Chapter 7- Sedimentary Rocks SUNY Planet Earth
During a hike in Zion National Park, you pick up a sedimentary rock sample. You see that the rock consists mainly of rounded glassy particles that appear to be quartz. You make two basic tests to check. You check for hardness and it easily scratches glass, which quartz does. However when you place a drop of acid, it fizzes. Explain how a rock that appears to be rich in quartz could effervesce with acid.
A rock that appears to be rich in quartz could effervesce with acid in the case that it is cemented together with other particles such as calcite that do not react with the acid.
Why might a single layer exhibit different types of sedimentary rocks? What term applies to the different parts of such a layer?
A single layer may exhibit different types of sedimentary rocks because we can see the successive changes in environmental conditions that occurred in environmental conditions during a particular passage of time, and they are traced laterally. The term facies is used to describe how different parts of such layers possess a distinctive set of characteristics that reflect the conditions in a particular environment.
Outline the successive stages in the formation of coal.
Accumulation of Plant Remains One important environment is a swamp, because dead plants easily decompose when exposed to the oxygen rich atmosphere, and swamp water is oxygen deficient. The plants are then attacked by certain bacteria that partly decompose the organic material and liberate oxygen and hydrogen. This causes the carbon levels to go up. Formation of peat The partial decomposition of plant remains lead to layer of peat, which is a soft brown material in which the plants are easily identifiable still. Peat then slowly changes to lignite with shallow burial. This burial raises the temperature of sediments and pressure on them. Formation of lignite and bituminous coal The high temperatures lead to chemical reactions within the plant material that yields water and organic gases (volatiles). And as the load increases, the water and gases are pressed out, and the proportion of fixed carbon increases. This coal becomes more compact and the greater the carbon the greater the energy source it is. Formation of anthracite coal When these coals are folded and deformed with association to mountain building, the heat and pressure lead to further loss of water and volatiles. Turning this coal into anthracite, a very hard, shiny, black metamorphic rock.
List three common cements. How might each be identified?
Calcite, silica and iron oxide are the most common cements. Calcite will effervesce with dilute hydrochloric acid. Silica is the hardest cement and thus produces the hardest sedimentary rocks. An orange or dark red color often indicates that iron oxide is present in sedimentary rocks.
Distinguish between clastic and non clastic. Which texture is associated with all detrital rocks?
Clastic rocks display a texture of discrete fragments and particles that are cemented and compacted together. Although cement is present, these openings are rarely completely filled. All detrital rocks have a clastic texture. Non clastic rocks have minerals that form a pattern of interlocking crystals. These particles probably consisted of shell fragments and other fragments rich in calcium carbonate and silica.
What minerals are most abundant in detrital sedimentary rocks? In which rocks do these sediments predominate?
Clay minerals and quartz are the most abundant in detrital sedimentary rocks. Clay minerals are most abundant products of the chemical weathering of silicate materials, especially in feldspars. When igneous rocks such as granite are attacked by weathering processes, individual quartz grains are formed.
What is the "raw material" for coal? Under what circumstances does it accumulate?
Coal often consists of plant structures such as leaves, bark and wood that have been chemically altered but are still identifiable. Meaning that it accumulates by large amounts of plant material in a swamp usually, buried for millions of years.
Compaction is the most important as a lithification process with which sediment size?
Compaction is the most important as a lithification process with fine-grained sedimentary rocks. This is because sands and other coarse sediments are less compressible.
Distinguish between conglomerate and breccia.
Conglomerate consists largely of gravels, and these particles are usually large enough to be identified as specific rock types. They are usually poorly sorted because the opening in between contains either sand or mud. They contain coarse particles that reflect the action of energetic mountain streams or strong wave activity along a rapidly eroding coast. Breccia consists largely of large angular particles, the pebble and cobbles indicate that it did not travel far from their source area before being deposited.
List specific examples associated with each category.
Continental Sand dunes, landslides, playa lakes, flood plains, alluvial fans, eolian Marine Swamps, bogs, coral reefs, shallow and deep marine environments Transitional Beaches, bars, spits, tidal flats, barrier islands, lagoons, deltas
What are the three broad categories of sedimentary environments?
Continental, marine and transitional environments.
What is the difference between cross-bedding and graded bedding?
Cross Bedding is when a sedimentary rock has layers within it that are accumulated in horizontal beds. Graded bedding is when particles within a single sedimentary layer gradually change form coarse at the bottom to fine at the top. They are also most characteristic of rapid deposition from water containing sediment of varying size
List and briefly distinguish among the three basic sedimentary rock categories
Detrital Sedimentary Rocks- Accumulation of materials that originates and is transported as solid particles derived from both mechanical and chemical weathering. Chemical Sedimentary Rocks- Soluble material produced largely by chemical weathering. When these ions in solution are precipitated by either inorganic or biological processes, it is chemical sediment. Organic Sedimentary Rocks- Form from the carbon rich remains of organisms. The primary example is coal, which forms from the remains of plants that died and accumulated over time.
