This document summarizes a study of the petrographic composition and depositional environments of Cretaceous coals in the Middle Benue Trough of Nigeria. Three coal facies were identified based on maceral analysis: a vitrinite-fusinite facies deposited in wet forest swamps, a trimaceritic facies deposited in delta plains, and a shaly facies deposited between dry-wet forest swamps and limno-telmatic clastic marshes. Tissue preservation indices and fossil evidence from interbedded sediments were used to interpret the depositional environments of each facies. The study provides insight into the paleoenvironmental conditions during formation of the coals in the Cretace

1. PETROGRAPHIC COMPOSITION AND
DEPOSITIONAL ENVIRONMENTS OF
CRETACEOUS COALS AND COAL
MEASURES IN THE MIDDLE BENUE
TROUGH OF NIGERIA
BY
AGBAJE MAYOWA
14/68EL001

2. OUTLINE
 INTRODUCTION
 GEOLOGY AND STRATIGRAPHY
 PETROGRAPHIC COMPOSITION
 DEPOSTIONAL ENVIRONMENT
 CONCLUSION

3. INTRODUCTION
 Coal is the oldest fuel known to mankind. It is a variety of solid,
combustible, sedimentary organic rocks that are composed mainly of
carbon and varying amount of other component such as hydrogen,
oxygen, sulphur and moisture.
 Coal is formed from vegetation that has been consolidated
between other rock strata and altered by the combined effect of
pressure and heat over millions of years.
 Coal beds are widely intercalated within the sedimentary
succession of the cretaceous system in the Central Benue Trough of
Nigeria. Visible among these coal beds are those in the area around
Obi/Lafia situated about 32km southeast of the town of Lafia (Fig 1)

4. Fig. 1: Map of Nigeria showing location of the study area
(Obaje et al.,1994)

5. GEOLOGY AND STRATIGRAPHY
In the Central Benue, the stratigraphic succession begins with the
basal Albian Arufu, Uomba, Gboko Formations, generally referred to as
the Asu River Group
. These are overlain by the Cenomanian Keana and Awe Formations
and Cenomanian-Early Turonian Ezeaku Formation. The Awgu Formation
lie conformably on the Ezeaku Formation and the Campanian-
Maastrichtian Lafia Formation capped the sequence.
 All six units are present in the Middle Benue Trough, but only the
Awgu and Lafia Formation are found in Obi/Lafia area.
 The coal bed occur within the Awgu Formation which consists of
Bluish-grey to dark black carbonaceous shales, calcareous shales, shaly
limestones, siltstones and coal seams.

6. Fig. 2. Benue trough and its origin. (a) Separation of the continents of Africa and South
America and initiation of the Benue Trough,
(b) The Benue Trough in relation to other major sedimentary basins in central West Africa.

7. Fig. 2c. Geographic location of the Benue Trough in
Nigeria (Modified from Petters, 1982)

8. Fig. 3. Stratigraphic successions in the Middle-Benue Trough of Nigeria.

9. PETROGRAPHIC COMPOSITION
Based on Maceral analyses, three Petrographic Facies
have been distinguished in the Obi/Lafia coals. They are:
 The Vitrinite-Fusinite Coal Facies.
 The Trimaceritic Coal Facies.
 The Shaly Coal Facies.

10. THE VITRINITE-FUSINITE COAL FACIES
This facies is High in Vitrinite; contains some
Inertinites but is poor in Liptinite.
 With Mineral matter counted, Vitrinite 67.3%,
Liptinite 4.2%, Inertinite 17.1% and Mineral matter
11.4%.
 Without mineral matter, Vitrinite constitutes 75.3%,
Inertinite 19.7%, and Liptinite 5.0%
 The facies is characterized Lower (deeper) coal units.

11. THE TRIMACERITIC COAL FACIES
 In this Facies, the three groups of Macerals: Vitrinite,
Liptinite and Inertinite are well represented.
With Mineral matter counted, Vitrinite is
42.3%,Liptinite 22.4% Inertinite 23.8% and Mineral
matter 11.5%
Without mineral matter Vitrinite averages 47.7%
Liptinite 25.6%, and Inertinite 26.7%
This facies is characteristic of the upper (shallower)
coals.

