Historical Geology
INTRODUCTION
CONTINENTAL
ARCHITECTURE: CRATONS AND MOBILE BELTS
PALEOZOIC PALEOGEOGRAPHY
Early Paleozoic Global History
EARLY PALEOZOIC EVOLUTION OF NORTH AMERICA
THE SAUK SEQUENCE
¬Perspective 10: The Grand
Canyon—A GeologistÕs Paradise
The Cambrian of the Grand Canyon Region: A Transgressive Facies Model
THE TIPPECANOE SEQUENCE
Tippecanoe Reefs and Evaporites
The End of the Tippecanoe Sequence
THE APPALACHIAN MOBILE BELT AND THE TACONIC OROGENY
EARLY PALEOZOIC MINERAL RESOURCES
SUMMARY
The following content objectives are presented in Chapter 10:
¬ Six major continents were present at the beginning of the Paleozoic Era and plate movement during the Early Paleozoic resulted in the first of several continental collisions leading to the formation of Pangaea at the end of the Paleozoic.
¬ The Paleozoic history of North America can be subdivided into six cratonic sequences, which represent major transgressive-regressive cycles.
¬ During the Sauk Sequence, warm, shallow seas covered most of North America, leaving only a portion of the Canadian shield and a few large islands above sea level.
¬ Like the Sauk Sequence, the Tippecanoe Sequence began with a major transgression resulting in widespread sandstones, followed by extensive carbonate and evaporite deposition.
¬ During Tippecanoe time, an oceanic-continental convergent plate boundary formed along the eastern margin of North America (known as the Appalachian mobile belt) resulting in the Taconic orogeny, the first of several orogenies to affect this area.
¬ Lower Paleozoic rocks contain a variety of important mineral resources.
To exhibit mastery of this chapter, students should be able to
demonstrate comprehension of the following:
¬ Reconstruction methods for paleogeography
¬ the formation of and evidence for cratons, domes, and mobile belts
¬ the six major Paleozoic continents and oceans
¬ the pattern and sequence of continental movement during the Early Paleozoic
¬ the major cratonic sequences of the Early Paleozoic
¬ the importance of transgressions and regressions in the cratonic history of North America, especially as seen at the Grand Canyon
¬ the major events of the Sauk sequence
¬ the major events of the Tippecanoe sequence, with an emphasis on modern and ancient reefs and evaporites
¬ the general evolution of the Appalachian mobile belt during the Early Paleozoic with emphasis on the evidence for the Taconic orogeny
¬ the types and occurrences of Early Paleozoic mineral deposits.
1. Most continents consist of two major components: a relatively stable craton over which epeiric seas transgressed and regressed, surrounded by mobile belts in which mountain building took place. Four mobile belts formed around the margin of the North American craton during the Paleozoic: the Franklin, Cordilleran, Ouachita, and Appalachian.
Figure 10.1 Major
Cratonic Structures and Mobile Belts
2. Six major continents and numerous microcontinents and island arcs existed at the beginning of the Paleozoic Era. During the Early Paleozoic (Ordovician-Silurian), Gondwana moved to a southward, as indicated by tillite deposits. The microcontinent Avalonia collided with Baltica. An active convergent plate margin formed along the eastern margin of Laurentia during the Ordovician. Avalonia-Baltica collided with Laurentia to form the larger continent of Laurasia, which closed the northern Iapetus Ocean.
Figure 10.2 Paleozoic
Paleography
3. The geologic history of North America can be divided into cratonic sequences that reflect cratonwide transgressions and regressions.
Figure 10.3 Cratonic Sequences of North America
4. The first major marine transgression onto the craton took place during the Sauk Sequence. At its maximum, the Sauk Sea covered the craton except for parts of the Canadian shield and the Transcontinental Arch, a few large islands above sea level.
