A Darriwilian (Middle Ordovician) graptolite fauna of the Lower Member of the Los Azules Formation, Cerro Viejo, San Juan Precordillera, Argentina
Gladys Ortega1 and Barrie Rickards2
1CONICET, Museo de Paleontología, Universidad Nacional de Córdoba, c.c. 1598, 5000 Córdoba, Argentina. E–mail: firstname.lastname@example.org
2 Department of Earth Sciences, Downing Street, Cambridge CB2 3EQ, Great Britain.
Key words: Graptolites. Los Azules Formation. Darriwilian. Ordovician. Argentine Precordillera
The Ordovician black shales of the Cerro Viejo of Huaco were originally mentioned by Borrello and Gareca (1951) who reported Nemagraptus gracilis in its upper part. The succession was known by different names in the geological literature (Harrington and Leanza, 1957; Turner, 1960) until Cuerda and Furque (1975) referred it to the Los Azules Formation, considering a Llanvirn to Caradoc age for these rocks. The unit crops out on the western flank of the Cerro Viejo, between 30º 11’ 40" and 30º 15’ 30" S Latitude, and 68º 34’30" and 68º 35’20" W Longitude, in the Central Precordillera, San Juan Province.
The Los Azules Shale is composed of three members informally named as lower, middle and upper members, cropping out in many sections on the western flank of the Cerro Viejo anticline (Ortega, 1987). The lower member (ca. 6 m thick) is made up of silicified black shales and K-bentonite levels which conformably overlie the San Juan Limestone. A rich graptolite fauna occurs. It was originally assigned to the Paraglossograptus tentaculatus Zone (Alfaro and Cuerda, 1986; Ortega, 1995) and subsequently to the Undulograptus austrodentatus (Darriwilian Da1) and Undulograptus dentatus (Da2) zones (Mitchell et al., 1998).
A study of the K-bentonites of the Cerro Viejo succession infers an isotopic age of approximately 464 Ma. for the lower levels of the Los Azules Formation (Huff et al., 1997).
These strata are covered by brownish micaceous sandstones of variable thickness, and succeeded by grey siltstones of the middle member. In these deposits the Pterograptus elegans and Hustedograptus teretiusculus zones (Darriwilian Da4) (Ortega, 1987) were recognized. The basal micaceous sandstone yielded conodonts of the Pygodus anitae Subzone (Eoplacognathus suecicus Zone) (Ottone et al., 1999).
The upper member comprises by calcareous siltstones and subordinate mudstone lenses. It is only recognized in the Los Azules, Amarilla, and Árbol Seco creeks. The graptolite fauna correspond to the Climacograptus bicornis Zone (Gisbornian, Gi2) and it is associated with conodonts of the Amorphognathus tvaerensis Zone (Ottone et al., 1999). Two different palynological assemblages were identified in the Los Azules Formation (Ottone et al., 1999; 2001).
A stratigraphic gap is present between the middle and upper members embracing the N. gracilis Zone interval, and possibly the uppermost part of the H. teretiusculus Zone. The Los Azules Formation is unconformably overlain by Carboniferous strata of the Guandacol Formation.
Figure 1. Stratigraphic ranges of graptolites in the lower member of the Los Azules Formation.
Graptolites present in the lower member of the Los Azules Formation are discussed below, and the ages of two different associations are defined (Figures 1, 2).
A rich graptolite fauna including Loganograptus logani, Pseudobryograptus parallelus, Pseudotrigonograptus ensiformis, Tetragraptus bigsbyi, T. serra, T. reclinatus, T. headi, T. quadribrachiatus, Xiphograptus lofuensis, Holmograptus bovis, Holmograptus sp. nov., Acrograptus sp., Glossograptus acanthus, Glossograptus sp., Paraglossograptus tentaculatus, P. tricornis, Cryptograptus antennarius, Arienigraptus zhejiangensis, Isograptus victoriae divergens, I. caduceus caduceus, Undulograptus austrodentatus, U. sinicus, U. dentatus, U. primus, and U. cumbrensis occurs in the basal part of the lower member of the Los Azules Formation. Most of these graptolites were recorded in the lower part of the Gualcamayo Formation, northern Precordillera (Ortega and Albanesi, 1999).
Approximately 0,60 m above the base of the member, Arienigraptus angulatus and Arienigraptus sp. make their appearance. Some forms, such as I. v. divergens are restricted to the base of the unit, while others, e.g., T. headi, are just localized in the middle part of the lower member. A. zhejiangensis, A. angulatus and U. dentatus range upwards throughout the middle part of the member but they were not recorded in our collections from the uppermost strata.
