Study Guide Lect 2

Stratigraphy & Correlation Chapter 5 

Neptunism- proposed in 1787 by German professor of mineralogy, Abraham Werner He declared that all rocks were formed from a primeval, worldwide ocean.

II. Catastrophism - Dominated European thinking from late 18th century to mid-19th

• The century Proposed by Baron George Cuvier

• Stated that all rocks came about through a series of catastrophic events (volcanism, earthquakes, etc.)

These two hypotheses were eventually abandoned because they were not supported by field evidence.

James Hutton – Father of Historical Geology

• Developed uniformitarianism

• This hypothesis / theory became the backbone of all future thoughts regarding sedimentary sequences.

Stratification – Layering of rock sequences (IR, SR, and/or MR)

• There can be vertical lithologic changes in an outcrop

Contact – Term used to indicate the interface between lithologies [ picture to follow ]

Differences in lithology could be due to:

A. Changes in depositional environment

B. Different rock types represented vertically (Igneous sill)

Vertical Relationship 

A formation is a definable stratigraphic unit of rock(s) given a name to describe it.

• Example: St. Louis Limestone Formation (Miss.)

• A formation is a mappable unit and has a distinctive upper and lower boundary.

• It could be the same rock type (limestone) but doesn’t have to be (interbedding) A specific fossil could represent the formation. 

A stratigraphic sequence is all of the vertical changes in rock formations found at a given locale

-For clarity, this sequence is usually represented by a stratigraphic column.

-Stratigraphic sequences are assumed to be conformable –no depositional breaks have occurred since being deposited 

-Bedding planes can represent small breaks of minutes, hours, or years but is inconsequential compared to the vastness of geologic time.

-An unconformity occurs when there is a break in deposition because significant geologic time has passed (millions of years)

This break is called a hiatus

The hiatus could also represent erosion of pre-existing layers

Lateral Relationships

Lithology can also change laterally for the following reasons:

A. Deposition of sediment in a basin

B. Truncation of layer(s) by erosion

C. Truncation of layer(s) by faulting

D. Pinching out –intertonguing (seafloor)

E. Lateral gradation (seafloor)

Transgression

Regression

Transgressions/Regressions

Causes: 

1. Continental uplift 2. Continental subsidence

3. Sea level rise (glaciers melting) or fall (ice age)

Fossils and Stratigraphy William “Strata” Smith – Father of Stratigraphy

He recognized the usefulness of fossils in determining the ages of rock units / layers

His conclusions:

1. Fossils lower in stratigraphic sequence are older than ones above (superposition)

2. Some fossils lived a brief period of geologic time (index fossils) and can therefore be used for absolute age dating

3. Even though similar fossils can occur in different rock types, they represent strata of the same age

Stratigraphy and Correlation

Correlation –Comparing two stratigraphic columns (from two separate geographic areas) to determine their equivalency to one another

Types:  1.Lithostratigraphic correlation

–Using only rock types to determine geologic history

2. Biostratigraphic correlation

- Using fossils Stratigraphy and Correlation Index fossil [ have already defined ]

Index fossils are good because:

1. Abundant

2. Easily distinguished from other taxa

3. Geographically widespread; good for large-scale correlation

4. Occurring in many kinds of SR’s

Range Zone

–How long a certain fossil has existed in the geologic past

End of Chapter 5

The Early Atmosphere Chapter 6 

Our atmosphere Today contains mostly nitrogen, oxygen, and Trace Greenhouse gases. 

•  Archean: Hydrogen and helium mostly these elements are very light and would have escaped into space.

•  once magnetic field establishment, volcanoes on Earth began out gassing and those gases began to accumulate (including water vapor)

. ;

 Two important processes began during the Archean

1.  Photochemical Dissociation

•  water vapor molecules began to be broken down by the sun to make ozone.

2.  Photosynthesis

•  the release of free oxygen into the atmosphere by primitive organisms like stromatolites 

Early Life

-Primitive

- What are the requirements for life?

• Metabolism (grows)

• Reproduction 

• Power to adapt to environment through internal changes

There are two requirements for the Origin of life:

1. Source of appropriate elements from which organic molecules could have been synthesized.

2.  an energy source to promote chemical reactions necessary to synthesize organic molecules.

