Abstract

Mudclast aggregates are documented in the Lower Triassic, anabranching fluvial deposits of the Katberg Formation, Karoo Basin, South Africa. Here, aggregates are: (1) coarse silt to fine sand size, (2) of a heterogeneous texture consisting of several clay clasts, some of which display the presence of clay skins, (3) uniquely shaped, not simply filling pore space between other grains as matrix, and (4) in association with reworked carbonate nodules and mud-chip rip-up clasts. They occur in basal channel lags, trough fills of megaforms, and at the tops of channel-fill sequences; there is no evidence of mud aggregates in paleosol intervals. Two chemical types of aggregates are identified. One type is enriched in carbon and depleted in calcium, and is interpreted to have originated in an upper soil horizon; the second is depleted in carbon and enriched in calcium, and is interpreted to have originated at some depth in the soil profile. These attributes, in conjunction with the presence of calcite-cemented nodules concentrated in channel lags, are used to interpret the former presence of paleoVertisols across the Early Triassic Karoo landscape prior to landscape degradation.