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Stratigraphic-paleogeographic evolution of Eastern Sardinia Jurassic passive margin carbonates: synthesis and future developments.
No full textDuring the Mesozoic, Sardinia belonged to the European Peritethys domain. Its Jurassic stratigraphic evolution is comparable, for sedimentary and climatic-paleogeographic characteristics, to coeval successions in NE Spain and SW Provence, that record the Middle Jurassic rifting phase affecting the south European margin of the Tethys. The 500-650 m thick Middle-Upper Jurassic carbonate succession of Eastern Sardinia documents the presence of a Jurassic structural high and the its sedimentary evolution along the eastern margin, close to an inferred erosional escarpment. This NE-SW trending structural high, about 75 km wide, was dominated by deposition of prevalent shallow water deposits, with low accommodation rates, up to the Early Oxfordian. The recent biostratigraphic redefinition of regional sedimentary events, the well-preserved depositional geometries and the carbonate facies associations permitted i) to define the geological history of the Eastern Sardinia Jurassic carbonate succession and ii) to reconstruct the tectono-sedimentary evolution of a well-preserved, but still poorly studied, passive margin of the Tethys.
The syn-rift Jurassic succession of Eastern Sardinia, mainly preserved in structural troughs, begins with discontinuous lenses (up to 40 m thick) of polygenic fluvial conglomerates, sandstones, lacustrine facies and coastal marine mixed calcarenites and quartzarenites at the top. The upper boundary of this Bajocian-Early Bathonian succession (Genna Selole Fm., Fig. 1) is still mater of discussoin (Dieni et al., 1983, 1985; Costamagna & Barca, 2004; Costamagna 2008, 2015). This siliciclastic unit is overlain by 500-650 m of Bathonian to Berriasian prevalent shallow-water carbonates: Dorgali, Mt. Tului and Mt. Bardia formations (Amadesi et al., 1967). This lithostratigraphic subdivision is still reliable: the three formations document the evolution of a persistent shallow-water carbonate depositional system characterized by different stratigraphic architectures and lithofacies associations. This succession records regional tectono-sedimentary, climatic and diagenetic events, that control lithostratigraphic boundaries and provides constraints for stratigraphic correlations. Recently, reviews of the lithostratigraphic classification have been cently proposed (Dieni e al., 1985, 2013; Costamagna & Barca, 2004; Costamagna et al., 2007; Jadoul et al., 2010, Lanfranchi et al., 2008, 2011; Casellato et al. 2012. The new lithostratigraphic schemes (Fig.1) evidence nomenclatural problems and the need of a redefinition of the litostratigraphic boundaries.
The first marine deposits, covering a pedogenized Variscan Basement or the Middle Jurassic continental succession, are Early Bathonian in age. The first marine depositional sequence (Bathonian, Perda Liana Mb., Dieni et al., 2013; and Dorgali Fm., Fig.2) documents a coastal paleogeography with a few open bays generally characterized by low accommodation. Transgressive, open marine, fine-grained bioturbated to bioclastic limestones with siliciclastic inputs at the base evolve to oolitic, mainly dolomitized, grainstones towards the top. This sequence, more than 100 m thick southward (Tacchi area, Dieni et al 2013), is thinner in the northern, eastern areas, (8-40 m in the Baunei-Dorgali Supramonte, N. M.Albo successions). The first regional depositional hiatus, recorded by Fe- rich hardgrounds (Upper Bathonian after Dieni et al, 1966) frequently associated with siliciclastic input, marks the top of this unit. The overlying Lower Callovian-Lower Oxfordian carbonates (upper Dorgali Fm. or the coeval basinal carbonates at the base of the S’Adde and Baunei fms.), are thinner, associated to the last terrigenous inputs and locally dolomitized. They are characterized by a few regional hardgrounds (Dieni et al., 1966; Casellato et al., 2012) developed during a transgressive trend, with long periods of non-deposition, and related to a regional crisis of carbonate production. In particular a Middle-Late Callovian hiatus is represented by thin Fe-Phosphate-Qz rich crust or by a very thin fossiliferous calcareous horizon (Dieni et al., 1966). The Early Oxfordian is also frequently condensed in the northern basinal successions and represented by oncolitic pelagic limestones (Massari & Dieni, 1983) near the base of the S’Adde Fm. (Casellato et al., 2012). This facies association, coeval with the condensed successions of other Middle Jurassic structural highs of the