Introduction

• 70% of rocks on the earth surface– Limestone, Sandstones, Shale, Salt deposits, Ironstones, Coal

• The study of modern environments, their sediments and processes contributes to the understanding of their ancient equivalents

• Once sediments are deposited, they are subjected to the processes of diagenesis, that is physical, chemical, and biological processes which brings about compaction, cementation, recrystalization and other modifications to the original sediments and form rocks

• Environments and processes of deposition and palaeogeography can all be deduced from study of sedimentary rocks.

• Sedimentary rocks are formed from the accumulation of broken down materials

• Weathering and erosion of a rock can produced different particles (which can vary in size)

• Chemical weathering – CO, CO2 NO2 H2S CH4

dissolve in moisture to produce mineral acid• Products of chemical weathering could be

particulate (coarse-very fine/colloidal) or neoformed/autigenic (one that was generated where it is found )

Transportation of sediments

• Finer sediments – Solution• Small Particles – Suspension/saltation• Coarse – Bedload

Terrigenous Materials – Land derived sedimentHydrogeous Materials – Materials derived from

water bodiesBiogenous – Materials derived from organisms, or

reminants of organisms

Classification of sedimentary rocks• Formed through physical, chemical or

biological processes• Detritus – Anything derived from weathering

of rocks• Can be grouped into 4 broad categories (by

virtue of their originSiliciclastic Sediments

Biogenic, Biochemical and organic sediments

Chemical sediment

Volcanistic sediments

Conglomerates, sandstones, mudrocks

Limestones, cherts, phosphates, coal

Evaporites, ironstones

Ignimbrites, tuffs

Scheme for identification and description of sedimentary rocks

• Color• Texture• Composition• Sedimentary structures• Fossils• Interpretation• Others • ………………………see photocopied paper

Classification of sedimentary rocks cont.

• Facies – A body or packet of sedimentary rocks that distinguish it from different facies. Facies is product of deposition….what are these factors? The most important factors that control and affect sediments deposited– Depositional processes and environments– Tectonic context – determines the depositional setting– Climate – weathering and erosion – Sediment supply– Sea level

Siliciclastic Sediments

• Made up of particles ranging from fine grained mudrocks to coarser grained conglomerates

• The sediments are made up of clasts derived from pre-existing igneous, metamorphic, and sedimentary rocks. The clastic grains are released through weathering processes, and transported to the depositional site by variety of mechanisms

• (wind, glacier, river currents, waves, tidal currents, debris flows, & turbidity currents)

• Sandstones contain rock fragments, mostly quartz and feldspar, abraded to various degrees.

• Composition of siliciclastic sediments is a reflection of weathering processes determined largely by the climate geology and the source area (the sediment provenance)

• Source areas are generally uplands, mountainous regions undergoing uplift, but detritus also may be supplied from erosion in lowlands and coastal areas.

• Sediment composition (and roundness) is also affected by distance of sediment transport and by diagenetic processes.

• Two important features of siliciclastic sediments is their sedimentary texture and sedimentary structure

Sedimentary Texture

• Study of sedimentary texture condisers– Grain size, and grain size parameters– Grain morphorlogy– Sediment fabric

• Sediments can be considered on the basis of their textural maturity – which is a reflection, largely of the depositional processes

Grain size and grain size parameters

• Grain size is the basic descriptive element of all sedimentary

• The most widely accepted scale for measuring grain size is the Udden-Wentworth scale.

• ………Draw the wentworth scale, and read about it.

• Units - milimeters• Phi = -log2d, where d is the diameter of the

grains (for mathematical calculations)• Scheme for classifying sand-gravel-mud

mixture

• When studying sedimentary rocks in the field, first approximation of grain size can be made with hand lens, conglomerates, breccias can be measured by tape

• For accurate grain size analysis, lab methods can be used – examples.

• Once grain size distribution has been established, sediments can be characterised by several parameters– Sorting (later)– Skewness (find the meaning)

• Trends in grain size over large areas can be used to infer the direction of sediment dispersal, with grain size decreasing away from source area. In other words, sediment size decrease mostly relates to selective sediment transport, rather than abrasion

• Sediments close to the source would be angular, and subsequently becomes rounded (due to erosion)

• Shape and size of gives information or helps us deduce some attributes of the rock– Sorting – Define – Permeability– Porosity

Pebbles 30 5 50

Sand 50 85 35

Clay/silt 20 10 15

Poorly sorted

Well sorted

Poorly sorted

Diagenesis• diagenesis is the changes in

composition, texture, and other physical properties that occur after sediments have been deposited in a basin, until the time it has been examined.

• Compaction, recrystallization and cementation are the major diagenetic process that converts sediments to sedimentary rocks. This process in consequence alters the original textural characteristics, this depends on a large number of other factors, most importantly depth of burial (km), and temperature (oC).

Q – WHAT IS THE EFFECT OF DIAGENESIS ON POROSITY

2mm 1mm 0.5mm 0.25mm 0.125mm

VC MC FS VF

• sorting is a function of grain size and its distribution. It shows the effectiveness of a depositional medium by separating the grains to different classes. The theory is by depositing certain sizes preferentially (Prothero and Schwab 1996) therefore just as grain size, it improves along the sediment transport path. Sediment provenance also affects sorting in that grains and grain size sourced from different environments varies greatly in the way they weather, gets transported and subsequent decomposition (e.g. igneous vs. sedimentary environments) (Tucker 2001).

