Ordovician Palynomorphs of Argentina: An Integrated Approach

Maria del Milagro VERGEL1, Lucia ARAOZ2 and Claudia V. RUBINSTEIN3

1 CONICET / Facultad de Ciencias Naturales e I.M.L. (UNT). Miguel Lillo 205 – San Miguel de Tucumán, Argentina. E-mail: vergelmar@tucbbs.com.ar

2 INSUGEO – CONICET. Miguel Lillo 205, 4000 San Miguel de Tucuman. Argentina. E-mail: insugeo@unt.edu.ar

3 CONICET - IANIGLIA – CRICYT. cc: 131- 5500 Mendoza, Argentina. E-mail: crubinstein@arlinkbbt.com.ar

Abstract: ORDOVICIAN PALYNOMORPHS OF ARGENTINA: AN INTEGRATED APPROACH. The knowledge of Ordovician Argentine palynomorphs increased substantially in the last decade. The temporal distribution of them (acritarchs, chitinozoans, scolecodonts, prasinophycean algae and cryptospores) embraces the entire Ordovician, from the early Tremadocian to the latest Ordovician (post Hirnantian) - Asghill. At the moment, they have only been documented in the Cordillera Oriental, Precordillera, Famatina and Puna Basins. Biostratigraphic and paleogeographic data with paleoecological inferences are summarized.

Resumen: PALINOMORFOS ORDOVICICOS DE ARGENTINA: UNA VISION INTEGRADORA. El conocimiento de palinomorfos (acritarchs, chitinozoans, scolecodonts, prasinophycean algae and cryptospores) ordovícicos de Argentina se ha incrementado en la última década. Su distribución temporal abarca desde el Ordovícico temprano (Tremadociano Inferior) al Asghiliano tardío. Hasta el momento, ellos han sido registrados en Cordillera Oriental, Precordillera, Famatina y Puna. Se resumen datos bioestratigráficos, paleogeográficos e inferencias paleoecológicas.

Key words: Ordovician. Palinomorphs. Argentina.

Palabras clave: Palinomorfos. Ordovicico. Argentina.


Although palynological researches were unknown in the Ordovician rocks of Argentina before 1970s, the situation dramatically changed during the following two decades. The incentive was the recognized biostratigraphic potential of such organic-walled microfossil group, mainly used to establish biostratigraphical correlations, as well as the palaeobiogeographical and palaeoenvironmental interpretation with applications in the field of geochemistry. The integrated palynological studies with other important Ordovician fossil groups (trilobites, graptolites and conodonts) highlighted its important applications.

In the present report we considered different palynomorphs: acritarchs, chitinozoans, scolecodonts, prasinophycean algae and cryptospores, all of them with record in the Ordovician strata of Argentina.

Due to the scarcity of contributions related to Ordovician palynomorpohs from Argentina, this synthesis is organized by geographic regions and stratigraphic ranges. The geographic location of records are shown in figure 1. Plates that illustrates this article have been extracted from the original papers cited in the text.

At the moment Ordovician palynomorphs from Argentina have only been documented in three basins, the Central Andean Basin (Cordillera Oriental and Puna of Jujuy and Salta provinces), the Precordillera Basin (La Rioja and San Juan provinces) and the Famatina Basin (La Rioja province).

In the Cordillera Oriental, the record of microflores ranges from the early Tremadocian to the early Llanvirn, in the Famatina basin palynomorphs have a middle Arenig age, while in the Puna and Precordillera, they are confined to the Upper Ordovician (Puna: Ordovician-Silurian boundary; Precordillera: Hirnantian and Llanvirn-Caradoc strata).

From a paleogeographical point of view, Argentinian acritarch assemblages of the Lower Ordovician belong to the Peri-gondwana Province (Tongiorgi et al., 1994). Vavrdová (1974) defined the former Mediterranean Province, later redefined and extended by different authors, and renamed as the Peri-Gondwana Province. The Peri-Gondwana Province, as considered by Playford, Ribecai and Tongiorgi (1995), comprises “a broad, latitudinally extensive, circumpolar, cold to temperate paleogeographical belt along the northern Gondwana border, from Argentina through eastern Newfoundland, north Africa, central and southern Europe, and southern Turkey to South China”.

Afterwards, the genera Arbusculidium, Coryphidium and Striatotheca have been added as diagnostic elements of the Peri-Gondwana Province (Li Jun, 1987). This province has also been identified in Sardinia (Albani, 1989), Pakistan (Tongiorgi et al., 1994) and Iran (Ghavidel-Syooki, 1995).

Subsequently, Servais and Fatka (1997) summarized the previously palaeobiogeographical information from early to Middle Ordovician acritarchs. Likewise, high latitude, cold to temperate water realm (climatically controlled) have been assigned to Perigondwana assemblages, and low latitude, warm water realm to Baltica and the East European Platform assemblages.