northern margin of the Tethys, is related to global climatic and oceanographic changes. The condensed Oxfordian basinal carbonates also suggest a regional carbonate production crisis, in particular during the Early Oxfordian, recently associated with a cooling climatic event. The youngest episode of condensed sedimentation is locally associated with sedimentary dikes, rare slumpings and an increase in accommodation space possibly related to local syn-sedimentary tectonics which led to the development of half-graben basins. The absence of evident tectonic escarpments and slope breccias suggest low angle slopes related to the development of blind normal faults in the Variscan Basement (Fig. 2,3) The presence of blind growth faults may also have controlled the accommodation and paleogeographic evolution of the overlying Upper Jurassic shallow-water carbonates (Fig.2,3). This tectono-sedimentary event (inferred Late Call.-Early Oxfordian) is associated with a regional transgressive trend, a paleogeographic reorganization and development of new shallow water carbonate factories (inner-middle ramp grainst.-pack of Lower Mt.Tului Fm., Fig. 1,2) covering fine-grained bedded carbonates (outer ramp to basin). In particular the shallow-water facies firstly nucleated on a wide, articulated structural high (Urzulei/Oliena, Codula Sesine/Codula Luna, Lula), facing to the N-NW (Cala Gonone, M. Tuttavista, Siniscola/Posada) and to the S-SE (Baunei, Pedra Longa, Jerzu), intraplatform basins (Fig.2). The basin/outer ramp facies consist of calci-mudstone and thin peloidal packstone (S’Adde Lmst. in the Northern Basin, Baunei Fm. in the SE Basin). The Upper Kimmeridgian basinal carbonates, rich in cherty nodules, represent a major stratigraphic marker
(Casellato et al., 2012).
The Upper Jurassic carbonate succession is characterized by a few transgressive-regressive trends (T\R ) (3rd order cycles), that are here described.
1) In the SE Basin a T\R cycle (up to ten meter thick) with at the top the first coral patch reefs could represent the top of the Oxfordian (Fig.2, Lower Tului Fm.?). A Late Oxfordian regressive trend, documented by basinward progradation of more shallow-water carbonates, is also observed in the NW Basin (S’Adde Lmst., Casellato et al., 2012).
2) A more regional Early Kimmeridgian transgression (Upper Baunei and S’Adde fms.) and a regression (Upper Mt. Tului Fm., Fig.1,2) at the base of the Tithonian (Late Kimmeridgian according to previous Authors) has been recognized . Higher frequency cycles (recorded by submarine Fe-hardgrounds and ramp progradation ) have been recognized in the uppermost Kimmeridgian succession of the Southern Basin (Fig. 2).
3, 4) An up to 500 m thick Tithonian - Berriasian T|R cycle (Pedra Longa or Urzulei fms. and overlying Mt. Bardia Fm., Fig.1) characterizes the middle-upper stratigraphic position of all the carbonate successions from Tortoli to Golfo degli Aranci. At the base, a higher frequency Tithonian T\R trend, (Lanfranchi et al. 2008, 3nd cycle) has been recognized both in the coeval Urzulei and Pedra Longa fms. but only in the southern areas (Fig.1,2). The 4th cycle is represented by the Late Tithonian- Early Berriasian Mt. Bardia Fm.
Fig. 1 – Stratigraphic schemes of the Middle Upper Jurassic succession of the North-East Sardinia (modified after Jadoul et al., 2010)
The shallow-water Kimmeridgian (2nd cycle) facies associations are dominated by ooids, coated/aggregate grains, oncoidal grainstones in the inner ramp, whereas fine-grained grainstone/packstone rich in crinoid fragments, peloids are more frequent in the middle ramp. Large coral patch reefs are well-developed at the top of this cycle (top of Tului Fm., near the Kimm.-Tith. boundary Fig.2). These reefal carbonates (1st coral marker) are characterized, at the base, by fine to coarse grained bioclastic grainstone with stromatoporoids, ooids and, at the top, by large coral colonies, stromatoporoid and chaetetid boundstone, rudstone (Jadoul et al., 2010). These bioconstructions could represent a fringing reef because they crop out along the peripheral areas of the central structural high (Urzulei Supramonte, and Su Conte-Cala Sesine Supramonte, Fig.1,2).
On the Urzulei carbonate paleo high, the 1st coral marker is overlain by continental carbonate breccias with black pebbles, peritidal calci-mudstone and wackestone with tepees, fenestrae and charophytes recorded by the base of the Urzulei Fm., (Fig.1,2, base of 3rd cycle) (Early Tithonian).
The last (4th) cycle records an increase both of accomodation space and skeletal carbonate production, as documented by up to 500 m thick Mt. Bardia Fm. shallow water carbonates.
