Sardic Phase Research Articles

The Ordovician of Sardinia (Italy): from the ‘Sardic Phase’ to the end-Ordovician glaciation, palaeogeography and geodynamic context

Abstract This review illustrates the most important features of the Ordovician successions of the Sardinian basement. We focus on the stratigraphy and tectonic structures in the tectonic units of the External and Nappe zones of the Variscan basement. The Ordovician successions are characterized by unconformities related to tectonic events ascribed to the Sardic and Sarrabese phases. The different durations of the unconformity-related gaps in the External (17 myr) and Nappe (6 myr) zones, recent work on the trilobite fossil content, and the occurrence of a volcanic arc only in the Nappe Zone (Sarrabus and Gerrei units) highlight significant discrepancies suggesting that these domains did not share the same geodynamic setting and palaeogeographical position during the Ordovician. This implies they were amalgamated only in Variscan times. Whereas for the external and nappe zones the Ordovician features are clear, the high-grade metamorphic Inner Zone, where numerous Ordovician ortho- and para-gneiss occur, more detailed studies are needed to define a complete framework for the Ordovician evolution of Sardinia. The present revision of data for the best-preserved succession of Sardinian tectonic units suggests that at least two distinct terranes, which did not share the same Ordovician evolution, were only amalgamated during the Variscan Orogeny.

NEW GEOLOGICAL AND STRATIGRAPHICAL DATA AND DISCOVERY OF LOWER ORDOVICIAN ACRITARCHS IN THE SAN VITO SANDSTONE OF THE GENN'ARGIOLAS UNIT (SARRABUS, SOUTHEASTERN SARDINIA)

The subject of this paper is new geological and stratigraphical data regarding the Paleozoic in the Sarrabus area (Southeastern Sardinia). More particularly Tremadocian (Lower Ordovician) acritarchs have been found in samples taken for microfossils research from the weakly metamorphosed terrigenous sediments of the San Vito Sandstone.
 Until recently only acritarchs from the Middle and Upper Cambrian has been found in the San Vito Sandstone. This new finding provides a more precise dating of the San Vito Sandstone, a more exact correlation between this Sarrabus formation and analogous formations with acritarchs in Central- Eastern Sardinia and also further confirmation of the Intraordovician age of the Sarrabese Phase and of the corresponding Sardic Phase of Iglesiente-Sulcis area (Southwestern Sardinia).

Deciphering the Sardic (Ordovician) and Variscan deformations in the Eastern Pyrenees, SW Europe

Detailed geological mapping of the La Cerdanya area (Canigó unit, Eastern Pyrenees) provides new data characterizing the different structural styles exhibited by Cambrian–Lower Ordovician (Jujols Group) and Upper Ordovician successions. Their unconformable contact, related to the Sardic Phase, ranges from 0° (paraconformity) to 90° (angular discordance). The Jujols Group rocks topped by the unconformity are affected by Sardic foliation-free open folds. The pre-Sardic succession, the Sardic Unconformity and the lower part of the post-Sardic succession (Rabassa Conglomerate and Cava formations) are cut and offset by several Late Ordovician NNE–SSW-trending synsedimentary extensional faults associated with hydrothermal activity, which dramatically affected the thickness of the lower part of the Upper Ordovician succession. We relate (1) the Mid-Ordovician Sardic uplift and erosion, and (2) a Mid- to Late-Ordovician upward propagating extensional fault system bounding the outline of half-grabens, subsequently infilled by alluvial deposits, to a thermal doming event (about 475–450 Ma) that led to the uplift and stretching of the Ordovician lithosphere. Thermal doming may be caused by mafic magma underplating and responsible for the coeval calc-alkaline magmatic activity broadly developed in the Eastern Pyrenees. We discuss the similarities between the Mid-Ordovician Sardic Unconformity and other Early Paleozoic unconformities described in neighbouring areas. Finally, we suggest a geodynamic scenario in which a regional-scale thermal event was related to the opening of the Rheic Ocean that led to the drift of Avalonia from the SW European margin of Gondwana.

