PART 1
Saturday, January 25, 2025 5:38 PM
Section C. Sedimentology
Classification of sedimentary rocks; sedimentary textures-grain size, roundness, sphericity, shape
and fabric; quantitative grain size analysis; sediment transport and deposition - fluid and sediment
gravity flows, laminar and turbulent flows, Reynold’s number, Froude number, grain entrainment,
Hjulstrom diagram, bed load and suspension load transport; primary sedimentary structures;
penecontemporaneous deformation structure; biogenic structures; principles and application of
paleocurrent analysis; composition and significance of different types of sandstone, limestone,
banded iron formation, mudstone, conglomerate; carbonate diagenesis and dolomitisation;
sedimentary environments and facies-facies models for fluvial, glacial, deltaic, siliciclastic shallow
and deep marine environments; carbonate platforms- types and facies models; sedimentation in
major tectonic settings; principles of sequence stratigraphy-concepts, and factors controlling base
level changes, parasequence, clinoform, systems tract, unconformity and sequence
Classification of sedimentary rocks
Pettijohn's Sandstone Classification
• Field Descriptions: Informal names like micaceous sandstone, calcareous sandstone, and ferruginous sandstone indicate distinctive minerals or chemical composition.
• Petrographic Classification: Based on Pettijohn et al. (1987) scheme, which considers texture (matrix content) and composition (quartz, feldspar, lithic fragments).
• QFL Triangle: Used in clastic sedimentology to classify sandstones based on relative proportions of quartz (Q), feldspar (F), and lithic fragments (L).
• Texture-Based Classification:
○ Arenite (<15% matrix)
○ Wacke (15–75% matrix)
○ Mudstone (>75% matrix)
• Compositional Classification:
○ Quartz Arenite (>95% quartz)
○ Feldspathic (Arkosic) Arenite (≥25% feldspar)
○ Lithic Arenite (≥25% lithic fragments)
○ Subarkosic & Sublithic Arenite (Intermediate feldspar/lithic content)
○ Quartz, Feldspathic (Arkosic), and Lithic Wacke (Wacke divisions)
• Greywacke: Older term for feldspathic/lithic wacke, consisting of mixed rock fragments, quartz, feldspar, and clay-silt matrix.
Folk's Sandstone Classification
• Philosophy: Rock names should convey maximum information without being overly complex. Classification is based on five properties:
1. Grain size (based on the Wentworth scale)
2. Cements (optional; authigenic minerals precipitated in pores)
New Section 1 Page 1
2. Cements (optional; authigenic minerals precipitated in pores)
3. Textural maturity (stages from immature to supermature)
4. Miscellaneous transported constituents (e.g., heavy minerals, fossils)
5. Clan designation (based on QFR diagram)
• Naming Format:
(Grain size): (Cements) (Textural maturity) (Miscellaneous constituents) (Clan designation)
○ Example: Fine sandstone: supermature quartzarenite
• Clan Designation (QFR Diagram):
○ Normalizes quartz (Q), feldspar (F), and rock fragments (R) to 100%
○ Further classification for arkosic and litharenite rocks based on feldspar or rock fragment types
• Textural Maturity Stages:
1. Immature: >5% clay, poorly sorted, angular grains
2. Submature: <5% clay, poorly sorted, subangular to subrounded grains
3. Mature: No clay, well sorted, not well rounded grains
4. Supermature: No clay, well sorted, well rounded grains
• Cements: Authigenic minerals dependent on pore water chemistry, mineralogy, temperature, and pressure.
• Grain Size Classification: Uses the Wentworth scale to determine grain size.
Carbonate Classification
General Principles
• Limestone classifications are arbitrary and overlapping.
• Practical classifications should be based on binocular microscopes or hand lenses.
• Classification is typically based on the most significant sedimentary particle in the rock.
• Example:
○ Oolitic limestone (dominantly ooids)
○ Skeletal-oolitic limestone (minor skeletal fragments present)
Folk's Classification (1959, 1962)
• Based on allochem content (transported carbonate grains).
• Limestones with >10% allochems are termed allochemical rocks.
• Further subdivision based on interstitial material:
1. Sparry allochemical limestone (Sparite) → Cemented by coarsely crystalline calcite.
2. Microcrystalline allochemical limestone (Micrite) → Contains micrite (carbonate mud, <5
µm).
• Grain-Based Classification:
○ Micrites = Mud-rich carbonates.
○ Sparites = Matrix-free, cemented by sparry calcite.
• Best suited for thin-section studies.
New Section 1 Page 2
Dunham’s Classification (1962) and Modifications
• Based on depositional texture (grain support vs. mud support).
• Better suited for hand lens or binocular microscope studies.
• Limestone Categories:
1. Grainstone → Grain-supported, no mud.
2. Packstone → Grain-supported, minor mud.
3. Wackestone → Mud-supported, >10% grains.
4. Mudstone → Mud-supported, <10% grains.
New Section 1 Page 3
• Modifications by Embry & Klovan (1971):
○ Floatstone → Wackestone with grains >2mm.
○ Rudstone → Coarse-grained grainstone.
○ Boundstone Classification Modifications:
▪ Bafflestone → Organisms trapped sediment.
▪ Bindstone → Organisms bound sediment.
▪ Framestone → Organisms built framework.
○ Useful but difficult to apply to ancient limestones due to diagenesis.
Comparison of Folk and Dunham Classifications
Folk (Thin Section-Based) Dunham (Depositional Texture-Based)
Micrite (Mud-rich carbonate) Mudstone or Wackestone
Sparite (Matrix-free, cemented) Grainstone or Packstone
Allochem-based names Texture-based names
Limitations
• Folk’s system provides detailed mineralogical insight but is more complex.
• Dunham’s system is practical for fieldwork but has broad mud percentage categories.
The Udden–Wentworth Grain-Size Scale, commonly known as the Wentworth Scale, is a widely used classification system for sedimentary particles based
on grain size. Originally introduced by Udden (1914) and refined by Wentworth (1922), it follows a logarithmic progression (base 2), meaning each class
boundary is double or half the size of the adjacent one. This scale provides a systematic way to categorize sediments into clay, silt, sand, and gravel,
offering a consistent framework for sedimentology and geological studies. Additionally, the phi (Φ) scale mathematically represents grain size using a
logarithmic transformation for easier analysis and graphical representation.
New Section 1 Page 4
logarithmic transformation for easier analysis and graphical representation.
roundness, sphericity
Clast roundness During sediment transport the individual clasts will repeatedly come into contact with each other and stationary objects: sha rp edges
tend to be chipped off first, the abrasion smoothing the surface of the clast. A progressive rounding of the edges occurs wit h prolonged agitation of
the sediment and hence the roundness is a function of the transport history of the material. Roundness is normally visually e stimated (Fig. 2.16), but
may also be calculated from the cross-sectional shape of a clast.
Clast sphericity In describing individual clasts, the dimensions can be considered in terms of closeness to a sphere (Fig. 2.16). Discoid or n eedle-like
clasts have a low sphericity. Sphericity is an inherited feature,hat is, it depends on the shapes of the fragments which form ed during weathering. A
slab-shaped clast will become more rounded during transport and become disc -shaped, but will generally retain its form with one axis much shorter
than the other two.
New Section 1 Page 5