Fig.2 Paleogeographic profiles with the evolution of the Middle-Upper Jurassic carbonate depositional systems developed on the North- Eastern Sardinia structural high.
The stratigraphic evolution of the Bardia depositional system can be divided in two stages, recorded by the “lower and upper Bardia”.
In the SE basin (Fig.2), the “lower Bardia” is characterized by basinward progradation of sigmoidal clinoforms (Mt. Punnacci, Cala Sesine) with reefal carbonates (coral - calcareous sponge floatstone and rudstone) at the slope break (Lanfranchi et al. 2011). The maximum thickness of the “lower Bardia” is up to 160 m, observed where it developed on thick outer ramp fine-grained packstone to mudstone of Baunei-Pedra Longa and S’Adde Lmst. successions of the SE and NW basins, respectively (Fig. 1,2). In the NW basin, the calci-mudstone facies of Pedra Longa Fm. (plattenkalk) are missing and the Mt. Bardia progradation begins earlier, during the Early Tithonian (Cala Gonone and Mt. Albo, Fig. 1; Casellato et al., 2012). The upper portion of the “lower Bardia” of Cala Gonone and M. Tuttavista is characterized by well-developed reefal limestones (“2nd coral limestone”, Late Tithonian). Reefs are dominated by many, different coral colonies, associated with thick microbialic envelops, frequent stromatoporoids, chaetetids, sponges, and a microframework matrix matrix characterized by skeletal grains coming from the back reef (echinoids, diceratids, nerineids, benthic foraminifera, dasycladacean algae)(Rusciadelli, Ricci et al., in prep.). The reefal carbonates of Cala Gonone and Orosei quarries also exhibit a network of syn- and early diagenetic tensional fractures filled with different generations of internal, shallow marine sediment.
In the southern Baunei basin, the base of the lower Mt. Bardia Fm. is characterized by up to a few decameter thick, chaotic, polygenic carbonate megabreccias recording a regressive trend marked by a sharp, erosional boundary between the thin-bedded Pedra Longa Fm. and the massive Mt. Bardia Fm. (Fig. 1). This erosional unconformity, observed in a wide area (western margin of the Pedra Longa southern basin, Fig.1), is characterized in the proximal area by deep erosional incisions in the underlying sediments associated with slump scars, and in the distal area by erosional canyons and debris flow breccias. Clasts and matrix of these breccias derive from prevalent shallow-water inner ramp, lagoonal oo-bioclastic grainstone/packstone (base of Mt Bardia Fm.) and calci-mudstone of the Pedra Longa Fm. Possible mechanisms for the megabreccia emplacement include: i) catastrophic gravity mass transport triggered by liquefaction processes in the upper Pedra Longa plattenkalk facies, ii) channelized to unchannelized debris-flow complexes generated by multiple gravitational platform margin collapses due to increasing accommodation space associated with a fast progradation of shallow water carbonate sands on a gentle slope. Gravitational failures were favored by the presence of a not rimmed platform margin and abundance of carbonate sand shoals and mud. Margin collapses were controlled by Late Tithonian syn-sedimentary tectonics (possibly related to a fault block tilting in the underlying Variscan Basement toward the east) that created a NE-SW flexure-monocline in the upper Jurassic succession of the present day coastal massif of the Orosei Gulf (Fig.2,3).
Fig.3 Depositional model for the Tithonian restricted intraplatform basin (Pedra Longa Fm.) bordered by prograding shallow water carbonates with breccias and reefal facies (Lower Mt. Bardia Fm.)
On the central carbonate high succession (Urzulei-Oliena,Cala Luna-Sesine) the “lower” and “upper Bardia” are not distinguishable. Here the main facies associations are represented by inner platform mudstone\wackestone to packstone with common dasycladacean green algae, which were interbedded with diceratids and nerineids floatstone to rudstone and subtidal skeletal packstone/grainstone.
The upper Mt. Bardia Fm. consists of up to 350 m thick, shallow-water carbonates, dominated by subtidal skeletal-oncoidal grainstone and packstone, stacked in shallowing-upward cycles. Locally up to 1 m intertidal cycle of stromatolitic bindstone, fenestral ooidal-intraclastic pisoidal packstone/grainstone with tepees (Orosei quarries) are present. Regressive metre-scale peritidal cycles cap the Mt. Bardia succession (Dieni & Massari, 1985; Dieni & Radoicic, 1999).
