Middle Ordovician graptolite biostratigraphy from the Los Azules Formation at Los Gatos Creek,Central Precordillera, Argentina

Edsel D. Brussa¹, Charles E. Mitchell², Gladys Ortega³, Jörg Maletz² and Ricardo A. Astini⁴  

¹CONICET. Cátedra de Paleontología I, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa, Uruguay 151, (6300) Santa Rosa, La Pampa, Argentina. E–mail: ebrussa@exactas.unlpam.edu.ar

² Department of Geology, State University of New York at Buffalo, Buffalo, NY 14260, USA.   E–mail: cem@acsu.buffalo.edu; jorgm@acsu.buffalo.edu

³ CONICET. Museo de Paleontología, Universidad Nacional de Córdoba, C.C. 1598, (5000) Córdoba, Argentina.    E–mail: gcortega@arnet.com.ar

Key words: Graptolites. Biostratigraphy. Los Azules Formation. Middle Ordovician. Argentine Precordillera.

The main objective of this work is to analyse the biostratigraphy of the Los Azules Formation in the Los Gatos creek, based on new information and the revision of previously studied graptolite specimens. Results are discussed in accordance with the recent biostratigraphical schemes proposed by one of the authors (Maletz, 1997a, b).

The outcrops of the Los Azules Formation are exposed along the western flank of the Cerro Viejo anticline, east of Jáchal, in the Precordillera of San Juan. This unit has a maximum thickness of 318 meters and is made up mainly of black shale partly calcareous and partly silicified. It overlies San Juan Formation limestones and is erosively truncated by carboniferous glacial diamictities. Borrello and Gareca (1951) gave the first descriptions, whereas Turner (1960), Cuerda and Furque (1975), Furque and Cuerda (1979) and Ortega (1987) considered its biostratigraphy. The formation is divided into three members (Ortega, 1987). The lower member is predominantly composed of massive, dark argillites with subordinate sandstone and siltstone and K–bentonite strata. The middle member includes thin dark sandstones in its basal part and gray laminated siltstones above. Thin beds of yellowish K–bentonites (averaging 2–5 mm) are present throughout this member. Twenty–six K–bentonites were recorded in the Los Gatos creek section where the middle member is truncated at 104,7 meters from its bottom. The upper member comprises gray calcareous–rich silty shales interbedded with mudstones. Ortega (1995) proposed that the Paraglossograptus tentaculatus Zone was present throughout the lower member, and suggested that the middle member contain a fauna belonging to the Pterograptus elegans and Hustedograptus teretiusculus zones. Mitchell et al. (1998) considered that the occurrence of U. sinicus together with Arienigraptus zhejiangensis and U. austrodentatus suggested that the beds immediately above the San Juan Formation belonged to the Undulograptus sinicus Subzone (Undulograptus austrodentatus Zone). These authors also noted for the first time the rocks of the upper part of the lower member, beginning at about 3.44 meters from the contact with the San Juan Formation, appeared to be referable to the Undulograptus dentatus Zone (Da2).

Figure 1. Graptolite species range chart for the middle member of the Los Azules Formation at Los Gatos creek. Graptolite distribution in the lower member only referred to diplograptids and arienigraptids. For complete distribution of species in the lower member see Ortega and Rickards (this volume). SJ FM: San Juan Formation.

The revision of the existing collections and new collections allowed confirmation that the first beds of the Los Azules Formation contain fauna referable to the Undulograptus dentatus Zone. In the Los Gatos creek, a section where only 110 m of the Los Azules shales are preserved, the Undulograptus dentatus Zone spans the entire first 4.5 meters of the unit. The beds referable to the U. dentatus Zone contain the following taxa, among others: U. dentatus, Dichograptus octobrachiatus, Tetragraptus arcuatus, Jiangshanites sp. cf. J. dubius, Pseudophyllograptus sp., Pseudobryograptus sp., Pseudotrigonograptus ensiformis, Acrograptus sp. 1, Acrograptus sp. 2, Paraglossograptus tentaculatus, Arienigraptus zhejiangensis, Isograptus caduceus, Anormalograptus reliquus, Undulograptus sinicus and U. austrodentatus. An additional key record may be the presence of Arienigraptus angulatus in the basal section of the formation. This is a species similar to A. zhejiangensis that develops a horizontal part of the manubrium and would be the youngest species of this genus (Maletz, 1997a). However, another young arienigraptid was recorded in this unit, i.e., Arienigraptus sp. cf. A. geniculatus, which ranges from the lowermost levels to the thick K– bentonite bed, at 3.44 meters above the contact with the San Juan Formation.

