Plate Tectonics and Crustal Evolution

Author(s):

Kent C. Condie

New Mexico Institute of Mining & Technology, Sacorro, U.S.A

About this Book

                This comprehensive text has established itself over the past 20 years as the definitive work in its fields, presenting a thorough coverage of this key area of structural geology in a way which is ideally suited to advanced undergraduate and masters courses. The thorough coverage means that it is also useful to a wider readership as an up to date survey of plate tectonics.The fourth edition brings the text fully up to date, with coverage of the latest research in crustal evolution, supercontinents, mass extinctions. A new chapter covers the feedbacks of various Earth systems. In addition, a new appendix provides a valuable survey of current methodology.

Contents-

Chapter 1 - Plate tectonics

Chapter 2 - The Earth's crust

Chapter 3 - Tectonic settings

Chapter 4 - The earth's mantle and core

Chapter 5 - Crustal and mantle evolution

Chapter 6 - The atmosphere, oceans, climates, and life

Chapter 7 - Comparative planetary evolution

References

Index

Download

Read More

Method of Mineral Exploration

Introduction:

    Economic viability of any mineral deposit is estimated by sampling the mineralised body. Initially, the rock chips from the mineralised outcrops exposed on the surface is sampled and later stage channel samples from mine faces are collected, but for exploration geologist the most important samples are from drilled boreholes.

    In mineral exploration programme, drilling in the initial stage is either for data collection or to test anomalies established by radiometric and non-radiometric survey methods (geophysical and geochemical) and to get information on stratigraphy like lithological succession, basinal structure or depositional or deformation history. Keeping the objective of drilling in mind the boreholes were carefully planned.

    As an example, where the dip of a mineralised outcrop is not clearly visible usually two inclined boreholes are drilled from either side. But still, there are problems of exploration of ore bodies having a steep inclination or variable inclination on either side. In such cases sometimes same platform is used to drill a number of boreholes with different inclination after taking into account the possibility of deviation of the boreholes. Sometimes the surface features or topography is an important limitation for borehole planning. For example, the deep nalas or escarpments restrict the movement of drill rigs and in such cases, the planning of borehole needs special skills.

    The initial or reconnoitory phase of drilling basically meant on testing the anomalies, identifying favourable target zones and the subsurface behavior of mineralisation on a broad sense for detailed evaluation. Once the mineralisation or nature of mineralisation is established, considerable meterage is drilled at the next stage during evaluation of deposit. Subsequently, different criteria are followed depending on the phases of exploration.

    The prime objective in drilling is to determine the quantity and quality of minerals. The quantity means the volume of ore, thus, the aim of exploration is to ascertain the nature and configuration of ore body or size or shape of the deposit which has to be established in all three direction by drilling. Hence, the location of boreholes needs careful planning depending on whether the ore body has a surface manifestation or it is a blind ore deposit.

    The geometry of ore body is very important as it is related to control of mineralisation and influences the decision about the spacing and inclination of boreholes. Geometrically, all mineral deposits fall into three morphological types -

   a. Bodies with one short and two long axis examples horizontal low dipping and steep dipping beds, veins, sheets, lenses, etc. This type is widespread in nature like coal beds etc.

   b. Bodies with one long and two short axis e.g. pipe like body, like kimberlite pipes.

   c. Bodies which are more or less isometric e.g. stock works and masses.

  Quality is generally determined by chemical and mineralogical composition of the deposit. Quantity and quality of mineral deposit are interrelated since the shape of the ore body is determined based on the accepted cut off grade.

Principles of Exploration

Exploration of all mineral deposit is governed by same set of principles. These are:

1. Successive approximation (including the principle of geological forecasting and                                                                  verification).

2. Completeness of investigations.

3. Uniform reliability.

4. Less outlay of time and means.

    According to the principle of successive approximation or geological forecasting and verification, every opening (borehole or sample point) either confirm the forecast or corrects it or rejects it for revision. Thus, geological model conceived through different stages of exploration needs verification and if required modification and re-verification. This process is continuous.

    The principle of completeness of investigation is based on the fulfilment of objective at different stages of exploration i.e. determination of grade and quantity of mineral. Besides fulfilling these primary objectives, it should provide information needed for mine planning and mineral treatment technology with successive stages of exploration. The main problem which require earliest attention is the determination whether the deposit is monomineralic or poly metallic, establishment of its full outline and dimension across the strike for which reliable cross section of the ore body and host rock are required. Exploitation by mining in future requires detailed investigation not only of ore body but also of the host rocks which will be involved in the operation to some degree.