Recently Rubinstein and Vaccari (2001) have reported acritarchs and cryptospores from the upper siliciclastic member of the Salar del Rincón Formation. This unit crops out in western flank of the Puna unconformably above Middle Ordovician volcanic sequences. The sequence mainly bears brachiopoda and mollusca faunas attributed to the latest Ashgill - earliest Llandovery (Benedetto and Sanchez, 1990). Although the palynomorph assemblages from the upper member of the Salar del Rincón Formation lacks enough markers to identify the stratigraphic Ordovician-Silurian boundary, according to Rubinstein and Vaccari (2001), palynomorphs suggest a latest Ordovician (post Hirnantian)-earliest Silurian (Rhuddanian) age, supported by the presence of acritarchs as Eupoikilofusa striata and Dactylofusa spinata (partial synonym of Dactylofusa estillis), and by the occurrence of the genus Imperfectotriletes. The first occurrence of Imperfectotriletes, as well as the absence of true trilete spores allow the correlation with the Imperfectotriletes spp. Interval Biozone (sub-biozone a) of the Imperfectotriletes spp.- Laevolancis divellomedia Assemblage Biozone of Steemans et al. (2000). This zone is believed to correspond to the Hirnantian (uppermost Ordovician). The difference between

the faunal and palynological dates should be clarified with further study.

The discovery of oldest cryptospores (probable bryophyte-like plants, sensu Gray, 1985; Wellman, 1999) can contribute much to evolutionary scenario of the early land plants. The mention of them in Ordovician sequences of Argentina (Ottone et al., 1999, Rubinstein, 2001, with references) similary to those found from coeval deposits from around world (such as North Africa, the Prague Basin, Sardinia, southwestern Wales, China, a.o. sensu Wang et al., 1997 and Wellman, 1999) suggests a cosmopolitan character of cryptospores, probably confined to periodically damp environmental (Wellman, 1999)

Cordillera Oriental

The Cordillera Oriental constitutes the southern part of the larger Central Andean Basin. The Santa Victoria Group represents the early Ordovician deposits. It includes the Santa Rosita Formation of a late Cambrian to late Tremadocian age, followed by the Acoite Formation (Lower to Middle

Figure 1. Localities with Ordovician palinomorphs of Argentina (Puna, Cordillera Oriental, Famatina System and Precordillera basins).

Arenig). Both, Santa Rosita and Acoite Formations are composed of a thick sequence of over 3.000 m dominantly clastic.

1.1. Tremadocian

- Yacones and La Caldera Region (San José Shales and Caldera Sandstone - La Caldera department, Salta province). In the San José Shales and Caldera Sandstone from the Yacones and La Caldera locality, eastern side of the Cordillera Oriental, Bultynck and Martin (1982) presented the first discovery of Tremadocian acritarchs in South America. The taxa mentioned in this region were Acanthodiacrodium sp., A. angustum, Cymatiogalea cuvillieri, C. sp., Saharidia sp., Vulcanisphaera sp. Although the authors mentioned the impossibility to refer the taxa specifically to the Lower or Upper Tremadocian, comparison with British, Spanish and French forms allowed them to assign a Lower Tremadocian age. Associated invertebrate faunas confirm this age (e.g. Harrington and Leanza, 1957).

- Nazareno Region (Santa Victoria department, Salta province). The microflore of this region (eastern slope of the Cordillera Oriental) has been analyzed by Manca et al. (1995) and Rubinstein (1997). The palynomorphs come from black shales of the lower part of the Santa Rosita Formation and has been interpreted as transitional early to late Tremadocian age from trilobite evidence. Complementary conodonts data from other horizons of this formation indicated a late Tremadocian age (Manca et al. 1995). The palynological sample yielded a poorly preserved acritarch assemblage with a relatively high degree of thermal maturation. Despite their poor preservation the following taxa have been identified: Acanthodiacrodium cf. simplex, A. cf. spinum, A. tuberatum, A. spp., Actinotodissus achrasi, A. sp., Aremoricaniun sp., Cristalinium cambriense, Cymatiogalea cf bouvardii, C. cristata, C. cf. multarea, C. cf. velifera, C. sp., Elektoriskos? sp., Goniosphaeridium cf. akrochordum, G. pungens, Impluviculus cf. lenticularis, I. Stellaris, Leiofusa spp., Leiosphaeridia spp., Micrhystridium cf. nannacanthum, M. cf. robustum, M. shinetonense, Polygonium sp., Solisphaeridium sp., Stelliferidium cf. stelligerum, Stelliferidium? sp., Synsphaeridium sp. and Vulcanisphaera cf. frequens. According to Rubinstein (1997), the acritarchs clearly indicate a Tremadocian age, although has not been possible to assign either to a Lower or Upper Tremadocian. Paleogeographically, the assemblage showed affinities with the Mediterranean Paleoprovince (Vavrdová 1974; Martin, 1982) and thus a high paleolatitudinal location have been inferred to the Tremadocian Nazareno assemblage.

