Week of October 1st, 2001
The assembly of North America:
what's a craton, what's
an orogeny?
This week you should:
| During
Precambrian times, prokaryotic microbes oxygenated the shallow oceans and
the atmosphere, invented cellular reproduction (asexual and sexual) and
developed a range of metabolic pathways to extract energy from chemical
compounds or from reactions driven by solar energy (photosynthesis). This
early Earth also developed plate tectonics. This provided further impetus
for biological evolution, by causing environmental change at a pace to
which life could adapt.
Associations of prokaryotes gave rise to aerobic eukaryotes. Some eukaryotes adopted colonial modes of life and symbiotic relationships developed among different types of cells. Some microbial colonies probably gave rise to the earliest, soft-bodied animals of the Ediacaran (also called Vendian) fauna. The Proterozoic life forms show no evidence of predation and limited ability to hide into or feed from sediment. Towards the end of the Proterozoic Eon, algal mats and the stromatolitic structures they left behind became scarcer, while deeper and more complex burrows appeared. Fossils of Proterozoic life are scarce because most lacked the ability to secrete hard parts. Their discovery in Australia dates from 1946 but the recent discovery that similar fauna also thrived in Africa and North America rekindled interest in the cause(s) of the Cambrian explosion. |
The earliest animal life, documented in Precambrian rocks, vanished or declined by the beginning of the Cambrian. Were there dramatic environmental changes that might explain its appearance and its decline?
Precambrian fossils are relatively hard to find, but not necessarily because Precambrian rocks are scarce or strongly deformed and metamorphosed. Canada has exceptional sedimentary sequences of Proterozoic age, some recently (or now) examined by Professor Narbonne's group (Queen's University) and Hans Hofmann (U. Montreal-McGill). Where are these sequences, and why haven't they been worn out by erosion or buried deep by younger sediments?
Thick sequences of Proterozoic sediments are preserved on the Canadian Shield, but no oceanic crust of that age is preserved. Why not?
The Appalachians and the
Rockies are familiar sights of Canadian landscapes, produced by the collision
of tectonic plates. The stable "core" of our continents grew by similar
processes. How can earth scientists identify the various "plates"
or blocks of crust that came together billions of years ago? Was tectonic
activity back in Proterozoic times responsible for mineral deposits that
are now part of our economy?
| Keywords: |
The review questions relevant to this week's lectures are:
A few more questions to think about: