Microbial primary dolomite from a Norian carbonate platform: northern Calabria, southern Italy

The origin of fine‐grained dolomite in peritidal rocks has been the subject of much debate recently and evidence is presented here for a microbial origin of this dolomite type in the Norian Dolomia Principale of northern Calabria (southern Italy). Microbial carbonates there consist of stromatolites, thrombolites, and aphanitic dolomites. High‐relief thrombolites and stromatolites characterize sub‐tidal facies, and low‐relief and planar stromatolites, with local oncoids, typify the inter‐supratidal facies. Skeletal remains are very rare in the latter, whereas a relatively rich biota of skeletal cyanophycea, red algae and foraminifera is present in the sub‐tidal facies. Some 75% of the succession consists of fabric‐preserving dolomite, especially within the microbial facies, whereas the rest is composed of coarse dolomite with little fabric preservation. Three end‐members of dolomite replacement fabric are distinguished: type 1 and type 2, fabric retentive, with crystal size <5 and 5–60 μm, respectively; and type 3, fabric destructive, with larger crystals, from 60 to several hundred microns. In addition, there are dolomite cements, precipitated in the central parts of primary cavities during later diagenesis. Microbialite textures in stromatolites are generally composed of thin, dark micritic laminae of type 1 dolomite, alternating with thicker lighter‐coloured laminae of the coarser type 2 dolomite. Thrombolites are composed of dark, micritic clotted fabrics with peloids, composed of type 1 dolomite, surrounded by coarser type 2 dolomite. Marine fibrous cement crusts are also present, now composed of type 2 dolomite. Scanning electron microscope observations of the organic‐rich micritic laminae and clots of the inter‐supratidal microbialites reveal the presence of spherical structures which are interpreted as mineralized bacterial remains. These probably derived from the fossilization of micron‐sized coccoid bacteria and spheroidal–ovoidal nanometre‐scale dwarf‐type bacterial forms. Furthermore, there are traces of degraded organic matter, probably also of bacterial origin. The microbial dolomites were precipitated in a hypersaline environment, most likely through evaporative dolomitization, as suggested by the excess Ca in the dolomites, the small crystal size, and the positive δ¹⁸O values. The occurrence of fossilized bacteria and organic matter in the fabric‐preserving dolomite of the microbialites could indicate an involvement of bacteria and organic matter degradation in the precipitation of syn‐sedimentary dolomite.     

Weathering and morphogenesis in a mediterranean climate, Calabria, Italy

Deeply weathered plutonic rocks occur widely in the Sila Massif, Calabria, southern Italy. Three representative weathering profiles developed on the hilltops indicate that sand represents more than 50% by weight throughout most of the weathering profile, and silt- and clay-size fractions are also well represented, filling the interstices among corestones. Both sand, and finer fractions of grus, and soil horizons, are thought to be derived from a combination of granular disintegration and chemical decomposition, developed on relatively flat terrains of the Sila massif. These slopes are now experiencing transport-limited morphodynamics, under a montane-modified Mediterranean climate. The depth of the weathered layers, that have suffered little erosional truncation, typically exceed 15 m, and may reach 50–60 m or more. Major isolated or grouped exfoliation boulders are the most common minor landform feature developed on the Sila massif granite. Boulders have developed as a result of spheroidal weathering and by removal of the sandy-textured granite. The Schmidt hammer (SH) test on boulders and corestones, suggests three distinctive degree of weathering (from moderately to completely weathered rock), and that biotite content is the major controlling factor of the granite mechanical behaviour. Understanding of the dynamics of this weathering system is crucial to the interpretation of the complex suite of variables that control landscape evolution of granitoid terrains.     

Geomorphology and tectonics of uplifted coasts: New chronostratigraphical constraints for the Quaternary evolution of Tyrrhenian North Calabria (southern Italy)

The Tyrrhenian coastal sector of North Calabria, stretching between Torre S. Nicola and the Lao river, belongs to the inner extensional sector of the Neogene Apennines thrust belt. It is characterised by a stair of Quaternary marine and fluvial terraces representing the geomorphic response to the interaction between the Quaternary sea level fluctuations and the regional trend of tectonic uplift experienced by the margins of the Tyrrhenian back-arc basin. Since the last century, several authors studied the North Calabria coasts, where the flight of terraces preserves significant marine and continental successions, and proposed several paleo-geomorphological and tectonic reconstructions. In this paper we present a new stratigraphic and morphostructural setting of the North Calabria coasts based on both chronostratigraphical constraints obtained from marine deposits and detailed geomorphological analysis. A ten order stair of marine terraces, stepping between 240 and 0 m a.s.l., was recognized and time-constrained by the age of the Fornaci S. Nicola marine succession which was ascribed by integrated paleoecological, biostratigraphical and paleomagentic analyses to the early Middle Pleistocene (MIS 19–15). In particular, the 240, 200 and 160 m a.s.l. high strandlines were ascribed to the Early Pleistocene and the ones between 100 and 15 m a.s.l. to the Middle Pleistocene. The total amount of the vertical motion experienced by the studied area was estimated, and evaluation of the average rates of uplift for the Middle and Late Pleistocene times were also given. Considering the elevation a.s.l. of the oldest terraces, a tectonic uplift of at least 240 m was calculated for the North Calabria coasts since the Early Pleistocene times, 100 m of which gained from the beginning of the Middle Pleistocene. On the other hand, the 8-m high Late Pleistocene strandlines display a negligible vertical displacement affecting the area during the last 130 ka. The entire staircase of terraces preserves a record of slowing down in the rate of uplift, which attained an average value of 0.15 mm/year during the Middle Pleistocene.     

