7 Siliciclstic Sediments II: Mudrocks
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7 Siliciclastic sediments II: Mudrocks 7.1 Introduction Mudrocks are the most abundant type of sedimentary rocks, constituting about 45-55% of the sedimentary sequences. But since they are easily weathered, they are covered with vegetation and are poorly exposed. Mudrocks can be deposited in all sedimentary environments, but the majority are deposited in river floodplains, lakes, large deltas, the more distal areas of clastic shelves, basin slopes, and deep sea floors. The main constituents of mudrocks are the clay minerals and silt-grade quartz. In terms of grain size, clay refers to particles less than 4 µm in diameter, whereas silt is between 4 and 62 µm. On the other hand, clay as a mineral is a hydrous aluminosilicate with a specific sheet structure (to be discussed below). The term mud (also lutite) loosely refers to a mixture of clay- and silt-grade material. The mudstone, the indurated or lithified equivalent of mud, is a blocky, non-fissile rock, whereas shale is usually laminated and fissile (fissility is the ability of rock to split into thin sheets). Argillite is used for more indurated mudrock and slate is the metamorphic mudrock possessing a cleavage. Claystone is the sedimentary rock consisting of a clay- grade material. Siltstone consists of more silt-grade particles than clay. Calcareous mudrocks are called marls. Fig. 7.1 gives a scheme for mixed sand-silt-clay deposits. 1 Fig. 7.1: Classification of siliciclastic sediments based on sand, silt and clay content. In the field, the terms mudstone, shale, claystone and siltstone are best qualified by attributes referring to color, degree of fissility, sedimentary structures, and mineral, organic and fossil content (Tab. 7.1). Tab. 7.1: Features to note in the description of mudrocks 2 7.2 Textures and structures of mudrocks The particle size on unconsolidated muds can be measured using sedimentation chamber or settling tube, whereas lithified mudrocks can be studied by the scanning electron microscope. The use of grain-size data in interpretation of the depositional environments is complicated due to the fact that clay minerals are deposited as floccules and aggregates, and that sediment feeding organisms may generate pellets of muds. Also bioturbation disrupts the depositional textures of mud. However, the following are the important textures and structures of mudrocks. Preferred orientation of clay minerals and mica flakes parallel to bedding is a common texture of mudrocks. It is the result of deposition of clay flakes parallel to bedding, and less commonly due to compaction and dewatering. Related to preferred orientation is fissility of shale (Fig. 7.2). Fig. 7.2: a- Shale characterized by fissility (left); b- Massive non-laminated mudstone (right). 3 The origin of fissility is mainly due to alignment of clay minerals as a result of compaction, in addition to the presence of lamination. Absence of fissility, as in mudrocks, can be explained by bioturbation, the presence of much quartz silt or calcite, and the flocculation of clays during sedimentation that produces random fabric which could be retained on compaction. The most common sedimentary structure of mudrocks is lamination (Fig. 7.3). Fig. 7.3: Lamination in mudrock: rhythmites of glacial verves, where graded silt passes upward into clay-grade material Lamination is the result of variation of grain size and/or changes in composition. Size- graded laminae may be deposited from low-density turbidity currents followed by deposition from suspension currents in relatively short periods of time (hours or days). Other laminae develop over longer periods of times (months or years) if there is a seasonal or annual fluctuation in sediment supply and/or biological activity. Organic laminae in mudrocks, for example, may be produced by seasonal microbial blooms. Also the varved couplets of glacial lakes are taken to reflect the annual spring melting. Small-scale current ripples occur in siltstones and give rise to cross-lamination (Fig. 7.4). Symmetrical wave-formed ripples also can form in siltstones. 4 Fig. 7.4: Photograph showing ripple cross-lamination in siltstone indicating a flow from right to left; the ripple structure is picked out by alternation of dark clay-rich and pale clay-poor laminae. In tidal flats, mud and fine-sand to silt are deposited alternatively through fluctuating current regimes and/or sediment supply giving rise to flaser (ripple-shaped fine sand-silt occur in mudstone), and lenticular bedding (ripple-shaped mud occurs within fine sand- silt) (Fig. 7.5). Fig. 7.5: Flaser-lenticular bedding in Ordovician Dubaydib Formation of Jordan. 