The Ordovician Brachiopod Faunas of Argentina: Chronology and Biostratigraphic Succession

Juan L. BENEDETTO

Abstract. THE ORDOVICIAN BRACHIOPOD FAUNAS OF ARGENTINA: CHRONOLOGY AND BIOSTRATIGRAPHIC SUCCESSION. This article summarizes brachiopod biostratigraphic data from Ordovician basins of Argentina. Taxonomic composition of the five brachiopod biozones recognized by Herrera and Benedetto (1991) in the Argentine Precordillera is updated on the basis of new information accumulated in the past ten years, especially from silicified material present in the lower part of the San Juan Formation. The Archaeorthis Zone is subdivided into three ‘subassemblages’ and the new Ranorthis niquivilensis Zone is recognized. After the drowning of the carbonate platform sedimentation becomes dominantly clastic and stratigraphic units underwent strong lithofacial changes through the Precordillera basin. As a result, late Llanvirn to late Ashgill brachiopod assemblages are comparatively more restricted in space and time. In the Arenig volcano-sedimentary sequences of the Famatina Range three distinct brachiopod assemblages can be recognized (in ascending order): Suriorthis depressus, Mollesella planiventralis and Famatinorthis turneri. The brachiopod fauna reported from western Puna is probably coeval with the Famatinian S. depressus assemblage. The Central Andean brachiopods are still poorly known. On the basis of available data from the Cordillera Oriental, a preliminary stratigraphic succession of eight brachiopod assemblages spanning the early Tremadocian to midlle Arenig interval is proposed.

Resumen. LAS FAUNAS DE BRAQUIÓPODOS ORDOVÍCICOS DE ARGENTINA: CRONOLOGÍA Y SUCESIÓN BIOESTRATIGRÁFICA. Este trabajo resume los datos disponibles sobre biostratigrafía de los braquiópodos ordovícicos de Argentina. Se actualiza la composición taxonómica de las cinco biozonas de braquiópodos reconocidas por Herrera y Benedetto (1991) en la Precordillera Argentina en base a la nueva información obtenida en los últimos diez años, especialamente la proveniente de los braquiópodos silicificados de la Formación San Juan. La Zona de Archaeorthis es subdividida en tres ‘sub-asociaciones’ y se reconoce la nueva Zona de Ranorthis niquivilensis. Después de la inundación de la plataforma carbonática la sedimentación se tornó dominantemente clástica y las unidades estratigráfiacas experimentan fuertes cambios litofaciales a través de la Precordillera. Como resultado, las faunas de braquiópodos del intervalo Llanvirniano tardío-Ashgilliano tardío están comparativamente más restringidas en el tiempo y el espacio que las del Ordovícico temprano. En la secuencia volcaniclástica Arengiana de la Sierra de Famatina se han reconocido tres asociaciones de braquiópodos (en orden ascendente): Suriorthis depressus, Mollesella planiventralis y Famatonorthis turneri. La fauna de braquiópodos de la Puna occidental (Vega Pinato) es relativamente similar y probablemente contemporánea de la asociación de S. depressus de Famatina. Los braquiópodos de la cuenca Andina Central son aún poco conocidos. Sobre la base de los datos disponibles de la Cordillera Oriental se propone en forma preliminar una sucesión de ocho ‘asociaciones’ que abarcan el lapso Tremadociano temprano- Arenigiano medio.

Key words: Ordovician. Brachiopods. Argentina

Palabras clave: Ordovícico. Braquiópodos. Argentina

Introduction

Among benthic invertebrates, trilobites have been used, to a greater and lesser extent, for biostratigraphical purposes, especially since the publication of the Harrington and Leanza (1957) monograph on Ordovician trilobites in which they proposed a formal biozonation of Ordovician rocks of Argentina. The first attempt to use brachiopods for a biostratigraphic zoning was carried out in the thick and continuous succession of fossiliferous limestones of late Tremadocian to early Llanvirn age (San Juan Formation) exposed everywhere in the Precordillera basin. The increasing knowledge on taxonomy and stratigraphic ranges of the abundant and diverse brachiopods from the San Juan Formation led to propose a biozonal scheme based on five assemblage biozones, each named for their dominant components (from lowest to highest): Archaeorthis Zone, Huacoella Zone, Niquivilia Zone, Monorthis Zone and Ahtiella Zone (Herrera and Benedetto, 1991). Although variations in taxonomic composition among successisve ‘assemblages’ may have been environmentally controlled, as their close correspondence with biofacial and lithofacial changes suggests (Carrera et al., 1999, 2001), they are not time-recurrent. Environmental changes through deepening and shallowing sequences affect principally the proportion of higher-taxa. For example, diversity of plectambonitoids attains higher values in the open shelf settings prevailing during the Huacoella-Niquivilia biozones and the Llanvirn Ahtiella biozone. Changes among biozones involving apparition of new genera and species were produced by a combination of phenomena such as evolution within a given lineage (e.g. the Nothorthis-Ranorthis lineage, Benedetto, in press b; Benedetto et al., in press), immigration of new taxa, and extintion of others.

Since the brachiopod-based biozones were proposed, much additional taxonomic work has been published (Benedetto, 1998a, in press b, and references in both papers). Nevertheless, correlation among sections of the San Juan Formation based on brachiopod zones has still high confidence, especially in the lower and midlle part of the unit. Correlation of its upper part (Ahtiella Zone) is somewhat more problematical because of lithofacial and biofacial changes between sections located in the northern and southern parts of the basin (Sánchez et al., 1996).

Most of new taxonomic information comes from the silicified material from the lower half of the Archaeorthis Zone, the Niquivilia Zone and the Monorthis Zone (Benedetto, 2001a; 2002; Benedetto et al., in press). A preliminary refinement of the biostratigraphic succession within the lower part of the Archaeorthis Zone was recently made by Benedetto et al. (in press). On the other hand, the highdiversity brachiopod assemblages from both the Ahtiella Zone and the succeeding Skenidioides assemblage (defined herein) are actually under revision, thus a substantial increase in number of taxa can be expected inasmuch as this study is concluded.

