conodonts from the andes

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upper view, sample MT 7; 9, Palmatolepis gracilis expansa Sandberg and Ziegler, 1979, MDLCA 30209, upper view, sample MT 6; 10, Pseudopolygnathus.
Publicación Especial Nº 13

ASOCIACIÓN PALEONTOLÓGICA ARGENTINA

CONODONTS FROM THE ANDES Proceedings of the 3rd International Conodont Symposium & Regional Field Meeting of the IGCP project 591

Edited by Guillermo L. Albanesi and Gladys Ortega

BUENOS AIRES 2013

CONODONTS FROM THE ANDES - Publicación Especial Nº 13 - Julio 2013 - PALEONTOLOGICAL NOTE

FAMENNIAN-TOURNAISIAN CONODONTS FROM THE MONTE TACCU SECTION (SARDINIA, ITALY) MOSSONI, A. 1, CORRADINI, C. 1

AND

SPALLETTA, C. 2

1

Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari, via Trentino 51, I-09127 Cagliari, Italy; [email protected], [email protected] Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Alma Mater Studiorum Università di Bologna, via Zamboni 67, I-40126 Bologna, Italy; [email protected]

2

Keywords: Conodonts. Famennian. Tournaisian. Devonian/Carboniferous Boundary. Sardinia.

INTRODUCTION

sequence of the whole island is preserved, refer to Corradini et

THE Monte Taccu section is the only known section that

al. (2002) and Corradini and Ferretti (2009). The base of the

spans the Devonian/Carboniferous boundary in Sardinia. It is

Devonian is here represented by a few meters of black graptolitic

located in the southeastern part of the island, close to San

shales. This unit grades into an alternation of dark pelites and

Nicolò Gerrei village. Corradini et al. (2003) studied the upper

nodular marly limestone (Tentaculitic shales and limestones) of

part of the section, focused on the uppermost five meters across

Early-Middle Devonian age. The calcareous content progres-

the D/C boundary, illustrating a rich conodont fauna from the

sively increases and the Upper Devonian-lowermost Carbonif-

Lower expansa (Upper Devonian) to the Upper duplicata

erous sediments are represented by massive limestones. Above,

(Lower Carbonifeorus) zones. In connection with the activities

several dozens of meters of metasandstones and metaconglom-

of the "International Working Group on the Revision of the

erates (“Conglomerato di Villasalto”, Auct.) are present. They

Devonian/Carboniferous Boundary" we restudied the section,

represent the transition to the terrigenous sedimentation termi-

revised the fauna of Corradini et al. (2003), and collected a new

nating the pelagic sequence of the Palaeozoic in SE Sardinia.

sample at the boundary; also, data from the lower part of the section are here presented.

The "Clymeniae limestones" The Upper Devonian-Lower Carboniferous limestone con-

GEOLOGICAL SETTINGS In Sardinia an almost complete portion of the southern

sists mainly of grey massive limestones, known as "Clymeniae limestone" because of the occurrence of ammonoids in some

branch of the Variscan orogenic belt crops out, characterized

levels (Lovisato, 1894). The apparent thickness of this unit may

by non-metamorphosed to high-grade rocks of Early Cambrian

reach hundreds of meters, but tectonic repetitions of the se-

to Early Carboniferous age involved in a complex polyphase

quence are highly probable (Carmignani et al., 1986); the true

deformation. The main result of the Variscan orogeny in Sar-

thickness is more likely about 50-70 m. Apart from a few

dinia is a tectono-metamorphic partition with, from north to

crinoid stems, ammonoids concentrated in a few levels are the

south: an Inner Zone, with medium to high grade metamor-

only abundant macrofossils. The microfacies is a poorly fossil-

phism, thrusted over a Nappe Zone, with green schist meta-

iferous micrite with scarce fossil remains in the ammonoid-

morphism that overthrusted a Foreland Zone affected by very

bearing beds: ostracodes, small shells (bivalves? or brachio-

low grade regional metamorphism (Funedda and Oggiano,

pods?), fragments of echinoderms, gastropods, and rare trilo-

2009, and references therein).

