Lecture 4 - Weathering and Sedimentary Rocks

Pre-Sedimentary Rocks

  • Before sedimentary rocks can be formed, sediments need to exist. And for sediments to exist, rocks have to be weathered, eroded, transported, and deposited.

Weathering

  • It simply means being exposed to weather - but not just “weather” as a meteorological concept, but rather the forces that may change it on earth - like rains, winds, moving body of water, animals, plants, etc.

    • Physical Weathering - process that transform rocks into soil, fractures and exposes more surface area of the rock for chemical weathering

    • Chemical Weathering - destroys mineral structures to weaken rocks for physical weathering to be easier

  • Weathering will result to sedimentary clasts (or solid fragments), ions in solutions (which can form minerals later on), and soil

Physical Weathering

  • Exfoliation

    • When the material over a rock is weathered and transported over time, the rock gets exposed and this also REDUCES the pressure the rock experiences

    • The loss of pressure allows the rock to expand and this creates cracks on the rock

  • Disrupting Forces

    • Forces due to ice and root wedging

    • Plants growing through rock cracks further expanding the crack

    • Water freezing inside rock cracks further expands when becomes ice

  • Crystal growth

    • Salt water infiltrates rocks, as the water dries, the salt grow in the rock and crack it

    • Not confined to coasts since most environments have salt in them

  • Unequal Temperature

    • Bending of rocks due to unequal heating and cooling may cause rocks to break

Chemical Weathering

  • Results from minerals undergoing chemical changes as they become unstable when exposed to the surface

  • Greatly dependent on the environmental conditions (normally, greatest at warm and wet climates) and minerals present

  • Two main types:

    • Minerals into other minerals (ex. Feldspar becomes clay minerals through hydrolysis)

    • Minerals getting dissolved into a solution (ex. Calcite can be dissolved by acidic solutions)

Chemical Weathering Agents

  • Carbonic Acid (H2CO3)

    • It needs water and carbon dioxide

    • In the air, only very weak carbonic acid can be made but having more CO2 in the soil can make water passing through it considerably more acidic

  • Oxidation

    • For ex., ferromagnesian silicates (Olivine, Pyroxene, etc.) can be altered so that its iron becomes dissolved iron and undergo oxidation to create iron oxides

    • Another ex. is sulfide minerals becomes sulfuric acid

Erosion

  • Removal of weathered materials

  • Helps weathering happen more as weathered materials are moved and thus exposing the rock again

  • Erosion has several agents including:

    • Gravity

    • Wind

    • Water

    • Waves

    • Animals

Transportation

  • It is the movement of sediment and/or dissolved ions from point A (point where it was eroded) to point B (point where it will be deposited)

  • Velocity of the medium (water or wind, mostly) is the key factor affecting which sediments it can move. Medium’s velocity depends on various factors including but not limited to:

    • Geometry of path including meanders, slopes, etc.

    • Season

Deposition - Transportation stops and sediments are deposited.

Terrestrial Depositional Environments

  • Lacustrine - Sediments (sand mainly on edges, silt, clay, organic matter) are moved by water flowing into a lake

  • Evaporitic - Much like lacustrine but lakes are situated in arid areas and sediments are mostly salts and clays

  • Aeolian - Sediments are sand and silt carried by wind into deserts and coastal areas

Marine Depositional Environment

  • Tidal - Sediments (silt and clay) are moved by currents/tides to tidal flats

  • Deltaic - Sediments (sand, silt, clay, organic materials) are moved by water into deltas

  • Beach - Sediments (gravel and sand) are carried by waves and longshore currents into the beaches and sand bars

  • Lagoonal - Sediments (typically carbonates) are almost not moved and settles to the lagoon bottom

  • Reef - Carbonates are carried by waves and tidal currents to be deposited in reefs and basins around it

  • Shallow Water Marine - Sediments (carbonates, sand, silt, clay) are carried by waves and tidal currents to shelves and slopes

  • Deep Water Marine - Sediments (clays, carbonate mud, silica mud) are washed by ocean currents into deep ocean abyssal plains

  • Submarine Fan - Sediments (gravel, mud, sand) are carried by underwater gravity flows into the slopes (before abyssal plains) and abyssal plains

Sedimentary Rocks

  • Formed from the lithification of sediments

  • For the weathered materials to become rocks, the following must occur (in order):

    • Burial - Newer sediments bury the older sediments at point B causing covering and compacting of material

    • Compaction - As they are buried, the sediments are squeezed/compacted together

    • Cementation - At depths of hundreds to thousands of meters, minerals crystalizes at pores and contact points of sediments, cementing together to form sedimentary rocks

  • Most common cementing materials are quartz, calcite, and iron oxide

  • Can be classified as:

    • Clastic - made up of sediments that were transported as solid fragments or clasts

    • Chemical - made up of sediments that were transported as ions in solutions

Clast

  • Rock or mineral fragment of various sizes

  • Clasts can be described based on its diameter size (based on Udden-Wentworth grain-size scale)

Clastic Sedimentary Rock Groups

  • Mudrock - has at least 75% silt or clay composition and form in very low-energy environment (lakes, deep parts of the ocean)

