Middle Ordovician graptolite faunas of the Gualcamayo Formation (Middle Member) in the Corridita Creek Section, northern Precordillera, Argentina  

Beatriz Máspero Castro¹, Gladys Ortega^1,2 and Guillermo L. Albanesi^1,2  

¹ Museo de Paleontología, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba. Casilla de Correo 1598, 5000 Córdoba, Argentina. E–mail: gcortega@arnet.com.ar

² Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET).

Key words: Graptolites. Biostratigraphy. Gualcamayo Formation. Precordillera. Argentina.

The Gualcamayo Formation (Furque, 1963), which is mostly composed of graptolitic black shales, crops out in the Central and Eastern Precordillera, western Argentina. The type area of this unit is placed between the Guandacol and Gualcamayo rivers showing extensive exposures. The Las Plantas Creek succession was considered the most complete section of the Gualcamayo Formation by Astini (1995). This author divided the unit into three members. The lower member is made up of black shales and limestones, the middle member contains claystones and siltstones, and the upper member is integrated by mudstones and calcareous blocks. The age of the succession is referred to as the Middle Ordovician according to its graptolite faunas. The Isograptus victoriae maximus, Oncograptus upsilon, Cardiograptus morsus, Undulograptus austrodentatus and Pterograptus elegans zones were recognized in the Gualcamayo Formation (Cuerda and Furque, 1975; Ortega et al., 1993; Brussa and Astini, 1998; Ortega and Albanesi, 1999; Ortega and Máspero Castro, 2002).

The Corridita Creek is localized a few kilometers southwest of Guandacol Village, in the northern Precordillera. It is an approximately N-S trending tributary of the Gualcamayo River (Figure 1). The Corridita Creek is considered as one of the classic localities for studying the rich graptolite faunas of the Gualcamayo Formation (Turner, 1960; Brussa and Astini, 2001; Máspero Castro, 2002; Ortega and Máspero Castro, 2002).

The Gualcamayo Formation conformably overlies the San Juan Formation in the Corridita Creek section. In the basal part of the unit a succession of interbedded limestones and black shales, corresponding to the lower member, is identified. This sequence is overlain by claystones of the middle member. The lower part of the Gualcamayo Formation is folded and faulted through the studied section. About 160 m of grey siltstones are conformably overlying these levels. The sequence is interrupted by a fault that suppresses the upper member of the Gualcamayo Formation at this section. Fine gray sandstones of the Trapiche Formation (Caradoc-Ashgill) overlie the Gualcamayo Shale.

Figure 1. Location map and geology of the study area.

The studied graptolite fauna, collected in the middle member of the Gualcamayo Formation, is scarce and poorly preserved (Figures 2, 3). A graptolite assemblage assigned to the Arienigraptus zhejiangensis Subzone (Undulograptus austrodentatus Zone) was identified in the lower part of the Gualcamayo Formation at Corridita Creek, suggesting a Darriwilian (Da1) age for these layers (Brussa and Astini, 2001). Our first graptolite records are placed above the A. zhejiangensis assemblage. A graptolite fauna composed by Tetragraptus bigsbyi, Isograptus victoriae divergens, Arienigraptus zhejiangensis, Paraglossograptus tentaculatus, and Undulograptus sp. cf. U. dentatus occurs ca. 70 m above the base of the Gualcamayo Shale (point 4, Figure 2), in the middle member. Tetragraptus headi, Pseudotrigonograptus ensiformis, Paraglossograptus tricornis, Cryptograptus antennarius, Arienigraptus angulatus, and Arienigraptus sp. are recorded few meters above the former assemblage. The mentioned fauna corresponds to the U. dentatus Zone of Darriwilian (Da2) age, following the North American scheme (Maletz, 1997).

Similar graptolite assemblages are present in the lower member of the Los Azules Formation at Cerro Viejo of Huaco (Mitchell et al., 1998), in Central Precordillera, and in the Las Plantas Creek, southwest Guandacol town (Máspero Castro, 2002), in the northern Precordillera.

Figure 2. Stratigraphic column, species ranges, and biozones of the Gualcamayo Formation at Corridita Creek.