Abra de Zenta Region (Humahuaca department, Jujuy province). In the eastern flank of the Cordillera Oriental, within the lower levels of Santa Rosita Formation cropping out at the Abra de Zenta, Aráoz and Vergel (2001) and Aráoz (2002) described a marine plankton assemblage with the following

Plate I: 1-Micrhystridium shinetonense Downie, 1958. 2-Impluviculus stellaris Martin, 1977. 3-Vavrdovella areniga (Vavrdová) Loeblich y Tappan, 1976. 4-Stelliferidium? simplex (Deunff, 1961) Deunff et al., 1974. 5-Coryphidium sp. 6-Dictyotidium sp. 7-Athabascaella? sp. 8-Ordovicidium sp. cf. O. yangtzeense Tongiorgi et al., 1995. 9-Tectitheca sp. cf. T. additionalis Burmann, 1968. 10-Cymatiosphaera sp. 11-Stelliferidium sp. cf. S. brevipalmatum Albani, 1989. 12-Rhopaliophora sp. cf. R. pilata (Combaz y Peniguel, 1972) Playford y Martin, 1984. 13-Aureotesta clathrata var. simplex (Cramer et al., 1974) Brocke, 1997. 14-Coryphidium miladae - C. tadla group sensu Rubinstein et al., 1999. 15-Barakella sp. cf. B. rara (Lu, 1987) Tongiorgi et al., 1995. 16-Stellechinatum uncinatum (Downie, 1958) Molyneux, 1987. 17-Buedingiisphaeridium tremadocum Rasul, 1979. 18-Arbusculidium filamentosum (Vavrdova, 1965) Vavrdova 1972. 19-Dactylofusa velifera Cocchio forma brevis Albani, 1989. 20-Eisenackidium orientalis Rubinstein, 1999. 21-Saharidia fragilis (Downie 1958) Combaz 1967. 22-cf. Acanthodiacrodium angustum (Downie 1958) Combaz 1967. 23-Acanthodiacrodium tuberatum (Downie) Martin 1973. 24- Acanthdiacrodium crassus (Loeblich y Tappan 1978). 25-Acanthodiacrodium achrassi (Martin 1973) Yin Leiming, 1986 26- Polygonium gracile Vavrdova 1966. 27-Cymatiogalea velifera (Downie 1958) Martin 1968 28-Cristallinium cambriense (Slavikovà) Vanguestaine 1978. 29-Vulcanisphaera africana (Deunff 1961) Rasul 1976. 30-Vulcanisphaera turbata Martin 1981. (From: 1,2: Rubinstein, 1999 (not scale assigned). 3-20: Rubinstein et al., 1999 (3,4,7: 500x - 5,9,14-17,19: 375x - 6,8,10- 13,18,20: 250x). 21-30: Aráoz, 2002 (21,26,29,30: 250x - 22: 500x – 23-25,27,28: 375x).

taxa: Acanthodiacrodium achrassi; A. crassus, A. formosum, A. hirsutum, A. ignoratum, A. tuberatum, A. cf. angustum, Cristallinium cambriense, Cymatiogalea velifera, C. sp., Leiosphaeridia sp., Lophosphaeridium spp., Polygonium dentatum, P. gracile, P. symbolum, cf. P. pungens, Polygonium spp., Saharidia downie, S. fragilis, Stelliferidium spp., Vulcanisphaera africana, V. nebulosa, V. tuberata and V. turbata.

The microplankton assemblage is more diversified that others previously found in the early Tremadocian strata of northwestern Argentina. However this assemblage was considered as low to moderate diversity, dominated by Saharidia spp (76,2%), leiosphaerids (9,1%), Polygonium (5,1%) and acanthomorphitae forms (3,6%) as the less frequent. The assemblage was interpreted as indicative of a restricted marine setting of a shallow-water environment with Saharidia and leiosphaerids dominants (following Vecoli, 2000). The analyzed assemblage has been referred to the early Tremadocian, but did not provide yet enough diagnostic taxa to secure a full affinity with the Perigondwanian Province (Playford et al., 1995).

1.2. Upper Tremadocian – Arenig

- Santa Victoria Area (Santa Victoria department, Salta province). An integrated biostratigraphic scheme obtained from acritarchs, graptolites, and trilobites of an Upper Tremadocian to Middle Arenig age has been recently established in the Santa Victoria area belonging to eastern side of the Cordillera Oriental of NW Argentina (Rubinstein et al., 1999; Rubinstein and Toro, 1999; 2001).

Five palynological assemblages have been recognized from the uppermost Tremadocian (Santa Rosita Formation) and the Lower-Middle Arenig (Acoite Formation) at the Rio La Huerta and Rio Acoite sections (Rubinstein et al., 1999). In ascending order, the following biozones have been established:

Assemblage 1: From the Upper Tremadocian, equivalent to the Adelograptus-Bryograptus graptolite association, is characterized by Cymatiogalea cristata, C. velifera, C. cuvillieri and Vulcanisphaera sp.

Assemblage 2: From the Lower Arenig, equivalent to the Tetragraptus akzharensis graptolite zone.

The first appearance of the genus Coryphidium, and the following species has been registered:

Dactylofusa velifera forma brevis, Eisenackidium orientalis, Vavrdovella areniga and Veryhachium sp. cf. rhomboidium. In addition, Rhopaliophora palmata and R. sp. cf. pilata were restricted to this assemblage in the Río Acoite section. Other species exclusive to this assemblage were Acanthodiacrodium sp. cf. tasselli, Buedingiisphaeridium tremadocum, Cymatiogalea sp. cf. membranispina, Micrhystridium henryi, Ordovicidium sp. cf. yangtzeense, Stelliferidium sp. cf. brevipalmatum and Vavrdovella areniga.

Assemblage 3: This assemblage has been recognized in the lower part of the Baltograptus deflexus graptolite zone (Lower Arenig). The only species to appear is Cymatiogalea granulata.

