The effect of depositional environment on coal distribution and quality parameters in a portion of the highveld coalfield, South Africa

International Journal o[ Coal Geology, 10 (1988) 51-77 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

51

The Effect of D e p o s i t i o n a l E n v i r o n m e n t on Coal Distribution and Quality P a r a m e t e r s in a P o r t i o n of the H i g h v e l d Coalfield, South Africa H.H.B. HAGELSKAMP*, P.G. ERIKSSON and C.P. SNYMAN

Department of Geology, University of Pretoria, 0083 Hillcrest, R.S.A. (Received December 8, 1986; revised and accepted December 4, 1987)

ABSTRACT Hagelskamp, H.H.B., Eriksson, P.G. and Snyman, C.P., 1988. The effect of depositional environment on coal distribution and quality parameters in a portion of the Highveld Coalfield, South Africa. Int. J. Coal Geol., 10: 51-77. Eight successive lithofacies associations are identified and described; they are each laterally continuous and represent a certain depositional phase throughout the study area. These are re* lated to sedimentary processes, from which a three-dimensional paleoenvironmentalmodel is derived. Subsequently, coal distribution and coal quality characteristics are linked to the depositional features of the model. The identified depositional phases commenced with subglacial, glaciofluvial and glaciolacustrine settings, with associated Gilbert-type deltas. These are followed by meandering and minor braided fluvial settings, characterized by laterally and vertically highly variable lithofacies, in which the main coal-bearing strata were formed. Coal distribution and quality parameters (ash and volatile matter content, calorific value ) are closely related to paleoenvironmentalcharacteristics. Major changes of coal distribution and quality parameters are mainly associated with active and abandoned channels and these parameters are less variable in floodplain settings.

INTRODUCTION

In recent years it has become common to assist coal-exploration drilling programmes and mine production planning with depositional model studies. These are conducted in order to establish interrelationships between the coal distribution and quality parameters and the sedimentological characteristics of the paleoenvironment of the coal swamps. From these, areas of variable economic seam quality and of different mining feasibility can be delineated within single coalfields (Horne et al., 1978). *Present address: L + C Steinmiiller (Africa), Pty. Ltd., P.O. Box 1537, Rivonia 2128, South Africa.

0166-5162/88/$03.50

© 1988 Elsevier Science Publishers B.V.

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COALFIELDS OF SOUTH-AFRICA

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W/TBANK MIDDELBURG COALFIELD

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53 Investigations of recent and ancient coal-forming swamps indicate that these swamps develop under various environmental conditions, namely back-barrier lagoonal, deltaic, fluvial and alluvial fan settings (Heward, 1978; Horne et al., 1978; Flores, 1983; Galloway and Hobday, 1983; Fielding, 1985). These depositional environments have very different effects on coal seam geometry and coal quality. Recent studies on South African coalfields (Cadle and Hobday, 1977; Hobday, 1978; Cairncross, 1979, 1980; Le Blanc Smith, 1980; Cairncross and Winter, 1984) mainly concentrate on the Witbank, Middelburg and northern Natal areas (Fig. 1 ). They envisage marine deltaic, lacustrine-deltaic (Gilbert-type) and fluviodeltaic settings for the coal-bearing sequences in the northeastern Karoo basin. In this study a paleoenvironmental model of coal in a Secunda mine will be established. Subsequently, the properties and distribution of the economic coal seam will be linked to the sedimentological model. This will determine the influence that the depositional paleoenvironment has on grade and type of these coals and thus on the economic value of the seam. This correlation of depositional model and coal parameters should eventually enable a reduction in exploration drilling, which to date is the main investigative tool of coalmining companies in South Africa. LOCATIONAND GEOLOGICALSETTING The study area lies near the town of Secunda in the southeastern Highveld Coalfield, approximately 130 km southeast of Johannesburg and 100 km south of the Witbank and Middelburg Coalfields (Fig. 1 ). The area extends 20 km in an east-west direction and 10 km in a north-south direction. Coal-bearing sediments in this region belong to the Karoo Sequence. The main Karoo basin extends over a large portion of South Africa. The sedimentary rocks represent depositional paleoenvironments ranging from the Permo-Carboniferous glaciation of Southern Africa (Dwyka Formation), through an intracratonic basinal and partly marine phase (Ecca Group), to a period of terrestrial sedimentation with increasing aridity (Beaufort Group; Molteno, Elliot and Clarens Formations) (Fig. 2). The Triassic Drakensberg Group volcanics cap this sedimentary sequence and contemporaneous dykes and sills intruded and metamorphosed the coal measures of the Vryheid Formation of the Ecca Group (Tankard et al., 1982). The Karoo sediments of interest in this study include the Dwyka Formation diamictites and shales at the base and the coal-bearing Vryheid Formation of the Ecca Group at the top. The Ecca sediments comprise various gritstones, sandstones, siltstones and shales. The basal 80-130 m of these sediments exhibit sporadic seamlets and coalified plant debris. Above this, the main coal zone contains the economic seam No. 4L (lower) and the seams No. 3 and No.

54 INTERNA 50

Mill.

GROUP

TIONAL

FORMA

TION

Dolerlte dykes & slits within the study area are regarded as part of the Drakensberg volcanism

y.

Drakensberg Jurassic 95

Mill.

+_ 1 8 7

y.

Triassic 225

Mill.

y.

Clarens

LU

Elliot Molteno Beaufort

Permian 0 0 Mill.

Ecca

Volksrust Vryheid ~: Pietermaritzburg

present study

in area

y.

Dwyka

Upper Carboniferous Pre-Karoo

y.

0 (n

285

Mill.

LU

rocks

:

Transvaal Ventersdorp Witwatersrand

Sequence lavas Supergroup

2 +2 2

300

- 2

300 700

000

Mill. -

2

Mill.

y.

Mill.

y.

y.

350

Fig. 2. Stratigraphy of the Karoo Sequence in the northeastern half of the Main Karoo Basin (compiled after SACS, 1980).

4U (upper) (Fig. 3). Another noncoal-bearing sequence of 40 m follows and is capped by seam No. 5, which is not economic in the present coalfield. After the northeasterly retreat of the Dwyka ice sheets, remnant glacial valleys with a north-south orientation reflected the major directions of previous ice movement (Tankard et al., 1982, pp. 368-369). Subsequent basinal sedimentation of the Ecca Group comprised a partly marine basinal paleoenvironment with flysch-type deposition in the south and a gently subsiding shelf platform towards the northeast (Ryan, 1968). The platform facies along the northeastern margins of the Ecca basin (where the southeastern Highveld Coalfield is situated) comprises widespread clastic fluviodeltaic deposits along the northeastern margins of the Karoo basin (Hobday, 1978). The underlying pre-Karoo rocks mainly belong to the lavas of the Ventersdorp Supergroup and the sedimentary strata of the Transvaal Sequence and the Witwatersrand Supergroup (Figs. 2 and 3). DATA ACQUISITION AND METHODS OF INVESTIGATION

The logs of some 400 boreholes that were drilled in the study area over the past two decades are the main source of data. Most of the drill holes penetrated the entire Karoo Sequence present here. Seam profiles were measured and sampled from various places underground in the mine, to provide insight into the effect of the surrounding sediments and their depositional environment on the coal seam itself. In addition, thin sections, mainly from argillaceous rocks

