Upper Llanvirn – Lower Caradoc Conodont Biostratigraphy, Southern Mendoza, Argentina

Susana HEREDIA1

Abstract: UPPER LLANVIRN- LOWER CARADOC CONODONT BIOSTRATIGRAPHY, SOUTHERN MENDOZA, ARGENTINA. The Ponón Trehué Formation (Mendoza Province, Argentina) is composed by a coarse clastic sequence transitionally followed by a fine dark carbonate sequence; the lower one includes Upper Cambrian to Arenig olistoliths and blocks, and Lower to Middle Llanvirn fragments of different sizes. A synthesis of the biostratigraphy of Upper Llanvirn-Lower Caradoc conodonts from the Ponón Trehué Formation is presented. The following conodont biozones can be recognized: Pygodus serra Zone with the Eoplacognathus robustus Subzone and the Eoplacognathus lindstroemi Subzone, and Pygodus anserinus Zone with the Sagittodontina kielcensis subzone and the Amorphognathus inaequalis subzone. Allochthonous conodonts recovered from clasts and from olistoliths are mentioned.

Resumen: BIOESTRATIGRAFÍA DE CONODONTES DEL LLANVIRNIANO SUPERIOR-CARADOCIANO INFERIOR DEL SUR DE MENDOZA, ARGENTINA. La Formación Ponón Trehué (provincia de Mendoza, Argentina) está compuesta por una secuencia clástica gruesa seguida, transicionalmente, por calizas oscuras de grano fino; la unidad inferior incluye olistolitos, bloques y fragmentos de diversos tamaños cuyas edades van desde el Cámbrico superior- Tremadociano, Arenigiano, al Llanvirniano inferior y medio. Se presenta una síntesis de la bioestratigrafía de esta formación basada en conodontes. Se han reconocido las siguientes biozonas de conodontes: la Zona de Pygodus serra con la Subzona de Eoplacognathus robustus Subzone y la Subzona de Eoplacognathus lindstroemi, y la Zona de Pygodus anserinus con la Subzona de Sagittodontina kielcensis y la Subzona de Amorphognathus inaequalis. Dentro de este contexto se mencionan los conodontes alóctonos recuperados de clastos y olistolitos.

Key words: Ponón Trehué. Mendoza. Conodonts. Ordovician. Biostratigraphy.

Palabras clave: Ponón Trehué. Mendoza. Conodontes. Ordovícico. Bioestratigrafía.

Introduction

Outcrops of the Precordillera terrane, extend from 29º S to 33º S latitude, and correlative rocks appear near San Rafael City (35º S / 68º 20 ‘ W) in the south of Mendoza Province, Western Argentina (Figure 1)(“Cuyania” of Ramos, 1995; Keller et al., 1996). These lower Paleozoic exposures occur in the Sierra Pintada range, which represent the San Rafael Block (Criado Roqué and Ibáñez, 1979).

In the San Rafael Block the lowest sedimentary unit is represented by the Ponón Trehué Formation, whose Llanvirn-Caradoc deposits suggest shallow clastic to deep carbonate facies. Astini (1999) suggests that these deposits represent an extensional episode postdating the collision of the Precordillera terrane.

The Ponón Trehué Formation overlies the Precambrian basement (Cerro La Ventana Formation), which is composed of Grenvillian (Bordonaro et al., 1996) rocks: gneisses, granitoids and siliciclastic metasediments. These rocks are alternatively overlain by Ordovician (Ponón Trehué Formation) and Carboniferous strata (Pájaro Bobo Formation), which present a northwest-southeast alignment.

The relationship between the continental reddish sedimentary rocks of Carboniferous age and the igneous-metamorphic Grenvillian complex is tectonic, while the contact between the latter ones and the Ordovician clastic sediments is an erosive unconformity (Criado Roqué and Ibáñez, 1979; Heredia, 1996) (Figure 2).

