Preliminary studies of Devonian microfloras of a borehole from the Tarija basin, northwestern Argentina

NOETINGER, SOL; DI PASQUO, M.M.

Otro; 4º European Meeting on Paleontology and Stratigraphy of Latin American; 2007

Instituto Geológico y Minero de España

INTRODUCTIONDevonian deposits in the southern Tarija Basin crop out mainly in the western Subandean Range to the eastern part of theCordillera Oriental of Argentina and are widely distributed from the northernmost of Argentina to Bolivia. In contrast, Devoniansubsurface deposits extend over between Bolivia, Paraguay and Argentina. They are represented by shale and sandstone facies deposited mainly in a shallow marine environment; less significant are littoral andcontinental facies that contain palaeontological evidences of having beenconnected with other coeval close deposits in Brazil, Peru, Uruguay especially during transgressions (Figure 1; Melo, 1989; Starck,1996; Limachi et al., 2002). Subandean zone, in this basin is a thin-skinned thrust belt characterized by elongated anticlines that run north-northeast?south-southwest, forming several continuous, and parallel ranges(Echavarria et al., 2006, and references therein). One of them is called San Antonio, where the borehole San Antonio X-1 is located. Most of the knowledge on Devonian successions from the Tarija Basin comes fromsubsurface data, from petroleum boreholes drilled during the pasts decades (see Limachi et al., 2002). The Los Monos Formation and its Argentinean equivalent, the Tonono Formation, range from the latest Eifelian or early Givetian through the early Frasnian (Melo, 2005 a, and references therein). This work presents a preliminary palynological survey of a Devonian microflora recovered from the Los Monos Formations in theSan Antonio X-1 well in the Tarija Basin, northwestern Argentina.COMPOSITION AND AGE OF THE ASSEMBLAGESThe San Antonio X-1 well was drilled to a maximum depth of about 3600 m (15700 feet). A hundred and fifty seven cutting samples were collected, around each seven meter depth along 1100 m of the borehole. Twenty-three samples were selected and processed for a preliminary overview, with an average distance of thirty-seven meters in between each one. The microflora recovered is composed of 68 species relatively well-preserved which is represented by diverse palynological groups such as trilete spores (34 species), cryptospores, palaeomicroplankton including several Prasinophycean (e.g., Cymatiosphaera, Duvernaysphaera, Maranhites, Polyedrixium, Pterospermella, Hemiruptia, Leiosphaeridia) and acritarchs taxa (29 species), the chlorophycean algae Chomotriletes and Quadrisporites, chitinozoans (5 species) and scolecodonts. Some of these species are new taxa that need to be described. Quantitative information about major groups is displayed in Figure 2. The stratigraphic distribution of the species allowed to distiguish three associations (Table 1).The palynoassemblage A3 (Fig. 2; Table 1) is characterized by trilete spores and a high palaeomicroplankton diversity represented mainly by acritarchs and prasinophytes (57% of the total) while the chitinozoans are less frequent (8%). Many species are exclusive to this assemblage such asVerrucosisporites bulliferus (Taugourdeau-Lantz) Richardson and McGregor, Ammonidium garrasinoi Ottone, Umbellasphaeridium deflandrei (Moreau-Benoit) Jardiné et al., Stellinium micropolygonale (Stockmans and Willière) Playford, Pseudolunulidia laevigata Brito and Quadros, Belonechitina holfeltzii Ottone, Angochitina galarzae Ottone and Maranhites mosesii (Sommer) Brito emend. Burjack and Oliveira, the latter, also present in A2. The presence of different species of Maranhites is typical from the LateDevonian assemblages in Bolivia, Brazil and Argentina (e.g., Ottone, 1996; Limachi et al., 1996; Quadros, 1999). Most of the species in A3 are registered in the upper section Los Monos Formation (northwesternArgentina) by Ottone (1996 and references therein) attributed to the late Givetian ? early