Geology 102 - Outline for Lecture 5
STRATIGRAPHY, CORRELATION and DATING ROCKS
STRATIGRAPHY - The study of stratified rocks, especially their geometric relations,
compositions, origins, and age relations.
stratified rocks - layers / strata
STRATA
Layers or beds of sedimentary rock easily distinguished from ones below and above.
relative dating
CORRELATION - The proceedure of demonstrating correspondence between
geographically separated parts of a stratigraphical unit.
CHRONOSTRATIGRAPHY
LITHOSTRATIGRAPHY
Nicholas Steno 1638-1686
STENO'S THREE LAWS
(1) PRINCIPLE OF SUPERPOSITION
(2) PRINCIPLE OF ORIGINAL HORIZONTALITY
(3) PRINCIPLE OF LATERAL CONTINUITY
William Smith (1769 - 1839)
First geologic map
Principle of Faunal succession
George Cuvier (1769 - 1832)
vertebrate paleontology
the extinction of past lifeforms.
lifeforms did not evolve over time
Led to the creation of the GEOLOGICAL TIME SCALE – systems / periods. Discussed the first day & later today
LITHOSTRATIGRAPHY
applies mainly to sedimentary rocks
Principle of Original Horizontality
vast majority of sedimentary rocks were laid down as horizontal sheets.
Bed - the arrangement of the particles into distinct layers of beds
Bedding plane - a change in depositional environment
Way-upwardness
Graded bedding - coarse to fine
Cross bedding
grains down the lee slopes
dunes, ripples, bars, fans or small deltas
deposited at an angle BUT still ok for correlation
- can be WATER DEPOSITED or WIND DEPOSITED -
RIPPLE MARKS rare in the rock record
Either be water or wind
Asymmetric ripple marks
Symmetrical ripple marks
sharp crest always peaks up
Fine cross beds when ripple cut
Mudcracks fine grained sediment - dries out.
Erosional Structures SCOUR and FILL & SOLE MARKS
CORRELATION:
Demonstrate correspondence between geographically separate parts of a stratigraphical unit
• Lithological correlation
• Temporal correlation
ROCK UNITS
rocks have a hierarchy too....
BEDS - the smallest division (formal unit) of a geological formation or a stratigraphical rock series.
BEDS are...
MEMBER - a lithological subdivision of a formation....
FORMATION - primary formal unit of lithostratigraphy
BED - MEMBER - FORMATION
FORMATION is
GROUP / SUPERGROUP
several formations
STAGE - a succession of strata laid down in a single age of the Geological Timescale.
SERIES
SYSTEM - a PERIOD of geological time
STANDARD / TYPE sections
correlate between other sequences
The rocks of the CAMBRIAN SYSTEM
Exposures in Wales form the type section
The standard section is Cambrian by definition - other sections are Cambrian by comparison / correlation
For every formation a type section exists
KEY BEDS
• readily distinguishable
• easily traceable - it is well exposed
BIOSTRATIGRAPHICAL UNITS
units defined by their fossil content
Range of a species
INDEX FOSSILS (GUIDE FOSSILS)
• abundant
• large range of environment
• occurrence is widespread
• short time span
• easy to recognize
MAGNETISM
Magnetic reversals p. 132.
can be used for correlation
as covered in GEOL 101
Facies - characterized by a distinctive set of physical and chemical parameters
Boundaries may be distinct or gradational
Biofacies - associated with a distinct fossil of group of fossils
Law of SUPERPOSITION
TRANSGRESSIONS AND REGRESSIONS
transgression
deeper, finer sediments on top of coarser material
a relative rise in sea level
regression
coarser material is stacked on top of finer
relative fall in sea level or increasing material due to erosion
WALTHER'S LAW
Those facies that occur in conformable vertical successions of strata also occurred in laterally adjacent strata.
Walter's Law and the Paleozoic
Stratigraphy, Dating
Geological time
two main methods
• relative dating
• absolute dating
RELATIVE DATING
Geological Time Scale (AS DISCUSSED)
1664 Archbishop Ussher
"Heaven and Earth, Centre and Substance were made in the same instant of time and clouds full of
water and man were created by the Trinity on the 26th day of October, 4004 B.C. at 9:00 in the morning"
late C18 - Hutton and the Theory of Uniformitarianism.
Gradualism
CHARLES LYELL expanded Hutton's ideas and publicized them.
CALCULATIONS OF THE AGE OF THE EARTH
1) sediment accumulation. age of 3-1.5 million years
problems -
2) ocean salinity - 90 million years
problems -
3) Lord Kelvin late C 19
Assumed Earth originally molten - calculated how long to cool
Earth - less than 100 million years old
CONCLUSION- Earth only hospitable for 20-40 million years.
problems for the evolutionists e.g. Darwin
RADIOMETRIC DATING
1903 - Radioactivity - Pierre and Marie Curie.
NOW calculations - about 4600 million years old
Matter is made up of elements which are in turn made up of atoms
definitions of –
Matter. The substance of which physical objects are composed
Elements. Any one of the simplest chemical substances that cannot be decomposed
in a chemical reaction
or by any chemical means and is made up of atoms
- all having the same number of protons
Atoms
Protons
Neutrons
Nucleus
Electrons
Atomic number
Atomic mass
descriptions of atoms of - CARBON, OXYGEN, HYDROGEN, URANIUM
ISOTOPES
definition
examples CARBON, OXYGEN, HYDROGEN, URANIUM
RADIOACTIVE DECAY
Rate of decay
Unstable Parent to a Stable Daughter
Rate is always constant
alpha decay -
beta decay -
electron capture -
HALF LIFE
HALF LIFE IS ALWAYS A CONSTANT
USES IN GEOLOGY
K-Ar (potassium - argon) Example of a simple system.
Problems of K-Ar dating
U-Th (uranium thorium) example of a complex system
Carbon isotopes and dating - half-life of 5730 years
Stable isotopes e.g. oxygen isotopes and temperature
Event Stratigraphy
e.g Bishop's Tuff
Marker Beds
K(Cretaceous) - T (Tertiary) Boundary
Transgressions & regressions
Global unconformities / Eustatic sea level changes