Sedimentary Rocks Are Derived from Sediment and Chemical Precipitates That Result from Weathering and Erosion Processes
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Sedimentary rocks are derived from sediment and chemical precipitates that result from weathering and erosion processes. Sediment and chemical precipitates accumulate in sedimentary basins, such as lakes or oceans, and are converted to sedimentary rock over time. Sediment and chemical precipitates can accumulate in a variety of sedimentary environments. The type of sediment or chemical precipitate can tell you a lot about the environment of deposition. For example think about where coal or limestone forms? What information can you infer from these rocks. Sediment is converted to sedimentary rock following three processes. 1. Accumulation of sediment in a sedimentary basin, such as this deltaic environment. 2. Burial and compaction of sediment as it accumulates. 3. Cementation of the sediment by cementing agents dissolved in groundwater.. Cementing Agents 1. Silica, 2. Fe-oxide 3. Calcite, 4. Clays 5. Tar (organics) How would you identify a specific cementing agent in rock. What is the predominant cementing agent of the sandstone shown in the upper image (note it does not react with HCl acid and is clear)? What is the predominant cementing agent of the sandstone shown in the lower image? Sediment may composed of various grain sizes that range from the smallest clays to boulder-sized clasts. The grain size of sediment provides information regarding the energy regime of transport. What can you infer about the stream’s energy regime during certain times of the year based on the size of the cobbles in the above image? This conglomerate formed from sediment deposited in a river channel. The clast lithology provides information regarding the eroded source rock and imbrication (“piggy- backing”) of clasts tells you the direction of paleo-stream flow. Note that this sedimentary rock classification chart is slightly different than the chart we provided in lab. It adds a “bioclastic” subdivision. Clastic sedimentary rocks are comprised of cemented “pieces” of weathered rock that was deposited into a sedimentary basing. Clastic sedimentary rocks are subdivided based on grain size. The conglomerate, shown above, may have grain sizes that range between gravels (diameter > 2 mm) and boulders. Sedimentary breccia has grain-sizes similar to conglomerate, except the clasts are angular and have not been subjected significant transport and erosion. Can you think of a natural environment of deposition where large clasts can be transported without significant erosion or abrasion. What does the degree of rounding or angularity tell you about distance of transport? Sandstone is clastic sedimentary rock comprised of cemented, sand-sized (0.5 – 2 mm) clasts. The sandstone pictured above is predominantly quartz clasts cemented with silica. How could you determine the cementing agent of given sandstone rock? What cementing agent holds the sand grains together in the above sandstone? What is the diagnostic property that helps you identify the cementing agent? Mineralogically mature sandstones are predominantly composed of quartz. Immature sandstones (arkosic), such as the sample shown above, will contain minerals, such as feldspar and micas, which over time will weather and be removed from the sediment. What does the maturity of a sandstone tell you about distance of transport? Siltstones form in low energy environments, such as lake of marine basins. They are often laminated (deposited in thin depositional layers). Shale or mudstone is deposited in the lowest energy environments and is typical of marine settings distal from the continental margin (i.e., terrerstrial sediment source). Note the fine-grained texture. The clay minerals (< 0.002 mm) are too small to see without magnification of a microscope. Chemical sedimentary rocks precipitate directly from an aqueous solution. Halite (common table salt) is produced an evaporite deposit in the desert lake basin in southeastern California. Large deposits of salt can accumulate in restricted marine basins, where water flow is restricted and evaporation high. The lighter salt can migrate upward into the denser rock forming bulbus domes or diapirs. Salt dome form important stratigraphic traps for Gulf Coast oil. Most limestones are biogenic and derived from calcite formed from biological processes, such as this fossiliferous limestone shown above. Inorganic limestone precipitates in warm waters because the solubility of calcite (calcium carbonate) is directly proportional to CO2 content in water. Warm water typically holds less CO2 than cold water, which causes the solubility of calcite to decrease. Chalk, such as the rock that comprises the “white cliffs” of Dover, England forms from the accumulation of microscopic calcareous marine organisms on the ocean floor. Oolitic limestone forms from oscillation (by wave action) of sand grains or shell fragments and the precipitation of concentric rings of calcite around the nucleating particle. Wave agitation causes the release of dissoved CO2 gas from ocean water, which reduces the solubility of calcite (it precipitates). The oolites are cemented together by calcite. Why does limestone tend to precipitation in shallow water and chert in deep ocean water? The calcium carbonate compensation depth (CCD) represents the abrupt transition of increased dissolved CO2 in ocean waters, which increases the solubility of CaCO3. Limestone will not precipitate below this depth and silica precipitation (formation of chert) dominates. Chert is composed of almost pure silica (SiO2) that tends to precipitate in deep ocean basins below the CCD. Chert, like obsidian and quartz, fractures conchoidally, which makes it an ideal material to construct arrowheads and axe blades Coal forms from terrestrial organics that accumulate in sedimentary basins where organic production exceeds organic decomposition. As organics accumulate peat form and is later converted to lignite and then bituminous coal with additional compaction and pressure. Anthracite coal is produced under the highest heat and pressure conditions. Why does sediment become stratified into depositional layers? Depositional layers signify the end of one depositional event and start of another. They can signal change in the sediment itself, such as the Triassic lake beds of the Painted Desert, AZ), or a shift in energy, such as waxing and waning flood of a river. Cross-beds consist of sedimentary layers deposited at an angle compared to the underlying beds. They form when sediment is deposited by a moving current such as a river or stream (shown on the left images) or wind. Cross-beds form when sediment is deposited by a moving current such as a river or stream or wind (such as the migrating dunes). Sand grains saltate (bounce) along the stoss- face and then avalanche down the steep, lee face, where wind velocities are reduced. The Navajo Formation in Arizona and Utah is comprised of sandstone that formed from migrating dune fields. Can you determine the paleo-wind direction based on the cross-bedding shown in the two images? Mudcracks will form when shrink-swell clays dry out during a period of dessication, such as an ephemeral lake. Deltas form when rivers flow into a still body of water (i.e., a lake or marine embayment) and deposit sediment as the stream’s energy is reduced. Deltas are subdivided into three distinct layers: 1. Topset, 2. Foreset beds, and 3. Bottomset beds. This post-glacial delta formed when melt water streams flowed from the Cascades into a proglacial lake dammed behind by the Puget Ice Sheet. Can you see the contact between foreset and topset beds? Ripples can from from currents (wind and water), such as those shown on the top right image or from wave oscillation, such as those shown on the image on the bottom right image. Which of these ripples patterns is comprised of asymmetric ripples and why do think asymmetric ripples form versus symmetric ripples? Graded beds form because large sediment grains settle faster in a water column than smaller sediment grain. In an event such as a submarine landslide, a turbidity current may be created with a chaotic mixture of sediment suspended in the water. It will settle out onto the bottom of the ocean floor and be graded with the largest grains on the bottom of the “turbidite layer.” Multiple turbidity currents can result in turbidite sequences that contain many units of graded beds. This turbidite sequence was deposited in a marine setting similar to the Puget Sound, which occupied the Central Valley of California. The turbidite sequence has been deformed by compressive stresses and compised the Vaca Mountains lying between the Central and Napa Valleys. Another important attribute of sedimentary rocks is the presence of fossils, most commonly found in fine-grained clastic sedimentary rocks or biogenic sedimentary rocks. Why would fine-grained sedimentary rocks most likely preserve fossils? Think about energy regime and presence or absence of oxygen. .