The significance of dropstones in a tropical lacustrine setting, eastern Cameros Basin (Late Jurassic - Early Cretaceous, Spain)
Résumé
Outsized clasts (mainly white quartzite pebbles) are found in carbonate deposits of the Enciso Group exposed on the northern border of the Cameros rift basin (Late Jurassic – Early Cretaceous, northern Spain). In the Arnedillo section, all the stones are enclosed in micrite, which was assumed deposited in littoral to open lacustrine environments, with minor inputs of deltaic sandstones. The clasts are found in fine-grained lacustrine sediments either as isolated stones or as loosely packed elongated clusters occasionally associated with quartz sands and gravels. The mean size of the clasts is 40 mm (range: 3 – 100 mm); they are polished, well-rounded, and lack any faceting or striae. Lithology suggests that the clasts were potentially supplied by three stratigraphic formations cropping out in the catchment area of the Cameros lake. The occurrence of outsized stones within featureless micrite indicative of low-energy conditions involves a hydrodynamic paradox which can only be resolved by their vertical or oblique emplacement in the host sediment as dropstones. The lack of mass-flow deposits in the Enciso Group precludes gravitational processes and suggests deposition from a rafting agent. Icebergs and stone-swallowing animals are the main transport agents identified in continental settings. Here, stone swallowing is rejected as the transport mechanism because the stones are loosely packed and are never associated with skeletal remains. Ice rafting is also rejected because of the low density of clusters, and the absence of faceting and striations which characterize glacial sediments. Furthermore, the subtropical position of Iberia, the absence of surrounding high relief, and global climatic models for the Aptian corroborate the absence of a glaciolacustrine environment. Consequently, we propose wood rafting as an alternative depositional mechanism for dropstones in non-glacial lacustrine environments. Although no direct evidence has yet been found (i.e. stones enclosed in the roots of rafted woods), several data support our interpretation. First, the occurrence of isolated stones and loosely packed elongated clusters is consistent with the fall of single stones or the disintegration of clods embedded among tree roots. The occurrence of large rafted conifer trunks in lacustrine deposits of the Enciso Group indicates that they may have rafted small clusters of stones from the catchment area into the lake. If wood rafting is a possible depositional mechanism, the occurrence of dropstones is consequently not a sound criterion for inferring the existence of glacial conditions in lacustrine environments. The reliability of wood-rafted dropstones in lacustrine settings as an indicator of paleoclimate is also discussed.