Recently, Bordonaro et al. (1996) proposed new stratigraphical formal units called Ponón Trehué Formation (Tremadocian-Arenig) and Lindero Formation (Upper Llanvirn-Lower Caradoc). Heredia (2001) and Astini (2002) suggested that the limestones of the Ponón Trehué Formation (Bordonaro et al., 1996) were blocks and fragmentary carbonate bodies discontinuously exposed floating in arkose conglomerate. In fact, different sections of these Ordovician outcrops have a few meters in thickness, and a problem of scale has been verified in mapping the succession. Only one formal stratigraphic unit is herein proposed: the Ponón Trehué Formation (Heredia, 1996). This formation involves two different deposits: the lower one comprises coarse siliciclastic deposits (including olistoliths) and the upper one consists of fine, dark carbonate-fine clastic deposits. The biostratigraphy of these Ordovician outcrops has been based on conodont assemblages (Heredia, 1982; Bordonaro et al. 1996; Heredia, 1996; Leslie and Lehnert, 1999; Lehnert et al., 1999; Heredia, 2001) The purpose of this study is to summarize the stratigraphical distribution of late Llanvirnlower Caradoc conodonts recovered from La Tortuga section of the Ponón Trehué region. Moreover, a brief comment on allochthonous conodonts from the coarse deposits and carbonate bodies (“phantom formation”) is included.

Ponón Trehué Formation at La Tortuga section

This is a normally graded siliciclastic-carbonate succession, 25 m in thickness (Figure 3). The basement of the La Tortuga section is composed of metamorphic rocks (Criado Roqué and Ibáñez, 1979), which are Grenvillian in age. An erosive surface separates it from the Ordovician sedimentary rocks. A normally graded, very thick conglomerate that contains pebbles and blocks derived from the underlying rocks occurs at the base of the unit. It passes transitionally into very thick sandstones with carbonate cement that show thick parallel laminations. They are also normally graded with thick grains of quartz chaotically interbedded among fine silts. These dark gray to black siliciclasticcarbonate beds exhibit remains of crinoids, trilobites and brachiopods on the bedding plane surface.

Progressively to the top, the beds are thinner (1 to 2 cm in thickness) and laminate-normally-graded carbonate sandstones are recognized. Transitionally, thin dark limestone beds occur, and grade into slumped limestones and black shales beds, with floating isolated rounded clasts of quartz. The top of this succession is truncated by faulting.

The Ponón Trehué Formation is characterized by an increasing amount of carbonate material to the top. The sequence represents a transition from a predominantly coarse clastic shallow regime (Astini, 2002) to a predominantly clinoform deep-water carbonate regime (Heredia and Beresi, 2000; Beresi and Heredia, 2000; Heredia, 2001; Astini, 2002).

Biostratigraphy

Considerable efforts have been made to establish global series and stage subdivision of the Ordovician System to replace the local classifications (Webby, 1998). The term “Llandeilo”, which is used in the present paper, was retained as a stage for the upper part of the British Llanvirn Series.

This comprises the upper part of the Pygodus serra Zone, Eoplacoganthus robustus and E. lindstroemi subzones and the lower part of the Pygodus anserinus Zone (Sagittodontina kielcensis Subzone) (Fortey et al., 2000).

Conodont zones and subzones

The conodont collections of the Ponón Trehué Formation are housed at the Museum of Paleontology of the National University of Córdoba, Argentina (CORD-MP 2220-2358) (S. Heredia collection), and at Ianigla, CRICyT, Mendoza, Argentina (IANIGLA PI 710-762 and 763-814) (O. Lehnert collection). The preservation of conodonts is good, and they frequently occur complete, except for those of large size or with long processes. Reworked conodonts are usually broken (Heredia, 2001) and partly recrystallized (Lehnert et al., 1999). The conodont elements exhibit a high Color Alteration Index (CAI of Epstein et al., 1977) 5 to 6, indicating a paleotemperature over 300º C. Present Upper Llanvirn - Lower Caradoc conodont collection appears to have a close affinity with those ones studied by Dzik (1994) and Bergström et al. (1987).

Numerous and well preserved conodonts occur in clastic-carbonate rocks in the Ponón Trehué Formation, being these levels assigned to the Llanvirn British Series (Llandeilo Stage) by index conodont species following the biostratigraphic scheme of the Argentine Precordillera proposed by Albanesi et al. (1998). Several hundreds of specimens of platform conodonts have been recovered from this section, and conodont zones and subzones can be recognized. The assemblage is dominated by typically North Atlantic species (showing rather high contributions of Eoplacognathus and Pygodus) with particular influence of taxa from the Midcontinent Realm. The conodont distribution in the Ponón Trehué Formation allows us to recognize two biozones: the Pygodus serra Zone and Pygodus anserinus Zone (Figure 3 and Plate 1).