55 LFA

SANDSTONE FACIES micaceous, shale clasts and coaly plant debris

08

SIL TSTONE= SHALE FACIES Jnterlam~nated No 5 COAL SEAM with torbanite and carbonaceous shale SANDSTONE FACIES rnarnty matrix-supported SlLTSTONE SHALEFACIES localized ~nd interlamtnated:wJth rare coat seam/ets SANDSTONE FACIES marniy mat[ix-supported

•- - L F A 0 6 - - SANDSTONE FACIES micaceous, coaly dedrts, trough cross-beds flaser beds and r~ppies SILTSTONE SHAEEFACIES mterlaminated

LFA 05 MANN COAL,ZONE

No 4U COAL SEAM si/tstone shale facies m the ricer rocks PARTING Gnts, Sandstones Slltstones interlam~nated NO 4L COAL SEAM FARTING Sandstones No 3 COAL SEAM sillstone - shale facies ~nfloor rocks

~05

SANDSTONE FACIES matrix - supported and

LFA 04

LFA 03

StLTSTONE SHALEFACIES Jntedaminated •gra~ned sandstone interbeds, coat seamlets (up to 20 cm) in black carbonaceous shale

SILTSTONE SHALEFACIES interlaminated and SHALE FACIES with coal seam/ets SANDSTONE FACIES mainly matrix supported rs and arkostc, locally gritty interbeds of fine slltstone

LFA 02 SANDSTONES FACIES mainly grain supported well sorted; shghtly arkossc

LFA

01

SHALE FACIES black carbonaceous, coaly debris, ] dropstones, varved sl DIAMtCDTE~AClES intefbedded wl reworked matrix- supported conglomerates and ~ 's,

PRE-KARO0

I

BASEMENT

L[GENO upwaldcoarse¢i~O

coal torbanili¢ shale

upward firing

skllI

Planar cress-bedding

$hale-silts~o~ e inlerlamma~ed

hmizontll bedding

Slltstone

bi°lurbati°n

~dlum-c~rze

slractureballand plll©w

grlls and coar~gralned sandsIo~ diamictrites w,fh conglDmerates in a shaly matrix ba~ment r~k

f~ne-medlum g[ained sandstone

~ ~

soft sediment defofmatiefl

ii

\

i

grained sanJsione

i

Fig. 3. Generalized sequence of lithofacies associations (LFA's) in the study area.

56 within the sedimentary sequence, were examined for their mineral content and grain size distribution. For the subsequent determination of the distribution of the coal parameters only data from the No. 4L seam were available. Reconstruction of the sedimentary history of the study area was approached by developing a three-dimensional model of the sedimentary rocks at the mine. Vertical lithofacies associations (LFA's) were identified and correlated from borehole to borehole in order to establish their lateral relationships. The compilation of isopach plans of each LFA, as well as present-day elevation maps of some interfaces between LFA's formed the basis for interpreting paleoenvironmental conditions. LITHOSTRATIGRAPHICDESCRIPTION The description of the sedimentary sequence at the study area is based on an observational (Reading, 1982, p. 4) concept of vertical lithofacies associations (LFA's). A series of eight LFA's has been established, based on major lithological changes within the sedimentary column. Each LFA is developed over the entire coalfield and represents a distinct vertical sedimentary rock unit within the present sequence of Karoo sediments (Fig. 3 ). The facies within one LFA are considered to be genetically related and are either randomly interbedded or in a sequential order, with mostly gradational transitions. Each LFA has a relatively constant thickness throughout the coalfield and thus represents a specific phase of deposition, with little lateral variation. However, a single facies within an association may only occur locally. The facies within one LFA are described on the basis of their lithology, their sedimentary and biogenic structures, their spatial distribution and their association with one another. The present-day pre-Karoo topography in the study area has a slightly undulating relief, with two north-south-trending paleovalleys, and slopes southwards (Fig. 4).

LFA 01 comprises a massive diamictite facies, with lesser matrix-supported conglomerates and coarse-grained sandstones, and a shale facies, with occasional siltstone and sandstone interbeds (Fig. 3). Both facies can alternate, but the diamictites and associated rocks generally occur as a massive layer underneath the shales. The diamictite facies mainly accumulated in the paleovalleys of the preKaroo topography (up to 50 m thick) and thins out rapidly over the paleohighs, in places to nil, while the shale facies is generally more evenly distributed. The present-day post-Dwyka topography reveals a subdued surface as compared to the pre-Karoo topography, and has a more gentle north-south slope.

57

PAL0lH ~

Fig. 4. Present-dayelevationof the pre-Karootopography (metersabovesea level). LFA 02 is dominated by a lower, 25-30 m thick, sandstone facies with rare bands of interstitial mica and clay minerals. The contact with the underlying LFA 01 is sharp (Fig. 3). Grain sizes vary randomly, but do display subtle fining-upward and coarsening-upward trends. The upper 10-20 m of this LFA are characterized by a generally coarsening-upward, matrix-supported sandstone facies, with grain sorting decreasing towards the top, and is terminated by a generally sharp upper contact. LFA 03 comprises a black carbonaceous shale facies and an interlaminated shale and siltstone facies; a sandstone facies occurs as occasional coarse-grained interbeds. Finely disseminated pyrite and pyrite nodules are abundant throughout these shales. Both facies alternate randomly in both vertical and lateral extent. Generally, the carbonaceous shale facies, however, dominates in the basal half of LFA 03, and the silty facies and sandy interbeds increase towards the top. Hence a subtle coarsening-upward trend exists within LFA 03. LFA 04 comprises two major components. Oneis a medium- to coarse-grained, partly matrix-supported sandstone facies. The other contains finely interlaminated siltstones and shales, with some black carbonaceous shales and few fineto medium-grained and coarser-grained sandstone interbeds. Mica and pyrite are common, coal seamlets occur sporadically in the carbonaceous shales. Load casts and ball-and-pillow structures commonly occur below contacts with the overlying sandstone facies; bioturbation is rare (Fig. 3). The two facies alternate in large scale units of up to 10 m thickness, with

58

sharp as well as gradational contacts. In general, fining-upward trends predominate; however, coarsening-upward sequences also occur. The sediment package of LFA 04 averages 22 m in thickness, but reaches a m a x i m u m of more t h a n 40 m and a m i n i m u m of less t h a n 10 m, over large areas in the eastern half of the study area. The paleotopography after deposition of LFA 04 resembles a muted version of the pre-Karoo paleosurface (Fig. 5). L F A 05 (the main coal zone) includes coal seams No. 3, No. 4L and No. 4U as well as their respective partings. The No. 3 and No. 4U seams are commonly underlain by shales and siltstones in the floor rocks, whereas No. 4L largely developed on fine- to medium-grained sandstones. The partings between the seams are characterized by sandy and gritty material, with minor amounts of silty and shaly sediments. The No. 3 coal s e a m consists mainly of mixed lithotypes, clarain and duroclarain, with only a few clastic lenses in the seam, mostly containing silty and shaly material and minor sandstone. The floor rocks of the seam are dominated by finely laminated micaceous shales and siltstones. These rocks mostly occupy depressions in the present-day elevation of the top of LFA 04, particularly in an extensive area over the eastern pre-Karoo paleovalley, and thin out to nil over flanks of these depressions (Figs. 5 and 6). The coal of No. 3 seam averages about 50-60 cm in thickness. Two narrow

LEGEND

~

LFA04 fhlcker fhan horn

~ ~

LFA04 fhIcker ~ fhan 25m ~

LFA04 fhinner fhan 20m

~ ~

LFA04 fhinner fhan lore

Fig. 5. Present-day elevation of the interface of LFA 04 with LFA 05 (meters above sea level), with isopach patterns of LFA 04.