Assemblage 4: From the upper part of the B. deflexus graptolite zone (Lower Arenig). It is characterized by the first appearance of the genus Striatotheca, with the species S. cf. principalis var parva and Ammonidium multipugiunculatum.

Plate II: 1-Cymatiogalea granulata Vavrdova, 1966. 2-Adorfia hoffmanensis (Cramer et al., 1974) Ottone, 1992. 3-Ammonidium ballistum Ottone, 1992. 4-Veryhachium trispinosum (Eisenack, 1938) Deunff, 1954. 5-Marrocanium simplex Cramer et al., 1974. 6-Striatotheca principalis Burmann, 1970. 7-Peteinosphaeridium trifurcatum intermedium Eisenack, 1976. 8-Vulcanisphaera tuberata (Downie 1958) Eisenack et al., 1973. 9-Asketopalla sp. 10-Multiplicisphaeridium multipungilatum Cramer y Diez, 1977 11-Baltisphaerosum peniguelii Ottone, 1992 12-Spathachitina sp. A. 13-Lagenochitina sp. A. 14-Conochitina sp. A. 15- Scolecodont sp. A. 16-Cristallinium chalalensis Ottone, 1995. 17-Virgatasporites sp. similar V. rudi Combaz, 1968. 18- Acanthodiacrodium cf. A. simplex Combaz, 1968. 19-cf. Baltisphaeridium hirsutoides hamatum Downie, 1958. 20-Pterospermella crameri (Volkheimer et al., 1980). 21-Pirea sp. cf. Pirea dubia Vavrdova, 1972. 22-Tetrahedraletes sp. 23-Gorgonisphaeridium granulobrachium Ottone, 1999. 24-Cyathochitina sp. cf. C. jenkinsi Neville, 1974. 25-Belonechitina sp. A. (From: 1-15: Ottone, 1992 (1-7,11,15: 250x - 8: 750x - 9: 375x - 10: 500x – 12-14: 75x). 16: Ottone et al., 1995 (x750). 17-19: Volkheimer et al., 1980 (17-19: 750x). 20-23: Ottone et al., 1999 (x375). 24, 25: Ottone et al., 2001 (26,27: 75x).

Assemblage 5: Assemblage recorded from the Didymograptellus bifidus graptolite zone (Middle Arenig). It is characterized by the increasing abundance and diversity of acritarchs and by the first occurrence of the following taxa: Acanthodiacrodium costatum, Adorfia hoffmanensis, Arbusculidium filamentosum, Aureotesta clathrata var. simplex, and Coryphidium miladae-C. tadla group. Also, restricted to lower D. bifidus graptolite zone, the following taxa appear: Barakella sp. cf. rara, Coryphidium australe, Petaloferidium sp. cf. florigerum, Polygonium sp. cf. delicatum, P sp. cf. symbolum, P. spp., Striatotheca microrugulata, S. sp. cf. rarirrugulata, S. transformata, and Tectitheca sp. cf. additionalis.

Recently, Rubinstein and Toro (1999; 2001) strengthened the former biostratigraphical scheme with two new sections (Quebrada Grande and Quebrada de La Curva), adding new data from La Huerta section (Jujuy province). From the middle part of the Quebrada Grande section Rubinstein and Toro (1999) noticed the first report of Cymatiogalea messaoudensis var. messaoudensis. The occurrence of this species which is diagnostic of the messaoudensis-trifidum Assemblage (latest Tremadocian to earliest Arenig) allowed some considerations. The record of Cymatiogalea messaoudensis var. messaoudensis is accompanied by Arbusculidium filamentosum, Coryphidium cf. bohemicum, Dactylofusa velifera forma brevis and Eisenackidium orientalis among others. So the occurrence of Cymatiogalea messaoudensis var.messaoudensis with Arbusculidium filamentosum suggests an age similar to that of the sub-assemblage 5 of the messaoudensis-trifidum Assemblage (base of the Arenig sensu Servais and Molyneux, 1997).

The first appearance of Arbusculidium filamentosum has been reassigned as corresponding to lowest B. defexus Zone (early Arenig age) equivalent to the former Assemblage 3. On the other hand, Rubinstein and Toro (2001) mentions Cymatiogalea messaoudensis var. messaoudensis from the Middle Arenig (Didymograptellus bifidus Zone) of the La Huerta section, suggesting that this species may range higher in Quebrada Grande section than previously thought. Acritarch assemblages from Quebrada La Curva section have been correlated with the Didymograptellus bifidus Zone and assigned to a Middle Arenig age.

According to Rubinstein and Toro (1999; 2001) some paleogeographical inferences can be done on the basis of an integrated study with acritarchs, graptolites and trilobites from Upper Tremadocian to the Middle Arenig in the Cordillera Oriental of Argentina. Perigondwanan affinities have been interpreted for the assemblages of NW Argentina, during the early Ordovician (early to middle Arenig). The palynomorphs from the Acoite Formation indicated a temperate paleolatitudinal location for northwestern Argentina, transitional, near the limit of the cold water Perigondwana Realm (units climatically controlled as suggested Servais and Fatka, 1997). Some elements are shared with those of intermediate paleolatitudinal locations such as South China and Pakistan. These data confirms the paleogeographical location of the Cordillera Oriental during the early Ordovician, previously determined by trilobites and graptolites (Waisfeld, 1993; Toro, 1996). An unrestricted communication between the eastern and western side of the Cordillera Oriental have been confirmed and clarified (Astini and Waisfeld, 1993; Waisfeld, 1996; Moya, 1988; Moya and Montero, 1999).

