The Mt Perda Liana Section (Middle Jurassic, Central-Eastern Sardinia): Revised Stratigraphy and Brachiopod Faunas
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Bollettino della Società Paleontologica Italiana, 52 (2), 2013, 123-138. Modena The Mt Perda Liana section (Middle Jurassic, central-eastern Sardinia): revised stratigraphy and brachiopod faunas Iginio DIENI, Francesco MASSARI & Vladan Radulović I. Dieni, Dipartimento di Geoscienze, Università degli Studi di Padova, Via Gradenigo 6, I-35131 Padova, Italy; [email protected] F. Massari, Dipartimento di Geoscienze, Università degli Studi di Padova, Via Gradenigo 6, I-35131 Padova, Italy; [email protected] V. Radulović, Department of Palaeontology, Faculty of Mining and Geology, Kamenička st. 6, P. O. Box 162, RS-11000 Belgrade, Serbia; [email protected] KEY WORDS - Middle Jurassic, Sardinia, Lithostratigraphy, Biostratigraphy, Brachiopods. ABSTRACT - The Jurassic in age stratigraphic section of Mt Perda Liana, the classic locality of the “Tacchi” area (central-eastern Sardinia), has been studied from a stratigraphic and palaeontologic viewpoint. A rich fauna is contained in a limestone interval comprised between the siliciclastic deposits of the Genna Selole Formation (Bajocian-lowermost Bathonian) and the typical dolostones of the Dorgali Formation. This limestone interval, also present in other localities of the “Tacchi” area, is here formally distinguished as member of the Dorgali Fm. (Perda Liana Member) due to its peculiar lithology and faunal content. It mainly consists of shelf-lagoonal micritic limestones, particularly rich in infaunal bivalves. The presence of the brachiopods Kallirhynchia oranensis (Flamand, 1911) and Holcothyris angulata Buckman, 1918, allows the attribution of the Perda Liana Member to the Lower Bathonian. RIASSUNTO - [La sezione di Monte Perda Liana (Giurassico Medio, Sardegna centro-orientale): revisione stratigrafica e fauna a brachiopodi] - Viene illustrata la sezione stratigrafica della classica località di Perda Liana, ubicata nella regione dei “Tacchi”. Una ricca fauna, in parte già nota, è contenuta nell’intervallo calcareo compreso tra i sedimenti silicoclastici della Formazione di Genna Selole, attribuita al Bajociano e al Batoniano basale, e le tipiche dolomie della Formazione di Dorgali. Questo intervallo calcareo, identificato in alcune altre località dei “Tacchi”, viene qui distinto per i suoi caratteri peculiari di fauna e litologia come membro della Formazione di Dorgali e formalizzato come nuova unità litostratigrafica (Membro di Perda Liana). Si tratta prevalentemente di calcari micritici indicativi di un ambiente di bassa energia identificabile come laguna aperta, il cui fondo ospitava una ricca fauna di bivalvi fossatori. Particolarmente significativi dal punto di vista biostratigrafico si sono rivelati i brachiopodiKallirhynchia oranensis (Flamand, 1911) e Holcothyris angulata Buckman, 1918, indicativi del Batoniano Inferiore. Terebratula lamarmorae Meneghini, 1857, viene considerata come nomen oblitum a favore di Holcothyris angulata Buckman, 1918, che diviene pertanto nomen protectum. INTRODUCTION The continental to marginal-marine sedimentation gave way to marine carbonate ramp sedimentation, more The lower part of the Jurassic succession of eastern completely preserved in the Ogliastra, Supramonte and Sardinia is generally made up of dolostones which failed Baronie areas of eastern Sardinia - as exemplified by generally to give information useful for a biostratigraphic the Dorgali Formation, Mt Tului Limestone, locally attribution, due to the scarcity or absence of fossils. grading basinwards into S’Adde Limestone, and Mt However, in places the succession includes calcareous Bardia Limestone (Amadesi et al., 1961; Casellato et al., lenses to layer bundles of limited lateral persistence, 2012, and references therein). The deposits share several containing rich fossil assemblages, mostly bivalves and characteristics of the Jurassic carbonate platforms of the less abundant brachiopods, allowing to establish some south-European margin (Stampfli et al., 2002; Costamagna firm chronostratigraphic points. One of the richest section, & Barca, 2004), for instance in Provence (Fourcade et useful to this purpose, is that of Mt Perda Liana in the al., 1977) and in the Corsica autochthonous succession “Tacchi” area of central-eastern Sardinia, which is taken (Peybernès et al., 2001). Similar record of Middle Jurassic into account in this paper, by illustrating stratigraphic and transgressive carbonates bounded at the base by a marked palaeontologic data. unconformity is known in the Briançonnais (Bourbon et The first unit of the local Jurassic cover is represented al., 1973), and part of the Swiss and French “Préalpes by continental to marginal-marine siliciclastic deposits médianes” (“Domain