Geology Reference
In-Depth Information
Fig. 14.4
“
Gr`s polymorphes
”
1
2
(Gp) and
(So)
Formations: Thin-sections and
Scanning Electron Microprobe
pictures of Mount Mougengenge
samples (10 km SE from the
Kinshasa International airport,
DRC). 1. Gp—thin-section of
coarse-grained sandstones,
subrounded to subangular
(finest—
arrows
) grains. 2. Gp—
thin-section of silicified
sandstones (quartzites); the quartz
grains are cemented by subhedral
and sutured silica overgrowth; the
original rounded shape of the
grain is sometimes recognizable
(
arrow
), underlined by dust rims.
3. Gp—thin-section of silicified
sandstones with vugs filled by
euhedral quartz (
arrows
). 4. Gp—
thin-section of silicified
limestones (microcrystalline
quartz or microquartzitic
matrix—(
a
) with vugs filled by
(
b
) flamboyant palisadic silica
(chalcedony) and (
c
) euhedral
quartz. 5. Gp—rounded medium
quartz grain coated by silica—
probable aeolian origin. 6. Gp—
detail of the grain surface:
evidences of dissolution
(
a
,
arrow
) and coating by
automorphic quartz and kaolinite
(
b
). 7: So—Subrounded irregular
medium to coarse quartz grain. 8:
So—detail of the grain surface:
evidences of dissolution (
a
,
b
)
and possible iron oxides/
hydroxides coating (
a
)
“
Sables ocres
”
200
µ
m
200
µ
m
3
4
a
c
b
200
µ
m
200
µ
m
5
6
a
b
1
µ
m
1
µ
m
100
µ
m
7
8
a
b
10
µ
m
10
µ
m
100
µ
m
pebbles or conglomerates (reworking of weathered
Gr`s
polymorphes
”
in Kasai according to de Ploey et al.
1968
).
In Katanga (Alexandre
2002
), the succession is as follows:
(i)
“
superimposed on breccia of corroded quartz and weathered
pebbles
of
Proterozoic
sandstones
and
“
Gr`s
polymorphes
. Iron duricrust 2 is a gravelly crust with
coated red shinning iron nodules with some corroded
coarse-grained sands. When directly located over the base-
ment, the iron duricrust constitutes the uppermost part of a
weathering profile of kaolin.
”
“
Gr`s polymorphes
”
, (ii) iron duricrust 1, (iii) gravels
bed (
“
Cailloutis des plateaux
”
), (iv) iron duricrust 2, (v)
“
Sables ocres
”
.
Iron duricrust 1,
sometimes directly
overlain by the
“
Sables ocres
”
Fm,
is a pisolitic crust
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