18 Sedimentology
A.R. Chaudhri
After reading this text, a student of Environmental Sciences will understand The concept and importance of size and shape of sediments Porosity and permeability – their significance Factors governing sedimentary rock mineralogical composition Process of soil formation – significance Underground water – contaminant – sediment interaction Air quality – terrain characteristics Hydroelectric projects – Reservoir siltationWatershed management – biodiversity conservation
Sedimentology is the study of rocks formed by erosion, transportation, deposition, compaction and cementation of sediments derived from pre-existing rocks. Of the three major categories of rocks, namely the igneous rocks, the metamorphic rocks and the sedimentary rocks, the last one interestingly surfacially covers more than 75 per cent of the Earth’s surface. The sedimentary veneer on the Earth surface, interestingly, is the most significant aspect which supports biotic components of our ecosystem. Although the major focus of a sedimentological study is the decipherment of provenance, environments of sedimentation, palaeogeographic and paleotectonic setup of the terrain, yet, the basic characteristics of the sediments, namely the mineralogy, texture and geometry are the ones which are of significance for students of environment science.
Human beings are unique creatures on the planet Earth. First they try to modify the natural environment to meet their objectives and then they try to undertake remedial measures to undo what has been done. The environmental management, besides other aspects, covers aquatic ecosystems, air quality monitoring, surface and sub-surface water resource evaluation and pollution monitoring studies, farmland geology, development of built-up structures including dams, tunnels etc., seismological terrain evaluation etc. In all these investigations sedimentological studies are of primal significance.
Size of sediments is a fundamental property. The packing characteristics of the sediments, the availability of surface area per unit mass of a sedimentary particle and its physico-chemical activity are of significance in understanding the behavioral aspects of a sedimentary unit. The understanding of the concept of sediment size has evolved through a long process of discussion amongst sedimentary petrologists. The size of the sediments has been linked with the measurement of actual dimensions, weight, volume, surface area and settling velocity. The size classification of sediments has been attempted by Hopkins (1899), Atterberg (1903), Udden (1914), Wentworth (1922), Cayeux (1929) and others. The Wentworth-Udden Scale (Table 1) is the mostly widely used notation.
The individual pebbles have been classified into tabular, equant, bladed and prolate shapes (Zingg, 1935). Roundness of sediments is a term associated with the sharpness of the edges and corners of the detrital particles. Pettijohn (1984) classified the sediments into angular, subangular, subrounded, rounded ad well rounded categories. Roundness of the sediments is an index of the distance of transport of the sediments from their source regions as also the cyclicity of the detrital particles. Although the mineralogy of the fragment plays an important role in influencing its roundness, usually, the sediments which have travelled greater distances from their source regions as also the multi-cyclic sediments are better rounded than their less travelled and single cycle counterparts. The manner in which the individual particles are in contact with each other in an assemblage is of significance in comprehending the porosity and permeability of the sedimentary assemblage. These two properties are quite important in understanding the bulk behavior of a sedimentary unit. Considering an ideal case of sediments as spheres, the rhombohedral packing represents the tightest scenario with least porosity whilst the cubic packing represents the loosest systematic packing. The porosity values are around 25 per cent in rhombohedral packing while these are a whopping 47 per cent for cubic packing (Fig. 1).
Fig. 1 Porosity variation in different sphere packing styles.
Initially the detrital particles exhibit tangential contacts but overlying pressure and intrastratal solution subsequently change the grain contacts to planar or concavo-convex types and the associated porosity/void spaces.
Porosity represents the percentage of pore space in the total volume of a rock. This pore space, which develops due to tangential grain contacts, is an important characteristic of the sediments that affects the manner in which fluids, namely water, petroleum or mineralizing solutions, are transmitted or stored in sedimentary horizons. The sorting characteristics of the detritus, its shape, depositional characteristics, packing configuration and physical as well as physico-chemical changes upon compaction and diagenesis significantly reduce the primary sediment porosity values. Carbonate rocks or those sediments which have a soluble cement usually exhibit increased secondary porosity on account of leaching away of the soluble component. In general, finer sized sediments exhibit higher porosity than coarse grained sediments. Uniform grain size sediments have a higher porosity as compared to a mixed sediment size population as the finer size grade sediments occupy the interstices between the coarser grains and result in lowering of the available void volumes. Certain types of limestones and freshly deposited clays exhibit porosity values of around 80 per cent. Apart from the anomalously high values listed above, usually sandstones have around 20 per cent porosity.
Permeability is a significant property of sediments which is of importance in conduction of fluids. A permeable rock has interconnected pores which allow appreciable quantity of fluids to pass through it in a given time frame. The permeability Q through a given porous medium cross-section is mathematically expressed as
Q= K.CP/VL
where K is the proportionality factor, C (cm2) is the cross-section, L (cm) is the length, P (atmosphere) is the pressure difference at the two ends of the system and V (centipoises) is the viscosity of the fluid. The coefficient of permeability, K, is influenced by grain size, sorting of sediments, shape of sediments and their packing characteristics. Coarser grained sediments having lower sphericities exhibit more permeability than their finer, more spherical counter parts. Consolidated sedimentary horizons exhibit greater permeability values along the bedding plane direction as compared to in a direction across the bedding plane apparently due to grain fabric orientation.
