Download Answers Neoarchean to Mesoarchean 2.5 – 3.4 Ga 3 of: tonalite

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Geology 1023 – Lab #7, Winter 2014
Shields & Precambrian life forms
Shields are the oldest (and most stable) parts of the continental masses. They consist of rocks of
Precambrian age. These are overlain by flat-lying (or gently dipping) sediments of the “platform”
(next lab). Despite the fact that the Precambrian amounts to 7/9th of geologic time, and that, in
Canada, rocks of Precambrian age supply a large part of our mineral wealth, they have not been
studied as extensively as younger rocks. The main reason is the virtual absence of fossil material.
What fossils are present have only recently been recognised and are of little stratigraphic value.
Detailed study of the shield has only become possible with the advent of reliable radiometric
dating (mostly in the past 20 years).
This lab focuses on shield rocks, some of the methods used to study them, and a short study of
the sort of organisms that were present in Precambrian times.
1. Use Map 2543 (Bedrock Geology of Ontario, East-Central Sheet), Map 2586 (Total
Magnetic Field of Ontario, East-Central Sheet), and Map 900A (Principle Mineral Areas of
Canada) to do the following exercises.
NOTE: The Total Magnetic Field (TMF) is mainly an indication of the magnetite content of
the underlying rocks. The TMF includes both the remnant magnetisation and the
magnetisation due to the present Earth's magnetic field. In most continental rocks, the latter
is much stronger than the former. Hence we can ignore the effects of remnant magnetisation.
a) The oldest rocks on this map sheet (units 4 to 9) are a typical “greenstone belt”
succession and form the Abitibi Greenstone Belt and other smaller greenstone belts.
What age are these rocks?
Neoarchean to Mesoarchean 2.5 – 3.4 Ga
b) The greenstone belts are closely associated with intrusive rocks of similar age. Name
three rocks that are abundant in these intrusions.
3 of: tonalite, granodiorite, granite, diorite, monzonite
c) Most of the map area is part of the Superior and Southern provinces of the Canadian
Shield. However, the southeastern corner of the map around Lake Nipissing is part of the
Grenville Province. What ages of rocks are characteristic of the Grenville Province?
Mesoproterozoic (0.9–1.6 Ga)
d) Name three rocks that are abundant in the Grenville Province.
Migmatite,
gneiss,
felsic igneous rocks
e) Swarms of mafic dykes (called diabase on this map) occur in this area. They also show
Shields & Precambrian life-forms — Winter 2014
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up clearly on the magnetic map (Map 2586), because they contain quite a lot of
magnetite. Are the most abundant diabase dykes older or younger than the greenstone
belts?
younger
f) How can you tell (without looking at the map legend!)?
Cross-cutting relationships
g) The youngest rocks in this area are Paleozoic and Mesozoic rocks of the James Bay area.
How can you tell that the Precambrian rocks extend under these Phanerozoic rocks?
(Hint: look at Map 2586.)
Magnetic pattern visible through younger rocks
h) What types of geological contacts occur between the Precambrian rocks and the
Phanerozoic rocks?
Non-conformities and faults
i) This map area includes many of Canada's most important mining areas. Use the Principal
Mineral areas of Canada Map (900A) to list the most important metals obtained from
each of the following areas:
(1) Kirkland Lake (ON)
(2) Timmins (ON)
Au, Ag
4 of: Au, Ag, Ni, Cu, Zn, Ag, Se, Te, In, Cd,
(3) Flin Flon (MB)
Cu, Zn, Au, Ag
(4) Fermont (QC)
Fe
(5) Sudbury (ON)
4 of: Ni, Cu, Co, PGM, Au, Ag, Se, Te.
Life in the Precambrian
Life in the Precambrian was primitive (by modern standards). Organisms in the Precambrian did
not possess “hard parts” and thus preservation is sporadic and very poor. With the exception of
stromatolites (see below) Precambrian fossils are rare. But the apparent “explosion of life” in
the Paleozoic with the appearance of organisms with hard parts obscures the fact that some very
important biological events occurred in the Precambrian. The evidence for what occurred is very
fragmentary but modern molecular biology has supported many of the inferences made from this
fragmentary evidence.
The landmark events are: the development of life, the prokaryotic cell, symbiosis and the
development of the eukaryotic cell, colonial life (unicellular), multicellular life.
Shields & Precambrian life-forms — Winter 2014
First, a little revision.
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2. Name the 7 major categories of the Linnaean classification system (in the correct order).
kingdom
phylum
class
family
genus
species
order
Kingdom Monera
The Monera are essentially single celled organisms with no cell nucleus. The most abundant
fossil representatives of the Kingdom Monera are “stromatolites”. They are the only fossils
present in the Archean, and Paleoproterozoic. Stromatolites are formed by the sediment-binding
and cementing action of cyanobacteria (photosynthetic bacteria). See Fig. 19.20, p. 493 (Fig.
