1. Figure 12.17: The Early Silurian was a time of extensive erosion of the Taconian Highlands. The resulting clastic sediments were deposited westward of the Taconian Belt. Sandstone and conglomerate are found near their source, the core of the orogenic belt.

2. Silurian iron-rich sedimentary deposits were deposited in the southern Appalachians due to intense tropical weathering in this region of the Taconian Mountains. Sediments and oolites are typically cemented and replaced by iron.

3. Silurian/Lower Devonian marine carbonates and shale were deposited throughout the craton and comprised the upper portions of the Tippecanoe Sequence.

4. Figure 12.18: Most of the North American continent was covered by the Tippecanoe shallow sea during the Middle Silurian. Note the reefs around the present Great Lakes region.

1. Figure 12.20: Modern reefs form in warm shallow seas between latitudes 30o N and 30o S.

2. Most marine organisms are also more diverse in warm, tropical latitudes.

3. Figure 12.19: Carbonate sediments form in shallow (<30 m) warm seas and as deep-basin accumulations (ooze) of calcareous microfossils.

4. There are two major types of reefs:

(a) Bioherms (organic buildups)

(b) True Reefs (began in Ordovician, important in middle Paleozoic)

5. Reef Growth Requires

(a) warm, shallow waters within the photic zone.

(b) abundant plankton for food.

(c) A well-developed food chain from plankton to fishes.

(d) somewhat agitated, oxygen-rich water but also adjacent quiet waters. Reefs typically grow in the surf zone only a meter or so below sea level.

(e) upwelling currents to continuously bring up fresh food supply.

Parts of a Typical Reef (Figure 12.23)

1. A Reef Core forms along the shelf edge where upwelling nutrients are brought up from deeper portions of slope. Reef cores consist of interlocking skeletal material (e.g. corals, sponges, etc.).

2. The Backreef consists of stratified lime mud, possibly evaporite and carbonate sand from reef the core.

3. The Fore-reef consists of an apron of poorly-sorted reef debris.

Figure 12.27: A typical Devonian reef complex consists of the following:

(a) A reef core of corals and stromatoporoid sponges.

(b) Backreef lagoon of gastropods, bivalves, bryozoa, stromatoporoid sponges and cyanobacterial mats.

(c) Reef slope (quieter water) include attached animals such as crinoids, sponges and brachiopods.

1. Figure 12.29: Evaporite Basins form as a result of restricted circulation of seawater. Inflow of seawater and evaporation are closely balanced.

2. Figure 12.30a: Supratidal (sabhka) deposition occurs slightly above high-tide level and form by infrequent washover onto supratidal flats. Seawater fills pores of sediment and precipitate salt. A significant thickness of salt can accumulate under conditions of balanced subsidence and deposition.

3. Figure 12.30b shows dolomization of limestone due to seepage of Mg-rich brines originating from a restricted lagoon. CaCO₃ + Mg²⁺ = CaMg(CO₃)₂

1. Figure 12.31: The Michigan Basin originally formed as an intracratonic basin during Taconian thrust loading.

2. The basin accumulated 1500 meters of Silurian dolomite + halite + anhydrite during subsequent subsidence. What would have caused such a great thickness of salt to be deposited?

(a) Tectonic subsidence due to Taconic thrust loading in addition to weaknesses in the lithosphere.

(b) Sediment loading.

(c) Seawater evaporation and precipitation of evaporites.

3. Figure 12.32: Great thicknesses of evaporites suggest a restricted basin with continuous replenishment of water into the subsiding basin.

4. Silurian barrier (fringe) reefs may have served to restrict seawater circulation. The reefs allowed seawater to reach the basin but blocked currents that would have provided circulation.

5. Slight lowering of sea level during Late Silurian time may have produced islands and shoals around basin margin which also aided in evaporite formation.

Devonian Paleoclimate and Paleogeography (Figure 12.33)

(a) Broad distribution of Devonian Forests worldwide

(b) Laurasia supercontinent

(c) Gondwana supercontinent

(d) Both supercontinents rimmed by subduction zones

(e) Orogenic belts (brown)

1. Figure 12.34: A major regression began during the Late Silurian and soon most of the craton was exposed. By Late Silurian and Early Devonian times, marine deposition was restricted to only a few basins connected by narrow seaways. Ordovician and Silurian carbonates were eroded away in many places. A major erosional unconformity capped the Tippecanoe Sequence and marked the beginning of the Kaskaskia Sequence.

2. Figure 12.35: During middle Devonian times, erosion of the central craton resulted in deposition of the Oriskany Sandstone (base of the Kaskaskia Sequence) in eastern North America during a subsequent transgression. The transgression first filled basins and then gradually encroached onto the arches. Widespread carbonate deposition soon followed.

3. The Michigan Basin continued to subside during the Devonian.

4. The presence of the Catskill sedimentary wedge indicates renewed uplift to the east (Acadian Orogeny).

5. The Chattanooga black (organic rich) shale originated just west of the Catskill deltas during Middle Devonian time and expanded across the North American craton by end of the Devonian (shale of Fig. 12.35). This organic rich shale is an important source of petroleum. The Chattanooga black shale was possibly deposited in deep, stagnant water during a high-stand of sea level or during a time of high salinity and density stratification in the epeiric sea.