    The principle of Uniform reliability is based on the variability of the deposit. It is well known that character like width, grade etc. are variable do not behave in similar manner in all directions and therefore borehole openings (sample points) are to be planned at larger spacing in the less variable direction and at shorter spacing in the more variable direction.

    The principle of least outlay of time and means is a basic proposition for all industries. Means is divided between labour costs and materials. The time factor is most important for national economy as a whole, and it is sometimes expedient to accept greater outlay on labour and materials in order to save time on exploration of deposit.

Exploration stages

Exploration of a deposit is divided into three stages.

1. Preliminary stage

2. Detailed stage

3. Mine or exploitation stage

Preliminary stage: The objective of Preliminary stage exploration is to establish the general size, approximate shape and quality of ore body. At this stage detailed topographic survey is completed and accurate geological map (1: 5000 to 1: 10000 scale) is prepared. Exploratory openings are planned according to a definite system. The data obtained at this stage helps in selecting areas for subsequent detailed exploration in case of a very large deposit. Smaller deposits are usually completely explored at once in detail. Reserves estimated with the help of data obtained at this stage are classified as “Inferred Reserve”.

Detailed stage: The detailed exploration is undertaken only where a deposit is to be developed in near future. Objective of drilling in this stage is the estimation of total tonnage of ore and metal content, fixing the average tenor based on particular cut-off, the depth wise distribution/outlining the configuration of ore body for selecting mining methods, collection of samples for hydrometallurgical studies for mill parameters and detection of associated metals which could come out as by product etc. All these help the management to take an “investment decision”. Most vital in this stage is the confidence limits of such estimates. At this stage, the contour and attitude of each mineral body or ore lode is outlined with greater accuracy. The results are plotted on 1:500 to 1:1000 scale maps depending on the size and complexity of the deposit. To obtain reliable information at this stage, new exploration opening were driven, thus thickening the exploration grid, especially in those sections with more complex geology and those with richer concentration of mineral. Each new borehole is a separate sample point to reflect block grade from a relatively small volume of sample. The precision of estimation depends directly on the number of samples taken. In order to estimate the grade of a block with extreme precision it is necessary to sample intensively. Results of various sampling intensities can be analysed visa-a-viz the deviation from true mean. Intensive sampling can be prohibitively expensive even for the most valuable ores. So after examining the consequences of having “imperfect” knowledge of true block grades, one may be in a position to compromise i.e., to take only limited samples or drill minimum number of boreholes. Geostatistical analysis is applied at this stage.

        Detailed exploration data provided a much more exact estimate of mineral reserve in the category of “Indicated Reserve”. The results obtained at this stage are utilized to draw up technical plan for exploiting the deposit. After this stage, a deposit may be handed over for development.

Mine or Exploitation stage: This stage begins as soon as mining operations start and continues until the deposit is almost worked out. At this stage, very accurate information is obtained in respect of grade and width of the deposit from the networks of levels, cross – cuts, raises, winzes etc. The outline of the mineral deposit is delineated most precisely. At this stage of exploration, an underground geological map is made from mine surveys on scales of 1: 100 to 1:500 scale, revealing detailed features of the deposit. Reserve estimated from the data obtained at this stage is classified as “Proved Reserve”.

CONCLUSION:

    Planning of borehole is a continuous evolving process related to the stage of exploration for acquisition of data as per the allotted budget i.e., “cutting your coat according to cloth”. The exploration stages are sometimes clearly separated from each other, but they generally merge into a continuous process and the line between the preliminary and detailed exploration stage can not be easily drawn.

Read More

DRAINAGE PATTERNS AND TEXTURE

DRAINAGE PATTERNS

Drainage pattern is the general arrangement of channels in a drainage basin. Drainage patterns reflect the influence of such factors as initial slope, inequalities in rock hardness, structural controls, recent diastrophism, and recent geomorphic and geologic history of the drainage basin. Looking at them in the most general manner, one can classify drainage patterns into the following categories:

Dendritic or branch-like pattern- This is probably the most common drainage pattern. This is characterised by irregular branching of tributary streams in many directions and at almost any angle usually less than 90°. Dendritic patterns develop on rocks of uniform resistance and indicate a complete lack of structural control. This pattern is more likely to be found on nearly horizontal sedimentary rocks or on areas of massive igneous rocks. They may also be seen on complex metamorphosed rocks [Figure 2.1 (a)].

