Paleogeography, Part 2:
Epeiric Seas
Epeiric seas are extensive bodies of marine water overlying continental crust rather than
oceanic. Because continental crust stands higher in the asthenosphere than oceanic they are
typically very shallow. Tens of meters is the average depth and they probably don’t get much
over a couple hundred meters in most places. The exceptions to this are very interesting, as
we will see.
Similarly, because the continental crust must be flooded by marine transgression these things
do not generally form where there is much topographic relief. Consequently the bottoms are
very nearly flat as well as shallow. The stable central cratons of the late Proterozoic and
Phanerozoic continents fit the bill very well: nearly flat lowlands not much above present sea
level.
The only good extant example we have is Hudson Bay in Canada. This region is flooded
because the thick ice sheet that covered it until about 10,000 years ago had caused it to
subside below sea level. Greenland is in a similar situation but that ice is still in place, so it
cannot flood. The land around and under Hudson Bay is actively rebounding and the Bay is
actively draining.
The present continental shelves are small versions of the same thing. When sea-level was
higher their shores transgressed inland pretty far, but they were always marginal seas and
never reached the extent we will see for true epeiric seas.
Synonyms for epeiric sea include “cratonic sea” or “epicontinental sea” in reference to their
typical location.
Ediacaran paleogeography (modified from Scotese)
sChn
nChn
Aust
Arab
Panthalassia
Ind
Ant
Rhodinia
nAf
sAf
N Asia
NAm
wAf
Grn
nSAm
Eur
Grenville Mts
The maps we will be most interested in in this slide show will be the North American part of
the global ones you saw in the previous show. This global one is included to show you the
extent of these seas and to impress upon you that they are generally global in scale, even
though we will pay particular attention to the North American ones. Certain of them in other
regions are of great economic interest and we will occasionally mention them as well.
Ediacaran paleogeography (modified from Scotese)
sChn
nChn
Aust
Arab
Panthalassia
Ind
Ant
Rhodinia
nAf
sAf
N Asia
NAm
wAf
Grn
nSAm
Eur
Grenville Mts
Notice that most of western North America is inundated on the map above (red arrow). The
Grenville Mountains in the east and the coastal plain that prograded westward from those
mountains, along with the fairly high elevation of the Canadian Shield kept the eastern half dry.
North America in the Cambrian (Modified from Scotese)
By the Cambrian
Period the Grenville
Mountains were
eroded completely
away. Except for the
Shield the remainder
of the craton was
covered by the Sauk
Sea, which persisted
into the early
Ordovician before
regressing and
leaving most of its
former area above
sea level.
Look back at the
global map to see
that the other
continents were
similarly flooded.
Equator
Sauk
Sea
Canadian Shield
(land)
North America in the Ordovician (Modified from Scotese)
In the Middle Ordovician
the land was again
transgressed forming a
new sea we call
“Tippecanoe” (but not
Tyler too). It persisted into
the Silurian.
Geologists name each of
these seas separately just
to bedevil intro students.
The names are derived
from some location where
the sediments that show
the seas’ existences can
be seen. This one comes
from a river of that name in
Indiana. The Sauk is a
stream in Wisconsin.
Equator
Canadian Shield
(land)
Tippecanoe
Sea
Taconic
Mts.
North America in the Devonian (Modified from Scotese)
Regression of the
Tippecanoe Sea in the
Silurian again left most
of the continent dry, but
it was again
transgressed in the
Middle Devonian. The
Kaskaskia Sea for a
river in Illinois.
The Kaskaskia was
drained by regression in
the Mississippian
Period.
Equator
Canadian
Shield
(land)
Antler
Mts.
Kaskaskia
Sea
Acadian
Mts.
North America in the Pennsylvanian (Modified from Scotese)
Once again sea level
rose in the
Pennsylvanian Period
and covered it with the
Absaroka Sea, named
for a mountain chain in
Montana and Wyoming.
This sea persisted into
the Permian, but as
deltas and coastal plains
prograded westward
from the huge
Appalachian chain it was
restricted farther and
farther southwestward –
to Texas, New Mexico,
and Mexico.
