Grutas del Palacio and other related Upper Cretaceous continental deposits (SW Uruguay): main sedimentary features and evidence for an old flooded forest

Abstract

The Grutas del Palacio is the sector where the Asencio Formation (Upper Cretaceous), which crops out discontinuously in SW Uruguay, is best displayed. The Asencio Formation consists of red terrigenous sediments modified by pedogenesis-lateritic processes. It is constituted from bottom to top by the members Yapeyú (lacustrine) and Palacio (palustrine). The Palacio Member is characterized by the presence of numerous oval-like caves, <2 m high, as well as several column-like structures made up of ferruginized pisolithic aggregates. These structures are encased within sparsely laminated whitish sand-rich mudstone horizons and covered by iron-rich siliceous sandstones. Several interpretations have been proposed for the column-like structures. In this article we suggest that the Asencio Formation was accumulated primarily because of lacustrine processes, under relatively stable tectonic conditions and a warm, seasonally humid climate. The column-like structures would represent the casts of rotting tree trunks, which were later infilled by sand and iron-rich materials. The upper parts of these structures were probably affected by water erosion related to recurrent floods.