Fossils and the Rock Cycle
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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 Sedimentation
Uplift
Erosion
Erosion by wind
Erosion by water
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
Fossils
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 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 Record
• All of the fossils that
have existed throughout
life’s history, whether
they have been found or
not.
Tully Monster; 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, nonrepeating 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
to matching sedimentary strata in widely separated
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
This was the very first (1789) example of an unconformity
Hutton's
unconformity
Ordovician/Silurian unconformity in New York state
Precambrian/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 Era
 Very
recent glacial sediments above ancient
bedrock
 Due
to long period of erosion of uplifted
sediments
 Big
gap in fossil record