The Sardic unconformity and the Upper Ordovician successions of the Ribes de Freser area, Eastern Pyrenees

The Ribes de Freser area of the Eastern Pyrenees is characterized by an Alpine antiformal stack that comprises the so-called El Baell and Bruguera tectonostratigraphic units. This work presents detailed geological mapping and stratigraphic and facies analyses of these units, in order to improve the understanding of the Mid–Late Ordovician paleogeographical features of this sector of the Pyrenean margin of North Gondwana, subsequently affected by the Sardic Phase and the Variscan Orogeny. The Katian El Baell Formation, which crops out in the homonymous unit, is revised, formally erected and proposed as a lateral equivalent of the Estana Formation. The El Baell Formation consists of shale/limestone alternations where the carbonate production was controlled by development of pelmatozoan–bryozoan meadows and mud-mounds with plano-convex geometries and preservation of ramose bryozoans in life position. The thicker limestone levels, marking the top of shallowing-upward sedimentary cycles, contain fissure networks occluded with Pb–Zn–As ore mineralizations, whose clast counterparts occur forming overlying lags. The El Baell/Ansovell contact is marked by karstic clayey byproducts. Both the drastic changes in thickness and facies of the El Baell and Estana formations, and the presence of contemporaneous epithermal and fissuring episodes in the former, point to the onset of Katian (Late Ordovician) extensional tectonics, which may have controlled sharp modifications in accommodation space. Finally, the internal structure of the El Baell and Bruguera units are different, pointing to the record of a pre-Variscan (Mid Ordovician) deformational episode responsible for the dispersion of Variscan mesostructures in pre-Upper Ordovician rocks.

The Sardic Phase: field evidence of Ordovician tectonics in SE Sardinia, Italy

AbstractDetailed geological mapping, field observations and structural analyses demonstrate that Early Ordovician (‘Sardic’) deformation occurred in the early Palaeozoic successions that are now incorporated in the Variscan Nappe zone of SE Sardinia. This deformation is represented by folds that formed at a shallow depth, lack a significant syn-folding axial planar foliation, and do not affect the overlying Late Ordovician – Devonian sedimentary sequence. These deformation features can be related to the development of the Sardic Unconformity and to calc-alkaline volcanism in several now-scattered terranes of Ordovician northern Gondwana. This reflects a convergent geodynamic setting that in the study sector appears to have failed to reach a continental collisional end-stage. Associating the structural data from this study with those of several published research studies, a preliminary evaluation about which tectonic setting could better fit is proposed. These conditions affected the eastern side of the northern Gondwana margin more or less contemporaneously with the opening of the Rhéic Ocean and the closure of the Qaidam Ocean, before the amalgamation of the Hunia terranes.

Very low-temperature metamorphism in Ordovician metasedimentary rocks above and below the Sardic unconformity, SW Sardinia, Italy

In the Iglesiente region, the Cabitza and Monte Argentu Formations are separated by an angular unconformity known as the Sardic unconformity. This is related to an early Ordovician mild compressional phase, known as the “Sardic phase”. The pelitic samples from the structurally lower Cabitza Formation consist of alternating reddish phyllosilicate-rich and whitish phyllosilicate-poor, sialic layers, whose S0 bedding plane is parallel to a pre-Variscan S1P schistosity overprinted by the Variscan S1V schistosity. Pelitic samples from the Monte Argentu Formation are characterized by a Variscan S1V axial plane schistosity. Samples from the two formations consist of quartz and phyllosilicates. The latter are potassic white mica, chlorite, paragonite, locally kaolinite, and pyrophyllite. The illite crystallinity values determined for the Cabitza samples are 0.25–0.31, with an average of 0.29; meanwhile, the Monte Argentu samples produce values of 0.33–0.38, with an average of 0.35. The chlorite crystallinity and b0 of potassic white mica values show greater heterogeneity in the Cabitza than the Monte Argentu samples. The b0 values and P–T pseudosections allow us to confirm that there is no significant difference in the P–T metamorphism conditions between the Cabitza and Monte Argentu samples. The Iglesiente region, which is considered to be the rift zone behind the Middle Ordovician Sarcidano–Barbagia volcanic arc, underwent the “Sardic phase”, giving rise to E–W folds. These were first overprinted by weak E–W, and then by stronger N–S-oriented Variscan deformation events.