Figure 3. A, Pterograptus elegans Holm, CORD-PZ 31195-A; B, Pseudophyllograptus sp., CORD-PZ 31242-b; C, Tetragraptus headi (Hall), CORD-PZ 31061-B; D, cf. Brachiograptus etaformis Harris and Keble, CORD-PZ 31133; E, F, Holmograptus lentus (Törnquist), E, CORD-PZ 31079; F, CORD-PZ 31072-B; G, Arienigraptus zhejiangensis Yu and Fang, CORD-PZ 31122; H, Arienigraptus angulatus (Mu), CORD-PZ 31072-A; I, J, Undulograptus sp. cf. U. dentatus (Brongniart), I, CORD-PZ 31066-B, J, CORD-PZ 31066-B; K, Archiclimacograptus sp. cf. A. riddellensis, CORD-PZ 31431; L, Haddingograptus sp. cf. H. oliveri (Boucek), CORD-PZ 31099; M, Glossograptus sp., CORD-PZ 31431; N, Cryptograptus antennarius (Hall), CORD-PZ 31063; O, Cryptograptus schaeferi Lapworth, CORD-PZ 31090-B; P, Kalpinograptus parallelus (Ni), CORD-PZ 31208-B.

The FAD of Holmograptus lentus is placed 130 m above the base of the Gualcamayo Formation (point 8). At this level Dichograptus sp., Tetragraptus quadribrachiatus, P. tentaculatus, P. tricornis, Isograptus caduceus caduceus, A. angulatus, and Undulograptus sp. cf. U. dentatus are also recorded. This graptolite assemblage is referred to as the H. lentus Zone, ranging up next 23 m. Acrograptus sp., Xiphograptus sp., Pseudophyllograptus sp., cf. Brachiograptus etaformis, and P. tentaculatus were collected in the upper part of the biozone. Taxonomic diversity and abundance are remarkably scarce in this interval.

The appearance of H. lentus suggests a Darriwilian (late Da2) age, in accordance with the Baltic and North American biostratigraphic schemes (Maletz, 1995, 1997). Strata of same age seem to be present in the uppermost part of the lower member of the Los Azules Formation at Cerro Viejo section (see Brussa et al. and Ortega and Rickards, this volume). The real extension of cited graptolite assemblage is unknown because the section is covered through the next 20 m (up to point 11). Representative taxa of the Da3 assemblages (e.g., I. forcipiformis, B. crawfordi, and N. fasciculatus) were still not recorded in the Precordillera. Da3 age horizons were not recognized at Corridita Creek; however, they may be present in the covered interval of the sequence. This interval is apparently suppressed by a hiatus in the Cerro Viejo section.

At point 11, the appearance of Pterograptus elegans, Cryptograptus schaeferi, Glossograptus sp., Hustedograptus sp., Archiclimacograptus sp. cf. A. riddellensis, and Haddingograptus sp. cf. H. oliveri indicates the P. elegans Zone, late Darriwilian (Da4a) in age (Ortega and Máspero Castro, 2002). Kalpinograptus parallelus is localized a few meters above the exposed base of the biozone. This graptolite assemblage ranges up to the uppermost layers of the middle member. It can be correlated with similar faunas from the Los Azules Formation (middle member) at Cerro Viejo section (Ortega, 1987; 1995; Brussa et al., this volume), and the Gualcamayo Formation (middle member) in the Cerro Potrerillo area (Ortega and Albanesi, 2000), about 40 km to the south of the studied locality.

Black shales of the Corridita Creek section yielded a few conodont elements. These specimens are preserved as casts on bedding plane surfaces from particular levels of the middle member. The presence of Polonodus magnum in strata of the P. elegans Zone is significant. This species can be considered a key taxon for the Precordillera because of its persistent association with Pygodus anitae, which indicates the upper part of the Eoplacognathus suecicus Zone (sensu Bergström, 1983), in numerous sections of the region (e.g., Albanesi et al., 1998; Ottone et al., 1999). The conodont assemblage of the middle member includes recurrent taxa, such as, P. magnum, P. anitae, Paroistodus horridus, Periodon aculeatus, Drepanodus reclinatus, Protopanderodus spp., Drepanoistodus spp., which represent a slope, deep-cold water, biofacies for the Gualcamayo Formation and equivalent units (Albanesi and Ortega, 1998).

A graptolite fauna referable to the U. dentatus Zone is mentioned for the Gualcamayo Formation at Corridita Creek.

The FAD of H. lentus is recorded for the first time in the Argentine Precordillera, allowing the recognition of the homonymous biozone.

The identification of the P. elegans Zone in the Gualcamayo Formation at Corridita Creek, extends its presence to the northern Precordillera.

Investigated graptolite assemblages allow us to assign the middle member of the Gualcamayo Formation, at Corridita Creek, an Early Darriwilian (Da2) to Late Darriwilian (Da4a) age.

The P. elegans graptolite Zone corresponds to the upper part of the E. suecicus conodont Zone, allowing a precise world-wide correlation.

The authors thanks to the Universidad Nacional de Córdoba and CONICET for support received to accomplish the present contribution.

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Received: February 15, 2003

Accepted: June 15, 2003