59

FLODDPLAN

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5

LEGEND

'h'cksandstones°fLFAO4"l not covered by No3 seam, and associated shales

:

I

shales before and confem= poraneous wlfh No 3 cool seam

m

~

thin coal

: :~

fhlck coal

medium fh/ck coal

Fig. 6. Isopach map of No. 3 coal seam, also showing the distribution of contemporaneous shales and LFA 04 sandstones. r

0

~

'~

~

~

~

LE6END thick porhng > 3m ~.~..~q

sondsf . . . . . f LFA 06 not covered by parting

~ 1

shales before and confem': poroneous wlfh No 3 seam

~ s h o l

. . . . . . [oppmg No3 seam ond'LFA 04

-

-

outline of porhng sandstones

~ cool of NO3 ~ ' , ~ ' ~ < ~ seam

contour mfervols in meters

Fig. 7. Isopach map of sandstone parting between No. 3 and No. 4L seams, also showing distribution of shales abutting and overlapping No. 3 seam and of LFA 04 sandstones.

the

60

bright coal stringers

L EG E N D

~--]

VlTRAIN BRIGHT CLARAIfi

bright coal stringers CLARAIN DORAIN TORaJ'JITL lustrous brtght coal strtngers

CARBONACEOUS SHALE

~

SILTSTON[

EI~ lustrous and brsttle bright coal str,ngers

i

bmgh[ coat slrmgers lustrous and brittle bright c.oal strsngers

V

SANOSPONE

SIOERITECONCRETIONS PYRITE CONCRETIONS

BRIGHT COAL STRINGERS ARE GENERALLY LESS THAN 3mm THICK

lustrous

'i

some

lustrous bands

I t~r~ght coal strlnqers

Fig. 8. Representative profile of coal bands in the No. 4L seam. zones of m a x i m u m accumulations in the central portion of the coalfield area trend north-south and east-west, respectively, and occur above and alongside thin and intermediate shale deposits in the floor. There is no coal overlying the thick shales and siltstones above the eastern pre-Karoo paleovalley or the thick occurrences of LFA 04 (Figs. 5 and 6). The parting between No. 3 and 4L coal seams contains a fine-grained, grey to brownish, very micaceous sandstone, with rare pyrite and siderite concretions and some trough cross-bedding. Where No. 3 coal seam is not present this sandstone parting overlies the shale and siltstone facies associated with the seam. It is particularly thick over the eastern paleovalley and in the northwestern corner of the study area, however, the parting is thin to absent above the thick sandstones of LFA 04 (Figs. 5, 6 and 7). The No. 4L coal seam consists of clarain and durain lithotypes, with a few prominent vitrain bands. Torbanites and torbanitic shales form sporadic lenses and bands. Pyrite concretions and calcite occur along cleavages throughout

61

the seam and siderite nodules accumulated in a number of distinct horizons (Fig. 8). Medium-grained, locally cross-bedded sandstones and minor shales form inseam partings, which are concentrated in the basal and top one thirds of the seam. Their cumulative thickness ranges between 10 and 50 cm. Individual lenses seldom exceed 10 m in lateral extent, but areas of their occurrences can be up to several 100 m across and are abundant in the eastern half of the study area. The No. 4L coal seam covers the entire coalfield. Relatively thin coals are concentrated in the eastern part of the coalfield, where the floor rocks are dominated by thick LFA 04 sandstones (Figs. 5 and 9). Coal accumulated to extreme thicknesses over the eastern pre-Karoo paleovalley, where the No. 3 seam is missing and the associated floor rock shales are thickly developed (Figs. 6 and 9). The parting between No. 4L and No. 4 U coal seams is an irregular succession of coarse-grained to gritty sandstones and gritstone, with medium- to finegrained sandstones, siltstones and shales. The sandstone and grit facies are generally quartzitic, rarely arkosic and micaceous. A glauconitic band occurs locally in the basal 30 cm of the parting. The siltstone and shale facies is finely interlaminated, locally carbonaceous and contains some sandy load casts. This entire facies association is poorly sorted; fining-upward and coarsening-upward sequences alternate rapidly and randomly. It averages about 5 m m

LEGEND

coal thicknesses

Fig. 9. Isopach map of the No. 4L seam.

62 in thickness and reaches extremes of up to 14 m along the eastern margin of the study area, where it becomes particularly gritty (Fig. 10). The No. 4 U coal seam is initiated by a finely laminated shale and siltstone facies (40-60 cm thick on average) in the southern parts of the study area. This facies pinches out towards the north against the preceding gritstone layers. The coal seam comprises mixed lithotypes and occupies almost the entire coalfield with an average thickness of 40-50 cm. In the southwest and southeast of the field area it is replaced by shales and siltstones, and along the eastern margin sandstones and grits split the seam by 10-160 cm. LFA 06 comprises a sandstone facies as well as a siltstone and shale facies

in its lower portion. The siltstone and shale facies is finely interlaminated and partly carbonaceous. Contacts with overlying sandstones often display slumping and ball-and-pillow structures. Generally, there is no overall cyclicity of these two facies; however, the basal portion of LFA 06 often begins with one or two coarsening-upward sequences, that are followed by a number of finingupward arrangements. The upper portion of LFA 06 is dominated by a locally very arkosic sandstones facies with rare pyrite and siderite. Grain sizes are variable and indicate a fining-upward tendency. Siltstones and shales, with associated coal seamlets, form a locally prominent band in the top of this sandstone sequence. LFA 07 contains coal seam No. 5, with clarain and vitrain lithotypes as well

as torbanitic and black carbonaceous shales. Pyrite concretions and calcite

/ \~

~,

~ ~

4 :::::::::::::::~

~lii!'t;~ ;~ .:::::' :: ~.~:x=y~ i!:::ii[:i!!:..~<:{ii

EGEND 'i::::::~

areas of parhcularly

iiiiii:.!:.:.j gr, tt~ p,~rt,~g

contour /ntervals

Fig. 10. Isopachmap of parting betweenseamsNo.

in meters

4L and No. 4U.

63 along cleavages, as well as minor siderite occur within the coals. Shaly and coarse sandy lenses split the seam up to 120 cm and locally replace the coal. The coals average 60-80 cm in thickness, and are succeeded by 1-2 m of black carbonaceous shale, which is often torbanitic in its basal 50 cm. The floor rocks of coal seam No. 5 are dominated by LFA 06 sandstones.

LFA 08 resembles LFA 04 and the lower portion of LFA 06, with an alternating sandstone and siltstone/shale facies. Within the siltstone and shale facies ball-and-pillow structures and load casts locally occur near the interfaces with overlying sandstones. MODEL OF DEPOSITIONALPALEOENVIRONMENT A coal-forming environment requires steady, long-lasting and balanced sedimentation (Stach et al., 1982, p. 9), during which normal depositional processes and in exceptions normal erosional processes (Reineck and Singh, 1980, p. 5; Reading, 1982, pp. 10-11) can be expected to predominate. The normal processes include pelagic settling, organic growth, tidal and fluvial currents, as well as compaction and diagenesis. It is considered that analogous processes formed the observed lithofacies associations at the coalfield discussed here. The previously described LFA's will be related to events (Reading, 1982, pp. 10-11), which delineate single sedimentary processes or several contemporaneous processes. These in turn, together formed the depositional paleoenvironment, in which the respective LFA was laid down. The chronological succession and partial overlap of these events in various parts of the study area is illustrated in Fig. 11. The pre-Karoo topography is the result of erosion by glaciers during the Permo-Carboniferous glaciation of Gondwanaland. The north-south-trending paleovalleys and paleoridges in the study area (Fig. 4) correspond with the major directions of ice movement reported by Du Toit (1954) and Tankard et al. (1982, p. 368) for the northeastern Karoo basin.