Trellised or lattice-like pattern- It displays a system of sub-parallel streams, usually along the strike of the rock formations or between parallel or nearly parallel topographic features recently deposited by wind or ice [Fig. 2.1 (b)].

Radial pattern- It is usually found on the flanks of domes or volcanoes and various other types of isolated conical and sub conical hills [Fig. 2.1 (c)].

Parallel drainage pattern- It is usually found in regions of pronounced slope or structural controls that lead to regular spacing of parallel or near parallel streams [Fig. 2.1 (d)].

Rectangular drainage pattern- It has the main stream and its tributaries displaying right-angled bends. This is common in areas where joints and faults intersect at right angle. The streams are thus adjusted to the underlying structure [Fig. 2.1 (e)].

Deranged drainage pattern- It indicates a complete lack of structural or bed rock control. Here the preglacial drainage has been affected by glaciation and new drainage has not had enough time to develop any significant degree of integration. It is marked by irregular stream courses that flow into and out of lakes and swamps and have only a few short tributaries [Fig. 2.1 (f)].

Centripetal pattern shown- This is encountered locally. Here the drainage lines converge into a central depression. These are found on sinkholes, craters and other basin like depressions [Fig. 2.1 (g)].

Highly violent pattern- It is characteristic of areas of complex geology [Fig. 2.1 (h)].

Drainage Texture

An important geomorphic concept about the drainage pattern is the drainage texture by which one means relative spacing of drainage lines. Drainage texture is commonly expressed as fine, medium or coarse. Climate affects the drainage texture both directly and indirectly. The amount and type of precipitation influence directly the quantity and character of runoff. In areas where the precipitation occurs primarily in the form of thunder showers, a larger percentage of rainfall will result in runoff immediately and hence, other factors remaining the same, there will be more surface drainage lines. The climate affects the drainage texture indirectly by its control on the amount and types of vegetation present which, in turn, influences the amount and rate of surface runoff. With similar conditions of lithology and geologic structure, semiarid regions have finer drainage structure than humid regions, even though major streams may be more widely spaced in semiarid than in humid regions. It is also noticed that drainage lines are more numerous over impermeable materials than over permeable areas. The initial relief also affects drainage structure; drainage lines develop in larger number upon an irregular surface than on the one that lacks conspicuous relief.

Bad-land topography promotes fine drainage structure. Impermeable clays and shales, sparse vegetation and existence of thundershowers are responsible for very fine drainage structure. Coarse drainage structure is in particular found on sand and gravel outwash plains. Gravel plains have fewer drainage lines on them than adjacent till plains underlain by relatively impermeable clay till.

Reference- RIVER MORPHOLOGY by R. J. Garde; pp. 12-14.

Read More

SEDIMENTOLOGY AND SEDIMENTARY ROCKS

Fig: Dolomites of Cuddapah Super Group, Cuddapah Basin India. Image courtesy Mr. Atanu Mukherjee.

Sediments and sedimentary rocks are formed from the breakdown products of pre-existing rocks and by chemical and biochemical precipitation. This results in two major sources for sedimentary rocks.

1. Particles (rock fragments, mineral grains) carried to the site of deposition by gravity, water, wind or ice. The term detritus is applied to solid products of weathering which have been removed from their site of origin. Particles produced by mechanical fragmentation of pre-existing minerals or rocks (including organic remains) are called clasts (such deposits are said to have clasts texture). Pyroclastic particles are those exploded from volcanoes. Bioclastic particles are broken up skeletal material.

2. The chemical composition of the solution (river, lake, ocean, groundwater) where the sediment is deposited or formed. The composition of the fluid (interstitial solution) surrounding the particles after deposition is important (together with temperature and pressure) in precipitating cements and altering the sediment.

Sedimentary rocks can be divided into three groups: A. Detrital sediments:                   clay and silt grade - argillaceous                   sand grade - arenaceous                   pebble grade - rudaceous B. Chemical and biogenic sediments: e.g. calcareous, dolomitic, siliceous, phosphatic, ferruginous, manganiferous, carbonaceous, saline, glauconitic. C. Residual deposits (remaining at the site of weathering): e.g. laterite, bauxite, silcrete.

