Lab 4- Archean fossils and rocks.docx
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Geology
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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.
Seems to be curvy laminate, perhaps wavy lamina shape. There is definitely a series of
parallel lines though.
A2.
Looks like there is some sort of spine-like structure in the middle, I would characterize
this as maybe a rib without any projections or bumps.
A3.
This seems to have very curvy almost circular parallel lines, I would characterize this
as a Non-Columnar Stromatolite, cumulate.
A4.
This seems to have wavy parallel lines which seems to break off at some points, I would
characterize this as a non-columnar stromatolites, columnar-layered.
GEOL 102
History of the Earth History
A5.
I don’t see any decipherable patterns in this one except some small circular
occurrences that appear to be whitish in color, I also see some rusted colored wavy
patterns.
A6.
I’m seeing orange outlines of rectangular patterns some of them protruding outside of
the rock.
A7.
This rock seems to have a number of pores as well as some crater shapes, there are also
a lot of triangular shaped pertrusions on the rock.
A8.
I am seeing column shapes and margin structure with hollowed out insides, I identify
this as most likely being a “tuberous shape”, I don’t see obvious lamina shape or
branching.
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.
This banded iron sample seems to be mostly grey with streaks of orange and yellow and
visible cleavage. The grey coloration in the sample may indicate the presence of minerals
such as hematite and magnetite. The yellow/orange coloration is typically associated with
the presence of goethite or limonite
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GEOL 102
History of the Earth History
B2.
this rock seems to be very similar to B1, it has one area fully yellow/organge with the
rest of the rock being the gray color. The grey coloration in the sample may indicate the
presence of minerals such as hematite and magnetite. The yellow/orange coloration is
typically associated with the presence of goethite or limonite
B3.
This rock has a reddish tone on all of the non-smoth sides, which are likely the sides
that were not exposed to oxygen. The reddish tone is characteristic of hematite, which often
presents a red to reddish-brown coloration.
1.
What are some characteristics that tell you this is a sedimentary rock, rather than an
igneous or metamorphic rock (5 points)?
The layering and stratification of these rocks, the presence of fossils, the grain size and
sorting, the mineral composition, and the sedimentary structures such as the ripple marks
or the cross-bedding all indicate sedimentary.
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)
Pilbara - 2.8 to 3.5 billion years ago.
Isua - 3.8 Ga
GEOL 102
History of the Earth History
2.
What are the rock types at each location? (4 points)
Warrawoona group
Kelly group
Sulpher Springs Group
Soanesville Group
Forescue Group
3.
Make some observations about the map patterns. What is similar between the two sites?
What is different? (6 points)
Boninitic amphibolites
IAB amphibolites and andesites
Felsic/magic schists
Chemical sedimentary rocks
Ultramafic rocks
Orthogneisses
mafic dykes.
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)
Pilbara:
●
The formation likely began with the accumulation of volcanic materials
●
Subsequent sedimentation led to the deposition of sandstones and carbonates
●
Continued volcanic activity and sedimentation occurred.
●
The region underwent tectonic activity, shaping the current geologic structure.
Isua:
●
The area started with the formation of volcanic and ultramafic rocks.
●
This was followed by the deposition of chemical sediments.
GEOL 102
History of the Earth History
●
The region was then affected by intense tectonic activity, leading to the intrusion of
mafic dykes and the formation of orthogneisses.
●
Metamorphic processes played a significant role in the development of the current
rock types.
You are more likely to find fossils in the Pilbara region.
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