In the Famatina basin (La Rioja and Catamarca provinces) great advances have been made recently in elucidating both the taxonomic composition and paleobiogeographic signature of brachiopod faunas. Of particular importance have been the extensive collections from the volcanoclastic successions exposed in the Chaschuil and central Famatina regions. As a result, 20 genera and 24 species have been identified and carefully located in measured stratigraphic sections (Benedetto, 1994, in press a).

Although most of Central Andean stratigraphic units contain brachiopods, they are still poorly known. Since the reports of Kobayashi (1937) and Harrington (1938) on some early Ordovician brachiopods from northwestern Argentina, only a few taxonomic works have been published (Benedetto, 1998c, 2002; Benedetto and Carrasco, in press), most of recently collected material has yet to be studied. For this reason, the stratigraphic succession of brachiopod assemblages given below only constitutes a first attempt to organize the available data. Three different kind of faunal associations are considered: (a) formally defined biostratigraphic zones, (b) informal biostratigraphic units, named here ‘assemblages’ or ‘sub-assemblages’ (e.g. Destombesium assemblage), and (c) faunas whose biostratigraphic significance has not been demonstrated, often including poorly preserved, sparse and/or incompletely known taxa (‘not zoned’).

Chronology of brachiopod faunas

Figure 1 shows the stratigraphic succession of brachiopod biozones and assemblages recognized in this paper. They have been referred to the standard chronostratigraphic chart on the basis of conodont and graptolite information. In the Precordillera basin, conodonts have been essential for dating the San Juan Formation brachiopod biozones (Lehnert, 1995; Albanesi et al., 1995b, 1998a, and references therein), while graptolites provide invaluable ties between carbonate and siliciclastic successions (Albanesi et al., 1999; Ortega and Albanesi, 1999 and references therein).

In the Sierra de Famatina, the age of the Suri and Molles formations has been based largely on conodont and graptolite evidence, and additionally on trilobite faunas (Vaccari and Waisfeld, 1994).

Graptolites from the lower member of the Suri Formation described by Toro and Brussa (1997) belongs to the early-middleArenig B. deflexus and D. bifidus biozones. Conodonts recovered from shell beds of the ‘Suri’ Formation in the Chaschuil region indicate the B. navis Biozone (Albanesi and Vaccari, 1994), while conodonts from the central Famatina span from the P. elegans to the P. originalis Zones (Lehnert et al., 1997). The brachiopod-bearing coquines from the upper Suri and lower Molles formations have yielded slightly younger conodonts (Oepikodus evae Zone) (Albanesi and Astini, 2000b).

Chronology of Tremadocian faunas of the Central Andean basin has not been accurately established. In the last years, however, the classical trilobite biozonation proposed by Harrington and Leanza (1957) has been substantially improved on the basis of conodont data (Rao and Hünicken, 1995; Rao and Flores, 1997; Rao, 1999; Tortello and Rao, 2000 and references therein). On the other hand, graptolites are by far the most important biostratigraphic group to calibrate the latest Tremadocian and Arenig successions (Toro, 1994, 1997; Albanesi et al., 2001, amongst others).

Acritarch and palynological assemblages have also been used to refinate the biostratigraphic schema (Ottone et al., 1992, 1999; Rubinstein and Toro, 1999)

Biostratigraphic succession

Precordillera basin

1) The lowermost currently known brachiopod association comes from the coalesced spongereceptaculitid reef-mounds developed near the base of the San Juan Formation (Cañas and Carrera, 1993). Only fragmentary specimens of a small unidentified pentamerid have so far been recovered (Benedetto et al., in press). Intensive sampling from these levels has yet to be carried out. Acording to the conodont assemblage recovered from the La Silla section, these levels may be correlative to the P. deltifer Zone of Baltoscandia (Lehnert, 1993, 1995; Albanesi et al., 1995b).

2) Archaeorthis sanroquensis Zone. (= Archaeorthis Zone defined by Herrera and Benedetto, 1991).

This is the lowermost brachiopod zone formally recognized in the Precordilleran carbonate platform. It is characterized by low-diversity assemblages dominated by Archaeorthis sanroquensis Benedetto, a species having great spacial continuity. Intensive sampling over the past three years in the Cerro La Silla, Cerro San Roque and Cumillango sections substantially increased our knowledge on taxonomic composition and stratigraphic ranges of brachiopods from the A. sanroquensis Zone (Cech, 2000; Esbry, 2001; Benedetto, 2001a, in press b; Benedetto et al., in press). Based on faunal variations through the zone, a number of ‘assemblages’ have been recognized in a previous work (Benedetto et al., in press). It should be noted that such a compositional changes appear to have been linked to the gradual deepening of the basin. Lithological and paleoecological evidence indicates a transition from high-energy, shallow-water settings punctuated by storm events at the base to a relatively more stable, inner shelf environment upward in the biozone. According to Cañas (1999) the maximum flooding was reached near the boundary between the Archaeorthis sanroquensis and Huacoella radiata zones.

The following successive sub-assemblages, whose ranges may be equivalent to ‘sub-zones’, have been recognized within the A. sanroquensis Zone, in ascending order: (a) the lowermost subassemblage, encompassing the lower 12-15 m of the zone, is characterized by the abundance of the pentamerids Syntrophia sanjuanina Benedetto and Parallelostrophia lasillaensis Benedetto; (b) a very low diversity sub-assemblage in which pentamerids are nearly absent and Orthidium prominens Benedetto and A. sanroquensis dominate; (c) a third sub-assemblage containing a relatively more diverse fauna is marked by the first appearance of Nothorthis marginata Benedetto, and the tritoechiids Tritoechia (Parvitritoechia) preandina Benedetto, Tritoechia (Tritoechia) prima Benedetto and other species of Tritoechia not yet determined. Nothorthis termalis (Herrera and Benedetto), recorded in the Agua Hedionda (Huaco) section, probably belongs to this assembalage, but its precise stratigraphic position within the A. sanroquensis Zone is in need of verification. The best stratigraphic sections to study the Archaeorthis sanroquensis Zone are Cerro Cumillango and Cerro La Silla, while at Agua Hedionda (core of the Huaco Anticline) and Niquivil only its upper half is preserved.