This graptolite assemblage can be referred to the U. dentatus Zone in accordance with the presence of the nominal species, and A. angulatus and Arienigraptus sp. It correlates with the U. dentatus Zone of the North America (Maletz, 1997 b) and the Undulograptus intersitus Zone of Australasia (VandenBerg and Cooper, 1992).
In the upper part of the lower member, a thick K-bentonite bed (ca. 0,50 m) contains Pseudobryograptus sp. (proximal ends and young colonies), P. ensiformis, Tetragraptus sp. (a reclinated form), X. lofuensis, X. cf. disermus, Brachiograptus etaformis, I.. c. caduceus, A. angulatus, Arienigraptus sp., Glossograptus sp., P. tentaculatus, Cryptograptus sp. (siculae and early stages), and U. primus. The appearance of Archiclimacograptus, Haddingograptus and Hustedograptus allow reference to the Holmograptus lentus Zone (upper Da2), following the scheme of Maletz (1995) (see Brussa et al., this volume). This author placed the base of the zone at Arenig-Llanvirn boundary. It is possible to correlate this interval with the H. lentus Zone from Scandinavia, Germany and Belgium (Maletz, 1995), and Point Lévis, Canada (Maletz, 1997 b). In the Precordillera, a similar assemblage was studied in the Corridita creek, to the north of the Cerro Viejo area (Máspero Castro et al., this volume), where scarce rhabdosomes of H. lentus were found in the middle member of the Gualcamayo Formation.
We do not have the definitive fauna of Da3 age, equivalent to the Nicholsonograptus fasciculatus Zone of Scandinavia and North America (Maletz, 1995; 1997a, b) or Diplograptus? decoratus of Australasia (VandenBerg and Cooper, 1992). It is possible that rocks of this age are not represented in the Cerro Viejo area. If that is the case, a hiatus might be present between the lower and middle members of the Los Azules Formation. The record of Holmograptus spinosus in the Sierra de La Invernada Formation (Brussa, 1999) seems to indicate that faunas of Da3 age are present in the Precordillera.
The basal coarse sandstone of the middle member bears conodonts, inarticulate brachiopods uniserial stipes and biserial graptolite remains. Immediately above this sandstone we record the entrance of Pterograptus elegans, Acrograptus euodus, Kalpinograptus parallelus and Wuninograptus sp., associated to Reteograptus geinitzianus, Cryptograptus schaeferi, and biserial graptolites (e.g., Archiclimacograptus, Haddingograptus, Hustedograptus, and Eoglyptograptus) which become more abundant constituents of the fauna. This graptolite assemblage was assigned to the P. elegans Zone, late Darriwilian (Da4a) in age (Ortega, 1995). Equivalent faunas were described in the Potrerillo Range and the Corridita creek, northern Precordillera (Ortega and Albanesi, 2000; Máspero Castro et al., this volume).
Figure 2. Graptolites of the lower member of the Los Azules Formation. a-b, Cryptograptus sp., a, CORD-PZ 18564-B, b, CORD-PZ 18574-A; c-d, Arienigraptus sp., c, CORD-PZ 18565-B, d, CORD-PZ 18578-B; e, Xiphograptus lofuensis (Lee), CORD-PZ 12368 ; f, Holmograptus bovis Williams and Stevens, CORD-PZ 11591; g, Isograptus victoriae divergens Harris, CORD-PZ 13989; h, Cryptograptus antennarius (J. Hall), CORD-PZ 12365; I, Loganograptus logani (J. Hall), CORD-PZ 14395; j, k, Undulograptus sinicus (Mu and Lee), j, CORD-PZ 12252, k, CORD-PZ 12273; l, Isograptus caduceus caduceus (Salter), CORD-PZ 11646; m, Glossograptus sp., CORD-PZ 11593; n, Undulograptus austrodentatus (Harris and Keble), CORD-PZ 14000; o, Undulograptus dentatus (Brongniart), CORD-PZ 13988; p, Arienigraptus angulatus (Mu), CORD-PZ 18591; q, Arienigraptus zhejiangensis Yu and Fang, CORD-PZ 11648; r, Xiphograptus cf. disermus (Ni), CORD-PZ 12387; s, Undulograptus cumbrensis (Bulman), CORD-PZ 11882-A; t, Undulograptus primus (Leg), CORD-PZ 12241; u, Archiclimacograptus sp., CORD-PZ18610. All drawings 4x, except for i: 0.7 x.
The authors thanks continued support by CONICET for graptolite studies, and Dr. G. L. Albanesi for constructive comments.
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Received: February 15, 2003
Accepted: June 15, 2003