Cold subduction zone in northern Calabria (Italy) revealed by lawsonite–clinopyroxene blueschists

Lawsonite blueschists are important markers of cold subduction zones, subjected to intense fluid circulation. This is because lawsonite preservation in exhumed blueschists and eclogites is typically linked to cold exhumation paths, accompanied by hydration. In the Catena Costiera (Calabria, southern Italy), lawsonite–clinopyroxene blueschists of the Diamante–Terranova Unit, affected by ductile shearing and retrogression, are exposed. Blueschists contain zoned clinopyroxene crystals, showing core–rim compositional variation from diopside to omphacite and hosting primary inclusions of lawsonite and titanite. Thermodynamic modelling of phase equilibria in the NCKFMASHTO system revealed peak metamorphic conditions of 2.0–2.1 GPa and 475–490°C for the Alpine subduction in Calabria. The subsequent post-peak metamorphic evolution mainly proceeded along a decompression and cooling path up to ~1.1 GPa and ~380°C. The final exhumation stages are recorded in the sheared blueschists where a mylonitic to ultramylonitic foliation developed at ~0.7 GPa and 290–315°C. Therefore, the P–T evolution of the Diamante–Terranova blueschists mostly occurred in the stability field of lawsonite, sustained by H₂O-saturated conditions during the exhumation path. The results of this study indicate that the blueschists underwent peak metamorphic conditions higher than previously thought, reaching a maximum depth of ~70 km under a very cold geothermal gradient (~6.6°C/km), during the Eocene subduction of the Ligurian Tethys oceanic crust in Calabria.     

Weathering processes affecting granitoid profiles of Capo Vaticano (Calabria,southern Italy) based on petrographic,mineralogic and reaction path modelling approaches

This work focuses on developing multidisciplinary research on weathering profiles of granitoid rocks related to the tectonic and landscape evolution of the Capo Vaticano area, Calabria, southern Italy. During the Pleistocene, the Mediterranean climate plays, on the already decomposed plutonic rocks, important processes of alteration, on both the highest and inland areas and the coastal areas of the Calabrian region, such as the studied area. Field observations coupled to chemical, minero‐petrographical features and geochemical modelling are used to characterize the weathering processes affecting the granitoid complex. The granitoid cut slopes show a generally simple weathering profile characterized by a progressive increase in weathering towards the top of the slopes. The completely weathered rocks (class V) and residual soil (class VI) contain a high percentage of altered minerals, microfractures, and voids. The main mineralogical changes are the partial transformation of biotite and the partial destruction of feldspars (mainly plagioclase) that are associated with the neoformation of secondary clay minerals and ferruginous products during the most advanced stage of weathering. These transformations produce a substitution of the original rock fabric. Geochemical modelling showing the precipitation of kaolinite, illite, vermiculite, ferrihydrite and calcite. These secondary solid phases are similar to those found in this natural system. Thus, the final results of the weathering process is a soil‐like material mainly characterized by mostly a sand to gravel grain‐size fractions related to microfabric changes and mineralogical and chemical variations. Copyright © 2014 John Wiley & Sons, Ltd.     

Low-pressure metamorphism of Palaeozoic pelites in the Aspromonte, southern Calabria: constraints for the thermal evolution in the Calabrian crustal cross-section during the Hercynian orogeny

During Hercynian low-pressure/high-temperature metamorphism of Palaeozoic metasediments of the southern Aspromonte (Calabria), a sequence of metamorphic zones at chlorite, biotite, garnet, staurolite–andalusite and sillimanite–muscovite grade was developed. These metasediments represent the upper part of an exposed tilted cross-section through the Hercynian continental crust. P–T information on their metamorphism supplements that already known for the granulite facies lower crust of the section and allows reconstruction of the thermal conditions in the Calabrian crust during the late Hercynian orogenic event. Three foliations formed during deformation of the metasediments. The peak metamorphic assemblages grew mainly syntectonically (S2) during regional metamorphism, but mineral growth outlasted the deformation. This is in accordance with the textural relationships found in the lower part of the same crustal section exposed in the northern Serre. Pressure conditions recorded for the base of the upper crustal metasediments are c. 2.5 kbar and estimated temperatures range from <350 °C in the chlorite zone, increasing to 500 °C in the lower garnet zone, and reaching 620 °C in the sillimanite–muscovite zone. Geothermal gradients for the peak of metamorphism indicate a much higher value for the upper crust (c. 60 °C km^?1) than for the granulite facies lower crust (30–35 °C km^?1). The small temperature difference between the base of the upper crust (620 °C at c. 2.5 kbar) and the top of the lower crust (690 °C at 5.5 kbar) can be explained by intrusions of granitoids into the middle crust, which, in this crustal section, took place synchronously with the regional metamorphism at c. 310– 295 Ma. It is concluded that the thermal structure of the Calabrian crust during the Hercynian orogeny – as it is reflected by peak metamorphic assemblages – was mainly controlled by advective heat input through magmatic intrusions into all levels of the crust.     

