SEDIMENTOLOGY

 

 

 

 

 

 

CHAPTER EIGHT

CHAPTER SUMMARY

 

 

I. Sedimentology

A. Characteristics of sediments

1. Texture

2. Color

3. Mineral Composition

4. Depositional Structures

B. Classification of sediments

1. Detrital sediments

a. Rudaceous

b. Arenaceous

c. Argillaceous

2. Precipitates and bio-chemical rocks

3. Biological sediments and fuels

I. SEDIMENTOLOGY

A. Characteristics of Sediments

From weathering to mass movement the next logical step is erosion and transportation of the debris via rivers, glaciers, and wind. This erosional stage gives raise to the sediment, which hardens into the sedimentary rocks: products of the erosion-transportation-depositional cycle. Sedimentary rock formations make-up about 75 percent of the rocks occurring at the surface of the earth. We can most easily classify the sediments and their lithified products [sedimentary rocks] by essentially recalling what happens to the igneous rock when it weathers. These produce essentially resistant minerals e.g. quartz survives mainly unchanged and is easily incorporated in sediments. This can be termed an inherited mineral. Non-inherited minerals are new minerals. These include clays, amorphous aluminum, and the aluminum and iron oxides amongst others. In addition, the weathered material taken into solution may be, and usually is, deposited somewhere by precipitation. Finally, dead organic matter may pile up on a weathered surface (i.e. on and in the soil) and this will be transported along with the normal mineral products by the erosional and transporting agent and deposited along with the sediment e.g. coal, peat and petroleum are essentially formed in this way. The products of weathering thus are sorted into fairly distinctive groups. This is termed sedimentary differentiation. The initial sedimentary differentiation provides us with sediments of two types. Detrital sediments are made up of the accumulation of fragments or clastic particles of minerals or rocks. We conventionally divide the detrital or clastic rocks according to size classes of their modal grains. The pebble sized grains form the Rudaceous detrital sediments. The sand sized particles form the arenaceous detrital sediments. The clay sized particles form the Argillaceous detrital sediments. The precipitation of material from solution or by organic processes give the chemical and organic sediments, represented mainly by limestones, evaporites, and coals.

The lithification process changes sediments into sedimentary rocks. Lithification may involve cementation by the precipitation of material as cement [particularly iron, quartz, and the carbonates calcite and dolomite]. This cement is precipitated around and between the mineral grains and precipitates from the percolating ground water. Compaction takes place by the squeezing out of the water [desiccation].

The two processes of cementation and compaction cause the sediment to lithify. This process of lithification is regarded as the first stage in diagenesis.

In order to be able to determine the depositional environment of a sedimentary rock it is often necessary to examine a large number of rock characteristics. One basic idea behind classifying sedimentary rocks is to provide an easy descriptive method of linking a knowledge of depositional environment and sedimentary process with the name of the rock. Characteristics of important in describing sedimentary rocks from the viewpoint of interpreting the depositional environment include the following.

Texture of the sediments

The textures of the detrital sediments are determined mainly by the size, shape and arrangement of the particles. Size may vary from a boulder down to a very fine rock flour. The shape of the particles can be generalized as angular, subangular, or rounded. The degree of sorting of particles is controlled mainly by the way in which the sediment was transported and deposited. The well sorted sediments are those in which the particles are all of roughly the same size. They are formed where the wind, waves, or current have time to work over the material and to separate finer and coarser material. The poorly sorted sediments in which the fine and coarse material is jumbled together, result from rapid deposition, from deposition by turbidity currents and from deposition by glaciers. Phanoclastic rocks are equigranular and Phenoclastic rocks are not equigranular.

The ways in which the constituent mineral grains fit together is termed the fabric of the rock. In some rocks the grains seem to be arranged quite haphazardly; in others we can see a definite pattern about the arrangement of their longer and shorter axes. This is termed preferred orientation e.g. pebbles transported by strong streams may come to rest with their longest axes pointing in the direction of the stream flow.

The textures of the chemico-organic rocks are very complicated. The chemical rocks are frequently strongly altered by recrystallization during diagenesis and the rock becomes a crystalline mass (in many ways similar to an igneous rock). This recrystallization takes place at normal temperatures and pressures existing at the earth's surface.

Two further important properties of sedimentary rocks that are related to particles size, shape and sorting are porosity and permeability. Porosity is the ability of a material to hold fluids. Permeability is the ability of a material to pass fluids. Porosity and permeability of surface rocks control the movement and storage of liquids in the earth's crust, particularly ground-water and oil. Porosity is really the ratio of the spaces (voids) not occupied by solid rock, to the bulk volume of the rock. The average porosity in rocks is probably around 10% Permeability depends on the size, shape and packing of the particles to a very high degree. The void spaces may be so minute that no continuity of voids exist and water cannot pass through. Poor sorting, cementation and recrystallization all can reduce permeability to almost nil.

