Evolution of the Earth

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Evolution of the Earth Chapter 4
MAIN TOPICS ROCK FACIES RELATIVE TIME GEOLOGIC
TIME SCALE
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RELATIVE GEOLOGIC TIME

• As geologic thought progressed in the early
  1800s it became necessary to classify and
  organize material (fossils) and concepts (maps))
  in a more orderly and manageable form.
• In 1835 Adam Sedgwick and Roderick Murchison
  proposed formal names for the entire European
  stratigraphic succession.
• Eras (Paleozoic, Mesozoic, Cenozoic)
• Periods (Vendian, Cambrian, Ordovician, etc.)
• Based solely on fossils (e.g. fossil life spans)
• Based on fossils and Stenos Laws

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GEOLOGIC TIME SCALE ORIGINATED IN ENGLAND,
FRANCE, GERMANY

• GEOLOGIC TIME SCALE
• PALEOZOIC ERA (383 My)
• LOWER PALEOZOIC
• CAMBRIAN (Cambria)
• ORDOVICIAN
• SILURIAN (Silures)
• UPPER PALEOZOIC
• DEVONIAN (Devonshire)
• CARBONIFEROUS (coal-bearing)
• MISSISSIPPIAN (N. America)
• PENNSYLVANIAN (N. America)
• PERMIAN
• MESOZOIC ERA (183 My)
• TRIASSIC (Trias, Germany)
• JURASSIC (Jura Mtns.)
• CRETACEOUS
• CENOZOIC ERA (66 My) (cont.)

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Fig. 4.2
Geology of Northwestern Europe where much of the
geologic time scale was developed. Note the
unconformities and lateral extend of major rock
units.
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FACIES DEFINITION RELATIONSHIPS

• Sedimentary FaciesOverall lithology (rock-type)
  reflecting or diagnostic of depositional
  environmentExamples
• Sandstone facies
• Mud facies
• Carbonate facies
• Salt facies
• General rule adjective describing depositional
  environment facies
• Note metamorphic petrologists use facies concept
  in a similar fashion, e.g. kyanite facies.

Relations between Old Red Sandstone in Wales and
marine facies in Devonshire. Intertonguing
relationships established Devonian age of the Old
Red Sandstone.
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Fig. 4.4
Source of nomenclature feud between Sedgwick and
Murchinson. As their field areas converged it
became apparent that each has included the same
rocks in his own classification. Sedgwicks top
of Cambrian overlapped Murchinsons lower
Silurian. After their deaths, the dispute was
resolved by naming a new system, the Ordovician.
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Fig. 4.5
Relative Geologic Time
Cross-section across Scotland showing
superposition, cross-cutting and
included-fragment relationships. What is the
sequence of events here? Included fragments Any
rock represented by frag ments in another rock
must be older than the host rock. Cross-cutting
relationships Any igneous rock or any fault must
be younger that the rocks it cross cuts.
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Fig. 4.5
Relative Geologic Time
Sequence of events here the primitive and
transition rocks were (1) folded, intruded by
granites, uplifted and deeply eroded before
deposition of Old Red Sandstone on unconformity
surface. This was followed by injection of dikes
and sills.
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Fig. 4.6
ROCK UNITS (LEFT) AND TIME CHART (RIGHT)
Observed rock unit (left) and interpreted time
chart on right. Note hiatus corresponding to
unconformities. (Hiatus is a time of
non-deposition and/or erosion.)
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Fig. 4.7
FORMATION a mappable unit either in the field
or by well logs A GROUP consists of 2 or more
FORMATIONS.
A formation is the basic rock unit in geology. IT
IS NOT A TIME UNIT. It is defined by its
properties type (sandstone, limestone, etc. e.g.
(Bell Shale), color (Brown Niagrian), texture,
geometry. The choice is fairly obvious in A, but
more difficult in B. In B and C the choice of
subdivisions is somewhat arbitrary.
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Fig. 4.8
Lateral Relationships different facies, same time
Lavoisiers 1789 diagram showing relationships
between littoral gravels (near-shore) and pelagic
(mud, off-shore) facies illustrating
transgression and regression. Lavoisier
recognized that gravel can only be moved in a
high-energy environment, such as the near-shore
where braking waves provide energy. He also
recognized that distinctive organisms inhabit
each environment and that rise and fall of
sea-level would cause the sediments to migrate
shoreward with rising sea-level and seaward with
falling sea-level.
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Depositional Environments and Sedimentray Facies

• Lateral variations of strata not fully
  appreciated until 1838
• Facies concept relates sediments to their
  depositional environment

