The following represents a summary of some of the major discussion
points we will have in class about these Paleozoic Periods.
Major evaporite and reef formation occurred during these periods, as
discussed in detail in the text. A subsequent lecture will focus entirely
on evaporite and reef formation, therefore, is not discussed in detail
herein.
II. Paleogeography.
North America remained astride the equator during this interval.
Much of the U.S. would have been under the influence of paleo-tropical
trade winds.
Gondwana moves increasingly over the South Pole and into higher latitudes
II. Paleoclimate.
The wide global distribution of reefs and evaporites indicates a more
uniform global climate than today.
Devonian seedless trees proliferated in wet lowland environments forming
coal swamps.
III Sea Level.
The Tippecanoe transgression which began during the mid-Ordovician
continued until the Late Silurian as the Tippecanoe Sea began to withdraw
from the craton.
During the Late Silurian-Lower Devonian marine sedimentation became
restricted to deeper regions (e.g. basins) of the craton.
After a period of epeiric sea withdrawal, a third major flooding episode
of the craton began in the Early Devonian. The rise of sea level continued
unabated throughout the Devonian resulting in deposition of the Kaskaskia
cratonic sequence.
The Devonian rise of sea level was counteracted by renewed uplift on
the east coast.
Elsewhere sea level was influenced by basin and arch subsidence and
uplift.
IV. Surficial Distribution of the Silurian and Devonian in the U.S.
As we previously discussed, the distribution of rocks today has greatly
and variably influenced by subsequent erosional events. As a consequence,
the distribution we see today may have been significantly reduced from
its for extent. Such is the case with the Silurian and Devonian rock record.
From the widespread remnants of limestones and marine shales, it would
appear that the Tippecanoe cratonic sequence was seriously eroded during
the low stand separating the Tippecanoe and Kaskaskia sequences.
With the exception of the eastern margin of the U.S. most of the craton
was low and covered by a vast epeiric sea
V. Depositional Environments.
Most of the shallow epeiric seas of this interval were sites of major
carbonate deposition.
The cratonic arches and basins (mentioned in the last lecture) underwent
most of their subsidence or uplift during this time.
Some of these basins (e.g. Michigan Basin, Williston Basin) became
sites of shale and evaporite deposition, while shales and clastics increased
toward arches.
Transgression at the base of the Kaskaskia sequence again formed a
major basal transgressive sand body, the Oriskany Sandstone.
How do such well-sorted, pure quartz sandstones form?
The Devonian Acadian orogeny created a replay of Ordovician clastic
wedge formation.
The Devonian clastic wedge, the Catskill Delta, formed west of the
Acadian highland.
Far to the west, clays flooded the epeiric sea changing carbonate to
shale deposition.
VI. Tectonics-Class Discussion.
A. Acadian Orogeny
began in early Middle Devonian
caused by collision of microcontinent Avalon terrane with eastcoast
between Newfoundland and West Virgina
thick folded sequences of turbidites with rhyolitic volcanic rocks
and granitic intrusions
formation of Catskill clastic wedge
B. Caledonian Orogeny
As in N. America, began with the closure of the Iapetus Ocean in the
Middle Ordovician when subduction zones developed along the margins of
the formerly separating continents.
culminates in Late Silurian-earliest Devonian with deformation in Canadian
Maritime Provinces, NW England, NE Greenland, and Norway.