Presentation on theme: "BACKGROUND MATERIAL AND FACTUAL CHRONOLOGY Field study of rocks involves measuring the thicknesses of different rock units, called formations, and plotting."— Presentation transcript:
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 Sinai Eastern (Arabian) Nile Valley and the Western (Libyan) Desert. Have a look on the map to recognise the topographic and geomorphologic features of each region.
EGYPT : Four major structural units: * Arabo-Nubian Massif or Craton“Shield” * Stable Shelf or the foreland facies. * Unstable Shelf. * The Gulf of Suez region.
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