Rocks, Fossils, Time  

Sedimentary rocks are really important in Historical Geology, because they have lots of information about the environment they formed in. They give very important key to the puzzle of earth history.

Characteristics of sedimentary rocks

1.Color

Color of a rock is not always meaningful, however sometimes especially in sedimentary rocks it can give useful information about the depositional environment.

Black or dark color

This color means the presence of organic carbon, and iron. The fact that organics could be still in there means that there was no oxygen around to destroy it. Usually this color is characteristic to deep water, quiet bottom environment. Organic decay in such an environment is related to anaerobic bacteria. In this environment Fe will have 2 charge FeO, and pyrite and hydrogen sulfur would accumulate. Bed smell......(Source rock for oil)

Red or light color

The iron in near surface environment where lots of oxygen is around, will have three charge Fe₂O₃ (hematite) RED or light color is characteristic. Organic decay in such an environment is complete, no any black left.

(Think  of why is most of the soil around your house is reddish?)

Texture

The size, shape and arrangement of mineral and rock grains in a rock constitutes its texture.

          Grains Larger particles

          Matrix Finer particles

          Cement Chemical precipitates

Size

          Wentworth scale  

          The larger the grains, the higher the energy of the transporting material.

SORTING

From the grain size distribution we can determine the sorting of the sediment.

          Well

          Poorly

Shape of grains

       Rounding

          The edges of particles

       Sphericity

          How closely the grain approaches the shape of a sphere

Arrangement of grains

          Orientation

Sedimentary structures

Larger features in the sediments that are forming during sedimentation or shortly after sedimentation and before lithification. These are extremely useful.

Mudcracks

Drying after deposition

Shrinkage of drying mud and clay

Cross bedding

It is an arrangement of sedimentary beds, in which one set of layers is inclined relative to the others.

Graded bedding

Repeated beds, in which each beds has the coarsest grains at the bottom.

Ripple marks

Develop along the bedding surfaces

symmetric

assymetric

Composition of sediments:

Classification of sedimentary rocks

CLASTIC OR DETRITAL SEDIMENTARY ROCKS

Goes by grain size

Conglomerates, breccia

Sandstone

Sandstone is very important, and can give us information about the source area (geology of the source area)

          -Amount of transport,

          -transportation material

          -distance of transportation

We will talk about this in detail in the lab.

Siltstone

Clay

CHEMICAL AND BIOCHEMICAL SEDIMENTARY ROCKS

Carbonates

Limestone

dolomite

chert

evaporites

          gypsum

          anhydrite

          halite

          sylvite

  Coal

STRATIGRAPHY

Is the branch of geology that is concerned with the composition, origin, age relationships and aerial extent of layered or stratified rocks. Stratification may occur in any of the major rock groups, however all the sedimentary rocks are stratified.

Vertical Stratigraphic Relationships

The bounding surface separating one layer of strata from another is called Bedding plane.

Bedding plane can be

1. Gradational (a rock gradually changes into an other)

2. Sharp (below and above rocks are different) meaning rapid changes in sedimentation, environment, or it could mean that some time is missing from the succession.

Superposition

If sedimentary layers are undeformed, than correct relative ages easy to determine by the position in the rock sequences. However if strata have been deformed, relative age determination is more difficult.

Most of the time however geologic structures help to solve the geologic problems (ripple marks, root traces etc.).

It is more difficult to find relative age  solution with igneous contacts, geologists are looking for signs such as baking.

People of course use the other principles also; we have already learned such as Principle of Crosscutting, Principle of original horizontality, principle of lateral continuity, Principle of inclusion,

and Principle of Unconformities.

This principle is about time breake or missing time. (we call it HIATUS, meaning during this time there was no rock layer preserved)

Three major type:

 1. Disconformity

surface of erosion or nondeposition between younger and older beds, that are parallel to each other

2. Angular unconformity

Erosional surface on tilted or folded strata over which younger strata have been deposited

 Unconformities of regional extent may change laterally from one to an other.

3. Nonconformity

Erision surface cut into metamorphic or igneous rocks and is covered by sedimentary rocks.

The Formation

The most basic local unit of stratigraphy is the formation.

Its definition:

distinctive series of strata that originated through the same formative process.

Stratum:

The thinnest observable rock layer. Formation would contain lots of stratum.

Characteristics chosen to define a formation are:

1. composition of mineral grains

2.color

3.textural properties

4. thickness and geometry of stratification

5. Fossils

6.Outcrop character.

The six together is referred as lithology of the formation.

