Middle Ordovician graptolites from the Sierra de Rinconada,
northern Puna, Argentina
and Gustavo Rodríguez3
1 Consejo Nacional de Investigaciones
Científicas y Técnicas (CONICET). Departamento de Geología y Paleontología,
CRICyT (IANIGLA), Av. Ruíz Leal S/N, Parque General San Martín, 5500 Mendoza,
Argentina. E–mail: firstname.lastname@example.org
2 Consejo Nacional
de Investigaciones Científicas y Técnicas (CONICET). Cátedra de Paleontología
I, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa,
Uruguay 151, 6300 Santa Rosa, Argentina. E–mail: email@example.com
3 Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). Instituto de Geología y Minería, Universidad Nacional de Jujuy, Av. Bolivia 1661, 4600 San Salvador de Jujuy, Argentina. E–mail: firstname.lastname@example.org
Keywords: Graptolites. Llanvirn. Ordovician. Puna. Argentina.
A new Ordovician graptolite assemblage from the northern Puna of Argentina is presented. The taxonomic study and the discussion of their biostratigraphic implications are the main objectives of this work. The fossils were collected in the northeastern sector of the Sierra de Rinconada, in Jujuy Province, northwestern Argentina.
The Sierra de Rinconada is characterized by the presence of a thick marine Lower Paleozoic sedimentary succession (ca. 3.500 m thick) which has been assigned to the Puna Turbidite Complex by Bahlburg et al. (1990). These authors proposed an stratigraphic scheme of two facies units: the Volcanosedimentary Succession (Middle and Upper Arenig) and the Puna Turbidite Complex (Uppermost Arenig – Llandeilo (? Lower Caradoc) from the northern Puna, and suggest that the sedimentary succession was deposited as a response to the change from an extensional regime to a foreland basin setting from the Lower to the Middle Ordovician.
In the studied area, regular trains of upright N–S – trending folds, associated faults and cleavage, developed as a result of a major episode of crustal shortening known as Ocloyic Orogeny (Turner and Mendez, 1979; Mon and Hongn, 1987). The base of the Ordovician succession of northeastern Sierra de Rinconada is not exposed. The outcropping levels are represented by two different lithofacies association. The lower unit, which has a minimum thickness of 1200 meters, is composed by a number of thinning – finning upwards cycles of thick – bedded lithic sandstones, shales and siltstones. The upper unit consists of an homogeneous succession (up to 200 m thick) of banded black shales, siltstones and sporadic sandstones. The lower unit is folded into a single east – verging, large, 16 km long, 1.2 km wavelength Minas Azules anticline (Rodríguez and Hongn, 2001). The graptolite fauna was collected from two fossiliferous localities on the eastern flank of this anticline. The first one is close to Santa Catalina town (A, in Figure 1), whereas the second one namely Minas Azules (B, in Figure 1), lies about 10 km to the south. The graptolite bearing beds lie about 100 m east of the axial plane of the Minas Azules anticline, and in accordance to the observed stratigraphic relationships they could be correlated with.
Figure 1. Location map of the Sierra de Rinconada and fossiliferous locations.
In the northern Argentine Puna graptolite records to date are scarce. In the last years, Gutiérrez Marco et al. (1996) reviewed the taxa described by Aceñolaza (1980) and the associated graptolites found in the northwest of the Cordón de Escaya, assigning an age close to the Arenig–Llanvirn boundary (Undulograptus austrodentatus Zone). In addition to these records Toro and Brussa (1997) mentioned the first diplograptids in the eastern outcrops of the same area, confirming the previously attributed age by Gutierrez Marco et al. (1996). In contrast, Arenig–Llanvirn graptolites from numerous stratigraphic sections, located to the south of the studied area, were reviewed by Bahlburg et al. (1990), and Monteros et al. (1996) described an Arenig graptolite fauna from the Huaytiquina area.
Paleontological and biostratigraphical aspects
The graptolites are regularly preserved and some specimens exhibit signs of tectonic deformation. They are generally flattened, as remnant of periderm material, and sometimes as piritized semirelief or moulds in fine sandstones and siltstones. Because of the above expressed, it was not possible to observe details of the proximal development of the colonies.