Both, biostratigraphical and paleogeographical implications of Arenig palynomorphs include data from Los Colorados Creek and Rio Cajas localities (Ottone et al., 1992) to the western side of the Cordillera Oriental.

- Parcha Region (La Caldera department, Salta province). In the southern part of the Cordillera Oriental, the Parcha section has been palynologicaly analyzed by Rubinstein at al. (1999) and Rubinstein and Toro (1999). Acritarchs were recovered from the Saladillo Formation (partially equivalent to the Santa Rosita Formation) and Parcha Formation (partially equivalent to the Acoite Formation). The palynofloras are poor in abundance, diversity, and preservation. Thus ages and correlations have been based on the graptolites. Samples from the lower part of the succession corresponding to Saladillo Formation (Tremadocian) yielded Bryograptus sp. and the following palynomorphs:

Polygonium dentatum and Baltisphaeridium cf. multipilosum. In the Parcha Formation (Lower Arenig at Parcha section) the basal sample equivalent to the Tetragraptus akzharensis graptolite zone has yielded Stelliferidium sp. and Actinotodissus? sp. The following Baltograptus deflexus graptolite zone contain Stellechinatum cf. uncinatum, Micrhystridium stellatum, Peteinosphaeridium sp., Cymatiogalea sp., Cymatiosphaera sp., Coryphidium sp., Leiosphaeridium sp., Acanthodiacrodium spp. and Polygonium spp. Recent acritarch findings in Parcha Region (Rubinstein and Toro, 2002) lead to support the presence of the messaoudensis-trifidum acritarch assemblage in the Tremadocian-Arenigian boundary of northwestern Argentina. Recorded species are Caldariola glabra var. glabra, Cymatiogalea messaoudensis var. messaoudensis, Cymatiogalea messaoudensis var. inconnexa, Cymatiogalea deunffi, Coryphidium sp., Stelliferidium cf. trifidum and Acanthodiacrodium-Vavrdovella transients. Acritarchs come from a level related to the Araneograptus murrayi graptolite biozone, which indicates an age not younger than late Tremadocian for the acritarch assemblage.

- Sierra de Cajas Region and Los Colorados Area (Humahuaca and Tumbaya department, Jujuy province).

The palynomorphs studied by Ottone et al. (1992) in this region (western slope of the Eastern Cordillera) have been recovered from the Acoite Formation. It is a clastic sequence mainly composed of thinly laminated claystones, siltstones and sandstones. This unit rests conformably above the Tremadocian Santa Rosita Formation and the Upper Arenig-Lower Llanvirn Sepulturas Formation conformably overlies it. The palynological samples are from the lower and upper section of Acoite Formation at Los Colorados Creek (Tumbaya department), and from the lower section of the unit at the Rio Cajas (Humahuaca department). Two distinct palynomorph assemblages with acritarchs, algae, chitinozoans, scolecodonts and graptolite siculae have been differentiated throughout the unit: a lower Acoite Palynological Assemblage (AL) and an Upper Acoite Palynological Assemblage (AU). The following taxa are present: Acanthodiacrodium cf. angustizonale, A. spp., Adorfia hoffmanensis, Ammonidium ballistum, Arbusculidium sp., Asketopalla sp., Baltisphaerosum peniguelli, Cristallinium sp., Cymatiogalea granulata, C. velifera, Eisenackidium sp., ?Gloecapsomorpha sp., Leiosphaeridia sp., Lophosphaeridium aequicuspidatum, Marrocanium simplex, Micrhystridium henryi, Multiplicisphaeridium multipungilatum, M. spp., Peteinosphaeridium trifurcatum intermedium, Polygonium gracile, Striatotheca principalis, S. transformata, S. triangulata, Veryhachium lairdii, V. trispinosum, Vulcanisphaera tuberata, Conochitina sp., Lagenochitina sp., Spatachitina spp., Cyathochitina sp. and Scolecodont sp. The AL Assemblage has been characterized by a relative scarcity of palynomorphs and a poor preservation. The most abundant and consistently represented form is Leiosphaeridia sp. (ca.50%); the minor constituents of the assemblage are Vulcanisphaera tuberata (ca. 5,5%), Cymatiogalea velifera (ca. 5%), and Polygonium gracile (ca. 5%). Graptolite siculae are relatively common and the Thermal Alteration Index (TAI) (Staplin, 1969) varies from 3 (rarely) to more than 4. An early Arenig age was referred to the AL Assemblage from the lower section of the Acoite Formation, equivalent to upper part of the Baltograptus defexus graptolite zone (Rubinstein and Toro, 2001).

The AU Assemblage has been characterized as moderately diverse and relatively well-preserved palynological assemblage. The commonest taxon is Polygonium gracile (ca. 33%), together with a consistent representation of Cymatiogalea granulata (17%), leiosphaerids (10%), Veryhachium lairdii (7%) and Adorfia hoffmanensis (5%). The TAI assigned to thinner-walled acritarchs was of about 2 +.

