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

tel-00534181, version 1 - 9 Nov 2010
Chapter II
4.2. Sandstone facies
Sandstone forms the most abundant facies in the MSF. Based on the dominant sedimentary structures,
4 types of sedimentary facies were recognized in the formation (See Table 10). They are described below
in order of relative abundance.
Facies Sm
Massive sandstone beds lacking sedimentary structures are the most abundant facies type in the MSF
and it occurs throughout the vertical section measured. Due to its general lack of sedimentary structure the top
10 m quartzitic bed at the very top of the measured section has been classified as belonging to the Sm facies.
Fine to medium sandstones are the dominant grain size range, while coarse and pebbly to conglomeratic sandstones
also occur but in smaller proportions. The samples are well to very well indurated. They range in colour
from white to light grey, yellow to purple and brown. The quartzitic bed at the top is highly silicified, it produces
a metallic ring when struck with a hammer, it contains fossil wood fragments and it is highly jointed and
faulted in places. Oxidation gives it a reddish-brown colouration. The white, pebbly, massive beds contain pebbles
of quartz and clay clasts that are 2-4 cm in diameter. The clay clasts form subhorizontal layers towards
the base of the beds. There is tendency for the beds to grade vertically from horizontally bedded facies Sh
(marked in most cases by the presence of horizontally aligned elongate clay clasts) to massive beds of facies
Sm. Transition from planar The sandstones range in granulometry from fine sandstones, medium to coarse and
very coarse. Some sandstones show bimodal grain size distribution with medium grained sandstone in fine
sand to silty matrix.
Facies Sm is interpreted as a deposit of subaerial hyperconcentrated flows or in subacqueous highdensity
turbidity currents. In this case, sand is deposited directly from turbulent suspension with insufficient
time for development of bedfoms (Horton and Schmitt, 1996).
Facies Sp
This facies is composed of fine, medium, coarse, very coarse and occasionally pebbly and microconglomeratic
sandstones, all with planar cross stratification. Clay clasts are present in the pebbly sandstones.
Samples are commonly well indurated. It forms beds with thickness ranging from 0.7 to a maximum of 6 m,
and their colour ranges from white, grey, to yellow with brown stains. Orientation of the planar cross stratifications
are in the direction N40°, N80°, N90°, N100°, N130°, N180°, N220°, N230° and N300°.
Facies Sp is deposited by the action of 2-D dune generation and migration on non-cohesive sand beds
(Miall, 1977). Lack of sigmoidal foresets signifies the absence of a strong separating eddy normally associated
with high water stage; indicating that these beds were deposited at the low water stage (Miall, 1977).
Facies Sh
Sh facies is composed of horizontally stratified fine, medium and coarse grained sandstones, with
clay clasts measuring up to a maximum of 20 cm in long diameter. Bed thickness varies from 80 cm to a maximum
of 7 m. Colour of the sandstones ranges from grey through yellow to brown. This facies sometimes is
association with the planar cross-stratified facies Sp and the massive sandstones facies Sm, where vertical gradations
between one facies to another is observed within one bed. In most instances, the horizontal stratification
is marked by zones containing abundant clay clasts that are elongated and aligned in a horizontal fashion.
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