Lateral Realtionships - facies

Steno formulated the principle of the lateral continuity, in which he stated, that sediment layers will extend in  each direction untill they terminate.

Termination can be:

1. Abrupt

          Edge of depositional basin

          fault

          or eroded away

2.Lateral termination may also occur when a rock unit becams very thin until it pinches ou

3. Intertonguing occurs, when a unit splits laterally into thinner units each of which pinches out. This occurs when different depositional processes are operating in adjacent areas.

4.Gradual change in rock type

Sedimentary facies

Each environment in a sedimentary basin produces a body of sediment which can be characterized by a distinctive set of physical, chemical and biological  attributes. The body of sediment is called sedimentary facies.

Transgressions and regressions

Transgression is the advance of the sea over the land, when sea level rises with respect to the continent. (shrinkage of the continent) This will produce a continuous shift of environments and their sedimentary products landward. This means, that shallow water sediments will be overlied by deeper water sediments. On the other hand individual rock units become younger in a landward direction. These sediments are time-transgressive meaning that their ages vary from places to place. These are all together are so called transgressive facies pattern.

Regression is the opposite, when there is a retreat of the sea from the land, when sea level falls with respect to the continent. In the sediments depositing during retreating sea we can see a regressive facies pattern.

This will reflect

1. enlargement of land (more fluvial, lake wind deposits)

2. Finished by erosional unconformity

3. seaward shift of facies through time

4. In one locality sediments tend to be coarsening upward.

WALTER’S law

About 100 years ago a german geologist Johannes Walter  studied the relationship of modern facies to their environment.He noted that environments shift position thorough time, The same facies following one another in a vertical sequence (which does not have time break) will also replace one another laterally. Most of the time it is easier to work with walter’s low from the vertical successoin, than in horisontal, because of the big distances.

Sedimentary cycle

This is the repetition of facies or sequence of facies, in other word the repetition of sedimentary environments.

Causes of transgression and regressions

There are different scales of sea level changes, causing these transgressive - regressive cycles.

-Global (eustatic)

          -Longterm sea-level ~100 million years

                    Ice age/nonglacial time

          -Mediumterm sealevel changes

                    These are related to plate tectonic movements

          -Short term sea level changes

                    Climate changes caused by gravitational movement between the planets (high frequency sea level     changes 100, 40, 20 ky cycles)

-local

          Uplifting

          subsidence

          Just for the local basins

FOSSILS

Fossils are the remnants or traces of prehistoric life, that have been preserved in the rocks.

1.They are important in global correlation

2. They give information on the depositional environment

3. Provide information about evolution

Types of preservation

1.Unaltered remains (retain original structure and composition)

Resin (amber)---insects

Freezing --- Mammuth in Berezovka

2.Altered remains

Fossils, that has been changed structurally, chemically or both

3. Molds, casts

Biostratigraphic concepts

The principle of fossil succession:

According to this principle fossil assamblages succeded one another through time in a regular and determinable order.

Index fossils:

Fossils unique to a limited thickness of strata. It has to be widespreaded geographically. (free floating larval stage of marine animals)They are very important in correlation (Ammonites).

Fossil zone:

An interval of strata characterized by disticnt index fossil. (Typical time duration is 0.5-2 million years.

The relative time scale

During the development of the relative time scale geologists through lots of fighting made up a composite geologic column, which is the relative time scale.

Stratigraphic terminology:

To deal with both rocks and time the terminology includes two fundamentally different kind of units:

1.Unit defined by its content

2. Units defined by geologic time

1. Units defined by their content:

A. Lithostratigraphic unit (Defined by the physical char. of the rocks)

Supergroup

                    group

                             Formation (mappable rock unit, basic lithostratigraphic unit)                      

                                       member

                                                bed

B. Biostratigraphic unit:

Based on fossil content

          Biozones

2. Units defined by geologic time

Time-stratigraphic units                    time units

eonotherm                                     Eon

Eratherm                                           Era

  System (fundamental unit)        Period

          Series                                                epoch

                 Stage                                           age

Systems are based on stratotype from the best locality of the certain system.

First of all you guys have to learn the ERA< PERIOD< EPOCH (pg.94).

Correlation

Correlation is the process demonstrate equivalency.

For ex.Correlation of lithostratigraphic units is the recognition of similar lithology.

We can also use the presence of KEY beds (lava flow)

-Oil companies, water comp. ---well cuttings, well logs give wonderful information about underlying lithologies.

-Seismic profiles

-biozones (extremly useful)