The coalification of South African coal

International Journal of Coal Geology, 13 (1989) 375-390

375

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

T h e c o a l i f i c a t i o n of S o u t h A f r i c a n coal C.P. SNYMAN 1and J. BARCLAY2

1Department of Geology, University of Pretoria, 0083, HiUcrest, Pretoria, South Africa 2Technikon Northern Transvaal, Soshanguve, South Africa (Received March 7, 1988; revised and accepted October 26, 1988)

ABSTRACT Snyman, C.P. and Barclay, J., 1989. The coalification of South African coal. In: P.C. Lyons and B. Alpern (Editors), Coal: Classification, Coalification, Mineralogy, Trace-element Chemistry, and Oil and Gas Potential. Int. J. Coal Geol., 13: 375-390. It has been generally accepted that the variation in rank of South African coal is essentially due to the metamorphic effect of dolerite dykes and sills. However, no satisfactory explanation could be given for the fact that true anthracite has not been formed adjacent to dykes and transgressive sills in those South African coal fields where the rank of the coal is normally low. In these areas, the coal close to the intrusives is often referred to as "burnt". A detailed examination of the contact metamorphism of South African coal by dolerite intrusives shows that a dyke or sill affects the coal to variable distances, generally from 0.6 to 2 times the thickness of the intrusive, and that this distance is independent of the rank of the coal outside the contact aureole. This is explained by an initial episode of contact metamorphism while the coal was still in the lignite stage of coalification, followed by burial metamorphism during which the paleogeothermal gradient increased in an easterly direction. This regional increase in paleogeothermal gradient is probably related to the large-scale magmatic activity that culminated in the extrusion of the Drakensberg basalts and the break-up of Gondwanaland.

INTRODUCTION

In the past, it has been generally accepted that the rank variations in South African coal seams are essentially due to the intrusion of dolerite sills and dykes (Du Toit, 1954, pp. 287, 367). Particularly in the Natal coal fields (Fig. 1 ) is has been estimated that 90% of the coal deposits have been affected by dolerite intrusions. In many cases, the whole range of rank, from high-volatile bituminous coal to anthracite is present, and graphite may even be developed at the contact between the coal and the dolerite. However, in the coal fields of the Orange Free State and the Transvaal (Fig. 1 ) commercially viable anthracite is normally not developed close to dykes and transgressive sills of dolerite, and the normal high- to medium-volatile bituminous coal changes over a relatively short distance to cinder coal with a high ash content close to the con0166-5162/89/$03.50

© 1989 Elsevier Science Publishers B.V.

376

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377

tact. These heat-affected coals are often referred to as "burnt". The question, therefore, arises why the intrusion of dolerites apparently led to the formation of anthracite in some cases and to the formation of low-quality, "burnt" coal in others. According to De Jager (1983), there is a gradual increase in coal rank in the South African coal fields from west to east (Fig. 2) as the major centres of dolerite magmatic activity are approached, but there are many exceptions to this rule due to the local effect of dolerite dykes and sills. This regional variation in rank suggests that the major cause of coalification may not necessarily be directly related to the presence of dykes and sills. However, in order to test this possibility it is necessary to investigate the local metamorphic effect of these intrusions. GENERAL GEOLOGY

According to the South African Committee for Stratigraphy (SACS, 1980) all the South African coal deposits occur within the Karoo Sequence which ranges in age from Late Carboniferous to Early Jurassic (Table 1 ). The radiometric age of the basalt of the Drakensberg Formation, which forms the top of the succession in the main Karoo basin (Fig. 1 ), is 187 Ma, whereas the radiometric ages of different intrusions of Karoo dolerites vary between 150 and 190 Ma (SACS, 1980). The sedimentary rocks of the Karoo Sequence in the main basin represent a depositional episode starting with Permo-Carboniferous glaciation (the Dwyka Tillite Formation), followed by an intracratonic basinal and marine TABLE 1 Simplified stratigraphic column of the Karoo Sequence in the northern portion of the main Karoo basin (after SACS, 1980) Period (Age)

Group

Formation

Rock types

Jurassic (150 Ma) Triassic (195 Ma)

-

Drakensberg ( _ 187 Ma)

Basaltic lava

-

Permian (225 Ma)

Beaufort

Clarens Elliot Molteno Tarkastad Estcourt Volksrust Vryheid Pietermaritzburg Dwyka

