28

CHAPTER 3

3 FACIES DESCRIPTION AND INTERPRETATION 3.1 THE FACIES CONCEPT The first use of the term “facies” was introduced into geology by the Swiss

geologist Gressly (1838), who applied this term to lateral changes within time-

stratigraphic units of the Mesozoic. This term was then broadened and widely

used by Johannes Walther the sedimentologist and stratigrapher, in his “Rule

of Succession of Facies” (Walther, 1894, In: Middleton, 1973). This rule was

referred to as the Law of Correlation (or Succession) of Facies. Walther (op.

cit.) points out that there must be no major breaks (unconformities) in the

stratigraphic sequence if the principle is to be applied. The practical

application of this law is applying it to determine the relationship between

present-day sedimentary facies and their lateral distribution, with those of

vertical successions of facies in the rock record.

Subsequently, the term facies has been used more broadly, with a number of

different meanings. Moore’s (1949) definition, has been most widely accepted:

“Sedimentary facies is defined as any areally restricted part of a designated

stratigraphic unit which exhibits characters significantly different from those of

other parts of the unit” (In : Blatt et al., 1980, p. 618).

This study uses the term facies in the same sense as proposed by Middleton

(1978). The facies description is objective, based upon observations of

physical features in the rocks. Middleton (op. cit.) outlined: “The key to the

interpretation of facies is to combine observations made on their spatial

29

relations and internal characteristics (lithology and sedimentary structures)

with comparative information from other well-studied stratigraphic units, and

particularly from studies of modern sedimentary environments” (p. 323-325).

This definition is similar to that used by Walker and James (1992) who relate

facies models to sequence stratigraphy.

Miall (1977, 1978) applied the concept of facies analysis to identify a variety of

fluvial systems, which were then successfully applied to submarine -fan

systems. This concept combines individual lithofacies at a macroscale and

microscale, and attempts to erect models used in the identification and

description of depositional systems.

Based on Miall’s (1977, 1978) work, Le Blanc Smith (1980) proposed a logical

letter code for facies nomenclature with the main objective of standardizing the

facies types found in the northern Karoo Basin, Witbank Coalfield. These

facies were identified from field exposures, and drill holes. The definition of

the facies was based on lithology, sedimentary structures, biogenic structures

and stratigraphically distinctive minerals, such as glauconite, which is of

significant importance for this study, as a stratigraphic marker mineral

occurring above the No. 4 and No. 5 seams.

For this study, most of the data used for defining the facies, came from

borehole logs and borehole core. It is therefore, a subsurface investigation.

One of the main aspects used in defining each lithofacies was grain size

variation. According to standard international practice, the grain size was

defined by applying the Wentworth scale illustrated in (Table 3.1).

30

WENTWORTH GRAIN SIZE SCALE

Limiting particle diameter

mm φ units Size class

2048 - 11 Very large

1024 - 10 Large

512 - 9 Medium

256 - 8 Small

128 - 7 Large

64 - 6 Small

1 0 Very coarse

µm

½ + 1 500 Coarse

¼ + 2 250 Medium 1/8 + 3 125 Fine 1/16 + 4 62 Very fine 1/32 + 5 31 Very coarse 1/64 + 6 16 Coarse 1/128 + 7 8 Medium 1/256 + 8 4 Fine 1/512 + 9 2 Very fine

Boulders

Cobbles

Pebbles

Granules

Sand

Silt

Clay

G R

A V

E L M

U D

Table 3.1. Wentworth grain size scale (modified after Davis, 1983, p. 7).

31

3.2 SEDIMENTARY FACIES

Although this study focuses primarily on the No. 4 seam and associated facies,

all of the facies types encountered in the study area are presented below for the

sake of completeness. These include lithofacies from the basal Dwyka Group

tillites to the argillaceous units of the Volsksrust Formation, overlying the

Vryheid Formation. However, the lithofacies directly above No. 4 seam, are

discussed in greater detail, rather than those of the entire stratigraphic column

present.

3.3 CONGLOMERATE FACIES The coarse-grained lithofacies includes clast-supported and matrix-supported

rocks in the study area, with a mean grain diameter greater than 4mm (Table

3.1).

Three conglomerate sub-facies are recognized in the study area. These are

defined by the presence or absence of sedimentary structures, i.e., whether

they are matrix supported conglomerate, massive or cross-bedded small-

pebble conglomerate.

