1. Historical Geology: the study of the history of the earth.
Stratigraphy and Historical Geology
Chapter 4:
The Relative Geologic Time scale and Modern Concepts of Stratigraphy
Stratigraphy: the study of stratified rocks, their nature, their occurrence, their relationships to each other and their classification.
Historical Geology: the study of the history of the earth.
It includes stratigraphy.

2. 4.1 Early Mapping and Correlation
How geologic mapping in northern Europe led to the
modern relative time scale??
4.1 Early Mapping and Correlation
How geologic mapping in northern Europe led to the modern relative time scale??
- by the application of the principle of
superposition.
fossil correlation.

3. First Geological time scale
Fig. 2.11

4. Standard Geologic Column as conceived by 1800
Based on the Neptunism theory by Werner 1787.
Alluvial rocks
Secondary rocks
Transition rocks
Primitive rocks
Youngest
Oldest

5. The result Geological map of all western Europe gradually developed.
After publication of Smith´s and Cuvier´s geological maps in the 1800s, the case was :-
- Wernerian chronology continued to be used.
- Local names of strata became more and more numerous.
Naming of distinctive rock bodies was a natural by-product
of mining and mapping.
These names developed as a shorthand and reflected
geographic localities or peculiar rock types.
As a result of the lateral continuity these names was
extended more widely.
- At the same time fossils were being collected and widely
separated strata were correlated.
The result Geological map of all western Europe gradually developed.

6. A group of secondary strata named Jura for the Jura
mountains of France and Switzerland.
When the principle of superposition were applied, the
Jura strata were found to overlie another group named
Trias in Germany and underlie a third group on France
named Cretaceous.
Quaternary
France Cretaceous
France and Switzerland Jura
Germany Trias
Britian Carboniferous
- The coal –bearing strata in Britian were named Carboniferous.
- Later , in 1829 the formal name Quaternary was proposed to
include the young deposits.
This scheme gradually displaced the older Wernerian scheme.

7. Fig. 4.2

8. 4.2 Modern Relative Time Scale
In 1835 Sedgwick and Murchison they named:-
- The Cambrian (for Cambria, the ancient roman name of
Wales).
The Silurian (for an ancient welsh tribe, the Silures).
Based on the correlations and fossils content Sedgwick and
Murchison defined the relative age of Devonshire strata
which lies between the Silurian and the Carboniferous,
they named these strata as Devonian layers. Fig 4.3.
Sedgwick proposed a large division which should include a
number of smaller subdivisions, for example Paleozoic Era.
- Based on the relative percentages of living species
Mesozoic Era and Cenozoic Era were named.
- The Permian Period was named next. Table 4.1

9. Table 4:1

10. Fig 4.3
Relations of the non-marine Old Red Sandstone facies of Wales to marine
Devonian facies of Devonshire, as inferred by Sedgwick and Murchison.
Inter-tonguing of marine and non-marine deposits proved the Devonian age
of the Old Red Sandstone.
These facies also proved that different environments of deposition existed
simultaneously.

11. The Geologic Time Scale

12. 4.3 Rocks Versus Time
There is a clear distinctions between the abstract geological time and the rock record by:
- time is continuous.
- rock record is riddled with unconformities of varying magnitude.
So what we known of history must be gleaned from the imperfect rock record.
Fig. 4.6

13. Eon Era Period Epoch Age Chron Eonothem Erathem System Series Stage
If elsewhere a more complete sequence of Cambrian rocks were
found with fewer and smaller unconformities and more fossils, it
might provide a better world standard of reference than does that
of Wales.
Thus, what we have are time divisions and rock division.
The most generally accepted stratigraphic classification scheme
is shown in the next table
Relative Time Division Equivalent Universal rock Division
Eon
Era
Period
Epoch
Age
Chron
Eonothem
Erathem
System
Series
Stage
Chronozone

14. 4.4 The Formation
Formation is the most basic local units of stratigrphy.
As definition: it is a distinctive series of strata that
originated through the same formative processes.
Formation must be:-
- distinctive in appearance in order to be easily recognizable.
- named for a type locality where they are normally
displayed in a well-exposed type section.
Characteristics chosen to define a formation include one or more of the following:
Composition of mineral grains.
Color.
Textural properties (size of grain, etc.).
Thickness and geometry of stratification.
Character of any organic remains.
Outcrop character.
Lithology

15. Group: rock unit consisting of two or more formations
So Formation include more than one stratum, and Group contains many Formations.
Important stratigraphical terms arranged from the biggest to smallest:-
Group: rock unit consisting of two or more formations
that are next to each other in a succession and
related to each other.
Formation: it is a distinctive series of strata that
originated through the same formative
processes.
Member: a part of a formation.
Bed or strata: a layer of sedimentary rock that is marked
off above and bellow by surfaces that can be seen
and made up of material that is the same in all parts.
Lamina: a thin layer (less than 10 mm thick) in a
sedimentary rock that can be separated from the
material above and bellow it.