The mineralogy of sedimentary rocks is dependent upon the source rock mineralogy, the agents of denudation, the modifying effect of intrastratal solutions and diagenetic changes and the time duration of existence of the rock. Usually a young sediment assemblage consists of pyroxenes, amphiboles, garnets, alumino-silicates, plagioclase felspars, micas, clays, quartz etc. This assemblage, which contains unstable and stable minerals, gradually decays over a period of geological time and gradually the ultrastable assemblage comprising quartz and miniscule amount of felspars and minor mineralogical constituents remains.
The biotic environment is grossly nurtured by soil. Soil is the weathering product of rocks which has been conditioned by the physical, chemical and biological processes for thousands of years. The role of plants and animals and their residues in the soil evolution process is significant. Soil profile represents the entire process of events from a buried bed rock at depth to the top layer of the soil, commonly referred to as the A horizon. Pedology is the branch of soil science dealing with soil formation and classification (Fig. 2). The model of soil development begins with physical disintegration of the parent rock. Gradually many microscopic and megascopic living organism inhabit this space and the decomposition of these gives rise to humus. The denudational process acting on rock particles results in decomposition of minerals present in the rock fragments. Fine clay results which either remains on surface or tends to move downwards, either to be lodged in the intermediate zone (B horizon) between the upper zone and the lower C horizon, or moves down below in the deeper parent material horizon. The process of washing out of the clay material is termed as eluviation and the mechanism of moving in of the clay into deeper horizons is termed as illuviation. Both these mechanisms result in compositional and structural variability of the different soil horizons.
the entire process of soil formation starts afresh. In certain cases, new soil formation may take place over mature soil profile. The periodic development of new soil results in the development of a soil succession. The old soil horizon is termed as paleosol. It supplies a host of information regarding the climate and vegetation that was supported by the old soil horizon.
The mineralogy of the bedrock which disintegrates in case of an in situ soil profile, and the mineralogy of the host rocks which have yielded the sediments, control to a large extent the dominant physical and chemical characteristics of the soil. The peculiar characteristics of soil like degree of compaction, porosity values, wet or dry nature, leached or nutrient rich character, warm or cool feel etc. are of significance in understanding and evaluating its potential for cultivation of specific crops as well as its utility in acting as a sink for varied environmentally toxic materials. Further, the depth of soil cover varies in different geomorphological settings and has a significant bearing upon soil fertility and the availability of underground water. In a hilly terrain, the hill slopes usually have a thin veneer as compared to the valley. The valley region with a thicker soil cover are, in general, more promising terrain for exploration of underground water.
Underground water exists in horizons termed as aquifers. These could be confined or unconfined in nature. The textural characteristics of the sand, gravel or fractured rock as also porosity, permeability, packing etc. govern the water yielding capability of the dug wells/bores in these water saturated horizons of sedimentary terrains. Further, the subterranean flow of water in the aquifers is of significance in comprehending the contaminant transport through the three major dispersal processes namely advection, diffusion and dispersion. Advection is the transportation of fluid contaminant through bulk downstream flow. It is different from transport by molecular diffusion. Diffusion refers to the net movement of molecules from a region of high concentration to a region of low concentration. In this process, bulk motion of material is not the major factor. Hydraulic gradient controls the dispersal through advection while concentration gradient is the major factor affecting diffusion of contaminants through underground water. The pore size, length of the transect under observation and the retarding affect on water velocity due to friction with soil particles creates differences in contaminant concentration on a micro-scale. The mixing or dispersion could be in a longitudinal direction along the flow path or in a transverse direction, normal to the direction of the major flow path. The mineral grains and organic particles in soil interact with contaminants. Sorption, which includes absorption, chemisorption and absorption characterizes the contaminant distribution between the solution and the solid phase. The major factors influencing the process includes the water solubility and polarity of the contaminants; the mineralogy, porosity, permeability, homogeneity, organic carbon content, surface charge and surface area of soil particles; and pH, alkali content and dissolved organic carbon content of the interacting soil media. These characters are of significance in landfills, impoundments, in situ barriers and in major ground water remedial geo-environmental projects.
The air quality at a location in terms of suspended particulate matter is governed by the terrain characteristics besides being influenced by emitted pollutants and climatic factors. The fine grained fluviatile sediments that comprise the Indo-Gangetic alluvial plains of northern India are responsible for the suspended atmospheric dust particles in the region. These compound the effect of climatic conditions and emitted pollutants on humans.
Storage hydro-electric dam projects involve huge fiscal investment over a very small time horizon. The projects have limited life span as their functionality is governed by the natural time taken by the siltation process to fill the storage reservoir. The sedimentology of the catchment area, especially the presence or absence of a weakly consolidated friable silt contributor horizon in the vicinity of a dam site is a significant issue. Usually, power generation in higher Himalayan dam projects is adversely affected during rainy season when silt inflow is high. Tehri Dam is not being able to put to optimum power production as the bounding hills around the dam reservoir site are friable and extraction of large quantities of water from reservoir for power generation would result in quick siltation of the dam reservoir.
In hard rock terrain with scanty rainfall, the soil cover is rather thin. Watershed management at micro- and mini- levels is required for soil conservation and management of soil productivity and food grain cultivation. Construction of check dams, gullies, hill slope contouring, growing grass and trees on slopes are some of the methods being employed to maintain and enrich the soil and the biotic diversity in such regions.
The interaction between soil, water and air in the sedimentary terrain, which has been built by nature over a period of millions of years, sustains plant and animal life on earth. Thus, sedimentology plays a key role in supporting the ecosystem in which we live.