From available records and data of colliery product samples, an area
within the Witbank and Highveld coalfields was selected for this study.
The petrographic and elemental nature of the coal seams have been
derived from the relationship between the volatile natter content and
the calorific value, both expressed on a dry ash-free basis (Snyman
et al, 1983; Barnard, 1987).
These derived values were complemented by ash_ analyses of colliery
product samples, paleotopographic data and seam thickness. These data
provided information on the paleoenvironmental conditions during peat
accumulation and deposition.
Computerized subsurface mapping techniques were applied to record
various chemical and petrographic parameters. The existence of major
channel complexes culminating in outwash streams towards the south is
postulated. During the formation of the basal seams these trunk
channels were up to 6 km wide and trended generally from east to west
and from north to south along loT.v gradients. The channels follow
topographical lows and skirt topographical highs. The vegetationstabilized
channels aggraded upwards at a rate consistent with peat
accumulation. The channels were fed by melt-water which originated from
the northwards receding glacial ice-cap.
Low energy currents led to the wide lateral distribution of
autochthonous coal seams. The channels led to the transportation and
redeposition of inertodetrinitric material of hypautochthonous origin.
The coal seams associated with channel areas are generally thinner and
the resultant coals are usually inertinite-rich. These coals have a
brittle character and tend to become more shaley towards the south.
A close relationship was established between the mineral matter
characteristics and coal type of the No. 2 Seam. The inertinite-rich
coals within the major channel areas contain mineral matter which is
richer in acidic minerals. These minerals are quartz, normative rutile and kaolinite. Coals which formed in interchannel areas and swamps
have generally higher vitrinite contents. The mineral matter of these
coals contains relatively more pyrite, carbonate minerals and illite.
This relationship between mineral matter and coal type is indicative of
different paleoenvironmental conditions in which the peat was formed.
Vitrinite-rich coals resulted from peats which were subjected to
brackish water conditions where moderately high pH and a lower redox
potential prevailed. The inertinite-rich coals were derived from peats
formed in acidic waters -in an aerobic environment.
Fluvial channel environments had a control on seam geometry and the
petrographic profile which characterizes coal seams. Most of the
vitrinite-rich
vitrinite-rich
coal seams
base which
have
grades
a distinctive cyclic pattern: a
upwards into inertinite-rich coal.
This petrographic pattern may be repetitive within the seam and is
mostly reversed in the coal seams which were deposited in channel areas
where a vitrinite-rich cap grades downwards into an inertinite-rich
base.
The influence of the topography of the pre-Karoo basement is evident
throughout the succession of major depositional units, especially in the
Witbank coalfield. Nevertheless, the paleoenvironments which existed
during the formation of the various seams differed in extent. The
different seams each have their own areal distribution, seam geometry
and petrographical and chemical characteristics.
Van beskikbare inligting asook data van steenkoolprodukte is n
studiegebied in die Witbank- en Hoeveldsteenkoolvelde uitgesoek. Die
petrografiese en elementsamestellings- eienskappe van die steenkoollae
is afgelei uit die verband tussen droe as-vry vlugstof en hittewaarde
(Snyman et al, 1983; Barnard, 1987).
Hierdie afgeleide waardes is aangevul met assamestellingsanalises van
steenkoolprodukte, paleotopografiese data en lac:tgdiktes. Uit hierdie
data is inligting oor die toestande wat tydens veenvorming en
afsetting geheers het, ingewin.
Gerekenariseerde ondergrondse karteringstegnieke is gebruik om verskeie
chemiese en petrografiese eienskappe aan te dui. Die bestaan van
hoofkanaalkomplekse wat na die suide uitspoelstrome tot gevolg gehad
het, word veronderstel.
hierdie hoofkanale tot
Gedurende die vorming van die onderste lae was
6 km breed en het die roetes met n matige
gradient oor die algemeen van oos na wes en van noord na suid gestrek.
Die kanale volg topografiese laagtes en om topografiese hoogtes. Die
plant-gestabiliseerde kanale het geaggradeer teen n tempo wat met
veenvorming tred gehou het. Die voedingsbron van die kanale was
smeltwaters afkomstig van die ·noordwaarts terugbewegende
gletserbedekking.
Lae-energie
steenkoollae
strome
tot
het n
gevolg
wye laterale
gehad. Die
verspreiding van
kanale het die
autochtone
vervoer en
herafsetting van inertodetrinitiese materiaal van hipautochtoniese
oorsprong te weeg gebring. Die steenkoollae wat met die kanaalareas
geassosieer is, is oor die algemeen dunner en hierdie steenkool is
gewoonlik inertiniet-ryk. Die steenkool is ook bros en is geneig om
suidwaarts skalierig te word.
n Goeie verband tussen die mineraaleienskappe en steenkooltipe van die
No. 2 Steenkoollaag is vasgestel. Die inertiniet-ryke steenkool wat in
die hoofkanaalgebiede voorkom bevat mineraalbestanddele wat ryker is aan suur minerale. Hierdie minerale is kwarts, normatiewe rutiel en
kaoliniet. Steenkool wat in tussen-kanaalareas en moerasse ontstaan
het, het oor die algemeen n hoer vitriniet-inhoud. Die mineraalbestanddele
van hierdie steenkool bevat relatief meer piriet, karbonaatminerale
en illiet.
Hierdie verwantskap tussen mineraalbestanddele en steenkooltipe is
aanduidend van verskillende paleo-omgewingstoestande waarin die veen
gevorm het. Vitriniet-ryke steenkool het ontstaan uit veen wat aan
brakwatertoestande
redokspotensiaal
onderwerp
geheers het.
was waar matige
Inertiniet-ryke
hoe pH en n
steenkool word
veenvorming in suur waters onder oksiderende toestande geassosieer.
laer
met
Rivierkanale het n beheer op laagverspreiding en die petrografiese
profiel wat kenmerkend van steenkoollae is, uitgeoefen. Die meeste van
die vitriniet-ryke steenkoollae het n kenmerkende patroon: n
vitriniet-ryke basis wat opwaarts gradeer na inertinitiese steenkool.
Hierdie petrografiese profiel mag herhaal word in n steenkoollaag en
is meesal omgekeer in steenkoollae wat in kanaalareas afgesit is waar n
vitriniet-ryke bocnste gedeelte van die laag afwaarts gradeer na n
inertiniet-ryke basis.
Die invloed van die topografie van die voor-Karoo vloer is deurgaans in
die opeenvolging van die hoofafsettingseenhede waarneembaar, veral in
die Witbank-steenkoolveld. Nietemin het die paleo-omgewings wat tydens
die vorming van die verskeie steenkoollae geheers het, van mekaar
verskil. Die verskillende lae het elk sy eie oppervlakteverspreiding,
laaggeometrie en petrografiese en chemiese eienskappe.
