H. Williams               HISTORICAL GEOLOGY

LATE PALEOZOIC GEOLOGY

The Late Paleozoic (Devonian-Mississippian-Pennsylvanian-Permian) is noted for a number of geologically significant events:

- widespread COLONIZATION of the land by large plants, reptiles and amphibians.

- a number of major OROGENIES, including the ACADIAN (Devonian), the ANTLER (Mississippian), the HERCYNIAN (Mississippian), the ALLEGHENY (Pennsylvanian), OUACHITA (Pennsylvanian) and the SONOMA (Permian).

- major non-marine deposits, including COAL beds (related to the development of LAND PLANTS), and sand dunes/evaporites (related to the widespread aridity of many parts of the northern continents).

The North American craton had continued its counterclockwise rotation relative to the Equator. Most of the craton was still experiencing hot, tropical conditions. Carbonates were forming in the quieter regions of epeiric (platform) seas; clastics were being shed by the newly emergent mountainous areas along the orogenic belts; coral reefs and evaporites were forming in the more restricted shallow marine areas.

A major feature that had developed during the Silurian was the great Caledonian Mountain range running down the northeast edge of the Craton. The convergence of cratons that would end in the reassembly of Pangaea was continuing.

The beginning of the Devonian is marked by the TIPPECANOE REGRESSION, causing widespread early Devonian erosion and leaving a major EROSIONAL UNCONFORMITY, used to identify the boundary between the TIPPECANOE and the KASKASKIA - the next major transgressive sequence. The rest of the Devonian is characterized by THE KASKASKIA TRANSGRESSION, which continued into the Mississippian. Familiar transgressive sequences were deposited in the advancing epeiric seas. Further west and northwest, away from the main land mass, carbonate deposition prevailed. The shallow margins of the Northern Seaway area in particular, are noted for Devonian age reefs.

The Late Paleozoic is noted for 3 major orogenic events:

1. THE ACADIAN OROGENY: the continental convergence that formed the Taconic Uplands of the Early Paleozoic continued into the Late Paleozoic as THE ACADIAN OROGENY. The Acadian Mountains run from Newfoundland right down the east coast to west Virginia. They consist of folded sedimentary and metamorphic rocks containing igneous intrusions.

Question: Describe the origin, location and composition of the catskill wedge.

2. THE ANTLER OROGENY:

On the west coast, the island arc that formed back in the Ordovician had now been pushed up against the continent forming the highlands and volcanic activity of the ANTLER OROGENY, running through (what is now) Nevada, Idaho and into British Columbia. These mountains became source areas for clastics that spread across the western states in the Pennsylvanian and Permian.

3. THE HERCYNIAN OROGENY: The collision between Gondwanaland and the northern continents happened slightly earlier in Europe than in North America. The result was the Late Mississippian HERCYNIAN OROGENY (roughly equivalent to the Allegheny Orogeny of North America, which occurred slightly later - in the Pennsylvanian). A great mountain chain was pushed up along the margins of Southern Europe. Some of the clastic wedges that formed around these mountains were covered by forest and went on to become some of the great European coal basins. Much of the Hercynian Mountain chain was subsequently eroded away - only small eroded stumps remain uncovered today.

The Pennsylvanian opened with the Kaskaskia Regression; the regression, combined with continuing orogenic uplift in the east, and the continuing deposition of clastics, resulted in a large land area forming down the eastern half of the North American Craton. Africa was now joined to the north east of the North American Craton, the SUTURE ZONE marked by a vast mountain chain (the Appalachians).

The KASKASKIA REGRESSION continued until mid-Pennsylvanian time; the rest of the Pennsylvanian, like the Cambrian, Ordovician and Devonian before it, is characterized by a marine transgression - THE ABSAROKA TRANSGRESSION. Erosion of the Appalachians formed low lands on the eastern craton. An important part of these deposits were COAL BEDS.

Question: What are cyclotherms; what is their relationship to coal deposits?

THE ABSOROKA REGRESSION characterizes much of the Permian; epeiric seas became shallow, resulting in common basins, reefs and evaporite deposits.

