Sedimentology Concepts: Grain Size, Transport & Diagenesis
Sedimentology Concepts: Grain Size, Transport & Diagenesis
(a) Krumbein’s Logarithmic Transformation (φ scale)
Why is Krumbein’s logarithmic transformation necessary? Grain-size data are not normally distributed but are skewed. Krumbein introduced the phi (φ) scale to convert grain size into a logarithmic form: φ = −log₂ (grain diameter in mm). This transformation simplifies statistical analysis of sediments and helps in easy comparison of grain-size parameters like mean, sorting, skewness, and kurtosis.
(b) River Sediment Transport Mechanisms
How are sediments transported by rivers?
Sediments are transported by rivers in four main ways:
- Solution: Dissolved materials carried in water.
- Suspension: Fine particles (clay, silt) carried within the water column.
- Saltation: Sand-sized particles move by hopping or bouncing.
- Traction: Coarse particles roll or slide along the river bed.
(c) Diagenesis Processes
How does diagenesis take place?
Diagenesis includes all physical, chemical, and biological changes after deposition. It occurs at low temperature and pressure conditions. Major processes include:
- Compaction: Reduction of pore space.
- Cementation: Precipitation of minerals binding grains.
- Recrystallization: Change in crystal size or form.
These processes lead to the conversion of sediment into sedimentary rock.
(d) Sedimentary Environments
What is a sedimentary environment?
A sedimentary environment is a specific area where sediments are deposited, controlled by physical, chemical, and biological conditions. It determines the nature and characteristics of sediments. Major types include:
- Continental
- Transitional
- Marine
Each environment produces distinct sedimentary structures and fossils.
(e) ZTR Index and Mineralogical Maturity
What is the ZTR index? The ZTR index is a measure of sediment maturity. ZTR stands for Zircon, Tourmaline, and Rutile, which are ultra-stable heavy minerals.
ZTR Index = (Z + T + R) / Total heavy minerals × 100.
A high ZTR value indicates high mineralogical maturity.
(f) Significance of Graded Bedding
What is the significance of graded bedding?
Graded bedding shows a gradual change in grain size from coarse to fine. It typically indicates deposition by turbidity currents, helps determine the younging direction of strata, and is useful in palaeocurrent analysis. Graded bedding is common in deep marine environments.
(h) Sphericity of Clastic Grains
How is the sphericity of clastic grains determined?
Sphericity refers to how closely a grain resembles a sphere. It is determined by measuring three mutually perpendicular axes: long (L), intermediate (I), and short (S). Sphericity can be calculated using Krumbein’s sphericity formula. Visual comparison charts (e.g., Krumbein chart) are also used. High sphericity indicates a long transport history.
(i) Dolomitisation Process
What is dolomitisation?
Dolomitisation is the process by which limestone is converted into dolomite. It occurs by replacement of calcium by magnesium in calcite and typically takes place under marine or hypersaline conditions. Dolomitisation can lead to an increase in porosity and is common in carbonate sedimentary environments.
(j) Cone-in-Cone Structure
What is cone-in-cone?
Cone-in-cone is a secondary sedimentary structure consisting of nested cone-shaped calcite crystals. It is formed due to pressure solution and crystallization, is commonly found in shales and limestones, and indicates early diagenetic processes.
(a) Chemical Weathering
Chemical weathering involves decomposition and alteration of rocks by chemical reactions. It operates mainly through water, oxygen, carbon dioxide, and organic acids. Major processes include solution, oxidation, hydrolysis, carbonation, and hydration. Chemical weathering is more effective in warm and humid climates and produces clay minerals and soluble ions, supplying sediments for deposition.
(b) Fluvial Transportation of Sediments
Fluvial transportation refers to movement of sediments by river systems. It depends on velocity, discharge, gradient, and sediment size. Modes of transport are:
Solution, Suspension, Saltation, Traction.
Fluvial transport results in sorting, rounding, and size reduction of sediments and produces structures like cross-bedding and graded bedding.
(c) Cementation as a Diagenetic Process
Cementation is a diagenetic process in which minerals precipitate in pore spaces. Common cementing materials are silica, calcite, iron oxide, and dolomite. Cementation occurs due to circulation of mineral-rich groundwater, binds sediment grains together to form solid rock, and reduces porosity and permeability of sedimentary rocks.
(d) Roundness of Sediments
Roundness describes the degree of smoothness of grain edges and corners. It develops mainly due to abrasion during transportation and is classified as angular, sub-angular, sub-rounded, rounded, and well-rounded. Roundness is estimated using Krumbein’s roundness chart. High roundness indicates long transport distance and reworking.