Figure 10.4 Cambrian Paleogeography of North America
Figure 10.5 Cambrian Rocks of the Grand Canyon
Figure 10.6 Time Transgressive Cambrian Facies
Enrichment Topic 1. Transgressing Seas
Although climate change and rising sea levels are a current topic in
the news, the transgression of the Sauk Sea reveals in the geologic record that
rising sea levels have occurred at different times in our geologic past. Have students investigate projected
coastlines if the East Antarctic Ice Sheet would melt. How do these projected
coastline changes compare with the amount of continental crust above sea level
when the Sauk Sea trangressed? Nova
Online provides graphics depicting the scientific interpretation of coastlines
(http://www.pbs.org/wgbh/nova/warnings/waterworld/)
5. The Tippecanoe sequence began with a transgression that deposited clean, well-sorted sandstone over most of the craton. This was followed by extensive carbonate deposition. In an addition, large barrier reefs enclosed basins, resulting in evaporite deposition within these basins.
Figure 10.7 Ordovician Paleogeography of North America
Figure 10.8 Transgressing Tippecanoe Sea
Figure 10.9 Organic Reefs
Figure 10.10 Silurian Paleogeography of North America
Figure 10.11 The Michigan Basin
Figure 10.12 Evaporite Sedimentation
Enrichment Topic 2. The St. Peter Sandstone
The St. Peter sandstone is one of the chief deposits of glass sand of the north-central US. This Ordovician sandstone is derived from the weathering of Precambrian quartzites of the Canadian Shield, which may have been a second-generation sandstone in itself. The St. Peter Sandstone is a clean sandstone, meaning that it was reworked for many years, resulting in the removal of clay-sized material. The Ford Motor Company in St. Paul actually used the St. Peter Sandstone as a source for its windshield glass for several years. E.W. Heinrich, ÒGeologic types of glass-sand deposits and some North American representations,Ó Geological Society of America Bulletin, v.92. n.9, p.611-613.
A brief history of the glass industry may be found online at http://www.glassonline.com/infoserv/history.html.
6. The eastern edge of North America was a stable carbonate platform until the Middle Ordovician. During Tippecanoe time an oceanic-continental convergent plate boundary formed, resulting in the Taconic orogeny, the first of several orogenies to affect the Appalachian mobile belt.
Figure 10.13 Neoproterozoic to Late Ordovician Evolution of the Appalachian
Mobile Belt
7. The newly formed Taconic Highlands shed sediments
into the western epeiric sea, producing a clastic wedge geologists call the
Queenston Delta.
Figure 10.14 Reconstruction of the Taconic Highlands and Queenston Delta
Clastic Wedge
8. Early Paleozoic-aged rocks contain a variety of
mineral resources including building stone, limestone for cement, silica sand,
hydrocarbons, evaporites, and iron ore.
1. Point out the importance of river deltas in the growth of trading and cultural centers. Modern deltas such as on the Nile, Yangtze, and Mississippi Rivers have prominent cultural development. The Queenston Delta formed in much the same way as a fluvial system; it flowed from growing highlands into the proto-Atlantic Ocean.
2. Reinforce the importance of
time-stratigraphic markers by talking about the significance of Ordovician age
bentonite deposits in the Appalachians. These regionally extensive bentonite
deposits are interpreted to be volcanic ash layers related to island arc
volcanism during the early mountain building stages of the Appalachians. They
provide a widespread, datable, stratigraphic unit of uniform age within the
sedimentary rocks.
3. Review how geologists
reconstructed the paleogeography of the Early Paleozoic. What evidence exists for identifying
continents around a paleoequator?
1. How high were the Appalachian Mountains in the early stages of growth? Can you think of a modern analogy on Earth today? In your modern analogy, can you identify the location of where the clastic wedge is forming, or will form?
2. How does the size of one of SchlossÕ cratonic sequences compare with the lithostratigraphic units (formation, group, supergroup) that were introduced in Chapter 5?
3. If the organisms throughout the Early Paleozoic resembled the Ediacaran fauna, would the carbonate deposits of the Early Paleozoic have been formed organically, or inorganically? Keep this in mind as Early Paleozoic life is discussed, in the next chapter.
|
Appalachian Mobile Belt |
epeiric sea |
Queenston Delta |
|
Baltica |
Gondwana |
Sauk Sequence |
|
China |
Iapetus Ocean |
sequence stratigraphy |
|
clastic wedge |
Kazakhstania |
Siberia |
|
Cordilleran mobile belt |
Laurentia |
Taconic orogeny |
|
craton |
mobile belt |
Tippecanoe Sequence |
|
cratonic sequence |
organic reef |
Transcontinental Arch |
|
|
Ouachita mobile belt |
|
CHAPTER 10 – ANSWERS
TO QUESTIONS IN TEXT
|
1. b |
6. e |
11. c |
|
2. d |
7. a |
12. e |
|
3. b |
8. c |
13. d |
|
4. b |
9. d |
|
|
5. b |
10. b |
|
14. Paleozoic paleogeographic reconstructions are
based primarily on structural relationships, climate-sensitive sediments such
as red beds, evaporites, and coals, as well as the distribution of plants and
animals.
