of the Tremadocian graptolite sequence of the Famatina System, NW Argentina
Juan Carlos Gutiérrez–Marco1 and Susana B. Esteban2
de Geología Económica (CSIC‑UCM), Facultad de Ciencias Geológicas, 28040
Madrid, España, and Academia Nacional de Ciencias, Av. Vélez Sarsfield 229,
Casilla de Correo 36, 5000 Córdoba, Argentina.
2 Instituto Superior de Correlación Geológica (CONICET‑Universidad Nacional de Tucumán), Facultad de Ciencias Naturales e IML, Miguel Lillo 205, 4000 Tucumán, Argentina. E–mail: email@example.com
Key words: Graptolites. Tremadocian. Famatina System. Biostratigraphy. Palaeoenvironment. Palaeogeography.
Tremadocian graptolites were first reported from the Famatina System (La Rioja and Catamarca provinces, NW Argentina), in the early XX Century (Bodenbender, 1911 to 1922) and were later identified or described by Turner (in Harrington and Leanza, 1957; Turner, 1960), Toselli (1975), Aceñolaza and Durand (1984), Esteban and Gutiérrez–Marco (1987) and Tortello and Esteban (1999), among others. According to these studies, the graptolites occurred mainly in the black shales of the upper member of the Volcancito Formation, which were correlated with some uncertainty to the upper Lower Tremadocian on the basis of the graptolite biostratigraphy and also to the range of the Parabolina (N.) frequens argentina and the Kainella meridionalis trilobite biozones. Erdtmann (1982) regarded the graptolite association to be representative of the Matane–type ‘dictyonemids’ and to be allochthonous to deep areas of a back–arc basin.
Renewed interest in the Volcancito Formation during the last decade resulted in additional detailed knowledge of trilobite and conodont distributions, which allowed for the precise correlation of the Cambrian–Ordovician boundary within the middle of the lower member of the formation (Tortello and Esteban, 1999; Albanesi et al., 1999, 2000, a.o.) and for the first identification of upper Tremadocian conodonts and graptolites from the younger Bordo Atravesado Formation (Esteban, 2002), previously considered to be an upper member of the Volcancito Formation (Albanesi et al., 2001).
The present work is a taxonomic and biostratigraphic reappraisal of the abundant graptolite record from the upper member of the formation, which is merited by Cooper’s (1999) recent review and correlation of the Tremadocian chronozones and global graptolite assemblages.
The graptolite sequence
The oldest Ordovician graptolites occur in the upper part of the lower member of the Volcancito Formation in its holostratotype section at Volcancito river on the eastern slope of Sierra de Famatina in La Rioja province (Figure 1). The assemblage was figured by Tortello and Esteban (1999) and is now considered to be representative of the Early Tremadocian Rhabdinopora flabelliformis parabola chronozone. Graptolite assemblages from the upper member of the formation are mainly from the complementary section of Peña Negra, 4 km from the type section at Volcancito river. Bed–by–bed collecting of graptolites through the 400 m thick sequence of monotonous black shale of the upper member of the Volcancito Formation in both sections resulted in the discovery of a long vertical range for Rhabdinopora flabelliformis flabelliformis in the Peña Negra section (Figure 2). A shorter distribution was documented for two other homeomorphic forms of R. praeparabola and R. f. parabola (herein provisionally named as R. turneri n.n. and R. f. acenolazai n.n., respectively), which also extend upwards to the lower beds in the Volcancito river section. Although the morphological resemblances of the new taxa suggest correlation with Tremadocian graptolite "Assemblage 1", the occurrence of Anisograptus matanensis at the basal levels on the Peña Negra section indicating that R. turneri n.n. and R. f. acenolazai n.n. clearly postdate the globally distributed "Assemblage 1", the top of which is defined by the appearance of the rhabdosomes with triradiate development exemplified by Anisograptus.
On the basis of this biostratigraphical review, the lower part of the upper member of the Volcancito Formation is correlated with the A. matanensis and R. f. anglica graptolite biozones of the upper Lower Tremadocian (Figure 2). The upper part of the Peña Negra section (see Figures 2) and also the entire black shale sequence of the Volcancito river section probably correlate with the lowest Upper Tremadocian (?Adelograptus Zone). The black shale of the upper member of the Volcancito Formation is in fault contact with the lower member in the Volcancito river section with the fault cutting out a large part of the stratigraphy recorded at the complementary Peña Negra section. Both sections were considered largely time equivalent until the present study. Our correlation of the Volcancito Formation is consistente with the occurrence of Upper Tremadocian conodonts and graptolites, namely the Paltodus deltifer conodont biozone and the graptolitic "Assemblage 4", in the overlying Bordo Atravesado Formation as reported by Albanesi et al. (2000, 2001).
