Abstract

SPALLETTI, Luis et al. Sedimentology, palaeobotany and geochronology of the Piedra del Águila Formation (Lower Jurassic, Neuquén). Rev. Asoc. Geol. Argent. [online]. 2010, vol.66, n.3, pp.305-313. ISSN 0004-4822.

At its type locality, the Piedra del Águila Formation is a 71.6 m thick homoclinal succession that lies on a weathering palaeosurface developed on the igneous-metamorphic basement (Mamil Choique Formation), and is unconformably covered by the Lower Jurassic volcanic and volcaniclastic Sañicó Formation. The sedimentary succession of the Piedra del Águila Formation is dominated by mudrocks (shales, mudstones and siltstones). Coarse- and very coarse-grained sandstones (and subordinated pebbly sandstones and fine-grained conglomerates) intercalate in the fine-grained succession. These deposits appear as thick (1-3 m) lenticular bodies and thin (0.2-0.4 m) lenticular/tabular beds showing very common through cross-stratified sets. The sandstones are feldarenites essentially composed of quartz and K-feldspar derived from the disintegration of the local crystalline basement. The studied deposits were accumulated in the flood plain and channels of a mixed-load fluvial system. Eleven plantiferous taxa were recognised in the Piedra del Águila Formation. Two taxa are new for the Argentinean fossil record (O. groeberi y O. oldhami, being the last species characteristic of the Liassic of India), one is representative of the Triassic-Early Jurassic (Cladophlebis indica) and two other taxa are typical of the Early Jurassic (Scleropteris sp. and Otozamites bechei). Equisetites sp.,Sphenopteris sp., Cladophlebis sp., C. hairburnensis, Otozamites hislopi and Ptilophyllum acutifolium have an ample biochron. A 0.5 m thick chalazolitic vitric tuff bed was identified in the lower part of the studied succession. The zircons of this tuff show a variety of forms, but most of them are euhedral with bipyramidal terminations. The calculated U-Pb SHRIMP age for the time of deposition of the tuff is 191.7 ± 2.8 Ma (Sinemurian).

Keywords : Piedra del Águila Formation; Neuquén Province; Argentina; Sinemurian; Sedimentology; Palaeobotany; Geochronology.

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