Sedimentary rocks form by the breakdown (both physical and chemical) of pre-existing rocks (these may be of igneous, metamorphic or sedimentary origin). The main factors that control this breakdown are:
- properties (physical and chemical) of the rock.
The particles that are broken down are called sediments. Sediments are classified according to their size, ranging from silts and muds up to gravels and boulders. The main factors that control the transportation of sediments are:
- wind (particularly in arid regions)
- gravity (with all sediments flowing downhill regardless of the slope)
Horizontal layering in sedimentary rocks is called bedding or stratification. It forms by the settling of particles from either water or air (the word sediment comes from the Latin sedimentum, meaning settled). Layer boundaries are natural planes of weakness along which the rocks can break and fluids can flow. As long as the sequence of layers has not been deformed or overturned, the youngest layers are at the top and the oldest are at the bottom. This sequence of stratification is the basis for the stratigraphic time scale. These observations were first made by a Danish physician, Nicolaus Steno, who in 1669 formulated the principles of horizontality, superposition (younger layers on top of older ones) and original continuity (sedimentary layers represent former continuous sheets).
Sedimentary geologists (known as sedimentologists) tend to study both present-day sediments and older sedimentary rock sequences.
- Laminations are thin discrete layers of rock.
- Formations are groups of sedimentary rocks which have formed at the same time and contain similar sedimentary rocks. They are mappable units that formed under distinctive environmental conditions.
- Unconformities are major time-gaps between layers.
Principle of uniformitarianism
The principle of uniformitarianism is that processes which operate on the Earth’s surface today are similar to those that operated in the past. It is a fundamental principle in sedimentary geology and was first proposed by Charles Lyell in 1830. Using this principle when studying present-day sedimentary environments (e.g. coral reef systems, delta systems, river systems), we can determine fundamental principles such as:
- rates of sedimentation
- geometry of sediment sequences
- rates of compaction
- amount of water present in the sediments
which can then be applied to much older sedimentary rock sequences.
- Weathering and feldspars
- Weathering and carbonates
- Resistance of minerals to weathering
- Weathering in different rock types
- Shapes of weathered fragments
- Rates of erosion
Weathering is the process where rocks break down under the effects of water and air. It consists of two processes which always act together:
- fragmentation (known as mechanical or physical weathering)
- decay (known as chemical weathering)
Erosion is the process of the movement of weathering products, by water and air.
The smaller the pieces (or fragments), the greater the surface area available for chemical attack and the faster the pieces decay. Important agents in weathering are rainfall, wind, ice, snow, rivers, seawater, vegetation and living organisms.
Soil is both a factor in weathering and the result of it. Once soil starts to form, rock weathers more rapidly and more soil is formed. Rainwater, which is mainly H2O, also contains small amounts of dissolved CO2 and H2S which react strongly with many rock materials.
Weathering and Feldspars
Feldspar is the most common mineral in many igneous and metamorphic rocks, particularly in granites. In temperate climates, granites are quite resistant to erosion whereas in humid to tropical regions, granites decay easily. This is because in areas of greater rainfall, the feldspars decay into clay minerals that are then easily released from the rock. Feldspars generally only remain untouched in very arid climates.
The most common clay mineral formed is kaolinite. In the weathering of feldspars to kaolinite, potassium, sodium, calcium and silicon are released from the feldspar structure and go into solution. As the speed of chemical reactions increases with increasing temperature, weathering occurs at a much faster rate in tropical areas than in more temperate ones. Under extreme weathering conditions in tropical climates, the clays themselves decay further into the mineral gibbsite (Al (OH)3) the main ore of aluminium. The resulting rock is a bauxite or laterite.
Weathering and carbonates
Whereas feldspar minerals decay into clays, carbonate minerals can completely dissolve. Caves are a characteristic product of the weathering of limestone (rocks made up predominantly of the carbonate minerals calcite and dolomite) in humid climates.
The main agent of weathering in this case is groundwater containing dissolved CO2. This dissolution of limestone results in the enrichment of the groundwater in Ca ions, leading to what is known as ‘hard water’. CO2 is extracted from the atmosphere and from organic material in the soil and dissolves in the groundwaters. The greater the weathering of limestone, the more CO2 is removed from the atmosphere. As limestone dissolves faster than silicate rocks, the chemical weathering of limestone accounts for more of the total chemical erosion of the land surface than any other rocks, even though much larger areas of the Earth consist of silicate rocks.
