PALYNOASSEMBLAGES IN THE SIERRA DE MAZ AREA, LA RIOJA PROVINCE, ARGENTINA: AN OLDER AGE FOR THE BASE OF THE GUANDACOL FORMATION

VALDEZ, V., DI PASQUO, J.P MILANA, B. KNELLER, P. PAIM

P. Madryn

Congreso; Reunion Argentina de Sedimentología, P. Madryn, 2014; 2014

Asociación Argentina de Sedimentología

The Guandacol Formation, in the homonymous area, consists of an alternation between proglacial and postglacial marine strata, including deltaic and turbidite sandstones, black shales, and resedimented diamictites. In order to characterize the palynology at Cerro Guandacol, a systematic sampling was undertaken through the entire succession in the south of Sierra de Maz, La Rioja Province (Fig.1). Twenty one samples were taken and processed following standard palynological methods; just four associations were obtained with rather good preservation of palynomorphs. Another seven barely productive samples present only poor palynological remains, fungal hyphae and phytoclasts of terrestrial origin, with or without fine particles of amorphous organic matter. A sample from an ice-rafted debris unit taken from the base of the Guandacol Fm. yielded the first palynoassemblage (CICYTTP-Pl 513) of scarce palynomorphs composed of spores and Botryococcus. Among the spores, Cordylosporites sp. cf. C. absimilis is mostly recognized in the Tournaisian (Strunian?) of Brazil, Densosporites regalis is known from the Malimán Fm. (Tournaisian-early Visean of Precordillera (Amenábar et al. 2007) to the late Visean (Mag Zone) in Brazil (Melo and Playford 2012), Densosporites pseudoannulatus is Strunian-Visean up to Mag Zone of Brazil (Melo and Playford 2012, Playford et al. 2012), Cristatisporites sp. cf. C. mattheusii, recorded from the Malimán, El Ratón and Cortaderas formations (Amenábar et al. 2007, Pérez Loinaze 2007, Amenábar and di Pasquo 2008), Cristatisporites peruvianus is documented from the Tournaisian-Visean in the Malimán Fm. (Amenábar et al. 2007) and Brazil (Playford et al. 2012; Melo and Playford 2012), and in the late Visean of Perú and Bolivia (Azcuy and di Pasquo 2005), Vallatisporites ciliaris , and Cristatisporites rollerii Ottone and Cristatisporites stellatus are recorded in the Cortaderas and Guandacol formations and the latter in the El Ratón Fm. as well. A second assemblage (CICYTTP-Pl 477) is recovered from a confined turbidite deposit at the top of the lower of two mass transport deposits (MTD) at Sierra de Maz. Few species were confidently identified such as, Apiculiretusispora tuberculata, A. variornata, Cristatisporites scabiosus, C. inordinatus, C. matthewsii, C. chacoparanensis, C. stellatus, Cyclogranisporites microgranus, C. rinconadensis, ?Schopfipollenites sp. Most of these species are known from the Malanzán and Guandacol formations (Raistrickia densa-Convolutisporta muriornata SubZone a- (DMa SZ) (Fig.2). The remaining two assemblages (MP-P 8931 and 8926) are taken from a thin unit of shales with dropstones immediately overlying the upper of the two mass transport deposits at Sierra de Maz. Identified spores are Calamospora sp., Cristatisporites stellatus, C. menendezii, Cristatisporites sp., Verrucosisporites sp. Considering this information and previous palynologic data from the Cerro Bola area (Césari and Limarino 2002, Valdez et al. 2013) and the well-established lithostratigraphic correlation between the successions at Cerro Bola and Sierra de Maz (Valdez et al. in rev.), we interpret that the first assemblage is the oldest for the Guandacol Fm., probably akin to the Tournaisian-Visean, and a younger Serpukhovian ? ?Bashkirian age is suggested for the second one, correlated with the lower DMa SZ. The other two assemblages are also assigned to the DMa SZ. This conclusion endorses our previous interpretations made from lithostratigraphical analysis (Valdez et al. in rev.), that Guandacol Fm. at its type section started accumulation well before the rest of Paganzo Basin areas. Thus, this finding supports the definition of a Guandacol-La Peña sub basin that started evolving before the widespread intracratonic Paganzo basin and probably under a particular tectonic regime making possible to preserve older glacial events than in the rest of the basin. [Projeto BG-01]