Sedimentary rocks constitute about 75% of the total exposed rocks on the earth’s crust. They are found in different forms/structures. The important structural features of sedimentary rocks are stratification, lamination, graded bedding, current bedding, ripple marks. Besides these, there are some minor structures such as mud cracks, rain prints, tracks of terrestrial animals, etc. These structures give clues to the past environment. The most important structural features of sedimentary rocks are discussed below;
All sedimentary rocks are, in general character, stratified into layers. Deposition of sediments into layers or beds is called “stratification”. The planes dividing different beds are called “bedding planes”. The thickness of a bed may vary from a few centimeters to many meters. Different beds are, clearly distinguished from one another due to (i) differences in their mineral composition, (ii) texture (variation in grain sizes), (iii) difference in colour, and (iv) variation in thickness.
Thin beddings less than one centimeter in thickness are called Lamination. Lamination is usually found in very fine-grained rocks like shale and gives them the characteristic property of fissility. In laminated rocks, the clay and other flaky minerals tend to lie with their flat surfaces parallel to the plane of lamination. It should be noted that lamination refers to the parallel arrangement of minerals within a bed whereas stratification refers to a succession of beds separated by bedding planes.
(3). Graded Bedding
Graded bedding simply identifies strata that grade upward with coarse-textured clastic sediment at their base and finer-textured materials at the top. In graded bedding, each bed shows a gradation in grain size from coarse below to finer above. The graded bedding results in rapid sedimentation in water. This structure is commonly found in greywackes. The bottom of the graded bed is enriched with coarse gravel, while the top is enriched with finer grains. The lower part of the shale bed generally consists of coarse grit. It then shows an upward transition towards finer material. At the top, it commonly ends in shale.
(4). Current Bedding
Current bedding is also called “cross-bedding”. In this structure, minor beds or laminations lie at an angle to the planes of general stratification. These minor beds commonly terminate abruptly at the top, where they are overlain by the next current bedded deposits. Current bedding is commonly found in shallow and wind-formed deposits. This structure indicates rapid changes in the velocity and direction of flow of streams or winds carrying sediment. In current bedding, the minor beds are inclined and stacked up in the form of a wedge in the direction of water currents or prevailing wind.
(5). Ripple Marks
The wavy undulations on the surface of bedding planes are known as the “ripple marks”. They are produced by the action of waves and currents in shallow water. This structure may also be formed on the surface of deposits formed by winds. Ripple marks are of two types; (i) asymmetrical/ current ripple marks, and (ii) symmetrical/ oscillation ripple marks. The oscillation ripple marks are useful in determining top and bottom of deformed beds.
(6). Minor Structures
The surface of the bedding plane may show some minor structures such as mud cracks, rain prints, and tracks and trails of animals. These structures are commonly preserved as casts. “Mud cracks” are often found in the fine-grained sedimentary rocks, that have been exposed to drying under sub-areal conditions. They form a network of fissures enclosing polygonal areas. Mud cracks are characteristic of flood plains of large rivers. A “rain print” is a slight shallow depression rimmed by a low ridge, which is raised by the impact of the raindrops. It is formed when a brief rain shower falls on a smooth surface of fine-grained sediment. “Tracks and trails”are the markings indicating the passage of some animals over soft sediment. All these minor structures, if found in formations that have been disturbed by severe folding, are of great help in determining the top and bottom of beds.
“Concretions” are variously shaped masses or nodules of mineral matter found within sedimentary rocks. Their shape may be spherical, ellipsoidal, lenticular, or irregular. Concretion. Concretions generally consist of calcium carbonate or silica and often possess an internal radiating or concentric structure. They are formed by the deposition of mineral matter from percolating solution about a nucleus. Their chemical composition is generally different from the enclosing rock. They often represent a concentration of one of the minor constituents of the host rock. For example, in limestones, there are concretions of chert or flint, in clays concretions of calcium carbonate or iron sulfide, and in sandstones, the concretions are common of iron oxide or calcium carbonate.
You may like to read;
- Sedimentary Rocks and their formation
- Classification of Sediments
- The texture of the Sedimentary Rocks