Lab 06 Sedimentary Rocks
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Geology
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Apr 3, 2024
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Name: Section: 1pm or 3pm (circle one) Lab 6: Sedimentary Rocks Problem 1: Sedimentary Rock Textures:
Give the texture of each of the following samples. The following textural terms will apply: clastic
(mineral grains or unidentified debris), bioclastic
(made almost entirely of whole fossils or fossil debris), very fine grained
, or fissile
. You may use more than one for some samples. Sample Texture(s) 100 101 102 103 104 112 Rusty brown flecks are plant debris. 113 Look for a fossil. 116 Problem 2: Seeing the big differences.
Put your samples into three groups: Group 1 Group 2 Group 3 100 103 102 101 112 104 116 113 117 You have probably already figured out which samples are sandstone and conglomerate. It is more challenging to distinguish fine-grained limestone or dolostone from shale or mudstone because both are very fine grained and both can contain fossils. Here are some distinguishing clues: •
If you break shale, it, being fissile, breaks into many flat pieces. Limestones break into more irregular, blocky pieces. Mudstones also form irregular blocky pieces, but they tend to be softer than solidly cemented limestone and dolostone. •
If you have a silty shale or mudstone, you may see tiny shiny flecks of muscovite. In some coarse-grained limestones you may see calcite cleavages, but you will not see muscovite flakes. •
If present, silt-sized quartz grains in shale or mudstone will scratch glass if the rock is solid enough to not crumble against the glass. Limestone and dolostone do not scratch glass.
2 •
Of course, limestone and dolostone fizz in acid, but this test is not intended to distinguish between carbonate rocks and other rocks. It is only to be used to distinguish between limestone and dolostone, which otherwise are identical in appearance. a.
Which of your three groups consists of sandstone and conglomerate? b.
Which group consists of shale and mudstone? c.
Which group consists of limestone and/or dolostone? Problem 3: Sedimentary Rock Identification.
To make things
simple, restrict your choices to the rock types listed below: Clastic (text Table 4.4) Chemical/Bioclastic (Table 4.5) Conglomerate Micritic Limestone Sandstone (fossils possible) Bioclastic Limestone Shale (if fissile) (fossils possible) Dolostone (fossils possible) Mudstone (if not fissile) (fossils possible) Chert Rock Gypsum Rock Salt Coal a.
Examine sample 100
with a hand lens. It looks like sugar. What type of rock is it? What type of mineral makes up 100% of this rock (apart from occasional rust-colored Fe stains)? b.
Sample 101
is what type of rock? Name the mineral that comprises one of the common clasts. c.
Sample 116
. The surfaces of feldspar grains in this type of rock often weather and break down, making it tough to find a vitreous luster or cleavage. The most obvious grains in this sample give silvery flashes. What mineral gives these flashes? What type of rock is this? d.
The "acid test" is used ONLY to distinguish limestone from dolostone. Calcite fizzes rapidly in dilute hydrochloric acid (1M HCl) whereas dolomite fizzes very slowly. It is
3 often necessary to use a steel tool to scrape together a small pile of powder on a dolostone sample in order to see any bubbling take place. The acid test is actually used by geologists to tell limestone from dolostone. In contrast, some people use the acid test on every sample placed in front of them, even on samples that are obviously conglomerate or sandstone. Sometimes these rocks as well as shale and mudstone contain some carbonate and hence fizz. This does not make them limestone. Most lack carbonate, and since this acid doesn't react with non-carbonates, it is then available to burn hands and eyes and to bleach clothes. A quick look with a hand lens is a faster and safer way to distinguish conglomerates, sandstones, limestones/dolostones, mudstones, shales, evaporites, and coal. Only use the acid test if you already know it is either limestone or dolostone.
Compare samples 102
and 106
. Use a hand lens to see if 106 has a clastic or crystalline texture. Do you see distinct grains or not? Use the acid test to determine which sample is limestone and which is dolostone. 102
: 106
: You will not need the acid bottle for the rest of the lab because the rest of the carbonate rocks are all limestone. e.
Identify the following two common sedimentary rocks: 108
: (Just the dark grey or dark brown areas; note fracture and hardness) This rock is found as irregular blobs in limestones, dolostones, and chalks. It can be great for making stone tools. See Figure 2.4. 110
: Give both the rock type and the depositional environment (Table 4.2). The next few questions give you an idea of how sedimentary rocks are used to infer the depositional environment in which the rock was formed. There is a whole field of geology devoted to such work. Problem 4:
Fossils are extremely useful when trying to figure out where a given rock was deposited. GEOL 1020 (Sedimentology and Stratigraphy) teaches you how to read the sedimentary rock record so you can figure out whether a given sequence was deposited in a river, delta, beach, etc. (Table 4.2 lists some possible depositional environments), but nothing beats a good fossil. Since well-preserved leaves are generally too delicate to survive transport into the ocean, fossil leaves indicate a terrestrial environment. Most fossil shells indicate a
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4 marine environment (lake sediments may have a low-diversity shelly fauna, but lake sediments are relatively rare in geologic record). a.
