Origin and interpretation of Sedimentary rocks (Ecology, environments)

Environments and life

In historical geology were 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.

 

ECOLOGY

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.

TERRESTRIAL REALM

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:18-20C (64-68F) 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.

MARINE REALM

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.

Depth of the sea

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 .

FRESHWATER ENVIRONMENT

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.

b. Lakes as depositional environments

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.  

c. Glacial environments

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.

d. Desert and arid basins

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.

Sand dunes

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).

e. River systems as depositional environments

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

f.Transitional environments

 1.DELTAS

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

g. Marine environments

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