H. Williams HISTORICAL GEOLOGY

EXAM 2. REVIEW

Orogenesis; Continental Growth by Accretion:

When continental plates converge, subduction does not occur because the continental masses are too thick and buoyant. Instead, the 2 continents become welded together along a SUTURE ZONE. The force of the collision folds up the crustal rocks and former oceanic sediments, creating metamorphic rocks and uplifted mountains. This period of mountain building is termed an OROGENY. An example is the creation of the Himalayas mountain chain about 25 million years ago, when the Indian-Australian plate collided with the Eurasian plate. 
Mountain building also occurs during continental-oceanic plate collisions. The oceanic plate is subducted beneath the continental plate. All orogenies have common features:
1. compression, buckling and uplift at the edge of the continent.
2. emplacement of igneous rock masses
3. folding, faulting and metamorphism
4. Erosion of the growing mountains and deposition adjacent to the mountains
5. (in oceanic-continental collisions)scraping off, folding and uplift of sea floor rock and sediments onto the edge of the continent - resulting in:
Continental Growth By Accretion
Caused by the "plastering on" of material at the edge of a continental plate adjacent to a subduction zone. The material that accumulates in this fashion can be small continental masses MICROCONTINENTS or oceanic features such as volcanic arcs and seamounts. The Seychelles Bank in the Indian Ocean is an example of a microcontinent, which has apparently become detached from Africa. These larger masses of rock are scraped off the subducting plate and plastered onto the continent to form ALLOCHTHONOUS or EXOTIC TERRANES (meaning they had originated elsewhere). Example: Wrangellia consists of basaltic island arc volcanic rocks, deep marine shales and shallow marine limestones. This material originated in the area presently around New Guinea in Triassic time (225 million years ago) - travelling some 10,000 km to its present location.

The mantle is made of ultramafic rock - dense rock made of ferromagnesium minerals such as olivine and pyroxenes. It is likely that patches of komatiite crust formed during the early Archean, but being denser, sunk and were "recycled".

The Proterozoic (2.5 - 0.54 billion)

The Early Atmosphere:  the Earth's early atmosphere was formed by the release of gases contained in minerals during the melting of the Earth and later volcanic activity (4.6 - 3.6 BYBP) - termed OUTGASSING. Evidence = marine sedimentary rock 3.8 BYBP - proves existence of oceans. 
Composition = carbon dioxide, water vapor, carbon monoxide, hydrogen, hydrogen chloride (similar to modern volcanic gas). There was very little OXYGEN - when it was emitted it probably combined very quickly with iron -> iron oxide. No oxygen meant no OZONE and therefore ULTRAVIOLET RADIATION bathed the surface of the Earth (deadly today).

Life originated sometime prior to 3.5 BYBP - the exact process is still not known; however experiments have shown that carbon + oxygen + hydrogen + nitrogen + phosphorus + sulfer + U.V. + electrical discharge (lightning) + heat --> AMINO ACIDS, one of the major building blocks of PROTEINS - a basic component of life. Single-celled marine organisms developed, HETEROTROPHS. However, this is an inefficient process (e.g. shortages of organic molecules were likely) and probably spurred the evolution of AUTOTROPHS - the most important group were the PHOTOAUTOTROPHS -  cyanobacteria, that added oxygen to the atmosphere. Evidence of the resulting change in the atmosphere is found in the form of banded iron formations. STROMATOLITES are the most common trace fossil of the Precambrian, used as guide fossils in late Precambrian rocks.

Introduction: Cambrian + Ordovician + Silurian = early Paleozoic - geologic history characterised mainly by large-scale transgressions and regressions. Each period can be examined in terms of the major events that shaped the geologic record - these events are Paleogeography, Transgressions and Regressions, Orogenies.
CAMBRIAN

1. Paleogeography: The North American craton lay along the Equator during Cambrian time. The climate was hot. The craton land area was towards the (present) northeast and eroding. Shallow platform seas covered much of the craton, especially in the south. The deposits forming in these seas were gradational from sand-shale-carbonate, depending on depth of water/distance offshore. The break-up of Rodinia had crated deep marine basins (seaways) around the edge of the craton: Appalachian seaway to the S.E./E;Cordilleran seaway to the W; Franklinian seaway to the N. These were sites for SHALE deposition.

