Ediacaran and Cambrian paleosols from central Australia

Abstract

Ediacaran to Cambrian red sandstones of the Northern Territory, including Arumbera Sandstone, and Grant Bluff and Central Mount Stuart formations, have been reexamined and sampled in order to reconstruct paleoenvironments from sedimentary facies and paleosols. Sedimentary facies include green-gray calcareous sandstone (Member III of Arumbera Sandstone only), red siltstone with calcareous nodules (also only Member III), and also non-calcareous red-green siltstone, interbedded red siltstone and sandstone, and trough bedded sandstone. The green-gray facies has diverse and distinctive Cambrian marine trace fossils, not found in the red beds which have instead an assemblage of problematic body fossils including Arumberia, Noffkarkys, Hallidaya, Ernietta, Trepassia, and Dickinsonia. The red facies are interpreted as fluvial paleochannels (trough cross bedded), fluvial levees (interbedded siltstone and sandstone), poorly drained floodplain (red-green siltstone), and well drained floodplain or low terrace (calcareous nodular). The carbonate nodules have micritic, replacive fabrics and a significant positive correlation of δ13C and δ18O characteristic of pedogenic carbonate of Calcid soils. Other red beds have pseudomorphous sand crystals after gypsum, like those of Gypsid soils. Individual red beds show negative strain and mass transfer for cationic bases typical of soils, as well as gradational alteration down from fossiliferous tops characteristic of soils. Their varied degree of pedogenic differentiation is identified as Psamment, Aquent and Ochrept paleosols. Phosphorus depletion in the paleosols is evidence that organisms lived in the paleosols, and were not introduced by transient marine incursions. The problematic quilted fossils Arumberia, Noffkarkys and Hallidaya formed a distinctive inland terrestrial polsterland during the Ediacaran and early Cambrian.