(3) Ranorthis niquivilensis Zone. The base of this zone is drawn at the first apparition of the eponymous species, which occurs in beds immediately overlying the silicified interval. R. niquivilensis is associated with entirely calcified brachiopods, and persists approximately 10-15 m above the last apparition of A. sanroquensis. The lower part of the zone is distinguished by the co-occurrence of A. sanroquensis and R. niquivilensis. Upward in the zone is noticeable the first apparition of leptellinids, represented by Leptella (Leptella) variabilis Benedetto and Herrera and other still poorly known species.

Associated forms are Hesperonomia sp., Hesperonomiella sp., Paralenorthis sp., Orthidium sp., Tritoechia sp., and undetermined pentamerids. The informal name ‘Ranorthis-Leptella’ assemblage may be provisionally used to account for the distinctive co-occurrence of these genera in the upper part of the R. niquivilensis Zone. This assemblage fills the gap between the last apparition of Archaeorthis and the first apparition of Huacoella radiata.

(4) Huacoella radiata Zone (= Huacoella Zone of Herrera and Benedetto, 1991). Huacoella radiata Benedetto and Herrera is a widespread leptellinid which has been chosen to name this biozone. This species typically occurs in a 25-30 m thick succession of burrowed skeletal wackestones deposited in a low-energy, open marine environment below the fair-weather wave-base, punctuated by sporadical storm events. Key species in this still relatively poorly known assemblage include Hesperonomia cf. planidorsalis Ulrich and Cooper, Orthidium cf. gemmiculum (Billings), Paralenorthis n. sp.(not yet described), Leptella (Leptella) alata Benedetto and Herrera, and Tritoechia gigas Benedetto and Punctolira? sp. The H. radiata Zone is well exposed in the Cuesta de Huaco and Niquivil sections.

(5) Niquivilia extensa Zone (= Niquivilia Zone of Herrera and Benedetto, 1991). This zone occurs in a 30-35 m thick stratigraphic interval overlying the H. radiata Zone. In the Niquivil section -one of the best exposures of this zone- a small onlap between the stratigraphic ranges of Huacoella radiata and Niquivilia extensa has been detected. Typical associated brachiopods are Hesperonomiella sulcata Benedetto and Leptella (Leptella) cf. alata. Other species sporadically present are Trondorthis? sp., Hesperonomia? n. sp., Orthidium sp., Notoscaphidia sp., Tritoechia sp., Leptella (Leptella) plana Benedeto and Herrera , Leptella (Leptella) costellata Benedetto and Herrera, and Leptella (Petroria) sp.

(6) Monorthis cumillangoensis Zone (=Monorthis Zone of Herrera and Benedetto, 1991). Everywhere in the basin this zone developed in agitated shallow water settings linked to the second episode of reef-mound development. It occurs in a 50-60 m thick succession of bioclastic grainstones and stromatoporoid patch reefs (Cañas, 1999; Carrera et al., 1999, amongst others). A rather different, highly distinctive suite of brachiopods appears for the first time in this zone, including Ffynnonia spondyliformis Benedetto, Oligorthis zondaensis Benedetto, Polytoechia? sp. and Aporthophyla neumani Benedetto (Benedetto, 2001a). M. cumillangoensis, however, is absolutly dominant and often occurs

as monospecific concentrations which may indicate sorting and accumulation in relatively shallow envirnonment. Other associated taxa, such as Orthidium sp., Leptella (Leptella) sp. and Paralenorthis sp., come from the underlying zones.

Biostratigraphic evidence demonstrates that the transition from carbonate ramp facies to basinal black shales is clearly diachronous (Hünicken, 1985; Albanesi et al., 1998b). Such a diachronism, which becomes evident since the late Arenig, results from the development of a homoclinal ramp inclined northward (Cañas, 1999). In the depocenter located in the Guandacol area the trasitional beds consist of a ritmic succession of bioclastic micrograded wackestones and black mudstones belonging to the Tripodus laevis Zone (Albanesi et al., 1999), which correlates with the Monorthis cumillangoensis Zone (and probably with the upper part of the Niquivilia extensa Zone). Brachiopods from these levels have not yet investigated in detail. Some preliminary identifications include Tritoechia sp., Orthidium sp., Leptella (Petroria) sp., Leptella (Leptella) sp., Taffia? sp., and Productorthis? sp.

(7) Ahtiella argentina Zone (= Ahtiella Zone of Herrera and Benedetto, 1991). This zone is separated from the preceeding zone by a 50-60 m thick succession of scarcely fossiliferous bioturbated wackestones containing a low-diversity brachiopod fauna dominated by Paralenorthis vulgaris Herrera and Benedetto and Petroria rugosa (the Petroria-Paralenorthis Biofacies recognized by Carrera et al., 1999). The uppermost 30-35 m of the San Juan Formation consists of dark-gray burrowed bioclastic wackestones, often with nodular stratification, bearing a highly distinctive brachiopod assemblage formed, besides the index fossil of the zone, by Sanjuanella plicata Benedetto and Herrera, Inversella (Reinversella) arancibiai Herrera and Benedetto, Taffia anomala Benedetto and Herrera, Productorthis cienagaensis Herrera and Benedetto, Rugostrophia sp. (probably new, not yet described), Camerella sp., Idiostrophia sp., and several species of Ffynnonia, Tritoechia and Martellia. Brachiopods from the Ahtiella argentina Zone have been described by Benedetto, 1987; Benedetto and Herrera, 1986a, 1986b, 1987a, 1987b; Herrera and Benedetto, 1987, 1989. An oustanding feature of this zone is that species of Ffynnonia, Tritoechia, Martellia and probably Sanjuanella and Productorthis, are different in the northern and southern portions of the basin. As stated above, no evident changes in facies associations are recorded in the Precordilleran carbonate shelf until the late Arenig, when an homoclinal ramp with a depocenter located to the north developed (Cañas, 1999). By the early Llanvirn, lithologic and paleoecologic evidence from the southern Precordillera sections (e.g.Villicum, Talacasto) indicates a shallow subtidal environment in a mid-ramp setting sporadically affected by storm-wave base

PLATE 1.