Resistance of minerals to weathering
Measurements of weathering in the field can be combined with experiments in the laboratory to determine the relative resistance of minerals to weathering. The products of weathering are generally more resistant to further weathering than other minerals, particularly iron oxide and clay minerals. As quartz is generally insoluble and chemically stable, it tends not to weather readily under most conditions. The order of mineral stability under weathering conditions is related to the stabilities of chemical bonds and crystal structures under different temperatures and pressures.
Weathering in Different Rock Types
Layered rocks break into slabs or plates along bedding planes whereas massive rocks break along regularly spaced planar cracks, called joints. In some igneous rocks, the joints take the form of sheets – sets of parallel closely-spaced planar surfaces. In areas of large daily temperature gradients, thermal expansion often accompanies frost action and chemical weathering.
Exfoliation is the peeling-off of large curved sheets or slabs of rock from the weathering surface of an outcrop.
Spheroidal weathering is a similar phenomenon in which rounded boulders split off layers or shells from the weathered surface.
The shapes of fragments caused by weathering and erosion are largely inherited from the patterns of joints, bedding and other structures in the parent rock rather than being produced by transport. The size of fragments is a good clue to the intensity of mechanical erosion. In general, the higher or steeper the topography, the larger the fragments. Once fragments have been moved from their source rock, they enter new environments of weathering and erosion and undergo further breakdown. Once boulders fall into streams they break and abrade quickly, and the size of pebbles downstream decreases rapidly with distance from the source. The different sizes and shapes of eroded particles (from huge boulders to clay-sized particles) are attributable to the characteristics of their source rocks and their distance from the source.
Rates of Erosion
Rates of erosion can be averaged over regions and are called rates of denudation (measured in mm per thousand years). These rates are greatest in valley glaciers and badlands (deeply eroded areas) and lowest in areas of low relief and temperate and rainforest regions. The rate of denudation is primarily controlled by topography and climate. Human influences accelerate the rate of denudation by three to ten times, with the highest figures being recorded in areas of intensive land use.
One of the highest rates of denudation measured is in the Tamur Basin of the Himalayas. The combination of steep slopes, unconsolidated material (sediment), a glaciated terrain and human modification has resulted in a rate of 4700 mm per 1000 years.
What is soil?
Soil is a thin layer of loose sediments (usually very fine-grained) and organic matter that lies above a layer of partly-weathered broken rock (called subsoil). Subsoil lies above the bedrock. The term regolith describes all of the loose, incoherent sequence of rock fragments, sand, alluvium, and soil that rests upon solid rock (also known as bedrock).
Soils are made up of three layers and are thickest where they are older and in warm and wet environments. The layers are called horizons. These are known as:
- A-horizon: The top layer of soil. This is usually the darkest as it contains the greatest concentration of organic material. This layer mainly consists of clay minerals and quartz with soluble minerals being absent.
- B-horizon: The second soil layer, which contains little organic matter but abundant iron oxide and soluble minerals.
- C-horizon: The bottom soil layer. This is really just slightly altered bedrock (i.e. subsoil) and contains broken fragments of the bedrock, along with clay minerals.
Formation of soils
It is important to conserve the soil horizon as it takes many thousands of years to develop a mature soil. The A-horizon can take up to 10 000 years to fully develop and B-horizon may take as long as 100 000 years. A mature soil is one that is in chemical equilibrium. As the soil is slowly eroded from the top by natural processes, it is gradually deepened by chemical reaction. However, if the erosion is rapid, the rate of chemical decay is not fast enough and the soil is drastically thinned. Soil formation involves the response of mineral decay and mechanical erosion to variabilities in time, temperature, rainfall and biological activity. Palaeosols are ancient soils recognised in the sedimentary rock record.
Three major soil types can be classified based on the minerals present in their A and B horizons:
- Pedalfers: (Ped is Greek for soil, al for aluminium and fer for iron). In areas of high rainfall, the A and B horizons of the soil are leached, resulting in the soil being rich in quartz, clay minerals and iron oxide minerals. These soils are rich in aluminium and iron.
- Pedocals: In warm and dry climates, the soils are enriched in soluble minerals, particularly calcium carbonate.
