Download Twenty-third lecture

The Ordovician Period: 488-444 Million Years Ago
(Part I: Tectonics)
Laurentia and
Baltica, which were
diverging in the
Cambrian, begin to
converge (move
together) once
again. This will
have major impacts
on the interior of
North America.
Western North
America was
largely
submerged,
with an offshore
subduction
zone.
The Iapetus
Ocean was
closing between
Laurentia and
Baltica.
Gondwana
stretches from
the South Pole
to north of the
Equator.
Overall distribution of Ordovician rocks in the U.S.A.
Early Ordovian sea level
was high
Canadian Shield margins
were being covered by
clastic and carbonate
sediments.
Shallow-water carbonates
were accumulating in the
mid-continent regions - with
mudcracks and
stromatolites.
The St. Peter Sandstone is an ultra-mature quartz sandstone deposited in shallow
nearshore marine environments with relatively high energy levels, which washed out all
silts and clays. The fact that it is virtually 100% pure quartz suggests it may have been
derived from older sedimentary units, such that all other minerals were weathered.
This unit is a mainstay of the American glass manufacturing industry because of its
purity. More recently, it has been mined as a source of sand for hydraulic fracturing of
tight shales for oil and gas production.
The sand in the St. Peter Sandstone is highly rounded and smooth,
and virtually 100% quartz.
0.0
0.5
1.0 mm
Distribution of the St. Peter Sandstone (Ordovician) in the mid-continent of the
U.S.A.; darker color indicates greater thickness.
Implied direction
of source
Carbonate deposition
ended in the Middle
Ordovician
Region was downwarped
This was caused by
closing of the Iapetus
Ocean along the Taconic
subduction zone, and
upwarping of the
continental margin.
This subduction along the Laurentian margin initiated the formation of
the Appalachians in this part of the continent – the Taconic Mountains.
(Laurentia)
Subduction
shallow-water marine deposits
(Epicontinental sea)
The Taconic Orogeny added land to Laurentia that is now the western portions of New
England and the Canadian Maritime provinces. This collision added land and raised
mountains southward through northern New Jersey, south-eastern Pennsylvania,
Virginia and North Carolina. This occurred approximately 460-480 myBP
Ordovician metavolcanic rocks at Friendship, on the Maine coast. Minerals in adjacent
layers are chemically incompatible – implying that the original volcanic ash package came
from highly variable eruptions. The volcanic pile here is approximately 2 kilometers thick.
Prior to the
Taconic Orogeny,
the "east" coast
of what is now
the United States
was located near
the Hudson
River valley,
Philadelphia, and
Washington,
D.C., and
extended to
western South
Carolina.
Baltica
Laurentia
Taconic Mountains
Nashoba Volcanic Arc and Avalonia
The Taconic Mountains, including their volcanoes, became
accreted to Laurentia, as the Nashoba Island Arc and Avalonia
approached ....
Queenston
clastic wedge
Queenston Clastic Sedge
sediments exposed in
a Pennsylvania roadcut
The Utica Shale is a black Ordovician shale
deposited in the nearshore environment
at the outermost part of the Queenston
Clastic Wedge, that represent rapid
burial of organic carbon under anoxic
(oxygen-free) conditions.
(It, and the overlying Marcellus Shale of
Devonian age, are currently major
targets for natural gas exploitation.)
WHY?
1. Low Solar output
2. Concentration of land at South Pole
3. Dropping atmospheric CO2 levels
At least 4-5 separate expansions
and retreats of glacial ice are
known in modern North Africa
from the Ordovician.
These glacial striations
(scratches on the rock surface)
were photographed in Libya, in
northwestern Africa.
NEXT: Ordovician life ...
..... ending with a BANG!