Sedimentary Rocks, Processes, and Environments Sediments are loose grains and chemical residues of earth materials, which include things such as rock fragments, mineral grains, part of plants or animals, and rust (oxidized iron). Formation of Sediments All sediments have a source or provenance, a place or number of places of origin where they were produced. Chemical weathering is the decomposition or dissolution of earth materials through chemical processes, and takes up certain chemicals into aqueous solution. Physical or mechanical weathering is the cracking, scratching, crushing, abrasion, or other physical disintegration of earth materials. Soil is an amalgamation of decaying organic matter and sediments. Erosion and Transportation of Sediments Weathered materials are transported via wind, water and other erosional processes, eventually to a site of deposition. *Transportation of sediments and distance from source affect sedimentary rock textures, i.e. grain sizes, shapes and arrangement. Deposition of Sediments Sedimentary Rocks are formed when sediments have been deposited and undergone lithification, a process that includes compaction (through burial) and cementation (mineral precipitation from intergranular aqueous solutions), or when masses of intergrown crystals precipitate from aqueous solution. Detrital sedimentary rocks form when buried mineral and/or rock fragments are compacted and perhaps cemented (e.g. sandstones), Chemical sedimentary rocks form when masses of intergrown mineral crystals precipitate from solution (e.g. limestones, cherts). Organic sedimentary rocks have a high proportion of fragments of shells, carbon, plants or any other organic remains. Sedimentary Rock Textural Descriptions Texture is the description of a sedimentary rock’s constituent parts and their sizes, shapes, and arrangement. Textural Features of Sedimentary Rocks and GeoTools sediment grain size scale 1. Grain Sizes Gravel: > 2 mm (granules, pebbles, cobbles and boulders) Sand: 1/16 mm – 2 mm (grains are visible and feel very gritty when rubbed) Silt: 1/256 mm – 1/16 mm (grains are not visible and feel gritty) Clay: < 1/256 mm. (grains are not visible, rock feel smooth) 2. Grain Shapes (angular, rounded or well rounded) 3. Grain Arrangement (well or poorly sorted) Environmental Interpretations The environment of deposition of a sedimentary rock can be inferred from the rock type, texture, fossil types, and sedimentary structures. The interpretation of environmental changes in the past from sedimentary rocks can help us understand environmental changes that are occurring right now. Fig. 1: The rock cycle once more. From Sediment to Sedimentary Rock – – – Transportation Movement of sediment away from its source, typically by water, wind, or ice Rounding of particles occurs due to abrasion during transport. Angular clastic fragments indicate very short travel, for example during a rock avalanche. Sorting occurs as sediment is separated according to grain size by transport agents, especially running water Fig. 2: Clastic sedimentary particle size response to gradient. Coarse sediment can only be transported when the gradient is steep. • • Preservation – Sediment must be preserved, as by burial with additional sediments, in order to become a sedimentary rock Lithification – General term for processes converting loose sediment into sedimentary rock – Combination of compaction and cementation Fig. 3: From sediment to compacted sediment to cemented sedimentary rock Graded bedding – Progressive change in grain size from coarsest at the bottom to finest at the top of a bed- deposition from a gradually slowing current Mud cracks – Polygonal cracks formed in drying mud- deposition in a subaerially exposed environment Ripple marks – Small ridges formed on surface of sediment layer by moving wind or water Fossils – Traces of plants or animals preserved in rock – Hard parts (shells, bones) more easily preserved as fossils • • • • Sedimentary Rock Interpretation • • Sedimentary rocks give important clues to geologic history of an area Source area – Locality that eroded and provided sediment – Sediment composition, shape, size and sorting are indicators of source rock type and relative location Depositional environment – Location where sediment came to rest – Sediment characteristics and sedimentary structures (including fossils) are indicators • – Examples: glacial valleys, alluvial fans, river channels and floodplains, lakes, deltas, beaches, dunes, shallow marine, reefs, deep marine Continental Environments: glacial, alluvial fans, river channels and floodplains, lakes, dunes Transitional Environments: beaches, deltas Marine Environments: shallow marine, reefs, deep marine Plate Tectonics & Sedimentary Rocks • • • • Tectonic setting plays key role in the distribution of sedimentary rocks Occurrence of specific sedimentary rock types can be used to reconstruct previous platetectonic settings Erosion rates and depositional characteristics give clues to each type of tectonic plate boundary Fig. 4: Tectonic environment around a subduction zone and areas of sediment deposition Fig. 5: Tectonic environment around a rift zone and places of sediment deposition. Note: evaporates can also form in intercontinental basins and cut-off ocean arms. Fig. 6: Depositional areas around a mountain belt and marine environment. Table 1: Sedimentary Rock Samples # Rock Name 1 Conglomerate 2 Breccia 3 Sandstone 4 Arkose 5 Shale 6 Rock Gypsum 7 Chert 8 Dolostone (dolomite) 9 Micrite (limestone) 10 Fossiliferous limestone 11 Coquina 12 Bituminous coal Type of sedimentary rock (detrital, chemical, or organic) Distinguishing properties Question/assignment 1: become familiar with 12 samples. Identify the type of sedimentary rock (detrital, chemical, organic) and the distinguishing properties in the table above. Assignment 2: Now try these 6 unknowns. Sample ID Name Type of sedimentary rock Environment of deposition Texture Distinguishing properties UK-1 UK-2 UK-3 UK-4 UK-5 UK-6 Question 3: What does the presence of feldspar in an arkose tell you about the conditions of the environment when this sediment was formed and eventually became a sedimentary rock? Question 4: Comparing conglomerate and breccia, what do you think were the conditions of their formation? Question 5: From what you learned in the previous labs (igneous rocks and minerals), and from the stability of minerals that form early in a magma, versus those that form late, why is it so rare to find beach sands that olivine in them? Question 6: List 3 locations (countries or states) in the world today where evaporates are forming. a) b) c) Question 7: What was the environment when the old forests grew that eventually turned into coal deposits? Let’s now move on to an application! One of the ways sedimentary geologists can quantitatively describe the grain size distribution of a sediment sample is by ‘sieving’. Sieves are used to divide the sediment sample into classes of similar sizes. - At the front of the room there is a stack of sieves. Each sieve has a different mesh size that will only allow grains smaller than it to pass through. The meshes range from 1400 micrometers on the uppermost sieve to (1.4mm) to a catch pan at the bottom for particles smaller than all the sieves. - First, take a small portion of either sample alpha or beta and weigh Record this bulk weight here: _____________ grams. (remember to tare the cup first) - Pour that sample into the stack of sieves and shake the column gently for about 2 minutes. - You will then remove each sieve pan, turn it over onto a piece of paper, brush the sieve to knock off any lodged grains, tare the cup, dump the sediment from the paper into the cup, and weigh that sample. Repeat for each pan and record your results here: Size Class Grain Diameter (d) A d ≥ 1400 µm B 1400 µm > d ≥ 600 µm C 600 µm > d ≥ 425 µm D 425 µm > d ≥ 250 µm E 250 µm > d ≥ 150 µm F 150 µm > d ≥ 90 µm G 90 µm > d Weight Weight Percent it. -use the values of weight percent from your table to make a histogram for the grain size distribution (remember, in histograms the bars should touch each other) GRAIN SIZE DISTRIBUTION FOR SAMPLE ______________ 75 50 25 0 A B C D E F G Size Class (A > G) Question 8. Using the graph above, explain whether you think your sample is poorly sorted, moderately sorted, or well sorted. Now make an interpretation: do you think this sediment was proximal or distal to its source?
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