Abstract

LANES, Silvia; GNAEDINGER, Silvia C; ZAVATTIERI, Ana María  and  LEZAMA, Luis. Sedimentary paleoenvironment and fossil plants of the El freno formation (early jurassic) in Las leñas valley, Neuquén basin. Rev. Asoc. Geol. Argent. [online]. 2013, vol.70, n.4, pp.465-476. ISSN 0004-4822.

Non-marine Early Jurassic successions in Las Leñas valley and their paleofloristic fossil content have been known since late nineteenth century, though they are scarce in the bibliography. It led us to study the sedimentology and paleobotanical content of El Freno Formation outcrops in the surroundings of the Portezuelo and Peuquenes creeks, report the first finding of fossil plants there and interpret their taphonomic features and enclosing sedimentary environment. The studied section is a lensoidal, fining- and thinning-upwards, conglomerate and sandy succession, with carbonaceous plant impressions and silicified trunks. It records the evolution of a gravel braided fluvial system (with longitudinal and transverse bars, abandoned channels and strong topographic irregularities) into a sand braided fluvial system (with transverse bars, overbank deposits and no evidence of lateral migration). Both flowed mainly towards the NNW and show a continuously increasing accommodation probably driven by a relative base level rise and regional sag or erosional lowering of the topography. Collected fossil plants include Dictyophylum (Dictyophylum) sp., Goeppertella sp. and undetermined Equisetopsida. Goeppertella sp. is recorded for the first time in this unit. Equisetopsida would have thrived in semi-permanent water bodies on abandoned channels and Dipteridaceae, in well-drained zones of the channel belt above the permanent channel level. Conversely, the trees would have lived in higher and well-drained areas with well-developed soils, probably outside the channel belt. Based largely on lithostratigraphical considerations, the age of the studied deposits was limited to the Hettangian?-Middle Sinemurian without identifying hiatus inside the fluvial succession or between it and the overlain marine beds.

Keywords : Fluvial deposits; Paleobotany; Early Jurassic; Precuyo mesosequence.

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