3.3.1 Diamictite sub-facies DESCRIPTION

The diamictite sub-facies or matrix-supported conglomerate lithofacies is

located at the base of the Karoo Supergroup, and forms a thin layer of

32

sedimentary rock overlying pre -Karoo basement lithologies (Tankard et al.,

1982). The diamictite forms part of the Dwyka Group. The diamictite consists

of clasts, which are diverse in shape and size and range from rounded to very

angular. The clast colours are commonly red, grey-olive, brown and blue

(Figure 3.1). Almost all are exotic, extrabasinal in origin. The matrix ranges

from dark-grey mudstone to light-grey siltstone. The diamictite is overlain by

massive and cross-bedded conglomerate or coarse to very coarse cross-

bedded sandstone. This sub-facies is variable in thickness but has a

maximum thickness of 30m in palaeovalleys. The thickening of diamictite in

the study area is confined to the south-western section.

INTERPRETATION The Karoo Basin diamictites originated as gravity driven debris flows, slumps

and glacial fill deposits. Le Blanc Smith (1980) interpreted the Dwyka

diamictites in the Witbank Coalfield, as being of terrestrial glacial origin.

Winter (1985), Cairncross (1986) and Cadle (1995) came to similar

conclusions with respect to these basal diamictites.

3.3.2 Massive conglomerate sub-facies

DESCRIPTION

The massive conglomerate sub-facies can be distinguished on the basis of the

absence of sedimentary structures, poor sorting and proportions of quartz and

feldspar clasts vs. matrix. When the conglomerate clasts are well-sorted it

tends to be clast-supported, and the clasts are well rounded, with a high

33

Figure 3.1. Diamictite sub-facies comprising a siltstone matrix and

extrabasinal pebbles.

34

sphericity. If any matrix is however present, it is generally well -sorted and

composed of coarse silt or fine sand. This sub-facies (Figure 3.2) is most

commonly interbedded with the cross-bedded conglomerate sub-facies (see

sub-facies 3.3.3).

The second conglomerate sub-facies is a matrix-supported, small-pebble

conglomerate. This has a high percentage of siltstone matrix and a wide range

of clast sizes, varying between 2mm and 5cm in diameter. Figure 3.3 is Folk's

(1974), roundness (sphericity) scale, showing both high and low spherically

shaped particles ranging from very angular to well rounded and its pattern is

associated with clast composition in the study area. Pebbles are composed of

amygdaloidal lava, siltstone, coal spar, quartz, chert and granite. This sub-

facies is predominantly upward-fining. The matrix-supported conglomerate

sub-facies compromises linear bodies over 40m thick, along palaeovalley’s.

The matrix-supported, small-pebble conglomerate (Figure 3.4) occurs

stratigraphically between No. 4 seam and No. 4 A seam.

INTERPRETATION

This sub-facies is interpreted from deposition under high -energy aqueous flow

conditions. Adequate flow velocities are required to winnow out the finer

matrix, particularly with reference to the clast-supported sub-facies. Within

gravels, clasts are unable to respond to fluid stresses (Harms et al., 1982).

Massive and crudely stratified gravels have been described by Gustavson

(1975), Miall (1977, 1978), states that, a high matrix content is caused by

high-discharge events, followed by rapid deposition with little or no reworking.

Glacio-fluvial processes are inferred from the association of massive

conglomerate sediments with Dwyka Group diamictites. Massive, matrix-

supported conglomerate (diamictite) commonly forms by gravity flow

35

Figure 3.2. Clast supported sub-facies of the conglomerate facies

containing small-pebbles of quartz and feldspar.

36

Figure 3.3. Roundness and sphericity scale (modified after Folk, 1974).

processes, such as debris flows (Nicholas, 1999). Little or no reworking

preserves the high matrix content and results in very poor sorting of the clasts.

37

Figure 3.4. Matrix-supported sub-facies of the conglomerate facies

comprising quartz and feldspar grains. The unit is coarsening-

upward.

38

3.3.3 Cross-bedded small-pebble conglomerate sub-facies

DESCRIPTION

This sub-facies, is light-grey to white, consisting of angular to sub-rounded

quartz pebbles in a silty or sandy matrix. The cross-bedded conglomerate

may occasionally incorporate coal-spar and siltstone rip-up clasts. The

thickness of foresets varies between 1cm-5cm and set thicknesses between

30cm-1.5m. This sub-facies (Figure 3.5) is commonly intrabedded with the

massive conglomerate sub-facies and cross-bedded sandstone sub-facies or

is sharply overlain by carbonaceous siltstone.

INTERPRETATION

Cross-bedding is produced by migrating sand waves under different flow

regime conditions. Considering the grain size of this sub-facies, bedforms are

restricted to dunes, sand waves and lower flat beds (Harms et al., 1982).

Cross -bedding in sand originates from the downstream migration of two-

dimensional and three-dimensional dunes, respectively. As these structures

are observed in this sub-facies, small-pebble conglomerates, the flow velocity

must have been relatively high, to transport the sediment. The presence of

cross -bedding in this facies is probably attributable to the poorer sorting of the

sediment and generally, sediment has a high proportion of matrix. (Miall,

1978). Colella and Prior (1990) described deposition of this facies via the

migration of gravel longitudinal bars and dunes.

