Rend. online Soc. Geol. It., 3 (2008), 270-271, Note Brevi
Stratigraphy and Depositional Architecture of the continental Middle Eocene – Early Miocene successions of Southwestern Sardinia: changing tectostratigraphic significance of the “Cixerri Fm.” Auct. along times? LUCA GIACOMO COSTAMAGNA (*) & SEBASTIANO BARCA (*)
ABSTRACT Stratigrafia ed architettura deposizionale delle successioni continentali dell’Eocene medio – Miocene inferiore nella Sardegna sud-occidentale: variazioni di significato tettono-stratigrafico della “Fm. del Cixerri” Auct. nel tempo? È stata eseguita l’analisi stratigrafico-sedimentologica e dell’architettura deposizionale degli affioramenti più significativi delle successioni continentali affioranti nella Sardegna sud-occidentale ed in gran parte riferiti alla Fm. del Cixerri. Questa generalmente si depone in un sistema di piana a meandri ad energia decrescente verso E: la presenza di contesti di fiume intrecciato è sporadica e ristretta ad alcuni affioramenti occidentali. Lo studio di dettaglio di alcune di queste ultime successioni ha evidenziato nella parte alta degli affioramenti riferiti alla Fm. del Cixerri un brusco incremento dell’energia deposizionale, che passa nell’arco di qualche metro da ambienti di fiume intrecciato sabbioso distale a quelli di fiume intrecciato ghiaioso di alta energia o di conoide alluvionale. Il rinvenimento in questi affioramenti di piroclastiti, sia come elementi clastici che come intercalazioni, riferibili ad un ciclo vulcanico ben datato (Unità di Corona Maria) consente di riferire la loro età al Miocene basale. Questo cambiamento è quindi collegabile ad un ringiovanimento morfologico causato dalla coeva nascente morfologia ad horst e graben relativa all’apertura del bacino algero-provenzale. La mancanza di superfici di separazione chiaramente individuabili, le strette affinità litologiche e l’assenza di fossili rende difficile la separazione di questi affioramenti dalla Fm. del Cixerri s.s.. Questa “Fm. del Cixerri” Auct. accrescerebbe cosi notevolmente la sua estensione cronologica mutando contemporaneamente il suo significato tettonostratigrafico: da unità molassica nell’Eocene medio ad unità di margine di rift nell’Oligocene superiore Miocene inferiore, qui eteropica con la coeva Formazione di Ussana, che contemporaneamente si sviluppa nella parte centro-meridionale dell’isola, e che in questo settore ne verrebbe ad essere il corrispondente laterale.
Key words: Depositional Architecture, Pyrenean Orogeny, Sardinian Rift, Stratigraphy, SW Sardinia, Tertiary. INTRODUCTION The Cixerri Fm. (PECORINI & POMESANO CHERCHI, 1969) is a terrigenous, continental unit built of siltites, sandstones and subordinated conglomerates until now referred to an Eocenic-Early Oligocenic? age (PITTAU D EMELIA, 1979). It is a molassic unit (CHERCHI, 1979; BARCA & COSTAMAGNA, 2000) whose deposition has been mainly related to the dismantling of the Pyrenean Chain. A drilling campaign run in the area by Carbosulcis company (ASSORGIA et alii, 1992) for exploitation of coal evidenced the Cixerri Fm. has a thickness ________________________ (*) Dipartimento di Scienze della Terra, via Trentino 51 Cagliari
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comprised between 210 and 320 m. The Cixerri Fm. is posed unconformably over the Paleocene?