Figs. 1-3. Archaeorthis sanroquensis Benedetto. 1, exterior of ventral valve, CEGH-UNC 17825, x2; 2, exterior of dorsal valve, CEGH-UNC 17824, x2; 3, interior of ventral valve, CEGH-UNC 17801, x2.

Figs. 4, 5. Syntrophia sanjuanina Benedetto. 4, interior of ventral valve, CEGH-UNC 20867, x 1,6; 5, interior of dorsal valve, CEGH-UNC 20866, x1,4.

Figs. 6, 7. Orthidium prominens Benedetto. Exterior and interior of dorsal valve, CEGH-UNC 20776, x6,5.

Figs. 8-10. Nothorthis marginata Benedetto. 8, interior of dorsal valve, CEGH-UNC 20745, x4; 10, exterior and interior of ventral valve, CEGH-UNC 30745, x4.

Fig. 11. Ranorthis cumillangoensis Benedetto, exterior of ventral valve, CEGH-UNC

Fig. 12. Huacoella radiata Benedetto and Herrera, interior of dorsal valve, CEGH-UNC 175, x1,2.

Figs. 13-15. Niquivilia extensa Benedetto and Herrera. 13, exterior of ventral valve, CEGH-UNC 210, x1,8; 14, interior of ventral valve, CEGH-UNC 172, x3; 15, interior of dorsal valve, CEGH-UNC 227, x2.

Fig. 16. Leptella (Leptella) costellata Benedetto and Herrera, exterior of ventral valve, CEGH-UNC 5115, x2.

Figs. 17-19. Monorthis cumillangoensis Benedetto. 17, exterior of dorsal valve, CEGH-UNC 17924, x1,6; interior of dorsal valve, CEGH-UNC 17935, x 1,6; interior of ventral valve, CEGH-UNC 17932, x1,5.

Figs. 20, 21. Ahtiella argentina Benedetto and Herrera, 10, interior of dorsal valve, latex cast, CORD-PZ 8295, x3; exterior of ventral valve, CORD-PZ 8283, x2.

Figs. 22, 23. Skenidioides n.sp., 22, interior of ventral valve, CEGH-UNC 20951, x4; conjoined specimen, dorsal view, CEGH-UNC 20952, x3,5.

Figs. 24, 25. Anchoramena cristata Benedetto. 24, internal view of dorsal valve, CEGH-UNC 13686a, x1,5; 25, exterior of dorsal valve, CEGH-UNC 13695, x2.

Figs. 26, 27. Destombesium argentinum Benedetto. 26, internal mold of dorsal valve, CEGH-UNC 17471, x1,5; 27, internal mold of ventral valve, CEGH-UNC 17465, x2,3.

Fig. 28. Trucizetina chanarensis Benedetto. Internal mold of dorsal valve, CEGH-UNC 17539, x2.

Fig. 29. Anisopleurella cf. gracilis (Jones). Internal mold of dorsal valve, CORD-PZ 8238, x4.

Fig. 30. Hirnantia sagittifera (M’Coy). Internal mold of dorsal valve, CORD-PZ 8259, x 0,7

(Sánchez et al., 1996; Carrera, 2001). The presence in these levels of abundant demosponges, especially of large discoidal forms such as Patellispongia, suggests soft or shifting substrates (Carrera, 1994, 1997). Conversely, in the sections located to the north (Jáchal-Cerro Viejo area) the dominance of elongated sponges and plectambonitoid brachiopods along with their occurrence in nodular wakestones and marlstones are indicative of low-energy, deeper water conditions. Hence, taxonomic differences between the northern and southern portions of the basin can be attributed to an environmental gradient at the time the Ahtiella argentina Zone developed (Sánchez et al., 1996).

(8) Skenidioides assemblage. In the Cordón de las Chacritas the San Juan Formation is conformably overlain by a 50 m thick succession of dark-gray nodular limestones and marlstones assigned to the Las Chacritas Formation. Compared to the preceeding Ahtiella argentina Zone, the Skenidioides assemblage has in common several genera, such as Productorthis, Sanjuanella, Ffynnonia, Petroria and Ahtiella, but all of them are represented by different species (not yet described). One of the most distinctive taxa is Skenidioides n.sp., which makes its first appearance in this assemblage. Moreover, this species is always abundant at all localities in which this faunal association has been reported.

Among the plectambonitoids, Leptellina n. sp. and Benignites? n. sp. (unpublished) are also restricted to this assemblage. The varied plectambonitoid fauna, including the probably epiplantic Benignites (Havlicek, 1991), suggests a deep-water, open-shelf environment. This is in agreement with the dominance throughout the interval of the marginal-platform to upper-slope Periodon-Protopanderodus conodont biofacies (Albanesi and Astini, 2000a). The Skenidioides assemblage has also been recorded from many of the huge olistholiths embedded in the Rinconada Formation exposed on the eastern slope of the Sierra Chica de Zonda. The lower 50 m of the Las Aguaditas Formation are probably contemporary with the Skenidioides assemblage, but this unit has not intensively been sampled for brachiopods.

