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.