Fine-grained sandstone Red sandstone, mudstone Sandstone, subordinate coal Sandstone, shale Sandstone, shale, subordinate coal Shale, sandstone, subordinate coal Sandstone, shale, coal Shale Tillite, varved shale

Ecca

Upper Carboniferous (285 Ma)

-

378

phase (the Ecca Group), and finally by a period of terrestrial sedimentation with increasing aridity (Beaufort Group, Molteno, Elliot and Clarens Formations). The Drakensberg Formation consisting of volcanic rocks cap this sedimentary sequence and were fed by many dykes and sills, which also affected the coal measures of the Vryheid Formation (Tankard et al., 1982). The Dwyka Formation and the Ecca Group are of particular interest with regard to coal deposition. After the northward retreat of the Dwyka ice sheets, remnant glacial valleys with a predominantly north-south orientation reflect the major directions of the previous ice movement (Tankard et al., 1982, pp. 368-369). These features of the pre-Karoo topography controlled later sedimentation and, to some extent, peat deposition (see Cairncross, 1989). The subsequent basinal development during Ecca times comprised a marine palaeoenvironment with flysch-type deposition in the south and a gently subsiding shelf platform towards the northeast (Ryan, 1968 ). The platform facies along the northeastern margin of the Ecca basin is represented by widespread, generally coarse, clastic, fluviodeltaic deposits derived from the north, and which wedge out into siltstones and mudstones of the offshore facies towards the south. These coarse fluviodeltaic deposits of the platform facies are known as the Vryheid Formation and host the main coal seams in the different coal fields i

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Fig. 3. Coal fields within the northern platform facies of the Ecca Group.

379

Witbank Highveld EasternTransvaal

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Fig. 4. The tentative correlation of depositional sequences in the major coal fields within the main basin (after A.B. Cadle, pers. commun., 1983 ). The more important coal seams are also shown.

presently distinguished (Fig. 3). The number of coal seams varies from one coal field to the other, but A.B. Cadle (pers. commun., 1983) suggested a tentative correlation of depositional sequences in the major coal fields in the main Karoo basin, based on upward-fining and upward-coarsening cycles (Fig. 4 ). The outpouring of the Drakensberg basalt and the intrusion of the dolerites are largely contemporaneous. The dolerite sills cut through the sedimentary sequence at any angle, up to vertical. Being strongly undulating, they may form domes and basins. These intrusions generally are between 10 and 100 m thick, but extremes of less t h a n 1 m and more t h a n 600 m are known. Successive sills may occur, one above the other, and this can often be ascribed to repeated intrusions because of marked petrological differences. In the Vryheid area of Natal (Figs. 1, 3) at least seven intrusive episodes have been distinguished by Blignaut et al. (1940) on the basis of intrusive contacts and chilled margins. R A N K V A R I A T I O N S IN R E L A T I O N TO D O L E R I T E I N T R U S I O N S

Blignaut (1952) concluded that in general South African dolerite intrusions affect coal seams over a distance equal to their thickness, and this is still regarded as a general rule of thumb. However, du Toit (1954) mentions an extreme case where a 4.8-m-thick dolerite affected a coal seam over a distance of 90 m. In this case, all stages of coalification are present, from bituminous coal, through semianthracite and anthracite, to natural coke at the contact. In m a n y

380 cases the natural coke has a columnar structure, and the dolerite adjacent to the coal is converted to "white trap" (Kirsh and Taylor, 1966). The petrographical and chemical characteristics of coal in contact metamorphic aureoles have been described in detail by many previous authors (e.g., Dutcher et al., 1966; Teichmiiller, 1973; Roberts, 1987; Hagelskamp, 1987) and need not be repeated here. However, it should be mentioned that carbon dioxide is commonly enriched in proximity to a dyke and is attributed to the secondary precipitation of carbonate minerals in the degasification pores from percolating groundwater. Therefore, it is essential to correct the analyses of such coal samples from the contact aureole for inorganic carbon dioxide before interpretation of rank on the basis of volatile matter.