• Urey and Miller experiment 

What elements are  necessary for life?

1. Carbon 2. hydrogen 3. nitrogen 4. oxygen

 These four elements in different combinations will form organic molecules called monomers.

 polymers- linked monomers (DNA and RNA)

 Stromatolites

Cyanobacteria That trap sediment as they compete for the sunlight

 -They formed Reef structures and are among the oldest known fossils.

  -3.0 billion years old all this known stromatolites in South Africa.

  -3.3- 3.5 billion possible stromatolites in Australia.

Archean Mineral Deposits 

1. Gold- South African gold accounts for 50% of world's gold since 1886.

2.  Massive sulfide deposits- zinc, copper, and nickel (hydrothermally formed).

3.  Chrome- greenstone belts in Zimbabwe.

4.  Some BIFs (band iron formations)  however the majority of them formed during the proterozoic.

The Early Paleozoic 

Phanerozoic eon encompasses the remaining 13 % of geologic time. 

• Life is now flourishing in the oceans.

•  There are land masses larger than the microcontinent of proterozoic, much of it near the south pole. 

• Transgressions and regressions are prevalent throughout the remainder of time (sea level fluctuations).

Because transgressions occur, portions of cratons are being covered by shallow Seas other protons of cartons will experience mountain building episodes.

• Shallow seas = Epeiric sea 

Paleogeographic Reconstructions 

How is this possible?

1. Paleomagnetism- Ancient record of earth's magnetic field. 

2. Biogeography- distribution of faunas and flora.

3. Tectonic Patterns- location of deformed sediment.

4. Climatology- The specific climate sediment forbes.  

5. Absolute age dating technique 

6. Sequence of event ( law of cutting relationship)

Early Paleozoic Continents 

Scientists think that 6 major continents existed at the beginning of cambrian.

1. Baltica 2) China 3) Gondwana 4) Kazakhstan 5) Laurentia 6) Siberia 

Geologic Time vs Cratonic Sequence

1963: American geologist Lawrence Sloss determined that, during the Paleozoic there were four complete cycles of transgression-regression.

•  Each cycle called a cratonic sequence

Transgression:  Finding upward sequence

 Regression: Coarsening upward sequence

 High stand:  Level of Maximum transgression (sea level high)

 low stand:  Sea level lowest (cratonic sequence ended) 

Sauk Sequence 

From late Proterozoic- Early  Ordovian

• First major transgression onto N.A. Craton

Early Cambrian:  Cortana being eroded- sea level rising but not covering yet.

Middle Cambrian: Epeiric sea encroachment began.

Late Cambrian: Sauk sea covered most of N.A.

Summary 

1. Equator 

2. Shallow seas covering most of N.A

3. Sands- clean (mature) with ripple marks 

4. Carbonate D ( limestone) 

Sauk→Tippecanoe 

Early Ordovician: Sauk Sead regression rocks exposed: LS and DS (being eroded thus, unconformity created.

Middle Ordovician- Early Devonian: Clean well sorted sand deposited over most crab best example: st peters sandstone (aquifer).

Tippecanoe Sequence 

Late Ordovician: Deposited on top of the sandstone were carbonates

Early to middle silurian: Tippecanoe transgressive highstand 

Late Silurian: Tippecanoe regression 

• It was during this time that thes Michigan basin formed

• It was a “bow” of sediments (heavy)- created a basin around which corals grew (coral reefs) 

• Lagoonal Regressional Tippecanoe sea concentrated salts and precipitated evaporites ( Halite, Gypsum, Anhydrite) 

A Salt Lesson 

In general, salt precipitate in order beginning with the least soluble and ending with the most soluble.

Sequence: Carbonates (calcium carbonate) gypsum ( calcium sulfate), halite (sodium chloride) 

Early Devonian: Tippecanoe Sea regressed (low stand)

• Lots of mild deformation resulted of exposed sediments before the kaskaskia sea transgressed. 

Taconic Orogeny

During the Ordovician, the iapetus ocean began to close because the iapetus oceanic plate was subducting beneath the Laurentian plate.’

• This formed the first major mountain range in the eastern US

• Called Taconic Orogeny 

• This was first phase of the more formal caledonian orogeny (includes Taconic, Acadian and Alleghenian orogenies that formed Appalachian mountains).