Essay: Reconstructing Environment of Deposition using Sedimentary Rocks

Experiment 1: Reconstructing Environment of Deposition using Sedimentary Rocks
Sedimentary rock types and structures can be used to reconstruct environments of deposition. In this experiment, you will identify hand samples of sedimentary rocks and use your knowledge of weathering and deposition to determine how the rocks were formed.
Materials
Hand Lens
*Sedimentary Rock Specimens
Unknown Rock E
Unknown Rock G
Unknown Rock H
Unknown Rock I
Unknown Rock J
Unknown Rock K
Procedure
1. Begin by populating the information you have already gathered from the Earth Materials and Sedimentary Rocks Lab into Table 3.
2. For any information not previously collected, such as sorting and color, examine each rock specimen with a hand lens. Indicate whether the sample is poorly-sorted, well-sorted, or moderately-sorted. Record your observations in Table 3.
3. Use Table 3 to populate Table 4, and refer back to your Earth Materials and Sedimentary Rocks Lab for mineral composition.
4. Use the following questions to classify whether the rock was deposited near/far from its source:
a. Is the rock chemical or biogenic? If so, it formed near the source.
b. Does the rock have angular grains? If so, it formed near the source.
c. Does the rock have angular grains? If so, it formed near the source.
5. Use Figure 7 to help determine whether the rock was deposited in a high or low energy deposition environment. Record your answer in Table 4.
6. In Post-Lab Question 1, write a full description of the environment of deposition, including relevant details such as climate, wildlife, landforms, and surface water features. Include any details you can infer about the rock’s weathering history and its journey prior to deposition.
Table 1: Common Sedimentary Rocks
Rock Type Particle Size Composition Comments Rock Name
Clastic Very Coarse —Coarse
> 2.0mm Any rock (quartz, feldspars most common minerals) Rounded clasts Conglomerate
Angular clasts Breccia
Clastic Coarse —Medium
0.06—2.0mm Quartz, feldspars Feels sandy, some grains may be present Quartz Sandstone
Clastic Fine
0.004—0.06mm Clays, quartz Feels gritty, some grains visible through hand lens Siltstone
Clastic Very Fine
< 0.004mm Clays, quartz May be foliated, no visible grains Shale
Chemical
Varies
Microcrystalline
Varies Halite No visible grains, tastes like salt, has three perfect right angles Halite
Gypsum Softer than fingernail, can be pink, white, or clear Gypsum
Dolomite Reacts with dilute acid only when powdered Dolostone
Chemical or Biogenic Quartz and/or quartz based fossils Exhibits sharp edges and breaks in glass like patterns, smooth to the touch Chert
Biogenic Calcite and calcite based fossils Small fossils or fossil impressions may be present, reacts with dilute acid Fossiliferous Limestone
Carbon-based material Plant remains may be visible, black-brown color Bituminous Coal
Table 2: Sedimentary Rock Classifications
# Clast Shape Grain Size Other Observations Mineral Composition Rock Name
(If Applicable)
E clastic Medium Grains are visible Iron sulfide Pyrite
G Clastic Fine Some grains visible through hand lense Clay, Quertz Siltstone
H Clastic Very Fine No visible grains Clay, Quartz Shale
I Clastic Medium Some grains are visible Calcite Limestone
J Chemical Microcrustalline Very smooth to the touch Quartz or quartz based fossils Chert
K Clastic Varies Rough to the touch. No visible grains Calcite Limestone
Table 3: Sedimentary Rock Characteristics
# Clast Shape Grain Size Sorting Color Rock Name
(If Applicable)
E Rectangular Fine Brown Pyrite
G Pyritohedral Fine Gray Siltstone
H Flat Very Fine Gray-Black Shale
I Triangular Medium White Limestone
J Octahedral Microsrycalline Light brown Chert
K Modified Cubic Varies White Limestone
Table 4: Environment of Deposition Characteristics
# Rock Name Mineral Composition Near/Far from Source High/Low Energy of Deposition
 
E Pyrite Iron sulfide Far High
G Siltstone Clay, quartz Near Low
H Shale Clay, quartz Far High
I Limestone Calcite Far High
J Chert Quartz Near Low
K Limestone calcite Near Low
Post-Lab Questions
1. Environments of deposition
a. Rock E – Can be found in sedimentary deposits to hydrothermal veins.
b. Rock G- Siltstone can be found near a lake or sea
c. Rock H- Shale can be found near the shores of large lakes or continental shelves at sea edges.
d. Rock I- Limestone can usually be found in shallow waters.
e. Rock J- Generally occurs as nodules, lenses or beds withing sedimentary sequences.
f. Rock K- Limestone can usually be found in shallow waters
2. Did any of the rocks exhibit evidence of an organism-rich environment? If so, indicate the rock(s) and ex-plain the evidence. Yes, Siltstone and Shale because they can usually be found near small lakes or seas all the way up to large lakes.
3. What is the parent material for Rock G? Explain your reasoning.
The parent material for Siltstone is Weikert, berks and Leck Kill soils
4. Field examinations are often more useful in determining environment of deposition than hand samples. Why do think this is true? Give one example of a sedimentary structure you might see in the field but would not see in this lab. Explain how this structure is created.
If you can study something in its environment you can see how it was formed and deposited. In general you can learn more about it. So yes I would agree because being in the environment provides more chances.