(9) There are very few records of late Llanvirn (Llandeilo) brachiopods. Deposits of that age have not yet been recorded in situ in the Precordillera basin, with the exception of small outcrops in the

PLATE 2

Figs. 1-3. Suriorthis depressus Benedetto. Latex cast of ventral exterior, CEGH-UNC 19029, x2,5; ventral internal mold of juvenile specimen CEGH-UNC 15919, x5,5; internal mold of dorsal valve, CEGH-UNC 15762, x4.

Figs. 4, 6. Famatinorthis turneri Levy and Nullo. 4, internal mold of dorsal valve CEGH-UNC 11004, x1; internal mold of ventral valve, CEGH-UNC 19700, x1,5.

Fig. 5. Mollesella planiventralis Benedetto. Internal mold of ventral valve, CEGH-UNC 19649, x1.

Figs. 7, 8. Kvania n.sp.1. 7, internal mold of dorsal valve, CEGH-UNC 19465, x7; 8, internal mold of ventral valve, CEGH-UNC 19466, x7. Figs. 9, 10. Kvania n.sp.2. 9, latex replica of exterior, CEGH-UNC 19116, x4; 10, internal mold of dorsal valve, CEGH-UNC 19470, x4.

Figs. 11, 12. Nanorthis n.sp. 11, latex cast of dorsal exterior, CEGH-UNC 18938, x3,5; 12, internal mold of ventral valve, CEGH-UNC 18934, x3.

Figs. 13-15. Lipanorthis santalaurae Benedetto. 13, látex cast of dorsal exterior, CEGH-UNC 20463, x2; 14, internal mold of ventral valve, CEGH-UNC 204744, x2; 15, internal mold of dorsal valve, CEGH-UNC 20483, x3.

Figs. 16-18. Nanorthis purmamarcaensis Benedetto. 16, latex replica of dorsal exterior, CEGH-UNC 18864, x2,5; 17, internal mold of ventral valve, CEGH-UNC 18810, x1,7; 18, internal mold of dosal valve, CEGH-UNC 18814, x2.

Figs. 19, 20. Nanorthis saltensis (Harrington). 19, latex replica of dorsal exterior, CEGH-UNC 18820, x2; 20, internal mold of ventral valve, CEGH-UNC 18821, x2.

Figs. 21-23. Desmorthis segnis Havlicek and Branisa. 21, látex replica of incomplete ventral exterior, CEGH-UNC 14411, x4,6; 22, internal mold of dorsal valve, CEGH-UNC 14388, x4; 23, internal mold of ventral valve, CEGHUNC 14382, x4.

Figs. 24, 25. Paralenorthis altiplanicus Benedetto. 24, látex replica of ventral exterior, CEGH-UNC 13844b, x3,8; 25, internal mold of dorsal valve, CEGH-UNC 13840, x3,5.

Figs. 26, 27. Dalmanella cf. salopiensis Williams. 26, látex replica of ventral exterior, CEGH-UNC 21001, x3; internal mold of dorsal valve, CEGH-UNC 21002, x3.

Fig. 28. Drabovinella mojotoroensis Benedetto, Internal mold of dorsal valve, CEGH-UNC 17386, x1,5.

Ponon Trehue area, near the San Rafael village, which are thought to represent the southern edge of the Cuyania Terrane (Ramos, 1995). In the Sierra de Villicum, the lowest 15 m of the La Cantera Formation consists of clast-supported and matrix-supported conglomerates which include a relatively high percentage of fossiliferous clasts containing reworked bryozoan zoecia and fragmented brachiopod valves. The age of these clasts, according to their conodont content, is early Llandeilo (Eoplacognathus lindstroemi, the uppermost subzone of the P. serra Zone, Albanesi et al., 1995a). In general, preservation of brachiopods is too poor for taxonomic purposes.

The lower part of the Lindero Formation exposed in the Ponon Trehue area, south of Mendoza province (Bordonaro et al., 1996), is almost coeval with the fossiliferous clasts from the base of the La Cantera Formation (E. reclinatus-E. lindstroemi subzones of the P. serra Zone, Lehnert et al., 1998).

The thin leveles of crinoidal limestone have yielded the brachiopods Ptychoglyptus virginensis Cooper and Nugnecella rafaelensis Levy and Nullo (Levy and Nullo, 1975). The latter is an enigmatic taxon which is in need of revision. According to Williams and Harper (2000), the type specimens of N. rafaelensis probably include different taxa.

(10) The next interval corresponds to the latest Llanvirn-earliest Caradoc Pygodus anserinus Zone.

It is partially represented by the deep-water, carbonate slope-apron deposits of the Las Aguaditas Formation (Keller et al., 1993; Astini, 1995). The hemipelagic mustones and calcareous shales contain graptolites, conodonts, trilobites and rare radiolarians. Strongly reworked shelly faunas are present only in the gravity mass flow deposits, especially in the matrix of the lower megabreccia horizon, which contains abundant specimens of Dinorthis sp. and other forms not yet described.

(11) Anchoramena assemblage. Beds of early Caradoc age containing a varied brachiopod fauna are exposed in the northern Precordillera (Las Plantas Formation; Benedetto, 1995) and in the Sierra de Villicum (La Pola Formation; Benedetto in press c). Fossiliferous clasts embedded in the basal glacigenic member of the Don Braulio Formation have also yielded approximately coeval brachiopods (Benedetto, 1998b). The age of the Las Plantas Formation is well constrained on the basis of graptolite content which is indicative of the C. bicornis Zone (Ortega and Brussa, 1990; Albanesi and Ortega, 1998). Graptolites recovered from the upper part of the La Pola Formation suggest a similar age (Gisbornian to early Eastonian; Brussa, 2000). Despite taxonomic composition of the three faunas is clearly different, they have in common at least four genera, of which the Precordilleran endemic genus Anchoramena has been chosen to nominate this assemblage. It should be noted that these faunas include two environmentally different assemblages. The authochthonous fraction, recorded in the deep-water black shales of the Las Plantas Formation, is characterized by Oanduporella alamensis Benedetto, Bystromena? protegula Benedetto and Sowerbyella cf. sericea (J. de C. Sowerby). The genus Sowerbyella is also widespread in the hemipelagic facies of the upper part of the Las Aguaditas Formation, in the upper member of the Gualacamayo Formation (Benedetto et al., 1991) and in the slope facies of the La Pola Formation, where is associated with Anoptambonites villicumensis Benedetto.