Canadian
Shield
(land)
Absaroka
Sea
Equator
Appalachian
Mts
North America in the Jurassic (Modified from Scotese)
The “Sundance Sea” was
really a small gulf that
transgressed southward
from the northwest part of
the continent to about
Wyoming. It is
sometimes recognized as
a separate sea, but really
it is just the initial
transgression of the sea
we will see on the next
slide. it was named for a
stream and small town in
NE Wyoming where an
annual film festival is held
and where the Sundance
Kid got his name.
30°N
Uplift over
intrusions of
Granitic magma
that will become
the Sierra Nevada
and Cascades.
North America in the Cretaceous (Modified from Scotese)
By the Cretaceous that sea had
transgressed southward to
connect the Arctic Ocean to the
Gulf of Mexico, despite the
uplift of the Rockies on its
western coast.
Laramide
(Rocky)
Mts
This is called the Zuni Sea for a
tributary of the Little Colorado in
Arizona and New Mexico. It is
also sometimes called the
“Cretaceous Interior Seaway”.
30°N
By the Paleocene it had filled
with sediment from the Rockies.
Mesa Verde is built in some of
those deltaic sediments.
Treat the Zuni as Jurassic to
Cretaceous.
Zuni
Sea
North America in the Eocene (Modified from Scotese)
I want to mention one last “sea” that turns out not to have been a sea. Clearly aquatic deposits
are very common in Paleogene rocks of the eastern Rockies and Great Plains region, and
were initially interpreted as the “Tejas Sea”. If you have ever seen a complete fish fossil, it
probably came from there. Eventually, however more sedimentologic work demonstrated that
these sediments represent vast freshwater lakes on the coastal plain that developed east of
the Rockies, then filled the Zuni Sea and became simply the Great Plains.
Though epeiric seas were generally shallow and had little bathymetric relief, there
were noticeably shallower parts (or large Islands) and deeper parts, as evidenced
by unconformities (islands) or thinning/thickening of the strata in those regions. The
principle epeiric domes and basins of North America are shown here.
Williston/Alberta Basin
Adirondack Dome
Michigan Basin
Illinois Basin
Appalachian Basin
Cincinnati Arch
Permian Basin
Ozark Dome
Nashville Dome
These basins and domes are interesting from a paleogeographic perspective, but they are also of
great economic importance, particularly the basins.
Most of them were present in most of the Paleozoic epeiric seas and the exceptions are listed
below. For this class simply remember that the Era(s) that they existed and don’t worry about
more detailed ages.
Surrounding the edges of both domes and basins, at the point where depth reached wave-base
level, coral reefs formed. Above that depth coarse-grained sediments accumulated. In many
cases, because of slow subsidence in the basin or no additional uplift on the dome, these reefs
remained small. In others, greater subsidence rates allowed the reefs and coarse sediments to
accumulate to substantial thicknesses.
The smaller deposits have trapped modest amounts of oil and natural gas. There are many
people around the southern Michigan Basin and the western Appalachian Basin with little pump
heads on the back 40 pulling small fortunes out of the ground. This has been going on at fairly
constant rates for decades.
In others, bigger reefs have accumulated truly huge petroleum stores and vast fortunes have
been made from them.
The deeper parts of the basins also tended to accumulate evaporites when the climate was right.
Mr. Morton dug tons and tons of salt out of the Michigan Basin (and his company still does) and
KCl from the Williston/Alberta Basin is the world’s go-to source for K in synthetic fertilizers.
WILLISTON/ALBERTA BASIN
This basin persisted through all the Paleozoic seas and into the Mesozoic. It was
the most extensive, geographically, of all the American ones, and is often treated as
two separate basins – the Williston Basin and the Alberta Basin.
When I was in college the capitol of Alberta, Calgary, was a small cow and market
town. The oil that came into production there a little later has made it one of the
biggest cities in Canada and has made Canada an oil exporting country. Part of
the decline in OPEC power in the 1980’s came on the heels of development of
these reserves.
The Williston Basin also produces in
the USA. There have been small fields
around its southern margin for some
time. The present oil and gas boom in
North Dakota is also based in this basin.
