lab 5 (1)

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George Mason University *

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1302

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Geology

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Dec 6, 2023

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GEOL 1302 Lab 05 Clastic Sedimentary Rocks The first task in this lab is to make and describe some “clasts”. Then you are going to describe and identify some common clastic (or detrital) sedimentary rocks. Your lab instructor will summarize the main characteristics of the different rocks. In the third part of the lab, you should list some possible environments in which the rocks that you identified might have formed. Some help to perform the rock identification and information about clastic sedimentary rocks can be found below. Please submit both papers to your instructor. Don’t forget to put your name on the worksheets! Grain Size: Gravel > 2 mm Sand 0.06-2 mm Silt 0.004-0.06 mm Clay < 0.004 mm Rounding: Angular Sub-rounded Well-rounded Sorting: Poorly sorted Well-sorted Other Observations: Layering Fissility Color Identification and Formation: 1
http://geology.about.com/od/rocks/a/Rock-Tables.htm http://ratw.asu.edu/aboutrocks_clasticsedimentary.html http://www.earthsci.org/education/teacher/basicgeol/sed/sed.html 2
Clastic Sediment Use the two pieces of granite provided in the lab and tap them together over a piece of paper. The broken pieces that fall onto the paper are called “clasts”. Observe the pieces with a hand lens. Are your clasts fragments of crystals or rock fragments (or both)? they are both crystal and rock fragments What kinds of sediment did you just make – pyroclastic, siliciclastic, or bioclastic sediment? Siliciclastic Sketch at least three of your pieces (at a larger scale). What is the rounding of these pieces? Angular What is/are the size(s) of your pieces? Use the grain size descriptions given on the first page of this lab. 2 Coarse-grained = 0.06 – 2mm What is the sorting of your pieces? Poorly sorted 3
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Examining and Classifying Clastic Sedimentary Rocks Sample Number Grain Size Rounding Sorting Other Observations Rock Name 1 Gravel > 2mm Sub-rounded Poorly sorted Sedimentary rock Clastic Rounded particles Conglomerate 2 Clay <0.004 mm Too small to see Sub-angular Well sorted Fissile Sedimentary rock Laminated Shale 3 Sand 0.06– 2mm Well rounded Well sorted Light-brown sandy color Particles are very fine- grained Clay-sized particles keep the layers of sand together Sandstone 4 Gravel >2mm Angular Unsorted Angular Particles Grayish color Breccia 5 Clay <0.004mm Sub-rounded Well sorted Particles too fine to see Reddish color, might mean oxidation of Iron Siltstone Rocks provided: Breccia, Conglomerate, Sandstone, Shale, Siltstone 4
Clastic Sedimentary Rock Formation In what environment might the rocks that you identified in the first part of the lab have formed? Sample #/Rock Name Formation/Environment Conglomerate Strong water currents like beaches or flowing streams Shale Chemical weathering of rocks breaks down to clay material. muddy material is carried downstream, settles at the bottom of a lake, ocean, or swap. accumulation could possibly create shale Sandstone Sand is carried by rivers, wind, or waves. Deposited by its source rock beaches Breccia Commonly formed at the base of an outcrop where mechanical weathering debris accumulates Alluvial Fan siltstone Any places where a current, wave, or wind accumulates sand and mud Old delta or lake 5