39

Figure 3.5. Cross-bedded small -pebble conglomerate sub-

facies comprising poorly-sorted quartz and feldspar grains.

40

3.4 SANDSTONE FACIES

The sandstone facies is composed of clastic grains ranging in size from

0. 0625mm to 2mm (Table 3.1).

3.4.1 Massive sandstone sub-facies

This sub-facies can be subdivided on the basis of textural criteria and colour.

The sandstone is well-sorted with grains that vary from fine sand (0,25mm) to

very coarse-grained (2mm) sand. The colours vary between white and light-

grey, green and brown (Figures 3.6 and 3.7). The massive sandstone sub-

facies is commonly intrabedded with siltstone, cross-bedded and wave-ripple

cross -laminated sandstone. Stratigraphically, the well-sorted massive

sandstone occurs immediately above the No. 2, No. 3 No. 4 and No. 5 coal

seams.

INTERPRETATION

Blatt et al. (1980) have shown that some sandstone that appears to be

lacking in textural features, do in fact have lamination or cross-lamination,

when x-rayed. However, this facies in the study area appears to be massive,

with no structure present, or it can be reworked.

Colella and Prior (1990) showed that secondary destructive reworking

processes, such as bioturbation, could destroy primary structures thereby

producing homogenized sediment. Colella and Prior (op. cit.) further

41

Fig

ure

3.6

. (A

) W

ell-s

ort

ed c

oar

se- t

o m

ediu

m-g

rain

ed s

and

sto

ne.

Th

e ab

sen

ce o

f str

uctu

re a

ppea

rs to

be

pri

mar

y. (

B) C

oar

se s

and

sto

ne

wit

h s

hal

e ri

p-u

p c

last

s.

(A)

(B)

42

Fig

ure

3.

7

Mas

sive

sa

nd

sto

ne

sub

-fac

ies,

(A

) Ill

ust

rate

s w

ell-

sort

ed,

fin

e-g

rain

ed

san

dst

on

e, (

B)

Wel

l-so

rted

, m

ediu

m-g

rain

ed s

and

sto

ne.

(A)

(B)

43

demonstrated tha t massive sandstone can be formed either by rapid

deposition from suspension, contemporaneous deformation or by bioturbation.

The lack of grading and fluid -escape structures would suggest that the

sedimentation rate was low. The poorly-sorted massive sandstone of this

study, containing a high siltstone matrix, formed from the destruction of

bedding through bioturbation by organisms, and it can be associated with

bioturbated siltstone and sandstone facies (Cadle, 1995). The massive

sandstone sub-facies, in the study area is closely associated with the flaser-

laminated sandstone sub-facies, the interlaminated sandstone-siltstone facies,

and the lenticular-laminated siltstone sub-facies, particularly where they are

bioturbated.

3.4.2 Planar cross-bedded sandstone sub-facies

DESCRIPTION

This sub-facies is composed of grey-white arkosic sandstone, with minor

amounts of silt and mica. The grain size of this facies is fine to very coarse-

grained sandstone, and the sandstone is structured by planar cross-bedding.

Set boundaries are flat, and foresets are graded, from coarse- to medium-

grained sand, with a generally steep foreset angle of between 20° and 30°.

Set heights vary between 20cm and 1.5m (Figure 3.8). Stratigraphically, this

facies is situated below the No. 2 seam and above the No. 4 seam.

44

Fig

ure

3.8

. (

A)

Pla

nar

cro

ss-b

edd

ed s

and

sto

ne

sub

-fac

ies.

Th

e cr

oss

-bed

din

g i

s d

efin

ed b

y fi

nin

g-

up

war

d f

ore

sets

, (B

) P

lan

ar c

ross

-bed

din

g w

ith

cm

-intr

abed

ded

co

ng

lom

erat

e la

g a

nd

co

al-s

par

len

ses

at th

e to

p.

(A)

(B)

45

INTERPRETATION

Planar cross-bedding is produced by the downstream migration of two-

dimensional bedforms (Harms et al., 1982). Blatt et al. (1980) stated that the

mechanism that produces most cross-bedding is encroachment by

avalanching down the lee slope of dunes, small deltas, ripples, fans or bars.

The size of the sedimentary structures in this sub-facies suggests the latter

(Harms et al., 1982).

3.4.3 Trough cross-bedded sandstone sub-facies

DESCRIPTION

This sub-facies compromises grey-white, fine to coarse-grained sandstone

with a high silt and mica component. The trough cross -bedding is defined by

concave erosional scours filled by curved foreset layers that define the

structure. Thin gravel lags are frequently present at the base of sets. Set

heights vary between 10-50cm. Foreset laminae are characteristically concave

upward. This sub-facies (Figure 3.9) is commonly intrabedded with the planar

cross -bedded sandstone facies or is intrabedded with wave-ripple laminated

sandstone sub-facies. This trough cross-bedded sandstone sub-facies, occurs

stratigraphically below the No. 2 and, above No. 4 seams.

