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Lab 5: Sedimentary Rocks
Attempt History
Attempt Time Score LATEST Attempt 1 73 minutes 14.92 out of 15 Score for this attempt: 14.92 out of 15 Submitted Feb 9 at 2:04pm This attempt took 73 minutes.
Introduction
In the igneous rock lab, we focused on tectonic processes that influence rock formation inside of the earth’s crust. Now we will turn to the earth’s surface, where rock weathers into smaller rock fragments and minerals. The solid particles and dissolved ions (charged atoms and molecules) freed by weathering processes are transported by water, wind, gravity, and glaciers from their source (that is, the original rock) to depositional basins (the places where sediment accumulates). This collection of loosely packed, unconsolidated mineral or rock fragments is called sediment. In time, sediment is buried and lithified to form sedimentary rock. Careful examination of the mineral composition and texture of many sedimentary rocks provides clues to the: (1) original source of the sediment ( provenance ); (2) type and extent of the weathering processes by which the source rock was broken down; (3) agent (water, wind, gravity, or ice) that transported the sediment and, in some cases, the duration of transport; (4) physical, chemical, and biological environment in which the sediment was deposited, and; (5) changes that may have occurred after deposition ( diagenesis ). Sedimentary rocks are a window into ancient environments. They contain traces of evidence that reflect what the earth was once like. For example, fossilized bones in some sedimentary rocks show that dinosaurs once roamed the earth. Therefore, the characteristics of a sedimentary rock reflect the environment in which that rock formed.
The texture, size and shape of sedimentary clasts (or rock fragments) can tell us about the environment in which a sedimentary rock formed. For example, if the clasts that make up a sedimentary rock look like rounded river rocks, then we might conclude that that sedimentary rock formed in a riverbed (Figure 5-1). Figure 5 - 1 Rounded rocks in a conglomerate, a type of sedimentary rock that usually forms in a river environment. The mineral composition of sedimentary clasts can tell us where the clasts came from (provenance). For example, if sedimentary rocks in Oregon contain clasts that are mineralogically identical to igneous rocks in British Columbia, then we could infer that a river or some other transport mechanism once connected the two locations.
Sedimentary rocks can be divided into three different categories,
depending on what type of sediment(s) they are made of:
- Clastic (or detrital ), composed of clasts (broken pieces) of other rocks.
- Chemical and biochemical , formed by the precipitation of ions.
- Organic , composed of organic materials.
Section 1: Weathering
Weathering is a family of rock breakdown processes that occur at or near the earth’s surface. Mechanical weathering (also called physical weathering) is when rock physically breaks into smaller pieces. Chemical weathering is when chemical reactions, such as oxidation or hydrolysis, dissolve or alter minerals.
Mechanical Weathering
halite orthoclase calcite muscovite 0: Mineral 1 Mineral 2 Mineral 3 Mineral 4 Mineral 5 Question 2 1 / 1 pts 0:00 / 0: quartz
Use the following diagnostic properties to identify Mineral 6 (pictured below), a mineral commonly found in sedimentary rocks. Refer to your mineral identification charts (Appendices A- 1 (https://canvas.uw.edu/courses/1699809/files/113584328/download) , A- 2 (https://canvas.uw.edu/courses/1699809/files/113584462/download) , and A- 3 (https://canvas.uw.edu/courses/1699809/files/113584462/download) ) and the diagnostic properties of sedimentary rock-forming minerals (Table 5-2). Luster What is the luster of this mineral? vitreous Streak This mineral has a white streak. Hardness
This mineral often breaks down into individual sheets. How many directions of perfect cleavage does this mineral have? 1
This mineral is gypsum.
Answer 1: metallic vitreous resinous Answer 2: yes no Answer 3: 1 2 3 Answer 4: quartz biotite muscovite calcite halite gypsum kaolinite Question 3 1 / 1 pts
Different minerals weather and break down at different rates depending on how stable or
reactive they are at the surface of the Earth. A given mineral’s stability or reactivity at the
surface of the Earth can be predicted from the conditions in which it formed. For minerals
that form in igneous rocks, the closer the the conditions of formation are to
conditions at the earth's surface, the more stable the mineral. We can relate this
concept back to Bowen’s Reaction Series.