bites. Conodonts are abundant; fish teeth and very rare

The studied area is located in the External Nappes Zone,

brachiopods have been reported from acid-insoluble residues

and belongs to the Gerrei tectonic unit. For a complete descrip-

(Corradini, 1998; Derycke et al., 2003). Biofacies and micro-

tion of this unit, where the most complete mid-Palaeozoic

facies suggest a pelagic environment for these limestones. This 85

CONODONTS FROM THE ANDES - Publicación Especial Nº 13 - Julio 2013 - PALEONTOLOGICAL NOTE

unit has been biostratigraphically investigated by several au-

The microfacies is a mudstone-wackestone. Some fossil re-

thors in the last fifty years (see Corradini 2008 for a summary).

mains, mainly trilobites, brachiopods and ammonoids, have been observed in thin section. Bioclasts, mainly fragments of

THE MONTE TACCU SECTION The Monte Taccu section is located on the northern edge of Monte Taccu, a few km northeast of San Nicolò Gerrei village (Fig. 1), at coordinates 39° 30' 38.953" N, 9° 19' 27.258" E. The section is overturned and exposes about 25 meters of Clymeniae limestone: a grey and rather massive limestone in beds and banks up to 3 m thick. The upper part of the section is more tectonically deformed than the lower one. At the De-

goniatites more frequent in the uppermost levels, are readily recognizable despite the low grade metamorphic overprint. Beside conodonts, the insoluble residues produced a few fish teeth (Corradini, 1998; Corradini et al., 2003). CONODONT DATA Considering also data from Corradini et al. (2003), twentythree samples, weighting 1-6.3 kg, have been collected from the Monte Taccu (MT) section, for a total of about 60 kg of

vonian/Carboniferous boundary an irregular thin level of dark

limestone. Sample MT 4 of Corradini et al. (2003) is here re-

shaley limestone is present. The upper limit of the section is

named MT 5 for practical reasons of correlation with the new

marked by a tectonic boundary with a highly deformed sand-

sample set. The samples were dissolved with conventional formic acid technique. All the samples were productive, yielding almost

stone with silica nodules, probably belonging to the Ordovician San Vito Formation.

Figure 1. Location map of the Monte Taccu section.

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MOSSONI ET AL.: FAMENNIAN-TOURNAISIAN CONODONTS

Figure 2. Stratigraphic log, biozonation and conodont occurrences of the Monte Taccu section. Abbrevations of conodont genera: Alt. = Alternognathus; Br = Branmehla; Bi. = Bispathodus; M. = Mehlina; Pa. = Palmatolepis; Po. = Polygnathus; Pr. = Protognathodus; Ps = Pseudopolygnathus; Sc = Scaphignathus, Si = Siphonodella.

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CONODONTS FROM THE ANDES - Publicación Especial Nº 13 - Julio 2013 - PALEONTOLOGICAL NOTE