In the Santa Catalina locality (A, in Figure 1), large rhabdosomes (up to 30 mm long) which rapidly widen (up to 2.5 mm) and, exhibit conspicuous apertural excavations and short, straight supragenicular walls, are common. They are assigned to Pseudamplexograptus sp. cf. P. distichus (Eichwald, 1840). Their proximal ends are blunt; a virgellar spine and two strong subapertural spines, developing at th11 and th12 respectively, are usually observed (Figure 2). Other specimens with similar proximal end type are associated to the previously described, but they differ from these on the smaller size of their rhabdosomes (up to 1.6 mm width; up to 24 mm long). However, these different dimensions could be preservational, due to the distinct orientation respect to the slab deformation, as previously was noted by Maletz (1997). Pseudamplexograptus sp. cf. P. distichus was recorded in the uppermost Llanvirn in the Albjära, Sweden, and is common at comparable levels in Scania and Norway (Maletz, 1995, 1997).
In the Minas Azules locality (B, in Figure 1) a number of mature, gracile (up to 0,6 mm width) rhabdosomes are associated with several proximal ends, which exhibit classic spines on the th 1. They are assigned to Dicellograptus salopiensis Elles and Wood, 1904. This species is commonly recorded in the Nemagraptus gracilis and Climacograptus bicornis zones, but its first records occurs in the Hustedograptus teretiusculus Zone in Scandinavia and Great Britain (Williams, 1995). Other biserial graptolites, which show pseudoclimacograptid proximal end type, and progressive widening, are present in the same levels. However, the characteristic zigzag median septum was not observed in our specimens, therefore they were assigned with reserve to the genus Pseudoclimacograptus (Figure 2).
Correlation with other graptolites sequences
In the stratigraphic scheme presented by Bahlburg et al. (1990: Figure 8) most of the analyzed sections range from the uppermost Arenig to the Llanvirn. These authors inferred a Llanvirn age for the Sierra de Rinconada section, based on one fragmented specimen of Didymograptus s. l. associated with poorly preserved biserial graptolites.
The assemblage presented in this paper constitutes the first graptolites record in the northern part of the Sierra de Rinconada. Its biostratigraphic significance allows to add new data to the above mentioned scheme. The presence of Pseudamplexograptus sp. cf. P. distichus in the Santa Catalina locality and the records of Dicellograptus salopiensis associated with Pseudoclimacograptus? sp. in the Minas Azules locality, would indicate the uppermost Llanvirn (Da 4), and therefore extends the cronostratigraphic range of the Sierra de Rinconada sedimentary deposits.
Ramos (1972) mentioned the presence of Glyptograptus euglyphus var. linensis nov. var. in the Sierra de Lina section, suggesting a Caradoc age to the bearing–strata, whereas Bahlburg et al. (1990) inferred a Llanvirn to Llandeilo age for the same section due to the finding of similar forms, assigned to Glyptograptus sp., associated with "Glossograptus hincksii fimbriatus" and Eoglyptograptus cf. dentatus. The age from the studied graptolite assemblage suggests that the bearing levels could be correlated, partially at least, with those from the Sierra de Lina. Moreover, the new data allows to establish a possible correlation with the bearing levels with dicellograptids from the Sierra de Cochinoca / Cerro Queta section, assigned to a Llandeilo and probably Lower Caradoc by Bahlburg et al. (1990).
Müller et al. (1996) and Erdtmann and Suarez–Soruco (1999) listed Nemagraptus gracilis, Dicellograptus spp., Dicranograptus spp. and Orthograptus sp. cf. O. calcaratus in the westernmost part of the Bolivian Cordillera Oriental. Erdtmann (personal communication) has noted that the so–called Atocha Segment is the direct northern continuation of the Puna of Jujuy (Santa Catalina, Sierra de Rinconada). Therefore, until the present we can not verify a younger age than Late Llanvirn (Llandeilian) for the studied area strata.
Figure 2. Late Llanvirn graptolites from Sierra de Rinconada. A, D, Pseudoclimacograptus sp., IANIGLA-PI 1020 (x6), IANGLA-PI 1021 (x7), respectively; B-C, Dicellograptus salopiensis, IANGLA-PI 1022 (x7,5), IANIGLA-PI 1023 (x7), respectively; E-G, Pseudamplexograptus sp. cf. P. distichus, IANIGLA-PI 1024 (x6), IANIGLA-PI 1025 (x5,5), IANIGLA-PI 1026 (x5,5).
The fossiliferous samples were collected during the field work of the Ph.D. thesis of G.R. by Carlos Zavala and himself. We are grateful to Charles E. Mitchell for his valuable suggestions.
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Received: February 15, 2003
Accepted: June 15, 2003