LANDSCAPE VARIABILITY WITHIN THE EOCENE PIEDRA CHAMANA FOSSIL FOREST, PERU

The Piedra Chamana fossil forest of northwest Peru is a distinct assemblage (39 Ma) preserved by fluvial deposits and volcanic ash fall and represents a unique point in time and space for low latitudes of the late Middle Eocene of South America. Previous paleobotanical research at Piedra Chamana suggests that these forests represent coastal environments composed of seasonally flooded lowland forests and dry tropical forests. The paleopedology of these late middle Eocene landscapes was investigated to test these interpretations. Paleosols were recognized by their association with in situ fossil trees. Two trenches were excavated along the same paleo-landscapes to expose fresh profiles in order to document roots, soil structures, colors and mottling, micromorphology, and mineralogy.

Soil profiles developed on fining upward sequences of volcaniclastic materials characterized by white euhedral plagioclase grains within a light olive matrix that changed upward into salmon pink and orange altered plagioclase grains within a pinkish-orange matrix. Other than the obvious silicified horizontal tree roots associated with in situ stumps, most roots are several millimeters in diameter and preserved as a combination of downward branching and horizontal clay infills and drab haloed traces, some of which are associated with intense reddish orange staining. Zones of mottling are preserved as reddish-orange discolorations that occur at different levels in each profile.

Comparison of these profiles along the same paleo-landscape suggest slight differences in geomorphic position. Both sites preserve the same thickness of individual fining upward packages, but mottling is more pronounced in Site 1, suggesting a greater amount of hydromorphic influence. The lowermost paleo-landscape at both sites is overlain by a uniformly thick fining upward package of volcaniclastic sediments, which also has rare in situ trees, and is in turn capped by a widespread volcanic ash that contains accretionary lapilli and fossil leaves. As with the underlying profile, this thinner, overlying volcaniclastic interval displays greater hydromorphic influence at Site 1 and suggests that these profiles formed on different parts of the same landscape.