ESCI 203 Geologic Time

All geologic time categories in order of descending length.

Eon, era, period, epoch, age

List all eons from oldest to newest.

Hadean, Archean, Proterozoic, Phanerozoic

List all eras from oldest to newest.

Paleozoic, Mesozoic, Cenozoic.

Periods in the Paleozoic era from oldest to newest

Cambrian, Ordovician, Silurian, Devonian, Carboniferous (Mississippian and Pennsylvanian), Permian

Periods in Mesozoic era from oldest to newest

Triassic, Jurassic, Cretaceous

Periods in Cenozoic era from oldest to newest

Tertiary (Paleogene, Neogene), Quaternary

Precambrian

Hadean, Archean, Proterozoic

Permian epochs (oldest-newest)

Cisuralian, Guadalupian, Lopingian

Cretaceous epochs

Early and Late

Jurassic epochs

Early, middle, late

Triassic epochs

Early, middle, late

For what is the Phanerozoic named?

It’s named for phaneros (visible) and -zoic (life); it is the geologic layer with fossil records.

What does Ma stand for?

Millions of years (from present)

What does it mean for a fossil to be dated 113 m.y.r?

The fossil is 113 million years old (from present)

Principle of Original Horizontality

Layers of sediment are always originally deposited horizontally.

Law of Superposition

In any succession of strata not disturbed or overturned, younger layers lie above older layers.

Does the principle of original horizontality still hold true?

Not necessarily; tilted strata exist. However, the principle is still analytically useful.

Principle of Lateral Continuity

Layers of rocks are laterally continuous.

What does the Principle of Lateral Continuity tell us?

Layers of rocks that are separated by erosional features but that are otherwise similar can be assumed to be originally laterally continuous.

How do we determine the end of a sedimentary basin?

Abrupt changes in rock type or thinning.

Principle of Cross-Cutting Relationships

A rock that cuts across another rock must be the younger of the two.

What forms cross-cutting relationships?

Faults or fractures, intrusion, unconformities/erosion, animal activity or plant growth (paleontological), erosion caused by water flow or impact craters (geomorphological)

Law of Included Fragments

Clasts (fragments) in a rock are older than the rock itself.

Xenolith

A piece of rock trapped in igneous rock. Infrequently found in sedimentary rocks.

Principle of Faunal Succession

Fossils go from oldest (deepest) to newest (closest to surface), and fossils with large chronological gaps between them will never be found in the same strata.

Process of xenolith/xenocryst formation

Xenoliths/crysts are pulled from magma as it travels through the crust; if they melt they lose their unique properties, and can become metamorphic if exposed to enough temperature and pressure.

Xenoliths vs. Xenocrysts

Xenoliths are rocks, xenocrysts are crystals.