REVISTA TECNICA DE YPFB - VOLUMEN 21 - SEPTIEMBRE 2003
LOWER TO MIDDLE ORDOVICIAN GRAPTOLITE BIOSTRATIGRAPHY OF SOUTHERN BOLIVIA lORG MALETZ (1) & SVEN O. EGENHOFF (2) (I) Department of Geology. State University of New York at Buffalo. 772 Natural Sciences and Mathematics Complex. Buffalo. New York 14260-3050. U.S.A .• e-mail:
[email protected].; (2) Institut fUr Geologie. TU Bergakademie Freiberg. Bemhard-von-Cotta-Str. 2. 0-09599 Freiberg. Germany. e-mail:
[email protected].
ABSTRACT The graptolite biozonation of Southern ' Bolivia includes the Rhabdinopora flabelliformis. Adelograptus sp.• Araneograptus murrayi. Hunnegraptus copiosus. Tetragraptus phyllograptoides. Expansograptus protobalticus. Expansograptus holmi. Baltograptus minutus. Azygograptus lapworthi and lsograptus victoriae Biozones. In the Middle Ordovician the Urbanekograptus retioloides Biozone of Llandeilian age is differentiated. The Lower Ordovician succession of southern Bolivia is the most complete one documented from South America. but the Middle and Upper Ordovician is poorly known. The graptolite faunas belong to the Atlantic graptolite faunal province and show distinct differences to Pacific faunal province faunas of the Argentine Precordillera.
INTRODUCTION An extremely thick Lower to lower Middle Ordovician succession of siliciclastic rocks with strongly diluted. but biostratigraphically highly significant graptolite faunas are present in southern Bolivia (Figs. 1.2). Egenhoff (2000) documented the lithostratigraphy and palaeogeography of the region. encompassed by the provinces of Tarija. Potosi and Chuquisaca and redefined the lithostratigraphic units. The graptolite faunas are moderately well preserved. but often strongly tectonized. They belong to the Atlantic faunal province. like most of the faunas from South America. except for the Pacific type faunas found in the Argentine Precordillera (Maletz & Ortega 1995) and a still unexplained Didymograptellus bifidus lJiozone fauna from the eastern Cordillera of Argentina (Toro 1994). Similar Atlantic type faunas are well known from northern Argentina (Moya et al .• 1994; Monteros et al .• 1996; Toro. 1993; 1996; 1997a.b; 1999a.b). Here. the successipn is more incomplete and is cut off in the early Arenig. whereas in southern Bolivia younger mid- to late Arenig faunas are preserved below a postPalaeozoic unconformity. The presence of Ordovician graptolite faunas in Bolivia is known since Wood (1906) described Llanvirn pendent didymograptids. phyllograptids. diplograptids and glossograptids from Cule (northern Bolivia). Two dendroid graptolites when described shortly afterwards by Courty (1907) from the Tarija area and Steinmann & Hoek (1912) added further to the record of Lower and Middle Ordovician graptolite faunas from southern Bolivia. Bulman (1931) and Turner (1959) gave the first more comprehensive description of the grapt6lite faunas of the whole of South America. Silurian graptolite faunas were more recently discovered in Bolivia and are described by Maletz. Suarez-Soruco & Egenhoff (2002).
The discovery of a useful and complete succession of early Ordovician graptolite faunas in the thick siliciclastic succession of southern Bolivia also had considerable impact on the lithostratigraphic correlation and palaeogeographic interpretation of the region (Egenhoff et al. in press). It enables for the first time to describe a relatively complete succession of faunas and to correlate the lithostratigraphic units (Fig. 2).
GRAPTOLITE BIOSTRATIGRAPHY. Suarez-Soruco (1975) described the Dictyonema flabelliforme to Didymograptus murchisoni zones of the early and middle Ordovician and the Dicranograptus nicholsoni and Orthograptus truncatus zones of the late Ordovician for Bolivia, including data from the southern and northern parts of the country. Maletz. Kley & Reinhardt (1995) identified faunas of the Tetragraptus approximatus Zone of early Arenig age in the Tarija region of southern Bolivia and established a biozonation for this interval. Maletz. Egenhoff & Erdtrnann (1999) and Maletz & Egenhoff (2001) discussed the graptolite faunas around the base of the Arenig and concluded that the faunal succession in southern Bolivia is remarkably complete. These papers also gave range charts for the two most important sections in the region. Cieneguillas and Culpina. The faunal lists include all species collected from the intervals. but not necessarily all faunal elements are associated and the various levels within the individual biozones might include a number of slightly different associations. The definition of the biostratigraphical intervals is always based on local first occurrences (FAD) and the defined units are intended to be local biozones. not chronostratigraphic units. As the individual faunal intervals are fairly thick and the faunas have only been recorded from comparably few beds.
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lORG MALETZ & SVEN O. EGENHOFF
The Adelograptus Biozone.