intermédiaire”; Septfontaine, 1983). of the Genna Selole Formation (defined by Dieni et al., 1983), transgressive on the Palaeozoic and locally Triassic substrate and bearing palynomorphs and plant macrorests GEOLOGIC AND STRATIGRAPHIC SETTING indicating a Bajocian-Bathonian age (Dieni et al., 1983; (I. Dieni & F. Massari) Del Rio, 1984). The basal, coarse-grained conglomerates record alluvial fan sedimentation accounting for denudation Geologic setting of a rugged relief, with deep erosion of the substrate. The Monte Perda Liana (sometimes written as This episode is conceivably related to the rifting which Perdaliana or Perda ’e Liana; hereafter simply called affected the south-European margin in the Middle Jurassic, Perda Liana) is an isolated, tower-shaped mount (1293 m generating a swell-and-basin topography, concomitantly a.s.l.), located in central-eastern Sardinia, in the territory with the initial Tethys opening (Lemoine et al., 1978). of Gairo (boundary area between the Barbagia of Seulo ISSN 0375-7633 doi:10.4435/BSPI.2013.21 124 Bollettino della Società Paleontologica Italiana, 52 (2), 2013 remains of a much wider carbonate platform covering most part of Sardinia. Their morphology somewhat recalls that of the mesas of the Monument Valley (U.S.A.). As outlined above, the Perda Liana section, like most part of the “Tacchi”, is the record of the lower part of the Jurassic succession of eastern Sardinia, more completely developed in the Ogliastra, Supramonte and Baronie areas. An appreciable synthesis on the Middle Jurassic succession of the “Tacchi” area, highlighting the state of the art on these deposits, especially from the viewpoints of the palaeogeography and regional geology, was presented by Costamagna & Barca (2004). They distinguished three units within the Genna Selole Formation on lithostratigraphic grounds: the conglomeratic “Laconi- Gadoni Lithofacies”, the sandy-clayey “Nurri-Escalaplano Lithofacies” and the mixed siliciclastic-carbonate “Ussassai-Perdasdefogu Lithofacies”, grading upwards into the Dorgali Formation. They argued that the Genna Selole type-section, located in the Ogliastra region, is Fig. 1 - Distribution of Jurassic formations in central-eastern Sardinia (from Dieni et al., 1985, modified). poorly representative and inadequate to illustrate the full range of lithotypes in the Tacchi area. For this reason they proposed a formal redefinition of the formation, which would include the three above mentioned lithofacies, and Ogliastra). It is one of the so called “Tacchi” (Fig. 1) introducing two parastratotypes in the areas of Escalaplano of central-eastern Sardinia, hills generally rising from the and Perdasdefogu, characterised by good exposures Palaeozoic basement (Fig. 2) and consisting of carbonate and continuity. The threefold subdivision proposed by mesas. They are true natural monuments of characteristic Costamagna & Barca (2004) was accepted by Scanu et al. aspect, resembling castles, towers and ruins (Barrocu (2012) in a preliminary study of the Jurassic plant fossils & Gentileschi, 1966; Fig. 3a), and represent isolated based on the revision of the Lovisato Collection. Fig. 2 - Geologic sketch-map of the Mt Perda Liana area (from Atzeni, 1967). 1: limestone and dolostone (Dorgali Formation, Middle Jurassic); 2: quartz-conglomerate, arenite and carbonaceous claystone (Genna Selole Formation, lower Middle Jurassic); 3: quartz-porphyry in dykes and subvolcanic bodies (Variscan cycle); 4: Variscan granite; 5: graptolite-bearing slate (Wenlock, Silurian); 6: phyllite (Llandovery, Silurian); 7: Orthis-bearing slate and quartz-phyllite (Ordovician); 8: fault. I. Dieni et alii - Bathonian of Mt Perda Liana and its brachiopods 125 Fig. 3 - View of the Perda Liana tower with trace (dashed line) of the measured section of Fig. 4 (a) and the first stratigraphic description (b) due to La Marmora (1857, fig. 29 at p. 157). The stratigraphic succession of Perda Liana commonly marly, with conchoidal fracture, in places The stratigraphic succession of Perda Liana is containing a small fine-grained quartz fraction. A lower palaeontologically well known as it yielded abundant and plane-parallel-bedded interval 19 m thick shows muddy varied macrofaunas, formerly studied by La Marmora interbeds, plant remains, molluscs (among which very (1857; Fig. 3b), Meneghini (1857), Fucini (1894, 1899), abundant infaunal bivalves, usually in life position), Pampaloni (1900) and Dainelli (1903). A preliminary study common brachiopods and characteristic bioclastic infilling of the section was performed by De Biasi (1985). of burrows. It grades into an interval 20.5 m thick, The lower siliciclastic part of the succession (of which where compaction on thorough bioturbation resulted in a only the uppermost portion is represented in the log of Fig. predominantly flaser to nodular