Trace fossils as sedimentological and palaeoenvironmental indices in the Ecca Group (Lower Permian) of the Transvaal

Trace fossils are described from the Lower Permian Vryheid Formation (Middle Ecca) in the Transvaal. They are identified from surface outcrop in the Witbank Coalfield and next to the Smitfifield Ridge, a pre-Karoo granitic ridge that periodically acted as an island archipelago. In this area, the trace fossils are found between the 2 and 4 coal seams. They are Diplocraterion parallelum, Skolithos, Monocraterion, Scalarituba, and a new species of Rhizocorallium. Trace fossils associated with the progradational phase of delta out-building can be distinguished from those associated with an abandonment phase, whereby the delta subsided due to compaction and was probably transgressed by marine waters. In the case of the latter, the position of a transgressive sand barrier can be recognised, which protected swamp areas that are now preserved as a coal seam of enhanced thickness. A beach environment. can be recognised as part of this barrier system, which fronted one of the islands of the Smithfield Ridge. Wave and probable tidal processes operated on this beach. I!l the Eastern Transvaal Coalfield from a borehole core comprising most of the Vryheid Formation (Middle Ecca), several trace fossils are identified. These include Skolithos, Planolites and a new trace fossil named here Siphonichnus eccaensis ichnogen and ichnosp. nov. The latter is a vertical tube burrow, with backfill laminae which indicated fairly rapid burrowing in response to erosion. A central tube which pierces these laminae probably accommodated a siphon for breathing and feeding from suspension. Similar traces from the Recent are ascribed to the bivalve Mya arenaria, and if S. eccaensis similarly represents the burrow of a bivalve, this bears the important implication that diagenetic removal of shell material was an important process in the Ecca Group. Various bioturbated layers can be reconised in the borehole core, the majority of which occur just above a coal seam. These are interpreted as representing abandonment phases in agreement with recent work on ancient delta complexes. S. eccaensis is associated with two of these abandonment phases and this would agree with the persistent erosion implied by the structure of this trace fossil. Glauconite pellets are associated with one of the bioturbated layers, interpreted as glauconitised faecal pellets. Again, the marine nature of the abandonment phase is indicated.