Thesis submitted for the degree
of Master of Science in the
Faculty of Science, University
of the Witwatersrand / 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 . / AC 2018
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/24244 |
| Date | January 1969 |
| Creators | Turner, Brian, Ronald |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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