Late Ordovician (post-Sardic) rifting branches in the North Gondwanan Montagne Noire and Mouthoumet massifs of southern France

Upper Ordovician–Lower Devonian rocks of the Cabrières klippes (southern Montagne Noire) and the Mouthoumet massif in southern France rest paraconformably or with angular discordance on Cambrian–Lower Ordovician strata. Neither Middle-Ordovician volcanism nor associated metamorphism is recorded, and the subsequent Middle-Ordovician stratigraphic gap is related to the Sardic phase. Upper Ordovician sedimentation started in the rifting branches of Cabrières and Mouthoumet with deposition of basaltic lava flows and lahar deposits (Roque de Bandies and Villerouge formations) of continental tholeiite signature (CT), indicative of continental fracturing.The infill of both rifting branches followed with the onset of (1) Katian (Ka1–Ka2) conglomerates and sandstones (Glauzy and Gascagne formations), which have yielded a new brachiopod assemblage representative of the Svobodaina havliceki Community; (2) Katian (Ka2–Ka4) limestones, marlstones, and shales with carbonate nodules, reflecting development of bryozoan-echinoderm meadows with elements of the Nicolella Community (Gabian and Montjoi formations); and (3) the Hirnantian Marmairane Formation in the Mouthoumet massif that has yielded a rich and diverse fossil association representative of the pandemic Hirnantia Fauna. The sealing of the subaerial palaeorelief generated during the Sardic phase is related to Silurian and Early Devonian transgressions leading to onlapping patterns and the record of high-angle discordances.

Transient inversion during the opening stageof the Wilson cycle “Sardic phase” in the Iberian Variscides –Stratigraphic and tectonic record

The Variscides of Iberia have a bilateral symmetry with east vergence in the eastern branch and west vergence in the western, on both sides of a Centro-Iberian Zone (CIZ), with predominant steep axial planes. All the structures curve around the Ibero-Armorican Arc (IAA). Unconformities in the sedimentary sequences of Cambrian to Early Ordovician age were ascribed to “Sardic phase” by correlation with similar tectonosedimentary events in Sardinia. Recent studies showed diachronism between these events in Sardinia and Iberia but migration of major geodynamic regime in time may be due to regional variation of major events at plate tectonic scale. We studied in detail two critical areas in the CIZ, the Marão anticline in the NE and the Amêndoa-Carvoeiro synform in the SW. Two unconformities can be put in evidence, as elsewhere in CIZ. A stronger lower unconformity of a Volcano-Sedimentary Complex of Lower Arenig (and Tremadocian?) age on top of a Cambrian clastic sequence with flysch characteristics; and a milder upper unconformity of Armorican Quartzite of Arenig age on both the Volcano-Sedimentary Complex and the Cambrian sequences. The lithostratigraphy of the studied areas is described and correlated with other areas in Iberia. The Volcano-Sedimentary Complex and coeval magmatic bodies with bimodal composition are briefly described. The Sardic event corresponds to folds with steep axial planes at high angles to Variscan structures that produce the penetrative cleavage that cut across the unconformity surfaces. Sardic thrusts are also present and can be explained by thin-skinned compressive tectonics. Sardic folds and thrusts suggest a brief period of transient inversion between a major extensional regime from Cambrian to Devonian. The obliquity of Sardic structures to Variscan compression suggests a component of transpression during the Sardic tectonic event, corresponding to a tectonically enhanced unconformity near the Cambro-Ordovician boundary. The transient Sardic inversion is interpreted in terms of a break-up unconformity related to the migration of an intracratonic rift; in the Ordovician this rift moves into the SW of Ossa Morena Zone (OMZ) and since then become the SW Iberia suture during the Variscan Wilson cycle. This migration induced transient compression and dextral strike-slip in the major boundary between CIZ and OMZ due to presence of incipient primary curvature in this segment of IAA.

