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

[摘要] A stratigraphic and sedimentological study of the Moltenosediments in the vicinity of Aliwal North and Burgersdorpwas made in order to determine stratigraphic relationshipsand sedimentary history.The stratigraphic study shows that the Molteno stratais characterised by a cyclic pattern of deposition asfollows: (i) pebble conglomerate overlying an erosionalsurface of low relief; (2) coarse - to medium- grainedsandstone; (3) fine-grained sandstone~siltstone, andsilty shale and (4) shale with thin coal lenseso Threesuch cycles have been recognised in the Burgersdorp areaand a single cycle at Aliwal North. The succession thinsnorthwards from about 500 feet around Burgersdorp to 300feet at Aliwal North and on the basis of cyclic relationshipsand sedimentary tectonics it is suggested that thetwo upper cycles at Aliwal North are missing througherosion or non-deposition and the lower cycle at Burgersdorpis the stratigraphic equivalent of the Aliwal Northcycle.The importance of the lower pebble bed arises fromthe ease with which it can b e recognised and tracedthroughout the area; moreover, it is tectonically significantand represents a distinct stratigraphic break . Asa result the base of the Molteno is redefined in terms ofthe lower pebble bed.The limitations of the Indwe sandstone as a regionalstratigraphic marker are demonstrated. It is also recommendedthat the Indwe sandstone be more strictly definedin terms of modern stratigraphic nomenclature or droppedcompletely from the literature .The pattern of sedimentary transport was determinedmainly from cross-bedding measurements, though other sedimentarystructures such as erosion channels, ripple marks,current lineations, and fossil logs have also been used .These dimentary structures indicate shallow water conditionsof deposition with the dominant direction of transportfrom the south and south east.The sandstones have been classified petrographicallyas subgreywackes. Metamorphic quartz and metaquartziterock fragments together with minor amounts of feldsparare important constituents. Excluding micas the heavymineral suite is simple and consists of garnet, zircon,tourmaline and rutile .The Molteno sediments wer e derived chiefly fromhigh- rank metamorphic and granitic source rocks with onlyneglible contributions from pre - existing sediments .Integration of the stratigraphic and sedimentologicalevidence shows that the upward-fining cycle of theMolteno was deposited under both upper and lower flowregime conditions. Facies analysis of the cycle suggeststhat the conglomerate is the result of erosion and depositionin a braided river channel wandering across a floodplain.The overlying sandstone shows all the characteristicsof a modern point bar complex.The fine sandstone,siltstone, and silty shale represent a transitional faciesdeposited mainly from suspension in the quiet parts of thechannel or in abandoned channels during low water.If thesandstone facies represents channel deposits then the shaleand coal facies probably records overbank deposits fromflood waters in the quiet backswamp areas of the flood plain.The environment was probably permanently inundated by waterof such a depth as to allow for the growth of plants and theformation of peat swamps.The key to cyclic repetition appears to be related tosedimentary tectonics and diastrophic movement associatedwith the second phase of the Cape Orogeny. In terms ofsedimentary tectonics, and the classification scheme ofKrumbein and Sloss (1963), the lithologic association ofthe Molteno sediments probably represents unstable shelfor interior basin conditions .

[发布机构] University of the Witwatersrand