The nature of material in a sediment can tell us about the source of the sediment and the type and degree of transport (in the case of detrital sediments) and of conditions in the depositional environment.

      After deposition, sediments are subjected to changes (diagenesis) leading to lithification or consolidation into sedimentary rocks and it is important to unravel the changes they have undergone. Many features of the original sediment, however, are preserved and we can still study the history and origin of the sediment.

In the two practicals on sedimentary rocks you will take this semester you will examine lithified sedimentary rocks from the following four groups : clastic rudites (conglomerates); clastic arenites (sandstones); clastic argillites (mudstones) and various carbonates (fossiliferous and non-fossiliferous limestones). You will learn how to classify and name these rocks and how interpret their features that tell us about their sedimentary history.

EXAMPLES OF SEDIMENTARY ROCKS AND THE DEPOSITIONAL ENVIRONMENTS IN WHICH THEY FORM

BRECCIA	CONGLOM- ERATE	ARKOSE	DIRTY SANDSTONE	CLEAN SANDSTONE	SILT-SHALE	LSTONE'
Alluvial Fan River Channel (head waters) Volcanic	Alluvial Fan River Channel (head waters) Glacial (ice melt)	Alluvial Fan River Channel _____________ LITHIC SANDSTONE _____________ Delta Complex Alluvial Plain etc. _____________ GREYWACKE _____________ Delta Front Submarine Fan (turbidity currents)	River Channel Delta Complex Alluvial Plain Lagoon Delta Front Continental Shelf	Beach Strand Dunes	Shelf Tidal Flat Lagoon Delta Front Swamp Basin Lake	Tidal Flat Lagoon Carbonate barrier reef Shelf Basin Beach

Naming and Classifying Sedimentary rocks

Sedimentary rocks are named and classified on the basis of their: dominant grain size, grain composition, grain size distribution (sorting), and texture. The term TEXTURE refers to a combination of the grain size, shape, sorting and fabric.

Grain size in detrital sediments

Grain size is the size of the grains present in the rock and depends on the size of the parent materials which were eroded and then transported to a site of deposition. The transport of these grains in turn depended on the energy available for transport - the more energetic the transport conditions the larger the grains transported.

To some extent the grain size of a sediment becomes finer in the direction of transport and with greater reworking and abrasion. However, caution must be exercised in applying this generalisation. For example, if the source rocks are already fine grained, as in a fine-grained sandstone, coarser grained material will not result when these are fully broken down during weathering and the early stages of transport.

Grain size is generally measured in terms of the Wentworth Scale, a geometric scale with a constant ratio of two between successive classes (See Fig.1). Grain size can be measured by sieving or settling through a column of water.

Particle Size Range	phi unit	Sediment grade Name	Rock Name(Visually Discernible)
4096 2048 1024 512 256 mm 128 mm 64 mm 32 mm 16 mm 8 mm 4 mm 2 mm 1 mm 1/2 mm 1/4 mm 1/8 mm 1/16 mm 31 microns 16 microns 8 microns 4 microns less than 4 microns	-12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8	Very Large Boulders Very Large Boulders Medium Boulders Small Boulders Very Large Cobbles Very Large Cobbles Very Coarse Pebbles Coarse Pebbles Medium Pebbles Fine Pebbles Very Fine Pebbles Very Coarse Sand Coarse Sand Medium Sand Fine Sand Very Fine Sand Coarse Silt Medium Silt Fine Silt Very Fine Silt Clay	Conglomerate if the fragments are round in shape Or Breccia if the fragments are angular Sandstone Mudstone

Note the use of a geometric size scale having a constant ratio of 2 between successive sieve aperture sizes, i.e. the particle diameter which is assigned to each of the sample (or "grades") is twice that of the next smaller grade.

Grainsize Nomenclature for Granular Sediments

Prepare a grainsize comparator using the materials provided. Examine your grainsize grades and thereby become familiar with the Wentworth Scale.

Note that most silt, and clay size material cannot be resolved by the naked eye.

Sorting (Grain Size Distribution)

Most fragmental deposits of sediment are comprised of material which displays a range of grain sizes. Sedimentary deposits whose grains are of an approximately uniform size are formed under special conditions and are said to be well sorted, for example, a clean (sand and mud free) beach gravel whose grains are all the same size (say 5 + 1 cm in diameter). More commonly, however, sediments are a mixture of two or more of the four grainsize grades (gravel, sand, silt, & clay). Depending on the degree to which these grades are mixed we term the sediment sample to be :

I) well sorted - a very uniform grainsize distribution with a very distinct mode and little variation about that mode, (such a distribution has a very narrow and tall "bell curve" or histogram). Samples which are well sorted are discerned visually with great ease.