3. Orogenies

The major orogenic episode occurred during Early Pennsylvanian time - THE ALLEGHENY OROGENY. This was essentially the continuation of the (slightly) earlier HERCYNIAN OROGENY in Europe. Northwest Africa collided with eastern North America, producing mountain building, mainly in the Southern Appalachians. It was this compression that caused widespread thrust-faulting throughout the Appalachians, resulting in the "valley and ridge" province.

At about the same time, South America collided with North America crumpling up the Ouachita Trough and forming a great mountain chain along the southern margin of North America - THE OUACHITA OROGENY.

Erosion has since removed much of these mountains leaving only the Ouachita Mountains of Oklahoma/Arkansas, the Arbuckles of Oklahoma and the Marathon Mountains of Texas.

Question: "The Hercynian, Allegheny and Ouachita orogenies are really just separate parts of one big continent-continent collision." Explain this statement.

This collision between Gondwanaland and Luarussia not only threw up mountain belts around the edge of the craton, but also transmitted stress into the interior of the craton, resulting in INTRACRATONIC OROGENESIS and formed the COLORADO MOUNTAINS (the Ancestral Rockies). The west coast ANTLER OROGENY continued into the Pennsylvanian. Subduction also continued, building island arcs down the entire west coast. These uplands provided large amounts of clastic and volcanic deposits to this region. Many of these volcanic uplands continued moving eastward, pushing up onto the North American Craton and becoming part of it in the late Permian SONOMA OROGENY (the CASSIAR OROGENY in Canada). The mountainous CORDILLERA region was clearly beginning to take form.

Other notable events at the end of the Paleozoic:

1. the URALIAN OROGENY - Siberia finally collided with Laurussia, forming the north-south trending URAL MOUNTAINS.

2. The Caledonian, Hercynian, Alleghenian, Ouachita and Uralian suture zones joined all the world’s continents in one super-continent - Pangaea.

Introduction:The Mesozoic lasted about 180 million years. The first two of the three periods that make up the Mesozoic - the Triassic and the Jurassic lasted about 100 million years, or about the first half of the Mesozoic Era.

1. Paleogeography Mountains of the Allegheny and Ouachita orogenies now formed a continuous belt along the east and south coasts. The Triassic was a time of a major marine regression - the ABSAROKA REGRESSION, which exposed a land area over about the eastern 3/4 of the Craton. Only down the west coast there remained dominantly marine conditions. This regression ended with the ZUNI TRANSGRESSION, which again established marine conditions through the central part of the Craton. A major difference in geography between the Paleozoic and Mesozoic is that no sooner had Pangaea become assembled in the Paleozoic, it promptly begun to break up again in the Mesozoic.

2. Tectonics and Sedimentation Major tectonic activity was related to the breakup of Pangaea, which begun in the Triassic. In N. America there were 2 main effects:

1. the east coast became a divergent margin, characterised by tensional forces.

2. subduction on the west coast was enhanced, causing increased orogenic activity in the Cordillera.

The East Coast

In the Late Triassic (ca. 200 mybp), Gondwanaland began to separate from North America, opening a new ocean basin along the east coast - part of the modern Atlantic Ocean. This divergence caused fault-bounded tensional troughs to form throughout the Appalachian region. These troughs filled with arkosic red sandstones, from the erosion of the adjacent granitic uplands, forming the NEWARK GROUP of the east coast.

Question: What are the PALISADES of the Hudson River, and how are they related to the development of a divergent margin on the east coast around 200 million years ago?

Question: What structural features resulted from tension on the east coast and compression on the west coast during the Mesozoic?

The rest of the Triassic/Jurassic consists of erosion of the Appalachians to a more subdued surface.

Question: What are the modern Atlantic Coastal Plain and Continental Shelf, and how were they formed?

The South Coast The early Mesozoic saw the erosion of much of the Ouachita Mountains. A new depositional basin - THE GULF OF MEXICO - begun to form at this time. The Gulf probably formed a shallow embayment between the separating North and South Americas. In the Triassic, sedimentation consisted mainly of evaporites - particularly SALTS (shallow seas, restricted circulations). In the Jurassic, the seas deepened with the Zuni transgression and carbonates started to dominate.