46

Figure 3.9. Trough cross-bedded sandstone sub-facies defined by

scoured surfaces, with concave laminae.

47

INTERPRETATION

Harms et al. (1975) states that, hydrodynamically, trough -cross bedded

sandstone forms from the downstream migration of three-dimensional dunes.

These multidimensional bedforms migrate at relatively high flow velocities, the

uppermost part of the lower flow regime. However, the arkosic variant of sub-

facies often overlies the planar cross-bedded sandstone and possibly

represents the migration of dunes over sandwaves (Harms, op. cit.). This sub-

facies is commonly documented at relatively high velocities, within the lower

flow regime (Rust et al., 1984; Harms, op. cit .).

3.4.4 Planar laminated sandstone sub-facies

DESCRIPTION

This sub-facies is characterized by light-grey, medium- to very fine-grained

sandstone containing minor silt, mica and heavy minerals. This facies

distinguishing feature is that sandstone laminae are parallel to sub-parallel

with the underlying set boundaries. This sub-facies ranges between 5-50cm in

thickness. Individual laminae vary in thickness from a few millimeters to 1cm.

The sub-facies (Figure 3.10) is commonly intrabedded with planar cross-

bedded and cross-laminated sandstone or with wave-rippled and trough cross-

bedded sandstone. Stratigraphically, this sub-facies occurs towards the top of

coarsening-upward sedimentary units, below No. 3 and No. 5 seams. The sub-

facies also overlies cross-bedded sandstone below the No. 2 and above No. 4

seams. This sub-facies is often related to the upper-most parts of fining-

upward cycles.

48

Figure 3.10. Planar laminated sandstone sub-facies illustrating well

defined horizontal to sub-horizontal laminae.

49

INTERPRETATION

The interpretation of planar horizontal or nearly horizontally laminated

sandstone can be reviewed in several ways. Planar lamination includes

sediment particles originating via suspension settling in a fluid, in the absence

of any currents or lateral transportation. Harms et al. (1982) stated that planar

lamination can be produced under different conditions such as unidirectional

flows in strong currents, unidirectional flows with low velocities, settling fine-

grained sediment and symmetrical oscillatory flows when velocities are large.

Where the facies overlies trough and planar cross-bedded sandstone no

graded lamination is present. This suggests deposition of sediment at high

velocities as documented by Cadle (1995). Therefore, this facies originates

under upper flow regime conditions (Harms et al., 1982), where sediment is

transported laterally, as horizontal layers, under high flow velocities.

3.4.5 Cross-laminated sandstone sub-facies

DESCRIPTION

This sub-facies comprises cross -laminated fine- to medium-grained grey-white

sandstone with minor siltstone and mica present. The set size of laminae

ranges between 0,5-5cm, and sets that are less than 1cm thick are estimated

to have a cross-sectional dimension of 5 -10cm. There are two different types,

which can be distinguished. The first bedding style is cross -lamination where

laminae are concave and convex upwards. The second type of bedding is

represented by cross-laminae which are tabular parallel to flow, as well as

transverse to flow and concave upward as shown in Figure 3.11.

Stratigraphically, the first type of cross-lamination is present in the sandstone

50

Fig

ure

3.

11.

(A)

Cro

ss-la

min

ated

sa

ndst

one

sub

-fac

ies,

w

ave-

rip

ple

la

min

atio

n,

cap

ped

b

y

silts

ton

e,

(B)

Cro

ss-l

amin

ated

sa

nd

sto

ne-

silt

sto

ne

wit

h

som

e so

ft-s

edim

ent

def

orm

atio

n

stru

ctu

res

pre

sen

t.

(A)

(B)

51

intervals between the No. 2 and No. 3 seams and the No. 4 and No. 5 seams.

The second type is situated below the No. 2 and above the No. 4 seams.

INTERPRETATION

Harms et al. (1975) stated that oscillatory flow produced by waves interacts

with the sediment substrate and produces symmetrical wave ripples. These

ripples are typically symmetrical or slightly asymmetric, with peaked crests

and rounded troughs. Harms (op. cit.) noted that symmetric or wave ripples

with a spacing of 10cm-30cm are common in a shallow marine environment,

while closely spaced wave ripples between 1cm-3cm are typical of shallow

ponded water. Sandstone structured by small-scale trough cross-lamination is

present towards the top of upward-fining sandstones between the No. 2 and

No. 4 depositional sequences.