Rank and distance~thickness relationships On the basis of the volatile matter content (daf) of vitrinite-rich coal samples, Blignaut (1952) has shown that the rank of the Bottom Seam in Natal is related to the ratio distance/thickness (D/T), where D is the distance between the coal seam and the Ingogo dolerite sill and T is the thickness of the dolerite. In Figure 5 Blignaut's data are given, except that volatile matter (daf) has been converted to the mean maximum reflectance of vitrinite under oil (Rmax) by means of suitable diagrams (Snyman et al., 1983). Mathematical manipulation of the data in Figure 5 indicates that Rm,x can be related to D/T by an exponential equation of the form:

Rmax=Ro exp b(D/T)

0.~

3 e~K-I.BIZlD/T)

°e I

Thickness(D/T)

OAf 0.2 i

2

3

4

%Rmax

5

6 i

T i

Fig. 5. Relationshipbetweenthe ratio distance (D) to thickness (T) of the Ingogodoleritesill and Rmax(%) of the Bottom seamin Natal (basedon data in Blignaut et al., 1952).

381 TABLE 2 Derived values of Ro, b, the width of the contact aureole in terms of D/T and Rmax (%) at the boundary of the aureole for a number of examples Example Ingogo sill, Klip River coal field Nyembe Colliery, Vryheid coal field: ( 1 ) Seam above sill (2) Seam below sill Msebe Colliery, Nongoma coal field Utrecht coal field (Blignaut et al., 1952) Longridge Colliery, Vryheid coal field Zoetmelk Colliery, Utrecht coal field Alpha anthracite mine, Vryheid coal field Burnside Colliery, Klip River coal field (Roberts, 1987): ( 1 ) 0.4-m-wide dyke (2) 2.0-m-wide dyke Durban Navigation Colliery, Klip River coal field (Roberts, 1987): ( 1 ) 40-m-wide dyke (2) 2.7-m-wide dyke Kilbarchan Colliery, Klip River coal field, 11-m-wide dyke (Roberts, 1987 ): ( 1 ) Top bench of seam (2) Bottom bench of seam Twistdraai Colliery, Highveld coal field, 2.75-m-wide dyke (Hagelskamp, 1987) Cape Verde Rise (Simoneit et al., 1978): (1) Kerogen above sill (2) Kerogen below sill Colorado (Dutcher et al., 1966)

Ro (%)

b

D/T

Rmax (%)

5.64

- 1.812

1.0

0.95

9.63 3.90 6.22 3.21 13.70 8.59 11.42

- 1.536 -0.316 - 1.554 - 1.824 - 1.937 - 1.392 - 1.727

0.8 2.3 0.8 1.0 1.0 1.1 1.1

1.70 2.00 1.70 0.65 2.00 1.85 1.80

8.65 9.21

- 1.059 - 1.953

1.5 1.2

1.10 1.10

6.20 7.26

- 4.323 - 1.438

0.6 1.4

0.95 1.00

7.11 7.77

- 1,681 -6.083

1.2 0.6

1.10 1.10

5.50

-0.586

3.7

0.67

4.25 2.24 13.40

- 3.589 -3.009 - 0.085

0.8 0.5 21.0

0.40 0.40 1.00

Ro = mean maximum reflectance of vitrinite at the intrusive contact. b=a constant (see text). D/T= distance/thickness of the intrusive. Rmax = mean maximum reflectance of the vitrinite outside the metamorphic contact zone.

where Ro --the mean maximum reflectance of the vitrinite at the intrusive contact (Ro = 5.64% ) b --a constant determined by the average thermal diffusivity (Jaeger, 1965) of the rocks between the coal and the intrusive and by the temperature of the intrusive (b -- - 1.812 ). Similar relationships were found between Rmax and D/T at several other localities in South African coal fields, and could also be calculated from data given in the literature (Table 2). However, this fact was apparently not recognized by the original authors. Where the metamorphism was caused by dykes, this exponential relation-

382 3C

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:1; r o e)

--e-- Top portion of seam

e) cu

- - x - - Bottom portion of seam

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i

i

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% R max.

Fig. 6. Variation in Rma~ (%) of the top and bottom portions of a coal seam with respect to the distance (D) /thickness (T) ratio of a 11-m-wide dyke, Kilbarchan Colliery (data from Roberts, 1987).

30

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Fig. 7. Relationship between the ratio distance (D) to thickness (T) of (1) a 40-m-wide dolerite dyke and (2) a 2.7-m-wide dolerite dyke and Rmax (%), Durban Navigation Colliery (data from Roberts, 1987).