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Definition: The base of the zone is taken at the first occurrence of Adelograptus spp. at the 475 m level in the Cieneguillas section (coli. Cd-97-1). Fauna: Adelograptus spp. Remarks: A number of Adelograptus species (Fig. 5g) might be present as is indicated from the quite variable size of the specimens, but the material is too poor for a specific identification at the moment. Even though the faunas are poorly known and the taxonomy of the middle to late Tremadoc graptolites is still vague, the interval can easily be identified. In the Cieneguillas 1 section and at Culpina a number of levels with the fauna of this zone have been discovered. (Egenhoff, 2000).
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The Aorograptus victoriae Biozone Fig. 1. Geological map of the study area in southern Bolivia. The investigated Lower Ordovician localities are marked with white stars, the Middle Ordovician lurcuma section is indicated by a black star.
Definition: The base of the zone is taken at the first occurrence of Aorograptus victoriae at Clp97-1 in the Culpina section. The Tremadoc to early Arenig part of the succession was figured by Maletz, Egenhoff & Erdtmann (1999). The complete section was discussed in Egenhoff (2000). The Aorograptus victoriae Biozone is .also present at Cieneguillas (Egenhoff. 2000; Maletz & Egenhoff,2001).
the FAD's of the recorded graptolite species are considered preliminary and more detailed investigation may have to modify the biostratigraphic ranges. Each graptolite zone is defined in a certain section, where the local earliest occurrence of the index species of the interval is documented. The detailed sections with collection levels Fauna: Aorograptus victoriae, Adelograptus spp., ?Tetragraptus sp. are given in Egenhoff (2000). Specimens from the sections were figured previously by Maletz, Egenhoff& Erdtmann (1999) and . Remarks : The zone is easily identified by its name-giver Maletz & Egenhoff (2001). All figured graptolite specimens are Aorograptus victoriae (Fig. 4i), a generally common, but poorly preserved in the type collection at Forschungsinstitut Senckenberg preserved species in certain layers. A number of Adelograptus specimens are associated with Aorograptus victoriae, but due to their (Frankfurt/Main, prefix SMF). poor preservation, specific identifications were not possible. The Rhabdinopora flabelliformis Biozone Aorograptus victoriae ranges into the Araneograptus murrayi Definition: The base of the zone is taken at the first occurrence of Biozone, but is not found in the overlying Hunnegraptus copiosus Rhabdinopora flabelliformis parabola at the 570m level in the Biozone (Maletz & Egenhoff, 2000) in Bolivia. Taraya section. The same species also appears in sample Ca-96-3 in the Cieneguillas section (Egenhoff, 2000: 142). Fauna: Rhabdinopora flabelliformis parabola, Rhabdinopora flabelliformis ssp. Other subspecies may be present, but have not been differentiated so far. Remarks: Rhabdinopora flabellifonnis (Fig. 3c) is fairly common and can be identified in a number of localities where it generally is found in monospecific assemblages. There is no confirmed record of Staurograptus in the basal Tremadoc of Bolivia. The genus Anisograptus was also not recorded from Bolivia and, thus, the Anisograptus Zone cannot be differentiated. A finer subdivision of the broad Rhabdinopora flabelliformis Biozone might be possible with new collections. Cooper et al. (1998) re-defined the early Tremadoc graptolite biozonation, and differentiated a number of biozones, but it is not possible to identify these in Bolivia, as the faunal record is poor and the graptolites are too strongly tectonized for unequivocal identification of the subspecies of Rhabdinopora flabellifonnis at the moment.
The Araneograptus murrayi Biozone Definition: The base of the zone is taken at the first occurrence of Araneograptus murrayi at the 1200m level in the Cieneguillas 1 section (sample Cd-97-33). Fauna: Araneograptus murrayi, Paradelograptus norvegicus, Kiaerograptus sup remus (Fig. 3a), Paradelograptus sp., Tetragraptus bulmani. Remarks: Araneograpus murrayi (Fig. 3b) is a common and highly characteristic species in the Lower Ordovician of Bolivia and was found in large masses of specimens in a number of mud- and siltstone layers. Often the material clearly indicates current transport of the rhabdosomes. The specimens are even preserved as casts in sandstone beds. Lindholm (l991b) redescribed Araneograptus murrayi from the type material and specimens collected from drillcores in Scandinavia. Due to its robust rhabdosome it should not be misidentified with the much older and less robust species of the genus Rhabdinopora.
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REVISTA TECNICA DE YPFB - VOLUMEN 21 - SEPTIEMBRE 2003
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Fig. 2. The Bolivian sections and their biostratigraphic correlation.
The Hunnegraptus copwsus Biozone Definition: The base of the Hunnegraptus copiosus Biozone is taken at the first occurrence of its index species at the 1400 m level (CIN 12) in the Cieneguillas I section. Fauna: Hunnegraptus copiosus(Fig. 5f), Paradelograptus norvegicus (Fig. 3g), = I ~
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