The asymmetric or current ripples (Figure 3.12) are characterized by low relief

and linear crests. Rieneck and Singh (1980) stated that asymmetrical ripples

are commonly two-dimensional. According to Reineck and Singh (op. cit.) the

presence of bifurcating ripples may indicate interference patterns produced by

waves interacting with currents

Cadle (1995) noted that the presence of small-scale oscillation-ripple

lamination in the Vryheid Formation suggests that wave processes were an

important factor in reworking fine-grained sand. Small-scale trough cross-

laminated sandstone commonly represents the migration of current ripples

over the steep side of larger dunes. These ripples are formed due to a

reduction in water depth, velocity and grain size.

52

Figure 3.12. Cross -lamination produced by current ripples.

53

3.4.6 Glauconite sandstone sub-facies

DESCRIPTION

This sub-facies is easily defined lithologically, as it has a characteristic green-

flecked colour, with fine to granular green glauconite g rains. The sandstone is

well-sorted, fine- to medium-grained. The glauconite-bearing facies thickness

ranges from a few centimetres to over 15m. Glauconite forms granules, and

crudely elliptical grains, averaging one -half of a millimetre in diameter (Figure

3.13). The glauconite sandstone facies is a useful stratigraphic marker above

the No. 4 and No. 5 seams, presenting a valuable correlation between the coal

seams, and even between coal facies (Cadle et al., 1995).

INTERPRETATION

Pettijohn (1957) no ted that glauconite is presently forming in deep and shallow

marine waters. Takahashi (1939) states "Glauconite seems to be formed

under marine conditions by a process of hydration of silica and subsequent

absorption of bases and loss of alumina. Glauconite may originate from a

number of primary materials, such as faecal pellets, clayey substances filling

cavities of foraminifera, radiolaria and tests of other marine organisms, or from

silicate mineral substances, such as volcanic glass, feldspar, mica or

pyroxene. In salt water ….. the primary substances during glauconitisation

lose alumina, silica and alkali’s except potash, and green ferric iron. Sea

water, therefore, seems essential …….". Cloud (1955) stated that glauconite

is formed only in marine waters of normal salinity, whose formation is

facilitated by the presence of organic matter. However, glauconite is now

54

Fig

ure

3.1

3. (

A)

Cro

ss-b

edd

ed

gla

uco

nite

san

dst

on

e su

b-f

acie

s w

ith

a f

ew s

iltst

on

e le

nse

s, a

nd

(B)

wit

h a

silt

sto

ne

laye

r at

th

e b

ott

om

. No

te t

he

gre

en g

lau

con

ite

gra

ins

that

are

cle

arly

vis

ible

on

th

e cr

oss

-sec

tio

n o

f th

e co

re.

(B)

(A)

55

known to also form in non-marine lacustrine settings (McRae, 1972). It forms

from micaceous minerals or pelagic muds rich in iron. However, the bulk of

glauconite does originate under marine conditions. Winter (1985) described

ubiquitous and widespread glauconite beds in the northeastern Karoo Basin.

This sub-facies close association with cross-laminated sandstone may be

indicative of shallow water environments. The presence of glauconite-bearing

sandstone immediately above the No. 4 and the No. 5 seams, suggests that

the sediment was deposited during high-stand, transgressive marine episodes.

The close association of marine bioturbation (see facies 3.7.3) also supports

this conclusion.

3.4.7 Bioturbated sandstone sub-facies

DESCRIPTION

This sub-facies compromises sandstone ranging from fine - to coarse-grained

and contains significant percentages of silt, which gives the facies a grey

colour (Figure 3.14). The sedimentary structures may be partially to

completely destroyed by the activity of organisms. Where present, the primary

structures are cross- and flaser-laminated sandstone, and interlaminated

sandstone-siltstone. Bioturbation occurs as vertical and, less commonly, as

horizontal burrows. Where bioturbation is very intense, individual trace fossils

cannot be identified. Stratigraphically, this sub-facies occurs toward the top of

coarsening-upward sequences, between the No. 2 and No. 3 seams. It is also

present within sheet sandstones above the No. 2, No. 4 and No. 5 seams.

56

Figure 3.14. Bioturbated sandstone sub-facies, with

interlaminated dark-grey siltstone. Both lithologies have been

intensively bioturbated.

57

INTERPRETATION

After deposition, sedimentary strata may be disturbed by organisms. These

structures are termed biogenic trace fossils (Basan, 1978). This sub-facies is

very important, as it represents trace fossils that are usually grouped into

depth-controlled communities (Seilacher, 1967), although other factors such

as salinity, water temperature, and food supply also control ichnofacies

activity.

Trace fossils may be used as palaeoenergy indicators, according to whether

the animal was moving up, down or cross-current and resting tracks may have

a preferred orientation in the Vryheid Formation. Horizontal burrows are

indicative of low-energy environments, such as interdistributary bays, in which

significant percentages of silt were deposited (Stanistreet et al., 1980).