- Laterites: In tropical regions, the soils are so highly leached that all silicate minerals are completely altered leaving the soil enriched in aluminium and iron oxides. This type of soil is usually deep red in colour.
Sedimentary processes and structures
- Sedimentary transport
Most sedimentary sequences that are preserved in the rock record are formed from catastrophic deposition such as floods, mud flows, rock slides and melting of glaciers. For a sediment sequence to be preserved and lithified (turned into rock), it must be covered over by younger sediments soon after it is deposited and water within the sequence must be expelled (this usually achieved through compaction by the weight of overlying layers).
Sediments are transported downhill by either water or air until they meet a depression in the surface of the Earth. Most sediments are eventually deposited in large depressions which we call sedimentary basins. As more sediment is deposited within a sedimentary basin, subsidence occurs (brought about by the weight of the sediments) and the basin becomes deeper, thus allowing even more sediment to be deposited. Sedimentary basins can form in a number of different environments, both on land (e.g. lakes) or sea (e.g. ocean basins).
Sediments can be transported in three ways:
- Suspension load is when sediments are carried in suspension (usually fine-grained sediments that can be carried along easily by the flow)
- Bed load is when the forward force of the moving current acts more directly on the larger particles at the bottom as it pushes, rolls, and slides them along
- Saltation is more complex and usually affects sand-sized particles. Here, the particles are sucked up by eddies into the flow, travel with the flow for a while, and then fall back to the bottom
Water and sedimentary processes
Water plays a vital role in most sedimentary processes. Most of the water on Earth occurs in the large ocean basins. However, water is also produced by volcanic activity on the surface of the Earth and some of it actually disappears into space each year. As well as carrying sediments in suspension, water also carries large quantities of sediments in solution (e.g. dissolved components) and this sometimes exceeds the total amount of sediments carried by other processes.
Pure H2O itself has little effect on rocks. It is the dissolved gases within water – particularly CO2 (carbon dioxide) – which cause the chemical decay of minerals and mineral dissolution.
The water table
The groundwater or water table is the surface between two zones of rock or soil below which the pores are completely filled with water. This is also called the saturated or phreatic zone.
Above the water table is the unsaturated or vadose zone. This zone is also called the zone of aeration because the pore spaces are filled partly by air and water.
The water table in any area is rarely stable, even for a few months. In periods of low rainfall, the water table falls, while in periods of heavy rainfall, it rises, eventually leading to flooding when all the pore spaces are filled with water.
Aquifers are permeable beds or rock masses (e.g. vesicular basalt) whereas aquicludes are relatively impermeable beds or rock masses (e.g. quartzite). In some areas, the water in the aquifer is very old (e.g. in the Great Artesian Basin of Australia, much of the water is believed to be older than 50 000 years) and the aquifer recharge may not balance the removal of water from the aquifer. This will eventually lead to the aquifer becoming depleted of water.
Organic matter and sedimentary processes
The biosphere (all biological activity such as plants, animals, and their remains) also plays a vital role in sedimentary processes. All organic matter eventually decomposes, releasing vital nutrients (such as N, Ca, C) into the soil and sea. Both coal and oil are formed by the interaction of buried organic matter with sedimentary processes.
Diagenesis is the alteration of the mineralogy and/or texture of sediments at low temperatures and pressures. It affects sediments close to the Earth’s surface. There are two main processes operating,
- compaction by overlying sediments (involving the close-packing of the individual grains by eliminating the pore space and expulsion of entrapped water)
- cementation – development of secondary material in the former pore spaces which then binds the sedimentary particles together. This material may be introduced from the passage of groundwater or derived from solution.
Characteristics of sediments include:
- Roundness and Sphericity
Porosity is the volume of voids within a rock which can contain liquids. The porosity of a sedimentary rock made up of perfectly closely-packed spherical particles is 27%, with open-packing this rises to 47%. Because sedimentary rocks are never perfectly packed and also contain cement, the porosity generally varies from 1 to 50%. In sandstones, porosity varies from 5 to 15%, while in loose sands and gravel it may reach 45%. Clays are exceedingly porous, up to 50%. The degree of porosity largely depends upon the geometrical arrangement of particles in the sediment.
