The stratigraphy and sedimentary history of the Molteno stage in part of the North-East Cape Province

Turner, Brian, Ronald
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A stratigraphic and sedimentological study of the Molteno sediments in the vicinity of Aliwal North and Burgersdorp was made in order to determine stratigraphic relationships and sedimentary history. The stratigraphic study shows that the Molteno strata is characterised by a cyclic pattern of deposition as follows: (i) pebble conglomerate overlying an erosional surface of low relief; (2) coarse - to medium- grained sandstone; (3) fine-grained sandstone~siltstone, and silty shale and (4)" shale with thin coal lenseso Three such cycles have been recognised in the Burgersdorp area and a single cycle at Aliwal North. The succession thins northwards from about 500 feet around Burgersdorp to 300 feet at Aliwal North and on the basis of cyclic relationships and sedimentary tectonics it is suggested that the two upper cycles at Aliwal North are missing through erosion or non-deposition and the lower cycle at Burgersdorp is the stratigraphic equivalent of the Aliwal North cycle. The importance of the lower pebble bed arises from the ease with which it can b e recognised and traced throughout the area; moreover, it is tectonically significant and represents a distinct stratigraphic break . As a result the base of the Molteno is redefined in terms of the lower pebble bed. The limitations of the Indwe sandstone as a regional stratigraphic marker are demonstrated. It is also recommended that the Indwe sandstone be more strictly defined in terms of modern stratigraphic nomenclature or dropped completely from the literature . The pattern of sedimentary transport was determined mainly from cross-bedding measurements, though other sedimentary structures such as erosion channels, ripple marks, current lineations, and fossil logs have also been used . These dimentary structures indicate shallow water conditions of deposition with the dominant direction of transport from the south and south east. The sandstones have been classified petrographically as subgreywackes. Metamorphic quartz and metaquartzite rock fragments together with minor amounts of feldspar are important constituents. Excluding micas the heavy mineral suite is simple and consists of garnet, zircon, tourmaline and rutile . The Molteno sediments wer e derived chiefly from high- rank metamorphic and granitic source rocks with only neglible contributions from pre - existing sediments . Integration of the stratigraphic and sedimentological evidence shows that the upward-fining cycle of the Molteno was deposited under both upper and lower flow regime conditions. Facies analysis of the cycle suggests that the conglomerate is the result of erosion and deposition in a braided river channel wandering across a flood plain. The overlying sandstone shows all the characteristics of a modern point bar complex. The fine sandstone, siltstone, and silty shale represent a transitional facies deposited mainly from suspension in the quiet parts of the channel or in abandoned channels during low water. If the sandstone facies represents channel deposits then the shale and coal facies probably records overbank deposits from flood waters in the quiet backswamp areas of the flood plain. The environment was probably permanently inundated by water of such a depth as to allow for the growth of plants and the formation of peat swamps. The key to cyclic repetition appears to be related to sedimentary tectonics and diastrophic movement associated with the second phase of the Cape Orogeny. In terms of sedimentary tectonics, and the classification scheme of Krumbein and Sloss (1963), the lithologic association of the Molteno sediments probably represents unstable shelf or interior basin conditions .
Thesis submitted for the degree of Master of Science in the Faculty of Science, University of the Witwatersrand
Sedimentology -- History -- South Africa -- Cape of Good hope, Stratigraphy -- History -- South Africa -- Cape of Good hope, Geology -- History -- South Africa -- Cape of Good hope