Trace fossils may be used to determine rates of sedimentation. Vertical,

unbranched burrows, such as Skolithos (Seilacher, 1967) and Siphonichnus

Eccaensis (Stanistreet et al., 1980) indicate a rapid sedimentation rate where

organisms had to burrow vertically to keep pace with the depositing sediment.

The distribution of this facies between the No. 2 and No. 3 seams, which

contains Siphonichnus Eccaensis traces, has been interpreted by Stanistreet

(op. cit.) and Winter (1985) to have been deposited during high rates of

deposition. In some strata, the bioturbation is intense (Figure 3.14) suggesting

that although sedimentation ra tes were high, the organisms had sufficient time

to totally rework their substrata.

58

3.5 INTERLAMINATED SANDSTONE -SILTSTONE FACIES

DESCRIPTION

The interlaminated sandstone-siltstone facies encompasses a variety of

lithologies, ranging from siltstone to fine-grained sandstone. The facies can

be subdivided into three sub-facies:

3.5.1 Flaser-laminated sandstone sub-facies

3.5.2 Wavy-laminated sandstone-siltstone sub-facies

3.5.3 Lenticular-laminated siltstone sub-facies

The boundaries between these three sub-facies are gradational (Figure 3.15).

These three subfacies compromise units 2-3m thick above the No. 2, No. 3,

No. 4 and No. 5 coal seams.

3.5.1 Flaser-laminated sandstone sub-facies

DESCRIPTION

This sub-facies comprises fine- to medium-grained, grey sandstone. The

characteristic feature of this sub-facies is that wisps of dark-grey siltstone

occur scattered and disconnected in a matrix of sandstone. The siltstone

drapes are a few millimetres thick and seldom more than a few centimetres in

length. The siltstone drapes may be single entities or can be occasionally

bifurcated. This sub-facies generally occurs in units less than a few metres in

59

Figure 3.15 Block diagram showing (a) Flaser, (b) Wavy, and (c)

Lenticular lamination (modified after Davis,1983).

60

thickness. The sub-facies can be distinguished from the other sub-facies on

the basis that siltstone drapes are discontinuous over several centimetres

(Figure 3.16).

INTERPRETATION

Flaser lamination can form by thin, incomplete mud laminae trapped in ripple

troughs and draping ripple crests, during periods of slack water (Blatt et al.,

1980). Flaser lamination is most commonly developed under low-energy

conditions (Davis, 1983). Flaser lamination therefore forms by deposition of

sand and silt during episodic events of bedload traction and suspension

settling, respectively.

Flaser-lamination forms where the sediment supply is rhythmic or periodic

(Winter, 1985). Flaser lamination is described by Le Blanc Smith (1980) from

the Witbank Coalfield. He described periods of current activity during which

sand is transported and forms ripples, while mud is held in suspension. When

the current abates, the suspended sediment settles into troughs and over the

ripples. During renewed current flow, previously formed ripple crests are

eroded and the next layer of sand is deposited, burying the underlying ripple

with mud flasers in the ripple troughs.

3.5.2 Wavy-laminated sandstone-siltstone sub-facies

DESCRIPTION

This sub-facies comprises interlaminated sandstone and siltstone. The

61

Figure 3.16. Flaser laminated sandstone within the cross-laminated

sandstone sub-facies.

62

sandstone is fine- to medium-grained, and white -grey, while the siltstone is

dark-grey, highly carbonaceous and micaceous. The sandstone laminae vary

in thickness between a few millimetres to 5cm. Dark-grey siltstone laminae

vary between 2mm-3cm in thickness and drape over rippled sandstone

surfaces. This sub-facies (Figure 3.17) is characterized by undulatory (wavy)

bedding surfaces that separate individual sets seldom more than several

millimetres thick.

INTERPRETATION

The wavy-laminated sandstone-siltstone forms as a result of alternating

episodes of suspension settling of silt and bedload deposition of sand (Harms

et al., 1975). These two sediments are present in more or less equal quantities

and this sub-facies is an intermediate stage between the flaser and lenticular-

laminated sub-facies. Sand is either transported along the bed by current flow,

forming asymmetric ripples, or surface wave action reworks the sandy

substrate into symmetrical ripple forms. During periods of low wave activity

and reduced current flow, mud settles from suspension, draping the previously

formed ripples, thereby producing wavy-laminated sub-facies (Reinecke and

Singh, 1980).

3.5.3 Lenticular-laminated siltstone sub-facies

DESCRIPTION

The lenticular-laminated siltstone, consists of dark-grey carbonaceous

siltstone with thin, interbedded sandstone lenses up to 3cm thick. Lenses vary

from 10mm to 20cm in length.

63

Fig

ure

3.

17.

(A)

Wav

y-l

amin

ated

sa

nd

sto

ne

-silt

ston

e su

b-f

acie

s an

d

ero

sio

nal

co

nta

ct,

(B)

San

dst

on

e-s

iltst

on

e fa

cies

illu

stra

tin

g w

ell d

efin

ed h

ori

zon

tal l

amin

ae.

