EL BOSQUE PALEONTOLOGICO PIEDRA CHAMANA (NORTHERN PERUVIAN ANDES): A RECORD OF LOWLAND TROPICAL FOREST PRESERVED IN VOLCANICLASTIC ROCKS

Volcanic and volcaniclastic rocks of the Huambos Formation (Sexi volcanics) in the northern Peruvian Andes (northern Cajamarca; 79°10'E,6°35'S) contain a diverse assemblage of permineralized woods known as El Bosque Paleonotologico Piedra Chamana. Preliminary 40Ar/39Ar dating of associated rocks yields a date of 39 million years (Middle Eocene). The fossiliferous sequence includes a paleosol overlain by ashfall and lahar deposits containing large (1-2 cm) accretionary lapilli. Woods and leaves associated with the paleosol and ashfall deposits, including trees buried in situ, provide a highly localized representation of the paleovegetation. Fossil wood is also present in high abundance and diversity in the overlying lahar. Current elevation at the site is 2500 m. Various lines of evidence – diversity of monocots (palms and other monocots); low incidence of growth rings among the dicot woods; as well as other wood-anatomical characters – indicate that the assemblage represents wet tropical forest growing at an elevation at or close to sea level. The amount of uplift subsequent to deposition may have been as much as ~2000-2500 m.