On the other hand, brachiopods from the calcareous blocks of the Las Plantas Formation, as well as those from the bioclastic sandstones of the La Pola Formation (associated with bryozoans and solenoporacean algae), were resedimented from shallower water settings located probably to the east of the basin (Astini, 2001). These para-authochthonous or allochthonous concentrations include thick-shelled rhynchonellids (Rostricellula robusta Benedetto), orthids (Dinorthis polaensis Benedetto, Dinorthis cf. flabellum (J. de C. Sowerby), Glyptorthis sp., Hesperorthis sp., Howellites cf. macrostoma (Barrande), Tissintia robusta Benedetto, Campylorthis gualcamayensis Benedetto, Onniella inversa Benedetto), clitambonitoideans (Atelelasma sp.), triplesiids (three species of Bicuspina), plecatambonitoideans (Aegiromena glacialis Benedetto), and strophomenoideans (Oepikoides notus Benedetto, Anchoramena cristata Benedetto, Glyptomena cf. sculpturata Cooper).

(12) Destombesium assemblage. In the northern Precordillera the Anchoramena assemblage is succeeded by a low-diversity brachiopod assemblage present in the lower part of the overlying Trapiche Formation (Benedetto, 1999a). The fauna was recovered from a 50-60 m thick interval of debris flows laying approximately 150 above the base of the unit. These beds contain Amorphognatus aff. superbus, which suggests a late Caradoc-earliest Ashgill age (Albanesi et al., 1995c). Three species are typical of this assemblage: Destombesium argentinum Benedetto, Reuschella sp., and Rhynchotrema sp. Geographycally, it is restricted to the Rio Blanco area, north of Jáchal village. No other timeequivalent brachiopod faunas have been documented in the Precordillera basin.

(13) Trucizetina assemblage. The uppermost levels of the Trapiche Formation contain a different assemblage dominated by Trucizetina chanarensis Benedetto (Benedetto, 1999a). Preservation of associated brachiopods is not good enough to make precise taxonomic identifications. Benedetto and Herrera (1987c) reported from the same levels Holtedhalina? sp., Cyphomena? sp. and Dalmanella aff. testudinaria, the latter re-assigned to T. chanarensis (Benedetto, 1999a). The age of this assemblage is not older than early Ashgill owing its stratigraphic position well above to the Destombesium assemblage. Although known species of Trucizetina from China and Bohemia are Hirnantian in age, no typical elements of the Hirnantia Fauna have been found together Trucizetina in the Trapiche Formation. It thus appears that this assemblage may be late, but not latest, Ashgill in age.

(14) Hirnantia sagittifera assemblage. It has long been known that shelly faunas from the Don Braulio Formation exposed at the Sierra de Villicum are late Ashgill in age (Levy and Nullo, 1974; Baldis and Blasco, 1975). This assemblage includes most of typical elements of the widespread Hirnantia Fauna such as Hirnantia sagittifera (M’Coy), Dalmanella testudinaria (Dalman), Eostropheodonta hirnantensis (M’Coy), Plectothyrella crassicosta (Dalman), Paromalomena polonica (Temple) and Cliftonia oxoplecioides Wright (Benedetto, 1986, 1990). Leptaena trifidum Marek and Havlicek is also present (unpublished). Drabovia undulata has only been reported from the Cerro La Chilca outcrops (Astini and Benedetto, 1992).

(15) Anisopleurella assemblage. A low diversity fauna composed by Anisopleurella cf. gracilis (Jones) and Reuschella sp. has been recovered from the fine-grained succession underlying the early Llandovery La Chilca Formation at the Cerro del Fuerte (Benedetto et al., 1986b; Benedetto, 1987b). The latest Ashgill age of the Anisopleurella faunule is well constrained on the basis of its association with Normalograptus persculptus. However, its precise relationships with the above mentioned Hirnantia assemblage are not clear. One interpretation of faunal differences between them is that the Anisopleurella assemblage represents a deep-water lateral equivalent of the Hirnantia Fauna (Sánchez et al., 1991).

Another possibility is that the Anisopleurella fauna post-dates the Hirnantia assemblage. This is supported by the fact that in the Don Braulio Formation N. perscultus appears in the dark gray mudstones deposited just above the calcareous siltstones bearing the Hirnantia Fauna, to which coexists in an interval of 1-2 m. It thus appears that, as in southwest Wales (Cocks and Price, 1975), A. gracilis is present in between the Hirnantia Fauna and the earliest Silurian (Rhuddanian) fauna.

Sierra de Famatina assemblages

The early Ordovician volcano-sedimentary rocks of the Sierra de Famatina are represented by the Suri and Molles formations, excelently exposed along the Saladillo Grande, Cachiyuyo and Molles rivers in the central region of the Famatina Range, La Rioja province. Less continuous but equally richly fossiliferous successions crop out in the Chaschuil area, Catamarca Province (Aceñolaza and Toselli, 1977; Mángano and Butois, 1996; Benedetto, 1994). Most of the Famatinian brachiopods come from the bioturbated mudstones of the upper member of the Suri Formation and the lower third of the overlying Molles Formation, the latter formed by green pyroclastic breccias and volcanic sandstones interbedded with tuffaceous fossiliferous siltstones (Astini and Benedetto, 1996; Benedetto, in press a).

Three successive brachiopod assemblages can be recognized in the Arenig volcano-sedimentary rocks of the Sierra de Famatina.