2. Transgressions/Regressions: The Cambrian was a time of TRANSGRESSION - the SAUK TRANSGRESSION. As the coastline moved inland, a typical transgressive sequence was deposited:

3.Orogenies: The Cambrian was a stable period for North America. The fragments of Rodinia were moving APART, so the edges of the craton were PASSIVE MARGINS.

ORDOVICIAN

1. Paleogeography: The major difference in geography between the Cambrian and Ordovician is that the craton margins became ZONES OF CONVERGENCE during the Ordovician; fragmented Rodinia was just beginning to reassemble. The main result of this was the formation the TACONIC HIGHLANDS of the east coast.

2. Transgressions and Regressions: The beginning of the Ordovician is marked by the SAUK REGRESSION, which left a major EROSIONAL UNCONFORMITY, used to identify the boundary between the SAUK and the TIPPECANOE cratonic sequences.

The rest of the Ordovician, like the Cambrian, is characterized by a marine transgression - THE TIPPECANOE TRANSGRESSION.

Orogenies: The major orogenic episode occurred during the middle-late Ordovician - THE TACONIC OROGENY. This was associated with the subduction zone that had formed down the east coast of the craton. The development of a convergent margin caused the formation of the TACONIC HIGHLANDS, down the N.E. coast (remnants of which form the Taconic Mountains of New York State). These uplands consisted of folded/metamorphosed sedimentary rocks + accreted and intruded volcanics; and probably reached over 13000 feet in height. To the west, the crust was downwarped by compression, forming a back arc basin that collected thick marine shales and graywackes. To the N.W. (New England) the compression resulted in great thrust faults. Later, intense erosion of the Taconic highlands resulted in the QUEENSTON CLASTIC WEDGE.

SILURIAN

1. Paleogeography: The east coast from Newfoundland to Florida was now continuous highlands due to the continuing growth of the Taconic orogeny.

2. Transgressions/Regressions: Most of Silurian time was characterized by the TIPPECANOE REGRESSION. The major effect of the regression was the formation of landlocked, reef-fringed marine basins in the region now occupied by the Great Lakes. Evaporation from these inland seas created vast EVAPORITE DEPOSITS (salts) up to 500 m thick - these are presently mined.

3. Orogenies: The closure of the Proto-Atlantic continued during Silurian time, culminating in a continent-continent collision between Laurentia (N. America) and Baltica (Europe) in late Silurian time -> THE CALEDONIAN OROGENY. This resulted in LAURUSSIA and the Old Red Continent. The most intense effects of the Caledonian Orogeny were in Scandinavia and Northern Europe (Norway, Scotland). On the North American craton the effects were in the present Northeast and were less severe, but helped to create the mountainous terrain of the Northern Appalachian Mountains. This mountain-building episode continued into Devonian time as the ACADIAN OROGENY along the present east coast (the focus of the collision shifted from NE to E).

(Multiple Choice)

1. The earth's early (~4 billion years ago) atmosphere:

a. was extremely rich in oxygen b. resembled modern volcanic gases

c. was mainly nitrogen d. was over 80% ozone e. was entirely hydrogen and helium gas

2. In which of the following are the time periods in the correct order (oldest to youngest)?

a. Cambrian Ordovician Silurian Devonian b. Ordovician Cambrian Devonian Silurian c. Cambrian Devonian Silurian Ordovician d. Devonian Silurian Cambrian Ordovician e. Silurian Cambrian Ordovician Devonian

(Short Answer)

1. Describe and explain the origin of stromatolites.

2. Describe Greenstone Belts and their interpretation.

(Longer Answer)  

1. How did the modern crust originate? (basalt oceanic crust; granitic continental crust). The answer is probably cooler temperatures and partial melting. Explain:

2. With the aid of a sketch, describe common features of orogenic belts.

3. What is a typical transgressive sequence? Why does it form? (Use Cambrian-age rocks from the Grand Canyon as an example):

4. Describe a major event involving India around 25 million years ago.

5. Describe the origin, appearance, structure and significance of stromatolites.

6. Explain the connection between photoautotrophs, stromatolites and BIFs.

7.  Explain the difference between heterotrophs, autotrophs photoautotrophs.

8. Where and when did metazoans originate? How were they different from earlier forms of life?

The exam will be 1 hour and 15 minutes and consist of 10 multiple choice questions, 3 short answer questions and 1 longer answer question out of a choice of 2 (the 2 questions will be selected from the list of 8 above).