Sedimentary Rocks Classification PDF

Sedimentary Rocks ES301 3rd level Lectures 4 & 5 Edited and collected by Safiya M. Hassan Beni Suef University 2024  The Classification of Clastic Sedimentary Rocks A very basic classification of all sedimentary rocks is based on the type o...

Sedimentary Rocks ES301 3rd level Lectures 4 & 5 Edited and collected by Safiya M. Hassan Beni Suef University 2024  The Classification of Clastic Sedimentary Rocks A very basic classification of all sedimentary rocks is based on the type of material that is deposited and the modes of deposition. Classification based on grain size A simple classification of terrigenous clastic rocks and sediment is based on the predominant grain size of the material: Grain Sediment Rock Name Adjectives Size1 name (mm) >2 Gravel Rudite Cobble, pebble, well sorted, etc. 0.0625-2 Sand Arenite Coarse, medium, well sorted, etc. < 0.0625 Mud Mudstone Silt or clay or Lutite 1For the purposes of this general classification we will assign the rock or sediment name shown if more than 50% of the particles are in the range shown. More detailed classification schemes will limit terms on the basis of different proportions of sediment within a given grain size. Classification of Sandstones Most sandstone classifications are based on the composition of the rock. Dott’s classificaton scheme is used in most courses at Brock. It is based on the relative proportions of: Martrix (fine-grained - 95% Quartz = Quartz arenite 25% Feldspar = arkosic composition Arkose >25% Feldspar = mineralogically immature Feldspar Lithics PETTIJOHN CLASSIFICATION Based on percentage of matrix Matrix poor (15%) – wackes Wackes or (greywackes) Tend to be dark in colour Are poorly sorted Clastic sedimentary rocks can be made up of “multicycled” particles. i.e., have passed through the rock cycle several times. Each time through the cycle the sediment becomes more and more mature. Sediment texture and mineralogical composition all reflect the maturity of a sediment. Most changes are related to transport distance, nature of weathering at the site of sediment formation and number of passes through the rock cycle. i) Textural Maturity Changes in grain size and shape. Increasing textural maturity Increased sorting Increased rounding Increased sphericity  The name of a sandstone tells you something of its maturity. E.g., a Quartz arenite has less than 15% matrix and is better sorted than a Quartz graywacke. The quartz arenite is more mature (greater transport distance and/or more times through the rock cycle) than the Quartz graywacke. ii) Compositional Maturity Compositional maturity is reflected by the relative proportion of physically soft or chemically unstable grains. The fewer the soft or unstable grains, the more mature the sediment. The most “mature” sediment would be made up of 100% quartz grains. With increased transport and number of times through the rock cycle the less stable minerals are lost. The “average” igneous and metamorphic rocks contain 60% feldspars. The “average” sandstone contains 12% feldspars. This reflects the fact that many sandstones are made up of particles that have been through several passes of the rock cycle. b) Provenance of a sediment Provenance: where something originated. The Provenance of a sediment is inferred from aspects of composition that reflect the source rock and tectonic and climatic characteristics of the source area for the sediment. i) Tectonic setting The source rock of a sediment and the tectonic setting are closely linked: the tectonic setting determines the relative abundance of different types of rock that is available for weathering and the production of clastic sediment. e.g., An arkosic sandstone (rich in feldspars) would have a source area that is rich in granites. A mountain chain adjacent to a convergent margin (e.g., modern Andes)? An exposed craton (e.g., the Canadian Shield)? Not foolproof! These are two very different tectonic settings. e.g., a sandstone with abundant volcanic and low grade metamorphic rock fragments. Island arc setting. Quartz arenite: sedimentary source rocks; uplifted sediments in an orogenic belt. Major types of sandstone described by Levin: Sandstone Environmental Interpretation Each type of sandstone implies something about depositional history and environment: Quartz sandstone implies a long time in the depositional basin. Deposition typically in tectonically stable, shallow-water environments. Common sedimentary structures are ripple marks and cross-bedding. Arkose implies a short time in the depositional basin (because feldspar typically weathers quickly to clay). Also implies rapid erosion, arid climate, tectonic activity, steep slopes. Commonly deposited in fault troughs or low areas along granitic mountains. Often has a pinkish color due to oxidized iron, suggesting continental deposition. Graywacke implies a tectonically active source area and depositional basin, with rapid erosion. Graded bedding is common. Associated with volcanic rocks, shales, and cherts of deep water origin. Lithic sandstone is characteristic of deltaic coastal plains, and may be deposited in nearshore marine environments, swamps, or marshes. Associated with coal and micaceous shales. Geologic settings where various sandstones are deposited. A = Quartz sandstone B = Arkose C = Graywacke D = Lithic sandstone ii) Climate Climate exerts a strong control on the type of weathering that takes place in the source area of a sediment; this, in turn, influences composition. Cold, arid climate: predominantly physical weathering, producing abundant detrital grains (unaltered mineral grains and rock fragments). Sandstones produced in such settings will be relatively immature, depending on the source rocks. Warm, humid climate: chemical weathering predominates. Unstable minerals removed from the sediment that is produced by weathering. Will produce a more mature sediment than a cold climate. Climate: colder in the north so that physical weathering is important, producing immature sediment. Many sediments were produced during glaciation which only breaks down source rocks by physical processes. Warmer in the south so that chemical weathering produces a more mature sediment. III. Which classification should you use? This depends on the purpose of the study that you are participating in. Most studies aimed at determining ancient depositional environments can classify sandstones on the basis of grain size only. Studies that aim to reconstruct ancient tectonic settings require a detailed analysis of the composition of the sandstones. Some studies require compositional classification in order to understand the mechanical properties of the sandstone (e.g., if the study aims to determining excavation costs). References 1)Tucker, M 1981.Sedimentary Petrology: An Introduction.252 pp, Oxford:Blackwell. 2) Tucker, M. (2001). Sedimentary Petrology.3rd edition, Blackwell publishing company. 3) Pettijone, F.J., Potter and Siever, R. (1987).sand and Sandstones 2nd edition, springer-verlag 4) Cheel, R.J. (2005). Introduction to Clastic Sedimentology. 5) Adams,A.E., Mackezie, W.S. & Guilford, C. (1984) atlas of sedimentary Rocks under the Microscope. Additional References Nichols, G. (2009). Sedimentology and stratigraphy. 2nd edition. Blackwell Publishing company.