Identify the following rocks: 113
: 114
: b.
Sketch one fossil from each sample. 113
: 114
: c.
Which sample was deposited on land and which was deposited in an ancient ocean? 113
: 114
: Problem 5:
Use Table 4.2 and Fig. 4.2 to infer the environment of deposition for the following samples. a.
104
: b.
109
: Gypsum. Problem 6:
In the field, geologists nibble shale samples in order to see if shales and mudstones are siltstones or claystones. If it’s gritty between the teeth, it’s a siltstone. The grains are so tiny in a claystone that you do not feel any grit. I will spare you the fun of chewing on rocks. Instead, compare samples 103 and 112. Look for any tiny silvery reflections of light. These are muscovite flakes. The larger dark grey and reddish/brownish flecks in 112 are fossil plant remains. Based on what you see, which rock has coarser grains and is therefore a siltstone? Sample 103 or 112?
Sample 112 with a star next to the number has a nice fern stem on it.
5 Problem 7: Inferring relative current strength a.
Clastic particles are transported and deposited by water or wind currents. Using grain size, arrange the samples below from the strongest to the weakest currents that led to their deposition: 100, 101, 103, 112 b.
Comparing sample 112 and 114, which was deposited under stronger current conditions. Consider both grain size and the preservation of the delicate plant materials. Sample 112 or 114?
Problem 8:
Identify samples 118
and 119. 118:
119:
Both of these samples came from just north of Miami, Florida. Did these samples form on land or in the ocean? What is your evidence? Which sample was deposited under higher energy current conditions (i.e., under stronger waves and/or tidal currents)? Explain your reasoning. Problem 9:
The waves crashing on a beach continuously stir up beach sand and wash away the finer particles. Although muscovite is very resistant to chemical weathering, its flat, platy shape allows it to be carried away by the waves like leaves on the wind. Similarly, plant debris (leaves and twigs) is readily broken up and carried away from the beach. Thus, beach sands tend to be poor in both muscovite and plant debris. In contrast, river and delta sands often display muscovite and plant debris. Examine samples 100 and 116. Which is more likely to have come from a beach and which from an ancient delta or river environment? Beach: 100 or 116? River/delta: 100 or 116?
6 Problem 10:
Sketch each of the following: a. Look at one of the samples with ripple marks. Carefully sketch the sample to show the shape of the ripple marks in cross-section (from the side) and identify the type of ripple mark. If it is a sample of current ripple marks, indicate with an arrow the current direction that formed the ripples. If it is a sample of oscillation ripple marks, indicate currents going in both directions. b. Look at a sample with mud cracks. Sketch the mud cracks to get the pattern in your mind. There are some much nicer specimens in the rock display outside Thaw 203. Problem 11: Fossils and depositional environments:
The sedimentary rocks of southwestern Pennsylvania are dominated by delta and river deposits that include sandstone, mudstone, and, most famously, coal. These were deposited as sediments some 300 million years ago. Sample 117
is a sample from one of several thin sedimentary formations found all across southwestern Pennsylvania and northern West Virginia. You can find this layer down in Panther Hollow, in the roadcuts on Interstate 279, and at various exposures across the North Hills, for example. The figure on the next page represents outcrops exposed north of Camp Horne Road along I-279. a.
Examine sample 117
and name the texture and rock type: b.
Was this sample deposited in a marine or non-marine (terrestrial) environment? What is your evidence? In case you are curious, these fossils are 300 million years old. Pretty old, eh?
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7 c.
Sample 110
was collected a few hundred feet above sample 117
. Was 110
deposited above or below sea level? d.
Sample 117
is from the “Ames Ls” layer shown above. The Pittsburgh red beds contain fossil root remains. Are these red beds (red-colored shales) likely to be marine or non-
marine? The Morgantown/Birmingham Sandstone layer shown above contains plant debris and muscovite. Are these terrestrial or marine? d.
How did sea level vary to deposit 117
across a broad area otherwise dominated by sedimentary rock layers containing root casts, plant debris, muscovite, and other evidence for terrestrial sedimentation? What did sea level do after the Ames Ls was deposited?