Permeability is the ability of water or other liquids (e.g. oil) to pass freely through a rock. It is important to note that some rocks (e.g. sandstones) may have a high porosity but may still be impermeable. A rock is pervious if it is permeable by mechanical discontinuities such as joints and bedding planes.
Roundness refers to the roughness of the surface of the sedimentary grain. In general, grains become more rounded the further they are from their source rock. This is an important tool in mineral exploration, particularly for diamonds and other gemstones. Sphericity refers to the shape of the grain and is largely inherited from the host rock.
Sorting refers to the range of particle sizes in a sediment or sedimentary rock. In general, sediments which have travelled relatively long distances from their source are well sorted while those that haven’t travelled far are poorly sorted.
Matrix is the fine-grained material (usually clays or silt) that is deposited originally with the coarser-grained material (e.g. sands and gravels) in a sediment. It is also derived from the weathering and erosion of the source rocks.
Sedimentary structures are very important as they provide us with information on the palaogeography and palaeoclimate of the areas in which they occur. They can also indicate the direction of palaeocurrents of rivers and seas. Sedimentary structures can be of either physical (e.g. wave action) or biological (e.g. disruption of sediments by animals) origin.
Physical structures: The movement of sand grains in a current creates ripples and dunes on the stream bed. These are known as bedforms. Ripples are the low narrow ridges that are separated by wider troughs. Physical structures form on sand dunes, on underwater sandbars in rivers and streams, and under the waves at beaches. They come in a wide variety of shapes and sizes which are characteristic of the currents that form them:
- Asymmetrical ripples are ripples that have a gentle slope upstream and a steep slope downstream.
- Cross-bedding is inclined bedding and commonly forms in alluvial environments.
- Potholes are rounded depressions caused by swirling currents and eddies.
- Mud cracks are formed by evaporation on mudflats or in shallow lakes.
Biological structures: include worm burrows (usually in soft-sediment, particularly at high tide levels along beaches), crab burrows (particularly those of hermit crabs), tubeworm colonies and animal tracks on soft sediment.
Sedimentary environments can be classified according to the climate in which they occur and/or the geometrical arrangement of the sediments. In general, we classify ancient sedimentary rocks according to their similarity to current sedimentary environments. When describing sedimentary sequences, we usually do so in terms of facies (the sum total of all sedimentary features) columns in which the sequence is graphically illustrated from the base to the top. Facies columns include all details on sedimentary structures, variations in grainsize, contacts between the individual beds and fossils present.
In geology, river environments are known as fluvial environments. Within fluvial environments deposition occurs with normal settling of relatively coarse to medium-grained sediments on the river bottom and along bends in the river, and also during times of flooding when finer-grained sediments are deposited along the river banks.
When rivers flow into lakes, the coarser-grained sediments are usually deposited at the toe of the deltas while the finer-grained sediments are carried in suspension to be deposited in the centre of the lakes. In many cases, the river breaks up into tributaries when it enters into the lake, and the banks between these tributaries are mainly composed of sand-sized sediments. As it contains a relatively high porosity, detrital organic material (particularly plant material) is deposited into the centre of the lakes along with the clay and silt-sized sediments.
All major rivers (along with many minor ones) eventually reach the sea. As they approach the sea, they generally divide into many channels before eventually reaching it. The banks of these channels are generally composed of sand deposits along with minor mud which is deposited in times of flooding. Between the channels, shallow bays are filled by fine-grained sediments such as sands and muds. When the river eventually reaches the sea, the coarser sand-sized particles are deposited first (sometimes forming bars between the river mouth and sea) and the finer-grained silt and clay particles are deposited in deeper water out to sea.
Coral reefs only occur in warm shallow water environments. They are formed by the interaction of biological (i.e. corals) and sedimentary (i.e. silts and muds) processes. Many coral reefs are formed on pre-existing drowned volcanic necks (i.e. many of the islands in the Pacific have formed in this way) while others form on pre-existing masses of rock (e.g. the Great Barrier Reef). Coral reefs grow far more rapidly on the windward side as the currents driven by the wind bring in abundant nutrients.
