HISTORICAL GEOLOGY
LECTURE 9. EARLY PALEOZOIC GEOLOGY II.
ORDOVICIAN
(488-443 MYBP)
1. Paleogeography
The major
difference in
geography between
the Cambrian and
Ordovician is that
the craton margins
became ZONES OF
CONVERGENCE
during the
Ordovician;
fragmented Rodinia
was just beginning
to reassemble.
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The result of this was the formation of subduction
zones on the craton margins:
-> TACONIC HIGHLANDS of east coast
-> highlands bordering Franklin Seaway
-> volcanic island arc along the margin of the
Cordilleran seaway.
Rocks deposited adjacent to these areas are
characteristic of subduction zones:
- erosion of rugged rapidly growing uplands ->
graywackes, conglomerates, shales.
- volcanic activity -> volcanic rocks
- no carbonates (in areas adjacent to subduction).
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2. Transgressions and Regressions
The beginning of the Ordovician is marked by the
SAUK REGRESSION, which left a major EROSIONAL
UNCONFORMITY, used to identify the boundary between
the SAUK and the TIPPECANOE strata sequences.
The rest of the Ordovician, like the Cambrian, is
characterized by a marine transgression - THE
TIPPECANOE TRANSGRESSION. Typical transgressive
sequences were deposited in the advancing epeiric seas.
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3. Orogenies
The major orogenic episode occurred during the middlelate Ordovician - THE TACONIC OROGENY
This was associated with the subduction zone that had formed
down the east coast of the craton.
The development of a convergent margin caused the
formation of the TACONIC HIGHLANDS, down the N.E. coast
(remnants of which form the Taconic Mountains of New York
State). These uplands consisted of folded/metamorphosed
sedimentary rocks + accreted and intruded volcanics; and
probably reached over 13000 feet in height.
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Later, intense erosion of the Taconic highlands resulted in the
QUEENSTON CLASTIC WEDGE
This is a great wedge of non-marine clastic sediments which
spread westward from the growing highlands
(contains 600,000 km3 of eroded rock).
PROXIMAL =
DISTAL =
CONGLOMERATES
SANDSTONES
SHALES
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The other noticeable event
of this period was the
beginning of thick
sedimentation in the
OUACHITA TROUGH the deep marine S.W.
extension of the
Appalachian Seaway.
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Silurian (443-416
mybp).
1. Paleogeography
The east coast from
Newfoundland to
Florida was now
continuous highlands
due to the continuing
growth of the Taconic
orogeny.
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2. Transgressions/Regressions
Most of Silurian time was characterized by the
Tippecanoe regression. The major effect of the regression
was the formation of landlocked, reef-fringed marine
basins in the region now occupied by the Great Lakes.
Evaporation from these inland seas created vast
EVAPORITE DEPOSITS (salts) up to 500 m thick these are presently mined.
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3. Orogenies
The closure of the
Proto-Atlantic
(Iapetus) Ocean
continued during
Silurian time,
culminating in a
continent-continent
collision between
Laurentia (N.
America) and
Baltica (Europe) in
late Silurian/early
Devonian time. This
resulted in the
Caledonian Orogeny
and LAURUSSIA.
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The most intense effects of the Caledonian Orogeny were in
Scandinavia and Northern Europe (Norway, Scotland). On the
North American craton the effects were less severe, but helped
to create the mountainous terrain of the Northern Appalachian
Mountains. This mountain-building episode continued into
Devonian time as the ACADIAN OROGENY (more on this
later).
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A major result of the
Caledonian Orogeny in
Northern Europe was the
building of mountains
(CALEDONIDES) which
underwent intense erosion
and formed a wedge of nonmarine clastics over much of
what is now Northern
Europe. This was the "Old
Red Continent" - composed
mainly of rusty-colored
sandstones (old red
sandstones) - similar to the
Queenston Clastic Wedge of
North America.
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Part of the Caledonian orogenic belt, Scotland.
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The other notable event of late Silurian time was that Siberia begun
to converge on Laurussia - forming subduction zones at their
margins and beginning the closure of the Uralian Seaway.
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