Lecture 10
Early Paleozoic Mountain Building
Focus Question-How did North America change during the early Paleozoic Era?
1.
In the
first period of the Phanerozoic Eon, the Cambrian Period,
we find North America lying on the equator, and the midcontinent
(including Ohio), completely submerged in a shallow sea (epeiric or
epicontinental sea).
See this link for a view of the Late Cambrian (500 million years ago)
from Ron Blakey’s plate tectonics reconstructions (Ron Blakey, NAU Geology)
http://jan.ucc.nau.edu/~rcb7/namC500.jpg.
Note
A.
The
volcanic island arcs offshore of the east coast of north America
B.
The trench visible along those volcanic island
arcs, between the islands and North America
C.
The passive margin of the west coast, compared to
the active margin of the east coast
2. By Late Ordovician time (about 450 million years ago) note what has happened to North America on Ron Blakey’s (Ron Blakey, NAU Geology) plate tectonics reconstructions here http://jan.ucc.nau.edu/~rcb7/namO450.jpg.
A.
The
volcanic island arc offshore has now collided with North America, and the
mountain building event from this collision is the Taconic Orogeny (orogeny =
mountain building event) and the Queenston Delta Clastic Wedge. This is
the first big mountain building event that starts the development of the
Appalachian Mountains.
B.
The Taconic Parkway in eastern New York runs
through exposures of this Ordovician event.
The Taconic unconformity is an angular unconformity showing the folding
and faulting at this time.
C.
Look at Ohio-still a tropical, shallow sea.
Look at Michigan-see how that is deeper water-this is a basin, and is
colored a darker blue to indicate deeper water.
D.
Note how the west coast is still a passive
margin. Note that there is NO
FLORIDA! This predates the
formation of the big carbonate platform that becomes Florida.
E.
Tropical
Ohio has a record! The shallow
tropical seas (rather like the Bahamas today) of late Ordovician Ohio are still
visible ! The fossil rich carbonate
layers are exposed at Caesar’s Creek State Park, just south of Wilmington, Ohio,
off I-71. Go wading in those
tropical seas of long ago, and collect 450 million year old seashells!
These fossils are typical of shallow marine environments with lots of
carbonate muds.
a.
Top predator-the Nautiloid-a large cephalopod
with a long straight, chambered shell.
b.
Common coral-the horn coral (a Rugosa coral)
c. Other common marine organisms: bryozoans, brachiopods, other mollusks, and trilobites.
3.
The
Ordovician ended with an extinction event, and the shallow seas started to dry
up. Glaciers formed as Gondwanaland
moved south over the pole. Look at
the difference between the Late Ordovician map (above link) and the Silurian map
(Ron Blakey, NAU) here:
http://jan.ucc.nau.edu/~rcb7/namS430.jpg
.
a.
Note how
the shallow seas have retreated in the midcontinent, and much of Ohio is
exposed.
b.
Note the appearance of another volcanic island
arc, this time coming in from the west.
c.
Note the appearance of land masses to the east of
North America.
d.
Still no Florida!
4. Devonian
Period-Acadian Orogeny-affected primarily the northeastern coast of the U.S. and
eastern Canada. Collision between
Baltica (western Europe) and Laurentia (North America) as well as a small
continental mass known as Avalonia caused the Acadian Orogeny-see this Late
Devonian Period image by Ron Blakey (University of Northern Arizona) here:
http://jan.ucc.nau.edu/~rcb7/namD360.jpg
b.
The Catskill Delta, an immense clastic (sands and
silts) wedge of sediment poured off the rising mountains in the Acadian Orogeny,
and shed westward across New York and Pennsylvania.
a.
These
sediments are much thicker towards the east, and thinner towards the west.
They are facies deposits ranging from stream deposits to shallow marine
deposits.
b.
The Marcellus Shale, currently an important
source of natural gas for Ohio, Pennsylvania and New York is a lower Devonian
shale bed that can be seen in the lower section of the Catskill Delta.
5.
Ohio is a
state dominated by Paleozoic rocks.
If we look at the bedrock map of the state of Ohio (see link to this at
http://www.ohiodnr.com/geosurvey/pub/maps/bgmap/tabid/7224/Default.aspx
and download the free map)
we can see that
a.
The
oldest rocks are Cambrian and Ordovician, exposed in the southwest of the state.
b.
The youngest rocks are Permian, exposed in the
southeast of the state.
c.
Progressively, as we move from west to east, we
cross a roughly north-south
pattern of stripes of these Paleozoic
rocks.
d.
The cross section shows the anticlinal shape (the
"A" shape appearance) of the Cincinnati arch.
6.
Ohio
brings in a considerable income
(about 700 million dollars/yr) in sales of its Paleozoic oil,gas,coal and salt.
The major minerals /resources include
a.
Salt-mined
as solid crystalline salt, or pumped as a brine from deep groundwater-Silurian
b.
Silurian salt is deposited in a sequence of
precipitated minerals that formed as a shallow sea dried out during Silurian
time.
c.
The shallow sea was a bit of flooded continent
that included what is now Ohio, Michigan, western Pennsylvania and western New
York.
d
As the seawater dried out, the first mineral to
precipitate out was calcium carbonate (calcite).
This was followed by gypsum, and finally sodium chloride (salt).
e.
To reach the salt, we mine into the Silurian
rocks (the Salinas Group) beneath modern Lake Erie, or we pump brine (salty groundwater) up from
Silurian layers and let it evaporate, and collect the salt.
7.
Oil-pumped from Cambrian (Rose Run Sandstone) and
Ordovician (Trenton Limestone) rock
a.
Look at
the map. You can see that Cambrian
and Ordovician rocks are exposed at the surface in the southwestern part of the
state-near Cincinnati. The Trenton
Limestone was a major producer of oil for the U.S. in the early 20th
c.
b.
The Rose Run has a strong facies change-there is
a shallow water facies (sandstone) to the east (where it is subsurface), and a
deeper water facies (limestone and shale) to the west, where it is closest to
the surface. The sandstone facies
is a gas and oil producer, so those wells are to the east, into the Rose Run.
8.
Gas-(that is, methane-CH4)-pumped conventionally
with oil, and in unconventional gas wells in gas shales-Devonian
1.
Conventionally
pumped gas is methane extracted along with oil extraction.
Unconventional is direct gas extraction
from shales. Ohio has a lot of this
shale gas.
2.
The Devonian organic rich shales forming in
basins along the western side of the Appalachian front (think Acadian Orogeny
here) are the sources of shale gas.
3.
The Marcellus Shale is a Devonian shale extending
in a large area across western NY, western PA, WV, eastern Ohio, and eastern KY.
4.
The Marcellus is actively drilled now to extract
the methane gas, and is one of the most productive and potentially gas-rich
shales in the U.S. This Devonian
gas-bearing shale has 262 trillion cubic feet of technically recoverable gas,
according to the Department of Energy’s NETL lab
http://geosurvey.ohiodnr.gov/mineral-resources/minerals-home and download
the 2009 pdf report or just open the maps
The Shale Gas Primer-publication of the DOE-NETL
lab-on line to download as a pdf