The AU Assemblage from the upper section of the Acoite Formation has been referred to Middle Arenig, equivalent to Didymograptellus bifidus graptolite zone (sensu Rubinstein and Toro, 2001).

Both AL and AU palynological assemblages have been related to the Mediterranean or Perigondwana Province (Vavrdová, 1974, Martin, 1982; Li Jun, 1987; Albani, 1989) with an inferred high latitudinal setting.

- Chalala Creek (Tumbaya department, Jujuy province). Also on the western side of the Cordillera Oriental from the Chalala Creek, near Purmamarca locality, Ottone et al. (1995) described a scarce a relatively low-diversified acritarch assemblage from the basal section of the Acoite Formation. The following taxa have been identified: Acanthodiacrodium spp., Baltisphaerosum sp., Cristallinium chalalensis, Cymatiogalea velifera, Leiosphaeridia sp., Lophosphaeridium aequicuspidatum, Polygonium gracile, Vulcanisphaera tuberata, and undetermined chitinozoans and scolecodonts.

This assemblage showed some resemblance with the previously defined AL Assemblage from the Acoite Formation at Rio Cajas and Los Colorados sections, which has been referred to early Arenig (Ottone et al., 1992). However, owing to the absence of Striatotheca and conspicuous forms of the AL Assemblage such as: Peteinosphaeridium trifurcatum intermedium and Baltisphaerosum peniguelli the authors suggested a probably Tremadocian age to the assemblage, but not similar to the previously known assemblage from Tremadocian strata of the Cordillera Oriental (Bultynck and Martin, 1982).

This microflore have also been included in the Mediterranean or Perigondwanian Province 1.3. Llanvirn - Mojotoro Region (Quebrada del Gallinato and Corral de Barranca, Sierra de Mojotoro, Capital department, Salta province). The first Ordovician acritarchs mentioned in South America were localized in this region. In the upper part of the Mojotoro Formation two palynoassemblages have been documented by Volkheimer et al. (1980b). Both assemblages belong to an only microfloristic unit and the palynomorphs (acritarchs and prasinophytes) recorded are Acanthodiacrodium cf. latizonale, A. cf. simplex, Baltisphaeridium cf. redondensis, cf. Baltisphaeridium hirsutoides hamatum, Cymatiosphaera spp., Leiovalia sp., Leiosphaeridia spp., Multiplicisphaeridium cf. varians, Pirea cf. dubia, P. sp. Pterospermella crameri, Veryhachium sp. and Virgatasporites cf. rudi, and chitinozoans (gen. et sp. indet.).

According to Volkheimer et al. (1980b) the stratigraphic value of palynomorphs indicated a Lower Ordovician to Llanvirn age for the Mojotoro Formation. In addition, it was correlated to other Lower Ordovician assemblages from the Sahara. A narrow paleogeographical relationship between both, northwest Argentina and North Africa, have been inferred for the early Ordovician.

2.- Precordillera Basin

The Precordillera of western Argentina has been a matter of intense debate during the last decade. This debate is focused on its supposed paraautochthonous – allochtonous origin. These problematic are suggested on the basis of tectonic, stratigraphic and paleontological basis (Bond et al., 1984; Ramos, 1986; 1988; Dalla Salda et al., 1992; Dalziel et al., 1996; Astini et al., 1996; Pankhurst and Rapela, 1998; Benedetto et al., 1999; Aceñolaza and Toselli, 1999; Finney et al., in press, with references).

Ottone et al. (1999) and Ottone et al. (2001) interpretates a Gondwanan link during the Llanvirn for the supposed allochtonous Precordilleran terrane on the basins of palynomorphs Carbonate and siliciclastic rocks ranging in age from the Tremadocian to the Ashgill mainly characterize the Ordovician sedimentary sequences from Precordillera Basin. An extensive carbonate platform with thick limestone sequences (San Juan Formation) was established during the Cambrian and early Ordovician for the Eastern and Central Precordillera. In the western part of the basin, the carbonate sedimentation persisted until the early Caradoc (Las Aguaditas Formation). Late Tremadocian and Arenig alternating clastic sandstones, mudstones, and black shales assigned to the Gualcamayo and Los Azules formations were deposited in several places of Precordillera; these sequences were related to an Arenig-Llanvirn transgresive event. The following Middle and Upper Ordovician successions include shales, sandstones, and conglomerates (Las Plantas, Trapiche, Don Braulio, and La Chilca formations) were deposited in several parts of the basin until the uppermost Ashgill.

The Ordovician sequences display abundant fossil faunas (graptolites, conodonts, trilobites, brachiopods, a.o.) that have been treated in several papers and provided valuable data to understand the regional paleoenvironmental evolution. For the present, palynomorphs have only been recovered from early Ordovician levels (San Juan, Gualcamayo and Los Azules Formations) and for the Hirnantian, uppermost Ashgill (La Chilca Formation).

2.1. Arenig/Llanvirn

- Jachal Region (Jachal department, San Juan province). Enigmatic organic-walled microfossils have been documented from two sections near Jachal locality (Heuse et al., 1997). The incertae sedis microfossils from Cerro La Silla section come from lower levels of San Juan Formation (early Arenig) correlated with upper part of the P. elegans / O. communis conodont zone. The other section outcropping at the Cerro La Chilca yielded material from early Llanvirn strata correlated with E. suecicus conodont zone from the top of the San Juan Formation and the basal part of the Gualcamayo Formation. These problematic microfossils were described as black, spherical to subspherical shaped forms with an opening and cell-like wall structure. The possible biological affinities was discussed by Heuse et al. (1997) and a hypothetic origin, linked to chaetognath specimens have been assigned from similar Bohemian microfossils founded inside two specimens of the chaetognath Titerina rokycanensis.

