Download Shields and Precambrian fossils

Geology 1023
Shields and Precambrian fossils
Continental building-blocks (see
map at side of lab)
• Shield: central core of Precambrian rocks
– Virtually all shield rocks are igneous or highly
deformed metamorphic rocks
• Platform: a thin (a few hundred metres) skin of
undeformed sedimentary Phanerozoic rocks
– Divided into various sequences mentioned in class
– Sauk, Tippecanoe, Kaskaskia, Absaroka, Zuni
• Mobile (orogenic) belts: currently (or recently)
active mountain chains, not yet fully worn down
– Cordillera and Appalachians in North America.
This week’s lab is in two parts
• Rocks of the Shield
– In the exercise you will look at maps and
answer questions that pertain to the study of
these ancient and mostly highly-deformed rocks
• Precambrian life:
– A first look at organisms that first arose in the
Precambrian
– Note that most of the specimens are of
Phanerozoic fossils but they represent groups
that first developed in the Precambrian
Doing the lab.
• Q. 1. Based on 3 maps (2543, 2586 & 900A).
• (a) – (f) relate to reading information from map 2543
of Central Ontario
– Mostly related to seeing the distribution of the rocks
(shown on the map) and their ages (as shown on the
legend)
• (g) asks you to compare the geological map (2542)
and the map showing the magnetic fields in the area
(2586).
– Magnetic fields are produced/influenced by the rocks and
help us understand rock distribution, even when the rocks
are hidden from view.
Doing the lab.
• Canada’s wealth and status in the world today comes
from our role as a resource-rich country.
• A huge percentage of Canada’s wealth has come
from mining. The Precambrian shield has provided a
considerable amount of Canada’s mineral wealth.
• Q. 1 (i). Asks you to look at map 900A, which shows
the distribution of current (or recent) mining and oil
and gas operations in Canada
• At the side of the map is a legend for each Province
• Determine the major minerals/elements mined in the
the Precambrian shield areas of ON, QC, MB
Precambrian life
• Life arose in the Precambrian
• Oldest known preserved life forms are colonial
single-celled organisms of the Kingdom
Monera which formed cabbage-shaped mounds
called stromatolites
• Oldest stromatolites are from 3.5 Ga (billion
years ago)
• Stromatolites are still found in parts of the
world today.
Modern stromatolites
Ancient stromatolite
Multi-cellular life
• Multicellular organisms (like ourselves)
first developed in the last part of the
Precambrian
• One famous group of multicellular
organisms is known as the Ediacaran fauna,
first found in Ediacara, Australia
• Ediacaran fauna are now extinct
• We do not have a collection of these
bizarre-looking organisms which resemble
primitive sea-pens, sea jellies etc.
Multi-cellular life
• The two most primitive groups of
multicellular organisms which are still
living today are the Porifera, sponges, and
the Cnidaria, which include sea jellies, sea
anemones and corals.
• Sponges and corals both have skeletal
elements and thus are well-represented in
the fossil record
Sponges
• Sponges are probably the most primitive multicellular organisms around today
• Consist of a largely undifferentiated mass of
gelatinous cells supported by a variably rigid
structure (your modern bath-sponge is the
skeleton of a modern sponge animal)
• Sponges absorb water through their sides,
extract food, and emit the water through their
top – hence the chimney-type shape of most
sponges.
A modern funnel-shaped sponge
Bath sponges, the skeletons of
primitive animals
A fossil sponge
Cnidaria
• Phylum that includes sea jellies, anemones,
and corals
• All are jelly-like organisms with “stinging”
cells
• Only corals produce a skeleton (a calcium
carbonate structure attached to the bottom)
• Most corals are colonial, but some
Paleozoic (and probably Precambrian ones)
were solitary. Many produced a conical
cup-shaped skeleton (“cup corals”)
Doing the lab.
• Q. 2. Refresh your memory as to the
Linnean classification scheme
• Q. 3. Draw a representative portion of the
stromatolite at the back of the lab.
• Drawings do not have to be highly artistic,
just show the relevant features
• All drawings must have a scale, and label
what you can (laminations in the case of the
stromatolite)
Doing the lab
• Q. 4. Examine the examples of sponges at
the back of the lab.
• At the back of the lab are some specimens
of the calcareous sponge Girtyocoelia.
Make a drawing of it with scale and labels.
• At the back of the lab is a second handout
“Diagrams showing simple body plans…”.
Use it to determine features of interest.
Various sponges showing inhalant
pores and exhalant osculum.
Doing the lab
• Q. 5. Examine the specimens of corals at the
back of the lab. There are both “solitary” and
“colonial” forms.
• Examine the set of fossils in the drawers at the
back and identify the 4 that are corals.
• Determine the genus and age range for two of
the solitary forms.
• Draw a solitary and a colonial form. Use the
handout “Diagrams showing simple body
plans…” to help you assign labels.
Two fossil groups of corals
• Rugose corals:
–
–
–
–
Solitary and colonial forms
Prominent radiating septa in the cup (calice)
Calice large (commonly 1 cm or more across)
Solitary forms clearly show growth lines on the
outside giving a wrinkled (rugose) appearance
• Tabulate corals:
– Colonial forms only
– No (or minimal) radiating septa
– Calices are small (a few mm across)
A solitary rugose coral showing: polyp,
calice, septa, rugae (wrinkled growth lines).
Solitary rugose corals
Colonial rugose coral with prominent septa
Tabulate coral showing: tabulae and
pores, but with minimal septa
Questions?