19.19, p. 514, 5ed) of Monroe & Wicander. The earliest stromatolites are about 3.5 Ga and
there are stromatolites living today. These organisms have been around for 3,500,000,000 years!
3. Study the stromatolite specimens provided (264A and 264B at the back of the lab). Draw a
portion of one, with a scale. Label the laminations.
Ediacaran fauna
Though stromatolites are “colonial” (i.e., they consist of multiple individuals) each of the
individuals is a single identical cell. True multicellular (metazoan) organisms developed in the
Proterozoic. Metazoans (both individual and colonial) contain a range of cell types. In addition,
with true metazoans, different cells are organised into different “tissues” (muscles, nerves,
digestive tissue, etc.). True metazoan (multicellular) animals arose in the Neoproterozoic and
there are a number of localities worldwide that preserve animals from the latest Neoproterozoic.
These are commonly referred to as “Ediacaran faunas” (after the first discovery in Ediacara,
Australia) and date from about 570 Ma.
Three important groups of organisms with modern representatives, which appear to have arisen
in the latest Proterozoic are: Porifera (sponges), Cnidaria (a large phylum which includes jelly
fish, sea anemones, and corals) and Annelida (segmented worms).
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Sponges (Phylum Porifera; Late Precambrian to Recent) are very primitive organisms. They are
not really “metazoans”. They contain a few different types of cell but these aren’t organised into
tissues and there is no central nervous system. Sponges are often simple bag or bowl shaped
structures of varying sizes. The organism inhales water through a porous structure, extracts food
and then exhales the water. There are/were many variations on this theme. Many sponges had
skeletal (reinforcing) elements composed of either calcareous or siliceous needle-like
microscopic structures called “spicules,” some have organic fibres called spongin as a skeleton
(classic bath sponge). Some fossil sponges, however, had massive vesicular calcareous skeletons,
such as the stromatoporoids, which often formed reefs (e.g., Devonian in Alberta). Note that the
forms you are examining here are not Precambrian in age. Precambrian Porifera all lacked
skeletons (some modern sponges also have only jelly-like support structures).
4. Examine the fossil sponges at the back of the lab. Specimens 258A and 258B are
Girtyocoelia (Carboniferous-Permian). It is a fossil calcareous sponge with a fused skeleton,
which preserved inhalant pores and exhalent osculum. Draw a diagram with scale.Use the
“morphology handout” to help you label the inhalant pores and the osculum.
Phylum Cnidaria (=Coelenterata) (Late Precambrian to Recent) includes corals, sea anemones,
and jellyfish. Only the corals produce a skeleton today and there are/were solitary and colonial
forms. Three types of corals were important fossils and contributed to reef formation. The living
coral animal is called a polyp and it inhabits a cavity called a corallite. Note that the forms you
are examining here are not Precambrian in age. Precambrian Cnidaria all lacked skeletons (as
do modern jellyfish and sea anemones). There are three important groups of corals – rugose,
tabulate and scleractinian corals.
Rugose (wrinkled) corals were very important in the Paleozoic. These consist of single or
colonial “cups” each of which had a radiating array of walls called “septa” and basal floors
“tabulae” on which the organism sat (see diagram in handout). As the individual organism grew
the cup became bigger (giving solitary rugose corals their distinctive horn shape). Corallites in
colonial forms are separated from one another by walls. The solitary forms (often called “horn
corals”) show the external rugae (wrinkles) more clearly.
Tabulate corals were also important in the Paleozoic. All tabulate corals were colonial and the
individual corallites were smaller than the rugose corals. Corallites looked like tubes partitioned
by prominent tabulae (horizontal partitions). Septae were small or absent.
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Modern (scleractinian) corals are similar in some respects to the rugose corals. They have
prominent septa with a radiating pattern. Modern colonial corals differ from the colonial tabulate
and rugose corals in that they commonly do not have separating walls between corallites.
5. Examine the fossil corals at the back of the lab. The fossil set in the drawers has four genera
of coral.
a) Which ones are they?
F10,
F11,
F12,
F13
b) Identify two using the fossil identification chart and give the age (or age range) for each.
Any 2 of: Favosites (Sil-Penn); Eridophyllum (L-M Dev);
Name:
Age:
Lophophyllidium (Penn-Perm); Caninia (Miss-Penn);
Heliophyllum
(Dev); ZaphrentisAge:
(Dev)
Name:
c) Make a labelled drawing of one solitary coral and of one colonial coral from the display
at the back of the lab. Use the “morphology” handout to help you label each
appropriately. Labels should include one or more of septum (pl: septa), calice, tabula (pl:
tabulae), ruga (pl: rugae). Make sure you have a scale.
Solitary form:
Colonial form:
or