Fossils and the Rock Fossils and the Rock CycleCycle

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The rock cycle explains how one type of rock can be transformed into another in nature.

The Geologic Cycle

3 key events: deposition, uplift, erosion

Deposition or SedimentationDeposition or Sedimentation

UpliftUplift

ErosionErosion

Erosion by waterErosion by water

Erosion by windErosion by wind

Geologic Cycle

Cycle repeats over and over through geologic time

Forms Strata: layers of rock

The rock cycle explains how one type of rock can be transformed into another in nature.

Index Fossils

Use fossil layers to date rocks: index fossils

FossilsFossils

What is a Fossil?

Any evidence of past life, including remains, traces, imprints as well as life history artifacts. Examples of artifacts

include fossilized bird's nests, bee hives, etc.

Mastodon at Wheaton CollegeMastodon at Wheaton College

How Fossils are Formed

About 67 million years ago -- A Tyrannosaurus rex died and its body was quickly covered by riverbed sand and mud.

Over time, pressure and some remineralization turned its bones to fossils.

The Fossil RecordThe Fossil Record

• All of the fossils that All of the fossils that have existed have existed throughout life’s throughout life’s history, whether they history, whether they have been found or have been found or not.not.

Tully MonsterTully Monster; IL State Fossil; IL State Fossil

Fossils

Usually found in sedimentary rock Sometimes in metamorphic rock

These are often distorted, hard to interpret

Upper layers - younger Deeper layers – older

Principle of Fossil Succession

There is a unique, non-repeating pattern (history) of fossils through stratigraphic time.

All rocks containing fossils of the same species were deposited during the duration of that species on Earth.

Correlation by fossils. Certain index fossils are keys . Certain index fossils are keys to matching sedimentary strata in widely separated to matching sedimentary strata in widely separated outcrops outcrops

Used in conjunction with radioisotope techniques to age rock layers

Unconformities Unconformity: contact between layers of rock

that should not touch; represents a gap in the fossil/geological record

Layers of sandstone over pink granite

Hutton's unconformity at Siccar Point Berwickshire, Scotland Hutton's unconformity at Siccar Point Berwickshire, Scotland This was the very first (1789) example of an unconformityThis was the very first (1789) example of an unconformity

Hutton's Hutton's unconformityunconformity

Ordovician/Silurian unconformity in New York stateOrdovician/Silurian unconformity in New York state

Precambrian/Cambrian unconformity near St. Louis – 1 billion years is missingPrecambrian/Cambrian unconformity near St. Louis – 1 billion years is missing

Unconformities

Disconformity: gap in geological record Caused by long period

of erosion between episodes of sedimentation

Or by faulting and uplift Fossils in the two layers

may be very different

Unconformity of Cambrian sandstone on Precambrian gneiss. North of Kingston, Ontario, Canada.

Unconformities

Disconformity: missing strata between layers that are parallel to each other. Often difficult to detect in the field. Must use fossils or other correlation methods. Angular unconformity: younger strata overlie an erosion

surface on tilted or folded rocks. Implies a specific sequence of events.

Nonconformity: contact with overlying sedimentary rocks on top of an erosion surface of plutonic or metamorphic rocks. Implies long-lived erosion prior to burial and re-deposition.

Angular Unconformity

Nonconformity

Angular

Nonconformity

Angular Unconformity

Nonconformity

Great Lakes Lost Interval

Mesozoic EraVery recent glacial sediments above ancient

bedrock

Due to long period of erosion of uplifted sediments

Big gap in fossil record