Figure 3. 1, Palmatolepis marginifera utahensis Ziegler and Sandberg, 1984, MDLCA 30201, upper view, sample MT 15; 2, Palmatolepis marginifera marginifera Helms, 1961, MDLCA 30202, upper view, sample MT 10; 3, Polygnathus glaber bilobatus Ziegler, 1962, MDLCA 30203, upper view, sample MT 10; 4, Palmatolepis glabra ssp., MDLCA 30204, upper view, sample MT 14; 5, Palmatolepis rugosa cf. ampla Müller, 1956, MDLCA 30205, upper view, sample MT 9; 6, Polygnathus granulosus Branson & Mehl, 1934a, MDLCA 30206, upper view, sample MT 9; 7, Pseudopolygnathus communis renatae Corradini and Spalletta, 2003 (in Corradini et al., 2003), MDLCA 30207, upper view, sample MT 6A; 8, Polygnathus diversus Helms, 1959, MDLCA 30208, upper view, sample MT 7; 9, Palmatolepis gracilis expansa Sandberg and Ziegler, 1979, MDLCA 30209, upper view, sample MT 6; 10, Pseudopolygnathus irregularis Traghelen and Hartenfelds, 2011, MDLCA 30210, upper view, sample MT 6; 11, Pseudopolgnathus bidentatus Hartenfelds, 2011, MDLCA 30211, upper view, sample MT 6; 12 Pseudopolygnathus micropunctatus Bishoff and Ziegler, 1956, MDLCA 30212, upper view, sample MT 6A; 13, Polygnathus duolingshanensis Ji and Ziegler, 1993, MDLCA 30213, upper view, sample MT 6B; 14, Scaphignathus velifer velifer Helms, 1959, MDLCA 30214, upper view, sample MT 6C; 15, Alternognathus regularis Ziegler and Sandberg, 1984, MDLCA 30215, upper view, sample MT 5; 16, Polygnathus pennatuloideus Holmes, 1928, MDLCA 30216, upper view, sample MT 6C; 17, Polygnathus nodoundatus Helms, 1961, MDLCA 30217, upper view, sample MT 15; 18, Pseudopolygnathus primus Branson and Mehl, 1934b, MDLCA 30218, upper view, sample MT 4; 19, Bispathodus costatus (Branson, 1934), MDLCA 30219, upper view, sample MT 4; 20, Bispathodus aculeatus aculeatus (Branson and Mehl, 1934a), MDLCA 30220, lateral view, sample MT X3.

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MOSSONI ET AL.: FAMENNIAN-TOURNAISIAN CONODONTS

5000 conodonts. The state of preservation is quite good, even if some specimens are slightly deformed or broken. Conodonts are black (CAI = 5-5.5). The abundance is highly variable, from a maximum of 491 conodonts/kg in sample MT X1 to a minimum of 19 conodonts/kg in sample MT X3; the average abundance is 97conodont/kg. Seventy-eight taxa, between species and subspecies, belonging to ten genera: Alternognathus, Branmehla, Bispathodus, Mehlina, Palmatolepis, Polygnathus, Protognathodus, Pseudopolygnathus, Scaphignathus and Siphonodella have been recognized (Fig. 2). The whole conodont collection is housed in the Museum of Palaeontology "Domenico Lovisato" of Cagliari University (MDLCA); catalog numbers of figured specimens (Fig. 3) can be obtained from the plate caption. Since conodonts from the younger part of the section have been illustrated in detail by Corradini et al. (2003), only middle-upper Famennian conodonts from the new samples are figured. Biofacies Palmatolepis is the dominant genus in the lower part of the section, up to the Lower trachytera Zone (sample MT 6B). Then bispathodids increase up to the latest Famennian, with the only exception of sample MT 4 at the top of the Lower expansa Zone, when Palmatolepis and Polygnathus represent almost 90% of the association. In the praesulcata Zone Bispathodus is highly dominant. In general the Devonian part of the section belongs to the palmatolepid-bispathodid biofacies of Sandberg (1976). The association of the lower Tournaisian is dominated by polygnathids with a predominance of the Polygnathus purus and Po. communis groups making up about 80% of the assemblages (Corradini et al., 2003). Biostratigraphy The conodont zonation used is the scheme proposed for Sardinia by Corradini (2008), that is a rielaboration of the Late Devonian Standard Conodont Zones (Ziegler and Sandberg, 1990) and the Late Devonian-Early Carboniferous Zonation of Sandberg et al. (1978). The following ten biozones have been recognized in the Monte Taccu section (Fig. 2). - The Upper marginifera Zone (samples 15-6A) has been discriminated from the base of the section by the occurrence of the index taxon Pa. Marginifera utahensis. Palmatolepis is

the dominant genus, mainly represented by specimens of various subspecies of Palmatolepis glabra and Palmatolepis marginifera. The genus Polygnathus is less abundant than Palmatolepis, and the predominant species is Polygnathus glaber glaber. A few specimens of Pa. glabra are left in open nomenclature, because they do not fit in any of the subspecies already described. -

The velifer Zone (sample 6C) is defined by the presence of the marker Scaphignathus velifer velifer, and by the first occurrence of Alternognathus regularis.