Event 01 is controlled by an interaction of sedimentary processes of various energy levels, which resulted in the complexity of LFA 01. Such variable processes commonly coexist in a glacial environment (Reineck and Singh, 1982, p. 192). The diamictites are regarded as subglacial tillite deposits, interbedded with reworked, glaciofluvial sandstones and conglomerates, and proglacial lacustrine shale deposits. This assemblage of coarse-grained, crudely bedded sediments usually derives from braided channel systems on a glacial outwash plain (Eyles and Miall, 1984, p. 19). Similar settings are suggested for the Dwyka

COAL SEAM No 4L

SANDSTONES ":--~- SILTSTONES/SHALE" COAL SEAM No 4U PARTINGBETWEEN ~ :.i~ NO 4L and No 4U SANDSTONES .... SLTSTONE, GRIT

backswarnp

mainlyrneanderin§ Ioca#ylacustnne with Gilbert type deltas

SEMENT

I

i

flood plain

Basal

Upper

~,

i

J

...................\

carries on locally

...........v , y \

backswamp peat

and backswamp

continue activity

//

floodplain

"~ flood plain & play deposits erosional braided channels

~--

flood plain

/"

•

[ oxbow lake

/ /

NOTES

erosional achvity

E Waxis ~ ' ~ /

~-

_

Jsnotscaledandmerely~ndicates relativesuccess,'onand temporal overiapsof events indJcatesaneast-westcross-sechonacrossthe coalfield (approx 20kin mte~al extent)

~"

•

_

Verhcalaxis

localized glacio-lacustflne and glacso-fluvial reworking subglacial till deposflion

Glacio - lacustnne

~

~

main meandenng river belt conbnues

meander belt shifts out to East otstudy area i

backswamp over late abandoned channels

distebutocy channels ...... u~flvity deltas are tide and wave dommated thus reworked deposlts

deltas are fluwally dominated shift back and forth alOng shoreline

Iower portion

upper portion

delta switching and avuls~on

delta plain and marsh deposits

SANDFAClES active channels, point bars SIL TSTONE/ SHALE floodplain, oxbow lake clay plugs, levees COAL localized backswamp, peat bogs

play depostts

J

clastic influx at htgh flood stages

erosion

clastic influx at high floodstages /

crevasse splays

flood plain

backswamp

mainly meandering rivers locally lacustnne enwronment wsth wave actw~ty Sandstones channels and pomtbars Stflstones/shales flood plain, oxbow clay plugs, levees

malnly braided rlvers shot? ~ntervalof Iocahzed Jnterchannel and swamp deposits

gradually flooded with formation of extenswe flcodplam lakes and ponds #ackswamp ~nbottom porbon

.-,,-E

shifting back of meander belt towards West crevasse splays early

RELATIONSHIPOFEVENTSACROSSTHESTUDYAREA

meandering, locally lacustnne environment wtth small scale deltas

ENVIRONMENTALDETAILSWITHCHRONOLOGICALANDSPATIAL

_/

W

Fig. 11. Diagram of the chronological succession of all eight events and their temporal and spatial relationships•

O0

PR~ . . . . .

SUBGLAClAL T( ~ PARAGLAClAL

SHALES SHALES CONGLOMERATES DIAMICTITIES

01

SHORELINE Gilbert type deltas fed by braided rivers on glacial outwash plato

INTERDISTRIBUTARY EMBA YMENTS ON DELTA PLAIN

FLUVIAL

coal

Shales/Stltstones: floodplain, clayplugs oxbowlakes, levees and I crevasses

mayorchannelsand

mlnor eroslonal braided channels

SANDSTONES

SHALES/ SlL TSTONES

SANDSTONES

COAL SEAM No 3 with preceed~ng and contem I shales & siltstones

ETWEEN NO 3andNo 4L

Sandstone:

Gnts

FLOODPLAINANDBACKSWAMP OFA MEANDERING RIVER SYSTEM

02

(: : : ;

": :i~

: :

..=

I

~ : i : ~ SANDSTONE PARTING

:

Locallylacustnne withsmallscaledeltas

FLUVIAL

SANDSTONES SHALE/COAL

SILTSTONE SHALES COAL SEAMLETS

"

~-~,~

:

masnlymeandermg

FLOODPLAN&BACKSWAMP ON ABANDONED BRAIDEDRIVEBPLAIN

FLUVIAL

PRESUMEDPALEO ENVIRONMENT

COALSEAMNo 5 carbonac shale torban,ticshale coal

SILTSTONES SHALES

SANDSTONES

GENERAL LITHOLOGICAL DESCRIPTION

03

04

05

os

07

08

L/THO = LOGICAL PROFILE

J

0"~

65 sediments in the northern Karoo basin by Von Brunn and Stratten (1981) and Tankard et al. (1982, p. 368). Massive more evenly spread shale deposits above tillites (top of LFA 01) reflect sedimentation in periglacial paleoenvironments (Eyles and Miall, 1984, p. 20 ). They commonly cover glacial lake bottoms (Reading, 1982, p. 416 ), and reflect deposition from suspension (Easterbrook, 1982, p. 8). Bioturbation, coal fragments and pyrite indicate quiescent water basins with marginal peat swamps, as found in modern northern temperate lakes by Fouch and Dean (1982). Increasing fluvial activity probably produced the sandstone and siltstone interbeds and soft sediment deformation structures observed in the top of the shale facies (Fig. 11). Event 02 suggests an intercalation of fluvial and deltaic activity (Cadle and Hobday, 1977; Reading, 1982, p. 48 and 98), resulting in an alternation of fining-upward and coarsening-upward sandstone sequences in the basal portion of LFA 02. The sedimentary structures and variable grain sizes suggest lateral accretion, rapid sedimentation and fluctuating discharges, which are typical of meltwater streams and glacially related deltas (Easterbrook, 1982, p. 7). Similar Gilbert-type deltas (Stanley and Surdam, 1978) are reported by Cairncross and Winter (1984) from the southern Witbank Coalfield. The sheet-like distribution and the maturity of the sandstones, as well as the lack of elongated sandstone bodies, suggest rapidly shifting active delta lobes along a shoreline that exceeded the boundaries of the study area (Fig. 11 ). Fine particles were removed by the winnowing effects of basinal processes such as tidal and wave reworking (Reineck and Singh, 1980, p. 432; Reading, 1982, p. 63 ). These processes predominated over fluvial and deltaic influences for most of the time during Event 02, but decreased towards the end of the event, as indicated by a final coarsening-upward cycle and by poorly sorted matrix-supported sediments. These coarse-grained sediments point to sandy braided river deposition, related to irregular discharge and a high proportion of bedload sediments, which commonly occur in distributary channels of deltas in a polar climate (Reading, 1982, p. 103). Analogous sedimentation is reported from modern proglacial environments in Alaska and Iceland (Boothroyd and Nummedal, 1978). Event 03, succeeding the fluviodeltaic system of Event 02, is characterized by an interdistributary embayment environment with localized marsh deposits (Reading, 1982, pp. 38 and 125 ). Quiet water shale deposition from suspension, reducing conditions (pyrite) and burrowing organisms are characteristic of lacustrine environments in a delta setting (Coleman and Prior, 1982, p. 147). Soft sediment deformation structures and laterally variable amounts of silty sediments as well as minor sandstones are probably the result of overbank deposition and crevassing from the distributary channels of Event 02 (Fig. 11 )

