In
historical geology we’re learning about the history of the earth.
The
question is, how can we do that?
We have
already discussed this question somehow, and got to the fact, that we would use
Geologic Principles, and fossils, to put together he puzzle.
First
we have to learn to listen to the stories of ancient environments, climate
and paleogeography (paleolatitude, paleoaltitude).
One of
the remaining question is how are we going to make the rocks, and the fossils to
talk, to tell us their own stories.
Before
we do that, we have to know about the present day environments, to be able to
use the great idea of uniformitarianizm.
Learn
the Hypsometric curve of the earth, which represents the surface of the earth.
The way
living things relate to their environment is called ecological niche. For a
specimen its environment has to give food, and particular physical, and chemical
conditions. Some specimens have much broader niche than others. Every specimen
is restricted in its natural occurrence by certain environmental conditions.
(fern----humid
conditions)
Cactus------dry
conditions)
Almost
all of the specimens have to share their environments, and frequently have to
compete for food and living space.
Communities
of organisms:
Populations
of several species living together are called ecological
community.
In
these communities the eating habitat is depend on the food chains or food web in
the given environment.
The
organisms of a community and the physical environment they occupy called ecosystem. The animals of an ecosystem are called: FAUNA
and the plants are called FLORA.
Diversity is used to describe the number of species that live
together in an ecological system. It is nothing else than the account of the
number of different specimens
present.
BIOGEOGRAPHY
The
distribution and abundance of organism on a board geographic scale are studied
within the field known as biogeography.
The
most important limiting factor is temperature. Some to polar, some to
equatorial. More species are adopted to tropical regions than to polar areas.
Other
limiting factors are land barriers for aquatic forms or water barriers to
terrestrial forms.
The
fauna and flora of the continents. Today sea-level is lower than during most of
the geologic time, and the world has a steep temperature gradient between the
poles and the equator. Climatic
conditions have a very important influence on the distribution of organisms on
the land.
CLIMATES
AND VEGETATION
It is
remarkable how closely the distribution of terrestrial vegetation corresponds to
the geographic pattern of climates. This correspondence, coupled with the fact
that plants are dominant producers for food web, thus strongly effect the
distribution and abundance of animals.
Tropical
climates:
Rain
forest:
Av.
temp:18-20°C (64-68°F) or higher, and usually found at latitudes within 20°.
Lots of different kind of plants and animals.
The rocks in such a climate environment are chemically weathered. The
soil zone well developed, rich in nutrients.
Tropical
desert:
Between
20-30° latitude the tropical desert zone occur. Ex. Sahara. Much less
diversities. Big temperature different during a day. A few
plants, therefore just a few animals. Max 10 inches of rain/year. The
sediment here only physically weathered, mostly sand transported by winds.
Desert
also can occur in other latitude, such as mountain shadow (Arizona) or far away
from the ocean (Gobi)
Savannah
(grassland)
Moisture
enough for grass and scattered trees, but not enough for dense forest. They are
positioned between wetland and deserts usually. (Great Plain). They are famous
about the large amount of relatively large animals; bosons, antelopes, zebras,
elephants.
Physical
and chemical weathering common, well developed soil.
Polar
climates (arctic)
Big
contrast, from rain forest, Savannah. Today two icecaps exist. One on
Antarctica, the other one on Greenland. In ancient times, where there were no
continents on the poles, there were no existing icecap on the earth. If there is
no icecap, it is easy to imagine, that there is no temperature contrast either.
In these cases the climate is much more equal on earth.
The
life on the ice caps are very scattered, almost no plants, and just a few animal
can survive the very cold climate there.
No
weathering, no soil.
Tundra
(subarctic)
The
layer of the soil beneath is frozen even during the summer. In these conditions
water is rare. Here the plants are low, and they do not need too much water.
mosses, sedges, lichens and low growing trees. North America-Eurasia,
small diversity animal life.
Few
physical, and chemical weathering, poorly developed soil, mostly from grass.