Further detailed studies investigating the chronostratigraphic and lithostratigraphic architecture, carbonate facies types and their spatial distribution would be essential to better understand the spatial variability and evolution through time of the Jurassic carbonate depositional systems of Eastern Sardinia. However, to improve our understanding of the detailed biological-sedimentological response of these shallow-water carbonates to different controlling factors, new, high-resolution ages are required.
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Sedimentological and paleontological evidences of a “Mid Carnian” transgression in the Western Southern Alps (S. Giovanni B. Fm. Lombardy, Italy: stratigraphic and paleogeographic implications).
No full textA "mid-Carnian" transgressive succession, developed between the Breno carbonate platform and the semiarid coastal carbonates-sabkhas facies of the S. Giovanni Bianco Fm., is recorded in the northern Bergamasc Alps. This episode is characterized by the presence of two stratigraphic markers: a) Dark grey shales and siltstones ("Black Pelites"), considered previously as the northern closure of the Gorno-Lower S. Giovanni Bianco Fms., but re-interpreted as the western pinch-out of the Lozio Shale depositional system. The Early Carnian Lozio Shale was deposited first in the Valle di Scalve-Lozio trough and later covered the carbonate platform (Breno Fm.). b) Fossiliferous, open subtidal limestones, marls and burrowed marly limestones ("Bioclastic Horizon") of the northern Bergamasc Alps. The spreading of shales and siltstones represents the first transgressive stage of the last Carnian sequence in Lombardy, after the "mid-Carnian" (Julian substage) regional carbonate platform crisis (top of the Valcamonica Breno Fm.). The "Bioclastic Horizon" records the mfs represented by normal, open marine facies, identified and correlated throughout the Bergamasc Alps. Different petrographic and chemical characters between the Lozio Shale - "Black Pelites" and the Gorno-San Giovanni Bianco Fms. suggest different source areas: the former units are characterized by clasts derived from a metamorphic-intrusive area (placed northward and westward), whereas the latter units are characterized by prevailing volcaniclastic material. A climatic change (from arid to relatively humid conditions) may be invoked to explain the crisis of the "mid-Carnian" carbonate platforms in the western Southern Alps and the regional spreading of fine-grained terrigenous material
Applied stratigraphy and carbonate petrography of the Arabescato Orobico dimension stone from the Bergamasc Alps (Calcare Rosso, Italy)
No full textThe Arabescato Orobico is a decorative stone and building material from the Bergamasc Alps, Italy. From the geological standpoint it belongs to the Calcare Rosso, latest Ladinian-Early Carnian in age, which is a peritidal tepee-rich limestone strongly modified by superimposed early diagenetic processes, outcropping in Lombardy Southern Alps. In the last twenty-five years four commercial varieties have been available on the market, namely Grigio, Grigio-Rosa, Rosa, and Rosso, whose working and abandoned quarries, are located around the extraction districts of Camerata Cornell, and San Giovanni Bianco in the median Brembana Valley. Until the 1970s of the last century some different historical types, namely Rosso Antico, Rosso Venato, and Venato Finissimo, where extracted around Ardesio, in the Seriana Valley, as well. This paper deals with the stratigraphic analysis based on detailed physical correlations of several marker horizons mainly recognized along abandoned and active quarries, and their carbonate facies analysis. Stratigraphic results permit to divide the Calcare Rosso unit in three lithozones (CR1, CR2, and CR3), with a very low angle onlapping geometry at the top of the Esino Limestone. Southward (basinward) the Calcare Rosso gradually passes to dominant grey peritidal carbonates poor in tepee horizons corresponding to the lower Breno Fm.. The petrographic characterization of representative samples from Mecca and Cadei quarries, permits to distinguish peculiar textural and compositional features of the four contemporaneous commercial types. As concerns the Grigio we identify almost five different subtypes, respectively denominated Grigio Laguna, composed of fossiliferous subintertidal grey to dark grey and black wackestone-packstones, Grigio Granulare, composed of inter-supratidal pisolitic grey grainstone-rudstones, Nero Raggiato, very rich in "raggioni", dark grey to black fibrous-radial early diagenetic calcite, Grigio Laminato composed of poorly to middle deformed antiformal structures, namely embryo to mature tepees, and, finally, Grigio Brecciato, a grey-colored diagenetic-pedogenetic breccia. Remaining types, ascribed to pinkish-grey, Grigio-Rosa, pinkish, Rosa, and reddish, Rosso, colored diagenetic supratidal facies, can be divided into laminated or brecciated subtypes. The former are composed of antiformal syndepositional to early diagenetic structures, i.e. embryo to mature tepees; the brecciated subtypes consist of more diagenetically modified senile tepees passing to breccias. These types present different amounts of sediment filling, including internal calcitic and dolomitic sediments, pisolites, terra-rossa paleosols and green/reddish clays, and early cementation, such as fibrous isopachous crust, "raggioni", sparry calcite, of the primary and early diagenetic cavities. The stratigraphic organization, and the facies analysis of this carbonate succession, have been used to evaluate the areal and vertical distribution of the Arabescato Orobico commercial types; in this way the Calcare Rosso depositional model is also suitable for geological prospecting and quarrying activities