Orthoclase (Felsic) Biotite(Intermediate) Pyroxene (Mafic) Chemical Weathering Rank the following minerals from most stable to least stable: Most stable Intermediate stability Least stable Question 4 0.5 / 0.5 pts Weathering, both mechanical and chemical, is the first step towards forming sedimentary rocks. It is also the first step in processes that form landscapes. Below are two images of rock that have undergone weathering. Rock A is composed of mostly of mineral M3, while Rock B is an intrusive igneous rock. What form of weathering is primarily responsible for the shape of each rock that you see. Rock A Rock B
Table 5 - 1. Grain size chart for very fine-, fine-, medium- and coarse-grained sediments. Question 5 0.5 / 0.5 pts Below are images of two rivers. Note the humans for scale. River A has a fine grain size which indicates it is farther from the river's headwaters. River B has grains that are and likely the river's headwaters. Answer 1: coarse medium fine Answer 2: coarse-grained medium-grained fine & very fine-grained grain size >^2 mm^2 mm to^62 microns^ <62^ microns names granules (2- 4 mm), pebbles (4-64 mm), cobbles (64- 256 mm), and boulders (>256 mm) sand silt^ (2-^62 microns)^ and clay (<2 microns) photo drawing
farther from closer to Answer 3: smaller than 2mm diameter larger than 2mm diameter Answer 4: farter from closer to
Grain Rounding
In Lab 3, we saw that minerals can have different hardness and that harder minerals can scratch softer minerals. When mineral clasts first break off of a source rock, they are typically angular due to cleavage and crystal habit patterns. However, as clasts travel through sedimentary systems like rivers and glaciers, they come into forceful contact with other clasts, resulting in scratching. Scratching, also called abrasion, causes originally angular particles to become rounded over time. As a particle spends more time in a sedimentary system, it becomes increasingly rounded as it accumulates collisions. This concept of rounding applies to individual mineral grains, as well as rock fragments. Figure 5-2 shows how we classify grain roundness. Figure 5 - 2 Roundness Classification Scheme. Question 6 0.5 / 0.5 pts
is softer than the sand on the left has undergone chemical weathering Answer 5: more rounded more angular Answer 6: harder softer
Grain sorting
Grain sorting refers to the distribution of grain sizes within sedimentary rocks. If the clasts are all about the same size, the grains are well sorted, whereas if the clasts are different sizes, the grains are poorly sorted. Figure 5-3 shows examples of how we classify grain sorting. Figure 5 - 3 Grain sorting in sedimentary rocks. How does sorting occur in natural environments? Since it requires more energy to move larger, coarse - grained particles, low-energy stream systems often leave them behind. As a result, the grain size of the clasts in clastic sedimentary rocks can tell us about the energy of the environment wherein the rock was formed. Imagine a wave crashing into a beach. The relatively high energy of the wave stirs up the beach sediment, and then the seawater drains off of the beach. At this stage, the water has lost enough energy that it can now only carry silt and clay away from the beach, leaving the heavier sand and pebbles behind. As this process continues for many years, it results in a well-sorted sediment deposit. This is why many beaches have clean, well-sorted sand. In contrast, processes like landslides and glaciers result in little to no sorting of sediment. The velocity of a stream or river is directly proportional to how much energy it has. Therefore, faster streams can transport larger particles than slower streams. In water, coarser clasts travel by rolling or bouncing along the river bed (bedload) while finer particles float along in suspension (suspended load). Dissolved elements travel as dissolved load. Question 7
0.5 / 0.5 pts The images above shows an exposure of a sedimentary rock composed of sand-sized particles with a lens of larger (2-10cm) clasts. Inset: close-up of quartz clasts removed from outcrop. Describe the following: Grain Size: Coarse Roundness: well-rounded Sorting: poorly-sorted When taken together, particle size, roundness, and sorting suggest that this sedimentary rock was deposited in an environment that experienced low to moderate energy flow with occasional high velocity events such as range-front river Answer 1: Fine Medium Coarse Answer 2: angular subangular subrounded well- rounded Answer 3: well-sorted poorly- sorted Answer 4: constant high-energy flow such as a mountain stream low- energy flow such a lake low to moderate energy flow with occasional high velocity events such as range-front river high- energy flow such as a landslide
Composition
As mentioned above, different minerals weather and break down at different rates depending on how stable or reactive they are at the surface of the Earth. A given mineral’s stability or reactivity at the
PartialQuestion 8 0.42 / 0.5 pts The image above shows a sandstone with the mineral composition of its clasts labeled. Which minerals would you expect be weathered away first? Rank them below: Most susceptible to weathering: Amphibole Biotite Orthoclase Least susceptible to weathering: Quartz The source material for this rock included granite and diorite. Given the composition you observe above, would you say that R1 is mature or immature? Mature Which texture characteristic does this most agree with? Rounding Answer 1: Orthoclase Amphibole Biotite Quartz Answer 2:
Orthoclase Amphibole Biotite Quartz Answer 3: Orthoclase Amphibole Biotite Quartz Answer 4: Orthoclase Amphibole Biotite Quartz Answer 5: Mature Immature Answer 6: Grain Size Rounding Sorting
Section 3: The Formation of Sedimentary Rocks
Clastic Rock Formation
When a sediment clast has run its course and transport has ceased, it is said to have been deposited. Given the pervasive force of gravity, sediment collects in low elevation basins, but may be preserved in a variety of points in the landscape. Most sediment ends up in the largest basin of all: the ocean. Consequently, most sedimentary rocks exposed on the earth's surface today are marine in origin. For clastic sedimentary rocks, as sediment becomes buried, sometimes by many kilometers of new sediment, it undergoes two processes which cause the transition from sediment to sedimentary rock, cementation and compaction. Together these comprise the process of lithification. Very fine-grained pieces will “stick” to one another simply through Van der Waals forces (electrostatic interactions) and thus can be bound together by just compacting the material. Compaction occurs as more sediment piles on top of the first deposition.
Question 9 0.5 / 0.5 pts Composition What is the composition of this rock? Grain Size What size grains does this rock have? Grain Rounding
How rounded are the grains in this rock? Sorting What degree of sorting does this rock have? Fossils Does this rock contain fossils?
R1 is sandstone.
Answer 1: quartz mica, orthoclase, fragments clay minerals Answer 2: fine medium to coarse Answer 3: rounded subrounded angular Answer 4: poor moderate to well Answer 5: yes no Answer 6: breccia chert coal conglomerate limestone rock salt sandstone shale siltstone