Frasnian. Moreover, Lunulidia micropunctata Pöthe de Baldis and Maranhites brasiliensis Brito emend. Burjack and Oliveira were recorded in the Frasnian Ponta Grossa Formation of the Paraná Basin (Quadros, 1999). The presence of Verrucosisporites bulliferus that has been quoted in the early Frasnian of northwestern Argentina (Ottone, 1996) as well as in Bolivia (Pérez Leyton, 1991; Vavrdová et al., 1996) and Brazil (Melo, 2005 b), allow to attribute the same age to our association. The palynoassemblage A2 (Fig. 2; Table 1) comprises predominantly continental elements (91%) while the other taxa are strongly subordinated (Fig. 2). Pseudosaccate spores of the genus Grandispora are dominant and especially Grandispora pseudoreticulata, endemic of the Middle to Late Devonian of South America (di Pasquo et al., 2007). Even though Geminospora lemurata (Balme) Playford, which appears in the earliest Givetian of Euramerica (see Richardson and McGregor, 1986), was not registered, Verrucosisporites scurrus (Naumova) McGregor and Camfield, Samarisporites sp., Acinosporites macrospinosus Richardson, Acinosporites ledundae Ottone, Dibolisporites farraginis McGregor and Camfield, Cymbosporites catillus Allen, Leiotriletes balapucencis di Pasquo, Chomotriletes vedugensis Naumova, and Biharisporites parviornatus Richardson are shared with the Association 2 of Middle-Late Givetian age at Balapuca (Los Monos Formation, di Pasquo, 2007), and correlate both assemblages. Regarding on the presence of Ramochitina ramosi Sommer and van Boekel, quoted for the late Eifelianmiddle Givetian of the Paraná Basin (Grahn et al., 2002) and due to the fact that there are not exclusive Eifelian taxa registered; we propose a Givetian age to this assemblage.The palynoassemblage A1 (Fig. 2; Table 1) is composed mostly of continental elements (88%). Strong markers occurring in this section are: Verrucosisporites loboziakii Marshall and Fletcher quoted for the Eifelian of the Orcadian Basin (Marshall and Fletcher, 2002) together wih other Euramerican index taxa such as Grandispora douglastownense McGregor that appears in the Grandispora douglastownense ? Ancyrospora eurypterota Assemblage Zone (Late Emsian and earliest Eifelian) following in the Densosporites devonicus ? Grandispora naumovii Assemblage Zone (Mid-Eifelian to Early Givetian), and Dibolisporites eifeliensis (Lanninger) McGregor, Densosporites inaequus (McGregor) McGregor and Camfield and Corystisporites multispinosus Richardson that are recorded from the Eifelian and lower Givetian (see Richardson and McGregor, 1986). On the basis of these ranges and regarding on the fact that most of these taxa are also recognized in the Association 1 attributed to the Late Eifelian by di Pasquo (2007), we suggest a Middle to Late Eifelian age to our assemblage 1.PALAEOENVIRONMENTAL AND FINAL CONSIDERATIONSThere is no evidence for intraformational contamination due to the drilling, supported by the fact that taxa like Veryhachium spp. and Maranhites spp., abundant in the upper levels, do not appear further down together with asserted indigenous species. The Los Monos Formation was mainly interpreted to characterize offshore facies (Limachi et al. 1996). The data displayed above, show strong evidence for palaeoenvironmental changes during the deposition of the succession. It is likely that the low proportion of marine elements in the A1 and A2 are coincident with a drop in the sea level, a tendency that Albariño et al. (2002) registered from the Early Eifelian and appears to continue along the Givetian (assemblage A2),towards the end of this period and mainly in the Early Frasnian, the increase of palaeomicroplankton in the assemblage 3, supports a new transgressive cycle. Finally, it is outstanding as well, to state that there areseveral common species recorded in the Argentinean Precordillera that reinforce the connection of both marine and continental areas during the Middle Devonian (Amenábar, 2007; di Pasquo et al., 2007, andreferences therein).