Abstract
Storm-wave influenced glacimarine deposits of the Playa Hermosa Formation (Neoproterozoic) crop out along the beaches of Playa Grande and Playa Hermosa (town of Piriapolis, southern Uruguay). These deposits are characterized by fining-upward cycles composed, from the base to the top of successions of: (i) conglomerates and sandy-rhythmites in decimetric layers showing similar processes of rain-out, which pass to, (ii) finely laminated and rippled centimetric sandstones with deformed beds and outsized clast up to 71cm of size, with associated bending and rucking structures, (iii) well-sorted fine centimetric sandstones with swaley and hummocky cross-stratification and wave ripples interpreted as sandy tempestites which are recovered by (iv) finely laminated rhythmtes affected by intrusions of trachytes, syenites and rhyolites of the Sierra de Animas Suite (570-550 Ma). The main evidences for a glaciomarine origin includes: (i) the confirmed presence of a dropstone in fine-grained sediments indicating a hydrodynamic paradox, (ii) the presence of massive conglomeratic sandstone facies formed by rain-out processes and (iii) according to specialized literature the paleomagnetic data indicative of a low to intermediate position for the Rio de La Plata Craton in the Neoproterozoic. The loneclast was interpreted as a dropstone due to the anomalous size, the hydrodynamic paradox and the composition, farther that other forms of transport are rejected here ( e . g . organic transport).
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