Figure 1. Location of the studied area. 1. Map of the South America showing in black the placement of next figure in the NW of Argentina (Catamarca and La Rioja provinces). 2. Geology of the Famatina System with main outcrops of sedimentary Ordovician rocks: A. Sierra de las Planchadas; B. Chaschuil; C. Cerro Negro de Rodríguez; D. La Alumbrera river; E. Morro de Las Planchadas; F. Cerro El Pintado; G. Cachiyuyo river; H. Volcancito river–Peña Negra; I. Cuchilla Negra; J. Bordo Atravesado. Symbols: a. post–Paleozoic rocks; b. Ordovician sedimentary rocks; c. other Paleozoic rocks; d. provincial administrative border. 3. Detail of the area west of the Famatina city, showing the location of the graptolite localities and fossiliferous sections of Peña Negra and Volcancito river.
Figure 2. Stratigraphic column for the lower and upper members of the Volcancito Formation (uppermost Cambrian to Tremadocian) in the Volcancito river and Peña Negra sections, respectively. Vertical ranges of the graptolites reported and the general record of other fossil groups are indicated, as well as the correlation of some biostratigraphical and chronostratigraphical units based on graptolites, conodonts and trilobites. Younger upper Tremadocian graptolitic beds from the upper Member of the formation at the Volcancito river section (with Rhabdinopora and ?Bryograptus) are not included in this scheme. Symbols: a, black shales; b, marls; c, fine sandstones; d, calcareous shell beds; e, grey shales; f, lowest stratigraphic record of the trilobite genus Jujuyaspis. Abbreviations: A, Anisograptus; R., Rhabdinopora; R.f., Rhabdinopora flabelliformis; R. pr., Rhabdinopora praeparabola; C., Cordylodus; «Iapet.», Iapetognathus (local biozone defined by the first record of I. aengensis); N., Neoparabolina; K., Kainella. Conodont biostratigraphy after Albanesi et al. (1999, 2000)
Figure 3. Graptolites from the upper member of the Volcancito Formation at Peña Negra section. Stratigraphic position for each specimen is indicated by the stratigraphic level in meters above the base of the member. Compare with Figure 2 for additional details on the age and biostratigraphy. a–b. Rhabdinopora turneri n.n., 9 m, PIL 14491a and b (both x 3); c. h. Anisograptus matanensis Ruedemann, 110 m, PIL 14513 and 14805, respectively (both x 4,2); d. ?Dictyonema sp. 2, mature stipe with long bifurcate processes (ex “Airograptus”), 246 m, PIL 14814 (x 9); e. Rhabdinopora flabelliformis acenolazai n.n., outline of the proximal region of a typical specimen, 221 m, PIL 14482 (x 3); f. Rhabdinopora flabelliformis flabelliformis (Eichwald), detail of the mesh of a “patulum” morphotype, 168 m, PIL 13781 (x 3); g. Aspidograptus sp. nov., 27 m, PIL 14815 (x 0,8).
In addition to planktic graptolites, some transported remains of poriferans and benthic graptolites were collected from the upper member of the Volcancito Formation at Peña Negra. The benthic graptolites are herein identified as Dendrograptus sp., Dictyonema sp. 1, ?Dictyonema sp. 2 (ex "Airograptus"), Aspidograptus sp. and ?Pseudocallograptus sp. We report the first occurrence of a benthic graptolite in the Bordo Atravesado Formation. It is identified as ?Dictyonema sp. cf. D. cordillerensis and has remarkably sinuose and parallel stipes that also resemble those of Dictyonema yichangensis.
Environmental and paleogeographical setting
The black graptolite shales of the upper member of the Volcancito Formation are a record of sedimentation under anoxic conditions in an outer shelf to basin plain along the marging of western Gondwana. The anoxic conditions probably developed under a restricted upwelling area with high productivity that was a favourable habitat for some endemic holopelagic and mesopelagic rhabdinoporinids. The decrease in graptolite diversity towards the middle and upper parts of the upper member is illustrated by the generalized absence of typical batipelagic forms such as Psigraptus or some delicate anisograptid members, as well as the adelograptids that predominates in the epipelagic shelf biotope in beds that have been tentatively attributed here to the lower Upper Tremadocian.
Although sedimentation of the Volcancito Formation is considered to have occurred in a back–arc basin in the western part of Gondwana, graptolite paleoecology and the absence of Lower to Lower/Upper Tremadocian volcanic rocks indicates that, instead, sedimentation took place before birth and emersion of the Famatinian Volcanic Arch.
Authors acknowledge F.G. Aceñolaza and G. Ortega for their constructive suggestions. We also thank G. Valdivieso for their help in the field and D. Ruiz Holgado for the drawings. S. Finney edited the manuscript. Financial support has been provided by the Instituto Superior de Correlación Geológica.
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Received: February 15, 2003
Accepted: June 15, 2003