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The Lower trachytera Zone (sample 6B) is recognized based on the joint occurrence of Palmatolepis perlobata helmsi, Polygnathus subirregularis, Palmatolepis glabra lepta, and Polygnathus glaber glaber. The first two taxa have their first occurrence, while the others became extinct within this zone (Ji and Ziegler, 1993).

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The Upper trachytera Zone (sample 6A) is discriminated by the presence of the marker, Pseudopolygnathus granulosus, that is here recorded for the first time in Sardinia, and by the occurrence of Palmatolepis minuta schlezia that has its last occurrence within this zone (Ji and Ziegler, 1993).

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The styriacus Zone has not been documented and probably occurs in the thin interval between samples 6A and 6. In other Sardinian sections this biozone is very thin, and is less than one meter thick, like in the Corona Mizziu I section (Corradini, 1998, 2003).

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The Lower expansa Zone (samples 6-5) is recognized by the first occurrence of the marker, Palmatolepis gracilis expansa, and by the presence of Polygnathus styriacus and Pseudopolygnathus irregularis that have their last occurrence within this zone (Ji and Ziegler, 1993; Hartenfelds, 2011).

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The Middle expansa Zone (Sample 4-3) is here discriminated by the entry of Bispathodus costatus. Pseudopolygnathus primus first occurs.

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The Upper expansa Zone (Sample 2B) is recognized by the first occurrence of Pa. gracilis gonioclymeniae and Po. marburgensis trigonicus, which make their entry within this zone (Ziegler and Sandberg, 1984; Ji and Ziegler, 1993).

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The Lower praesulcata Zone (sample 2A-X3) is discriminated by the entry of the first representatives of the genus Siphonodella. The fauna includes several taxa whose range is restricted to the Devonian: Bispathodus aculeatus, Bi. costatus, Bi. ultimus, Branmehla suprema, Mehlina strigosa, Palmatolepis gracilis, and Polygnathus vogesi). 89

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The Upper praesulcata and the sulcata zones have not been recognized in the Monte Taccu section. The Lower duplicata Zone (sample X2) is documented by the entry of Siphonodella duplicata Morphotype 1. The Upper duplicata Zone (samples X1-X) has been recognized by the occurrence of Si. cooperi M1. In this biozone Polygnathus is the prevailing genus, especially Po. purus purus, Po. biconstructus, and Po. tenuiserratus.

NOTES ON THE D/C BOUNDARY Corradini et al. (2003, p. 232) reported a narrow interval, 2-4 cm thick, of dark shaley limestone between the Lower praesulcata Zone documented in sample MT2a and the Lower duplicata Zone noted in MT X2, that could be resolved in more detail through further sampling. Sample MT X3 was collected in this interval. The recovered fauna is scarce and poorly preserved with fragmentary specimens of Bispathodus costatus and Bi. ultimus indicative that this level still belongs to the Lower praesulcata Zone and probably represents a Hangenberg Shale equivalent. In the Monte Taccu section the youngest resolvable Devonian bed belongs to the Lower praesulcata Zone, and the oldest Carboniferous strata are Lower duplicata Zone. The Upper praesulcata and the sulcata zones are not present. CONCLUSIONS The results of this research on the Monte Taccu section are:

• • •

Seventy-eight conodont taxa belonging to eleven genera have been documented from the Monte Taccu section. Ten biozones, from the Upper marginifera to the Upper duplicata have been discriminated. The Devonian/Carboniferous boundary lies between samples MT X3 and MT X2. However, a hiatus spanning the Upper praesulcata and the sulcata zones is documented.

ACKNOWLEDGMENTS This paper is a contribution to IGCP 596 "Mid Palaeozoic climate and biodiversity". This study was supported by R.A.S. (grants LR7/07 - 2010 Resp. C. Corradini). REFERENCES Bischoff, G. and Ziegler, W. 1956. Das Alter der "Urfer Schishten" im Marburger Hinterland nach Conodonten. Notizblatt des Hessisches Landesamt für Bodenforschung 84: 138-169. Branson, E.B. and Mehl, M.G. 1934a. Conodonts from the Grassy Creek shale of Missouri. Missouri University Studies 8, 3: 171-259.

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