The early Hettangian shallow- water carbonates after the Triassic-Jurassic biocalcification crisis (Albenza formation , Westernm Southern Alps)
No full textPaleogeographic evolution of the ladinian platforms in the Western Southern Alps (Lombardy, Northern Italy)
No full textComments on “The Cenozoic fold-and-thrust belt of Eastern Sardinia: Evidences from the integration of field data with numerically balanced geological cross section” by Arragoni et al., 2016
Full text linkArragoni et al. (2016) suggest in their paper published on Tectonics that the carbonate succession of Eastern Sardinia represents a Cenozoic fold-and-thrust belt, related to the Alpine orogenesis. According to these Authors, this supposed fold-and-thrust belt represents the southward prosecution of the Alpine Corsica collisional chain and the missing link between the Alpine Chain and the Calabria-Peloritani domain. Field evidence and the published literature document instead that all the surfaces that Arragoni et al. interpret as thrust are actually stratigraphic contacts. The balanced geological section of Arragoni represents thus a geometric exercise missing the basic data needed to nurse the proposed model and it does not reflect the geology of eastern Sardinia. The data provided by Arragoni et al. (2016) do not support the presence of an Alpine thrust and fold belt in eastern Sardinia and this paper may suggest to the geological community a misleading interpretation of the geodynamic evolution of the Alpine and Mediterranean area
Environmental control on the end of the Dolomia Principale/Hauptdolomit depositional system in the central Alps: Coupling sea-level and climate changes
Full text linkThe Norian in the Western Tethys is characterised by the deposition of early-dolomitised inner platform facies (Dolomia Principale/Hauptdolomit, DP/HD), bordered on the landward side by terrigenous coastal deposits (Keuper) and on the seaward side by calcareous backreef and reefal facies (Dachstein Limestone) passing basinward to open-sea sediments (Hallstatt facies). The inner carbonate platform is locally (Lombardy Basin, Carnic Alps, Central Austroalpine) dissected by normal faults leading to the development of intraplatform troughs.
Close to the Norian-Rhaetian boundary, sedimentation records an abrupt environmental change both on platform top and basins all over the Western Tethys (e.g. Western Carpathians, Transdanubian Range, Alps, Central Apennine). The top of the Dolomia Principale locally emerged, reflecting a major eustatic sea-level fall. Emersion is recorded in favourable settings by the development of polycyclic paleosols up to 30 m thick. In the Norian intraplatform basins, the succession is capped by 4 to 8 m of thin-bedded, fine-grained limestones yielding abundant remnants of fishes and terrestrial reptiles. Fossil concentration as well as sedimentological features is indicative of reduced sedimentation rates due to decreased carbonate production, induced by the emersion of the platform top. The sea-level fall was followed by deposition of mixed fine-grained siliciclastic-carbonate successions (e.g. Riva di Solto Shale, Kossen beds, "Rhaetavicula contorta beds", Fatra Formation).
Stratigraphic evidence indicates a dry climate in the Western Tethys during the Norian, as indicated by the presence of evaporites (Burano, Apennine) and arid to semi-arid coastal to playa settings (Upper Keuper, Germany). In contrast, the basal layers of the basinal shales show evidence of wet climate.
The end of the Norian depositional system records two different phenomena: (1) an important sea-level fall was responsible for the emersion of the platform top and deposition of a condensed horizon in the basins: and (2) transition from dry to humid climate. The observed evolution is explained with a global cooling which caused the rapid sea-level fall responsible for the abrupt end of the DP/HD depositional system and the shift of the boundary between arid and temperate climate belts, which modified the distribution and amount of rainfall, triggering the deposition of shales along the Western Tethys margin
Early diagenetic fracturing in shallow subtidal environments from the Berriasian carbonate platform of Eastern Sardinia (Italy)
No full textLate Triassic biostratigraphic constraints in the Imagna Valley (western Bergamasc Alps, Italy)
No full textMiddle Jurassic stratigraphic evolution of basinal carbonates of Eastern sardinia (european passive margin)
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