66 into these secluded areas (Reineck and Singh, 1980, p. 298; Reading, 1982, pp. 32-39). E v e n t 04 comprises fluvial processes, which commonly succeed interdistributary bay and marsh conditions (Cant, 1982, p. 118). Current velocities were less than in Event 02, as indicated by the generally finer sediments of LFA 04. The combination of lithologies and sedimentary structures in LFA 04, particularly the occurrence of bioturbation and coal seamlets, suggests a highly variable fluvial paleoenvironment, with localized lakes (Reading, 1982, p. 53 ). The complex vertical and horizontal facies changes, including both fining-upward and coarsening-upward sequences, are attributed to a meandering river system (Jackson, 1978, p. 543; Flores, 1983), where the sandy sediments indicate channel and point-bar deposits as well as crevasse splays. The siltstones, shales and associated coal seamlets represent floodplain deposits and clay plugs from oxbow lakes and abandoned channels. A meandering river system is more likely for Event 04 than a braided river setting, due to the generally high proportion of suspended load clastics (Reading, 1982, p. 31); the latter setting would have produced an almost entirely sandy succession. The lateral facies variation within LFA 04 further supports a meandering river setting, as this normally shows less lateral shift than a braided river (Cant, 1982, p. 119). Towards the end of Event 04, continuous channel and crevasse sandstone deposition along the eastern margin of the study area, as well as early channel abandonment over the eastern pre-Karoo paleovalley re-emphasized the preexisting topographic features (Figs. 5 and 6). Contemporaneous to this, Event 05 commenced with the deposition of its lowest layers in other parts of the coalfield (Fig. 11). E v e n t 05 exhibits all the characteristics of Events 03 and 04, with the exception of massive coal seams, which represent long lasting and widespread backswamp conditions (Stach et al., 1982, p. 376). This implies that a meandering river paleoenvironment prevailed during Event 05; however, at different periods, either floodplain and backswamp conditions or channel processes controlled the deposition in the coalfield. The No. 3 coal seam, with associated shales and siltstones, represents a phase of widespread floodplain and backswamp paleoenvironments during the initial stages of Event 05. A predominance of mixed-coal lithotypes indicates relatively inferior peat preservation conditions, due to contact with oxygen-rich waters (Stach et al., 1982, p. 34). The northwestern part of the study area was characterized by shale deposition under lacustrine floodplain conditions. In the east of the coalfield the meander belt was still active (Fig. 11) and abandoned channels and oxbow lakes were found above the pre-Karoo paleovalley, where fine channel-fills (clay plugs) were deposited before and contem-

67 poraneous with the peat of the No. 3 seam. The proximity of such an abandoned channel is also indicated by the rapid pinching out of the seam towards the east (Figs. 5 and 6). The parting between seams No. 3 and No. 4L, with widespread fine-grained sandstones covering the preceding backswamp and floodplain deposits, is commonly the result of major phases of crevassing, in an overall meandering fluvial system. Trough cross-beds develop in such sandstones as a result of small-scale channel system on top of the crevasses (Coleman, 1969; Reading, 1982, pp. 39 and 105) and horizontal lamination typically results from overbank deposition. The abundance of mica within the sandstones suggest that no basinal winnowing processes affected the deposits. These crevasse sandstones were mainly deposited from the meandering channel systems east of the study area (Fig. 11) and filled up depressions in the floodplain with relatively thick deposits, such as into the presumed oxbow lake over the eastern pre-Karoo paleovalley. The No. 4L coal seam is the result of long-lasting, stable and widely uniform backswamp conditions, which formed laterally persistent lithotype bands within the seam (Stach et al., 1982, p. 376), particularly the vitrain bands. Laterally continuous siderite marker horizons within the seam give evidence of stable freshwater-dominated periods during this part of Event 05 (Stach et al., 1982, p. 164). In-seam clastic lenses settled out from suspension into swamp lakes and ponds in the eastern parts of the study area, within the reach of channel influences, and probably reflect over-bank sedimentation at flood peak periods (Reading, 1982, p. 38). Torbanites and torbanitic shales within the seam derive from increased algal growth in deep swamp lakes and abandoned channels (Stach et al., 1982, p. 264). The proximity of an actively shifting channel belt is indicated by thin coals in the eastern regions; these reflect late abandonement of the active channels in the initial phase, early termination of the peat growth and erosional activity by a reactivated meander channel during the final stage of the backswamp (Figs. 7, 9, 10 and 11 ). The western and central portions of the seam developed with few or no channel influences. The extremely thick coal above the eastern pre-Karoo paleovalley most likely resulted from higher compaction rates of the underlying thick shales compared to the sandstones in other parts of the floor rocks; frequent crevassing from the nearby eastern channel belt provided ample nutrient supply to improve plant growth (Stach et al., 1982, p. 31 ). The parting between seams No. 4L and No. 4U reflects a complex interfingering of channel, levee and rapid crevasse deposition with steady floodplain sedimentation (Reading, 1982, p. 53 ). The coarse and gritty material, concentrated in the east of the study area, points to channel erosion and rapid, highenergy fluvial sedimentation above the coal seam, probably related to channel switching (Figs. 9, 10 and 11 ). The fining-upward characteristic of the parting sediments into the mainly silty and shaly floor rocks of seam No. 4U can be

68 attributed to a gradual cessation of channel activity and a re-occupation of the study area by quiescent floodplain conditions. Glauconite occurrences above coal seams are regarded as an indicator of marine conditions in studies of the Witbank Coalfield (Le Blanc Smith, 1980; Cairncross and Winter, 1984). However, glauconite and glauconitic mica are also reported from nonmarine sediments (Porrenga, 1968) and an explanation for its formation in an alluvial and lacustrine paleoenvironment is given by Keller (1958). The wide range of environmental conditions favorable for glauconite formation make it unsuitable for paleoenvironmental interpretation (McRae, 1972). Therefore, as its occurrence is neither persistent throughout the study area nor concentrated in channel belts, where a marine ingression should first have an effect, glauconite is not taken as an indicator for marine conditions in this study. Glauconitization is regarded as an alteration process of a degraded 2 : 1 layer mica lattice in a potassium- and iron-rich environment, with a favorable oxidation potential (Burst, 1958). The No. 4 U coal s e a m developed under similar floodplain and backswamp conditions as the No. 4L coal seam, and its formation is related to another abandonment of the study area by the active channel belt. The predominance of mixed-coal lithotypes suggests a peat swamp with relatively deep water conditions, similar to coal seam No. 3. Towards the east, channel, levee and crevasse deposition, as found in the underlying parting, caused seam splitting and partial removal of the coal. Thickly developed shales and siltstones in the floor rocks outline oxbow lakes of a meandering river system, preceeding the peat growth. E v e n t 06 initially developed under the continuous control of a meandering

river system. The uncommon flaser-beds and ripples in these sediments are attributed to temporary wind-driven currents flowing against the regional paleoslope in localized floodplain lakes (Reineck and Singh, 1980, p. 112; Reading, 1982, p. 74). The previous coal swamp is regarded as having gradually drowned and been succeeded by extensive lacustrine conditions, with local crevasse deltas (Flores, 1983), indicated by the one or two coarsening-upward sequences at the base of LFA 06. The succeeding fining-upward cycles in the lower portion of LFA 06 were probably formed by channel and abandoned channel sedimentation. The later phase of Event 06 is controlled by higher, irregular discharge through the channels (Reineck and Singh, 1980, p. 311 ). Thin horizontal bedding and planar cross-bedding are ascribed to upper flow regime plane-bed deposition and bar migration (Reineck and Singh, 1980, pp. 122 and 109). The previously suggested meandering river system thus appears to have been succeeded by a sand-dominated braided river system, which deposited a more uniform sediment package (Rust, 1978, p. 608), where the finer fraction (sporadic shales and coal seamlets) accumulated in localized swampy interchannel re-