Color of the sediments

The color of a sedimentary rock is an immediate characteristic. The color is dependent upon the particles and cement. It also may be a reflection of a coating of some material over the particles. Another color is quantified using a special method [ the Muntzel Scale] two colors are particularly important at a very general level. The blackness of a sedimentary rock is mainly due to the presence of organic matter. Black shales are formed only if organic material is preserved and for this to be so the organic material must not have decayed (when the organic particles are buried rapidly so that decay does not set in ). Red sedimentary rocks on the other hand are usually due to the presence of the iron oxide mineral hematite and they are indicative of an oxidizing province and environment.

Mineral composition of sediments

The mineral composition of the sedimentary rocks can indicate the kind of rock that was being eroded and thus provide information on the area which was the source of the sediments [the provenance]. The weathering of granite gives typically quartz and the clay minerals. On the other hand the weathering of metamorphic rock also gives minerals produced by metamorphism e.g. Corundum.

Depositional structures of sediments

The depositional structure of a sedimentary rock is determined by the arrangement of particles on a large scale. The normal sequence of sedimentary rocks has the various layers separated by stratification of bedding planes. Each layer is known as a stratum or bed. A bed normally represents the product of a single act of sedimentation. No two beds are ever totally identical but a succession of beds may all have a common general component and are then grouped under the term formation. Bedding is of four main types:

1. Regular bedding which is simply shown by a sequence of parallel sided beds separated by bedding planes. This regularity may be emphasized by the development of parallel laminations within the bed as a whole, these are due to slight-to-abrupt but minor changes in composition. A diastema is a minor break in the depositional sequence and may be a feature of a regular bedding pattern e.g. as on tidal flats.

2. Cross bedding which is more complicated and produced when layers of sediments are deposited on an inclined surface such as the frontal slope of a small delta or the lee slope of a sand dune. Slight erosion by the current on the upper surface carries away their thin upper parts leaving a surface of erosion truncating the inclined laminations that are developed.

3. Graded bedding which this is characterized by a regular change in grain size within a single bed from coarse at the bottom to fine at the top. The settling of particles through deep water, or the drying-up of stream flow as when varves are laid down in lakes fed by streams which freeze up in winter are two common causes of graded bedding.

4. Slump bedding which is produced soon after the sediment has been deposited by the sliding or slumping of layers, of as yet unconsolidated material, down a slope. During this process the original bedding is crumpled and layers of different kind come together.

Additional sedimentary features that may be observed in the rocks which tell us something about the nature of the original environment include the following.

Depositional features

1. Scour and fill

2. Ripple marks

3. Mud cracks

4. Rain cracks

5. Fossils

Post depositional features

1. burrows

2. concretions

3. geodes

CLASSIFICATION OF SEDIMENTS

The major types of sedimentary rocks are Argillaceous clays, which form 45 percent of sediments; Arenaceous quartz sandstones, which form 32 percent; and, the Precipitated and biological calcareous limestones, which form 22 percent. All of the other sediments together only form about 1 percent of sedimentary rocks.

A rock or sediment normally has three main components: particles, cement, matrix. The matrix is the very fine grained detritus that lies between the obvious particles. Matrix may actually act as the binder between grains instead of cement.

The Detrital Sediments

A. Rudaceous rocks. There are two major types of Rudaceous rocks: conglomerates (large rounded particles) and breccias (large angular particles).

B. Arenaceous rocks. There are four types of sandstones: graywacke, subgraywacke,

arkose, and orthoquartzite.

GRAYWACKE: These have a high content of matrix, particularly of clay minerals, chlorite, and sericite (new minerals). The arenaceous particles consist of quartz, rock fragments, feldspars and a few minor constituents. The heterogenous nature and poor sorting indicates instability of both the source and depositional environment. Beds of graywacke usually vary from a few inches to a few feet in thickness and normally do not show any laminations. However, one may get thousands of feet of such beds in a formation. Graded bedding is common. Because we know that graded bedding is only readily achieved by settling out from a liquid, we believe that turbidity currents are largely responsible for graywacke. The fact that graywackes is associated with radiolarian cherts, submarine volcanic deposits, and other rocks (shales) containing deep water or bathyal faunas suggests that they were deposited i an unstable environment i.e. at the foot of the continental slope.

