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

tel-00534181, version 1 - 9 Nov 2010
Chapter I
occur mainly in the silty claystone facies, and are marked by extensively burrowed horizons (Fig. 20a, b) or
by distinctly mottled levels with alternating burgundy and grey colours and abundant root traces (Fig. 20c-e).
Large columnar pipes form an especially prominent zone within the palaeosol zone of the multi-coloured cliff
in the upper Lokitaung Gorge and Kerral sections (Figs. 11a, d, 12 and 20f, g). These pipes occur within
medium to coarse grained sandstones. Similar cylindrical columns were observed in the Jurassic sandstones
of southeastern Utah and can be explained in terms of permeability heterogeneities, presence of nuclei for precipitation,
followed by a self organization process or the influence of microbes. Where the sandstone within
the pipes is more permeable than the host sandstone thereby acting as conduits for fluid flow, this creates areas
of preferential cementation (Chan et al., 2000; Beitler et al., 2005; Parry et al., 2009). Fluid flow is more efficient
in the vicinity of faults and in the case of the LSF, the formation outcrops in the vicinity of the main
Lapur-Murua Rith boundary fault which should serve as a main conduit of groundwaters.
The colours visible in such types of facies are indicative of the drainage prevailing within the flood-plain or
abandoned channel environments where the sediments lay exposed for extended periods without deposition,
the red representing good drainage and an oxidising early diagenetic environment, while grey beds suggest
water-logged and reducing pore water environments (Collinson, 1996). Red hematite-rich palaeosols are interpreted
as having developed under semi-arid to arid conditions (Reed, 1991; Khalifa and Catuneanu, 2009).
The “ferruginous crust” facies is concentrated around the level 150 to 300 m above the base of the type-section
(Fig. 10a; north Lokitaung Gorge, type-section). These crusts are generally thin, normally measuring between
1 and 2 cm with a maximum of 5 cm, and occur at the top of sandstone beds and occasionally in the middle.
They are dark reddish-brown to black-brown in colour and are very well indurated (Fig. 20h, i).
4.3.2. Facies associations and depositional environments
The LSF is characterized by a predominantly coarse-grained facies association with only few and
laterally discontinuous fine-grained facies. The most common facies association consists mainly of bedload
deposits, predominantly sands and gravels being the main grain sizes, though pebbles are sometimes
abundant. The facies associations observed in the type-section of the LSF have been attributed to 2 depositional
environments:
1) Distal alluvial fan environments represented by the lower and upper parts of the LSF; and 2)
Sandy braided river depositional environments represented by the middle part of the LSF (Fig. 10).
4.3.2.1. Distal alluvial fan facies associations. Facies assemblages associated with distal alluvial
fans are dominated by massive ungraded gravely conglomerates with trough cross-bedded (St) and planar
cross-bedded (Sp) sandstone facies with minor amounts of horizontally stratified sandstone (Sh), planar
cross stratified conglomerates and massive claystone (facies Fm) (Miall, 1977; Collinson, 1996). The
conglomerate facies are both clast-supported (facies Gcu) and matrix-supported (facies Gm). Grading may
be present (facies Gcn), suggestive of deposition from dilute and turbulent flows, while the presence of
horizontal stratification in the conglomerates may represent deposition either at the top of braid bars or as
lags on channel floors (Collinson, 1996).
This facies association was observed to be the dominant association in the lowermost (the basal
150 m) and the uppermost (above the 340 m mark) parts of the LSF in the type-section as well as the top
part of the Lokitaung Gorge section (Fig. 10). In the basal parts of this section, thin beds of medium- to
coarse-grained and pebbly sandstones as well as silty claystones occur. The coarse-grained sandstones are
54