-Middle Eocene (PITTAU, 1974; FANNI et alii, 1982) “Lignitifero” succession. Up to now, the Cixerri Fm. has been believed terminated by the superposition of the calc-alkaline volcanites of OligoceneMiocene age (ASSORGIA et alii, 1992) related to the Apennine subduction under the European plate which the SardiniaCorsica block was still connected. MAJOR NEW DATA All the available outcrops of the Cixerri Fm. in Southern Sardinia have been considered and analyzed using a revised and adapted version of the Depositional Architecture MIALL’s concepts (1996 and references therein). Detailed sedimentological-stratigraphical and architectural reconstructions have been drawn. Generally, the presently Cixerri Fm. surfacing outcrops in the Iglesiente- Sulcis area most likely represents the upper part of the unit and mainly figures out a meandering stream environment formed by ribbon channels wandering along the alluvial plain and gradually loosing energy towards East. Here channels, frequently featured by sandy point bar sequences vertically separated each other by quite thicker overbank pelitic successions containing scattered crevasse splays, have been evidenced. Nevertheless, in the Western Iglesiente-Sulcis localized distal sandy braided river facies have been also characterized. They are formed by frequently interconnected tabular sandy channel bodies migrating rapidly sideways, alternated with pelitic overbank bodies: here, channel and overbank successions are fairly balanced in thickness. This improves furtherly the slope gradient direction and the prevailing western provenance of the main bulk of the detritus at these times. Frequently, along the border of the Giba and Cixerri plains scattered outcrops of coarse, polygenic and often poorly sorted Cixerri Fm. figuring out the development of scattered gravel bars, directly posed over the Paleozoic Variscan Basement take place. Rare paleodirectional indicators suggest a NS transport trend. They develop in response to the start of the Oligocene-Miocene extensional (trans-tensive) movements giving birth to the Cixerri and Giba plain. Most of these coarse outcrops mirror the development of a rejuvenating relief run by bedloadcarrying stream tributaries that join quite perpendicularly the main stream of the Paleocixerri river catchment basin, most likely oriented EW. At these times it is possible the Giba plain, for the developing uplifting relieves, separates from the Cixerri
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plain and so starts to have a different catchment basin with flow direction towards SW. In some western outcrops (Acqua sa Canna, Porto Paglia, Flumentepido, M. Sirai) almost suddenly the sandy-pelitic, distal braided environment sedimentation is replaced by coarse deposits formed by immature conglomerates and sandstones sized as lens or tabular bodies. Besides, intercalations of pyroclastic deposits of Early Miocenic age (Corona Maria Unit; A SSORGIA et alii, 1992) as well as pyroclastic pebbles have been found in this part of the Cixerri Fm. In the Flumentepido area (Carbonia), the deposits can be reported to gravelly bars and intercalated limited sandy bars configuring an high-energy braided environment. At Acqua sa Canna and Porto Paglia the Cixerri Fm. succession, essentially built of alternation of hyperconcentrated debris flows and sheet floods with rarer stream floods, is referable to environments comprised between the upper to middle alluvial fan. Collected imbrications suggest extremely variable provenance of the feeding along times. Nevertheless, reversely from other localities of the IglesienteSulcis, reporting a Western provenance of the feeding, here a Eastern source can be often plainly stated. PRELIMINARY RESULTS The above exposed data allow us to define a more precise paleodepositional framing of the Cixerri Fm. and to suggest a much more younger age than previously supposed of the topmost outcrops formerly referred to the Cixerri Fm. An attempt to divide these outcrops from the Cixerri Fm. s.s. failed because of the absence of clear separation surfaces, the litological affinities and the lack of fossils. In this way, this “Cixerri Fm.” Auct. would expand its lasting from the Lutetian (Middle Eocene) up to the Early Miocene, and would assume a changing tecto-sedimentary meaning along times: in fact, the “Cixerri Fm.” Auct. starts as a molassic unit flanking the southeastern border of the Pyrenean chain and feeded by its dismantling (BARCA & COSTAMAGNA, 2000). The investigations on an Eocenian Turtle (RIGHI & DELFINO, 2003) found in the Flumentepido quarry (Carbonia, specimen recovered by S. Barca) suggest an Eocenic age for the lower part of the local succession, perhaps representing a middle part of the “Cixerri Fm.” Auct.. Along Eocenic times, as indicated by the Carbosulcis drills (ASSORGIA et alii, 1992), the unit gradually fines up due to the progressive smoothing of the Pyrenean relieves. Nevertheless, the sudden coarsening of its upper part, together with its intimate association with magmatic products of Early Miocenic age (Corona Maria Unit, ASSORGIA et alii, 1992), and the frequent eastern provenance direction of the detritus, implies their products are related to the uplift linked to the development of the Sardinian Tertiary Rift (CHERCHI & MONTADERT, 1983) in the frame of the coeval Western Mediterranean rifts. So, at that time, this “Cixerri Fm.” Auct. terminates to exist as Pyrenean Molasse (pre–rift unit) and gradually starts to play the new role of rift margin succession (syn-rift unit), so being similar and coeval of the Ussana Fm. (PECORINI & POMESANO CHERCHI, 1969) of Central Sardinia In this setting, the tectonic deformations (reverse faults) mentioned in the Flumentepido quarry by BARCA & COSTAMAGNA (2000) and by CARMIGNANI et alii
(2004), and up to now framed with troubles into a precise tectonic context, could be linked to the transpressivetranstensive tectonics that give place to the Giba and Cixerri subsiding (pull-a-part?) depositional basins. REFERENCES ASSORGIA A., BARCA S., COCOZZA T., DECANDIA A., FADDA A., GANDIN A. & OTTELLI L. (1992a) – Characters of the Caenozoic sedimentary and volcanic succession of Western Sulcis (SW Sardinia). IGCP No. 276, Newsletters Vol. 5, Siena 1992, 17-20. BARCA S. & COSTAMAGNA L. G. (2000) - Il bacino paleogenico del Sulcis-Iglesiente (Sardegna SW): nuovi dati stratigrafico-strutturali per un modello geodinamico nell'ambito dell'orogenesi pirenaica. Boll. Soc. Geol. It., 119, 497-515. CARMIGNANI L., FUNEDDA A., OGGIANO G. & PASCI S. (2004) - Tectonosedimentary evolution of Southwest Sardinia in the Paleogene: Pyrenean or Apenninic Dynamic? Geodinamica Acta, 17, 275-287. CHERCHI A. (1979) - Microfaune aptiano-albiane dei ciottoli urgoniani della "Formazione del Cixerri" (Sardegna SW) e loro interesse paleogeografico. Riv. Ital. Paleont., 85, 2, 353-410. CHERCHI A. & MONTADERT L. (1983) – Il sistema di rift oligomiocenico del Mediterraneo occidentale e sue conseguenze paleogeografiche sul Terziario sardo. Mem. Soc. Geol. It., 24, 387-400. FANNI S., MURRU M., SALVADORI A. & SARRIA E. (1982) Nuovi dati strutturali sul bacino del Sulcis. L'Ind. Min., 4, 25-31. MIALL A.D. (1996) - The Geology of Fluvial deposits. Springer, 584 pagg.. PECORINI G. & POMESANO CHERCHI A. (1969) - Ricerche geologiche e biostratigrafiche sul Campidano meridionale (Sardegna) - Mem. Soc. Geol. It., 8, 421-451. PITTAU P. (1974) - Studio palinologico-stratigrafico di un foro di sonda perforato nel bacino lignitifero del Sulcis (Sardegna sud-occidentale). Boll. Soc. Geol. It., 93, 937943. PITTAU DEMELIA P. (1979) - Palinologia e datazione della sezione di Tanca Aru nella valle del Cixerri (Sardegna sudoccidentale). Boll. Soc. Paleont. It., 18 (2), 303-314. RIGHI D. & D ELFINO M., (2003) - Una tartaruga "malgascia" nel Paleogene della Sardegna. In: Pavia M. & Violanti D. (eds.): Abstracts and Programme of the "Giornate di Paleontologia 2003 (SPI), Alessandria, Italy, 44.