The West Coast During the early Mesozoic, tectonic activity was most active on the west coast. As the North American Craton was now moving away from Gondwanaland, it was actively "pushing" into the Pacific Plate, greatly increasing subduction and compression in the west. Late in the Permian, this subduction had formed an offshore island arc, surrounded by great thicknesses of greywackes and volcanics, and the thrust belt mountains of the Sonoma orogeny further inland.

In the Mesozoic, this island arc and the surrounding marine sediments became progressively pushed up against the edge of the continent, becoming part of it. This Late Jurassic deformation took place in the west, resulting in the NEVADAN OROGENY - the results of which (mainly metamorphosed greywackes) can be seen in California and Nevada.

Question: What are Granitic Batholiths and why are they characteristic of the Nevadan Orogeny?

The other major tectonic event in the west was the accretion of ALLOCTHONOUS TERRANES - microcontinents, island arcs and submarine basaltic plateaus were all scraped off the subducting Pacific Plate and thrust up onto the continent. 50 of these exotic terranes have been recognized, making up over 70% of the Cordilleran Region (especially in B.C. and Alaska).

Paleogeography: The Cretaceous begun with the continuation of the ZUNI TRANSGRESSION, which resulted in the largest marine invasion since the end of the Pennsylvanian, 190 million years before. The result, in North America and elsewhere, was the establishment of large EPEIRIC SEAS, characterised by carbonate deposits especially CHALK.

Question: What was responsible for forming so much chalk in the Cretaceous, and what’s the relationship between the words "chalk" and "cretaceous"?

North America was cut in two by a seaway reaching from the Gulf of Mexico to the Arctic Ocean.

By the Late Cretaceous, the mountainous Cordillera of the west coast of North America was well established. Two new orogenies (periods of intense mountain building) occurred in the Cretaceous:

The "crumpling up" of the west coast, which intensified with the break up of Pangaea, was transmitted from west to east through the Mesozoic. The Sevier Orogeny is notable for low angle thrust faults which formed further inland (e.g. Nevada, Utah, Montana, B.C. and Alberta), as compressional stress from the west coast was transmitted eastwards.

Orogenic activity continued into the Late Cretaceous, again further eastward than previously. This resulted mostly in folding of strata in the site of the present-day Rocky Mountains of New Mexico, Colorado and Wyoming, producing uplifted domes and anticlines, and basins. Most of the underlying structures of the present rocky mountains were the result of the Laramide Orogeny; however, the actual landscape we see today resulted from episodes of uplift and erosion in the later Cenozoic era.

Apart from the chalk deposition already mentioned, much of the sedimentation in North America occurred in the backarc basin of the growing Cordillera region. This basin stretched from Utah to Kansas. It contains a gradation of clastics eroded from the Cordillera, from proximal conglomerates and coarse sandstones to distal marine shales (sound familiar? – another clastic wedge).

Question: What is back-arc sedimentation? What kinds of rocks formed (in this manner) in Colorado in the Mesozoic?

The Tethys seaway was a deep marine depositional trough between Gondwanaland and Eurasia during the Mesozoic. The trough was a site of mainly limestone deposition. Soon after the break up of Pangaea, Africa begun to converge on Eurasia. The result, mainly in the Cretaceous, was the closing and compression of the Tethys, along what is now southern Europe. This caused orogenic activity and volcanism in the region, which continued and intensified in the later Cenozoic. At the same time, India had broken free from Gondwanaland and was heading towards the site of the present-day Himalayas.

Introduction: The MESOZOIC was a time of several important changes in the evolution of life. The most significant was the dominance of the Dinosaurs, the appearance of mammals in the Triassic, birds in the Jurassic and massive extinctions at the end of the era.

MARINE PLANTS: Golden-brown algae appeared in the Jurassic; the coccoliths of the algae (calcium carbonate platelets) formed the abundant CHALK of the Cretaceous.