Evergreen
coniferous forest:
These
are located next to the tundra. Pine trees. In this zone the summers are very
short.
Acidic
soil, chemical and physical weathering, good soils.
Temperate
forest
This
replaces the evergreen forests. Among some pine trees these are characterized by
maple oak trees. Ground animals are more diverse in temperate forest, than in
evergreen forests. Birds are especially well presented,
Well
developed soil zones.
Mediterranean
climates
Dry
summer and wet winter. They mostly lie about 40° from the equator.
(California,
Mediterranean see). In these areas chaparral vegetation is common which consist
of plants with waxy leaves, that retain water during dry summer. Lots of animals
as well.
Why climate is so important to interpret rocks and
fossils????
Because
plants are so sensitive of environmental changes, the fossil plants are such
good climate indicators. Geologists are often using fossil plants in climate
studies.
For
example Cycads, are today living in warm climate, and bec. Of uniformitarianizm,
we think that they also lived in warm climate during geologic times. Therefore
their distribution would help to draw climate boundaries of the past. Reptiles,
the cold-blooded animals also suggest warm climate.
Ocean
basins are the area for most of the sedimentation throughout the earth history..
This is the place where the sediments and the fossils have more chance to be
preserved.
It is
between 0-10km. Most of the seafloor lies between 3-6 km below sea level.
Continental
shelf:
Submarine
extension of a continent. The edge of the shelf is called shelf break.
Along
the sea margins barrier islands are
frequent, because the longshore current, and the wind. Behind barrier islands
there are lagoons bays, or marshes.
These marshes are populated by plants and animals especially in warmer
latitudes. Peat, coal formation is
frequent, especially on mid latitudes.
When
the sea occupies the land area as well, it is called epicontinental sea. Today
any of those exists, but in ancient time they were very common.
Seaward
of the continental shelf is the continental
slope.
At the
base of the continental slope there is now continental crust, but oceanic.
Continental
rise: seaward of
the continental slope
Abyssal
plain: below the
continental rise
The
upper layer of the ocean where the light can penetrate is called PHOTIC zone. There are sea animals and plants for which photic zone
is the only place to live. The depth of photic zone is about 100-200 m, which is
about the same distance seaward as the shelf break.
For
life the hardest conditions exist in the near land zones. Here depending on the
climate the chemical and temperature conditions are frequently changing, so
there is no diverse biota. Also close to the land because of the waves biotas
have to survive high energy conditions.
In the
deeper sea conditions are more stable.
MARINE
LIFE HABITATS
Organisms
that are floating in the water are called PLANKTON.
Phytoplankton: plantlike organism (photosynthesis).
Feeding
on the phytoplankton are floating animals ZOOPLANKTON.
Some of the animals in the plankton are able to swim, however most of the just
drifting with the flow.
Animals
moving, swimming in the water are called NEKTON.
(fish)
Plankton
and nekton together is called pelagic
life.
Organisms,
living on the floor are called Benthos. The
seafloor itself is the substrata.
The
distribution of marine life depend on:
1.The
temperature of the sea water
For ex.
Corals are restricted to the tropical latitudes.
2.Salinity
of the sea water
The
salt content of natural water is called salinity.
Oceanic
water is normally contains 35 parts of salt per thousand parts of water.
Water
of lower Salinity is called brackish
Water
of higher salinity is called hypersaline
3.Energy
level
It is
also a limiting factor
For
ex.At the shoreline which is very high energy environment, just certain type of
plants and animals can live
The
difficulty of living in water of low salinity is a major reason, that the
diversity of life in freshwater is very low.
SEDIMENTARY
ENVIRONMENTS
1.TERRESTRIAL
ENVIRONMENTS
a.Soil
environments:
Soil is
the loose sediment forming on the top of the rocks by weathering.
Remember
we learned in physical geology, that soil in humid climate is Al, Fe rich mostly
clay mineral (Pedalfer) and on arid climate it always contains carbonate (Pedocal).
The tropical soil, which has no SIO2
in it is called laterite.
