Classification of Sandstone

Geol 370: Sedimentology and Stratigraphy
Topic 4: Classification of Sandsdtone

1. There have been over fifty classification schemes proposed for sandstone. Most of those currently in use involve a ternary QFL (quartz, feldspar, lithic) plot for framework grains and a major division based on the relative amount of matrix. Classification of Sandstone

2. Dott Classification Because of its simplicity and added detail, I prefer this version (with some modification of terminology) of the Dott classification.

3. Point Counting Sandstone composition is typically determined microscopically by identifying mineral composition at 300 equally spaced points across a thin-section.

4. Total Rock Composition Component Proportion (%) Quartz 78 Feldspar 60 Rock fragments 36 Matrix 126 Total: 300 Total Q, F, and L: 174 Framework Grain Composition Component Proportion (%) Quartz (78/174) 45 Feldspar (60/174) 34 Lithic (36/174) 21 Total: 100 Sample Point Count Composition at 300 points is identified. The relative abundance of framework grains to matrix determines whether the rock is an arenite, a wacke, or a mudstone. The framework composition is normalized to determine the type (quartz, feldspathic, lithic) of arenite or wacke.

5. Ternary Diagrams The normalized framework percentages are plotted on ternary diagrams to determine rock composition. This sample plots as lithofeldspathic. Because of its high matrix content, it would be a lithofeldspathic wacke or lithic arkose wacke.

6. Quartz Monocrystalline Polycrystalline Quartz grains occur in three forms, monocrystalline (igneous source), polycrystalline (metamorphic source) and chert (sedimentary source). Some workers consider chert to be a lithic fragment. Chert

7. Feldspars Orthoclase crystal undergoing dissolution Feldspar grains occur in four forms, orthoclase, plagioclase, microcline, and sanidine and repesent a variety of igneous and metamorphic sources. Feldspar rapidly alters to clay (kaolinite, sericite) and calcite and is highly subject to dissolution. Plagioclase altering to calcite Dissolution along cleavage planes

8. Lithic Grains Volcanic Rock Fragment Chert Plutonic Rock Fragment Metamorphic Rock Fragment Sedimentary Rock Fragment (shale) Lithic fragments are grains that are recognizable as particles of pre-existing rocks and are classified as igneous, sedimentary, or metamorphic. Some workers consider plutonic and gneissic rock fragments to be feldspar grains.

9. Provenance of Sandstone (framework grains) Influence of Source Rock Influence of Climate The primary control on sandstone composition is the source rock. However, climate and transport distance also have significant impacts.

10. Provenance of Accessory Minerals in Sandstones Igneous Metamorphic Ig/Mc mixed Aegerine Actinolite Biotite Augite Andalusite Enstatite Chromite Chloritoid Hornblende Ilmenite Cordierite Hypersthene Muscovite Diopside Magnetite Topaz Epidote Sphene Garnet Tourmaline Sedimentary Glaucophane Zircon Calcite Kyanite Chert Rutile Sillimanite Staurolite Tremolite Accessory minerals typically have a much more restricted range of formation than quartz or feldspar; therefore, even minor occurrences can be very useful in determining the sediment source. Provenance of Sandstone (accessory [mostly heavy] minerals)

11. Matrix The matrix portion of sandstone consists of mud-size particles, particularly of clay and mica in composition, though quartz and feldspar can play a significant role.

12. Cement Common cements: Quartz Calcite Dolomite Siderite Hematite Limonite Mn oxides Pyrite Gypsum Barite Cement is a diagenetic precipitate formed mostly during burial and reflects changes in temperature, pressure, and ion concentration. Quartz sandstone with calcite, silica, and pyrite cements

13. Quartz Arenite At least 95% quartz Most common along passive continental margins and in epicontinental seas Often found in eolian, beach, and shelf depositional systems Are typically well-cemented and texturally mature to supermature Usually represent multiple recycling events (polycyclic) Can be first generation if the result of intense weathering

14. Feldspathic Arenite/Wacke (arkose) At least 25% feldspar (any type) Arenites have less than and wackes more than 15% matrix Most common in proximity to granitic source terranes, particularly those in arid or cold climatic regions or humid regions with high relief Often found in alluvial fan depositional systems Are typically immature to sub-mature texturally Usually from first generation sediment

15. Lithic Wacke (greywacke) At least 25% lithic framework grains At least 15% matrix Most common in proximity to volcanic source terranes, particularly those in arid or cold climatic regions or humid regions with high relief Often found in alluvial fan depositional systems Almost always immature to sub-mature texturally Nearly always from first generation sediment

16. Glauconitic Sandstone Glauconite is an iron-rich clay mineral related to illite that forms on marine shelves under mildly reducing conditions and very slow sedimentation rates prior to burial, often coating the surfaces of shells, fecal pellets, and sedimentary (clastic and carbonate) particles. Deposits are often some times referred to as “ green sands. ”

17. Phosphatic Sandstone Phosphate nodules and cements develop through dissolution of organic materials, such as bones and teeth, under conditions of low pH. Phosphate is commonly found in association with glauconite.

18. Porosity Porosity is a measure of the open space within a rock and can be either primary (intergranular) or secondary (dissolution) in nature. Primary Porosity Secondary porosity

19. Porosity and Grain Size Grain size and sorting significantly affect the primary porosity of sand.