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Sedimentary Environments, Rocks, and Structures

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Sedimentary Environments, Rocks, and Structures Lab 2: Sedimentary Environments, Rocks, and Structures Sedimentary rocks account for a negligibly small fraction of Earth’s mass, yet they are commonly encountered because the processes that form them are ubiquitous in the near-surface environment. Thus, they preserve the history of that portion of the planet that is most familiar. Sedimentary rocks indicate paleoenvironment, i.e. ancient climates and ecology. Sometimes they provide the only remaining evidence of former mountain ranges or shallow seas. Sedimentary rocks are also essentially the only type of rock that contains fossils, which not only are indicative of previous environments, but also are crucial in dating and correlating rock units. Sedimentary rocks also provide a record of previous geologic hazards such as seismic events, volcanic eruptions (ash deposits), storms, and fluctuations in climate. Furthermore, key economic natural resources involve sedimentary rocks. Resources such as coal, oil, natural gas, gypsum, aggregate (sand and gravel), and salt are all found within sediments. Sedimentary Structures Sedimentary structures such as stratification (layering), ripple marks, cross-bedding, and mudcracks can be preserved in sedimentary rocks. These structures provide important information about depositional environments such as flow direction, climate (arid, semi-arid, or humid) and setting (e.g. fluvial, lacustrine, or marine). These structures also may indicate which direction was originally up within the rock. Tectonic forces can fold and overturn rocks, so establishing the original orientation is not always easy, but is often useful. Bedding Sedimentary rocks will often be deposited in discrete layers, which leads to a particularly important sedimentary structure called bedding. Bedding layers can range in thickness from millimeters to tens of meters. Typically, though not always, bedding is originally horizontal in orientation; tilted bedding indicates that the rock has been deformed in some way. Lithification Sedimentary rocks start out as loose sediment. To become a sedimentary rock, the sediments must be lithified, which involves compaction and cementation. Compaction occurs through pressure via deep burial. Water is removed and the grains are packed tightly together. During cementation, minerals such as quartz, calcite, or hematite precipitate out of water and fill the spaces between the clasts, locking them together. The term friable describes a poorly cemented rock that falls apart easily. Note: rocks can become friable either because they were never cemented thoroughly, or because the cement has been re-dissolved and removed. Depositional Environments A sedimentary environment is a geographic location characterized by a particular combination of geologic processes and environmental conditions. Geologic processes include the currents that transport and deposit sediments (water, wind, or ice) and the plate tectonic settings that affect sedimentation. For example, the geologic processes of a beach environment include the dynamics of waves crashing against the shore, shoreline currents, and the distribution of sediments on the beach. Environmental conditions include the kind and amount of water (ocean, lake, river, arid land), the landscape (lowland, mountain, coastal plain, shallow ocean, deep ocean), and biological activity. Location Depositional Characteristics Rock Type(s) Formed Environment Lakes Lake deposits are generally low-energy shale, siltstone (Lacustrine) environments where fine-grained sediments are deposited in thin layers. Lakes that freeze over seasonally may develop varves: alternating layers of light (coarser) and dark (finer) sediments. Large lakes can also have higher-energy, sandy beaches. Glacial Glacial deposits form along the margins of and Conglomerate and sometimes beneath glacial ice. Because ice can transport any breccia size grain (unlike water or wind), deposits are If glacial origin is known, the typically very poorly-sorted rock is called a “tillite” Alluvial Fan Alluvial fans are fan-shaped wedges of sediment Conglomerate, sandstone, and deposited along the margin of a steep slope. sometimes breccia They often contain a lot of coarse-grained, moderately to poorly-sorted sediment. Continental Rivers and Streams Rivers and streams typically deposit medium- to Sandstone, siltstone, some Environments (Fluvial) coarse-grained (sand to cobble-sized) sediment in conglomerate their channels. River sediment is often moderately- to well-sorted. Floodplains Floodplains are relatively low-energy Siltstone, shale environments where finer (clay, silt, fine sand) sediments are deposited in well-defined layers. These plains are only periodically