Abstract

The Piedra Clavada Formation (PCF), Austral Basin, Argentina, reaches 70 m in thickness in the Santa Cruz area and has been subdivided into two Members (lower and upper) and 9 facies associations. These are represented by 22 coarse- to fine-grained deposits from bioclastic, polygenic orthoconglomerates, to lithoarenites, pelites and heterolitic (sand and pelite) facies. In the type locality, near Tres Lagos, PCF spans from Albian to the Cenomanian-Turonian boundary. In this area four logs have been made recording lithostratigraphy, sedimentology and ichnology (Piedra Clavada PPC, Tres Lagos PTL, Arroyo los Paisanos PAP and Quebrada Don Nilsen QDN). Invertebrates, vertebrate fossils and plant remains (large log) have been sampled, mainly bivalve shell beds (Eriphyla, Corbula, Triton, Mytilus, Panopea), ostreids, gastropods, fish teeth or scales of semionotiform Lepidotes, and plates of turtles. Facies assemblages of the lower Member suggest a coastal, estuarine-deltaic environment with minor tidal-storm influences. In the lower part of PCF, alluvial (ebb) currents of the deltaic system, mainly from land through open shelf, prevailed over the open-sea influence of waves, tides, combined flows and storms, as indicated by palaeocurrents, facies distributions and taphonomic features of shell bed/plant remains. In the upper Member the paleoenvironmental system of PCF evolved progressively through a typical estuarine-bay environment, with tidal/storm (flood) waves and combined flows prevailing over the deltaic/alluvial land influences, as indicated mainly by paleocurrents, facies distribution and taphonomic features of shell beds. Therefore, the narrow mouthed channel system with a sandy bar complex which characterized the lower Member seems to be replaced by a wider and relatively uniform area which in the upper Member was dominated by tidal/storm wave currents of the estuarine system.

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