    • If it is mostly clay, then it is CLAYSTONE

    • If it is mostly silt, then it is SILTSTONE

    • If it has beddings/lamination, then the mudrock is SHALE which is fissile

  • Sandstone - Made up mostly of sand (other part is silt and clay)

    • If sandstone has:

      • Silt and clay <15%: “Clean Sandstone” or “Arenite”

      • Silt and clay is >15%: “Wacke”

    • Its sand-sized particle components are mostly made of Quartz, Feldspar, and Rock Fragments

    Sedimentary rocks with significant amount of clasts bigger than 2mm will be known as:

  • Conglomerate - if the clasts are rounded (formed in high energy environments like swift rivers, hence the more rounded shape)

  • Breccia - if the clasts are angular (formed in environments where clasts not transported significantly, hence little effect on shape)

  • Coal - often classified as organic sedimentary rock due to its composition of decayed material (peat moss and humus).

Chemical Sedimentary Rock

  • Limestone

    • It is composed mostly of the minerals: calcite and aragonite, the crystal form of calcium carbonate

    • Forms when these minerals precipitate out of water containing dissolved calcium through biological or non biological processes

    • Can form around reefs as well as in deeper waters where there is abundance of carbonate shells of organisms

  • Chalk - a limestone composed of calcite minerals made from biochemical processes

  • Dolomite Rock

    • another carbonate rock primarily composed of mineral dolomite

    • dolomites are recrystallization of older limestones

  • Chert

    • some organisms like radiolarians and diatoms use silica to make their shell. When they die, the shells settle down and become chert

    • Chert is chemically precipitated silica from biological remains

  • Evaporites - In dry areas, when waters in lakes and rivers evaporate, they become concentrated with dissolved ions which then crystallize

    • Rock Salt - composed mainly of the mineral halite

    • Rock Gypsum - composed mainly of the mineral gypsum

Sedimentary Structures

  • Sedimentary rocks are usually observed in the environments as large sedimentary structures

  • These structures are formed at the time of deposition of sediments and follow some geological principles

Geological Principles for Sedimentary Structures

  • Principle of Original Horizontality - sediments are deposited and accumulated horizontally. Any layers that are tilted must have been subject to tectonic forces

  • Principle of Superposition - layers are deposited in sequence and older is always at the bottom (unless turned over by tectonic process)

  • Principle of Inclusions - any rock/fragment in a layer is older than the layer itself much like the rocks in a conglomerate is older than the conglomerate itself

  • Principle of Faunal Succession - fossils may be used to determine the age of the rock it is in

Sedimentary Structures

  • Bedding - layering of sediments that can be seen in changes in texture, color, composition, etc. It indicates changes related to the deposition of materials that may be due to seasonal, climates, or tectonic changes among others

  • Partings - narrow gaps in between layers shown in a bedding. These may indicate periods with no deposition

  • Cross-bedding - Bedding that has angled layers within horizontal beds. This forms due to the sediments being deposited by water or wind

  • Ripples - linked to cross bedding. Sediments in a flowing water body may become IMBRICATED — they tile towards one direction, with their tops tilting towards the direction of a flow

  • Graded Bedding - A bed where gradation/gradual change of grain size from large to small is seen as from bottom to top (known as normal graded beds). Some have large grains at the top and finer grains at the bottom known as reverse/inverse graded bed

  • Mudcracks - when a shallow body of water dries up, the mud at its bottom dries up and cracks due to the clay in the mud shrinking as it dries

Engineering Considerations

  • Weathering

    • Most engineering projects are situated in shallow depths, within the zone of weathering — where rock properties are greatly altered.

    • Even fresh rock excavated by engineering works are eventually exposed to weathering

    • Rocks in zones of weathering are highly variable and may change within the lifespan of the project

  • Grades of Weathering

  • Slaking of Mudstones and Shales

    • Slaking is the deterioration of a rock after exposure, often by excavation

    • Mudstones and shales that are not fully cemented (called compaction shales) often are deficient of shear strength and experiences slaking

    • Additionally, compaction shales loosen when wet and approximates soil behavior

  • Sandstones and Conglomerates

    • Clastic sedimentary rocks with large sediments tend to have higher compressive strength

    • Moisture content of sandstones may indicate degree of compaction. Higher moisture content have generally poorer compaction

    • Hydraulic conductivity of sandstones are usually measured and is an indicator of degree of cementation

  • Dissolution of Limestone and Evaporites

    • Limestones and evaporites are composed of minerals that are soluble in acidic water

    • Dissolution of these rocks depends on the solubility and solution rock properties with local waters

    • Older formations are less soluble than younger counterparts

    • This poses problems in water storage projects and foundations. Often, engineers may choose to select another site with better subsurface conditions than deal with soluble rocks

  • Karst Topography and Sinkholes

    • Karst is a topography formed from dissolution of soluble rocks. It is characterized by underground drainage with sinkholes and caves

    • A sinkhole is a depression or hole in the ground caused by some form of collapse of the surface layer

  • Limestone as Construction Materials

    • Crushed limestone and dolomite are used as aggregates in concrete and bitumen

    • Limestone serves as source of lime in making cement