Lab 4- Archean fossils and rocks (2)
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Apr 3, 2024
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GEOL 102
History of the Earth History
Name: Lab 4: Archean Fossils and Rocks
Purpose: The Precambrian, jointly comprised of the Archean and Proterozoic Eons, represents the first 75% of Earth history. The Earth during this time was in some ways similar to today, but in many ways was quite different. Tasks: Today, we’ll become oriented with some typical Archean rocks. These include stromatolites
, banded iron formations, and
greenstone belt units. We will also look at two geologic maps, from the Pilbara craton, Australia and from Isua, Greenland, to speculate on what
geologic processes were occurring at each location.
Part A: Precambrian fossils (16 points)
Since shells, bones, and other hard parts didn’t evolve until close to the Phanerozoic, there are relatively few fossils from the Precambrian. One of the most common ones, though, is a stromatolite
! The word stromatolite really refers to a specific morphology. That is, a fabric, or texture in a rock that is characterized by: “Laminations that are characterized by periodic or episodic accretion/formation”
Where a lamination is a small-scale (cm or less) layer. Many stromatolites are interpreted to have
been formed by microbes, or consortiums of microbes. These rocks are typically made of a carbonate mineral (like calcite or dolomite) or silica. They come in a variety of shapes and sizes, and we can classify them based on their morphology:
GEOL 102
History of the Earth History
Task 1
. Observe the eight (8) samples laid out in lab, and characterize them based on the figure above. Include as many descriptive terms as you deem appropriate (2 points each). A1.
A2.
A3.
A4.
A5. A6.
GEOL 102
History of the Earth History
A7.
A8.
Task 2. Speculate on the environment in which you think stromatolites could form. Rely on your knowledge from Geology 100 and your TA. What observations do we make to infer depositional environment? What could that mean about where these organisms were living? Part B: Chemical sediments (11 points)
One of the most characteristic sedimentary rock, common to the Precambrian, is a Banded Iron Formation (BIFs).
These are rocks that have alternating iron oxide and silicate layers. Typical minerals include hematite and magnetite in the iron rich layers, and clays and chert in the silicate
layers. You’ll learn more about how they are thought to have formed in the Take Home lab for this unit, so let’s focus on making observations here. Task 1
. Investigate and observe the following three (3) samples of BIFs, and try to determine the minerals in each sample (2 points each): B1.
B2. B3. 1.
What are some characteristics that tell you this is a sedimentary rock, rather than an igneous or metamorphic rock (5 points)?
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GEOL 102
History of the Earth History
Part C: Geologic maps of the Pilbara and Isua (23 points)
Two places on the Earth where Archean rocks are preserved are the Pilbara Craton in Western Australia and western Greenland, in a location called Isua. The rocks in Pilbara are between about 2.8 to 3.5 Ga, while the rocks at Isua are even older, back to about 3.8 Ga. Using the attached geologic maps, and the following information, answer the following questions. Pilbara units: Warrawoona group (chert, a chemical sediment made of silica, might have fossils);
Kelly Group (sandstone and carbonate); Sulphur Springs Group (komatiite, basalt, andesite–
dacite–rhyolite); Soanesville Group (greenstone belt: sedimentary rocks and some basalts); Fortescue group (combination of sedimentary rocks and basalts). 1.
What is the range of ages found in the Pilbara and Isua? (2 points)
2.
What are the rock types at each location? (4 points) 3.
Make some observations about the map patterns. What is similar between the two sites? What is different? (6 points)
GEOL 102
History of the Earth History
4.
Given your observations, write down a proposed short geologic history for each area. That is,
what happened in what order. Also, at which location do you think you are more likely to find fossils? (11 points)