How can
we recognize ancient soils?
Sometime
it is difficult, because diagenetic alteration.
But
most of the time roots, animal burrows are diagnostic features. To recognize
ancient soil horizons is important, because it tells us, that the rocks below
the soil were exposed to air.
Usually
the percentage of lake sediments is not too big in sedimentary rocks, however
they are very important to recognize. Most of the time to separate lake
sediments from marine can be done by the fossils!!!!!!!
The
freshwater animals are very different from marine (plants also).
Lakes
are usually also very close to other terrestrial environments such as river.
To
recognize glacial environments is very important for paleoclimatology studies.
It is
easy to recognize the areas which were once occupied by glaciers. As glaciers
move they erode the rocks. They live characteristic U shape valleys, and
sediments. The sediment of the glaciers are usually the mixture of boulders,
pebbles, to mud (UNSORTED) is easy to recognize. This material is called TILL. Sediment particles in till will never be rounded. Lithified
till is called tillite. In lakes farther from the glaciers fine grained sediment
accumulate called varves. Each coarse
layer forms during summer, when meltwater carry sand into lake. During winter,
when everything is frozen, just very few organic rich mud would deposit in this
lakes from suspension.
No soil
like in humid environment. The most important feature of arid environment is
carbonate precipitation+the presence of evaporites (gypsum, anhydrite, rock-salt (halite).
The
presence of mudcracks are important signs for alternating wet/dry season. Dry
polygonal structures.
Death
Valley a good modern example.
The
sand in sand dunes are piled up by wind, therefore it is well sorted, all the
grains are well rounded. The sand dunes are always cross bedded.
If the
prevailing wind direction is changing the cross beds are angular, called trough
cross-stratification.
This
environment always indicates arid climate. (If it was humid, soil would form,
and dunes would become stabile).
1.Alluvial
fan
Sediments
of alluvial fans form at the feet of the mountains and steep hills in both arid
and humid regions. This sediment is usually unsorted and grains are angular.
Alluvial fan has characteristic delta shape like delta.
2.Braided
stream deposit
This
forms just below the alluvial fan. It has a complex network of channels and
islands. The bars between the channels are filled up by coarse grained, hardly
rounded sediment.
In the
channel sedimentation is cross bedded.
3.Meandering
stream
The
channels of the stream migrates. Point bar, cut bank..Natural levee,
Relatively
sorted, well rounded sediment, mostly clay, and q-sand
Around
the delta sediment becomes finer grained, lots of clay accumulates. The
sediments in the delta frequently influenced by marine processes, such as waves
and tides. When a river enters into a large body of water the flow slows down,
and sedimentation happens.
Delta
has some characteristic parts: Prodelta, delta front and delta plain.
Because
of the sedimentation delta builds toward the ocean:Progradation.
As
the progradation happens the earlier prodelta will be overlied by deltafront,
and than delta plain.
(Ancient
Deltas are very important places to find bec. of the clay the channels are good
closed reservoars for everything.)
2.Barrier
islands and lagoons
(salinity)
3.Tidal
flats (salinity)
4.Tidal
channels
1.Organic
reefs
Reef
flat, talus, back reef lagoons
Atolls
(ancient atolls are also good reservoars).(bomba)
2.Carbonate
platforms (inter, supra, subtidal)
stromatolites
3.Submarine
slopes and turbidite
4.Pelagic
sediments (deep sea sediments)
Calcareous
ooze
planktonic
organisms
Carbonate
compenzation depth
siliceous
ooze
Environmental
interpretation and historical geology
Parameters
of environmental significance
1.
Geometry: three dimensional shape of rocks
2.
Lithology
a. detrital grainsize (energy level)
b. Carbonates-sedimentary facies with characteristic grain types (grainsize also show energy level).
3.
Fossils
a. macrofossils as rock builders (reef)
b.Macro-micro fossils as environment indicators
c.Trace fossils (sign of specific environments)
4.
Sedimentary structures
a. ripple marks
b. cross beds
5.Paleocurrents