Facies Analysis
Facies Analysis
medium to coarse grained pinkish white to greyish white Clayey Sandstones. The lower-most of this lithfacies in LI (unit 1) is very friable and possesses clasts and clay lamina (which is suggestive of a Tidal Current Deposit). Lithofacies (A), contains trace fine sands at the middle portion of L3 (unit 8) and displays a generally fining-upward arrangement. Bioturbated, Cross and Planar Bedded Sandstone Lithofacies (B): The Sandstone may be poorly sorted and pebbly in some places. The main characteristic Sedimentary Structures are Planar/ Cross-beds and Herringbone Structures. The colour ranges from whitish to dark grey and is typically Coarsegrained and occasionally fine-grained. This Lithofacies in L2 (Ajali Formation) within the study area may be heavily bioturbated (with both straight/branched channels and burrows as well as with tubes made up of Siliceous materials) and may also be characterized with a lot of trace pebbles (the pebbles align in the direction of the cross-beds) as well as an abundance of reactivation surfaces. Facies (B) is caped at the Upper Part of "L3" by a Ferruginized Hard-Ground. Cross-Stratified Siltstone facies (C): This Facies compose of fine/medium grained whitish-grey siltstone. The top contact between the Facies in L1 and the overlying Facies (D) is erosional and typified by a "Reactivation-Surface" (which could imply that the overlying Facies (D) may have been deposited by a current of higher-enregy). Dark-grey spots are obvious in this Lithofacies, which may be attributable to ferruginization. In L3, this Lithofacies mark a turn-around from an increasing thickness in the Silt and Sand content up-section to a corresponding increase in Shale thickness down-section, which may be indicative of a "transition zone" between the underlying Mamu Formation and the Overlying Ajali Formation, thereby presenting a change from Fluvial/Tidal environment to a more Marine environment respectively.
Intercalation of Lenticular Bedded Mudstone and Siltstone Facies (D): This Lithofacies was observed within the Middle Portion of LI only. It directly underlies the Flooding Surface in LI. The Mudstone in the lower part of this Lithofaciesi^Bis relatively more competent (protruding out) and the progressive increase o Mudstone content observed in this Lithofacies especially around nit 5 is indicative of a change to a more calm environment of deposition. Lithofacies (D), marks an abrupt turn-around from a previously fining-upwards profile relatively to a coarsening-upwards profile. Horizontal/Ripple Stratified Intervening Layers of Siltstone Facies (E): This Lithofacies (E) forms units within the upper parts of LI and L2 within the study area and are typically yellowish grey (in LI) to yellowish brown/brown (L3). This Facies may be heavily weathered especially in the uppermost part of Ll and may be heavily weathered, gravely and slightly bioturbated in some sections. This lithofacies shows internal coarsening in LI and is highly ferruginized at the uppermost part of L3. Laminated Carbonaceous Shale/Siltstone Facies (F): The Lithology of this Facies exhibits Shale-dominated Siltstone and is encountered (Within the study area) at L3 (Unit 5), it is dark grey in colour and fine-medium grain in size. Kaolinitic - Clay Lithofacies (G): This Lithofacies is observed only at L3 (unit 4). Its presence does not only mark an abrupt Basin-ward Shift in Facies but also mark the end of a rather more continental environment before the deposition of the overlying shaley unit (Facies F). It may be horizontally stratified, fine-grained and reddish brown in colour.
FACIES ANALYSIS
Boggs (2001) defined facies as the sum of the characteristics of a sedimentary unit resulting from some particular sets of physical, chemical, and biological parameters that work to produce a unit with specific textural, structural and compositional properties. This definition is generally interpretative, since it is not specific to a rock, but to a predictable
set of characteristics associated with some depositional environments, this is also because the nature of the material deposited anywhere will be determined by the physical, chemical, or biological processes which have occurred during the formation, transport, and deposition of the sediment, and these processes also define the environment of deposition (Nichols, 2001).The facies concept refers to the sum of characteristics of a sedimentary unit commonly at a fairly small scale (cm m). Facies analysis entails the study of strata in terms of depositional environments which is commonly based on a wide variety of observations. For the purpose of this work, i will imbibe both the descriptive and the interpretative meanings of facies by giving a description of the characteristic physical, chemical, and biological attributes of the sedimentary units which will in turn be used to interpret the depositional environment (s) of sediments within the area of study.
LITHOFACIES DESCRIPTION
A facies is defined by a particular set of sediment attributes; a characteristic lithology, texture, suite of sedimentary structures, geometry,paleocurrent pattern, etc. (Tucker, 2003). According to Walker and Janes (1992), the traditional descriptive stratigraphic schemes for subdividing ancient sedimentary rocks involve the definition of lithologically homogenous units. Facies are best referred to objectively in pure descriptive terms, using a few pertinent adjectives; examples could include cross-bedded, coarse sandstone facies or massive peebly mudstone facies. Facies can be numbered or referred to by letters (facies A,facies B etc) (Turcker, 2003).
This Lithofacies in sections of Locations 1 and 2 consists of mainly fine grained and occasionally medium to coarse grained pinkish white to greyish white Clayey Sandstones. In Location 3, it appears as intervening layers of Sandstones and thin bands of Clay. The Sandstone is friable especially in Location 1 where it is also characterised with the presence of Clay lamina (preservation of Clay lamina in sands is indicative of tidal current deposits.
beds and lenticular beds presents an average of 16o-210 and 10-80 respectively.