(1) Suriorthis depressus assemblage. The transition from the gray-bluish laminated shales to the bioturbated greenish mustones of the upper part of the Suri Formation is associated with the appearance of diverse brachiopod, trilobite and bivalve faunas. Typical brachiopods from these levels are Paralenorthis suriensis Benedetto, Crossiskenidium? stelzneri Benedetto, Protoskenidioides cf. revelata Williams, Hesperonomia orientalis Benedetto, Suriorthis depressus Benedetto, Hesperonomiella arcuata Benedetto, Panderina? ambigua Benedetto, Ffynnonia famatinensis Benedetto, Rugostrophia protoandina Benedetto, and Trigonostrophia reversa Benedetto (Benedetto, in pess a).

(2) Mollesella planiventralis assemblage. It occurs in the upper half of the Molles Formation, approximately 500 m above the Suriorthis depressus assemblage. The assemblage starts with lowdiversity coquines dominated by Paralenorthis riojanus (Levy and Nullo), reaching its maximum diversity in the overlying dark-gray bioturbated mudstones. Distinctive taxa are Tritoechia mollesensis Benedetto, Skenidioides kayseri Benedetto, Monorthis transversa Benedetto, Productorthis angulensis Benedetto, and Mollesella planiventralis Benedetto. The co-occurrence of several of these taxa in the Loma del Kilómetro Member of the Suri Formation (sensu Mángano and Buatois, 1992), in the Chachuil area, suggests that the two faunas could be time-equivalent. In the latter locality, however, Famatinorthis turneri is very abundant and Mollesella and Productorthis are absent, probably due to environmental differences. Both lithology and faunal content suggest that the Chaschuil fossiliferous beds are correlatable to the Molles Formation rather than the Suri Formation.

(3) Some isolated outcrops lithologically comparable to the Molles Formation yielded somewhat different brachiopod faunas. One of them, located to the north of the Loma del Kilómetro locality, contains Ahtiella sp., Paralenorthis riojanus and a different species of Famatinorthis. A comparable association composed by Ahtiella, Paralenorthis and Skenidioides was recorded in the Quebrada Honda (north of Rio Cachiyuyo section). On the basis of morphologic changes through the Famatinorthis lineage (unpublished) is believed that this fauna lies above to the Mollesella planiventralis assemblage.

However, no conodont data are still available to corroborate the age of these beds.

Puna assemblages It is widely accepted that the Ordovician Puna basin of northern Argentina evolved in a back-arc setting above an east-dipping subduction zone (Bahlburg, 1990). This north-trending, strongly subsiding basin accommodates more than 3,000 m of marine volcano-sedimentary rocks ranging in age from early Tremadocian to Arenig. In the northern Puna they are overlain by a turbiditic complex c.3,500 m thick reaching the Midlle Ordovician (Llandeilo-early Caradoc?). Shelly faunas and microfossils are almost completely absent, so age determinations have been based largely on graptolite faunas (Bahlburg et al., 1990). Brachiopod and trilobite faunas are restricted to the easternmost and westernmost parts of the basin. In the eastern belt, the early Tremadocian Taique Formation exposed in the Sierra de Cobres has yielded Apheorthis samensis, while the volcano-sedimentary Las Vicuñas Formation (early Tremadocian, Moya et al., 1993; Vaccari and Waisfeld, 2000) cropping out near the Argentina-Chile limit contains a new species of the genus Kvania (Benedetto, 2002). At Vega Pinato, located a few kilometers southward, a richly fossiliferous volcano-sedimentary succession of Arenig age is exposed (Koukharsky et al., 1996). The diverse brachiopod fauna includes Finkelnburgia sp., Oligorthis? sp., Hesperonomia sp., Tritoechia sp., Pinatotoechia acantha Benedetto, Punasptrophia multiseptata Benedetto, Jaanussonites? sp., Imbricatia sp., and Rugostrophia sp. (Benedetto, 2001b). The Vega Pinato assemblage shares four taxa (44%) with the Suriorthis depressus assemblage of the Famatina basin, including the distinctive spinose tritoechiid P. acantha, which until now is restricted to these two regions. This faunal affinity strongly supports the hypothesis that the western Puna volcanics represent the northern continuation of the Famatina volcanic belt (Benedetto, 1998a).

Cordillera Oriental assemblages

The southern part of the large Central Andean basin is superbly exposed in the Cordillera Oriental of northwestern Argentina, where the early Ordovician succession attains more than 3,500 m in thicknes. The stratigraphic relationships between the numerous informal and formally defined formations have not yet been completely elucidated because of lacking of basin-scale stratigraphic analyses and comprehensive sedimentologic and paleontologic studies. Turner (1964) recognized two main units, the Santa Rosita Formation (latest Cambrian-Tremadocian) and the Acoite Formation (early-mid Arenig), both forming the Santa Victoria Group. Moya (1999) redefined some infomal units erected by Harrington and Leanza (1957) and recognized along the western slope of the Cordillera Oriental (from lowest to highest) the Lampazar, Cardonal, Saladillo and Parcha formations.

In the eastern part, the stratigraphic succession is formed by the San José, Caldera, Floresta, Aspero and San Bernardo formations, which are approximately coeval with the western units (Moya, 1999).

(1) The lowest beds bearing brachiopods come from the Lampazar Formation, assigned to the Parabolina (Neoparabolina) frequens argentina Zone. According to conodont evidence, these beds are latest Cambrian to earliest Tremadocian in age (Rao and Hünicken, 1995). The small giraldiellid Kavania n.sp.1 (Benedetto, 2002) forms locally monospecific assemblages, but in general brachiopods are extremely rare in these beds.

(2) The overlying sandstone beds (Cardonal Formation and equivalents units) yielded several species of Nanorthis (not yet published) and a different finely costellate species of Kvania (n.sp. 2). As so far as I aware, the type material of Nanorthis putilliformis (Kobayashi) has been recovered from these levels at the Parcha section. Both morphology and stratigraphic range of this species are in need of revision. Nanorthis saltensis (Kayser), another poorly known species, has also been recorded from beds probably belonging to the Cardonal Formation. Preliminary collections from these sandy beds show clear taxonomic differences among different localities, which could be related to local environmental changes. However, until intensive sampling is accomplished, a vertical succession of these assemblages cannot be entirely rejected. These faunas span the uppermost part of the P. (N.) frequens argentina Zone and the Kainella meridionalis Zone.

