Early
Paleozoicum Ch. 10.
-Craton
-Mobil
belts
shield(Precambrian met. and
igneous rocks)
platform
Outward part of craton, where metamorphic rocks are
covered by shallow water sediments
Epiric
seas seawater
covers some part of the craton
Mobile
belts
Elongated
areas of mountain building activity. They occur along continent margins
Paleogeography
The
location of continents in the Earth history is not stable, everything is moving
all the time.
It is a
big buisness to reconstruct the paleogeography for a certain time.
What
can we use?
1.
Paleomagnetism (for younger ages than Paleozoic)
2.
Climate sensitive rocks
3.
Environmnet sensitive rocks
4.
Fossils
5.Tectonic
data
Paleozoic
Paleogeography
In
the beginning of Paleozoic, there were 6 continents:
1.
Baltica (west of Ural Mountains)
2.
China (China, indochina and the Malay Penninsula)
3.
Gondwana (Africa, Antarctica, Australia, Florida, India, Madagaskar)
4.
Kasakhstania (extension of Siberia)
5.
Laurentia (North America, Greenlans, Ireland, Scotland, part of Russia)
6.
Siberia (E of Ural, North of Kazahstan South of Mongolia)
Mobil
belts:
Franklin
Cordilleran
Ouchita
Appalachian
Sequence
stratigraphy
Laurence
Sloss in 1963 proposed that sedimentary rock record in North America could be
divided into six cratonic sequences.
A
cratonic sequence is a large scale (larger than supergroup) lithostratigraphic
unit representing a major transgressive-regressive
cycle bounded by cratonwide unconformities. Transgressive phase is well
preserved, regressive part is marked by unconformities.
For
the Early Paleozoic their name are: Sauk and Tippeconoe
The
global correlation of this super sequences is called sequence stratigraphy
Sauk
Sequence (This is
the first Phanerozoic transgression over the Precambrian american craton)
During
the latest Proterozoic and earliest
Cambrian the sea did not cover the craton, just along the border
(Appalachian, and Cordilleran borders). These borders were passive margin during
this time. Thick carbonate sequence formed, because the north american
continents were located around the equator.
The
other parts were emerged, and extensive weathering of Precambrian rocks took
place.
North
America was situated in the tropics at this time, and there is no evidence for
any kind of vegetation, therefore erosion rate must have been really high.
During
the Middle and late Cambrian a transgression started, and epiric sea covered
much of the craton living just a few land area behind. Such a high area was the
so called transcontinental arch (highland area) extended from Lake Superior to
New Mexico, and some part of hte Canadian shield.
This
time is characterized by wide spreaded shallow marine water sedimentation. Many
of the shallow water carbonates contain bioclastics, organic fragments of
fossils, and stromatolites (blue-green algae). They also frequently have oolitic
textures.
Grand
Canyon shows an excellent example for the Sauk marine transgression.
Carbonate
deposition dominated on the craton in the latest Cambriam, and even the
Transconintal arch was covered by seawater.
As the
Sauk sea regressed from the craton during the early Ordovician most of the
craton become emerged. The rocks exposed, were predominantly limestone and
dolostone, that experienced deep erosion (karst) (tropical climate). The result
of this was a widespreaded unconformity
giving the boundary between Sauk and Tippecanoe sequences.
The
Tippecanoe sequence
Tippecanoe
also started with a major transgression onto the craton
The
first product of the transgression was a very clean quartz sandstone all over
the craton.
St. Peter Sandstone almost pure Q ----glass industry
This sandstone is most likely recycled Proterozoic sandstone
The
sandstone is overlied by limestone,
as the second product of Tippecanoe transgression. This carbonate is mostly
biochemical in origin containing Corals, Brachiopods, Stromatoporoids, and Bryozoans. There were also
many dolomites. (dol. formation!!!!!)
Seaward
the carbonate deposition grades into shale, which is the deeper water facies.
Major
features of the Tippecanoe sequences are reef facies and evaporites.
In the
Middle Ordovician stromatoporoid-coral reefs become common in the low-latitudes.
During
the Middle Silurian reef and evaporite facies are even more common.
A great
example for this is the Michigan basin.
This
basin at that time was surrounded by large barrier reefs. Within the fast
subsiding basin Pinnacle reefs were formed. These are tall, spindly structures
up to 100 m high. They had to grow pretty fast to be able to keep up with the
subsidence rate.
On the
shallow water part of the platform, evaporite and anhydrite formed indicating
very arid climate.
During
the Late Silurian, as the Tippekanoe sea was regressing, about 1500 m sediment
deposited, and nearly half of it was salt and anhydrite.
At the
end of the Silurian and the earliest Devonian the Tippecanoen sea regressed even
further, and the whole craton became exposed, and experienced a large scale
erosion again.
During
the Middle Ordovician in the Appalachian mobile belt the first Phanerozoic
orogenic event started.
During
Sauk time, this area was a passive margin, similarly to the present day
situation. Sedimentation was balanced by subsidence, and thick shallow marine
carbonate formed (the big outcrop on 81.) During this time the Iapetus ocean (preatlantic)
was getting wider.
When
the subduction between Laurentia and the Iapetus ocean started, the Appalachian
area become similar to the Rockies.
The
period of subduction and mountain forming is called orogenesis.
This particular one is the so called Tacony orogeny.
In the
Middle Ordovician, the carbonate deposition ceased, and the basin become deep,
and started to be filled by graywacke, coarse sandstone which marks the onset of mountain building.
Adjacent
to the uplifted area a foreland basin formed, and filled up quickly by clastic
sediments (Clastic wedge) coming from the Taconi highland (qeenston delta).
q-sandstone,
gravel, limestone building
stones
evaporites
Michigan, Ohio, Virginia
Missisippi
Valley type deposits