Givetian- Frasnian microfloras from the Tarija basin, northern Argentina, and their relationship with the global eustasy and palaeoclimate

NOETINGER, S.; DI PASQUO, M.M.

Jujuy, Argentina

Congreso; XVII Congreso Geológico Argentino; 2008

The microfloras from Bolivia and Argentina have recently provided important evidence to extend the knowledge on the biodiversity and the succession of floristic events during the Middle Devonian (see di Pasquo et al., 2007). Two recently investigated boreholes, San Antonio X-1 (SA) and Tonono X-1 (To) (Figs. 1, 2) contribute with new information about palynofloral changes especially during the Givetian and early Frasnian in northwestern Argentina (Noetinger and di Pasquo, 2007, 2008). The reasons of these changes are briefly discussed in this contribution. A gradual change on the composition of the palynofacies from the Givetian to the early Frasnian palynofloras showing an increase of the content of amorphous organic matter and pyritization of palynomorphs and the dominance of marine microplankton over land-plant miospores, is recorded (Fig. 2). Hence, the early Frasnian palynofloras of both wells would reflect an increase in the relative sea level during a transgressive event that could have begun during the late Givetian. Sediment starvation and anoxic bottom conditions related to this event would have led to pyrite formation on most marine palynomorphs (see Tyson, 1995). The levels involved (Fig. 2) would represent a maximum flooding surface (MFS) in the context of a siliciclastic ramp-type basin model proposed by Albariño et al. (2002) and Alvarez et al. (2003). These authors had to infer this new transgressive event beginning at the end of the Givetian or the early Frasnian, because they could not find Frasnian deposits in the investigated localities of northern Argentina and Bolivia. They attributed the lack of these deposits to the important erosive phase occurring during the Late Devonian and Carboniferous. Therefore, this new information allows to confirm a palaeoenvironmental evolution from very shallow marine shelves and estuarine and near shore environments developed in the mid-late Givetian into a relatively deeper marine shelf setting (offshore) during the early Frasnian. These results are in agreement with the global sea level curve presented by Johnson et al. (1985) and especially with other MFS registered in the late Givetian / early Frasnian of Brazil (see Pereira et al., 2007). On the other hand, the presence of cosmopolitan and endemic palynomorphs in these two palynofloras encountered confirm that they belong to the Afro-Southamerican Subrealm (southwestern Gonwana) proposed by di Pasquo et al. (2007) and would reinforce palaeogeographic reconstructions where the South Palaeopole during the Mid-Late Devonian would have been near western to central part of Africa (e.g., Dalziel, 1997), and a narrow Rheic ocean between Euramerica and Gondwana. Additionally, a warmer global climate due to higher CO2 content in the atmosphere is accepted by many authors (e.g., Streel et al., 2000) for the Givetian until the Latest Frasnian interval, in comparison with nowadays. Hence, considering all these factors a mid to high latitudinal position of the palynofloras of Argentina and Bolivia where a temperate climate prevailed during the Givetian-Frasnian is supported.