69 gions. A modern analogue of such a change from a meandering system to a braided river is described by Coleman (1969). Event 07 formed the No. 5 coal seam. The rapid change from Event 06, and the subsequent predominance of arenaceous rocks in the floor of the seam, is ascribed to channel switching and sudden abandonment of the study area. Thus a backswamp paleoenvironment with relatively deep water conditions prevailed during Event 07. Initially, localized overbank sedimentation produced the in-seam clastic lenses, especially occurring along the eastern margin of the study area. Later, water depth slowly increased to form extensive lakes and ponds, in which torbanitic (Stach et al., 1982, p. 264) and eventually carbonaceous shales accumulated. Event 08 is a repetition of previous ones, such as Event 04. It is therefore also attributed to an intercalation of channel and floodplain deposition in an overall meandering fluvial paleoenvironment. The pre-existing lacustrine setting of Event 07 was subsequently covered by fluviolacustrine sediments of LFA 08, under conditions similar to those prevalent at the beginning of Event 06.

SUMMARYOF THE MODEL This study area underwent a paleoenvironmental history ranging from a subglacial period to a fluvial setting in which the main coal-bearing strata were formed. It was mainly characterized by continental influences which produced fluvial, fluviodeltaic and fluviolacustrine facies associations (Fig. 11 ). Preliminary subglacial conditions were followed by proglacial outwash plains with glaciofluvial and glaciolacustrine deposits (Event 01 ). These sediments caused the post glaciation paleotopography to be largely levelled. A lacustrine or shallow basinal paleoenvironment succeeded, which was dominated by Gilbert-type deltas within a glacially controlled paleoclimatic setting. The deltas were initially extensively altered by basinal shoreline activity, namely wave and tidal processes. Later, fluviodeltaic conditions with braided channels prevailed (Event 02). An alluvial plain paleoenvironment dominated the sedimentation during Events 03 to 08. Throughout the main coal-forming period, meandering rivers characterized the paleoenvironment; braided channels appeared towards the close of Event 06. A coexistence of channel, floodplain, backswamp and lacustrine settings, with localized crevasse deltas in the latter, resulted in a great variety of sediments, in both vertical and horizontal extent. Factors controlling sedimentation in this coalfield were as follows: ( 1 ) Initially a decreasing paleoslope, related to filling of glacial valleys, gave regional control over paleocurrent directions, which were orientated mainly

70 southeasterly and southwesterly during Ecca Group sedimentation (Ryan, 1968). This caused rapid active channel switching and subsequent sudden environmental changes in the study area during Events 01 and 02. (2 t The pre-Karoo paleotopography, which appears to be of major importance for sediment distribution in other South African coalfields (Cairncross, 1980; Le Blanc Smith, 1980), had an impact only during early sedimentation in the Secunda area (Event 01 ). Later, sediment distribution was the result of environmental changes or localized subsidence due to differential compaction. (3) A braided stream system was commonly present during the noncoalforming phases (Event 02 and upper portion of Event 06) and a meandering river system was dominant during major coal-forming phases (Event 03 to lower portion of Event 06). Thus lateral channel shifting was a significant process during noncoal-forming events and was restricted during coal-forming events. This caused laterally uniform sheet-like sedimentation during braided river periods and laterally variable facies associations during meandering river phases. N.D. Smith (1970) attributes braided streams to discharge fluctuations and easily erodible banks, with mainly arenaceous sediments, such as in LFA 02 and the upper portion of LFA 06. Contrary to that, meandering rivers require a lower slope, cohesive bank material, as provided by argillaceous sediments and peat, and a relatively steady discharge (Smith, D.G., 1976; Reading, 1980, p. 31), such as observed from LFA 04 to the lower portion of LFA 06. DISCUSSION OF THE MODEL Previous studies on depositional paleoenvironments in South African coalfields (Cadle and Hobday, 1977; Cairncross, 1979 and 1980; Le Blanc Smith, 1980) generally propose several superimposed deltaic cycles for the coal-bearing Ecca sediments. The peat swamps are generally placed in delta plain environments. Major fluviodeltaic paleoenvironments are considered controlling factors over coal formation by Cairncross (1980), and fluvial processes on an alluvial plain at times of major peat growth are proposed by Hobday (1978). This study differs from previously established models for South African coalfields, in that basinal influences and even deltaic control over the sedimentation are proposed to have been of minor importance, and then only during Event 02, and to have ceased before development of the main coal. They thus had no influence on coal distribution. Evidence for a fluvial paleoenvironment on an alluvial plain, as proposed here, has also been found by Hobday (1978). The meandering fluvial setting, proposed for the study area throughout most of the depositional phases and especially during peat accumulation, is only known to have been suggested for the Davel Coalfield near Ermelo (Fig. 1 ), southeastern Transvaal (Steyn, 1976). The model proposed for this coalfield is not necessarily contradictory to pre-

71

vious models for South African coalfields. Most of these investigations concentrated on the Witbank Coalfield (Cairncross, 1980; Le Blanc Smith, 1980), which lies to the north of the present study area, and on the northern Natal areas (Cadle and Hobday, 1977), which lie to the southeast of Secunda (Fig. 1). The mainly deltaic paleoenvironments suggested for the Witbank coals, specifically Gilbert-type lacustrine deltas (Cairncross and Winter, 1984), appear plausible, as that basin was almost entirely surrounded by pre-Karoo basement rocks, including the Smithfield Ridge to the south (Fig. 1). This paleogeographic factor most probably placed the Witbank basin in a unique and secluded situation as a major lacustrine basin. From there towards the south, into the main Karoo basin it appears likely that sedimentation and peat development took place on a very gentle paleoslope and within a fluvial paleoenvironment, further away from the source areas. Paleocurrent analysis in the Ecca Group by Ryan (1968), of the area south of the Smithfield Ridge, indicates that source areas for the Secunda Coalfields lay mainly towards the north and northeast, in and beyond the Smithfield Ridge. This again seems to place the Witbank Coalfield into the secluded position of a separate basin, which only had contact with the main Karoo basin via a number of prominent streams across a wide foreland, on which both the presumably meandering fluvial Secunda and Davel Coalfields developed. This study suggests a paleoenvironment analagous to that which can be observed in present-day Siberia, Finland, and northern Canada. Here, extensive peat swamps in fluvial and lacustrine environments dominate wide parts of the landscape, as, for instance, the muskeg in northern Canada {Radforth, 1969). These recent swamp and alluvial deposits in most cases succeed Quarternary glacial sediments (Smith, N.D., 1975; Smith, D.G., 1976). THE EFFECT OF DEPOSITIONAL ENVIRONMENT ON COAL DISTRIBUTION AND QUALITY PARAMETERS

