Evolution of the
Earth
Chapter 4
MAIN TOPICS:
ROCK FACIES
RELATIVE TIME
GEOLOGIC TIME SCALE
RELATIVE GEOLOGIC TIME
• As geologic thought progressed in the early 1800’s 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 Steno’s Laws
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)
–
–
•
•
PERMIAN
MESOZOIC ERA (183 My)
–
–
–
•
MISSISSIPPIAN (N. America)
PENNSYLVANIAN (N. America)
TRIASSIC (Trias, Germany)
JURASSIC (Jura Mtns.)
CRETACEOUS
CENOZOIC ERA (66 My) (cont.)
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.
FACIES – DEFINITION & RELATIONSHIPS
• Sedimentary Facies
Overall lithology (rock-type)
reflecting or diagnostic of
depositional environment
Examples:
–
–
–
–
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.
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 Murchinson’s lower
Silurian. After their deaths, the dispute was resolved by naming a new system, the
Ordovician.
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.
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.
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 nondeposition and/or erosion.)
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.
Fig. 4.8
Lateral Relationships: different facies, same time
Lavoisier’s 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 sealevel.
Depositional Environments and Sedimentray Facies
• Lateral variations of strata not fully
appreciated until 1838
• Facies concept relates sediments to their
depositional environment
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).]
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.
Fig. 4.11a
Illustration of restored facies map for Devonian of Europe. Note
how this illustrates Steno’s law of lateral continuity. Also, how
extensive the different depositional environments are. Lines refer to
cross-sections in following diagrams.
Fig. 4.11b
Restored cross section showing facies, thicknesses & unconformities
Time-stratigraphic chart showing time gaps represented by unconformities in B
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.
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?
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.
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.
Fig. 1.8
Fertile Crescent region of Middle East. Note position of
Tigris-Euphrates River delta.
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.
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.
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.
Fig. 4.18
Volcanic ash layers (bentonites) make excellent time markers and
permit correlations between facies (provided the ash layer is
preserved).
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.
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?
Fig. 4.21
Fig. 4.22
SLOSS SEQUENCES (6)
Six unconformity
bounded sequences from
which a world-wide sealevel 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.
Fig. 4.23
Effects of sea level change
on sediment accumulation and
unconformities at a continental
margin.
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.
Fig. 4.24
Magnetic reversals over past
80 million years. These
reversals are recorded in
sediments and can be used for
relative time dating.
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.
PALEO-RECONSTRUCTION
EARLY DEVONIAN 400 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 1storder tectonic
elements shows the
whole Earth for the
Early Devonian.
http://jan.ucc.nau.edu/~rcb7/paleogeographic.html
http://jan.ucc.nau.edu/~rcb7/paleogeographic.html
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 1storder tectonic elements
shows the whole Earth
for the Late Devonian.
http://jan.ucc.nau.edu/~rcb7/paleogeographic.html
http://jan.ucc.nau.edu/~rcb7/paleogeographic.html