New constraints to the geodynamic evolution of the southern sector of the Calabria–Peloritani Arc (Italy)Nouvelles données sur l'évolution géodynamique du secteur méridional de l'Arc Calabro-Péloritain (Italie)

New studies have been carried out on the Tertiary of the Stilo Unit, the uppermost of the Calabria–Peloritani Arc southern sector, and the Stilo–Capo d'Orlando Formation, sealing the whole nappe stack. The Tertiary terrains linked to the Mesozoic cover of the Stilo Unit consist of the lowermost Oligocene Palizzi Formation and the Late Rupelian–Aquitanian Pignolo Formation. The possibility that they deposited before the emplacement of this unit as the highest tectonic sheet of the sector is suggested. The base of the Stilo–Capo d'Orlando Formation resulted of Burdigalian age in both type areas. This interpretation, together with the existing and new data, allows proposing an age close to the Aquitanian–Burdigalian boundary for the stacking of the whole Calabria–Peloritani Arc southern sector. To cite this article: G. Bonardi et al., C. R. Geoscience 334 (2002) 423–430.     

Fiumara: a kind of high hazard river

Fiumara is a kind of gravel-bed river, very steep and short, which flow through the southern Italy mountain chains, in Sicilia and Calabria regions. These streams drain mountain areas only, and are subject to a Mediterranean climate. They have a great transport and erosion capacity because of their flow regime, dominated by episodic flash floods alternated with long periods of inactivity during which their beds become completely dry. During this last century, the natural equilibrium of these particular alluvial environments has undergone notable modifications caused also by human activities. The effects of such works are investigated in terms of river dynamics, and some local examples are reported and discussed; some suggestions are made for recovery measures too.     

Structural setting and tectonic evolution of the Apennine Units of northern Calabria

A new structural–stratigraphic synthesis of the Apennine units of northern Calabria is presented. The Meso-Cenozoic successions are grouped into two tectonic units, named Pollino–Ciagola Unit (PCU) and Lungro–Verbicaro Unit (LVU), comprising terrains formerly attributed to five different tectonic units. FeMg carpholite and blue amphibole record HP–LT metamorphism in the LVU, followed by progressive decompression leading to final greenschist facies re-equilibration during dominantly extensional deformation. Final tectonic emplacement of the LVU over the PCU post-dated the metamorphism of the LVU and was accompanied by intense ductile deformation along zones of strain localisation in footwall rocks. All of the units were later affected by folding and minor thrusting during subsequent Apennine tectonics. To cite this article: A. Iannace et al., C. R. Geoscience 337 (2005).     

Mid-crustal shear zone evolution in a syn-tectonic late Hercynian granitoid (Sila Massif,Calabria, southern Italy)

By means of petrogrological, meso- and microstructural analyses, the fabric of a syn-tectonic late Hercynian K-feldspar megacryst-bearing granodiorite is described in this paper. The granodiorite was emplaced at 293 Ma within migmatitic paragneisses which had reached the regional peak metamorphic conditions at 304–300 Ma. The granodiorite and the migmatitic paragneisses are both affected by the same ductile shear zone. In the core of the shear zone, mylonites show a clear grain-size reduction and microstructures related to deformation at high to medium temperature conditions. Migmatitic paragneisses, foliated granodiorites and mylonites mostly show concordant lineation and foliation orientations. In addition, the preferred orientation of euhedral feldspars in granodiorites indicates that the fabric anisotropy started to develop in the magmatic state. These features strongly suggest that shear deformation was active during crystallisation of granitoids and continued under subsolidus conditions. In wall rocks and mylonites, kinematic indicators such as - and -type porphyroclasts, S/C fabrics, shear bands and quartz (c) axis orientations suggest a top-to-the-W sense of shear. This is similar to the magma flow direction indicated by the tiling of euhedral feldspar megacrysts in granodiorites. Shear deformation developed, preferentially, by partitioning of strain in the granodioritic crystal mush. Geobarometry indicates that deformation took place at middle crustal levels (P=400–500 MPa). Whole rock-white mica Rb/Sr geochronological analysis of an undeformed pegmatite, crosscutting the mylonitic foliation, provided an age of 265 Ma. Timing of deformation is therefore bracketed between 293 Ma and 265 Ma.