- Guandacol Region (Quebrada Potrerillos, General Lavalle department, La Rioja province). The first data on Ordovician chitinozoans from Argentina belong to Volkheimer (1978) and Volkheimer et al. (1980a). The chitinozoans have been recovered from dark shales of the Gualcamayo Formation (early Llanvirn) at Quebrada Potrerillos, southwest of Guandacol locality. The assemblage has been characterized by dominant big sized forms (130-470 µm) with smooth or not much ornamented vesicle wall and a simple carina. Not appendices or siphoned forms have been observed. It belongs to the genera Lagenochitina, Conochitina, Cyathochitina, and Rhabdochitina? and Desmochitina.

Considerations about evolutionary successions on carina and appendices presence have been analyzed to establish an approximate age. Also the stratigraphic value of individuals forms such as Cyathochitina and Conochitina and its related species allowed to assign for the analyzed levels a Llanvirn age. The Gualcamayo Formation has been mainly dated as of an early Llanvirn age by graptolite record (Cuerda and Alfaro, 1986).

Equivalent levels on the west flank of Cerro Perico at the Potrerillos creek have yielded scolecodonts (Ottone and Holfeltz, 1992). The scolecodonts or fossilized elements of the proboscidal armatures of polychaetous annelids that occur in many types of marine sediments. They are abundant in shallow water and appear often associated with graptolites, chitinozoans, spores and acritarchs. The oldest scolecodonts have been found in Arenig rocks. To the Gualcamayo Formation the following taxa have been recognized: Anisoceracites sp., Arbellites sp., Leodicites sp., Nereidavus sp., Nereigenys sp. and Staurocephalites sp. The early Llanvirn age assigned to the assemblage have been obtained by the associated marine biota.

2.2. Llanvirn/Caradoc

- Cerro Viejo de Huaco Region (Jachal department, San Juan province). Middle Ordovician sequences belonging to Los Azules Formation crops out in the Cerro Viejo de Huaco region, between Huaco and Jachal localities, northern San Juan province, in the Central Precordillera. Los Azules Formation is mainly made up of argillites, shales and mudstones, and was divided into three informal members (Ortega, 1987). From these levels Ottone et al. (1999) reported a rich association of micro and macrofossils with palynomorphs, conodonts and graptolites associates. Several species of acritarch (Baltisphaerosum peniguelli, B. spp., Gorgonisphaeridium granulobrachium, Lophosphaeridium ruidum, Navifusa similes, Ordovicium elegantulum, Orthosphaeridium vibrissiferum, Polygonium echinatus and Solisphaeridium sp.), prasinophytes (Leiosphaeridia sp., Pterospermella sp.), cryptospores (Tetrahedraletes sp.) and different morphotypes of phytodebris were identified throughout the middle and upper members of the Los Azules Formation. Several species of conodonts have also been recorded within the middle member of the unit.

Considering the stratigraphic value and occurrence of each species in Los Azules section, two assemblages were defined: the Lower Los Azules Palynological Assemblage (LAL), and the Upper

Los Azules Palynological Assemblage (LAU). The first spans the entire middle member of the unit.

Leiosphaerids are the commonest forms (83-98%); Navifusa similis and smooth chitinozoans are subordinate. Terrestrial remain have not been observed. The second, in the upper member of the formation, yielded plentiful ornamented chitinozoans (60-81%); also Navifusa similis (1-10%); Ordovicidium elegantulum (1-7%), Leiosphaeridia sp. (2-5%), scolecodonts (2-5%) and Lophosphaeridium ruidum (3-4%) as the minor constituents. Terrestrial palynomorphs are rare (less than 1%).

According to Ottone et al (1999) paleoenvironmental inferences related to low diversity, leiosphaerids dominated assemblages are indicatives of restricted epicontinental deep-water settings to the middle member of the Los Azules Formation. In addition, the assemblage of the upper member of the unit is indicative as well as, to an offshore setting, in agreement with the sedimentological and tectonic data (Astini, 1994a,b; 1995; Baldis et al., 1989).

The LAL Assemblage lacks enough plant microfossils markers to confirm a precise stratigraphic rank, and was attributed to a late Llanvirn age on the basins of associated fauna. The presence of chitinozoans was later analyzed by Ottone et al. (2001) confirming this late Llanvirn age. On the other hand, the LAU Assemblage was biostratigraphically placed in the Llanvirn/Caradoc, although the chitinozoan assemblages suggested a younger age (early Caradoc). Chitinozoans species of the LAL Assemblage (middle member of the Los Azules Formation) are: Calpichitina megastrophica, C. sp. A; Cyathochitina dispar, C. sp. cf. campanulaeformis, C. sp. cf. jenkinsi, Conochitina dolosa, C. minnesotensis, Desmochitina minor forma amphorae, Desmochitina minor forma ovulum, Desmochitina minor forma typical, Hyalochitina sp., Lagenochitina cilindrica and L. sp. cf. baltica. Also the following chitinozoans have been reported from the upper member of Los Azules Formation or in the LAU Assemblage: Angochitina sp., Armoricochitina sp. cf. nigerica, Belonechitina punctata, B. sp., Colpichitina sp., Conochitina dolosa, C. minnesotensis, Conochitina? pygmaea, Cyathochitina sp. cf. campanulaeformis, C. sp. cf. jenkinsi, Eisenackitina yolei, Hercochitina volkheimerii, Kalochitina sp. cf. multispinata.