(e) Fabric of Sedimentary Rocks
Fabric refers to the arrangement, orientation, and packing of sediment grains. It includes grain size, shape, sorting, and alignment and is controlled by depositional processes and energy conditions. Fabric helps interpret palaeocurrent and depositional environments. Examples include imbrication, preferred orientation, and bedding.
(f) Turbulent Flow and Sedimentary Structures
Turbulent fluid flow is characterized by irregular, chaotic motion of fluid particles and occurs at high velocity and high Reynolds number. It has high erosive and transport capacity and produces sedimentary structures such as:
- Ripple marks
- Cross-bedding
- Graded bedding
Turbulent flow is common in river channels and shallow marine environments.
(g) Rose Diagram for Directional Data
Rose diagram is a circular statistical diagram used to represent directional data such as palaeocurrent measurements. The length of the petals indicates the frequency of directions. Rose diagrams are helpful in determining sediment transport direction and are widely used in sedimentological and structural studies.
(h) Calcrete and Duricrust Formation
Calcrete is a calcium carbonate–rich duricrust that forms in arid to semi-arid climatic conditions. It develops by precipitation of CaCO₃ from groundwater, occurs in soils and near-surface sediments, and indicates subaerial exposure and palaeoclimatic conditions.
(i) Dolomite as Rock and Mineral
Dolomite is both a carbonate mineral and sedimentary rock with the formula CaMg(CO₃)₂. It is formed mainly by dolomitisation of limestone and occurs in marine and hypersaline environments. Dolomite usually shows rhomb-shaped crystals and is important as a reservoir rock due to its high porosity.
(j) Allochems in Carbonate Rocks
Allochems are transported carbonate grains in limestone, formed by biological or chemical processes. Types include:
- Ooids
- Peloids
- Bioclasts
- Intraclasts
Allochems are embedded in a micrite or sparite matrix and are used in the classification of carbonate rocks.
(a) Udden–Wentworth Grain-Size Scale
The Udden–Wentworth scale is a standard grain-size classification for clastic sediments. Proposed by Udden (1898) and modified by Wentworth (1922), it is based on particle diameter in millimetres. Major classes include boulder, cobble, pebble, sand, silt, and clay. It is widely used in sedimentological description and rock classification.
(b) Packing in Sedimentary Rock
Packing refers to the spatial arrangement and closeness of grains in a sedimentary rock. It depends on grain size, shape, sorting, and compaction. Types include open packing and close packing. Packing influences porosity and permeability of rocks and is useful in interpreting diagenetic history.
(c) Provenance of Sediments
Provenance refers to the source area of sediments and includes information about rock type, tectonic setting, and climate. Provenance is determined by mineral composition, texture, and heavy mineral assemblage. It helps reconstruct palaeogeography and is important for understanding sediment dispersal systems.
(d) Graded Bedding Revisited
Graded bedding is a sedimentary structure showing a systematic change in grain size, typically with coarse grains at the base and fine grains at the top. It forms due to a decrease in current velocity during deposition and is common in turbidity current deposits. Graded bedding is useful for determining the younging direction of strata.
(e) Cementation (Repeated)
Cementation is a post-depositional (diagenetic) process occurring by precipitation of minerals in pore spaces. Common cements are silica, calcite, dolomite, and iron oxide. Cementation binds loose sediments into hard sedimentary rock and reduces porosity and permeability.
(g) Classification of Breccia
Breccia is a coarse-grained clastic sedimentary rock with angular fragments. It is classified based on origin:
- Sedimentary breccia
- Fault breccia
- Collapse breccia
- Volcanic breccia
Breccia is also classified based on cement or matrix and typically indicates a short transport distance.
(h) Bedding and Lamination
Bedding is the layering of sediments with thickness usually > 1 cm and represents a change in depositional conditions. Lamination consists of very thin layers (< 1 cm thick) formed due to minor variations in sediment supply or energy. The difference: bedding is thicker and more distinct, while lamination is thin and delicate.
(i) Limestone vs Dolomite Origins
Difference between origin of limestone and dolomite:
- Limestone forms mainly by biogenic accumulation or chemical precipitation of CaCO₃.
- Dolomite forms mostly by replacement of limestone through dolomitisation.
- Limestone forms in normal marine conditions; dolomite commonly forms in hypersaline or restricted environments.
- Dolomite is usually secondary in origin.
(j) Stalactites and Stalagmites
Stalactites and stalagmites are chemical sedimentary deposits formed by precipitation of calcium carbonate from dripping water. Stalactites hang from cave roofs, and stalagmites grow upward from cave floors. They are common in limestone caves and indicate karst topography.