16. Problems arise when a new formation defined :
The name.
Designation of lower and upper limits or contacts, may difficult as shown in fig. 4.7.
The type section.
Fig. 4.7

17. 4.5 Lateral Variations
The stratigraphic divisions being named in Europe were
universally present and of the same age worldwide.
In another word as long the geologist found the same
fossils in different and far stratified rocks they assigned
the same age for both of them .
- For example the Cambrian strata in Wales.
This mean that Geologist thought that any named
formation should laterally extend without change.
This principle was the cornerstone of Werner´s time scale.
In 1789 French writings show that ‘similarity of fossils in similar sedimentary rocks might reflect environmental factors rather than strict age equivalence.’
Based on fig. 4.8 it became clear that: the sedimentary products of each environment have aunique characteristics, even though they accumulate at the same time and grade gradually into one another.

18. The result in other words is: formations might vary in sediment type, or lithology, laterally as well as vertically.
Fig. 4.8

19. The last result was supported by sedgwick and Murchison in the 1830s.
The above two geologist determined that the dominantly non- marine deposits of the Old Red sandstone of Wales are Synchronous with marine Devonian deposits farther south based on the inter-tonguing interpretation.
So, inter-tonguing: two different lithologies can grade laterally into one another in a complex manner.
Fig 4.3

20. 4.6 Depositional Environment and Sedimentary Facies:
The lateral changes of lithology and fossils contents brings
the sedimentary facies concept after a well-exposed example of lateral changes of both lithology and fossils was described in 1883 in Switzerland.
4.6.1 Definition of Sedimentary Facies: the general
characters of a sedimentary rock, especially those
that indicate the environment in which it was
deposited.
This lead to define
The sedimentary environment which mean the conditions
under which a sediment is deposited; e.g.:-
The depth and temperature of the water.
The strength and direction of the currents.
The above factors can vary widely, and they affect the texture, composition, and structure of sediments that are formed.

21. Facies are identified simply by their dominant lithology,e. g
Facies are identified simply by their dominant lithology,e.g. the sand and mud facies, Fig. 4.9

22. Fig. 4.9
- Coarser sediments occur in strongly agitated, near shore environments.
Bottom-dwelling organisms in the different environments differ considerably.
The upper surface of the diagram is a map of modern bottom-sediment types
showing lateral variations only at a moment in time.
Sides are cross sections showing facies relationships through time.

23. Because the facies concept relates sediments to their depositional environments, the study of both sediments and organisms in modern environments provides important clues for understanding ancient facies.
For example: the ripple marks forming in modern environment.
Straight, sharp, symmetrical ripple crests are typical of those formed by waves in shallow water.

24. Fig. 4.11a
4.6.2 Regional analysis of facies: This map show present distribution of Devonian rocks

25. What can we say after looking to this cross section?
Mountain building disturbed the northwestern margin of Europe during the Silurian and Devonian time, whereas the region to the southeast remained more stable.
This is reflected by:-
More and greater unconformities in the west.
Coarse and thick non-marine sediments –Old Red- in the west.
Finer and more marine deposits in the east.
Erosion of the Caledonian Mountains shed immense volumes of coarse debris eastward, greatest thicknesses are nearest the mountains.
Fig. 4.11b

26. What can we say after looking to this Time diagram versus the large gap?
The longest gap due to erosion also is nearest the mountains.
uplift of the mountains was spasmodic, for a major unconformity divides the Old Red Sandstone at the left end of the section.
Fig. 4.11c

28. Transgression
Fig. 4.12

29. 4.6.3 Transgression and regression by the Sea
Preservation of large volumes of most strata requires one or both of the following conditions :-
- Subsidence of the earth crust during deposition.
- a rise of sea level.
example, the North Sea coast of Holland, the submergence is due to
both worldwide rise of sea level and local land subsidence. Fig. 4.12
Transgression
Fig. 4.12

30. Transgression: the advance of the sea over the land, it
caused a continuous shift of environments and their
sedimentary and biological products landward.
Regression: retreat of the sea from a land area, it has
the opposite effects.
Regression
Fig. 4.13