• Porosity is a measure of the amount of internal space in a rock (Fig. 2.3) that is capable of storing hydrocarbon, gases and water. Alternatively it could be defined as the total volume of space within a rock that is not occupied by grains and matrix material (Martey 2006). Quantitatively, porosity is defined as the ratio of the volume of void to the volume of the rock matrix (Selley 2006).

• Porosity (%) = Volume of Voids X 100

Total Volume of Rock

• Many reservoirs have porosity values ranging from 5% – 25%. Porosity values between 15% - 25% are usually accepted as excellent porosity.

• Permeability of a porous substance is the ability of the substance to permit fluid flow through the interconnected pore space. It is measured in Darcy. Since most reservoir rocks have permeability less than 1, it is usually expressed as millidarcy (mD).

• permeability is affected by grain size, grain shape, sorting, and grain surface texture. It is also controlled by non depositional feature such as fractures (Martey 2006).

Describe in terms of porosity

• Particles of similar shape

• Particles of different shapes

• Highly varied composition Quartz, feldspar, lithics

• The flow of water/fluid

Grain Morphology

• Grain shape• Sphericty and roundness• Dependent on– Nature of source & Initial minerology– Degree of weathering –abrasion– Degree of dissolution during diagenesis

Sediment Fabric

• Refers to orientation and packing• Orientation is a function of interaction with

the flowing medium unless it has been tectonically deformed. Therefore, can be used as paleocurrent indicator

• Packing affects porosity and permeability – small grains infill the pores

Textural maturity

• Sediments with poor sorting, and angular grains – immature

• Good sorting subrounded-rounded – mature• Porosity increases with increased maturity• Diagenesis affects maturity – compaction• Assignment – Read about diagenesis,

cementation, compaction

Sedimentary structures

• Stratification – caused by successive deposition

• Bedding Plane – Bed is the fundamental unit of a geologic formation.

Stratification plane

• A bed is recognised in cases where the deposits are different

• A bed will have uniform characteristics (texture, fossils, color, composition)

• Will have upper and lower bedding planes – often parallel to the earth surface

• Bedding and lamination

Types

• Simple beds- deposition are parallel to each other. Common in gentle current with laminar flow

• Cross beds – Occurs as a result of turbulent flow, therefore making the stratification plane inclined to the earth surface

• Lenticular beds – thickness at some point is different from thickness at another

• On the basis of thickness of a unit of deposition, there are some terms used– Lamination – less than 1cm– Parting – less than 1 mm– Thin lamination 1mm – 3mm– Thick lamination 3mm-10mm– Bed – greater than 1cm– Thin bed – 3cm– Thick bed – 3cm – 10cm– Very thick bed – 10cm – 1m– Massive – More than 1m (rocks without stratification planes)

Silicates – Micas, and clay minerals

Neosilicate

Sorosilicate

Chain silicate

Phyllosilicates/ sheet silicate

• Sheet silicates– Micas

• Muscovite• Biotite

– Clays• Kaolinite• Smectite – Swelling Clays• Illite

• Find the chemical formula of each of the clay minerals & their minerology

• Heavy Minerals– Zircon, Tourmaline, Topaz, Hematite, Magnetite,

Rutile, Ilmenite, Feldspar – IGNEOUS– Silimanite, Kyanite, Garnet, Epidote, Chlorite,

Stanrolite – METAMORPHIC– Calcite, Aragonite, Geothite, Quartz, Anatase –

SEDIMENTARY• Clay minerals– Kaolinite, Montmorillonite (smectite), illite,

Vermiculite, Palygorskile

Biochemical & Organic sediments

• Biological and biochemical processes are dominant in the formation of carbonate sediments, although inorganic precipitation of CaCO3. Once these sediments have been deposited, diagenesis can modify the carbonate sediment.

• Limestones occur in every geological period from Cambrian onwards – and they depict evolution and extinction of invertibrate with carbonate skeleton

• Correlates with sea level

• These are accumulation of remains of organisms (plant & animals)

• Tissues are dominantly organic materials made up of H, C, N, Fe etc

• Tissues can be lost from the surface. When rapidly buried, they will be preserved and form with the sediment

• Skeletons are diverse. They can be siliceous, Phosphorous, Carbonate, Calcium. When these organisms die, they become trapped and buried, thereby, becoming part of the sediments – fossiliferous

• Most organic matter impact dark coloration on sediments, making the clays black – called dark shales

• Skeletons is made up of abundant silicon – either the shell or the internal organs

• High silicon – Diatomaceous• High Phosphorus – Phosphatic• High CaCO3 – Limestone• Feaces of Fish – Coprolite

Trace fossils

• When organisms burrow into sediments, they– Feed on the sediment– Rest on the sediment– Move on the sediment– Inhabit the sediment

• Depending on these activities, they can leave imprints on the sedments. These could be track marks or burrows

• When trees are buried, dehydrogenation process takes place. The process starts from – Peat (some features are still present)– Lignite (Features are wearing out)– Sub bituminous– Anthracite

• Typical in swamp areas

Limestones

• Essentially carbonates • Originates from lime mud, or accumulation of

organic remains• If it is from lime mud, it is micritic• From organic remains it is bioclastic• Mud and organic remains could occur

together and it is called a biomicrite

Lime mud

• Fine grained carbonate – micrite (microcrystaline calcite) grain size less than 4 microns

• Formed from – Disintegration from green algae (release of

aragonite)– Bioerosion – when organisms such as sponges attack

carbonate grains and substrates– Mechanical breakdown of skeletal grains through

waves and currents

Classification of limestone