In fluvial environments peat accumulates in close proximity to the fluvial channels and forms elongated seams parallel to these channels (McLean and Jerzykiewicz, 1978, p. 441). These features can be readily recognized in the distribution of the No. 3 coal seam, and are less apparent, due to the limitations of the study area boundaries, in the No. 4L seam. Pronounced features of the distribution of both seams occur in the proximity of the active channel belt in the eastern half of the study area. There the coals are thickly developed on the floodplains, beyond natural levees, along the banks of abandoned channels and oxbow lakes and on top of crevasse deposits (Figs. 6, 7, 8 and 9). These environmental factors promote peat growth by regular nutrient supply, due to frequently occurring flood stages, and balanced, persistent water coverage to allow the peat to be preserved (Stach et al., 1982). Coal seam No. 4L is also thick above the proposed oxbow lake, abandoned

72 channel and crevasse deposits overlying the pre-Karoo paleovalley, due to high compaction rates of the thick silty and shaly sediments beneath. Peat is thinly developed in the western half of the study area, further into the backswamp away from channel influences, due to a lower nutrient supply and lesser water coverage. In the eastern portion coals are thin above natural levees, due to poor preservation conditions, and above channels, due to relatively late commencement of peat growth as compared to other areas. In addition post-swamp erosional activity, attributed to shifting channels, also caused diminished seam thicknesses. Peat is very thin to absent above floodplain lake deposits, where water depth was too great to allow plant growth, such as in the northwestern corner of the No. 3 seam. Peat accumulation at no time seemed to be significantly controlled by features in the pre-Karoo paleotopography, as sugggested for the Witbank Coalfield (Cairncross, 1980; Le Blanc Smith, 1980), where peat preservation was concentrated in pre-Karoo paleovalleys. The. coal distribution of seam No. 3 of this coalfield shows, rather, a contrary situation, with thick coals over a preKaroo ridge. Other seams, such as No. 4L, have thick coal in the eastern half of the study area related to differential compaction of underlying sediments. None of the seams here display features in the western half of the study area that can readily be related to the pre-Karoo paleotopography. The ash c o n t e n t of the No. 4L seam is generally about 20-26% on an air-dry basis (a.d.). It is conspicuously low (less 20% ) in the western part of the study area, away from channel influences (Fig. 12 ). The eastern half, however, shows minimum and maximum ash contents, with high values mainly along the margins of the underlying major crevasse deposits. Some high ash concentrations correspond well with the occurrence of in-seam clastic lenses (indicated by A or B in Fig. 12). This also coincides with the occurrence of the thickest coals of the seam. The ash is low in the No. 4L coal above the assumed abandoned channels, which preceded the swamp in the northeast of the study area. This correlates with a lack of crevassing into this area (Figs. 7, 9 and 12). The volatile m a t t e r c o n t e n t on a dry-ash-free basis (d.a.f.) is usually taken as a rank parameter of coals and therefore is not directly related to primary sedimentology. However, volatile matter is also dependent on the maceral composition of coals, which in turn is mainly the result of plant community and peat preservation (Stach et al., 1982, pp. 88 and 219). Both factors are controlled by depositional environment. Volatile matter (d.a.f.) in No. 4L seam is less than 32% in the western half of the study area; extremes of less than 30% are related to devolatilization by dolerite intrusions (Fig. 13). Values of more than 36% in the eastern half are associated with underlying crevasse, levee and channel deposits, which allowed a higher plant community to grow, due to more nutrient supply (Harvey and

73

A cot responds with in -seam clasbo lenses in top of the seam B = corresponds with in-seam clastic lenses in bottom of the seam

LEGEND

=

ash content (a.d.)

~<18%

~18"20%

~--~

2oOn~o26~t . . . . . . . .

~ 26-30%

I >30%

Fig. 12. Distribution of the ash c o n t e n t (a.d.) in the No. 4L coal seam.

LEGEND

volatile matter content (d.a.f.)

D~

D ~°-~ D:::=.

~~

Fig. 13. Distribution of the volatile m a t t e r c o n t e n t (d.a.f.) in the No. 4L coal seam.

74 Dillon, 1985) and a more stable rooting ground. This would have resulted in the accumulation and preservation of more vitrinite macerals, which usually have a relatively high volatile matter content compared to the inertinite macerals (Stach et al., 1982, p. 88). Extremely high volatile matter content of 38% (d.a.f.) is found in the coals above the abandoned channel, where deep water cover allowed mainly vitrodetritic and minor inertinitic material to be preserved. A high exinite content, presumably due to the occurrence of alginite in torbanites, may have added to an increased amount of volatile matter in the coals of that area. The calorific value is largely controlled by the carbon and hydrogen content, which in turn is partly the result of maceral composition and mainly that of rank (Stach et al., 1982, p. 467; Snyman et al., 1983). As especially the latter factor does not, under normal coalification, change significantly over the size of an area, such as investigated here, its effects on spatial changes in the calorific value will not be considerable. Therefore, components that diminish the calorific value, such as the ash content in the coal, will have a much higher effect. Thus, the calorific value of the No. 4L coals is inversely proportional to the ash content (Figs. 12 and 14). In the eastern half of the study area extremes of less than 22 M J / k g (a.d.) and more than 25 M J / k g (a.d.) calorific value correspond to highs and lows of the ash concentrations, respectively. However, there are divergences from this

LEGEND

calorific v a l u e (a.d.) > 26 MJ/kg

Fig. 14. Distribution of the calorific value (a.d.) in the

No. 4L coal seam.

75 relation, in t h a t the calorific value appears too high in comparison to the given ash content, such as in the northeastern corner and above the preceding crevasses above the eastern paleovalley (Figs. 7 and 14). This is attributed to possible variances in the maceral composition and subsequently the volatile matter contents in the coals of these areas, as discussed earlier. In the western half, away from the channel activity, the calorific value is constantly between 18 and 25 M J / k g (a.d.). CONCLUSIONS It is suggested t h a t this coalfield near Secunda developed in a meandering fluvial paleoenvironment. Coal distribution was controlled by fluvial and floodplain depositional and erosional processes, as well as by subsidence due to differential compaction. Coal parameters (ash, volatile matter and calorific value) show significant correspondence with depositional features. Major changes in coal distribution and quality parameters, and thus in the economic value of the seam, can be expected in the vicinity of major active and abandoned channels, whereas floodplains produce coal seams of laterally persistent thickness and quality. ACKNOWLEDGEMENTS The authors would like to t h a n k SASOL, Ltd., in particular Mr. P.P.A. Steyn and Mr. B.C. Hosking, who kindly gave permission for the publication of these data. The funding of this project by the CSIR (South African Council for Scientific and Industrial Research) is gratefully acknowledged. REFERENCES Boothroyd,J.C. and Nummedal,D., 1978. Proglacialbraidedoutwash:A modelfor humid alluvialfan deposits. In: A.D. Miall (Editor), Fluvial Sedimentology.Can. Soc. Pet. Geol., Alta., pp. 413-429. Burst, J.F., 1958. Glauconitepellets: their mineral nature and application to stratigraphic interpretation. AAPG Bull., 42: 310-327. Cadle, A.B. and Hobday, D.K., 1977. A subsurfaceinvestigation of the Middle Ecca and Lower Beaufort in northern Natal and the southeastern Transvaal. Trans. Geol. Soc. S.A., 80:111115. Cairncross, B., 1979. Depositional frameworkand control of coal distribution and quality, van Dykes drift area, northern Karoo basin. M.Sc. Thesis, University of Natal, Pietermaritzburg, 89 pp. Cairncross, B., 1980. Anastomosingriver deposits: Paleoenvironmentalcontrol on coal quality and distribution, northern Karoo basin. Trans. Geol. Soc. S.A., 83: 327-332. Cairncross, B. and Winter, M.F., 1984. High constructive lobate deltas in the Lower Permian Vryheid Formation, Rietspruit, South Africa. Trans. Geol. Soc. S.A., 87: 101-109. Cant, D.J., 1982. Fluvial facies models and their application. In: P.A. Scholle and D. Spearing (Editors), Sandstone DepositionalEnvironments. AAPG Publ., Tulsa, OK, pp. 115-137.

