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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