PEDOGENIC MUD AGGREGATES IN SEDIMENTARY LAYERS BETWEEN BASALT FLOWS

Jurassic (143 Myr) basalt flows occur in central Namibia (southwestern Africa) and record the rifting of the Karroo event where India, South Africa, and Antarctica began to separate. Known as the Kalkrand Formation, these porphyritic theolitic basalts represent several separate flow events followed by extensive weathering. In the Hardap Recreational Resort near Mariental, two sedimentary interlayers with a lateral extent of one to two kilometers are well exposed within faulted depressions on these basalt flows. With thicknesses of six to eight meters, these two sedimentary interlayers are composed of siltstone, sandstone, and minor mudrock, with the coarser siliciclastics containing pervasive calcite cement. Sedimentary structures include flat lamination, current ripple cross-lamination, and trough cross-bedding. Previously, these sedimentary rocks were interpreted as dominantly lacustrine deposits. A closer look, however, shows that these deposits instead could be mostly fluvial sheetflood deposits infused with calcite cement. Mudstone laminae associated with siltstone to fine sandstone lamina in a facies with parallel lamination suggest deposition in a high flow regime instead of a deep quiet paleoenvironment. In thin sections, this laminated rock shows mud lamina alternating randomly with silt to sand lamina. The mud lamina exhibit rounded mud (silt + clay) aggregates that are silt- to sand-sized. These rounded aggregates are interpreted as pedogenic mud aggregates which are common in rift basin sequences in the Triassic-Jurassic basins on both sides of the Atlantic Ocean. They are common in sheetflood deposits today in central Australia as well. The formation of these aggregates requires smectitic clays, usually originating from Vertisols derived from alkaline rocks. The presence of these mud aggregates does not support the collection of permanent aqueous water bodies on basalt flow surfaces since they would disaggregate in standing water. The highly-porous basalt probably could not maintain lakes and ponds on surfaces either. In addition, groundwater levels probably were very deep with the faulted depressions above the regional water table since coarse sand layers are present within large deep cracks in the basalt.