Time and Geology

Time and Geology

Important concepts

• Principle of Uniformitarianism (Hutton, 1790) “Present is key to the past”

• Principle of superposition – Order in which the strata were deposited is from bottom to top

• Law of horizontality – Strata deposited in horizontal/nearly horizontal and parallel/nearly parallel to the earth surface.                                                

• Unconformities – A substantial break or gap in sedimentary records – Three types

Geologic Age

• Relative age – Key beds – Index fossils (eg. Trilobites, 530 Ma ago)

• Highly abundant

• Widespread geographic occurrence

• Absolute age – C14, K40 radioactive dating

Stratigraphic Correlations

• Rock stratigraphic units – A body of rock forming a discrete and recognizable unit of reasonable homogeneity – Basic unit is “formation” – Divisions are group, formation, member and bed

• Time stratigraphic units – All rocks deposited in a given time – “System” is the primary time-stratigraphic unit – Divisions are erathem, system, series and stage

Geologic Time Scale

• Extends from the beginning of the earth to the present

• Consists of Eons, Eras, Periods, epochs, ages and chrons

– Eon

 • Largest time interval in the earth history

• Composed of 4 intervals

 – Hadean - no records – Archean

– oldest rocks of the earth

 – Proterozoic – multicelled organisms (not preserved)

 – Phanerozoic– clues of the life

 – Era

 • Subdivisions of eons

– Palaeozoic (Old life)

 – Mesozoic (Middle life)

– Cenezoic (Recent life)

Palaeozoic Era

• Lasted for 300 Ma

• By the end, most of species disappeared or evolved

• Lot of invertebrate sea species appeared from the Cambrian, especially trilobites

• Plants grew from algae to trees through swamps

• Vertebrates evolved from the Ordovician (Age of Fish), amphibians

• Climate changed at the end of the Carboniferous turning forests into coal (Oil seams)

Mesozoic Era (Triassic, Jurassic, Cretaceous)

• The supercontinent Pangaea came to the existence

• Nonfloweringseed plants dominated (conifers)

• Giant reptiles evolved from the Permian dominated the land

• Flying and marine reptiles appeared from the Jurassic

• Mammals evolved from the Cretaceous

• By the end of the ear, all dinosaurs became a victim of mass extinction

• Flowering plants first appeared

• Pangaea split to Laurasia and Gondwanaland during the Cretaceous

Cenozoic Era Tertiary and Quaternary

• Also called Palaeogene and Neocene

• From the Palaeocene, continentals started spreading

• Woolly mammals dominated (eg. Mammoth)

• Modern fish appeared (Whales, dolphins)

• Flowering plants dominated the lands

• Primates evolved through the era

• during the Pleistocene, modern human evolved.

• Pleistocene and Holocene are known are Ice Ages

Ice Ages

• At least 11 ice ages

 • Earliest in the Proterozoic (2.7 to 2.3 Ba ago)

• Followed by another between 850 and 630 Ma ago and minor ice ages in Ordovician to Permian

 • In Pleistocene Major Ice age exited forming ice sheets in northern hemisphere (20 Ma ago) and it ended 10,000 years ago.

• Usually a glaciations lasts for 18,000 years, thus during ice ages, glaciation and non glaciation periods exists. Hence, retreated and redeveloped.