II) moderately sorted - a more varied grainsize distribution with a definite mode but quite a deal of variation about that mode, (such a distribution has a fairly broad but definitely peaked "bell curve" or histogram). Samples which are moderately sorted are discernible visually as they possess an obvious mode but you may have to take care in detecting the mode.

III) poorly sorted - a varied grainsize distribution with no obvious mode

IV) unsorted - an extreme case in which all the size grades of sediment are discernibly represented ie. a gravelly-sandy-muddy sediment (note that clay and silt cannot be discriminated from each other by eye). 

Grain shape

Grain shape refers to the geometric form of grains. Note the important differences between the following:

I) Roundness - see the roundness grades illustrated

II) Sphericity - only grains of two very different states of sphericity are shown on the roundness chart (Figure 3). Obviously wide ranges of sphericity exist in particles. It is largely inherited and does affect the hydrodynamic behaviour of a particle, including its settling velocity.

III) Form - this differs from sphericity. For descriptive purposes, use the term sin Fig.4.

Platy (disc); bladed; elongate (rod); equant (spheres).

Grain shape partly depends on internal anisotropism of the material comprising the grain (e.g. cleavage of a mineral, schistosity of a rock) and the original shape of the particles. This can be modified by solution and abrasion. Roundness results from abrasion and breaking away of projecting areas of grains. Abrasion is a function of the degree of transport and the energy of the transporting medium. Hardness and size also affect the roundness. 

Grain Fabric refers to the orientation of individual grains and their packing with each other.

Diagrams Used For Naming Grain Shapes and Grain Surface Texture (Roundness)

Porosity and Permeability

These two properties are the most important physical factors involved in measuring the reservoir potential of a sandstone or limestone.

Porosity is the volume of pore space within the rock, and is expressed as a percentage of the total volume of rock mass. The total volume of pore spaces ultimately controls the maximum amount of hydrocarbons and/or water that can be stored in the rock.

Permeability is a measure of the resistance offered by the rock to the movement of fluids through it, and it reflects, to great extent, the degree of interconnection of the pore spaces. The following table is a guide to typical porosity and permeability values of sandstones:

Porosity Permeability

(f) (millidarcies) Qualitative estimates

5-10% <1 md Poor, not visible with hand lens

11-15% 1-10 md Fair, visible with hand lens

16-20% 11-100 md Good, readily visible

21-25% 101-1000 md Very good

> 1000 md Excellent, open framework, weakly lithified

Commercial oil and gas sands exhibit permeabilities ranging from a few millidarcies to several thousand millidarcies. In combination, the distributions of porosity and permeability in a reservoir exert very important controls on the amount and delivery of hydrocarbons.

A. CONGLOMERATES (rudites)

Rudaceous deposits comprise a heterogeneous group of sediments and rocks with over 25% of their particles (by volume) being greater than 2mm in diameter. Nomenclature and description of rudites is based on texture, composition, and source.

Texture: This gives clues as to the possible depositional processes and environment.

i) Orthoconglomerate: Clasts (gravel fragments) are in contact and are "self-supporting", that is the adjacent clasts are in point-to-point contact and hold each other up.

ii) Individual Clasts are separated by matrix (sand or mud), that is the gravel component appears to float in the matrix. If matrix is sand, the rock is a PARACONGLOMERATE; if matrix is mud, the rock is a DIAMICTITE. (Mud Matrix)

Composition: The clasts can be composed of either only one rock type (OLIGOMICTIC) or of many types (POLYMICTIC). Oligomictic conglomerates are generally the product of tectonically stable areas where extensive reworking removes all but the ore chemically stable silica clasts. Polymictic conglomerates are generally the product of aggradation in tectonically active source areas with greater relief.

Special names exist for some conglomerates:

Intraformational conglomerates are those composed of silt derived from penecontemporaneous sediment within the depositional basin. They are common in environments such as river beds and tidal mud flats. These pebble clasts are usually composed of soft but cohesive mud when deposited and are also called "intraclast", "mud-chip" or "shale-flake" conglomerates.

Tillite is a diamictite of glacial origin (a lithified till).