Coral reefs are very fragile environments. As the coral itself is the shell of a living organism, it needs abundant sunlight (for photosynthesis) and nutrients. If the coral is blanketed in sediments (such as silts produced by rapid erosion caused by intensive irrigation on land), sunlight will not penetrate and the coral (and reef community) will die out. Also, if there is too much nutrient in the water (sometimes produced on land by farmers using fertilisers, e.g. super-phosphate, to increase their crop yield), other predators (such as the crown of thorns starfish) will move in, eat the coral and eventually sterilise the reef.
In polar and cold climates, glaciers are produced. As glaciers move downhill, they grind down any underlying rocks into fine powder and carry along with them any pebbles or boulders that have fallen into the glacier from avalanches and rockfalls. When the glacier starts to melt, these sediments are deposited at the foot of the glacier (particularly the coarser-grained sediments) and are called moraine deposits. After glaciation, suspended valleys known as hanging valleys are formed, along with steep waterfalls and alluvial fan deposits.
In arid environments, there is little available water or vegetation to act on the rocks and sediments present. Most weathering and erosion is caused by wind action and frost. Because of the lack of water and vegetation, sedimentary structures produced in these environments last for a relatively long time period. The Namib Desert of Namibia contains the world’s largest sand dunes which are believed to be about 30 million years old. Most sediment deposits in arid regions are wind-blown in origin, and hence tend to be very well sorted. Other characteristic sedimentary deposits in arid environments are evaporite deposits that form in shallow lakes. Because of low rainfall the water table in these areas is very low, and soluble minerals occur near the surface and accumulate over time.
In areas of very high rainfall (i.e. tropical areas) the rainfall is so high and the vegetation is so dense that there are very few visible rock outcrops. The soil profile is very deep because the interaction of rain with the vegetation forms humic acids that strongly leach and rapidly break down any rocks present. Sedimentary deposits in these environments are largely formed by mudslides, particularly if the topography is steep.
Classification of sedimentary rocks
Prefixes can also be added to indicate the dominant mineralogy. For example, a quartz-rich sandstone is a quartzose sandstone, a feldspar-rich sandstone is an arkose, a mica-rich sandstone is a micaceous sandstone, and a lithic-rich sandstone or shale is a greywacke.
Conglomerate (7.5 cm x 4 cm). Newcastle District, New South Wales. Photo: S Humphreys © Australian Museum.
Conglomerates can be further subdivided according to their texture (grain-supported versus matrix-supported), composition (pebbles of one rock type or several different rock types) and source rocks (pebbles derived from within the depositional environment or from outside).
Limestones are a different case as they can either be of clastic or chemical origin. The clastic limestones can contain either:
- rock fragments
- fossils (e.g. fossiliferous limestone)
In contrast, the chemically-precipitated limestones consist solely of crystalline carbonates (e.g. microcrystalline limestone). The clastic limestones can be further subdivided according to their grainsize, much like other clastic sedimentary rocks.
Non-fossiliferous limestone (8 cm x 5 cm) Jenolan Caves, New South Wales. Photo: S Humphreys © Australian Museum.
Fossiliferous limestone (8 cm x 8 cm) Rockley, New South Wales. Photo: S Humphreys © Australian Museum.
- banded-iron formations
Zebra rock or Zebra stone is a distinctive reddish-brown and white banded claystone from the Ord River area of Western Australia where it forms discontinuous lenses within a Late Proterozoic shale sequence. Most occurrences are now submerged beneath the dam waters of Lake Argyle. Since its discovery in 1924, it has been used widely as an ornamental stone. It is composed of small particles of quartz, white mica, clay minerals, hematite and alunite. The striking colour banding was probably formed after the original sediments were deposited by the rhythmic precipitation of hematite-rich bands during alteration of the rock in a highly oxidising environment.
Some examples of sedimentary rocks:
Chalk is a fine-grained limestone, usually formed by compacted microscopic animals called foraminifera.
Chert is a fine-grained hard sedimentary rock composed of microscopic silica grains, and has a flat fracture.
Flint is a variety of chert (mostly of upper Cretaceous age) that has a conchoidal fracture.
Marl is a calcareous mudstone.
Micrite is a finely-crystalline calcite.
Mudstone is a fine-grained sedimentary rock that lacks a well-developed bedding plane.
Shale is a fine-grained sedimentary rock with a well-developed bedding plane.
Siltstone is similar to mudstones but consists predominantly of silt particles.
Sparite is a coarsely-crystalline calcite.