383

0.8

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Samples above sill R: 4.25 exp,- 3.589 D/T) Samples below silt R=Z.24exp.~- 3.009 D/T)

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Fig. 8. Relationship between the ratio distance (D) to thickness (T) of a 15-m-thick diabase sill and Rmax (%) of kerogen at the Cape Verde Rise. The two curves indicate that heat flow was mainly upwards (data from Simoneit et al., 1978). 26 24

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16

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12

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Fig. 9. Relationship between the ratio distance (D) to thickness (T) of a sill and Rmax (%) of a coal seam in Colorado (data from Dutcher et al., 1966). Note the very wide aureole in terms of the distance/thickness ratio.

ship does not necessarily apply as the shape of the metamorphic aureole is complicated by the presence of sills above or below the seam. This is illustrated by Figure 6 (based on data from Roberts, 1987) where the top portion of the coal seam has a distinctly higher rank than the b o t t o m portion, obviously due to the presence of a sill some distance above the seam. It is even possible that some of the values do not follow the exponential relationship at all (Fig. 7, data from Roberts, 1987). In this particular case (Fig. 7) a thin dyke 2.7 m wide had a much greater effect than a thick dyke 40 m wide.

384 Simoneit et al. (1978) investigated the thermal alteration of kerogen in Cretaceous shale from a borehole at the Cape Verde Rise in the Eastern Atlantic Ocean. A 15-m-thick diabase sill at a depth of 956.6-971.5 m below the seabed is believed to be responsible for the metamorphism. The exponential curves based on the data of Simoneit et al. (1978) indicate that the heat flow was mainly upwards, as would be expected (Fig. 8). Dutcher et al. (1966) examined the metamorphic effects of intrusive sills on a 2.5-m-thick coal seam in Colorado. The lower sill, which is only 0.08 m thick, affected the coal up to a distance of 1.95 m from the lower contact of the sill. According to the calculated exponential relationship, Ro at the intrusive contact agrees with that determined by Dutcher et al. (1966), namely 13.40% (Fig. 9). Given and Binder (1964) had previously used the same set of heat-affected coal samples to determine the temperature of metamorphism by means of electron-spin-resonance (ESR) techniques.

Temperature of metamorphism Although coal is a sensitive geothermometer, it is difficult to calibrate accurately on account of the variables involved. Where the metamorphism is caused by igneous intrusions, the specific metamorphic effect is governed by the maximum temperature of the intrusion, the period of cooling of the magma, the rank of the coal prior to the intrusion and the thermal diffusivity of the coal and the other rocks. It is also possible that the width of the aureole in a coal seam adjacent to a dyke may vary from one seam bench to the next because of variations in mineral and maceral contents which may influence the amount of heat absorbed by endothermic reactions (Hagelskamp, 1987). Karweil (1956) and Bostick et al. (1979) designed nomograms for the determination of temperature and duration of metamorphism under conditions of regional and burial metamorphism, whereas Johnson et al. (1963), Lowry et al. (1942) and Chandra (in Stach et al., 1982 ) related reflectance to carbonization temperature (see also Chandra and Chakrabarti, 1989, this volume). Given and Binder (1964) investigated the use of ESR behaviour of coal as a geothermometer and made use of the same sample set investigated petrographically by Dutcher et al. (1966). The ESR technique is based on the fact that the behaviour of coal does not change until it is heated to a temperature above that to which it had been exposed in its previous history. The minimum temperature at which ESR characteristics occur in laboratory heated coal would, therefore, be the maximum temperature to which the coal was heated during its geothermal history. A comparison of these methods of calibration on South African heat affected coals shows that artificial carbonization generally gives temperatures that are about 100-150 °C higher than those obtained by the ESR method (Fig. 10). Between Rm~ 1.7 and 3.4%, we found temperatures by means of the ESR method

385 (~) According to Given 8~ Binder (19641 (~) Artificial coalificalion Arnax : 1.02 exp.O.OOZ847t

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R max % 8

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zoo

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400

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700

BOO

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9O0

I000

Temperature *C

Fig. 10. The calibration of Rm,x (%) in terms of temperature of metamorphism; (1) ESR, according to Given and Binder (1964); (2) artificial carbonization (this paper). that agree closely with those given by Given and Binder (1964) for the same rank range.

