C., G., F. & G. (2005).- Paleontological evidence for Late Cambrian in the Arburese area, SW Sardinia. In: P. & E. (eds.), Pre-Cambrian to Paleozoic Palaeopalynology and Palaeobotany.- Carnets de Géologie / Notebooks on Geology, Brest, Memoir 2005/02, Abstract 08 (CG2005_M02/08)
New palynological investigations allowed a better definition of the age of terrigenous deposits of the Monte Fonnesu tectonic Unit, exposed in the Arburese area (SW Sardinia). The section investigated consists of a coarsening upwards positive sequence, up to 10 m thick, that is made up of quartz-arenitic sandstones and light gray quartzite beds (0.5 m thick), interbedded with thin beds of black shale. In the lower part of the section, two quartzitic levels are rich in lingulid (brachiopods) shell fragments. This is the first report of macrofossils from the Monte Fonnesu Unit. Samples collected for palynological analyses in the black shale at the base and at the top of the lowest quartzitic level yielded a Late Cambrian acritarch microflora that can be correlated with the Peltura trilobite Superzone. The Late Cambrian age assigned
to the lower part of the sequence allows us to define more precisely the age of Mt. Fonnesu sandstone, which
in consequence is bracketed between the Late Cambrian and the Early Ordovician.
Sardinia; Lower Paleozoic; Late Cambrian;
Peltura trilobite Zone; lingulids; acritarchs.
Sardaigne ; Paléozoïque inférieur ;
Cambrien supérieur ; Zone à trilobite Peltura ; lingulides ; acritarches.
This paper describes the first finding of lingulid-bearing quarztites
in a Lower Paleozoic low-grade metamorphosed succession of the Nappe zone (southwestern Sardinia). These quartzites have been dated using acritarchs.
From
a regional standpoint three parallel NW-SE trending orogenic zones have been identified in the Variscan Sardinian chain.
They were delimited on the basis of the relative degree of deformation and metamorphism
of each zone and its stratigraphic sequence. They are the: (i) External zone, (ii) Nappe zone and (iii) Axial zone. Deformation and metamorphic grade increase northeastward from the External zone to the Axial zone ( et alii,
1994, and Fig. 1 
).
The Nappe zone is made up of several southwest-facing tectonic units of low metamorphic grade (anchizone - epizone)
each with a very similar lithostratigraphic sequence. The sequence starts with Middle Cambrian-Early Ordovician metasandstone, phyllite and quartzite, followed upward by transgressive Upper Ordovician (Caradoc).
It continues with Silurian graptolite-bearing black shale, phyllite, and Orthoceras-bearing limestone, in turn covered by Devonian nodular limestone rich in tentaculites and Climeniida.
The sequence terminates with Lower Carboniferous Culm-like flysch deposits
containing olistoliths of Devonian limestone ( et alii,
1994).
In the Arburese area (SW
Sardinia), where the outcrop to be described is located, Paleozoic basement consists of five southwest-facing tectonic units ( & ,
1991) unconformably covered by Mesozoic and Cenozoic sedimentary and volcanic deposits (Fig. 2 ). These tectonic units are made up of low-grade metamorphic (anchizone-epizone) Upper Cambrian to Devonian sedimentary successions, very similar to those of the tectonic units of central and southern Sardinia
described above ( et alii,
1994).
Late Hercynian and Alpine brittle deformation affected all
of these stacked units. NW-SE strike-slip faults of late Hercynian age are often associated with mineralized seams. There are also normal faults related to the emplacement of late Hercynian granites. Alpine deformation consists of important E-W trending normal faults ( & ,
1991).
The Monte Fonnesu Unit (MFU, Fig. 2
and Fig. 3 ) crops out at the top of the tectonic pile. It consists mainly of terrigenous deposits, made up of alternating micaceous sandstones, grey quartzite, siltite and greenish-grey shale, that represent outer fan and basin plain depositional environments ( et alii,
1982a; & ,
1991). An Early Ordovician age, based on the palynological (acritarchs) content ( & ,
1980; et alii, 1982b; ,
1985) has been proposed for the terrigenous portion of the MFU.
