GEOS 234- Lab Report #3
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Apr 3, 2024
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Earth System History GEOS 234
Unit 3 Lab Identifying and Interpreting Clastic Sedimentary Rocks Mataya Topil 1/29/2023 Abstract- This lab exercise focuses on identifying and interpreting clastic sedimentary rocks. The primary objective is to familiarize oneself with the classification of clastic rocks and gain insights into the environments in which sediments are deposited. The analysis involves recognizing and applying textural criteria, classifying rocks based on texture and mineral composition, and describing potential geological interpretations. Various clastic rock specimens are examined, and their properties are assessed using standard field techniques. Purpose- The purpose of this lab is to develop proficiency in identifying and classifying clastic sedimentary rocks by examining the texture and mineral compositions in various rock specimens, we aim to deduce clues about their depositional environments. This exercise contributes to a better understanding of the process involved in the lithification of preexisting rocks. Methods- The examination used a rock kit, a simple magnifier, mineral hardness test items and a dropper bottle with diluted hydrochloric acid. The properties of each rock specimen, including grain size, composition and cementing materials, are analyzed. Equipment Used- -Rock Kit -Simple Magnifier -Mineral Hardness Test Items -Dropper bottle with dilute hydrochloric acid (or vinegar)
Data- Figure 1,. The Udden-Wentworth Texture Classification Scale, , directly from the lab. The Udden-Wentworth Texture Classification Scale Size range (average grain diameter) Sediment Name Rock Name
< 1/256 mm (0.004 mm) clay mudstone or shale 1/256 mm (0.004 mm)- 1/16 mm (0.0625 mm) silt siltstone 1/16 mm (0.0625 mm) – 2 mm sand sandstone >2 mm gravel conglomerate or breccia
Earth System History GEOS 234
Unit 3 Lab Figure 2- Classification table for clastic sedimentary rocks, directly from the lab. Answer to Questions- Q1- What is the average grain size of rock #2? Report your answer to the nearest 0.1 mm. Observing rock #2, I noticed 2-3 sand grains between each millimeter measurement, allowing me to approximate the grain size to be between 0.3 mm and 0.4 mm. Q2- Which among specimens 1-7 falls into the sandstone category? I posit that specimens #2, 3, 4, and 5 exhibit characteristics indicative of sandstone. Their
grain size falls within the sand grain range, categorizing all of them as sandstones. Specimens #3
and 5 can be identified as arkose sandstone due to the presence of feldspars, while #2 is recognized as quartz sandstone owing to its well-sorted quartz grains. Although #4 is also categorized as sandstone based on grain size, the specific minerals present, apart from quartz, remain undetermined. While reflective surfaces were observed, uncertainty arises regarding the
quantity of feldspar necessary to classify it as arkose. Q3- Which among rocks is 1-7 calcareous cement? Except for sample number one, all the samples exhibited some calcareous cement as evidenced by their reaction to vinegar. Comparing the reactions among the samples, 6, 3, and 4 demonstrated the most rapid responses, while samples 2, 5, and 7 fizzed, albeit weakly. The subdued reactions indicate a lower concentration of calcareous cement. Surprisingly, sample number one showed no reaction despite expectations to the contrary.
Earth System History GEOS 234
Unit 3 Lab Q4- Noting the color, what else can you conclude about the composition of the rock’s cement?
Specimen #2 exhibits a subtle pink hue, suggesting the presence of a minor quantity of iron oxide as an additional cementing agent. A deeper red coloration would signify a higher concentration of iron oxide in the rock. Q5- Identify rock #2 in the form of a full descriptive name
Sample #2 is a quartz sandstone with a slight calcareous composition and traces of iron oxide. The observable reaction to HCl indicates the presence of calcite. The rock, capable of scratching glass and displaying notable hardness, suggests quartz as the primary component of its grains. With a grain size falling within the sand-size range of 1/16mm-2mm, it qualifies as a sandstone. Additionally, the pinkish color further signifies the involvement of iron oxide, serving
as an additional cementing and coloring agent.
Q6- Using the sedimentary facies and examples of depositional environments sections of the reading, suggest possible depositional environments for specimens #1, 2, and 6. The conglomerate in sample #1 features notably large grain sizes, implying that the transporting mechanism must have been sufficiently powerful to move these sediments. Moreover, the rounded nature of the sediments indicates that they traveled a considerable distance to shape the gravel. A depositional environment suitable for this process could be near a riverbank. Gravel, carried by water, undergoes rounding and transportation, resulting in the accumulation of various-sized sediments. In contrast, the quartz sandstone (#2) exhibits uniformly shaped, well-sorted grains, suggesting transportation over a significant distance. The small size of the grains indicates they could be carried by wind or water. Depositional environments for sandstone include beaches and deserts. Rivers transporting sand particles to the sea can lead to their suspension and transport to coastlines. Alternatively, in deserts, wind action can accumulate sand particles into dunes. Moving on to specimen #6, shale consists of very small particles. For these particles to settle and lithify, a calm depositional environment is necessary, such as a still lagoon or lake. The fine particles require an undisturbed setting to undergo the process of settling and lithification.
Conclusion-
In conclusion, this lab exercise focused on the identification and interpretation of clastic sedimentary rocks. Through the analysis of various rock specimens using standard field techniques, we aimed to develop proficiency in classifying clastic rocks and gain insights into their depositional environments. The examination involved recognizing textural criteria, classifying rocks based on texture and mineral composition, and deducing potential geological interpretations. By utilizing a rock kit, a simple magnifier, mineral hardness test items, and diluted hydrochloric acid, we analyzed the properties of each rock specimen, including grain size, composition, and cementing materials. The Udden-Wentworth Texture Classification Scale
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Earth System History GEOS 234
Unit 3 Lab and a classification table for clastic sedimentary rocks were employed to facilitate the categorization of specimens.