(3) The uppermost part of the Cardonal Formation, well exposed in the Sierra de Cajas (Quebrada Amarilla section), bears monotypic assemblages of a species of Nanorthis characterized by its distinctive unequally multicostellate ornament (Nanorthis n.sp. 2, Carrasco, 2001, unpubl.). These beds have been referred to the C. angulatus Zone (Rao, 1999). A rather similar form occurs at the top of the Angosto del Moreno section (Moya and Monteros, 2000).

(4) The brachiopod composition of the lower part of the late Tremadocian succession (Conophrys minutula/ Bienvillia tetragonalis Zone, Floresta Formation) is still poorly known. These fine-grained beds contain one or more species of Nanorthis, Euorthisina sp. nov., and Lipanorthis santalaurae Benedetto (Carrasco, 2001, unpubl., Benedetto and Carrasco, in press).

(5) Nanorthis purmamarcensis assemblage. The late Tremadocian beds (Notopeltis orthometopa Zone, Aspero and Coquena formations) contain a more diverse brachiopod association characterized by Nanorthis purmamarcensis Benedetto, Nanorthis brachymyaria Benedetto, Lipanorthis andinus Benedetto, and Astraborthis quebradensis Benedetto (Benedetto and Carrasco, in press). The species N. purmamarcenis, which has been chosen to nominate this assemblage, is very abundant in the cross-bedded, stormdrived sandstones cropping out in the Purmamarca, Santa Victoria and Cerro San Bernardo sections.

(6) Nanorthis grandis assemblage. The succeeding early Arenig strata are dominated by Nanorthis grandis (Harrington) which usually forms monospecific assemblages. No associated brachiopos have been yet described. The transitional beds in between the above mentioned storm-related sandstones and the overlying fine-grained succession (referred to the Parcha, San Bernardo and lower part of the Acoite formations) have yielded in some places graptolites of the latest Tremadocian Hunnegraptus copiosus Zone (Albanesi et al. 2001). These units, however, are mostly early Arenig in age.

(7) Desmorthis segnis assemblage. Younger strata crop out along the westernmost part of the Cordillera Oriental, where a continuous, more than 2,300 m thick coarsening-up succession is superbly exposed in the Los Colorados-Espinazo del Diablo area. These successions have been matter of several biostratigraphic, sedimentologic, paleoecologic and paleobiologic studies (Waisfeld, 1993; Astini and Waisfeld, 1993; Sánchez and Waisfeld, 1995; Toro, 1997; Benedetto, 1998c, amongst others). The lower half of this succession is barren in shelly faunas. The graptolite association indicates the T. akzharensis Zone. It is ovelain by about 700 m bearing graptolites of the B. deflexus Zone (Toro, 1997) containig low-diversity brachiopod assemblages dominated by Desmorthis segnis

Havlicek and Branisa. This asemblage disappears before the first apparition of P. altiplanicus. (8) Paralenorthis altiplanicus assemblage. A notably more diverse fauna appears above the D. segnis assemblage. Besides the eponymous species, it is composed by Glyptorthis cf. imbrex Havlicek and Branisa, Hesperonomia orientalis Benedetto, Incorthis? sp., Pleurorthis? sp., Euorthisina sp., Camerella sp., and Panderina? cf. ambigua Benedetto. This assemblage falls within the midlle Arenig D. bifidus Zone (Benedetto, 1998c).

(9) The upper beds of the succession consist of pink to reddish cross-bedded sandstones (the so-called Sepulturas Formation) interpreted by Astini (1994) as deposited in an estuarine environment. The age of this unit is not well constrained because of lack of conodonts and graptoplites. Brachiopods recovered from these levels are Incorthis aff. marocana Mergl, Salopia? lipanensis Benedetto. Some other forms have yet to be described.

(10) Late Llanvirn (Llandeilo)-Caradoc brachiopod assemblages are relatively rare in northwestern Argentina. At the top of the Los Colorados succession there is a green-olive mudstone unit which overlies paraconformably early Ordovician strata. The base of this fine-grained unit has been interpreted by Astini (1994) as a regional flooding surface. According to Waisfeld (1996), trilobite evidence supports the correlation of these levels with the early Caradoc Santa Gertrudis Formation.

Brachiopods have not been fully studied. Until now, Monorthis coloradoensis Benedetto, Scaphorthis sp. and Dalmanella cf. salopiensis Williams have been recognized in the Los Colorados area. In the Quebrada Chamarra, north of the Los Colorados village, the resedimented boulders from the overlying Silurian diamictite contain coquines of Dinorthis sp. (Benedetto and Toro, 1996). Outside the Los Colorados area, Caradoc brachiopods are only known from the Santa Gertrudis Formation, exposed in the Sierra de Mojotoro, which contains a monotypic assemblage of Drabovinella mojotoroensis Benedetto, 1999a. Its early Caradoc age is supported by the conodont E. quadridactylus (Albanesi and Rao, 1996). Outside Argentina, Drabovinella has been reported from the Bolivian portion of the Central Andean basin, but since it is represented by different species the correlation is uncertain (Havlicek and Branisa, 1980; Suárez Soruco and Benedetto, 1996).

Aknowledgments. Discussion about Ordovician biostratigraphy, paleoenvironments and biofacies have been held with numerous workers of the Paleobiology Group of the Cordoba University including B. Waisfeld, M. Carrera, T.M. Sánchez, and N. Cech who provided invaluable discussion and comment. This research was supported, in part, by a grant from the Agencia Nacional de Promocion Cientifica y Tecnologica of Argentina (FONCYT, Grant PICT 5387).

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Recibido: 18 de Septiembre de 2002