MICHIGAN BASIN
As mentioned, the Michigan Basin has produced a lot of salt. In the Silurian
there was a tenuous connection (through the Appalachian Basin) to the
Iapetos/Rheic Oceans. This basin also sat right in the mid-latitude desert belt
at about 30° S, creating ideal conditions for high evaporation rates. The
hypersaline water at the surface sank as its salinity made it denser than the
water beneath, eventually making the entire basin’s water salty enough to
precipitate salt.
Around the margins, particularly the
southern margin in Indiana and Illinois,
reefs built up to great thickness. The
deposits of those reefs have produced
oil in modest, but consistent,
quantities since the early 1900’s.
The basin existed in seas throughout
the Paleozoic and caught some
sediment as late as the Jurassic.
ILLINOIS BASIN
Sediment thickenings in the Illinois basin are only obvious in the later Paleozoic
systems, particularly the Pennsylvanian.
With a huge mountain chain actively building to the east there should be nothing
surprising in the fact that it filled with, primarily, deltaic sediment.
The primary mineral resource of this
basin is therefore coal, which
supplied the power and some of the
raw material for the great steel mills
in Great Lakes cities like Detroit,
which processed mostly BIF ores
from the Archaean of the Superior
Province of the Canadian Shield.
APPALACHIAN BASIN
Like the Illinois Basin, the small Appalachian Basin has only sediment thickenings
from the later Paleozoic systems. Unlike the Illinois it has appreciable
Mississippian coarse sediments in it as well as Pennsylvanian and Permian coal
measures.
In addition, this basin was directly on the margin of the craton and mobile belt, and
so beneath it there are some thick basinal deposits from the Taconic and Acadian
Mountains. The organic-rich “Devonian Shales” are the more important of these,
serving as source for both oil and gas.
A drive on back-roads in eastern Ohio
or western West Virginia occasionally
takes you through a patch of land where
small oil tanks are densely scattered
among the farm and forest land, and
odor (the landowners would call it the
aroma of crude oil) is strong.
A school friend’s father told stories
when we were kids of scooping
high-test gasoline off the tops of these
stores. Just imagine, free gasoline!
PERMIAN BASIN
The small Permian Basin only existed very late in the Paleozoic, as the name implies.
Despite its size and short life, it has been a money machine.
The reef deposits in Texas and New Mexico that ringed this basin were phenomenally
large, and subsidence allowed them to build to great thickness. The center of the basin
filled with similarly thick black shales that eventually fed oil and gas into the reefs. The
University of Texas in the Permian Basin has its mission focused primarily on petroleum
geology, petroleum engineering, petroleum law, and petroleum whatever-else-there-is,
because there is a lot of petroleum there.
My first full-time teaching position was at UT San Antonio, where I learned of the PUF – the
Permanent University Fund..
The PUF is funded by a 1% royalty on
each drop of oil produced from the
Permian Basin. In 1980 the principle on
this fund was well in excess of a billion
1980 dollars. I can only imagine what it
is now. When UT wants a new building
they do not go to the TX legislature and
ask. They write a check from the
interest this fund has generated!
I want to mention two more epeiric seas that you should know about. Both were Cretaceous seas. Much of Arabia
(attached to Africa) was inundated by one of them, and much of the south Eurasian shelf, across the Tethys, was also
covered. Almost all of the Middle East’s oil originated in and is trapped in deposits of these epeiric seas.
Farther west, in the Paris/London basin and North Sea region, other tremendous oil deposits occur in their deposits.
The Middle Eastern oil led to the crreation of OPEC in the 1970’s which led to the “oil crisis” of that time. The North
Sea deposits, along with the Alberta Basin reserves, were instrumental in curbing OPEC power in the 1980’s and 90’s.
Cretaceous paleogeography (modified from Scotese)
Eurasia
Rifts
North
America
Laramide
(Rocky)
Mts
North
Atlantic
Ocean
Pacific
Ocean
Andes
Mts
Tethys
Ocean
Africa
South
America
South
Atlantic
Ocean
Madagascar
& India
Australia
Rifts
Antarctica