Geosc001_FA22_Sed_rocks
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Pennsylvania State University *
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Feb 20, 2024
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16 GEOSCIENCE 001 SPRING 2022 Name LAB 3: SEDIMENTARY ROCKS Section Date AND PROCESSES
During this lab, we will investigate sedimentation processes and the results of those processes, sedimentary rocks. Pick up a handful of sand on the beach and you will find that each of the individual grains that comprise it is unique. Different sizes, shapes, colors, densities, etc. all act as a sedimentary “fingerprint” that can be used to identify where grains came from (
provenance
), how they were transported (
sorting, roundness, sphericity
), and what chemical processes (
mineralogy, cementation
) they experienced during transport and deposition. In general, grain sizes decrease the longer they are subjected to sedimentary processes, making identification of the above characteristics more difficult. However, careful examination using a 10x hand lens (in the field) or a stereo microscope (in the lab) can improve accuracy of mineral identifications of most grains. For more accurate mineralogical studies of sedimentary grains, grain mounts or rock thin sections are studied using a petrographic microscope to identify mineralogy from optical characteristics of light transmitted through the grains. We’ll do this in upcoming labs!
After assessing the variety of minerals comprising a sample, the relative abundance of each is estimated. Qualitatively
, this is done using charts and relative proportions (attached). Quantitatively
, this is done by manually sorting each mineral type in unconsolidated sediments, or by conducting "point counts" across a thin section of lithified samples. Once this information is obtained, it is possible to identify the sedimentary rock. I. Determining Grain Size Distribution in a Sediment Take a close look at the two sediment samples provided. We will look at the sorting of these samples, not to be confused with the grading of a sample. In order to make the best use of everyone’s time, we have sieved the samples and share that data with you here. Q1 Would you characterize these samples as well sorted or poorly sorted? Sample A: Sample B: Estimate
the percentage of each the following grain sizes by mass in samples A and B. Sample A Sample B Pebbles Coarse Sand Medium Sand Fine Sand Silt
17 Calculate each sample’s total mass.
Sample Container and Sample Mass Mass of Container Total Mass of Sample A 963.2 g 130.1 g B 598.1 g 91.9 g We are going to calculate how much of each grain size makes up your sediment sample. Sieves are used to sort grains by size in sediment samples. Sieve sizes are provided below. #5 Five holes per inch Each hole is 0.508 cm captures pebbles #10 Ten holes per inch Each hole is 0.254 cm captures coarse sand #35 Thirty-five holes per inch Each hole is 0.0726 cm captures medium sand #60 Sixty holes per inch Each hole is 0.0423 cm captures fine sand Base captures silt Instructions for sieving: 1.
Nest the sieves by increasing sieve #, the largest # nests in the base. Pour your sample into the top sieve and place the cover on top of the nested set of sieves. 2.
To sieve a sample, gently shake the whole stack of sieves. One person should shake the sieve while the other taps the top of the sieve for approximately 1 to 2 minutes. Once you have shaken the sieves long enough to adequately separate the sediment, separate the sieves and place each on the table. 3.
Measure the mass of the contents in each sieve and the base using the container provided. Remember to subtract the mass of the container from each measurement. When youa r e finished, return sample A to its original container. It’s okay to remix the sample. Repeat the process with sample B. Sample Mass in #5 Sieve (Pebbles) Mass in #10 Sieve (Coarse Sand) Mass in #35 Sieve (Medium Sand) Mass in #60 Sieve (Fine Sand) Mass in Base (Silt) Total Sieved Mass (sum) A B Calculate the mass percent of each fraction of the samples by dividing the mass in the sieve by the Total Sieved Mass. Enter your results below.
18 Sample Weight % in #5 Sieve (Pebbles) Weight % in #10 Sieve (Coarse Sand) Weight % in #35 Sieve (Medium Sand) Weight % in #60 Sieve (Fine Sand) Weight % in Base (Silt) A B Q3 Were you surprised by any of these results? How did they compare with your estimates on the first page of this lab? Q4 Do you still agree with your assessment of the samples in Q1
? If not, explain. Q5 Which sample, A or B, do you think might have been deposited
in the highest energy environment? Explain your thinking. Q6 Why aren’t the sediments completely sorted (all fine sand or all gravel for example)? Why is there usually a mixture of sizes? Q7 You’ve looked at sorting in sediments. The same ideas can be applied to sedimentary rocks. Rank sedimentary rock samples 1 through 4 from well sorted to poorly sorted. well sorted poorly sorted
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19 II. Identifying Sedimentary Rocks and Environments
You’ve now taken a look at sediments before they become rocks. The above exer
cises should help you identify sediments that have become rocks. Use the Sedimentary Rock Identification section below to help. In the final two columns, include any observations you make about the energy of the environment or formation mechanisms, and make an educated guess at the depositional environment. Refer to some of the figures at the end of this lab. Sample Clastic or Chemical Grain Roundness (if clastic and visible) Grain Size (clay, sand, gravel) Other Characteristics Rock Name Observations about energy and structures Depositional environment interpretation 1 2 3 4
20 5 6 7 Q8 Samples 2 and 5 are both sandstones. Sample 2 comes from the Tuscarora Formation and Sample 5 comes from the Bald Eagle Formation. What is the difference between these two samples? Which one is made of sediments that have traveled further from their source?
