Subarkoses

Microlithofacial classification of the sandstones is based on Dott s classification modified by Pettijohn et al. (1972). They are mostly arenites and subarkose and quartz wackes (rare sublithic, sporadically lithic and arkosic). Quartz is the main component of the sandstones (about 60-70 vol. percent). Feldspars (6 vol. percent) are mostly represented by potassium feldspars with plagioclases in lesser amounts. Some micas (muscovite and biotite) and chlorites are observed. Mica content of arenites reaches 3 vol. %, but is higher in the wackes. Heavy minerals present include zircon, sphene, rutile and apatite. Magmatic rocks (volcanic more than Plutonic) are predominant among lithoclasts (about 2 vol. %), but some metamorphic and sedimentary clasts being present too. 

The Carboniferous sandstones in the boreholes in LCB are represented by arenites and wackes, mainly quartz, subarkose and sublithic ... 

Figure 1 Detrital orthoclase grains (K) in subarkosic sandstone, from Moodies Group, Barberton Mountains, South Africa. Apart from dissolution that may relate to modem outcrop weathering (blue), these feldspars have survived since the Archean (at least 3.2 Ga).

Fig. 7. Present and original detrital composition of representative Lunde sandstones plotted on McBride s (1963) diagram A, considering albitized feldspars as diagenetic constituents (not in F pole)—observe the shift from the arkose (original composition white dots) to the subarkose field (present composition black dots) B, plotting albitized feldspars in F pole—a less substantial field dislocation is seen from original (white squares) to present composition (black squares).

According to McBride s (1963) classification, the studied sandstones classify mostly as feldspathic litharenites or lithic subarkoses, with a few samples falling into the sublitharenite, litharenite and lithic arkose fields (Fig. 4). There appears to be little systematic variation between the detrital composition of different locations. 

The Namorado Sandstone is composed of fine- to medium-grained, moderately to well sorted arkoses and subarkoses (Fig. 5). The main framework constituents are quartz, potassium feldspar, plagioclase and quartz-feldspar rich rock fragments, most of Plutonic origin. Although previous work indicated the predominance of K-feldspars (Carvalho, 1990 ... 

Fig. 5. Detrital composition of the Namorado sandstone turbidites plotted on the McBride (1963) classification diagram. Sandstones are mainly arkoses and subarkoses.

Based on Folk s (1974) classification scheme, the average sandstone of the Hibernia Field is a sublith-arenite (Qgv 9 F3 g Rg.s). Individual samples are either quartz-arenitic or sublitharenitic, and some are subarkoses (Fig. 7 Table 1). If shale clasts, which are probably predominantly rip-up clasts of intrabasinal origin and do not carry provenance information with respect to terrigenous source areas, are excluded, then the majority of the samples would be classified as quartz arenites. 

Fig. 7. QFR triangular diagram of sandstone petrography of Folk (1974). Q, quartz F, feldspar R, rock fragments. 1, quartz arenite 2, subarkose 3, sublitharenite 4,5, arkose 6,7, litharenite.

Triassic sediments are deposited only in the eastern and northern parts of the Sahara, in the Triassic Province. Triassic sandstones are mainly a combination of deltaic to prodeltaic facies represented by upper deltaic fluvial system deposits, lower distributary channel rivers, mouth bars, beach sands and tidal flat and prodeltaic shales. Typical marine Triassic shales are restricted in extent in the study area. Triassic Lower, Middle and Upper sandstones are stratigraphically separated by shale intervals, but occasionally also by carbonates or volcanic interbeds. They are often medium grained, and range from friable to cemented with clay mineral, carbonate, anhydrite and siliceous cement. Porosity ranges from 12 to 22%. With respect to mineralogical composition and maturity, Triassic sandstones are mature sublitharenite (Lower Triassic), sublitharenite to submature subarkose (Middle Triassic) and submature subarkose (Upper Triassic). 

In general, sandstones with little detrital clay actually possess an elevated permeability and exhibit a notable secondary silicification as if they were part of a subarkosic... 

KMl Quartz, feldspar, serizite, muscovite, rock fragments (schist, quartzite, silicious sedimentary rock fragments) Few clay Fine-medium Subarkose/ arkose... 

AKT Quartz, feldispar, muscovite Carbonate and very few clay Fine-medium Subarkose... 

Arkose cemented with clay and subarkose cemented with carbonate are suitable for low and high strength concrete productions respectively. Subarkose/arkose, sublitharenite/litharenite, subarkose, arkose and quartz sandstone can be used in moderate strength concretes. 

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