Pygodus serra Zone (Plate 1)

Based on the recovery of the eponymous species of the Pygodus serra Zone from sample PT 3 to PT 10, and other index species, two subzones are recognized: Eoplacognathus robustus and E. lindstroemi.

Eoplacognathus robustus Subzone (Plate 1)

The La Tortuga Section starts with the Pygodus serra Zone/Eoplacognathus robustus Subzone. This interval shows E. robustus in diverse evolutionary stages. The stratigraphic interval that ranges from sample PT3 to PT8 (Figure 3) contains: Pygodus serra (Hadding), Eoplacognathus robustus Bergström, Baltoniodus prevariabilis (Fåhræus), Periodon aculeatus Hadding, Ansella sinuosa Stouge, Ansella biserrata Lehnert et Bergström, Pseudooneotodus mitratus (Moskalenko), Spinodus spinatus (Hadding), Phragmodus polonicus Dzik, Strachanognathus parvus Rhodes, D. reclinatus (Lindström), Drepanoistodus aff. suberectus, Erismodus sp, Erraticodon sp., Panderodus aff. sulcatus, Protopanderodus rectus (Lindström) and Costiconus ethingtoni (Fåhræus).

Eoplacognathus lindstroemi Subzone (Plate 1)

The lower limit is determined by the first appearance of E. lindstroemi (Hamar), where the amorphognatiform (sp) element of E. robustus changes to an earlier form of E. lindstroemi. This is pointed out by the change of the posterior-lateral processes form and the straight inner and outer processes carina, the Y and T-shaped elements shows little differences with those of the E. robustus. The conodont association (PT 9 to PT 10) includes P. serra, E. robustus, E. lindstroemi, Baltoniodus prevariabilis-variabilis transition (sensu Dzik, 1994), A. sinuosa, A. biserrata, C. ethingtoni, S. parvus, P. aculeatus, Erraticodon sp, D. reclinatus, Phragmodus? sp. and Panderodus sp. Subzones defined by Lehnert et al. (1999) such E. reclinatus and E. elongatus Bergström (early form) lack of appropriate place of sampling (no GPS coordinates). It should be noted that the occurrence of index conodonts such as P. serra, E. robustus and E. lindstroemi have been already mentioned for the Los Azules Formation (Hünicken and Ortega, 1987), the Yerba Loca Formation (Albanesi et al., 1995 b) and the allochthonous clasts of the La Cantera Formation (Albanesi et al., 1995 a), respectively.

Pygodus anserinus Zone (Plate 1)

Two subzones can be recognized on the base of conodont assemblages.

Sagittodontina kielcensis Subzone

The first occurrence of P. anserinus Lamont et Lindström marks the beginning of the Pygodus anserinus Zone (sample PT11, Figure 3) where forms of P. serra are still present. This association suggests the lowest part of the Pygodus anserinus Zone, i.e., the Sagittodontina kielcensis Subzone of the North Atlantic Scheme (Bergström, 1983). The Pygodus anserinus Zone includes: P. anserinus, B. prevariabilis-variabilis transition, P. aculeatus, S. parvus and P. serra.

Amorphognathus inaequalis Subzone (Plate 1)

When Pygodus anserinus occurs together with E. lindstroemi (late form), Cahabagnathus sweeti (Bergström) and Baltoniodus variabilis (Bergström) it is possible to refer to the A. inaequalis Subzone of the North Atlantic Scheme (Bergström, 1983, Lehnert et al., 1999) (Figure 3). Amorphognatiform (sp) elements of E. lindstroemi usually exhibit variable forms, but specimens from sample PT111 frequently show the postero-lateral process bent strongly toward outer side (being the inner and outer processes arranged no linearly). Some Eoplacognathus oz elements (both T-shaped and Yshaped forms) developed lobes in the right side of the internal process. This morphology was not

recorded until present and it could represent a new species of Eoplacognathus. Nevertheless, sp forms can not be clearly distinguished from this assemblage.

Cahabagnathus sweeti was already recorded for this locality (Lehnert et al., 1999). In my own collection a complete apparatus does appear, which is not a common form but a Midcontinent pelagic species (Bergström, 1983) (Figure 3 and Plate 1). This level is related to the maximum transgression recorded in the basin following Astini’s (2002) criteria.

Figure 2. Geological map of the Ponón Trehué area.

Figure 3. La Tortuga section with ranges of selected conodont species and biostratigraphic scheme for the Ponón Trehué Formation.