Genesis of polymetallic and precious-metal ores in the Western Mediterranean province (Cévennes, France–Sardinia, Italy)

An attempt has been made at correlation between the Lower Palaeozoic Iglesiente domain of southwest Sardinia and the southern Cévennes-Montagne Noire Variscan domain of France, with particular regard to the Cambrian-hosted economic lead-zinc concentrations and the spatially associated precious-metal (Au) occurrences. Geological, lithogeochemical and lead isotope investigations led to the following conclusions. (1) The major lead-zinc deposits of Iglesiente and southern Montagne Noire have a Mississippi Valley-type origin, which involved remobilization of pre-existing Cambrian syngenetic mineralization during the Ordovician distensive 'Sardic phase'. Lead isotope evidence suggests mixed crustal sources for the Iglesiente lead. (2) The southern Cévennes stratiform Minerai Zéro of the Malines district is not syngenetic with the Cambrian host rocks. For this ore type a synto late-tectonic Variscan formation model is proposed, similar to the model indicated for the genesis of the Salsigne gold deposit in the Montagne Noire. A reevaluation of the Vigan gold occurrences, regionally associated with the Minerai Zéro economic lead-zinc bodies, is therefore suggested. (3) The newly discovered Tertiary epithermal gold province of Sardinia is genetically quite distinct from the Palaeozoic lead-zinc province of Iglesiente.

Role of the Sardic tectonic phase in the metallogenesis of SW Sardinia (Iglesiente): lead isotope evidence

Role of the Sardic tectonic phase in the metallogenesis of SW Sardinia (Iglesiente): lead isotope evidence

Evolution of the Western European continental crust: implications from Nd and Pb isotopes in Iberian sediments

This paper presents Smz.sbnd;Nd and geochemical data on fine-grained sediments of the Iberian Massif, Spain, together with new and published Uz.sbnd;Pb results of selected sedimentary rocks. Profiles from two major zones (Cantabrian and Central Iberian Zone) have been investigated. Both profiles exhibit almost complete stratigraphic successions from uppermost Precambrian to Lower Carboniferous. Paleozoic shallow-depth platform sediments are essentially unmetamorphosed; the Precambrian syn-orogenic succession underwent a metamorphic overprint up to biotite zone. 147 Sm 144 Nd ratios range from 0.0988 to 0.1457 and Nd contents from 6 to 112 ppm. ϵ Nd[today]-values vary from −16.5 to −7.6, Pb contents from 1.7 to 97 ppm. Samples with high ϵ Nt[ T] ( > −3) are found to be low in μ 2[330Ma] ( < 9.75). Mean Nd crustal residence ages ( T DM) are 1.92 Ga (Cantabrian Zone) and 1.96 Ga (Central Iberian Zone). As juvenile Cadomian crust is present in the Iberian Massif, the oldest components have to be significantly older than 2 Ga. Within the European Variscides comparably old ( T DM) were so far only described from the Armorican Massif, France. An active margin of the Gondwana continent is the most probable provenance of the uppermost Precambrian strata as indicated by their Cadomian age, syn-orogenic facies and juvenile components. Precambrian as well as samples from the Precambrian-Cambrian boundary gave ϵ Nt[ T]-values > −4.8, significantly higher than average Paleozoic rocks. Contributions of juvenile Cadomian crust to the syn-orogenic sediments must be in the order of 30–50% to fit the Nd and Pb data. There is no evidence for any addition of mafic material according to major or trace elements, or to Sm Nd ratios. Lower Cambrian samples are already below −7.3 in ϵ Nd[ T]. Thus, compared to the total pre-existing crust, contribution of juvenile crust was very small. ϵ Nd[ T]-values of almost all samples ranging in stratigraphic age from 515 to 410 Ma are strikingly negative ( −12.9 to −10.1) and thus indicate the absence of newly differentiated continental crust in the provenance (s). Only the Tremadocian of the southern Central Iberian Zone gave high ϵ Nt[ T]-values ( −4.0 to + 1.0). Again there is no evidence for addition of mafic material to the strata. As Precambrian and Tremadocian ϵ Nt[ T] peaks are separated by the lowermost Cambrian of ϵ Nt[ T] > -7, juvenile crust may have been added related to the pre-Ordovician unconformity (Sardic phase). During the Middle Paleozoic (410–330 Ma), ϵ Nt[ T]-values increase from — − 12 to ∼ − 7, indicating a continuous contribution of juvenile continental crust of ∼20%. As major terranes of the Iberian Massif were not amalgamated until Late Paleozoic, it can be assumed that the respective juvenile crust was formed within the regime of one or several small subduction zones. Nd isotope data of this study compared with published results of Brittany, France, suggest that the Medio-North ArmoricanCentro-Iberian domain persisted from uppermost Precambrian to the Variscan.