31. There are two basic types of facies patterns:
Transgressive facies pattern, it is generally:-
- reflect shrinkage of land area.
- preceded by an erosional unconformity.
- show a landward shift of adjacent facies through time.
- tend to become finer upward at any one geographic locality.
Regressive facies pattern, results from a relative fall of sea level or a rise of land level, it has opposite characters than those of the transgressive pattern, thus:
- reflect enlargement of land area.
- followed by an erosional unconformity.
- show a seaward shift of facies through time.
- tend to become coarser upward at any one locality.
Transgression
Regression

32. Walther’s law
Walther noted that environments shifts position through geologic time.
As they do so, the respective sedimentary facies of adjacent environments succeed one another in vertical sequences.
As important result of this fact, Walther Law:
The vertical progression of facies will be the same as
corresponding lateral facies changes.
So, in transgressive facies pattern, just as the sequence becomes finer upward, the finer facies also spreads laterally in the direction of transgression, fig
Conversely, just as a regressive sequence becomes coarser upward, the coarser facies also spreads laterally in the direction of regression. Fig

33. 4.6.4 Local Versus Worldwide
Sea level changes may result from:
- fluctuating continental glaciation.
- large-scale warping of deep ocean basins.
Example, the Netherlands and California

34. Very rapid sedimentation could be a result from:
- uplift of a distant inland area.
- climatic change that accelerates erosion.
The above factors may cause local seaward retreat of a shoreline independent of structural warping of the crust or worldwide changes of sea level.
For example, the delta of Tigris- Euphrates which a retreat of the northern shore of the Gulf of Arabia from the land toward the gulf as much as 175 km in the past 3000 years even as sea level has risen about 4 meters worldwide. Fig

35. 4.7 Biostratigraphic Concepts
Fossil zone: an interval of strata characterized by a distinctive index fossil.
Fig. 4.16
Close attention must be paid to overlapping stratigraphic ranges of index fossils.
Both range and abundance of a single species vary from place to place.

36. Environmental changes may have been more rapid than
evolutionary ones. (ex. Lingulla fig.4.17).
Lithologic formation boundaries need not have any relationship to the
biostratigraphic boundaries of fossil zones.
The Lingula in the sand facies evolved very slowly, it is a poor index
fossil, it migrated with the sandy environment without any biological
change after millions of years.
Cephalopods are swimming forms found in the muddy facies. They
evolved rapidly, therefore, their species are admirable index fossils for
times 1,2 and 3.

37. Facies fossils: fossill type which restricted to one or a few
lithology.
The best index fossil for facies are those that lived independent of the bottom environment where sediments form, they should be floating or swimming organisms.
Fig. 4.18

38. Formation 3 is synchronous everywhere.
Fossil zone B represents the ideal case of an index fossil that occurs in
two different facies.
Many facies-dependent fossils are useful environmental indicators for
the sediments in which they occur.
Formation 3 is synchronous everywhere.
Formations 1 and 2
vary in age due to
lateral facies changes.

39. 4.8.1 Un conformities Unconformities can be : Types of Unconformities:
- Traced and mapped to established their physical continuity.
Studied from the standpoint of age, i.e. an unconformity surface varies
in age from place to place.
The total time interval represented by the discontinuity may vary
greatly.
An unconformity may even disappear laterally into a continuous,
unbroken, conformable sequence of strata.
this means that, unconformities show lateral and
vertical differences as important as those of rock units.
Types of Unconformities:
Angular unconformity: there is an angle of discordance between the older and younger strata. This type shows that severe deformation occurred before the unconformity was buried.
Disconformity: no discordance between strata below and above the discontinuity surface.
Nonconformity: when the underlying rocks are igneous or metamorphic. Fig 4.21

41. 4.8.2 Unconformity-Bounded Sequences
In the 1950s Sloss suggested a new way of studying the stratigraphic record using unconformity-bounded sequences which formed additional type of stratigraphic division.
As shown in the figure Six major unconformities clearly punctuate the phanerozoic record in a very meaningful way.

42. 4.9 Additional Relative Time Scales
Index fossils are no so far the only things that provide worldwide, synchronous punctuations of the rock record with potential for correlating and subdividing that record.
There are some alternatives that provides relative-age scales with special applications.
Examples:
1.The sequence unconformities which reflect the global
sea- level changes, fig This curve become a special kind of relative time scale.
2. Worldwide magnetic polarity reversals.
3. Chemical analyses for rare or trace elements and for isotopes in sedimentary rocks also provide potential for relative geochemical time scales.