Sublitharenites

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. 

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.

Tirrawarra Sandstones in the Moorari and Fly Lake Fields consist mainly of medium-grained, moderately sorted sublitharenites (mostly mica schist and phyllite, shale and siltstone clasts) (classification of Folk, 1974). A variety of authigenic minerals are recognized, including syntaxial quartz overgrowths, minor illite, patchy kaolinite and siderite. Attention is focused on the siderite and only a short description of the other diagenetic minerals is provided here. 

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). 

KM4 Quartz, feldspar, rock fragments (quartzite, schist, phyllite), serizite, muscovite, opaque min. Clay Coarse Sublitharenite/ litharenite... 

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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