The MFU is folded isoclinally with a northward-directed gently dipping syn-metamorphic foliation parallel to the original sedimentary bedding. Metamorphic assemblages, mainly of white mica and chlorite, are indicative of a lower greenschist facies. Later phases of deformation consist of open to concentric folds with upright crenulation cleavage developed ( & ,
1991).
The outcrop
that provided the fossils for this report is a 10 m thick sequence in the MFU. It is located along the Rio de Tremolia, near the coast, about 35 km south-southwest of the town of Oristano (Fig. 3 ). The sampled section (Fig. 4 ) is a coarsening upwards-positive sequence,
ranging up to 3 m in thickness. It consists of quartz-arenitic sandstone and light gray shell-fragment-rich quartzite beds, about 0.5 m thick, interbedded with thin black shales. In the section, the shaly levels decrease progressively in thickness and disappear in the upper portion. The occurrence of sedimentary structures (asymmetric ripples and concave low angle cross bedding) suggests a tidal-intertidal depositional environment for this part of the terrigenous sequence, with minor episodes of deeper marine environment, as indicated by the deposition of shales.
Macrofossil content - In the two shell fragment-rich quartzitic levels, samples (M13 and M3 in Fig. 4 ) were
collected for paleontological analysis. The fossiliferous content consists of collophanitic laminate fragments, with weakly arcuate shapes. In the upper quarzitic level, they are concentrated in thin layers. Detailed observation carried out with the scanning electronic microscope (SEM) to define the structure and ornamentation of the laminate fragments shows that they are composed of more than three laminae and in addition, that the rarely preserved shell ornamentation consists of arcuate parallel striae (Plate 1, Figs. 1-3 ).
Thus, the composition, ornamentation and laminate structures of the shells clearly indicate that they are lingulid (inarticulate brachiopod) fragments.
Palynological content - Six samples for palynological analysis were collected in the black shales. Approximately 50 g of each sample were processed using standard palynological techniques. Neither oxidation nor alkali treatment was used. Slides are stored in the Department of Earth Sciences of Pisa (Italy).
Only two samples (AR1 and AR3 in Fig. 4 ) located at the base and at the top of the lowest quartzitic level (M13) yielded a poorly preserved acritarch microflora (Plate 1, Figs.
4-13 ).
In the lowermost sample (AR1) Acanthodiacrodium spp., Cristallinium cambriense () ,
1978, Lusatia dendroidea ,
1970, Stelliferidium spp., ? Timofeevia pentagonalis () ,
1978, and Vulcanisphaera turbata in & ,
1981, were recognized.
In the uppermost sample (AR3) Acanthodiacrodium spp., Cymatiogalea virgulta
in & ,
1988, Dasydiacrodium caudatum ,
1973, Dasydiacrodium obsonum
in & ,
1988, Impluviculus multiangularis (
in & ) ,
1990, Ladogella sp., Lusatia dendroidea, Timofeevia phosphoritica ,
1978, Stelliferidium sp. and Vulcanisphaera africana ,
1961, are present.
According to the biostratigraphic scheme of &
(2000), based on Upper Cambrian successions from Random Island (eastern Newfoundland), Cymatiogalea virgulta, Dasydiacrodium caudatum and Vulcanisphaera turbata do not range higher than the RA5 microflora,
that has been correlated with the Protopeltura praecursor and Peltura minor trilobite Zones.
Also
found in the RA5 microflora are Impluviculus multiangularis, Ladogella
sp. and the form of Lusatia dendroidea with two processes (described as Orthosphaeridium? extensum in & ,
2000, see taxonomical remarks in et alii,
in press).
Similar assemblages have been recorded in Upper Cambrian sediments of the East European Platform (,
1990), the Rocroi Massif, northern France ( & ,
1993) and in the Cantabrian Zone, North Spain ( et alii,
in press).
