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The following are the twelve main characteristics of sedimentary rocks.
(1) Sedimentary rocks are formed of sediments derived from the older rocks, plant and animal remains and thus these rocks contain fossils of plants and animals. The age of the formation of a given sedimentary rock may be determined on the basis of the analysis of the fossils to be found in that rock.
(2) Sedimentary rocks are found over the largest surface area of the globe. It is believed that about 75 per cent of the surface area of the globe is covered by sedimentary rocks whereas igneous and metamorphic rocks cover the remaining 25 per cent area. Inspite of their largest coverage the sedimentary rocks constitute only 5 per cent of the composition of the crust whereas 95 per cent of the crust is composed of igneous and metamorphic rocks. Thus, it is obvious that the sedimentary rocks are important for extent, not for depth in the earth’s crust.
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(3) The deposition of sediments of various types and sizes to form sedimentary rocks takes place in certain sequence and system. The size of sediments decreases from the littoral margins to the centre of the water bodies or sedimentation basins. Different sediments are consolidated and compacted by different types of cementing elements e.g., silica, iron compounds, calcite, clay etc.
(4) Sedimentary rocks contain several layers or strata but these are seldom crystalline rocks.
(5) Like igneous rocks sedimentary rocks are not found in massive forms such as batholiths, laccoliths, dykes etc.
(6) Layers of sedimentary rocks are seldom found in original and horizontal manner. Sedimentary layers are generally deformed due to lateral compressive and tensile forces. The beds are folded and found in anticlinal and synclinal forms. Tensile and compressive forces also create faults due to dislocation of beds.
(7) Sedimentary rocks may be well consolidated, poorly consolidated and even unconsolidated. The composition of the rocks depends upon the nature of cementing elements and rock forming minerals.
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(8) Sedimentary rocks are characterized by different sizes of joints. These are generally perpendicular to the bedding planes.
(9) The connecting plane between two consecutive beds or layers of sedimentary rocks is called bedding plane. The uniformity of two beds along a bedding plane is called conformity (i.e., when beds are similar in all respect). When two consecutive beds are not uniform or conformal, the structure is called unconformity. In fact, ‘an unconformity is a break in a stratigraphic sequence resulting from a change in conditions that caused deposition to cease for a considerable time’.
There are several types of unconformity e.g.:
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(i) Nonconformity (where sedimentary rocks succeed igneous or metamorphic rocks),
(ii) Angular unconformity (where horizontal sedimentary beds are deposited over previously folded or tilted strata),
(iii) Disconformity (where two conformable beds are separated by mere changes of sediment type), and
(iv) Paraconformity (where two sets of conformable beds are separated by same types of sediments) etc.
(10) Sedimentation units in the sedimentary rocks having a thickness of greater than one centimetre are called beds. The upper and lower surfaces of a bed are called bedding planes or bounding planes. Sometimes the lower surface of a bed is called sole while the upper surface is known as upper bedding surface. There are further sedimentary units within a bed.
The units having a thickness of more than one centimetre are called as layers or strata whereas the units below one centimetre thickness are known as laminae. Thus, several strata and laminae make up a bed. When the beds are deposited at an angle to the depositional surface, they are called cross beds and the general phenomena of inclined layers are called cross lamination or cross bedding.
(11) Soft muds and alluvia deposited by the rivers during flood period develop cracks when baked in the sun. These cracks are generally of polygonal shapes. Such cracks are called as mud cracks or sun cracks.
(12) Most of the sedimentary rocks are permeable and porous but a few of them are also non-porous and impermeable. The porosity of the rocks depends upon the ratio between the voids and the volume of a given rocks mass.