Temperature and distance~thickness relationships The E S R temperatures obtained by Given and Binder (1964) and the metamorphic effect displayed by the same samples, as described by Dutcher et al. (1966), show that the relationship between D / T and the temperature of metamorphism is also an exponential one of the form:

t = to exp bD/T where: t = t e m p e r a t u r e of metamorphism in °C, to = temperature ( ° C ) at the intrusive contact, b --a constant determined by the average thermal diffusivity of the coal and other rocks, D = distance of the coal from the intrusion, T -- thickness of the intrusion (Fig. 11 ) In the case of this sample set from Colorado, the temperature of the intrusion is found to be 861 ° C and the value of b is - 0,0807. Assuming that such an exponential relationship generally applies, the E S R

386 28

0 24 o

=

2C

L6 ~ r n / ' T ' ! Thickness ..... i2

8

4

0

Loo

zoo

300

4o0

5o0

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eoo°c

Temperature ( t ) Fig. 11. The relationship between the ratio distance (D) to thickness (T) of an intrusive and the temperature of metamorphism of coal in Colorado (data from Given and Binder, 1964; and Duteher et al., 1966).

~ ~Oi(stance 0 /

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t= 841 exp.(-I.1402 D/T) (somples from various localities)

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200

300

400

500

600

700

soo

900

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Temperature ( t ) Fig. 12. The relationship between the distance/thickness ratio (D/T) of intrusives and the temperature of metamorphism of coal in the Natal coal fields: (1) Zoetmelk Colliery; (2) Samples from various localities.

387

temperatures obtained in this study for the samples from Zoetmelk Colliery are related to D/T by the following equation: t = 9 5 5 exp - 1 . 6 8 0 9

D/T

giving a temperature at the intrusive contact of 955 oC. Based on laboratory carbonization tests, the relevant equation is found to be: t = 8 4 1 exp - 1.1402

D/T

indicating a temperature of 841 ° C at the contact (Fig. 12 ). Both these values are within the temperature range for a basic magma. It is, however, clear that in the case of the South African example the drop in temperature with increasing distance from the intrusion was much more rapid than in the case described by Given and Binder (1964) and Dutcher et al. (1966). This is obviously due to the difference in thermal diffusivity in the two cases. T H E CAUSES OF COALIFICATION OF SOUTH AFRICAN COAL

Low rank coals of the Orange Free State, which were not affected by dolerite intrusions, have a reflectance of about 0.55%. At an age of the coal of about 250 Ma, this implies a maximum temperature of coalification of about 50 ° C, according to the nomogram of Bostick et al. (1979; Fig. 13). Considering the variation in age of the dolerites (150-190 Ma), the coal would have reached a reflectance of only about 0.45% at a temperature of 50 oC prior to the intrusion of the dolerites. This is typical of lignite with a moisture content of about 30% 350

........

300 '- 250 ~

200

~

~'150

5

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E

5 ~ 4

s

5_ 4

s

~ 2 - - 2

._E ~ oo

E

50

%

I0

K}O

I000 Ma

effective time

Fig. 13. The relationship between maximum temperature of metamorphism, effective heating time and vitrinite reflectance (R . . . . Rm) (after Bostick et al., 1979 ). The effective heating times of 60 Ma and 100 Ma are probable periods of burial metamorphism at 50°C prior to the intrusion of the Karoo dolerites, suggesting an Rmaxvalue of between 0.44 and 0.48% at the time of intrusion.