Late Cambrian palynological records in Sardinia have been reported previously only from the Nappe zone ( et alii,
1985; , 1991, et alii,
1993). The microflora occupies a range in age from the Agnostus pisiformis (?) trilobite Zone to the Parabolina spinulosa trilobite Zone. A Late or possible latest Cambrian age
has been hypothesized for the S. Vito Sandstones (Sarrabus Unit, et alii,
1982a, 1984) but the assemblage recorded lacks the species that characterize this interval.
In conclusion, the section studied here represents the first find of lingulid-rich quartzites in the Sardinian Paleozoic basement. The Late Cambrian age assigned to this
sequence allows us to define more precisely the age of Mt. Fonnesu sandstone,
now bracketed in age between the Late Cambrian and the Early Ordovician. In
addition, based on acritarch assemblages, the interval corresponding to the Peltura trilobite Zone of the Upper Cambrian is recognized
for the first time there.
The authors gratefully thanks
Prof. M. and N. for the kindly revision of the manuscript, their suggestions greatly improved it.
R., A., N. & M. (1985). Nuovi dati palinologici e e considerazioni geologiche sull'età delle Arenarie di Solanas (Cambro-Ordoviciano, Sardegna Centrale).- Atti della Società Toscana di Scienze Naturali - Memorie, Pisa, (A), vol. 91, p. 1-20.
R., G., E., J.C. & C. (in press). Late Cambrian acritarchs from the "Túnel Ordóvicico del Fabar", Cantabrian Zone, N Spain.- Review of Palaeobotany and Palynology, Amsterdam.
S., M. & P. (1982a).- Acritarchs in the "Arenarie di S. Vito" of South-East Sardinia (Italy): stratigraphical and geological implications.- Bollettino della Società Geologica Italiana, Roma, vol. C, p. 369-375.
S., T., M. & P. (1982b).- Discovery of Lower Ordovician Acritarchs in the "Postgotlandiano" sequence of southwestern Sardinia (Italy): age and tectonic implications.- Bollettino della Società Geologica Italiana, Roma, vol. C, p. 377-392.
S., M., N. & P. (1984).- Presenza di un Tremadociano ad Acritarchi in Unità tettoniche Erciniche a Sud del Lago di Mulargia (Sardegna Centrale).- Rivista Italiana di Paleontologia e Stratigrafia, Milano, vol. 89, p. 315-334.
S. & C. (1980).- Lineamenti litostratigrafici e sedimentologici del "Postgotlandiano" del Fluminese (Sardegna SW).- Memorie della Società Geologica Italiana, Roma, vol. 20, 1979, p. 203-213.
G. (1970).- Weitere organische Mikrofossilien aus dem unteren Ordovizium.- Paläontologische Abhandlungen, Abteilung B: Paläobotanik, Berlin, vol. 3, n° 3-4, p. 289-332.
L., R., A., G., G., G. & P.C. (1994).- The Hercynian chain in Sardinia (Italy).- Geodinamica Acta, Paris, vol. 7, n° 1, p. 31-47.
J. (1961).- Un microplancton à Hystrichosphères dans le Trémadoc du Sahara.- Review of Palaeobotany and Palynology, Amsterdam, vol. 4, n° 1, p. 37-52.
A. (1991).- Upper Cambrian Acritarchs from the Solanas Sandstone Formation, Central Sardinia, Italy.- Bollettino della Società Paleontologica Italiana, Modena, vol. 30, n° 2, p. 127-152.
A., M. & R. (1993).- Acritarchs findings in Early Paleozoic, low-grade metasediments of Sardinia (Italy).- Revista Española de Micropaleontología, Madrid, vol. 8, n° 2, p. 170-176.
F. & W.T. (1981).- Middle and Upper Cambrian and Lower Ordovician Acritarchs from Random Island, Eastern Newfoundland.- Bulletin of Geological Survey of Canada, Ottawa, vol. 343, p. 1-43.