76 Coleman, J.M., 1969. Brahmaputra river: Channel processes and sedimentation. Semdiment. Geol., 3: 129-239. Coleman, J.M. and Prior, D.B., 1982. Deltaic environments. In: P.A. Scholle and D. Spearing (Editors), Sandstone Depositional Environments. AAPG Publ., Tulsa, OK, pp. 139-178. Du Toit, A.L., 1954. Geology of South Africa, 3rd ed, Oliver and Boyd, Edinburgh, 611 pp. Easterbrook, D.J., 1982. Glacial sediments. In: P.A. Scholle and D. Spearing (Editors), Sandstone Depositional Environments. AAPG Publ., Tulsa, OK, pp. 1-10. Eyles, N. and Miall, A.D., 1984. Glacial facies. In: R.G. Walker (Editor), Facies Models, 2nd ed. Geol. Assoc. Can., pp. 15-38. Fielding, C.R., 1985. Coal depositional models and the distinction between alluvial and delta plain environments. Sediment. Geol,, 42: 41-48. Flores, R.M., 1983. Basin facies analysis of coal-rich tertiary fluvial deposits, northern Powder Basin, Montana and Wyoming. In: J.D. Collonson and J. Lewin (Editors), Modern and Ancient Fluvial Systems. Int. Assoc. Sedimentol., Spec. Publ., 6: 501-515. Fouch, T.D. and Dean, W.E., 1982. Lacustrine and associated clastic depositional environments. In: P.A. Scholle and D. Spearing (Editors), Sandstone Depositional Environments. AAPG Publ., Tulsa, OK, pp. 87-114. Galloway, W.E and Hobday, D.K., 1983. Terrigenous Clastic Depositional Systems, Applications to Petroleum, Coal and Uranium Exploration. Springer Verlag, New York, NY, 423 pp. Harvey, R.D. and Dillon, J.W., 1985. Maceral distribution in Illinois coals and their paleoenvironmental implications. Int. J. Coal Geol., 5:141-165. Heward, A.P., 1978. Alluvial fan and lacustrine sediments from Stephanian A and B (La Magdalena, Cinera-Matallana, and Sobero Coalfields, northern Spain. Sedimentology, 25:451-488. Hobday, D.K., 1978. Fluvial deposits of the Ecca and Beaufort Groups in the eastern Karoo Basin, South Africa. In: A.D. Miall (Editor), Fluvial Sedimentology. Can. Soc. Pet. Geol., Alta., pp. 413-429. Horne, J.C., Ferme, J.C., Caroccio, F.T. and Baganz, B.P., 1978. Depositional models in coal exploration and mine planning in Appalachian regions. AAPG Bull., 62; 12: 2379-2411. Jackson, R.G., 1978. Preliminary evolution of lithofacies models for meandering alluvial streams. In: A.D. Miall (Editor), Fluvial Sedimentology. Can. Soc. Pet. Geol., Alta., pp. 543-576. Keller, W.D., 1958. Glauconitic mica in the Morrison Formation in Colorado. Clays Clay Miner., 5th National Conference 1956, pp. 120-128. Le Blanc Smith, G., 1980. Genetic stratigraphy and paleoenvironmental controls on coal distribution in the Witbank Basin Coalfield. Ph.D. Thesis, Univ. Witwatersrand, Johannesburg, 242 pp. (unpubl.). McLean, J.R. and Jerzykiewicz, T., 1978. Cyclicity, tectonics and coal: Some aspects of fluvial sedimentology in the Brazean-Paskapoo Formations, Coal Valley Area, Alberta, Canada. In: A.D. Miall (Editor), Fluvial Sedimentology. Can. Soc. Pet. Geol., Alta., pp. 441-468. McRae, S.G., 1972. Glauconite. Earth-Sci. Rev., 8: 397-440. Porrenga, D.H., 1968. Non-marine glauconite illite in the Lower Oligocene of Aardeburg, Belgium. Clay Miner., 7: 421-430. Radforth, N.W., 1969. Environmental and structural differentials in peatland development. In: E.C. Dapples and M.E.. Hopkins (Editors), Environments of Coal Deposition. Geol. Soc. Am., Spec. Pap., 114: 87-104. Reading, H.G., 1982. Sedimentary Environments and Facies. Blackwell Scientific Publications, Oxford-London, 569 pp. Reineck, H.E. and Singh, I.B., 1980. Depositional Sedimentary Environments. Springer-Verlag, New York, NY, 439 pp. Rust, B.R., 1978. Depositional models for braided alluvium. In: A.D. Miall (Editor), Fluvial Sedimentology. Can. Soc. Pet. Geol., Alta., pp. 605-626. Ryan, P.J., 1968. Stratigraphy of the Ecca Series and lowermost Beaufort Beds (Permian) in the

77 Karoo Basin of South Africa. Ph.D. Thesis, Univ. Witwatersrand, Johannesburg, 210 pp. (unpubl.). SACS (South African Committee for Stratigraphy), 1980. Stratigraphy of South Africa Part I. Compiled by L.E. Kent. Lithostratigraphy of the Republic of South Africa, SWA/Namibia, and the Republic of Bophuthatswana, Transkei and Venda. Handbook Geol. Surv. S. Afr., 8, 690 pp. Smith, D.G., 1976. Effects of vegetation on lateral migration of anastomosed channels of a glacier meltwater river. Geol. Soc. Am. Bull., 87: 857-860. Smith, N.D., 1970. The braided stream depositional environment: Comparison of the Platte River with some Silurian clastic rocks, North Central Appalachians. Geol. Soc. Am. Bull., 81: 29933014. Smith, N.D., 1975. Sedimentary environments and late Quarternary history of a low energy mountain delta. Can. J. Earth Sci., 12: 2004-2013. Snyman, C.P., Van Vuuren, M.C.J. and Barnard, J.M., 1983. Chemical and physical characteristics of South African coals and a suggested classification system. NICR, Pretoria, 63 pp. Stach, E., Mackowsky, M.-Th., Teichmtiller, M., Taylor, G.H., Chandra, D. and Teichmtiller, R., 1982. Stach's Textbook of Coal Petrology, 3rd ed. Gebr. Borntraeger, Berlin, 535 pp. Stanley, K.O. and Surdam, R.C., 1978. Sedimentation on the front of Eocene Gilbert-type deltas, Washakie Basin, Wyoming, J. Sediment. Petrol., 48: 557-573. Steyn, P.P.A., 1976. Die Sedimentologie van die Davelsteenkoolveld in die Oos-Transvaal met spesiale Verwysings na die Vryheid Formasie van die Karoo Supergroep. M.Sc. Thesis, RAU, Johannesburg, 88 pp. Tankard, A.J., Jackson, M.P.A., Eriksson, K.A., Hobday, D.K., Hunter, D.R. and Minter, W.E.L., 1982. Crustal Evolution of South Africa - 3.8 Billion Years of Earth History. Springer Verlag, New York, NY, 523 pp. Von Brunn, V. and Stratten, T., 1981. Late Paleozoic tillites of the Karoo Basin of South Africa. In: M.J. Hambray and W.B. Harland (Editors), Earth's Pre-Pleistocene Glacial Record. Cambridge Univ. Press, London, p. 1004.