Carbonaceous High-Alumina Shale in the Transvaal Supergroup: Evidence of Early Proterozoic Karstic Wheathering in a Marine Environment
Extensive beds of carbonaceous shale occur within a thick sequence of Lower Proterozoic dolostone, the lower Transvaal Supergroup of South Africa. The shale contains such high concentrations of A12O3 (21.9%) and K2O (6.5%) that it is compositionally similar to Precambrian paleosols which developed on a variety of rock types. It is proposed that the alumina was concentrated from contemporaneously exposed dolostone through the removal of over 98 % of the original mass by weathering. The shale contains angular fragments and blocks of chert which are attributed to an insoluble component of the dolostone. The abundant Corg also may have been inherited from the dolostone but the abundant potassium is attributed to the introduction of diagenetic pore fluids.
Stratigraphic relationships reveal that the carbonaceous shale was deposited extensively in the photic zone of a shallow epeiric sea. The shale occurs throughout virtually the entire preserved extent of the Transvaal Supergroup (250000 km2), presumably indicating widespread emergence in adjacent areas and the weathering of voluminous dolostone. The dissolved ions may well have reprecipitated within the basin as dolostone which subsequently weathered, resulting in a chemical cycle. Chert breccia within the shale is attributed to locally high relief which induced debris flows.
KeywordsClay Dust Silicate Sedimentation Pyrite
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