Allochthonous conodonts

In the northern sector, at the Cerro Ai Sol section, Bordonaro et al. (1996) described a shallow to middle carbonate platform deposit. In this section, Lehnert et al. (1998) identified elements of Striatodontus prolificus (Ji et Barnes), Colaptoconus quadraplicatus (Branson et Mehl), Eucharodus parallelus (Branson et Mehl), “Drepanodus” gracilis (Branson et Mehl), Bergstroemognathus extensus (Graves et Ellison), Drepanodus arcuatus Pander, Juanognathus jaanussoni Serpagli, O. communis (Ethington et Clark), Reutterodus andinus Serpagli, Scolopodus rex Lindström, S.? krummi (Lehnert), Tropodus sweeti (Serpagli), Fahraeusodus marathonensis (Bradshaw), Oepikodus evae (Lindström), Periodon flabellum (Lindström), Protopanderodus gradatus Serpagli, D. forceps (Lindström), Erraticodon? balticus Dzik, Oistodus aff. lanceolatus Pander, Pteracontiodus cryptodens Mound, Drepanodus basiovalis (Sergeeva), Oistodus? tablepointensis Stouge, Spinodus sp. These elements can define the following assemblages zones (sensu Lehnert et al., 1998): P. striatus/C. quadraplicatus Zone, P. proteus/A.? deltatus Zone, O. evae Zone, O. intermedius Zone, H. altifrons/A. jemtlandica Zone.

In the southern sector, at La Tortuga section, the redeposited conodont elements are more abundant in the lower levels (PT 4 to PT 5) and they are identified as Eoplacognathus foliaceus (Fåhræus), Eoplacognathus pseudoplanus (Viira), Eoplacognathus suecicus Bergström, Eoplacognathus reclinatus (Fåhræus), Lenodus? sp, Dapsilodus sp., Microzarkodina sp., Histiodella? sp., Parapaltodus sp., Parapanderodus sp. and Protopanderodus sp.

The presence of reworked fauna in the lower part of the La Tortuga section allows us to fill, in part, the blank biostratigraphic interval between recorded faunas (Lehnert et al., 1998) in blocks and megablocks northward this section. Conodonts, whose vertical distributions cover this span of time, are Eoplacognathus pseudoplanus, E. suecicus, E. foliaceus and E. reclinatus. The latter species has also been recorded from autochthonous deposits (Lehnert et al., 1999). These redeposited conodonts can be used to prove the former presence of strata that are now absent in the area (“phantom formations”) (study in progress).

Acknowledgements. I thank to Consejo Nacional de Investigaciones Científicas y Técnicas de Argentina (CONICET) for support this research (PEI: 0308/97). I also thank to Alejandra Ortiz for her assistance in the laboratory.

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Recibido: 15 de Agosto de 2002

Aceptado: 15 de Octubre de 2002

Index conodonts from La Tortuga section (Llandeilo), Ponón Trehué Formation (Sierra Pintada, Mendoza).

Scanning electron microscope photomicrographs, scale bar 0,1 mm. 1, Pygodus serra (Hadding) (P. serra Zone).

CORD MP 2236 (1) PT8. 2 - 4, Eoplacognathus robustus Bergström (E. robustus Subzone). Pa element CORD MP 2220(1), PT 3. Dextral Pb element CORD MP 2223 (1), PT 10. Sinistral Pb element CORD MP 2222(2), PT8. 5 - 7, Eoplacognathus lindstroemi (Hamar) early form (E. lindstroemi Subzone). Pa element CORD MP 2269(1), PT 9.

Dextral Pb element CORD MP 2223(3), PT9. Sinistral Pb element CORD MP 2224 (3), PT 10. 8, Pygodus anserinus Lamont et Lindström (P. anserinus Zone). CORD MP 2359 (4), PT 111. 9 – 11, Eoplacognathus lindstroemi (Hamar) later form. Pa element CORD MP 2355 (5), PT 111. Dextral Pb element CORD MP 2361 (36), PT 111. Sinistral Pb element CORD MP 2362 (29), PT 111. 12, Baltoniodus variabilis Bergström. Pa element CORD MP 2356 (43), PT 111.

13, Cahabagnathus sweeti (Bergström). Pastiniplanate element CORD MP 2363 (3), PT 111.1 4 - 15, Eoplacognathus lindstroemi? Dextral Pb element CORD MP 2358 (2), PT 111. Sinistral Pb element CORD MP 2357 (3), PT 111.