5 Small-scale scour and fill structures, and micro-cross-lamiantion may be present in siltstones. Mudrocks lacking any internal sedimentary structures are called massive. This massive nature is due to deposition from high viscosity currents as mudflows and debris flows; or due to bioturbation that destroys any original structures as lamination, mass sediment movement (sliding), dewatering, soil processes (pedogenesis) and root growth. The structures in muds and mudrocks can be studied using X-ray radiography (photographing using X-ray) which may reveal lamination or bioturbation in an otherwise massive-looking rock. Other sedimentary structures that may occur in mudrocks include: erosional structures cut in muds and preserved on soles of overlying sandstones (grooves and flutes); slump structures; desiccation cracks formed through subaerial exposure; rain-spot prints and biogenic structures. 7.2.1 Nodules and concretions Many mudrocks contain nodules, also called concretions (Fig. 7.6). Fig. 7.6: Calcareous nodules in red mudrock. These nodules are regular to irregular, spherical, ellipsoidal to flattened bodies, commonly composed of calcite, siderite, pyrite, chert, calcium phosphate, together with some original sediment. Nodules may grow within the sediment during diagenesis, either just below sediment- water interface, or much deep in the sediment column. Early diagenetic nodules form in sediments that are still soft and uncompacted. These can be recognized from the presence of uncrushed fossils within the nodule and from the folding of laminae in the mudrock around the nodule, showing that compaction took place after growth of the noudle. 6 Nodules that form after compaction of the host sediment during burial diagenesis are characterized by laminae in the sediment pass unaffected through the nodule. The growth of nodules arises from the localized precipitation of cement from pore waters within the sediment. The composition, Eh, pH of these pore waters are important in controlling nodule mineralogy and growth rates. In some case nodules formed around a nucleus, a fossil for example, as a result of local chemical conditions. More commonly nodules are without nucleus, and form along definite horizons or within particular beds, reflecting a level at which supersaturation of pore waters was achieved. Elongate nodules may show a preferred orientation, reflecting the direction of pore water movement. 7.3 The color of Mudrocks The color of mudrocks depends on the mineralogy and geochemistry of the rock. The main controls of color are the organic content, pyrite and oxidation state of iron. With increasing organic matter and pyrite, mudrocks take a darker grey color and eventually become black. Many marine and deltaic mudrocks have a dark grey to black color because of finely disseminated organic matter and pyrite. Red and purple color results from the presence of ferric oxide, hematite, occurring chiefly as grain coatings and intergrowth with clay particles. Red color develops after deposition, though an aeging processes of a hydrated iron oxide precursor, as the case with red sandstones. The precursor is mainly detrital in origin, and may be in situ solution of metastable mafic mineral grains. Red color of flooding plain mudstones reflects oxidizing nature of depositional and early diagenetic environment. Green mudrocks contain no hematite, organic matter or iron sulfide, but the color comes from ferrous iron within the lattices of illite and chlorite. The green color may be original or may develop in mudrocks that were red originally but subjected to reduction of hematite by migrating ground water. Green spots and patches in some red mudrocks are sites of iron reduction from local occurrence of organic matter. Other colors in mudrocks result from mixing of color-producing components. For example, olive and yellow mudrocks may owe this color to a mixing of green minerals and organic matter. Some mudrocks have a color mottling, where there are different shades of grey that may be the result of bioturbation. In yellows/reds/browns mottles, pedogenic processes resulting by moving water through soil causes an irregular distribution of iron oxide/hydroxide and /or carbonate, and the effect of roots. The color mottling is common in lacustrine and floodplain muds and marls. 7.4 Mineral constituents of mudrocks 7.4.1 Clay minerals 7 Clay minerals are hydrous aluminosilicates with a sheet or layered structures; they are phyllosilicates like the micas. The sheets are of two basic types. One is a layer of silicon- oxygen tetrahedral with three of the oxygen atoms in each tetrahedron shared with adjacent tetrahedral and linked together to form a hexagonal network (Fig. 7.7). The basic unit is Si2O5 but within these silica layers aluminum may replace up to half of the silicon atoms. Fig. 7.7: Diagrams illustrating the structures of clay minerals. The second type of layers consists of Al in octahedral coordination with O2- and OH- ions, so that in effect the Al3+ ions are located between two sheets of O/OH ions (Fig. 7.7). In this type of layer, not all the Al (octahedral) positions may be occupied, or Mg2+, Fe and other ions may substitute for Al3+. Layers of Al-O/OH in a clay mineral are referred to as gibbsite layer because the mineral gibbsite (Al(OH)3) consists entirely of such layers.