Breccia is restricted to rudites which contain very angular fragments of locally derived rocks.

Agglomerate is angular pyroclastic material (= volcanic breccia).

Poorly sorted conglomerates/sandstone/mudstone mixture are called pebbly sandstones or pebbly mudstones.

B. SANDSTONES (Arenites and Wackes)

The classification in widespread usage (see figure below) , based on Dott, 1966) uses quartz (+ chert) at one apex, feldspars at a second, and "unstable" or "labile" lithic (rock) fragments at the third apex of a triangular diagram. The percentage of matrix present is also important; Matrix is <0.03mm in size.

Classification of the Sandstones, according to Folk (1980), note the position of quartz as the "ultimate" sedimentary mineral at the apex of the triangular diagram.

Chemical and Textural Maturity of Sandstones

Throughout the weathering of mineralogically-complex source area and transportation and diagenesis, relatively unstable minerals are destroyed (dissolved or altered to more stable minerals) and stable minerals thus increase proportionately. Quartz is the most abundant chemically and physically stable mineral. Feldspars and rock fragments are relatively unstable.

An index of chemical maturity is the ratio of quartz / (feldspar + rock fragments). As sediments are reworked, perhaps through two or more cycles of weathering, erosion and deposition, they thus tend to mature to pure quartz sands.

In contrast the textural maturity of a sediment is measured by its sorting and matrix content, and it reflects the processes of deposition. A measure of textural maturity is incorporated in the classification scheme using an axis with matrix (silt and clay, <0.03mm) percentage - an arbitrary value 15% has been proposed to divide the texturally mature sandstones (ARENITES) from the texturally immature sandstones (WACKES) (see the Figure above).

In field practice a more useful distinction between arenites and wackes similar to that made for orthoconglomerates and paraconglomerates is used. Instead of pondering the precise proportion of matrix the sample is examined and a decision as to whether or not the sand grains are in point-to-point contact is made.

If the sand grains are in contact and supporting each other we will call these rocks arenites.

If the sand grains appear to "float" in a muddy matrix then we will call these rocks WACKES.

If you are still uncertain about this characteristic then call the rock by the generic term sandstone.

The term "GREYWACKE" is a field term meaning a 'dirty' (poorly sorted), grey (lithic fragments and layer-lattice silicates) sandstone.

 Arkoses

An Arkose (arkosic sandstone) is an arenite with between 25-60% feldspar, little matrix and the rest of the particles are quartz. An arkosic wacke is a wacke with 20-50% feldspar, more than 15% matrix and the rest of the particles comprised of quartz.

C. MUDROCKS (lutites)

Rocks containing more than 75% mud size particles are known as the mudrocks. MUDSTONE is the general name used (equivalent to "lutite", "pelite" and "argillite"). These rocks may be divided into SILTSTONE and CLAYSTONE based on texture. If the rock is fissile it is called SHALE. Because of their fine grain-size it is difficult to identify the minerals with a hand-lens.

Quartz and mica are sometimes visible and the colour (black: carbonaceous or pyritic, red: ferruginous), hardness (very hard: siliceous), and a test with dilute HC (calcareous) may help. If the rock expands when wet it is rich in clay minerals (they also adhere to the tongue when the dry rock is licked). Chlorite often gives the rock a greenish tinge.

D. LIMESTONES

Calcite (hexagonal, CaCO3) is the stable polymorph and principal mineral in LIMESTONES. Aragonite, the orthorhombic polymorph of CaCO3, is precipitated inorganically and by some organisms but is metastable and alters to calcite with time. Dolomite [CaMg (CO3) 2] is precipitated inorganically and is more abundant in Early Palaeozoic and Precambrian rocks and forms the rock DOLOMITE (or DOLOSTONE).

Limestones often have non-carbonate components such as terrigenous mud and sand particles. Non-calcareous organic matter (opaline silica, phosphate, petroleum) is also common. Carbonate rocks are susceptible to diagenetic alteration through solution, precipitation and recrystallisation of both the carbonate and any silica present. The silica often precipitates as cryptocrystalline quartz to form charateristic chert nodules ("flint").

Grainsize is of only limited use in these rocks because many of the particles are created in situ and not transported to the site of deposition. Despite this the terms CALCIRUDITE, CALCARENITE and CALCILUTITE are used to indicate the dominant size of the carbonate particles in limestones particularly if these particles have been transported. Limestone composed only of shells is often called coquina.

Read More