(A)

(B)

64

Contacts between sandstone and siltstone are sharp, indicating distinct

episodes of alternating silt and sand deposition. This sub-facies (Figure 3.18)

generally overlies the carbonaceous siltstone sub-facies and underlies the

wavy-laminated sandstone-siltstone facies of No. 2, No. 3 No. 4 and No.5 coal

seams.

INTERPRETATION

This sub-facies consists of small isolated lenses of sandstone contained in

siltstone. These sedimentary structures are produced when there is

insufficient sand available. Isolated ripples (starved ripples) form on the

muddy substrate and become covered by more mud, thereby being preserved

(Harms et al., 1975). This sub-facies is thus a result of suspension settling of

mud, coupled with current flow that generates the isolated sandy ripples.

This spectrum of bedding is dependent on conditions such as availability of

both sand and mud and alternation with slack water conditions when silt and

clay are deposited (Reineck and Singh, 1980; Winter, 1985).

65

Fig

ure

3.1

8.

(A)

Len

ticu

lar-

lam

inat

ed s

iltst

on

e w

ith

san

dst

on

e le

nse

s, (

B)

Th

e sa

me

sub

-faci

es w

ith a

2cm

dia

gen

etic

pyr

ite

no

du

le.

(A)

(B)

66

3.6 SILTSTONE FACIES

Primary siltstone comprises a relatively small percentage of the Highveld

Coalfield stratigraphic column. The siltstone is usually massive and/or

bioturbated, and is commonly intrabedded with sandstone. Interbeded sets of

granule conglomerate, sandstone, and siltstone display lenticular-lamination

(Figure 3.19).

3.6.1 Gravelly siltstone sub-facies

DESCRIPTION

This sub-facies comprises grey to dark-grey carbonaceous siltstone, with

significant sand and gravel content, and particle sizes ranging between 2-

15mm (Figure 3.20). These coarse grains are poorly sorted, and may be

horizontally laminated throughout the siltstone matrix. The sub-facies occurs

above the No. 4 seam, within the carbonaceous siltstone sub-facies and is

occasionally overlain by the lenticular-laminated siltstone. This sub-facies is a

good stratigraphic marker horizon above the No. 4 Seam.

INTERPRETATION

Below No. 1 seam in the Witbank Coalfield, Le Blanc Smith (1980) has

interpreted cyclical stacking of depositional couplets to be caused by seasonal

sedimentation. A likely interpretation of this sub-facies is that of periodic debris

67

Figure 3.19. Siltstone facies. This core sample contains

interlaminated sandstone-siltstone at the top.

68

Figure 3.20 Gravelly siltstone sub-facies.

69

rain (Le Blanc Smith, op. cit .) of sand-sized and gravel-sized sediment onto

the underlying silty substrata. In the basal portion of the Vryheid Formation,

floating ice with entrapped sediment would melt, causing this coarse, poorly-

sorted clastic sediment to rain down onto the basin floor (Le Blanc Smith,

1980; Cairncross, 1986). It has been suggested by Cairncross (1979) that this

facies in the Witbank Coalfield, where it occurs above the No. 4 seam, may

have been deposited by organic rafting. If these rafts began to fragment or

became disturbed, the clastic material would become liberated and settle out

into the underlying substrata.

3.6.2 Glauconite siltstone sub-facies

DESCRIPTION

This sub-facies comprises a dark-grey to grey-brown siltstone. The glauconite

is present as anhedral matrix-filling material. The glauconite tends to be

associated with bioturbation and carbonaceous organic debris. This sub-

facies is commonly massive, but it can display some horizontally laminated

sandstone interbeds and lenticular lamination. The thickness of this sub-facies

averages a few meters and is stratigraphically approximately 20m above the

No. 4 and 5 seams.

INTERPRETATION

Glauconite is a green hydrous potassium iron phyllosilicate mineral, and is

associated with sedimentary rocks of marine origin. The presence of

glauconite most commonly suggests marine settings and sediments (Pettijohn,

70

1957). The repetitive occurrence of glauconite, in the Highveld Coalfield study

area, is found in the borehole core directly above No. 4 seam. Le Blanc Smith

(1980) stated that the close association of pelletiferous glauconite and

bioturbation structures in the Witbank Coalfield is suggestive of the

replacement of faecal pellets by glauconite.

3.6.3 Bioturbated siltstone sub-facies

DESCRIPTION

This sub-facies comprises dark-grey bioturbated siltstone with a lesser

percentage of sand. Horizontal burrows of Cruziana and Zoophycos

ichnofacies assemblage (Seilacher, 1967) are the most common ichnofacies

but there are also vertical and oblique burrows. Most of the vertical burrows

have been identified as Siphonichnus Eccaensis (Stanistreet et al., 1980).