wet and when they dry out mudcracks often develop. Swamps, Marshes, or Swamps are typically rich in organic material that Coal Bogs is buried and compressed to form coal Evaporite Basins Shallow basins in arid regions and lagoons may Rock gypsum, Rock salt, become supersaturated and precipitate evaporite crystalline limestone minerals. Desert (Eolian) Eolian environments are arid and typically have Sandstone, siltstone winds that transport and sort medium- and fine- grained sediment (sand to silt). Eolian sediments are often well-sorted and show well-developed cross-bedding. Deltas Deltas form where rivers and streams enter larger Sandstone, siltstone, shale bodies of water. They often contain fluvial-type Shoreline deposits as well as swampy environments. Environments Beaches Beaches occur on the margins of large bodies of Sandstone water. They generally contain deposits of well- sorted, medium-grained sediment with planar bedding. Shallow marine or Shallow marine environments are formed on the Sandstone, siltstone, shale, continental shelf margins of continents, on the continental shelf. diatomite, oolitic limestone, Associated deposits are typically medium- to chalk fine-grained and well sorted. In warm (sub- (Depends on supply of clastic tropical to tropical) environments these sediments and chemical sediments) may be calcite-rich. Deep marine Deep marine areas receive relatively little clastic Chert, shale Marine sediment. Common deep marine sediments are Environments either very fine-grained or microcrystalline (from recrystallization of microscopic silica-producing organisms). Reefs Organic structures composed of calcium Limestone (fossiliferous) carbonate-secreting organisms (i.e. coral) built up on continental shelves or oceanic volcanic islands Figure 7.2 Depositional Environments Clastic Sedimentary Rocks Clastic sediments are made of particles of mineral or rock fragments, known as clasts, that have been weathered from preexisting rock and transported by gravity, water, ice, or air. Chemical weathering involves the dissolution or decomposition of these minerals, whereas mechanical weathering consists of processes such as abrasion and cracking that do not change the mineral content of the material. During transport, clasts are abraded and become increasingly rounded (smooth surfaced) and equidimensional (spherical). Chemically and mechanically stable minerals, such as quartz, survive this transportation better than the less stable minerals and are therefore concentrated in sediments that have been transported for long times and distances. Transportation also tends to segregate particles by size. High-energy environments (e.g. rivers, coasts) can transport large clasts, while low energy environments (deep ocean) can transport only small clasts. Different transport processes can be more or less selective about which grain sizes are moved, therefore the degree of sorting can be indicative of the transport medium. For example, glacial ice is not selective at all and can move the widest range of clasts (tiny clay-sized particles to boulders the size of buildings), while wind typically moves grains sized within the narrow range of silt to fine sand. A sediment with clasts of uniform size is known as well-sorted, while one containing a wide range of clast sizes is poorly-sorted. Sediments that consist primarily of well-sorted, rounded, and spherical quartz grains indicate that the material has been subjected to long or repeated periods of transport and is designated as mature. On the other hand, sediments that consist of various minerals and rock fragments that are angular, non- spherical, and poorly-sorted are indicative of sediments that have not been transported far and are called immature. Factors relating to maturity are outlined in the following tables and figures: Stability of common minerals under surficial weathering conditions. Note that there is a relationship between this series and Bowen’s reaction series. The minerals formed at highest temperatures and pressures are the least stable, while those formed at lower temperatures and pressures (closer to surface conditions) are more stable. Most Stable Fe Oxides Al Oxides Quartz Clay minerals Muscovite Potassium Feldspar Biotite Sodium-rich Plagioclase Amphibole Pyroxene Calcium-rich Plagioclase Least Stable Olivine Degrees of rounding and sphericity. The degree of rounding is often a function of transport duration; the longer a clast is in transport, the more rounded it will become. The degree of sphericity also depends somewhat on transport duration, though some mineral grains start out more equidimensional than others. Quartz, for example, often is found in nearly equidimensional grains in granite, while amphibole and feldspar crystals are elongated. Guide to grain size. Note that (a) A conglomerate may instead be called
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