Ordovician alluvial fan to marine shelf transition in SW Sardinia, Western Mediterranean Sea: tectonically (“Sardic phase”) influenced clastic sedimentation

A localized Lower Ordovician, early Caledonian structural event (“Sardic phase”) is recorded in SW Sardinia by an angular and erosional unconformity, faulting, and mild folding. This event was accompanied by sedimentation of a transgressive, clastic unit (“Puddinga Beds”) which is particularly well developed along a major fault system (Gonnesa fault zone). There, the “Puddinga Beds” range from basal interfingering fans (some formed by coarse carbonate and schist-clasts others by schist-clasts only), to fluvio-lacustrine mud and gravelly sandstone, to coastal muddy and sandy deposits, and to fossiliferous restricted marine shelf sediments. The existence of the slightly compressive “Sardic phase” in SW Sardinia and quasi-coeval calc-alkaline volcanism and an extensional structural phase (“Sarrabese phase”) in central-east Sardinia, suggest transcurrent faulting. The calc-alkaline acid volcanism is also interpreted by some authors to indicate that the rifted basins developed in backarc position. The non-volcanic sequence of SW Sardinia analyzed in this paper was probably formed in basins developed on the northern margin of the Gondwanaland craton. This Sardinian system may be the eastward extension of transcurrent rifts which have also been recently reported from NW Spain. Structural activity has been the predominant control on sedimentation; however, worldwide, recurring, low sea-level stands may have influenced the sedimentation of the lower part of the “Puddinga Beds”, and the deglaciation of the Saharan Ice Sheet has contributed to the uppermost Ordovician-early Silurian deepening of the euxinic shelf.

Early Rifting Deposition: Examples from Carbonate Sequences of Sardinia (Cambrian) and Tuscany (Triassic-Jurassic), Italy: An Analogous Tectono-Sedimentary and Climatic Context: ABSTRACT

Lower Cambrian Ceroide Limestone (Sardinia) and Lower Jurassic Massiccio Limestone (Tuscany) belong to sequences deposited in analogous tectono-sedimentary context: the former linked to the Caledonian Sardic Phase, the latter to the Alpine Orogeny. Both units consist of massive pure limestone characterized by marginal and lagoonal sequences repeatedly interfingering in the same geological structure. This distribution indicates a morphology of the platforms composed of banks (marginal facies) and shallow basins (lagoonal facies) comparable with a Bahamian complex. Dolomitization affects patchily the massive limestone bodies, and karstic features, breccias, and sedimentary dikes occur at their upper boundary. Both units overlie early dolomite and evaporites (sabkha facies) containing siliciclastic intercalations in their lower and/or upper part and are unconformably covered by open-shelf red (hematitic), nodular limestone Ammonitico Rosso facies). The sedimentary evolution of the two sequences appears to have been controlled by synsedimentary tectonics whose major effects are the end of the terrigenous input, the bank-and-basin morphology of the platform, the irregular distribution of the dolomitization, and the nodular fabric of the overlying facies. The end of the Bahamian-type system is marked by the karstification of the emerged blocks and is followed by their differential sinking and burial under red-nodular facies. From amore » geodynamic viewpoint, sequences composed of Bahamian-like platform carbonates followed by Ammonitico Rosso facies imply deposition along continental margins subjected to block-faulting during an extensional regime connected with the beginning of continental rifting. Moreover, the variation from sabkha to Bahamian conditions suggests the drifting of the continent from arid to humid, tropical areas.« less

The Lower Cambrian of SW-Sardinia: From a clastic tidal shelf to an isolated carbonate platform