TERRESTRIAL PLANTS: Lowland swamps characterized by ferns, rushes and cycads (palm-like trees). Primitive CONIFERS were found in the cooler uplands and became more abundant as the climate cooled at the end of the Mesozoic. By the Early Cretaceous the ANGIOSPERMS appeared (carried enclosed seeds, "flowering plants") e.g. birch, maple, walnut, beech, sassafras, poplar, willow. These became dominant by the end of the Cretaceous, reflecting the success of INSECT POLLINATION.

MARINE INVERTEBRATES: The bivalves (mollusks) surpassed the brachiopods in dominance of the sea-floor e.g. oyster-like GRYPHAEA. Ammonites were particularly prominent in the Jurassic and Cretaceous and are used as guide fossils. Squid-like belemnites were also common.

MARINE VERTEBRATES: Some reptiles returned to the sea e.g. Plesiosaurs, Ichthyosaurs ("fish-lizards").

LAND VERTEBRATES:

REPTILES: MESOZOIC = Age of Reptiles; JURASSIC = Age of Dinosaurs. The early (Triassic) thecodonts were small primitive dinosaurs; from these, several major groups developed:

1. Saurischia – (lizard-hipped dinosaurs): this group included both carnivorous (theropods – for example: Allosaurus, Deinonyhus, Tyrannosaurus) and herbivorous (sauropods – for example: Apatosaurus, Brachiosaurus) dinosaurs.
2. Ornithischia – (bird-hipped dinosaurs): this group included quadruped and biped plant-eaters. The jaws were beaked at the front and had grinding teeth at the back (for example: Hadrosaurs, Stegosaurs, Triceratops).

Pterosaurs - flying reptiles (featherless). Archaeopteryx ("ancient wing") - The earliest known BIRD, appeared in the Jurassic Period. Had feathers, teeth, claws on the wing.

MAMMALS: Primitive shrew-like creatures appeared in the Triassic - probably insect-eaters. The big advantage they had over the reptiles is that they were ENDOTHERMIC (produced own heat internally from food). Reptiles are ECTHOTHERMIC (receive heat from sun - need warm climate).

EXTINCTIONS: The end of the Mesozoic is marked by mass extinctions: 1/4 of all known families were eliminated; marine plankton declined by 75%; Exinction of: Dinosaurs, Ammonites, Pterosaurs, Ichthyosaurs, Plesiosaurs, Belemnites, Causes?....

Asteroid Impact Theory: Examination of the Cretaceous/ Tertiary boundary ("K/T") has revealed a layer of red clay abnormally rich in IRRIDIUM - a metal that occurs rarely on the Earth's surface, but is frequently abundant in asteroids and meteorites -> theory that a large asteroid (10 km diameter) struck the Earth about 65 million years ago (possible impact site = Chicxulub on the Yacatan Peninsula) -> dust cloud (containing irridium) all over Earth -> abrupt climatic cooling -> death of many plants -> domino effect along food chains, killing off the most vulnerable species. Evidence = widespread irridium-rich clay layer at the K/T boundary; shocked quartz grains; tektites; soot layers at K/T boundary; Chicxulub crater.

Which of the following are in the correct order (oldest - youngest):

a. Devonian, Pennsylvanian, Mississippian b.Mississippian, Devonian, Pennsylvanian

c. Devonian, Mississippian, Pennsylvanian d. Pennsylvanian, Devonian, Mississippian

e. Pennsylvanian, Mississippian, Devonain

Throughout the late Paleozoic, the North American Craton:

a. Rotated clockwise and moved north b. Did not rotate and moved east c. Rotated clockwise and moved south

d. Did not rotate and moved west e. Rotated counterclockwise and moved north

Which period is noted for particularly large deposits of coal in North America:

a. Devonian b. Pennsylvanian c. Triassic d. Cretaceous e. Jurassic

A layer of ____________-rich clay is commonly found at the K/T boundary:

a. iron b. quartz c. sodium d. irridium e. silicon

Note: There is a lot of detail in the Late Paleozoic and Mesozoic – to give you a break, I will select about 3 short answers and 2 longer answers (you get to choose 1 out of 2) from the examples on this review sheet.