Lecture 2 Notes: Sediment Production

Sedimentary Rock and the Production of Sediment Sedimentology = scientific study of sediments and sedimentary rocks. Includes production of sediment, transport, deposition, & lithification. Provides the basis for geological interpretations of earth-surface signals. Stratigraphy = scientific study of sedimentary strata Sedimentary rocks have an average thickness of about 1800 m on the continents. This thickness is quite variable, with some areas like the Canadian Shield having no cover of sedimentary rocks, and other areas, like the Louisiana and Texas Gulf coasts, having more than 20,000 m of sedimentary rock cover. About 66% of all continental areas have a cover of sedimentary rocks. Older sedimentary rocks are exposed over a smaller area than younger sedimentary rocks. Over 40% of the exposed sedimentary rocks are younger than Cretaceous in age. I. Three Rock Types: Sedimentary, Metamorphic, Igneous II. Types of Sedimentary Rocks: A. Clastics (fragmental) – Detrital Particles – Derived from preexisting rocks – Derived external to the depositional basin 1 B. Chemical: Allochemical Particles biochemical origin Ooids, fossil fragments, pellets, pelagic tests (siliceous and calcareous) C. Chemical: Orthochemical Components Chemical Precipitates Secondary cement Primary chemical sediments: halite, etc D. Organic Particulate Material (detrital organic matter ) terrestrial and particulate marine pelagic Structureless gypsum. Coal 2 GRAIN SIZE SCALES FOR SEDIMENTS III. Class sizes: The grade scale most commonly used for sediments is the Wentworth scale (actually first proposed by Udden), which is a logarithmic scale in that each grade limit is twice as large as the next smaller grade limit. For more detailed work, sieves have been constructed at intervals 2 and 2 . The φ (phi) scale, devised by Krumbein, is a much more convenient way of presenting data than if the values are expressed in millimeters, and is used almost entirely in recent work. φ Wentworth Size Class 4096 1024 -12 -10 Boulder (-8 to -12 φ ) Use wire 256 64 -8 -6 Cobble (-6 to -8φ ) squares 16 -4 5 6 7 8 10 12 14 16 18 20 25 30 35 40 45 50 60 70 80 100 120 140 170 200 230 270 325 4 3.36 2.83 2.38 2.00 1.68 1.41 1.19 1.00 0.84 0.71 0.59 0.50 0.42 0.35 0.30 0.25 0.210 0.177 0.149 0.125 0.105 0.088 0.074 0.0625 0.053 0.044 0.037 0.031 0.0156 0.0078 0.0039 0.0020 0.00098 0.00049 0.00024 0.00012 0.00006 -2 -1.75 -1.5 -1.25 -1.0 -0.75 -0.5 -0.25 0.0 0.25 0.5 0.75 1.0 1.25 1.5 1.75 2.0 2.25 2.50 2.75 3.0 3.25 3.5 3.75 4.0 4.25 4.5 4.75 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 Millimeters Analyzed by Pipette or Hydrometer 1⁄2 1⁄4 1/8 1/16 1/32 1/64 1/128 1/256 Microns 500 420 350 300 250 210 177 149 125 105 88 74 62.5 53 44 37 31 15.6 7.8 3.9 2.0 0.98 0.49 0.24 0.12 0.06 Pebble (-2 to -6φ ) Granule Very coarse sand Coarse sand Medium sand Fine sand Very fine sand SAND U.S. Standard Sieve Mesh # G R AV E L 4 Coarse silt Medium silt Fine silt Very fine silt Figure by MIT OCW. Courtesy of MIT OCW. Used with permission. MUD 2 clay 1 VI. Sorting: refers to range of sizes in a rock. Well sorted vs. moderately sorted vs poorly sorted Poorly sorted Very well sorted VII. Rounding: angular vs. subangular vs subrounded vs rounded Very angular Sub-rounded Well rounded Courtesy of SEPM (Society for Sedimentary Geology). Used with permission. after Powers, M . C., 1953, Journal of Sedimentary Petrology, v. 23, p. 118. Vlll. Sediment vs, Sedimentary rock: gravel vs. conglomerate sand vs. sandstone silt vs siltstone clay vs claystone (or shale) fine grained (silt and mud) rock is commonly referred to as mudstone 4 IX. Sandstone Composition Field composition terms for sandstone: Quartzite (>50% quartz grains); Arkose or Feldspathic sandstone (<50% quartz & more feldspar than lithic grains); Lithic sandstone (<50% quartz & more lithic grains than feldspar). provenance (determining source areas) Earth surface can be thought of as a giant chemical reactor. About 20% of earth’s crust is composed of