DRAB-HALOED FILAMENT TRACES IN PALEOSOLS AS EVIDENCE OF CAMBRIAN BIOLOGICAL SOIL CRUSTS

Early Middle Cambrian (Onaraspis rubra zone) paleosols of the upper Moodlatana Formation near Wirrealpa in the eastern Flinders Ranges, South Australia have abundant green-gray tubular features at the top of the profile, similar to drab-haloes common around root traces in post-Ordovician paleosols. The gray-green haloes are particularly striking against the red oxidized matrix of paleosols, in which geochemical indications of clay formation and leaching indicate formerly well drained soil. Drab haloes are thought to form by burial gleization of organic matter of a buried filament or root, because haloes scale closer to volume of the filament or root (πr² in exposed surface) than surface area (2πr) of the filament or root hole. Cambrian drab haloes from South Australia are polymodal and small (mean and standard deviation 1.73 ± 0.78 mm diameter) with a slender central hole of original filaments now filled with clay (0.56 ± 0.29 mm diameter). In contrast, Triassic drab haloes from the Bald Hill Claystone, near Long Reef, Australia, are unimodal and large (6.46 ± 3.74 mm around root traces 1.44 ± 0.85 mm). Eocene root traces from the lower John Day Formation near Clarno, Oregon, are similar (halo 5.47 ± 2.48 mm around root trace 1.54 ± 0.78 mm). Polymodal haloes in Cambrian paleosols may represent several species of organisms, whereas Triassic and Eocene samples were from limited areas (ca. 1 m3) of forested paleosols, perhaps from the root system of a single tree. Drab tubular features comparable with the Cambrian examples also have been documented in 1.8 Ga alluvial paleosols of the Lochness Formation of Queensland, so predate the Cambrian. Cambrian and Neoproterozoic drab-haloed filaments are most like rhizomorphs of lichens and filament bundles of cyanobacteria in modern biological soil crusts, where microbes are in intimate association with sediment, unlike many intertidal microbial mats.