REDOXIMORPHIC PALEOSOLS IN ALLUVIAL AND LACUSTRINE DEPOSITS, 1.8 GA LOCHNESS FORMATION, MOUNT-ISA, AUSTRALIA - PEDOGENIC PROCESSES AND IMPLICATIONS FOR PALEOCLIMATE

Paleosols in the Lochness Formation (1.8 Ga, Australia) include both r are, non-red and abundant, strongly reddened varieties that formed at subaerial exposure surfaces in both ephemeral-river and lacustrine set tings, Physical processes dominated non-red paleo sols, which were cha racterized by repeated episodes of desiccation, shrinking, and crackin g alternating with wetting and introduction of sand, silt, clay, and i ron oxyhydroxides into planar voids, Redoximorphic (oxidation-reductio n) processes were especially intense for the red paleosols; redox depl etions of Fe and Mn (hypoalbans) occur immediately adjacent to desicca tion-related macropores and peds, whereas redox concentrations of Fe a nd Mn (quasi-coatings) occur within paleosol matrix adjacent to redox depletions, Illuviated clay and Fe-oxide coatings and hypocoatings are also common in planar macropores and on ped faces, Redoximorphic feat ures indicate periodic water infiltration and saturation, accompanied by development of reducing conditions along planar macropores and ped surfaces in Lochness Formation paleosols, Variations in soil saturatio n were caused by seasonal fluctuations of lake level in lacustrine dep osits, and by formation of perched saturation zones within floodplain deposits, respectively. Occurrences of red, hematitic paleosols in the Lochness Formation are compatible with previous interpretations of a weakly oxygenated 1.8 Ga paleoatmosphere. Redoximorphic features in th e paleosols suggest a warm to cool temperate paleoclimate (mean annual soil temperature 5-20 degrees C) characterized by seasonal saturation , by analogy with Quaternary redoximorphic soils. A minimal concentrat ion of organic C (at least 1 wt %), possibly of microbial or bacterial origin, must have been present in these Proterozoic soils to allow fo r Fe reduction.