quartz, > 60% is feldspar. However, Quartz is dominant mineral in siliciclastic rocks. Bottom line. The geochemistry of sedimentary rocks is less complicated than that of igneous and metamorphic rocks. WEATHERING AND SEDIMENTARY ROCKS Weathering - Processes acting at the earth's surface to decompose and breakdown rocks. Types of Weathering Mechanical or Physical - the breakdown of rock material into smaller and smaller pieces with no change in the chemical composition of the weathered material. Chemical - the breakdown of rocks by chemical agents (e.g., water). Physical weathering breaks rocks down into smaller pieces thus increasing the surface area over which chemical weathering can occur. Relative Contributions: Mechanical: 5.6×1015 g/yr Chemical: 4.0×1015 g/yr Roughly equal, but not equally distributed over earth surface. Production of Sediment via Weathering Controls Precipitation Temperature Relief Dependant on Latitude Geography Oceanography 5 Physical Weathering is most significant in: Cold dry high relief areas Protolith composition influences sediment production rate Thermal Expansion and Contraction – leads to fracturing Unloading – causes fracturing. Example: Half Dome, Yosemite, CA. Frost Action - Water in cracks freezes and expands, wedging apart rocks. Abrasion – Impacts and grinding by moving particles. Organic - Cracking of rocks by plant roots and burrowing animals. Chemical Weathering Factors influencing rate of chemical weathering are: -Particle size - Smaller the particle size the greater the surface area and hence the more rapid the weathering -Composition -Climate - temperature, water -Type and amount of vegetation The three common chemical reactions associated with chemical weathering are dissolution, hydrolysis, and oxidation. 6 Dissolution Dissolution of soluble minerals, commonly in the presence of CO2. Cations and anions in solution are transported by fluid away leaving a space in the rock (e.g., caves in limestone). CaCO3 + H2O + CO2 ↔ Ca 2+ + 2HCO3− Formation of carbonic acid Carbon dioxide (CO2) from the air is dissolved in rainwater to create a weak acid, carbonic acid (H2CO3). All rain is mildly acidic (average pH ~5.6). Interlinked reactions for combining water and carbon dioxide and the two-stage ionization (dissociation) of carbonic acid: H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3− ↔ 2H+ + CO32− Hydrolysis Feldspar, the most common mineral in rocks on the earth's surface, reacts with free hydrogen ions in water to form a secondary mineral such as kaolinite (a type of clay) and additional ions that are in solution. 4KAlSi3O8 + 4H+ + H2O Æ 4K+ + 2Al2Si2O5(OH) 4 + 8SiO2 orthoclase + hydrogen ions + water Æ K+(aq) + Kaolinite (clay) + silica CaAl2Si2O8 + 2H+ + H2O Æ Ca++ + Al2Si2O5(OH) 4 anorthite + hydrogen ions + water Æ Ca++(aq) + Kaolinite (clay) Simplified global weathering equation (the Urey equation) CaSiO3 + 3H2O + 2CO2 ↔ Ca2+ + 2HCO3− + H4SiO 4 Calcium silicate system (including Ca-plagioclase) Oxidation Loss of an electron from an element (commonly Fe or Mn), typically forming oxides or hydroxides. Loss of electron by a metal {ferrous to ferric state} 4Fe+2 + 302 Æ 2Fe2O3 4FeSiO3 + O 2 ↔ 2Fe 2O3 + 4SiO 2 (pyroxene) (hematite) (silica) 7 Goldich Weathering Stability Series Predicts relative abundance of particulate residues produced in a (typical) weathering environment from rock-forming, protolith minerals. Approximately inverse of Bowen’s Reaction Series. Resistant Particulate Residues Stable (with respect to surface conditions) primary mineral grains; quartz, feldspar, rock fragments Ions in Solution Ions introduced into the surface and ground water by chemical degradation of primary mineral grains Congruent solution: Produces only ions in solution (NaClÆ Na+ + Cl-) Incongruent solution : Ions in solution + new mineral phase Insitu Minerals (minerals formed in place) Clay Minerals : hydrous aluminum silicates Oxides Hematite - iron oxide; Goethite/Limonite – hydrated iron oxide; Gibbsite - aluminum hydroxide Amorphous Silica 8