All palynomorph assemblages confirm the previous graptolite-based age for Los Azules Formation (early Llanvirn-early Caradoc) and exhibits similarities with coeval assemblages from the Northern Hemisphere and Gondwanaland. However the appearance of some taxa such as Conochitina? pygmaea and Armoricochitina sp. cf. nigerica in younger levels than in Laurentia and Northern Gondwana was interpreted as an earlier radiation originated in Precordillera with posterior migration and reaching their acme in Laurentia and Northern Gondwana.

2.3. Hirnantian

- Talacasto area (Ullúm department, San Juan Province). In this area three palynomorph assemblages have been described for the La Chilca Formation (late Hirnantian-Llandovery/Wenlock?) and the Los Espejos Formation (Wenlock to Pridoli) (Rubinstein and Brussa, 1999). The Assemblage 1 corresponds to the Normalograptus persculptus graptolite zone of Hirnantian (latest Ordovician) age. It is dominated by simple spheroidal and short-spined acritarch taxa, none of which have any stratigraphic value. Genera include Leiosphaeridia, Michrystridium, and Veryhachium.

3.- Famatina Basin

The Ordovician sedimentary rocks of the Famatina System are represented by isolated outcrops between widespread intrusives. The sedimentary sequences belong to clastic rocks (Upper Cambrian- Tremadocian) from the Central Famatina and volcaniclastic rocks (Arenig) from Northern and Central Famatina. The volcaniclastic sequences were included in the Famatina Group (Turner 1964) that is integrated by the Suri (early to middle Arenig) and Los Molles (middle to late Arenig) Formations. Later Aceñolaza and Toselli (1981) proposed the Cachiyuyo Group to unify the clastic and volcaniclastic rocks. The latter is integrated in ascending order by the following formational units: The Volcancito (recently assigned to uppermost Cambrian to Tremadocian), Portezuelo de las Minitas (= La Alumbrera, assigned to earliest Arenig), Suri (early to middle Arenig), Los Molles (middle to late Arenig) and Morado (= Las Planchadas, Llanvirn) formations. The sequence represent a continuous sedimentation during the early Ordovician, controlled by volcanism, tectonic and eustatic changes. The age assigned to the formational units have been extracted from unified paleontological and biostratigraphical data of a recent synthesis (In Aceñolaza et al., 1996; Astini, 1999 and Esteban et al., 1999 among others).

3.1. Arenig

- Saladillo Grande and Los Molles Creeks (Famatina department, La Rioja province). Strata belonging to the Suri Formation cropping out at the Quebrada Saladillo Grande and the sequences of Los Molles Formation from the Quebrada Los Molles have yielded acritarchs and chitinozoans (Rubinstein and Astini, 2000 and Rubinstein, 2001a,b). Acritarch assemblages from the Suri Formation have a middle Arenig age, independently dated by conodonts (upper part of the Oepikodus evae Zone).

Graptolites of the Didymograptellus bifidus Zone have also been recovered in this unit, bellow levels with acritarchs. This formation contains, among other acritarch taxa Arbusculidium filamentosum, Eisenackidium orientalis, Dactylofusa velifera forma brevis, Cymtiogalea deunffi, Vavrdovella areniga, Striatotheca sp., Acanthodiacriodium costatum, Rhopaliophora palmata, Poikilofusa striatogranulata and Peteinosphaeridium spp. The chitinozoans are actually in study. The abundance of Eisenackidium orientalis and Dactylofusa velifera forma brevis of an outstanding stratigraphic value in the Cordillera Oriental (early to middle Arenig), allowed a biostratigraphic correlation of both basins (Acritarch Assemblage 4 to 5 or Baltograptus deflexus-Didymograptellus bifidus graptolite zones of the Cordillera Oriental). Thereby, the Suri Formation showed clear perigondwanan affinities (moderate - cold water) with typical forms such us Arbusculidium, Striatotheca, and taxa from temperate to low paleolatitudinal location.

All typical perigondwanan acritarchs disappear towards the upper Los Molles assemblages.

These are replaced by intermediate to low latitudes taxa such as Chinese, Australian and Baltic forms with predominance of acanthomorphic elements (Peteinosphaeridium, Baltisphaerosum and Tongzia).

They are also included in levels of the upper part of the Oepikodus evae Zone, consequently corresponding to the Middle Arenig. According to Rubinstein (2001a) the differences between Suri and Los Molles acritarch assemblages were due to local environmental conditions related to the volcanic arc setting.

Aknowledgements. We are gratefull to G. Aceñolaza for valuable discussion on the stratigraphy of NW Argentina. We also thank the editor F. Aceñolaza, for the invitation to participate in the volume.


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Recibido: 15 de agosto 2002

Aprobado: 18 de diciembre de 2002