430
9 AUTOCHTHONOUS SEDIMENTS
very fine-grained macerated plant materials most of which are too small to be identified. The coal type vitrain is composed mainly of the maceral vitrinite and with an increase in the proportion of other macerals, vitrain grades into clarains and durains. The
durains are characterized by micrinite and/or exinite, and some of the grey durains are
very rich in exinite.
The degree of maturation of these macerals, vitrinite and exinite, can be assessed optically under the microscope by change in their color under ordinary transmitted light.
In the humic coals, vitrinite changes from translucent yellow, through orange and red
to deep red with increase in rank of coal. Exinite retains shades of yellow throughout
much of the range of humic coals and only darken to orange and red with increase in
rank of coal. Exinite retains shades of yellow throughout much of the range of humic
coals and only darkens to orange and red in the higher ranks, that is, semi-anthracite.
The color transmission of the macerals is affected by thickness of the thin section and
determinations have to be made on very thin sections, with strict control of thickness.
Vitrinite can also be studied by reflected light and, using the techniques of ore microscopy, its reflectance can be measured. The reflectance of vitrinite changes with increase
in rank, and can be used as an index of maturation. Spores and scraps of vitrinite are
present in minor traces in many sediments other than coal, and where they occur determination of their degree of maturation can provide a guide to the thermal history of
the rocks that contain them. This knowledge is important in the subsurface exploration
of potential oil-bearing formation because maturation of liquid hydrocarbon runs parallel to that of coal.
9.3.3 Environments of Coal Deposition
Most coal seams appear to have formed in place. That is to say, the coal-forming peat
accumulated where the plants lived and died. Some coal, however, appears to be of
"drift" origin, and the plant remains were transported, such as log rafts, to the site of
deposition. An in-place origin is demonstrated by the presence of roots and rootlets
that lead down from the coal into an underlying fossil soil or "seat earth" (refer back to
Fig. 2.6). Seat earths are of interest in their own right, by virtue of their refractory properties. Clayey seat earths, termed "fire clays," consist mainly of kaolin minerals (disordered kaolinite) and are of value for manufacture of refractory bricks. Sandy seat
earths, "ganisters," are almost pure quartz rocks and are used for producing silica fire
bricks. There can be little doubt that the seat earths were deposited as normal clays and
sands, but that they were leached of alkali metals and reconstituted by humic acids from
the overlying peats (see also Section 2.3.1.2).
Certain coals, such as some in the Karro Group (Permo-Carboniferous) of southern
Africa, lack any form of seat earth, abruptly overlying shale. It has been argued that
such coals may have formed from floating islands of vegetation, analogous to those seen
today in the Sud of Sudan, and the Tigris-Euphrates delta at the head of the large gulf
between Iran and Arabia.
Great attention has been paid to the environments of deposition of coals and their
precursors (Dapples and Hopkins, 1969; Wanless et al., 1969; Ethridge and Flores, 1981;
Galloway and Hobday, 1983; Rahman and Flores, 1984; Fielding, 1985; Lyons and Rice,
1986; Merritt and McGee, 1986; Scott, 1987). The physicochemical conditions that fa-
9 AUTOCHTHONOUS SEDIMENTS
very fine-grained macerated plant materials most of which are too small to be identified. The coal type vitrain is composed mainly of the maceral vitrinite and with an increase in the proportion of other macerals, vitrain grades into clarains and durains. The
durains are characterized by micrinite and/or exinite, and some of the grey durains are
very rich in exinite.
The degree of maturation of these macerals, vitrinite and exinite, can be assessed optically under the microscope by change in their color under ordinary transmitted light.
In the humic coals, vitrinite changes from translucent yellow, through orange and red
to deep red with increase in rank of coal. Exinite retains shades of yellow throughout
much of the range of humic coals and only darken to orange and red with increase in
rank of coal. Exinite retains shades of yellow throughout much of the range of humic
coals and only darkens to orange and red in the higher ranks, that is, semi-anthracite.
The color transmission of the macerals is affected by thickness of the thin section and
determinations have to be made on very thin sections, with strict control of thickness.
Vitrinite can also be studied by reflected light and, using the techniques of ore microscopy, its reflectance can be measured. The reflectance of vitrinite changes with increase
in rank, and can be used as an index of maturation. Spores and scraps of vitrinite are
present in minor traces in many sediments other than coal, and where they occur determination of their degree of maturation can provide a guide to the thermal history of
the rocks that contain them. This knowledge is important in the subsurface exploration
of potential oil-bearing formation because maturation of liquid hydrocarbon runs parallel to that of coal.
9.3.3 Environments of Coal Deposition
Most coal seams appear to have formed in place. That is to say, the coal-forming peat
accumulated where the plants lived and died. Some coal, however, appears to be of
"drift" origin, and the plant remains were transported, such as log rafts, to the site of
deposition. An in-place origin is demonstrated by the presence of roots and rootlets
that lead down from the coal into an underlying fossil soil or "seat earth" (refer back to
Fig. 2.6). Seat earths are of interest in their own right, by virtue of their refractory properties. Clayey seat earths, termed "fire clays," consist mainly of kaolin minerals (disordered kaolinite) and are of value for manufacture of refractory bricks. Sandy seat
earths, "ganisters," are almost pure quartz rocks and are used for producing silica fire
bricks. There can be little doubt that the seat earths were deposited as normal clays and
sands, but that they were leached of alkali metals and reconstituted by humic acids from
the overlying peats (see also Section 2.3.1.2).
Certain coals, such as some in the Karro Group (Permo-Carboniferous) of southern
Africa, lack any form of seat earth, abruptly overlying shale. It has been argued that
such coals may have formed from floating islands of vegetation, analogous to those seen
today in the Sud of Sudan, and the Tigris-Euphrates delta at the head of the large gulf
between Iran and Arabia.
Great attention has been paid to the environments of deposition of coals and their
precursors (Dapples and Hopkins, 1969; Wanless et al., 1969; Ethridge and Flores, 1981;
Galloway and Hobday, 1983; Rahman and Flores, 1984; Fielding, 1985; Lyons and Rice,
1986; Merritt and McGee, 1986; Scott, 1987). The physicochemical conditions that fa-