Alfredo Loi

A Cenozoic-style scenario for the end-Ordovician glaciation

The end-Ordovician was an enigmatic interval in the Phanerozoic, known for massive glaciation potentially at elevated CO2 levels, biogeochemical cycle disruptions recorded as large isotope anomalies and a devastating extinction event. Ice-sheet volumes claimed to be twice those of the Last Glacial Maximum paradoxically coincided with oceans as warm as today. Here we argue that some of these remarkable claims arise from undersampling of incomplete geological sections that led to apparent temporal correlations within the relatively coarse resolution capability of Palaeozoic biochronostratigraphy. We examine exceptionally complete sedimentary records from two, low and high, palaeolatitude settings. Their correlation framework reveals a Cenozoic-style scenario including three main glacial cycles and higher-order phenomena. This necessitates revision of mechanisms for the end-Ordovician events, as the first extinction is tied to an early phase of melting, not to initial cooling, and the largest δ13C excursion occurs during final deglaciation, not at the glacial apex.

Zircon typology combined with SmNd whole-rock isotope analysis to study Brioverian sediments from the Armorican Massif

The identification of source materials contributing to mature terrigenous clastic deposits is made difficult due to a lack of useful discriminant criteria. SmNd isotope analysis can provide some indication of the present of mantle-derived constituents in clastic rocks. However, since this method is based on whole-rock samples, it is only possible to obtain the averaged composition of the different source materials involved. Moreover, SmNd isotope systematics can be strongly influenced by the presence of heavy minerals rich in rare earth elements and/or displaying isotopic ratios very different to the sedimentary host. In this manner, 0.1% of zircon or 0.01% of monazite from an extraneous source is sufficient to modify significatively the SmNd signature of the whole-rock. The typological study of zircon populations is an extremely valuable tool, especially since this mineral is highly resistant and exhibits a morphology controlled by the physical and chemical conditions under which it crystallized. Zircons that have been reworked in a sedimentary deposit display typologies that make it possible to identify the different igneous rock-types present in the source region. Moreover the typological study can be associated with a single-grain PbPb dating. This twofold approach was applied to Brioverian sedimentary rocks (Neoproterozoic III to Early Palaeozoic in age) from the Central Brittany Domain (Armorican Massif, NW France). It is proposed that these deposits inherited a major component from juvenile crustal materials (eND(540) = − 1.4 to − 6.3), reflecting the presence of igneous precursors with a dominantly mantle-derived origin (zircon sub-types S19, S20, S24, S25 etc.), to which was added another component of crustal anatectic affinity (zircon sub-types S2, S6, S7 etc.). Two igneous source regions are identified on the basis of the present study: the anatectic granitic massifs of the Mancellian Batholith (540 Ma), along with a coeval ignimbritic suite of end-Proterozoic/Eocambrian age, and the syn- and post-tectonic dioritic massifs (590 to 530 Ma). Apart from these main sources of clastic supply, it is very likely that sporadic contributions arose from basic to intermediate volcanic activity of mantle origin linked to the Cadomian orogeny and from the Icartian basement.

Controls of sea level fluctuations on the formation of Ordovician siliceous nodules in terrigenous offshore environments

Abstract Siliceous nodules of the Ordovician Period, of the Armorican Massif and of southwest Sardinia are made of a mixture of terrigenous origin material (quartz and phyllosilicates) and of biogenic origin components (microcrystalline quartz, calcium phosphate and calcite). They are associated with fine-grained sequences, which were deposited between the distal part of the inner shelf and the outer shelf. Within these sequences, two facies can be distinguished, clayey–silty facies without nodules related to episodes of high sedimentation rate and nodule-bearing clayey facies linked to episodes of reduced sedimentation. The genesis of the siliceous nodules can be integrated into a model taking account of the fluctuations in terrigenous flux due to eustatic variations: during periods of rising sea level, the terrigenous flux decreases and the sedimentation is dominated by bioclastic elements (calcareous, siliceous and phosphatic). Stability of the sediment–water interface facilitates biogenic silica dissolution. During early diagenesis, silica precipitation leads to the formation of proto-nodules. The distribution of the two facies can be explained by the superposition of two eustatic signals, one of very high frequency (VHF) and the other of high frequency (HF): the silty–clayey facies could be accumulated during the period of falling sea level within the HF cycle and the nodule-bearing clayey facies could correspond to amalgamation of several VHF cycles related to periods of low sedimentation rate in the HF cycle.

Zircon typology and geochemistry in the palaeogeographic reconstruction of the Late Ordovician of Sardinia (Italy)

Abstract The present study concerns the Upper Ordovician sedimentary succession which crops out in the Sulcis-Iglesiente area autochthon and in the Sarrabus-Arburese allochthonous units, with the aim of contributing to a better understanding of the pre-Hercynian palaeogeography. This study is based on the typology of zircon and trace-element geochemistry of zircon and titaniferous minerals. The Upper Ordovician rocks cropping out in the Sarrabus-Arburese area (allochthon belt) mainly result from the erosion of pre-Caradocian volcanics (the so-called ‘Porfidi grigi’), which were emplaced in a magmatic arc setting. The lateral and vertical homogeneity of the terrigenous supply during the Caradoc and early Ashgill, and the argillaceous-carbonate sediments with shallow shelf faunas during Ashgillian times, suggest that stable geodynamic conditions persisted throughout this time span. In the Sulcis-Iglesiente area (autochthon belt), the nature of the terrigenous influx and the source-rocks remained the same from the Late Cambrian to the early Ashgill. The constant nature of the detrital zircon grains rules out any possibility that the ‘Porfidi grigi’ might have contributed to the sedimentary supply; the source-rocks were probably located within an internal cratonic area, distinct from the magmatic arc of the Sarrabus area. In the early Himantian, new source-rocks contributed to the terrigenous supply. They were volcanics with a strong mantle component. This new material was later progressively diluted by terrigenous supply of cratonic origin. The significant differences in the composition and origin of the terrigenous material deposited in the autochthonous and the allochthonous units may reflect distinct source-areas with different geodynamic histories. It is therefore possible that the two depositional areas were geographically separated. Location of the Sulcis-Iglesiente area within the North Gondwanan shelf is a. plausible scenario. But the occurrence of Baltic taxa in the Sarrabus area suggests that this area was originally located in an intermediate position between North Gondwana and Baltica.

