A new Devonian flora in Argentina: palaeobotanical and stratigraphic implications

PRESTIANNI, C; RUSTÁN, J.J.; VACCARI, N.E; STERREN, A. F.; STEEMANS, P; C. V. RUBINSTEIN

Bruselas

Simposio; 4TH International Geologica Belgica Meeting: Moving plates and melting icecaps. Processes and forcing factors in geology; 2012

Recently a siliciclastic sucession, was surveyed in the southernmost area of the Sierra de Las Minitas, a set of low ranges exhibiting stratigraphic and structurally complex settings, considered to be the northernmost continuation of the Sierra de la Punilla. One section of the Sierra de las Minitas (corresponding to the poorly known Jagüel Formation) is particularly interesting because it includes glacigenic diamictites deposits of discussed Devonian/Carboniferous age. Although radiometric ages of igneous bodies seemed indicate Upper Devonian-Lower Mississippian ages in nearby sections, palynological contents from this locality indicated a Middle Devonian age for the glacigenic deposits. The section is composed of pebbled dark mudstones at the base, a thick bed of (ca.70 m) diamictites above overlied by an hetherolitic interval (alternating sandstones, mudstones and shales) withbrachiopod and bivalves coquinas and scarce plants, a thick sandy interval. Towards the top, another transgressive-regressive cycle with faunal records was recognised. An interesting and surprisingly diversified plant assemblage was discovered in the thick sandy interval. Moderately diverse, it is mainly composed of axis of indeterminate affinities together withvegetative Lycophyte stems, dispersed pinnules and fertile organs. This assemblage does not fit within the previously recognized Devonian and Lower Carboniferous biostratigraphic scheme of Argentina. None of the guide fossil plants of these two periods were recognized. However a comparison with other coeval localities outside of Argentina allows us to hypothesize an Upper Devonian age at least for these deposits. Thus, this would represent the first direct evidence of cropping Upper Devonian rocks in Argentina with fossiliferousrecords. This evidence suggests a linkage between the diamictite and the Gondwana UpperDevonian glacial event. Hence, palynological material indicating a Middle Devonian age for this diamictite should better be interpreted as reworked on the bases of these new evidences.