12. THE SHALY COAL FACIES
 The Main characteristics of this Facies is the abundance
of mineral matter, mainly as clays and pyrites.
 With mineral matter counted, Vitrinite 29.9%, Liptinite
13.8%, Inertinite 22.0% and Mineral matter 34.3%.
 Without mineral matter, Vitrinite 43.4%, Liptinite 20.1%
and Inertinite 36.5%.
 This facies occurs below, between and above major coal
units.

13. Fig. 4. Petrographic composition and Facies Diagram of the
Obi/Lafia coals based on Maceral Group.

14. DEPOSITONAL ENVIRONMENTS
Current coal paleoenvironmental analyses are based mainly
on:
 the indices of the Tissue preservation and the Gelification
(Diessel, 1986, 1992).
 the indices of the groundwater table and the vegetation
( Calder et al., 1991)
 the Maceral associations and mineral matter contents
according to Kalkreuth et al., 1991
 the Maceral associations and petrographic indices
according to Marchioni and Kalkreuth (1991).
In this study, the depositional paleosubenvironements
of the Obi/Lafia coal facies are Interpreted based on the first
parameter mentioned above.

15. TISSUE PRESERVATION INDEX
The TPI is essentially the measure of the degree of tissue
breakdown and the proportion of woody plants in the original
peat forming plant assemblage (lamberson et al., 1991; Akande
et al., 1992).
A low TPI suggest either a predominance of herbaceous plants
in the mire or large scale destruction of wood because of
extensive humification and mineralization( Diessel 1992).
High TPI values indicate the presence of well preserved plant
tissue( telinite, fusinite and semifusinite), a high proportion of
arboreal vegetation( telinite, telocollinite, fusinite and
semifusinite) and aerobic (oxidizing, burning) conditions (fusinite
and semifusinite).

16. TPI = telinite + telocollinite + semifusinite + fusinite
desmocollinite + vitrodetrinite + inertodetrinite
GELIFICATION INDEX
 GI is a measure of the degree and persistence
of wet conditions.(Diessel 1992)
 GI = vitrinite +macrinite
semifusnite +fusnite+inertodetrinite

17. DEPOSITIONAL SUBENVIRONMENTS OF THE
PETROGRAPHIC FACIES OF THE OBI/LAFIA COALS.
(THE VITRINITE-FUSINITE COAL FACIES)
High to medium TPI indices (average TPI=1.64) which is relatively high
compared to Diessel 1992
Gelification indices (average=5.99)
Interbedded inorganic sediment comprises black carbonaceous shale
and dark grey limestones. High population but low diversity of
assemblages of Ostracods with few calcareous Foraminifera
corresponding to a lagoon environment.
With the high GI values (high moisture content) medium to high TPI
values (arboreal vegetation) and the nature of the Interbedded
sediment suggest that the coals of the Vitrinite-fusinite facies were
deposited in the wet forest swamp.

18. THE TRIMACERITIC COAL FACIES
 Low TPI(average =0.94) a moderate high GI(average 4.65)
 Interseam inorganic sediment within the Trimaceritic coal
facies contain very poor assemblages of Foraminifera with
relatively rich ostracods and palynomorph content
 With the low TPI values and on the pattern of microfossil
distribution within the Interseam sediment show that the
coals were deposited in the delta plains of the marine-
shoreline environment

19. THE SHALY COAL FACIES
 The coals have moderate to low TPI (average=1.30)
 Microfossil studies of the sediment within the shaly coal
facies show in places rich assemblages of Foraminifera and
ostracods corresponding to shallow marine condition
while some fossil are related to lagoon and delta plain.
 On the TPI/GI diagram the Shaly Coal Facies generally
plots between the dry-wet forest swamp and the limno-
telmatic clastic marsh environment.

20. Fig. 5. TPI/GI Facies diagram of the Obi/Lafia Coals
(modified from diessel,1986; Lamberson et al.,1991
Marchioni and Kalkreuth,1991).

21. CONCLUSION
 Petrographic coal analyses allow distinction of three coal facies
that were formed in different environments.
Thus, the Vitrinite -Fusinite Facies was formed in the wet forest
swamp subenvironment along and within the Lagoons while the
Trimaceritic Coal Facies was formed in marsh sub environment
within the delta plains
 Paleontological and Palynological investigations have shown
that the general environments of deposition of the Awgu
Formation, which contains the coal seam in the Central Benue
Trough of Nigeria range from shallow marine through lagoon to
delta plain.

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