Maletz (1997a) informally considered two subzones within the U. dentatus Zone in his biostratigraphic analysis of the slice of Pointe–de–Lévy, Quebec Appalachians, Canada. The upper subzone was characterized by the first appearance of A. angulatus and the disappearance of U. dentatus. This record of A. angulatus coeval with U. dentatus does not agree with Maletz’s (1997a) observations in Canada. A fauna similar to that one from Lévis is the one described from the Cow Head Group at St. Paul’s Inlet, western Newfoundland (Williams and Stevens, 1988; Mitchell, 1994).

The next record of an index species is the first appearance of Pterograptus elegans at 5.3 meters from the contact with the San Juan Formation (bed C18). The 0.80 meters lying between the U. dentatus and P. elegans Zones are tentatively referred to the Holmograptus lentus Zone, despite the fact that the nominal taxon was not found. The first appearance of Hustedograptus sp. is taken as the end of the U. dentatus Zone. This bed is characterized also by the presence of specimens assigned to Undulograptus sp. cf. U. cumbrensis.

In addition to the appearance of Hustedograptus sp., the H. lentus Zone was inferred through the presence of new forms such as Undulograptus sp. cf. U. camptochilus, and Haddingograptus. However, a large fraction of the fauna (such as P. tentaculatus, U. dentatus, Undulograptus. sp. aff. U. cumbrensis, U. sinicus, Isograptus caduceus caduceus, Cryptograptus antennarius, Brachiograptus etaformis) continues upwards from the U. dentatus Zone. The presence of P. tentaculatus and I. caduceus together with Hustedograptus indicates that these beds are likely to be no younger than lower part of the biozone (Upper Da2).This is supported also by the absence of P. proteus and I. forcipiformis and the overlap of Hustedograptus sp. with H. bovis. The higher beds of the lower member sensu Ortega (1987) are gray sandstones; this part of the section seems to be condensed. The biozone is represented by a thin interval that could correspond to a maximum flooding interval with a resulting marine hiatus in the distal, deep–water facies as a result of sediment starvation. The Da3 interval appears to be entirely missing at this level and Da4 Pterograptus elegans Zone directly overlies the Upper Da2 at the contact between the lower and middle members (bed C18).

Bed C18, at 5.3 meters from the contact with the San Juan Formation is constituted by a mica rich K–bentonite (mica flakes > 1 mm) probably a product of reworking by bottom currents that have slightly concentrated coarser grains from the bentonite layers below. This level is characterized by the simultaneous appearance of Pterograptus elegans, Kalpinograptus parallelus and Wuninograptus sp. as the most distinctive taxa. This micaceous sandstone contains numerous large conodont elements of the Pygodus anitae Subzone (Eoplacognathus suecicus Zone) (Ottone et al. 1999); it can be followed through several sections in the Cerro Viejo region, and is clearly associated with the first appearance of P. elegans. The appearance of the P. elegans Zone marks an important change in the fauna. The nominal taxon is a common element and is associated with Haddingograptus sp. aff. H. olivieri, Archiclimacograptus angulatus and Hustedograptus sp. cf. H. teretiusculus. The assemblage of the lower beds of the middle member includes several species that first appear in what Maletz (1997b) recognized as the upper part of the P. elegans Zone. Among them we count Kalpinograptus parallelus, Hustedograptus vikarbyensis, Archiclimacograptus angulatus and Reteograptus speciosus. Archiclimacograptus caelatus, however, is a species that in the Oslo region first appears in the upper part of the Nicholsonograptus fasciculatus Zone and reaches to the middle part of the upper section of the P. elegans Zone.

In other creeks of the Cerro Viejo, a stratigraphic gap is present between the middle and the upper members of the Los Azules Formation (Ortega, 1987; 1995). There graptolites of the upper member were assigned to the Climacograptus bicornis Zone of the early Caradoc. The upper member is absent (most likely removed by erosion prior to deposition of the unconformably overlying Carboniferous strata) from the Los Gatos section. The youngest beds of the middle member exposed in the Los Gatos creek, still lies within the Da 4a and the typical assemblages of the Hustedograptus teretiusculus Zone (e.g., Pseudamplexograptus distichus, the first appearances of the Dicellograptus and Dicranograptus clades) seem to be missing. As previously recognized (Maletz, 1997b) the name of this biozone should be changed as the nominal taxon makes its first appearance in earlier biozones.

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Received: February 15, 2003

  Accepted: June 15, 2003