6. Further west, the Elk Point (Williston) Basin acquired significant evaporite deposits and an immense barrier reef complex developed around its margin and extended to northwest Canada.

7. Main features of Figure 12.36

(a) Carbonate deposition dominated the Silurian and Devonian epeiric seas.

(b) During the Late Devonian, marginal tectonic lands had emerged to the north and east.

(c) Europe collided with northeastern North America.

(d) Africa approached from the southeast.

8. Figure 12.38: Widespread doming and warping of the North American crust began during the Silurian and continued into the Devonian, possibly caused by mountain building in eastern North America. The Ozark Dome, Missouri, is an example.

1. Figure 11.36: The Acadian Orogeny resulted from collision between northeastern North America and the Avalonia microcontinent during Devonian time.

2. Figure 12.39: Acadian deformation was superimposed on older Taconian folding.

3. Figure 12.39: Deposition of the Devonian Catskill clastic wedge is strong evidence indicating renewed uplift along eastern North America resulting from the Acadian Orogeny. The Catskills is a vast apron of alluvial sediment extending westward and resulted from erosion of the Acadian mountains to the east. Within the Catskill clastic wedge, coarse gravel and sands occurred to the east while finer sediments were deposited to the west. This vast alluvial apron of sediment was spread mainly by deltas and rivers. Flysch sediment was deposited by turbidity currents in deeper water along the western shoreline.

4. The Acadian Orogeny is generally recognized along eastern North America by regional metamorphism, granitic intrusions and thrust faulting.

Figure 12.41: Tectonic overprinting along northeastern North America Is recognized ny discordant K-Ar Ages From granites, gneisses and other metamorphic rocks.

(a) Grenville basement (850-730 Ma).

(b) Taconian ages to northeast (435-400 Ma)

(c) Overprinting of Acadian Orogeny (380-350 Ma).

1. Figure 12.44: The Caledonian Orogeny occurred during the Silurian, 10 million years before the Acadian event.

2. Continent-Continent collision between eastern Greenland and western Europe.

3. Deformation resulting from the Caledonian Orogeny is recognized today largely in Scotland and Norway where the suture zone is located. Remnants of the Caladonian Orogeny also occur in Newfoundland.

4. Figure 12.43 & 12.45: The Caledonian Orogeny resulted in an intracratonic orogenic belt with thrusting towards both margins.

1. The following lists the various evidence for the Caledonian Orogeny:

(a) Remarkable similarities in the middle Paleozoic geology of northwestern Europe and eastern Greenland, suggesting that these two continents were once connected during the Caledonian Orogeny.

(b) Figure 12.43: westward thrusts in eastern Greenland and Appalachians, eastward thrusting in Norway.

(c) On both sides of Atlantic, intense metamorphism and granitic intrusions occurred at about the same time (e.g. plutonic and metamorphic rocks of similar age are found along eastern North America and Western Europe.

(d) Crushed oceanic rocks are found within the suture zone and mark the boundary between the two plates. This central zone also contains deformed volcanic arc rocks, granitic plutons and shows high-temperature metamorphism.

(e) In Newfoundland, a central zone of intensely deformed Ordovician and Silurian oceanic rocks lies between two zones of mildly deformed continental-margin rocks.

(f) The Devonian Catskill-Old Red Sandstone clastic wedge deposits of eastern Greenland and northwestern Europe are similar to North American Catskill red beds. The sediments were transported by rivers in both directions, away from a single great Caledonian-Acadian mountain range. These various clastic wedges contain identical land plant and fish fossils.

2. The collision of northeastern North America with Europe did not significantly affect the Acadian belt of U.S. Instead, the eastern U.S. reflects collision with one or more microcontinents (including Avalon terrane).

1. Figures 12.44 & 12.48: The Franklin orogenic belt of Arctic Canada and northern Alaska contain Silurian and Devonian conglomerates and sandstone derived from Caladonian mountains. These sediments were carried westward along a deep-water trough and deposited as a huge, elongate deep-sea fan.

2. Subsequent collision occurred between northern Canada and possibly an arc or microcontinent as evidenced by middle Paleozoic granites, ultramafic and volcanic rocks. This collision was followed by southward thrusting during the Late Devonian-Early Mississippian and is called the Ellesmerian Orogeny.

1. Figure 12.49: During the Late Devonian and Early Mississippian, chert-bearing conglomerates were deposited in northern Alaska, suggesting nearby uplift. The chert and quartz fragments were possibly eroded from uplifted marine sequences and re-deposited by rivers and turbidity currents.

2. In Nevada, early Paleozoic oceanic sediments and pillow basalt were extensively folded. Soon afterwards, sandstone and chert-bearing conglomerates of Mississippian and early Pennsylvanian age were deposited unconformably over the early Paleozoic oceanic rocks. This sequence of events suggests initial folding, followed by uplift (thrusting?).

3. Emplacement of Late Devonian granites in western Canada further indicate a period of mountain building.

4. This episode of Late Devonian-Early Mississippian mountain building along western Canada and the United States is termed the Antler Orogeny.

5. The Antler Orogeny was possibly caused by collision between western North America and volcanic arc to the west.