BACKGROUND MATERIAL AND
FACTUAL CHRONOLOGY
Field study of rocks involves measuring the thicknesses of
different rock units, called formations, and plotting their
distribution on maps. As the geologist measures the
sections and prepares to map the area, he or she must work
out the stratigraphy, that is, the vertical and lateral
relationships of the stratified rocks. The following
stratigraphic principles help to establish the age relations
of formations.
i) Original horizontality: Sediments are normally
deposited in horizontal sheets; any dip must have
resulted from post depositional folding or tilting.
This principle forms the basis for all structural
interpretations in regions of sedimentary rocks.
ii) Superposition: Younger sedimentary units overlie
older ones; assuming that the layers were not
strongly folded or faulted to such a degree that older
layers rest above younger ones.
iii) Original lateral continuity: Most formations
interrupted by valleys or mountain ranges were
originally continuous
iv) Cross-cutting relationships: Any rock cut by faults or
intrusive igneous rocks is older than the fault or intrusion.
v) Fossil succession: Each stratum contains organised
fossils peculiar to itself. Older rocks may be expected to
contain more primitive fossils than younger rocks.
vi) Sedimentary environments: Sediments accumulate on
land in terrestrial environments or on the sea floor in
marine environments. Ancient environments are usually
recognised by comparing the rocks and fossils with
sediments forming today in similar environments.
vii) Facies are different sediment types that replace
one another laterally; they have usually accumulated
at the same time but in differing environments.
Facies are particularly evident where shallow seas
and their associated patterns of environments move
landward in transgressions or seaward in
regressions.
viii) Areas of erosion: Unconformities are gaps in the
sedimentary record caused by erosion, rather than
sediment deposition; the erosional surface may
represent a small or large time gap, and the length of
the gap may vary from place to place. Rocks below
the unconformity are older than those above.
ix) Bedding: Cross bedding shows the direction of
current. In the case of graded bedding, coarser
sediments are at the bottom of the stratum and the
sediments become finer upwards.
x) The geologic time scale is based on the changing
fossil life in sedimentary rocks. It provides a
worldwide rather than a local means of dating
discoveries. The Phanerozoic Eon is divided into
three eras and twelve periods.
Stratigraphic units:
Lithostratigraphic units are designated as formations,
members and groups; they are defined solely on the basis of
rock type. A formation is considered worthy of recognition
and naming if its rocks are distinguishable from units above
and below, and are thick enough to be plotted on a large scale
topographic map.
They are the fundamental units used in making geologic maps.
Formation names consist of two parts: the first identifies the
geographic locality where the rock is well exposed; the second
describes the general rock type.
The Esna Shale and the Sudr Chalk are typical formation
names. Where no single rock type predominates, the second
part of the name becomes simply 'Formation" as in the Raha
Formation.
Rock units that are distinctive enough to warrant
recognition, but are generally too thin to be mapped
are called members of formations e.g. the Abu Had
Member of the Raha Formation. For convenience in
small scale mapping, two or more formations may be
lumped into a single larger rock unit called a group,
e.g. the Gharandal Group in the Gulf of Suez
region.
A unit formally recognised as a formation in one area
may be treated elsewhere as a group or as a member
of another formation, without change of name, e.g.
Mokattam Formation or Mokattam Group. A unit
distinguished only by the taxonomy of its fossils is
not a lithostratigraphic but a biostratigraphic unit.
Biozones are the fundamental biostratigraphic
units and include three principle kinds: Interval
Zone, Assemblage Zone and Abundance Zone. The
kind of strata that contain the fossils does not enter
into the definition. For example, a zone may
encompass several units of limestone and shale, or
it may fall entirely within a part of a single unit of
shale.
Most commonly used is the Interval Zone which is
the body of strata between two specified
documented lowest and/or highest occurrences of
single taxa and includes three basic types; Taxon
range zone (A) concurrent or partial range zone (B)
and the evolutionary or lineage zone (C).
The Lithostratigraphic Units:
Group - Formation - Member - Bed
The Formation is the essential rock unit to which every
stratigraphic column should be subdivided;
“ Any lithologically distinct, geographically
extended (mappable) unit”
= Composed of one rock type,
= Two alternating lithologies, or
= Heterogeneous unit between two homogenous ones
How to describe ?
# Name; compound: geographic part +
dominant
lithology
# Type section and locality.
# Thickness.
# Lithology; dominant.
# Boundaries; Overlie - underlie.
# Extent ; geographic distribution.
# Fossil content and age; guide.
# Equivalent units or synonyms.
# Reference (author and year).
EGYPT:
Four Geographic Regions
@ Sinai Peninsula.
@ Eastern (Arabian) Desert.
@ Nile Valley and the Delta.
@ Western (Libyan) Desert.
Have a look on the map to recognise the
topographic and geomorphologic features of each region.
EGYPT : Four major structural units:
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*
*
*
Arabo-Nubian Massif or Craton“Shield”
Stable Shelf or the foreland facies.
Unstable Shelf.
The Gulf of Suez region.
EGYPT: Volcanic activity:
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=
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Early Palaeozoic.
Carbon.-Permo-Triassic.
Cretaceous.
Oligo-Miocene.
Pleistocene.
The Palaeozoic
• The Palaeozoic outcrops of southwestern Sinai
• The Palaeozoic outcrops of the north Eastern Desert
• The Palaeozoic subsurface rocks in north Western
Desert
• The Palaeozoic outcrops of southwestern Egypt
• Palaeozoogeography
• Economic aspects of the Palaeozoic deposits