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 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 multi-

cellular 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.

Various coral

diagrams showing,

calice, tabula(e), septum(a),

etc.

Questions?