21 III. Sedimentary Structures and Depositional Environments Three samples have been provided for you to use in this exercise. Please don’t take these samples back to your seat. Q9
This sample shows a sedimentary structure that is quite useful to field geologists. a.
Name the structure (using your textbook if necessary). b.
Hypothesize how field geologists use this structure. Hint: Structures like this only form on the surface of a sedimentary layer. Q10
This sample shows a different sedimentary structure that is quite useful to field geologists. a.
What type of structure does this sample show? b.
Which way (away from or toward the x) did water flow over the sediment that became this sample? c.
Why would this sample be useful to geologists? (There are a few reasons.) Q11
This sample shows yet another sedimentary structure that is quite useful to field geologists. a.
Name this rock. Begin with the sedimentary structures and then the sedimentary rock name. For example: rippled shale __________ b.
Note the sedimentary structures in this rock. Describe an environment in which this rock might have formed.
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22 SEDIMENTARY ROCK IDENTIFICATION Clastic vs. Chemical Clastic sedimentary rocks are rocks formed by deposition of sediment and compaction and lithification (rock forming). Chemical sedimentary rocks are formed by chemical processes, such as evaporation, and precipitation. Clastic Grain Size - the size of the grains in the rocks. Coarse - greater than 2 mm. in diameter. Pebbles, cobbles, boulders. Sand - 2 mm. to 1/16 mm. in diameter. Sand Silt - 1/16 mm. to 1/256 mm. in diameter. Dust, gritty. Clay - less than 1/256 mm. in diameter. Mud or modeling clay. The best way to differentiate between silt and clay clast size is to chew a small piece of the rock. If the rock feels gritty, it is silt, if it feels mushy, it is clay. Cement - the material holding (cementing) the grains together. Usually silicon or carbonate. Can be tested by dropping a small amount of dilute HCl on the rock. Sorting - the rock may be all one grain size (well sorted) or more than one grain size (poorly sorted). Mineral Identification Quartz - hardness 7, usually transmits light, may be grayish, may look glassy. Potassium Feldspar (Orthoclase) - hardness 5.5 –
6.5, pink to light gray, two good cleavages at right angles, opaque, rectangular minerals with good cleavage. Plagioclase - hardness 5.5 –
6.5, gray, two good cleavages at right angles, striations on one cleavage, opaque, rectangular minerals with good cleavage and striations. Clay - aphanitic, soft, usually gray, but can be red or green, looks muddy. Calcite - hardness 3, fizzes in acid. Dolomite –
hardness 3, fizzes in acid when it is powdered. To powder the mineral, scratch it with a knife, steel nail, or other metal object.
SEDIMENTARY ROCK IDENTIFICATION (CONT.)
23
24 Clastic Sedimentary Rocks (particles or granular texture) Texture Rock Identifying Characteristic Coarse Conglomerate Contains rock fragments (rounded particles) Coarse Breccia Contains rock fragments (angular particles) Sand Sandstone Mostly quartz Sand Arkose Usually red to pink in color, noticeable amount of feldspar Sand Graywacke Poorly sorted, lots of clay, often dark colored Silt Siltstone Foliated (layered in sheets) or massive Clay Shale Soft, foliated, may be red, black, green, gray, tan, depending on content
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25 Chemical Sedimentary Rocks (interlocking or jigsaw puzzle texture)
Mineralogy Rock Identifying Characteristic Calcite Limestone Hardness 3. Calcite fizzes in acid, frequently has fossils. Dolomite Dolostone Hardness 3. Dolomite fizzes when you powder it by scratching it with a knife. Usually does not have fossils. Halite Rock Salt Hardness 2. Tastes salty. Gypsum Gypsum Hardness 2. Doesn’t taste
salty. Silica Chert, Flint Hardness 7. Luster is glassy to waxy. Shows conchoidal fracture.