Stratigraphic-paleogeographic evolution of Eastern Sardinia Jurassic passive margin carbonates: synthesis and future developments.

During 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. REFERENCES Amadesi E., Cantelli C., Carloni G.C. & Rabbi E. (1967) – Carta geologica del Foglio 208-Dorgali, 1:100.000, Libreria dello Stato, Roma. Casellato C.E., Jadoul F. & Lanfranchi, A. (2012) - Calcareous nannofossil biostratigraphy of the S'Adde Limestone (Mt. Albo, Orosei Gulf): insights into the Middle-Late Jurassic Eastern Sardinia passive margin evolution. Rivista Italiana di Paleontologia e Stratigrafia, 118, 439-460. Costamagna L.G. 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