Etude stratigraphique, pétrographique et diagénétique des grès d ...

tel-00534181, version 1 - 9 Nov 2010
Chapter I
Sh can be attributed to planar bed flows of shallow high velocity (Miall, 1978; Harms et al., 1995) or traction
carpets (Postma, 1990).
- Facies Sm. This facies is composed of fine to very coarse-grained sandstone lacking sedimentary
structures. The facies is evenly distributed within the LSF although they are more abundant towards the base
of LSF. The beds vary in thickness from a minimum of about 0.2 m to a maximum of between 3 and 6 m on
average. Nodular horizons are common, mostly occurring at the base or the top of the respective beds. This
facies is most common in the lower parts of the LSF represented by beds LPR 13/04, 19/04, 25/04, 35/04 and
68D/04 (Fig. 10a). The sandstones show a variety of colours ranging from whitish grey, grey-purple, to yellowish
orange and brown. Clasts of 2-3 cm of grey argillaceous material are present in the massive units (See §
4.3.1.3.). Massive sandstone beds are interpreted as sand rapidly deposited directly from turbulent suspension
by hyperconcentrated mass flows or subaqueous high-density turbidity currents (Ghibaudo, 1992).
4.3.1.3. Claystone and silty claystone facies. Claystone and silty claystone facies occur more commonly
in the lower part of the LSF than in the upper section, where they only appear as thin beds (few tens of
cm in thickness) or as reworked clay pebbles. Attempts to recover organic debris such as pollens or plant
remains from these claystones that could be used for palynological and palynofacies investigation were unsuccessful.
Plant remains are however present at several levels in sandstones, in the form of silicified tree trunks
that locally are up to 7 m long.
- Facies Fl. This facies is made of laminated or thinly bedded claystones and is not very common in
the LSF, though it is represented in the lower part of the type-section (sample LPR 65/04; Fig. 10a), and in the
upper part of the formation at the Kerral section (sample KER 21/05; Figs. 12 and 19d, e). In LPR 65/04, the
facies passes downwards into the massive claystone (Fm) facies. In the type section of the LSF, there are several
horizons made of unexposed sections buried under screes (Fig. 10) and it is suspected that such horizons
may be made of fine-grained facies (either Fl or Fm) that have been selectively weathered.
- Facies Fm. This facies is composed of claystone and silty claystone and is not evenly distributed in the
formation, being confined mainly in the lower section. The colour of the claystones ranges from reddish to violet,
grey green, and dark grey to black. Bed thickness is quite variable with minimum of 0.2 m to a maximum of 10
m. Other occurrences of this facies are in the beds LPR 59/04 in the middle of the formation and LPR 133/04 near
the top of the type-section (Fig. 10). The contact with overlying beds is almost always strongly erosive (Fig. 19be).
Clay pebbles are locally incorporated in coarser facies overlying the claystones (Fig. 19c, f), perhaps as rip up
clasts indicating erosion of flood-plain deposits accompanied by sediment bypass. Facies Fm is interpreted as overbank
or abandoned channel deposits that accumulate in from stagnant pools of water during low stage channel
abandonment, including deposits of distal parts of the flood-plain ponds (Miall, 1996).
4.3.1.4. Palaeosols and associated deposits. Palaeosol horizons are common in the lower and upper
parts of LSF, both at the Lokitaung Gorge (type-section) and the Kerral sections (Figs. 9a, 10 and 12). They
Figure 20. (a) Vertical burrows within the lower parts of the LSF at the north Lokitaung Gorge section. The lower bed is
in an erosive contact with the overlying massive sandstone bed. (b) Burrows, plan view, within a purple palaeosol horizon
of the lower part of the LSF, right bank of the Karamukun Laga (Fig. 9a). They occur as white-grey sediment within a predominantly
purple sandstone. This bed possibly correlated with similar facies observed at the type–section (photo (a)). (c)
(d) Several palaeosol horizons showing alternating red and grey zonation within the upper parts of the LSF. (e) Plan view
of root traces within the upper part of the LSF at the very north end of the Lapur Range. (f), (g) Large columnar pipes
within the palaeosol zone of the multi-coloured cliff in the upper Lokitaung Gorge and Kerral sections. These pipes occur
within medium to coarse grained sandstones and can be interpreted as fluid conduits or water escape features. (h), (i)
Crusts of hematite that are common especially within the middle part of the LSF in the north Lokitaung Gorge section.
They form bed terminations within the sandstone unit of the braided river deposit.
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