Origins: Geology

Introduction

Job 38: 4-5 “Where were you when I laid the earth’s foundation? … Who marked off its dimensions? … Who stretched a measuring line across it?”

• Beautiful complexity – Accepted and gratefully acknowledged

• Awed by the order & purpose revealed within the complexity

• Earth’s past – the story preserved in the rocks themselves

• Process and Pattern, being and becoming.

• Deep time

• Large scale (though minuscule to the Universe)

Outline

• Composition

• Time Scale and salient events

• Cycles (Processes)

• How do we know?

• Pattern (Stratigraphy & Fossils)

• Age

• “Bosch” hidden & revealed past

• Climate reconstruction

Earth’s interior composition

Geological Time Scale(International Stratigraphic Chart. International Commission on Stratigraphy.)

major units of geological time• 1 Ma = 1 Million years (a) ago

• 1 Ga = 1 Billion a ago (1 000 Ma)

• Hadean eon – 4.6 to 4.0 Ga, till end of Late Heavy Bombardment• Chemical differentiation

• Core (Ni-Fe alloy)

• Mantle (Mg, Fe, Si, O)

• Crust (+Al, Ca, Hydrated minerals, Na, K)

• Moon and tilt of axis (seasons)

• Micro-continents, then Oceans (or vice versa)

• Atmosphere (Water, N and C dioxide)

• Plate tectonics begins

• Oldest remnant rock 4.1 Ga

• Aquatic early life

• Archean eon till 2.5 Ga• Oldest remnant rock in SA formed 3.6 Ga

• Photosynthesis => Oxygen initiation

Geological Time ScaleArchean eon till 2.5 Ga

• Oldest remnant rock in SA formed 3.6 Ga

• Photosynthesis => Oxygen initiation 3.2 Ga

• Micro-continents (cratons) accrete to form continents 3.0 Ga

Proterozoic eon till 541 Ma

• Oxygen boost at 2.3 Ga

• Multicellular life (Precambrian) 650 Ma

• Malmesbury Group forms on continental shelf 560—555 Ma

• Two “Snowball Earth” freezes

Geological Time ScalePaleozoic era (541-252 Ma)

• Cambrian explosion c. 530 Ma

• Table Mtn Group sedimentation starts 510 Ma

• Terrestrial plants, Insects (c. 400 Ma)

• Vertebrate terrestrial animals (380 Ma)

• Swamp Forests become Coal deposits (350 Ma)

• Pangaea supercontinent complete (280 Ma)

• Permian extinction ends the era

• (4% of species survive)

Geological Time ScaleMesozoic era 252-66 Ma

• Mammals and dinosaurs

• Gondwana break-up (130 Ma)

• First Angiosperms (120 Ma)

• Angiosperms proliferate (69 Ma)

• Cretaceous extinction ends the era (30% survive)

Cenozoic era 66 Ma till present

• India converges on Asia and starts the Himalayan Orogeny (56 Ma)

• Humanity (c. 5 Ma to present)

• Hominids

• Anatomically modern humans

• Us – “Anthropocene”

• Climate change

• Sixth extinction

Plate Tectonics and Recycling

The Rock Cycle of the Lithosphere

A diagram of the rock cycle.

Legend:

1 = magma

2 = crystallization (freezing of rock)

3 = igneous rock

4 = erosion

5 = sedimentation

6 = sediments & sedimentary rock

7 = tectonic burial & metamorphism

8 = metamorphic rock

9 = melting

Importance of biogenesis, a special case of

sedimentation, and unique to earth.

Other Cycles: Carbon & Silicon

Importance for regulation of Earth environment

But how do we know this?

• Observation

• Curiosity

• Scientific method• Descriptive documentation

• Seismology

• Magnetic phenomena

• Minerals

• Radio-active decay in minerals (phase related)

• Many more diverse sources of corroborative evidence

Stratigraphy

• Catholic priest Nicholas Steno (1669)• Law of superposition • Principles:

• original horizontality• Lateral continuity - Practical application

• William Smith (turn of the 19th century)• "Father of English geology“• Significance of strata• Importance of fossil markers (correlating

strata)• First geologic map of England

• Biostratigraphy followed by Chronostratigraphy

Fossils as markers to group rocks of similar origin

ZandrivierBrachiopod fossils (Seweweekspoort)

• SA has an exceptionally rich brachiopod diversity.

• They indicate a relative rise in sea level during deposition of the Bokkeveld Group.