The horizontal trails have not been specifically identified because they are

difficult to examine in borehole core. However, they fall within the Cruiziana

assemblage of feeding trails (Seilacher, op. cit.). This sub-facies is

lithostratigraphically associated with the glauconite sub-facies above the No. 4

and 5 seams.

INTERPRETATION

Stanistreet (op. cit.) stated that the bioturbated siltstone sub-facies is

indicative of deposition, from suspension settling under quiescent conditions.

The presence of horizontal ichnofacies in the silty sediments implies that the

trails were made preferentially in low-energy settings. Similar bioturbation from

71

delta-front and crevasse-splay palaeoenvironments in the Vryheid Formation

has been described by Christie (1988). The trace fossils and their association

with coal seams imply that these sediments accumulated in low-energy

environments. The bioturbated siltstone suggests suspension deposition in a

low-energy environment, and the presence of horizontal burrows support this

interpretation. Cadle (1995) described bioturbation in the Vryheid Formation,

stating that the association of trace fossils with carbonaceous siltstones

adjacent to coal seams implies that these sediments are also accumulated in

low-energy environments.

3.6.4 Carbonaceous siltstone sub-facies

DESCRIPTION

This sub-facies comprises dark-grey to black carbonaceous and micaceous

siltstone (Figure 3.21). The sub-facies is typically massive or may be vaguely

horizontally laminated. Finely disseminated pyrite, crystalline pyrite and

Glossopteris plant leaves are common in this sub-facies. Stratigraphically,

this sub-facies appears above the No. 4 seam and varies in thickness between

a few cm to less than 4m. The carbonaceous siltstone sub-facies has

occasional interbeds of coarsening-upward, fine -grained siltstone-sandstone.

INTERPRETATION

This sub-facies is formed by suspension settling of silt, under quiescent

conditions, most likely in paralic or continental settings where terrestrial

vegetation contributed to the carbonaceous content.

72

Figure 3.21. Carbonaceous siltstone sub-facies.

73

Occasional higher energy events generated the scattered interbeds of

sandstone. Within the same sub-facies Cairncross (1979) identified

Glossopteris leaves and Lycopodium and Phyllotheca stems in the Witbank

Coalfield. This abundance of terrestrial organic debris suggests that deposition

of this sub-facies occurred terrestrially, under relatively shallow water

conditions. The fine-grained sandstone may have been deposited under

slightly higher energy conditions.

Cadle (1995) described the presence of this sub-facies in the Witbank

Coalfield extensively at the base of coarsening-upward sequence, above the

No. 2, No. 4, No. 5 and No. 6 seams. This facies implies low-energy

deposition following transgression, as it is laterally distributed at the base of

coarsening-upward sequences.

3.7 COAL FACIES

DESCRIPTION

This facies comprises humic bituminous coal with individual coal seams, from

a few centimetres to over 2m thick in the study area. The facies is present as

alternating dull, lustrous and bright coal bands (Figure 3.22).

In the study area, coal seams No. 2, No. 3, No. 4, No. 4 A and No. 5 are

present within the Vryheid Formation. The No. 4 seam is the main exploitable

seam, due to its thickness (up to 2m), and lateral continuity.

74

Figure 3.22. No. 4 seam coal facies at New Denmark showing dull

and bright bands.

75

INTERPRETATION

Coal forms from peat that has accumulated from organic debris under water-

logged conditions. After burial, peat becomes transformed, with increasing

temperature and pressure, to become lignite, bituminous coal and ultimately

anthracite.

This facies comprises bituminous coal seams. Lyons and Alpern (1989)

produced a definition saying that, coal is the product of plant debris which has

been modified chemically and physically by natural agencies with notable

evidence of smaller amounts of inorganic matter (Francis, 1961).

Moore and Bellamy (1973) described the importance of optimum conditions

for the accumulation of organic matter and rapid descent into the reducing

zone of the mire. Therefore, the position of the water table relative to the peat

surface during deposition and in early diagenesis is a critical component in the

determination of coal type.

Snyman (1998) described South African Gondwanan coals as forming by

limited transportation and redeposition. He applied the term

“hypautochthonous”, meaning redeposition in the same area as growth, to

these coals. "Major in situ coalfields" (Francis, op. cit.) have been formed in

fresh or brackish swamps water or in swamps interspersed with shallow lakes.

An “in situ” origin is evidenced by the presence of shallow, rooted horizons at

the base of the No. 4 seam in the Highveld Coalfield and by “in situ” tree

trunks in the No. 4 seam in the Witbank Coalfield (Le Blanc Smith, 1980).

Cadle (1995) described the Vryheid Formation coals, stating that the No. 2

seam of the Highveld Coalfield is high in ash, with an average of 23%, but low

in vitrinite with an average of 11%.