Geology 209 Final Chapter 10

Stratigraphy

The study of the successions of strata and bodies of rocks in the Earth’s interior, represents the key towards understanding the Earth’s geological history

Nicolaus Steno

Discovered that there were two rock sequences in the Earth’s crust separated by a surface of discontinuity, principles of sedimentary layers

Principle of layer superposition

In any sequence of undisturbed layers, the older layer is at the bottom of the succession and the younger layers occupy a progressively higher position according to their age of formation

Principle of successive layer formation

At the time of formation of a layer only fluid was above it. Accordingly, none of the above existing layers in the succession existed at that time, lack traces of soft tissue

Principle of original layer horizontality

Layers were originally horizontal but crustal movements affected their position, older strata will reflect the irregularities of the sea bottom

Principle of lateral layer continuity

Originally deposited strata extend in all directions until they terminate by thinning at the margins of the basin

Steam erosion

Results in valley formation

Mud cracks

Sedimentary structures that form frequently at the surface of the Earth, opening is always upwards and narrows downwards

Ripple marks

Sedimentary structures that form at the top of the layer where unconsolidated particles are rearranged as a result of agitation by water or wind, top of layer indicated by narrow portion of ripple mark

Burrowings

Produced by organisms that develop their life cycles buried in sediments as adaptations to avoid predators or for feeding purposes, present opening indicates top of layer

Unconformities

Breaks in sedimentation and such features

Disconformity

Unconformity between sedimentary rocks which are parallel to each other, no significant crustal movements

Angular unconformity

Unconformity between sedimentary rocks, the layering below and above the unconformity form a distinct angle, significant crustal movements

Non-conformities

Unconformity between igneous or metamorphic rocks, encompasses a long time period

Uniformitarianism

The concept that changes at the Earth’s surface are imperceptible and gradual

Actualism

The idea that past geological processes, phenomena and their effects can be understood if we study those actually happening on our planet

Catastrophism

The idea that the present day Earth was shaped through catastrophes such as volcanic eruptions and earthquakes

Stratigraphical range

The idea that certain fossils occur in certain layers

How do you classify the stratigraphical range of a fossil?

By its first appearance and extinction, once a species is extinct, it does not appear in high stratigraphical layers

Principle of inclusion

States that the inclusion within an igneous rock are older than the rock that includes them

Principle of cross-cutting relationships

States that a cross-cutting rock is younger than the cross-cut one

Xenolith

An inclusion in igneous rocks

Principle of rock-cooking

states that an igneous rock which thermally alters a layer of body of rocks is younger than them

Sills

Rock layer that cook the rocks below and above them

Hadean

The oldest eon in Earth’s history, began with the formation of our planet and has no rock record, only molten matter existed

Archean

Includes the oldest rocks on Earth, devoid of fossils

Hadean/Archean boundary

Given by the age of the oldest rocks in the stratigraphical record

Proterozoic

Stratigraphical interval with the oldest fossils on Earth, fossils were microscopic, larger fossils were found in the later part of this period

Phanerozoic

Youngest eon, visible large sized fossils can be found in many of the sedimentary rocks accumulated during this interval

Paleozoic

Oldest in the Phanerozoic eon, small resemblances between fossils in this era and the modern floras and faunas

Mesozoic

Contains fossils that present some resemblances to modern life forms

Cenozoic

Fossil debris discovered in these sediments present clear resemblances with modern life forms

Radioactivity

The natural properties of some atoms to transform spontaneously into one or two different atoms in order to achieve a more stable nucleus structure

Radioactive decay

The idea that some isotopes are stable and others are not, unstable ones can be transformed spontaneously into an isotope with a stable nucleus configuration

Parent atoms

Stable isotopes before radioactive decay

Daughter atoms

Stable atoms after radioactive decay

Half-life

The _____ of a radioactive element represents the time it takes for one half of the atoms of the original unstable parent element to decay to atoms of a new and more stable daughter element

Paelozoic/Mesozoic boundary

Given by the most severe crisis in the history of life where 90% of the species in the seas and oceans became extinct

Mesozoic/Cenozoic boundary

Given by the meteorite impact that led to the dinosaur extinction