Origin of large-scale crossbeds in the late permian coal measures of the Sydney and Bowen basins,eastern Australia

Theories proposed for the origin of large-scale crossbeds may be generalised into two main categories. The first suggests the large sigmoidal shape of the sediment layers results from variable differential compaction during and after deposition, whereas the second infers that the high angle of the crossbeds is the result of primary depositional processes.Britten (1983) has proposed that large-scale inclined strata may be produced according to his cide model whilst Mallett (1983) has suggested that such structures are produced by channel automigration. Parbury (1976) and Conaghan (1981), both proponents of the primary depositional model, interpret the large-scale inclined strata as foreset beds of a Gilbert-type delta prograding into deep (50 m water depth) lakes within which floating islands produce allochthonous deposits from hydroponic vegetation. Marchioni (1976, 1982), however, envisages that the large-scale inclined strata are epsilon crossbeds which have tilted during differential subsidence of peat swamps over which active channel systems have encroached.Nine kilometers of almost continuous high wall exposures have been studied in the Moura-Kianga Coal Mine of the southeastern Bowen Basin and it is concluded that none of the above-mentioned processes can explain the present configuration of sedimentary layering observed in these high walls at the Moura-Kianga Coal Mine. It is proposed that there the high-angle large-scale inclined strata have originated as primary features associated with a crevasse sub-delta. Subsidence in excess of vegetation accumulation within the peat swamp provides the mechanism whereby a shallow lake developed and into which a crevasse sub-delta was deposited. Similar processes have been described by Coleman and Prior (1980) from the Mississippi delta.