15 . Cratonic sequences are large-scale
lithostratigraphic units representing major transgressive-regressive cycles
bounded by craton-wide unconformities. They are convenient for studying the
geologic history of the Paleozoic because the transgressions are commonly well
preserved. They are widespread and easy to map and correlate.
16. The
Cambrian Grand Canyon sequence includes the Tapeats sandstone, overlain by the
Bright Angel Shale, and the Muav Limestone. This typical transgressive sequence
shows the progradation of a shallow sea, from a nearshore environment, to an
offshore environment, to deeper offshore.
17. Sequence stratigraphy can be used to make global correlations because the stratigraphic units were deposited over a wide area and can be found relatively easily in various locations. It is useful in reconstructing past events because it allows geologists to subdivide sedimentary rocks into related units that are bordered by time-stratigraphically significant boundaries. They can also be used for interpreting and predicting depositional environments.
18. The Appalachian margin of the Laurentian craton changed from a passive to an active margin in the Ordovician. The Taconic orogeny has the earmarks of a continent-ocean collision including volcanic activity, intrusions, and facies patterns.
19. The Michigan Basin was a nearly circular, persistently negative equatorial cratonic basin during the Paleozoic. This area became ringed with barrier reefs, which controlled the influx of sea water. As a result of high evaporative rates, concentrated brines built up within the circling reefs.
20. Carbonate deposition ceased in the Middle Ordovician, and was replaced by clastic rocks including turbidites, and volcanic sequences. Paleocurrent indicators show the location of rising Taconic highlands. Numerous unconformities, plutonic bodies, and the Queenston clastic wedge all attest to mountain-building (orogenic) processes.
Apply Your Knowledge
1. Just as the Queenston Delta resulted from the erosion of the adjacent Taconic Highlands, the Catskill Delta is also the result of erosion of the adjacent Acadian Highlands. If we assume that all of the rocks from the highlands were deposited as sediments in the adjacent delta, then the volume of the sediments in the Catskill Delta was about three times the volume of the sediments in the Queenstown Delta. That means that the volume of the Acadian Highlands must also have been about three times as great as the volume of the Taconic Highlands. Because the volume of a block is length * width * height (we assume that the range is shaped like a block, which is a reasonable approximation for this exercise), we must know what the length and width of the Taconic Highlands were. We know what the volume of the Taconic Highlands is (600,000 km3 = volume of Queenston Delta, assuming the Queenston Delta represents the complete erosion of the Taconic Highlands), and the height (4000 m), so we need to know the length and width of the Taconic Highlands. If we assume that the Taconic Highlands were 600 km long, then their width would be 250 km. Thus, the size of the Taconic Highlands would be 600 km * 250 km * 4 km = 600,000 km3.
If we assume that the length and width of the Acadian Highlands are the same (which seems unlikely), then the height would be three times greater or 12,000 m, if the volume of the Catskill Delta was 1,800,000 km3. It also seems unlikely that all of the sediments from both sets of highlands would end up in the adjacent deltas.
2. Paleogeographic maps are created by
synthesizing various information, including paleoclimatic, paleomagnetic,
paleontologic, sedimentologic, stratigraphic, and/or tectonic data that are
available. However, the data are often imperfect and precise. The magnetic anomaly patterns preserved
in the oceanic crust were destroyed when much of the Paleozoic oceanic crust
was subducted during the formation of Pangaea. Depending upon which data are used in
the reconstruction of paleogeographic maps, the maps can vary. Also,
progressing research adds to our knowledge base, so older reconstructions will be
(hypothetically) based on less data than more modern reconstructions.
3. Students will include a variety of information about the Grand Canyon. Some
possible topics that students will
address may be rock types, transgressions, sequences, unconformities, fossil
information, plate tectonics, and paleogeography of the region.