Much of the Lower Cambrian sediments of SW-Sardinia were deposited on a clastic tidal shelf, which changed into an isolated carbonate platform. Terrigenous clastics with carbonate intercalations (Nebida Fm.) grade into the pure carbonates of the Gonnesa Fm. Algal-archaeocyathan carbonates of Atdabanian to Lenian age occur within the clastics of the Lower Cambrian Nebida Fm. Smaller algal-archaeocyathan mounds evolve with time into a more complex facies-association of stacked mounds. The last stage produces flat-topped algal-archaeocyathan biostromes, several hundred meters long and locally more than ten meters thick. They consist mainly of algal bindstones (Girvanella especially) and also contain (sometimes branched) archaeocyathans. These buildups are interfingered and interbedded with coated grains and are underlain and overlain by terrigenous clastics. This association is similar in age and facies to the Poleta Fm. of western Nevada and eastern California, from which ROWLAND (1984) has reported the first appearance in the history of the earth of a wave-resistant reef-framework. We cannot assume that the algal-archaeocyathan community had a considerable resistance to waves and that it was environmentally dominant in SW-Sardinia. This association of algal-archaeocyathan carbonates and coated grains is to be found in the W and N of Iglesiente-Sulcis, apparently marking the rim of a platform situated further to the E. In the westernmost part of Iglesiente this platform-margin facies of the Nebida Fm. is overlain by a slope facies of the Gonnesa Fm., unknown so far. The slope is the lateral equivalent of tidal and sabkha deposits of the Dolomia Rigata Member (laminated dolomite member) located further to the E. This slope facies is called the Planu Sartu Member, a new member of the Gonnesa Fm. The Planu Sartu consists especially of varve-like laminated limestones and dolomites and up to m-thick calcareous mudstones with cherts. Slumping features are common in this facies. The Planu Sartu also occurs E of the platform, in the eastern part of Sulcis, forming another slope characterized again by slumping features. Because of these two slopes the size of the platform in between can be estimated. It extended approximately 50–60 km in the present E-W direction (taking into account a shortening to half its original size). The slopes were formed by tensional tectonics, active throughout Gonnesa and lower Cabitza time. In contrast to the slope in the W, the eastern slope has no predecessor in Nebida time. Because of the newly developed basin in the E, the clastic supply to today's SW-Sardinia was interrupted. This lasted until the breakdown of the platform in Cabitza time, when clastic delivery was reinstated. At least during the lower Gonnesa time, the platform wasnot rimmed by a barrier which consisted of ooids and an algal-archaeocyathan community. Ooids occur only locally. This lack of a special marginal facies, together with the slope-facies mentioned above, indicates poor water circulation in the two basinal areas during Gonnesa time, and possibly an increase in salinity. The tensional tectonics in the Cambrian might have initiated the separation of northern parts of Gondwana, which subsequently drifted to the N. Microcontinental collision might have been responsible for compressional tectonics (“Sardic phase”), which was active most probably in the lower or middle Ordovician.

Structural setting and genesis of ore bodies in carbonate rocks in the M.te Atzei-M.te Ega mining area (Sulcis, SW Sardinia)

At least seven different types of ore bodies, with an ore stock of Ba, F, Fe, Pb, Zn, Cu and Sb occur in Paleozoic rocks (Lower-Middle Cambrian and Ordovician) in an ≃15 km2 area north of the Narcao village (SW Sardinia, Italy). The ore bodies are related both to syngenetic accumulations, and later epigenetic recirculation of the primary ores. In the Lower Cambrian the area studied shows evidence of synsedimentary tectonic instability, probably related to carbonate shelf margin features. The Upper Cambrian is lacking and the transgressive Ordovician sediments cover large areas of the previously gently folded Cambrian rocks (Sardic phase of Caledonian orogenesis). The main Hercynian tectonic phases had a much stronger compressive folding that is mirrored in a sequence of tight folds and upthrusts. The geometric setting resulting from the tectonic frame of the Cambrian and Ordovician sediments must be taken into account to understand the distribution and the genesis of most of the ore bodies.

La Fase Sarda dell'Orogenesi Caledonica in Sardegna

La Fase Sarda dell'Orogenesi Caledonica in Sardegna