fold Belt of New South Wales. Two stratabound massive sulfide zones, Main and Toms' occur in a tightly folded Upper Silurian succession of marine felsic volcanic and sedimentary rocks. Siltstone, mudstone and quartz crystal‐rich sandstone overlie thick, tabular beds of poorly sorted mixed‐provenance breccia, limestone‐clast breccia and pebbly granular sandstone. The sedimentary rocks unconformably overlie a succession of strongly foliated, quartz‐plagioclase‐phyric dacite. Hydrothermal alteration and deformation have obscured primary textures and contact relationships in the footwall volcanic succession. However, three lithofacies associations were distinguished using phenocryst abundance, overall texture and immobile element compositions. Thick (up to 100 m) conformable or fault‐bound units of poorly sorted limestone‐clast breccia, megabreccia and massive limestone occur throughout the Lewis Ponds host sequence. Clasts vary in size from small pebbles to 10 m–long boulders and 90 m–thick lenses. Where primary textures are preserved, the bioclastic limestone contains crinoid fragments, brachiopods, bivalves, solitary corals, volcanic quartz and feldspar crystals and rare Silurian conodonts. Although originally deposited in shallower water, the allochthonous limestone occurs in a host sequence of predominantly siltstone and mudstone. The mixed‐provenance breccia, limestone‐clast breccia, pebbly granular sandstone and quartz crystal‐rich sandstone were deposited on the flanks of a high‐level intrusive dacite centre in a moderately deep, below wave‐base slope environment. Volcanic and sedimentary components originated from multiple source areas at the basin margin and in the adjacent hinterland. The Lewis Ponds host sequence records a transition from coarse‐grained polymictic volcaniclastic breccia and sandstone, upwards into mudstone, siltstone and minor volcanogenic sandstone, indicating deposition in a deepening, more restricted environment as volcanism waned and the basin progressively subsided.

Silurian sections from the Rabat-Tiflet area (northwestern Moroccan Meseta) have been systematically sampled for conodonts. The conodont faunas are characterized by two index species of the European Silurian zonation (Ozarkodina sagitta sagitta and Ozarkodina remsceidensis eosteinhornensis) and two other index species of the north American zonation (Kockelella stauros and Kockelella variabilis). Recognition of the sagitta/stauros through lower variabilis zones in the lower part of the Silurian limestone and shale succession from the Rabat-Tiflet area, proves for the first time the presence of Wenlock below Ludlow-Pridoli, in the Silurian sequences of this area. Most of the Silurian from the northwestern Moroccan Meseta is represented by black graptolitic shales with intercalations of limestone (beds or lenses) in its upper part. The Silurian biostratigraphy of Morocco is generally based on graptolites [Willefert in Destombes et al., 1985]. In the present contribution the age of some Silurian sections (fig. 3) from the Rabat-Tiflet area is re-assessed by using conodonts. The stratigraphically important taxa have been described in Benfrika [1999]. Section Oued Grou I (G I) Situated near the barrage Mohamed Ben Abdellah, this section shows 30 m of alternating black shales and limestones. The lower part of this unit (I) provided : Ozarkodina sagitta sagitta, Kockelella absidata, Kockelella stauros, Ozarkodina bohemica, Ozarkodina excavata, Dapsilodus obliquicostatus, Panderodus unicostatus, Pseudooneotodus bicornis. The first appearance of Oz. sagitta sagitta and the K. stauros indicates the lower boundary of sagitta Zone established by Walliser [1964] in the Carnic Alps and also the lower boundary of stauros Zone proposed by Barrick et Klapper [1976] for North America (fig. 2). These species are of Wenlock age. Section Oued Grou II (G II): This section is located at approximately a hundred meters north of the preceding section. It is subdivided into 2 units : Unit IIA : 12 m of greyish to black limestones interbedded with green shales ; Unit IIB : 65 m of shales interbedded with some limestones. The conodonts identified in unit A are : Ozarkodina bohemica, Kockelella absidata and Kockelella stauros. This fauna is of Wenlock age. The first occurrence of Kockelella variabilis in sample indicates the lower variablis zone (lower part of Ludlow). Section Al Khaloua Iron Mines (AK) The limestones interbedded with shales of the lower part of the Al Khaloua section yield Polygnathoides emarginatus and Kockelella variabilis. This association is of Ludlow age. The conodonts from the upper part of this section (Caudicriodus sp. and Belodella devonica) indicate a Lower Devonian age. Section Oued Tiflet syncline (T) Two units are recognized : Unit A : 18 m of alternating black shales and greyish black limestones providing : Kockelella absidata, Ozarkodina bohemica, Decoriconus fragilis, Dapsilodus obliquicostatus, Ozarkodina excavata. This fauna is of Upper Wenlock to Lower Ludlow age ; Unit B : 34 m of black shales with some limestone intercalations, is probably of Ludlow-Pridoli age. Conclusion The Silurian conodont succession recognized in the Rabat-Tiflet area (northwestern Moroccan Meseta) allows to attribute the lower part of alternating limestones and shales to the Wenlock and the upper part to the Ludlow-Pridoli. The presence of the Wenlock in the Rabat-Tiflet area and also in the Oued Cherrat (under investigation) is demonstrated by conodonts. This suggests that the Silurian transgression has reached this area of the Moroccan Meseta during the Wenlock and not the Ludlow, as generally admitted. Furthermore, the conodont faunas allow to recognize affinities with those from North America and Central Europe. Two index species of the conodont zonation established by Walliser [1964] for the Carnic Alps have been recognized in Morocco, two other index species of the conodont zonation proposed by Barrick and Klapper [1976] for North America have been also identified in Morocco.