F. & W.T. (1988).- Middle and Upper Cambrian acritarch and trilobite zonation at Manuels River and Random Island, Eastern Newfoundland.- Bulletin of Geological Survey of Canada, Ottawa, vol. 381, p. 1-91.
F. & P.C. (1991).- Structural evolution of Northern Arburese (SW Sardinia).- Bollettino della Società Geologica Italiana, Roma, vol. CX, p. 747-755.
M.G. & M.M. (2000).- Acritarch microfloral succession from the Late Cambrian and Ordovician (early Tremadoc) of Random Island, Eastern Newfoundland, and its comparison to coeval microfloras, particularly those of the Eastern European Platform.- American Association of Stratigraphic Palynologists Foundation Contributions Series, College Station, n° 38, 123 p.
P. (1985).- Tremadocian (Early Ordovician) Acritarchs of the Arburese Unit, Southwest Sardinia (Italy).- Bollettino della Società Paleontologica Italiana, Modena, vol. 23, n° 2, p. 161-204.
C. & M. (1993).- Latest Middle-Late Cambrian Acritarchs from Belgium and Northern France.- Special Paper in Palaeontology, London, n° 48, p. 45-55.
M. (1973).- New Acritarcha from the Upper Cambrian of Belgium. In: Microfossils of the oldest deposits.- Proceeding of the third Palynological Conference, Novosibirsk (1971), Academy of Science of the U.S.S.R., Siberian Branch, Institute of Geology and Geophysics, Nauka, p. 28-31.
M. (1978).- Critères palynostratigraphiques conduisant à la reconnaissance d'un pli couché revinien dans le sondage de Grand-Halleux.- Annales de la Société Géologique de Belgique, Liège, vol. 100, p. 249-276.
N.A. (1990).- Middle and Upper Cambrian acritarchs in the East European Platform.- Ordena Trudovogo Ksasnogo Geologicheskii Institut, Trudy, vol. 454, 116 p. (in Russian).
Click on thumbnail to enlarge the image.
Figure 1: Distribution of Paleozoic basement in Sardinia (simplified after et alii, 1994).
Click on thumbnail to enlarge the image.
Figure 2: Tectonic sketch map of the Arburese area (simplified after & , 1991).
Click on thumbnail to enlarge the image.
Figure 3: Geologic sketch map of the area studied.
Click on thumbnail to enlarge the image.
Figure 4: Schematic lithostratigraphy of the section sampled.
Click on thumbnail to enlarge the image.
Plate 1: Scale bars= 10 µm if not otherwise stated.
Fig. 1.- Enlargement of the surface of a shell fragment showing the ornamentation. SEM, Sample M3, bar is 100 µm.
Fig. 2.- A shell fragment in optical view, showing the laminate structure. Thin section, Sample M3, bar is 100 µm.
Fig. 3.- An enlarged laminated fragment (almost four laminae). SEM, Sample M3.
Fig. 4.- Cristallinium cambriense () ,
1978. Sample AR1, slide AR1/TOT-2.
Fig. 5.- Lusatia dendroidea , 1970. Sample AR1, slide AR1/TOT-2.
Fig. 6.- Dasydiacrodium caudatum ,
1973. Sample AR3, slide AR3/TOT-2.
Fig. 7.- Dasydiacrodium obsonum in & ,
1988. Sample AR3, slide AR3/TOT-2.
Fig. 8.- Stelliferidium sp. Sample AR1, slide AR1/TOT-4.
Fig. 9.- Timofeevia phosphoritica ,
1978. Sample AR3, slide AR3/TOT-2.
Fig. 10.- ? Timofeevia pentagonalis (, 1974) ,
1978. Sample AR1, slide AR1/TOT-1.
Fig. 11.- Impluviculus multiangularis ( in & , 1971)
1990. Sample AR3, slide AR3/TOT-2.
Fig. 12.- Vulcanisphaera africana , 1961. Sample AR3, slide AR3/TOT-1.
Fig. 13.- Vulcanisphaera turbata in & ,
1981. Sample AR1, slide AR1/TOT-5.