Compositional affinities of volcanic fragments in sedimentary rocks using electron microprobe analysis

Abstract The identification of clasts of volcanic origin in terrigenous detrital rocks is generally based on the observation of textural criteria. In the absence of quantitative analytical data on the chemical composition of such fragments, it is often quite difficult to identify their specific origin. However, it is now possible to use a defocused microprobe beam to obtain high-precision analyses of microcrystalline elements. The application of this technique to the groundmass of various lavas shows that the distribution of certain elements varies as a function of whole-rock composition. Three ternary diagrams, using the most discriminant parameters (K 2 O/Na 2 O/CaO, Fe 2 O 3 /Na 2 O+K 2 O/MgO and SiO 2 /CaO × 10/MgO × 10), are proposed for the identification of different groundmass compositions (ultrabasic, basic, intermediate, and acid). The study of reworked clastic material from the Upper Ordovician sediments of Sardinia shows that fragments of volcanic groundmass can preserve their initial geochemical characteristics. The application of this method to microcrystalline fragments contained in Proterozoic sandstones of the Armorican Massif shows that it can be an additional approach for the identification of siliciclastic source-areas.

Les nodules silico-alumineux: une expression de la condensation sédimentaire sur une plate-forme distale

The Ordovician siliceous-aluminous nodules of the Armoricain Massif are made up of terrigenous constituents (quartz/phyllosilicate) and biogenic constituents (silica and apatite). The nodules were formed during the early diagenesis. Their formation could be linked to processes of sedimentary condensation governed by eustatic variations in the distal shelf.

Caractérisation des horizons manganésifères de l’Ordovicien supérieur de Sardaigne : relation avec la glaciation fini-ordovicienne

Abstract Mn-rich crusts interbedded within glacially-related Hirnantian deposits of Sardinia result from starved sedimentation in isolated sub-basins. These latter result from low glacio-eustatic sea-levels on the North-Gondwanian platform. During very low sea-levels, large supplies of fresh water are responsible for oxidizing conditions allowing Mn-oxides concretions to form. During relatively higher sea-levels, sea water inflows result in water stratification, reducing conditions and Mn-carbonate concretions. A hydrothermal origin associated with a rifting event is suggested for the manganese.

Geology of the Variscan basement of the Laconi-Asuni area (central Sardinia, Italy): the core of a regional antiform refolding a tectonic nappe stack

The study area extends in central Sardinia between the coordinates 8°56′00″/39°55′00″ (NW corner) and 9°03′00″/39°50′00″ (SE corner). It forms part of the Sardinian Variscides that are characterized here by the western-most aspect of a regional structure known as the Flumendosa Antiform, which runs ENE-trending for more than 50 km along its axis. It represents the envelopment of several, upright km-scale minor folds that refold the different tectonic units of the Variscan Nappe zone characterized by a number of isoclinal folds, with axial plane foliation and thick ductile shear zones. The antiform is in turn deformed by late-orogenic extensional structures, namely asymmetric folds and narrow, low angle ductile shear zones that generally reactivate earlier collisional structures. The 1:12,500 scale map completes the mapping of this important regional structure, meaning that a detailed survey of the entire mega-structure is now available. This enables more detailed structural analyses of poly-deformed areas in low-grade metamorphic conditions to be conducted, given the robust knowledge available of the lithostratigraphic succession and the geometric and kinematic outlines.

Crinoid columnal associations and sequence stratigraphy architecture: the Le Faou Formation, Lower Devonian of the Massif armoricain (France)

Crinoid columnals are major macrobenthic components of the Lower Devonian faunas of the Massif armoricain particularly in the Seillou section. Three crinoid columnal associations have been delineated based on R and Q-mode cluster analysis. Distribution of these associations is correlated to the sequence stratigraphic architecture, showing the close relationships between crinoid columnal association dynamics and relative sea-level variations. Comparing the distribution of benthic associations, we note a lesser diversity in crinoid columnal associations than in brachiopod ones. Crinoids (high-level suspension feeders) seem to occupy broader specialised niches than brachiopods (reclining and low-level suspension feeders) and are characterised by a different ecological space utilisation.

Environnement de dépôt des formations à phtanites interstratifiés du protérozoïque supérieur armoricain (France): conséquences sur la genèse des phtanites

Abstract In the Cadomian belt (Upper Proterozoic) some sedimentary formations contain interbedded gra- phitic cherts. The sedimentological study carried out on the Lamballe Formation (Armorican Massif) shows that the associated terrigenous rocks were laid down between the shoreface zone and the inner shelf and were associated with storm wave action. These results question hypotheses about the origin of the black cherts and the validity of using chert as a lithostratigraphic marker.