Relative Age Estimation

Radiometric dating

Local geological past (after Compton 2004)

Possible tectonic setting in the Proterozoic during deposition of the Malmesbury group (560-555 Ma).

• Subduction

• Accretion of ocean floor and continental shelf sediments

• Vulcanism

Early Paleozoic Assembly of Gondwana (550-515 Ma):Granitic magma intruded into the Malmesburyrocks as the ancient Adamastor ocean closed up and the South American and Antarctic continents collided with Africa. The collision resulted in uplift and deformation of the Malmesbury sediments. Erosion soon after exposed the granite at the surface.

Geological Time ScaleArchean eon till 2.5 Ga

• Oldest remnant rock in SA formed 3.6 Ga

• Photosynthesis => Oxygen initiation 3.2 Ga

• Micro-continents (cratons) accrete to form continents 3.0 Ga

Proterozoic eon till 541 Ma

• Oxygen boost at 2.3 Ga

• Multicellular life (Precambrian) 650 Ma

• Fragments of Rodinia begin reassembling in a new configuration (600 Ma)

• Adamastor Ocean between Atlantica and Ur closure progresses in a southerly direction

• Malmesbury Group forms on continental shelf 560—555 Ma

• Two “Snowball Earth” freezes

Erosion smoothed the surface of granite-intruded Malmesbury rocks.

Rifting (c. 510 Ma) caused subsidence and deposition of the Cape Supergroup on the flat eroded

surface (new continental shelf).

Alternating deposition of sandstone and shale reflects changes in relative sea level along an overall

subsiding, passive continental margin. (after Compton 2004)

Paleozoic era – Cape Supergroup deposition

Geological Time ScalePaleozoic era (541-252 Ma)

• Cape Granite intrusion (540 Ma)

• Erosion cycle

• Cambrian explosion c. 530 Ma

• Table Mtn Group sedimentation starts 510 Ma

• Gondwana complete @ S Pole (500 Ma)

• Agulhas sea rifting (450 Ma)

• Terrestrial plants, Insects (c. 400 Ma)

• Vertebrate terrestrial animals (380 Ma)

• Swamp Forests become Coal deposits (350 Ma)

• Cape Supergroup complete (340 Ma)

• Southern subduction Folding starts (310 Ma)

• Pangaea supercontinent complete(280 Ma)

• Permian extinction ends the era

• (4% of species survive)

Continents collide to form Pangaea. Volcanic ash deposits in the Karoo basin indicate

the presence of active subduction zones to the west and south as the South

American and Antarctic plates uplift and crumple the Cape Supergroup (310-200

Ma) and its underlying strata into the Cape Fold Belt Mountains. (after Compton 2004)

Paleozoic – Mesozoic era: Cape Fold Belt

Possible evolution of the Cape Town landscape: The nearly horizontal Table Mountain

sandstones represent the trough and the steeply dipping sandstones of the same

formations of the Hottentots Holland Mountains to the east the limb of a large fold that has

since eroded away to expose the underlying shale. Thus the modern landscape may

represent an inverted version of an earlier landscape (dashed lines) characterised by a

large mountain where the Cape Flats are now. (after Compton 2004)

Geological Time ScaleMesozoic era 252-66 Ma

• Cape Fold Belt complete (200 Ma)

• Mammals, then dinosaurs

• Dolerite feeding Karoo flood basalts(130 Ma)

• Gondwana break-up (130 Ma)

• Uplift and erosion start afresh

• First Angiosperms (120 Ma)

• Angiosperms proliferate (69 Ma)

• Cretaceous extinction ends the era (30% survive)

Cenozoic era 66 Ma till present

• Erosion continues to remove much of the Cape Supergroup

• Cooling and Aridification, Sandveld

• Phosphate deposits near Langebaan (5 Ma)

• Fossil-rich (West Coast Fossil Park)

• Short-neck giraffe, bear, sabre-tooth cats

The Cape Peninsula shoreline in the case of

(left) a 25m higher sea level 5 and 1.5 million years ago, and

(right) a 125m lower sea level at the time of maximum ice build-up

during cold periods – the most recent being 20 000 years ago (after

John S. Compton 2004)

Sea level fluctuation (Pliocene to Pleistocene)

Past climate reconstructions –Ice core analysis

• Volcanic ash layers

• Gas content• Isotope analysis

• CO2 level

• Methane level

• Temperature

• Insolation