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Fig. 4.9
Block diagram showing proximal (near source) and
distal (distant from source) facies
relationships in a shoreline environment. The
source area is the uplifted island which is
supplying sediment (gravel, sand, mud in that
order) as it erodes)
Note diagram OK for clastic (clasts
particles) sediments, but not carbonates
(precipitates).
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Fig. 4.10
Ripples developed on surface of a sand body. This
texture can be diagnostic of depositional
environment. These ripples are diagnostic of
near-shore tidal environment, but ripples also
develop in fluvial (river) and aeolian (air,
sandstorm) environments.
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Fig. 4.11a
Illustration of restored facies map for Devonian
of Europe. Note how this illustrates Stenos law
of lateral continuity. Also, how extensive the
different depositional environments are. Lines
refer to cross-sections in following diagrams.
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Fig. 4.11b
Restored cross section showing facies,
thicknesses unconformities
Time-stratigraphic chart showing time gaps
represented by unconformities in B
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Fig. 4.12
Two basic types of facies patterns transgressive
and regressive
Recent marine transgression (sea-level rise) on
Netherlands coast showing landward shift of
facies. Absolute ages from radiocarbon dating.
Note shift in facies patterns as sea
transgresses. Note how time-lines cross facies
boundaries.
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Fig. 4.13
Example of regression (falling sea level) caused
by glacial uplift rebound) during the past 14,000
years. Notice how unconformity follows the
retreating sea level. Also note how the
sedimentary (facies) patterns are same for
preceding figure (sea-level rise) as expected
since sediments were deposited during sea-level
rise. Where does eroded material go when
sea-level falls? (It is deposited locally, but
note that most of the material now above
sea-level simply remains in place as erosional
highs. Note we can have simultaneous regression
and transgression in different parts of the world
(see previous figure) due to different geologic
agents acting in different places e.g. sea level
rise and rebound. What happens when rebound
stops? When sea level stops rising?
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Fig. 4.14
Contrasting effects of sea level rise on shallow
(Bangladesh) versus steep (Vancouver) coastlines.
The shift in shoreline can be 10s to 100s of
miles in shallow shore lines compared to may be
only 10s of feet in steep areas. Present day sea
level rise is thought to be due to upwarping of
ocean basins.
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Fig. 4.15
Advance of Tigris-Euphrates river delta (175 km
into Arabian Gulf) during the past 3000 years in
spite of a worldwide rise in sea level of abut 4
meters during this time. Rapid sedimentation is
the cause, perhaps induced by human agricultural
practices? Note calculated average sea-level rise
is about 4 mm/yr.
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Fig. 1.8
Fertile Crescent region of Middle East. Note
position of Tigris-Euphrates River delta.
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Tigris and Euphrates River Delta
This image is a Landsat scene of the mouth of the
fabled Tigris and Euphrates Rivers as it empties
through a delta into the Persian Gulf in
southeastern Iraq. Those rivers meet into a
single channel, the al Arab, in the swamplands in
the upper left of the image. The Rivers Karun
(top center) and Jarrahi (right center) are both
in western Iran. The lower left corner is a
barren desert, with sand dunes. Several black
plumes of smoke emanate from the burning oil
fields in Kuwait.
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Correlation using three different index fossils.
A single fossil zone is shown in blue. Note that
range and maximum development (indicated by
pattern width) vary from place to place.
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Fig. 4.17
Significance of different rates of evolution and
changes in environment (due to transgression).
The brachiopod evolved slowly and stayed in/on
sand facies. It is a poor index fossil. The
cephalopods evolved rapidly and are free
swimmers. They were changing and widely
distributed and thus excellent index fossils.
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Fig. 4.18
Volcanic ash layers (bentonites) make excellent
time markers and permit correlations between
facies (provided the ash layer is preserved).
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Fig. 4.19
Conodont. A conodont is a preserved bony part
of an extinct eel. It evolved rapidly and is
widely distributed across many facies types. They
are excellent index fossils and can be used to
determine maximum burial temperatures as well.
Conodont specialists were once highly sought
after by oil companies.
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Fig. 4.20
Block diagram showing relationships between
formations and index fossils. Fossil zone C shows
that Formation 3 is synchronous everywhere, but
zones 1 and 2 vary in age. How can you tell?
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Fig. 4.21
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Fig. 4.22
SLOSS SEQUENCES (6)
Six unconformity bounded sequences from which a
world-wide sea-level fluctuation curve has been
inferred (the Vail curve). Note two maxima
(highs) at about 500 and 75 million years and
three minima at 600, 200 and the present. Max
350 to -150 m above and below present.
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Fig. 4.23
Effects of sea level change on sediment
accumulation and unconformities at a continental
margin.
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Additional Relative Time Scales

• In addition to index fossils, there are several
  other ways to determine relative time
• The sequence unconformities just discussed
• Magnetic reversals
• Isotope geochemistry e.g. strontium isotopes work
  pretty well back to end ot Miocene.

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Fig. 4.24
Magnetic reversals over past 80 million years.
These reversals are recorded in sediments and can
be used for relative time dating.
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Fig. 4.25
Graph of depositional rates of a hypothetical
sequence of strata. The continuous average rate
of deposition is computed by dividing the strata
thickness by the time interval. The actual rate
accounts for changes in rate of deposition as
well as for erosional events.
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EARLY DEVONIAN 400 MY
PALEO-RECONSTRUCTION

• 2 global hemisphere views one centered on North
  America and the other centered on the
  Tethys-Indian Ocean region. A global mollewide
  projection with labels and 1st-order tectonic
  elements shows the whole Earth for the Early
  Devonian.

http//jan.ucc.nau.edu/rcb7/paleogeographic.html
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http//jan.ucc.nau.edu/rcb7/paleogeographic.html
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PALEO-RECONSTRUCTION
LATE DEVONIAN 370 MY

• 2 global hemisphere views one centered on North
  America and the other centered on the
  Tethys-Indian Ocean region. A global mollewide
  projection with labels and 1st-order tectonic
  elements shows the whole Earth for the Late
  Devonian.

http//jan.ucc.nau.edu/rcb7/paleogeographic.html
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http//jan.ucc.nau.edu/rcb7/paleogeographic.html