SUB-GRAYWACKE: These are the commonest of the sandstones - more than one third of all sandstones. They have a high content of quartz and chert and contain more rock fragments than feldspar fragments. Feldspar makes up 0.10 percent. The fine grained detrital material is less than 15 percent and the voids are filled chiefly with cement, although there is some clay grade matrix. Sub-graywackes are better sorted than graywacke and the particles are fairly well rounded. The sediments are normally well stratified and show cross bedding and ripple marks of water origin. The sub-graywacke is commonly found associated with coal beds in the sub-areal and subaqueous part of a delta and in the associated marine environment.

ARKOSE: These are the sandstones containing 15 percent or less of detrital matrix. Feldspar is greater than rock fragments in particles. Quartz and feldspar are the main minerals and kaolinite and mica are also important. Arkoses are generally well sorted and the particles vary in roundness. The beds generally occur i either thin sheet-like units overlying a granitic terrain, or as a wedge-like unit deposited in basins adjacent to a granitic mass.

ORTHOQUARTZITE: These have a very high quartz content and therefore are often called pure quartz sandstones. There is little or no clay matrix and they may or may not have cement. Associated with the quartz are small amounts of other stable minerals such as zircon and tourmaline. The particles are normally well sorted and well rounded. Cross bedding and ripple marks are particularly common in these rocks. They are often associated with or grade into, limestones and dolomites. They occur most often as blanket deposits only a few hundreds of feet thick but of wide areal extent. The indications are that they were deposited in a relatively stable environment, and there was either time for weathering and transportation to take out all of the unstable minerals and clays and leave only the stable residues, or alternatively these may be sandstones that have been derived from earlier sandstones (second generation sandstones).

C. Argillaceous rocks. These are formed from the fine grained quartz particles and clay minerals. They are classified according to content of organic material or content of precipitate material (calcium, iron etc.).

The Precipitated and Bio-precipitated rocks

A. Precipitated rocks. The typical chemical type of sediment is an evaporite, although precipitate deposits also include the biochemical rocks. The principal precipitates are mono-mineralic rocks, and are composed either of calcite (CaCO3), dolomite (CaMg (C03)2), gypsum (CaS04.2H20), halite (NaCl), or chert (Si02). Evaporites are normally formed in salt lakes (playas). A salt lake is the result of interior drainage and the absence of, or limited outflow. Excess evaporation causes the precipitation of salts from saturated solution in the reverse order of their solubilities. The order of precipitation is:

1. Carbonates Calcite (CaCO3)

Sodium carbonate (NaCO3)

2. Sulphates Anhydrite (CaSO4)

Gypsum (CaSO4.2H2O)

Epsomite (MgSO4)

3. Chlorides Halite (NaCl)

KCl

MaCl2

CaCl2

4. Borates Na2B4O7.10H2O

In addition one gets dripstone and travertine in caves which are evaporite rocks.

The non-evaporites. These include the biochemical rocks and non-evaporite chemical precipitates. The essential rocks formed are the carbonate rocks limestone (CaCO3) and dolomite (MgCO3CaCO3). There are two principle types of sedimentary rocks. The biostromes are simply layers or beds of calcareous shells which form shell limestones; and biohermes [reefs] which are mounds of colonial organisms having calcareous shells. The coral reef is the most typical example.

The bio-clastic rocks. These are the rocks formed by the breakup of shelly material and therefore are detrital.

The biological sediments and fuels

A. The coaly-series of rocks.

The coals are the bedded carbonaceous debris. The effects of chemical and bacterial action at the earth's surface, followed by burial diagenesis convert the layer of loose vegetable matter into a compact coal seam. There are various recognizable stages in the conversion from plant to coal and each is a different looking rock. Simply this is from peat to lignite to bituminous coal to anthracite. This is the coal series. The change of rank from peat to anthracite is marked by an increase in density, hardness, and carbon content of the rock and also by a decrease in content of moisture and volatile substances. In general, the higher the rank of the coal in the series, the greater its value as a fuel.

In addition to the coal series there is the bituminous series (composed of hydrocarbons of which paraffins, napthenes, and aromatics are the chief ones). These vary in form from the liquid oils to the solid substances such as asphalts, and they are the parent materials from which we get petroleum. Bitumens are produced by bacterial and thermal decay from original organic material entrapped in the sediments. Most plant tissues decompose under atmospheric conditions because they are mainly formed of cellulose which is oxidized.

Cellulose + oxygen = Carbon dioxide + water

C6H10O5 + O12 = 6CO2 + 5H2O

However, when cellulose accumulates under water, the oxidation does not completely take place, and although there may be partial oxidation of the plant debris. CO2, H2O and CH4 (Marsh gas) are formed in the simplest case. In addition to the cellulose lipids proteins and lignins are derived from the decaying vegetation. These, and particularly the lipids are the main components that produce the bulk of the hydrocarbons found in the sedimentary rocks. This is done principally by thermal decay [cracking] during burial.