Speleogenesis and speleothems of the Guacamaya Cave, Auyan Tepui, Venezuela

In March 2009 the Guacamaya Cave was discovered on the Auyan Tepui. It represents one of the longest caves explored on this table mountain, the only one known today with a complete horizontal development, comparable with those of the Brewer Cave System, in the Chimanta Tepui (the world largest sandstone cave). Guacamaya Cave presents peculiar morphologies, developed along an obvious bed of iron hydroxides and amorphous silica (Banded Iron Formation, BIF) interposed between hard and massive quartzite banks. In the walls around this layer, a variety of opal speleothems, of unusual dimensions and shapes, together with gypsum flowers and crusts have been documented. Here we present a compositional and morphological characterization for the BIF layer and one tufa-like bio-speleothem. The speleogenetic control exerted by the BIF stratum is discussed, and in relation to collapse morphologies observed on the surface. An attempt to date the bio-speleothem with the U-Th system shows the difficulties of applying this method to these silica formations, because of post-depositional alteration in this porous material