What is diagenesis?
Diagenesis is the chemical, physical and biological changes that take place after sediments are deposited and buried/ lithification.
How do evaporites form? What are some examples?
Evaporites form as seawater continually moved into the bay to replace water lost by evaporation. Leaving the waters of the bay to become saturated, causing the salt deposits began. Some examples are in halite, and gypsum. Halite is often used in cooking, or to melt ice on roads. Gypsum is the chief ingredient in plaster of Paris, used for wallboard and interior plaster. In Death Valley, California streams flow into enclosed basins, and as the water evaporates, salt flats form when dissolved materials are precipitated as a white crust on the ground.
If you hiked to a mountain peak and found limestone at the top, what would that indicate about the likely geologic history of the rock atop the mountain?
If you found limestone at the top of a mountain it would indicate that at some point in that past, the mountain was underwater. As limestone originates primarily from marine biochemical origin.
One of the labeled sedimentary layers in the accompanying photo is sandstone and the other is mostly shale. How can you determine which one is which just by looking at the photo?
Layer 1 is sandstone and layer 2 is shale. You can determine this by looking at the formation, as shale crumbles very easily as seen in the photo.
Distinguish between limestone, dolostone, and chert. Describe several variables of each.
Limestone When placed with a drop of hydrochloric acid, it fizzes Composed mainly of the mineral calcite and forms by either inorganic and biochemical means Can result from coral or fossils Dolostone When placed with a drop of hydrochloric acid, it will not visibly react Produced when magnesium rich waters circulate through limestone and convert calcite to dolomite by replacing calcium ions with magnesium ions Chert Made of microcrystalline quartz One well known form is flint, which is usually of a dark color resulting from organic matter Chert is durable and can easily preserve shapes Usually found in layered deposits of bedded cherts or as nodules.
How might mud cracks and ripple marks be useful clues about the geological past?
Mud cracks indicate that they were formed in an environment that was alternately wet and dry. Ripple marks are produced by either a stream flowing across a sandy channel, or the wind blowing over a sand dune. Both giving clues to their geological past and where they formed, and their environments.
What is the primary basis for distinguishing among detrital rocks?
Particle size is the primary basis for distinguishing among detrital rocks. It is convenient and provides useful information about environments of deposition.
Sandstone is poorly sorted and rich in feldspar and quartz. Which is the most likely site and explain your choice. What type of name is given to this sandstone? Site #1: A nearby exposure of weathered basaltic lava flows Site #2: An outcrop of granite at the previous field trip stop up the road.
Site 1 because it is poorly sorted, meaning that it did not move very far from its origin, which is a nearby exposure. As well it is rich in feldspar which is common in basaltic lava rock. The type of name given to this sandstone is Greywacke.
Describe how sediments become sorted.
Sorting refers to the degree of similarity in a particle size in a sedimentary rock. If they are the same size, its well sorted. If they contain large and small particles, it is said to be poorly sorted. Deposits of windblown sand are usually better sorted than deposits sorted by wave activity, and waves are commonly better than streams. Poorly sorted rocks usually occur when the particles are distributed rapidly after being transported over a short period of time.
Explain how the formation of biochemical sediments differs from the formation of sediments formed by inorganic processes. Include examples as part of your explanation.
The precipitation of materials can occur by inorganic processes, through evaporation and chemical activity which produce chemical sediments. These are things such as dripstone that decorates many caves, or salt left behind as seawater evaporates. It can also occur by organic processes of of water- dwelling organisms, which is said to be of biochemical origin. These can be seen as water dwelling plant and animal species extract dissolved mineral matter to form shells and other hard parts.
What is the primary basis for distinguishing (naming) different chemical sedimentary rocks? How is the naming of detrital rocks different?
The primary basis for naming chemical sedimentary rocks is on texture, breaking them into subcategories of clastic and crystalline. Which is different compared to detrital rocks which are broken and named based on their particle sizes.
What is the single most characteristic feature of sedimentary rocks?
The single most characteristic feature of sedimentary rocks are layers they form in known as strata or beds.
Examine the accompanying sketch, which shows three sedimentary layers on the ocean floor. What term is applied to such layers? What process was responsible for creating these layers? Are these layers more likely part of an offshore lagoon or a deep sea fan?
These layers are known as facies, and the process responsible for creating these layers was transportation by dense water currents of suspended sediment. These are most likely part of a deep sea fan as each layer has coarser particles at the bottom and finer materials on the dop.
Outline the steps that would transform an expose of granite in the mountains into various sedimentary rocks
Weathering Of the granite begins the process with physical disintegration and chemical decomposition of preexisting rocks Soluble continents are carried away Solid particles of the granite are moved downslope by gravity by a process called mass wasting, and move from their origins to places where they accumulate Deposition Of the solid particles occur when wind and water currents slow down and as glacial ice melts Deposition continues Other sediments are buried beneath younger layers of granite and other rocks and gradually converted into sedimentary rocks by compaction and cementation. These changes are referred to as diagenesis.