388 (Stach et al., 1982). This suggests that most of the magmatic heat was consumed in the heating and evaporation of water in the lignite (and the associated clastic sedimentary rocks) so that the metamorphic aureole is very narrow. By contrast, the wide aureole in the case described by Dutcher et al. (1966) implies intrusion into an essentially dry environment. As basic magmas are relatively poor in volatile components, especially water vapour (Carmichael et al., 1974), it is conceivable that a chemical gradient may actually cause the vapour phase to migrate from the surrounding rocks into the cooling magma (Turner, 1968, p. 20), thus further lowering the metamorphic effect of the intrusive on the coal. This may be the reason for the low values of Ro in some examples listed in Table 2, e.g., at the Nyembe mine and at the Cape Verde Rise. These low values of Ro imply maximum metamorphic temperatures of about 400-500 ° C (Fig. 10). At several localities listed in Table 2, the effect of thin dykes appear to be more pronounced than that of thick dykes, e.g., at Burnside and Durban Navigation Collieries. These apparent anomalies may be due to either an age difference in the dykes where the thin dykes are younger than the thicker ones and therefore intruded when the surrounding rocks were already partially dehydrated, or due to the fact that the thin dykes acted as magma conduits for a relatively long time, thus increasing their "effective thickness". The high initial values of Rmax at the outer limit of the contact aureole (1.02.0% ) imply maximum metamorphic temperatures of 110 to 160°C if the age of the coal (about 250 Ma) is taken into account (Bostick et al., 1979). Considering the high negative values of the constant "b" in Table 2, i.e., the rapid decrease in temperature away from the intrusive contact, these initial values of Rmax must be regarded as apparent, as they are characteristic of high-rank coals that have a low inherent moisture content. Therefore, the decrease in temperature with increasing D/T ratio should have been more gradual, similar to the example described by Dutcher et al. (1966). The most plausible explanation for this apparent anomaly appears to be that burial metamorphism, under a fairly steep paleogeothermal gradient, was superimposed on the initial contact metamorphism of the original lignite. This metamorphic episode would not have affected coal that had previously been heated to a temperature above that of this burial metamorphism. The constants in the exponential equations relating Rm~xto the ratio D/T would, therefore, not be changed by this burial metamorphism, only Rm~xat the boundary of the aureole would be increased. The ultimate rank of the coal outside the contact aureole is thus ascribed to abnormally high heat flow associated with the magmatic activity and the breakup of Gondwanaland during the Jurassic. Even today, the geothermal gradient in the Somkele coal field (Fig. 3) is 2 9 ° C / k m (Mr. X. Prevost, Geological Survey of South Africa, pers. commun., 1985), compared to about 10-15 ° C/ km for the Transvaal and Orange Free State (Holmes, 1965; Bouwer, 1952).

389 CONCLUSIONS The investigation of the metamorphism of South African coal by dolerite intrusives shows t h a t the rank of the coal is in general exponentially related to the ratio D/T, where D is the distance between the coal and the intrusive and T is the thickness of the intrusive. These contact aureoles are generally very narrow and contact metamorphism is not a plausible explanation for the regional increase in rank from west to east (Fig. 2 ). These localized metamorphic effects were largely controlled by the low thermal diffusivity of the coal and the associated rocks at the time of th~ intrusion and not by an abnormally low magma temperature. The low thermal diffusivity is hypothesized as due to the intrusion of the dolerites while the coal was still in the lignite stage of coalification and, hence, rich in moisture, so t h a t the magmatic heat was mainly consumed by the evaporation of this moisture within the lignite and the associated sedimentary rocks. Superimposed burial metamorphism is held responsible for the ultimate rank of the coal, and as the paleogeothermal gradients increased in an easterly direction, anthracite was only formed in the far Eastern Transvaal and in Natal. Sporadic examples of high-rank coal in this eastern region may have resulted from local hot spots. Towards the west, where the paleogeothermal gradient was much lower, the effect of the superimposed burial metamorphism was very slight, so t h a t mainly " b u r n t " coal resulted from the contact metamorphism. ACKNOWLEDGEMENTS The authors t h a n k Marina Potgieter for the drafting of the figures and Maurine Fisher for typing the manuscript. The financial support of the National Geoscience Programme, through the Foundation for Research and Development of the Council for Scientific and Industrial Research (CSIR), is gratefully acknowledged. The paper was reviewed by R.M.S. Falcon and P.H. Black. REFERENCES Blignaut, J.J.G., 1952. Field relationships of dolerite intrusions in the Natal coalfields. Trans. Geol. Soc. S. Afr., 55: 19-31. Blignaut, J.J.G., Furter, F.J.J. and Vogel, J.C., 1940. The northern Natal coalfield. Coal Surv. Mem. 1, Pretoria, 328 pp. Blignaut, J.J.G., Furter, F.J.J. and Savage, W.H.D., 1952. The northern Natal coal-field (Area No. 2 ). Coal Surv. Mem. 2, Pretoria, 228 pp. Bostick, N.H., Cashman, S.M., McCullogh,T.H. and Waddel, C.T., 1979. Gradients of vitrinite reflectance and present temperature in the Los Angeles and Ventura basins, California. In: D.F. Oltz {Editor), Low Temperature Metamorphismof Kerogen and Clay Minerals. Pacific Section, Soc. Econ. Paleontol. Mineral., Los Angeles,pp. 65-96. Bouwer, R.F., 1952. Measurementof borehole temperatures and the effect of geologicalstructure in the Klerksdorpand Orange Free State areas. Trans. Geol. Soc. S. Afr., 55: 89-123.

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