Chapter 5 Rocks, Fossils, and Time

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Chapter 5Rocks, Fossils, and Time -

• Earth events are recorded in the geologic record.
  Page 72 photo shows 14 million years of geologic
  history. The horizontal layers of the Grand
  Canyon show approx. 300 m.y. of history,
  including a 100 million year gap.
• Example a 7,000 yr. history of El Niño events
  has been reconstructed from core samples from a
  lake in the Galapagos Islands.

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• Stratigraphy the study of layered rocks.
  Within strata (or individual stratum) we can
  study the thickness nature of sediments,
  presence and types of organics, sample for
  certain isotopes. This proxy data can be used to
  determine past environmental conditions.
• Within vertical successions of strata, individual
  layers are separated by bedding planes. The
  existence of the bedding plane suggests a minor,
  short-term environmental change.

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• The presence of bedding planes, fluctu-ations
  between sediment types, changes within given beds
  offer suggestions of environmental changes.

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• Vertical strata Principle of Superposition,
  Prin-ciples of Inclusions, Cross-cutting
  Relation-ships, Unconformities useful in
  determining relative ages (see pp. 74-75).

C
Sedimentary layer
B
A
Sedimentary layer
Lava Flow - Sill -
A is baked, C is not.
A C will be baked.
Upper surface of B may show effects of
weathering.
B may hold inclusions of A and C.
C may hold inclusions of B.
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• Unconformities interruptions in the
  geologic column.
• Angular unconformity strata below are folded or
  tilted
• Disconformity layered rocks above and below are
  parallel (undeformed), and a noticeable eroded
  surface is present.
• Paraconformity layered rocks above below,
  with a subtle erosion surface.
• Nonconformity layered rocks deposited over
  igneous or metamorphic rocks.

Paraconformities and Disconformities can be
local, regional, or continental. Shallow water
sediments are more susceptible to sea level vs.
land changes.
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• Lateral changes in layered rocks.
• Principle of Lateral Continuity Deposition
  occurs across the basin (topographic low), but
  environments change due to varying distances from
  the shore, water depth, currents, etc.. Each
  environment yields a different sedimentary
  facies, i.e., a sediment with distinctive
  characteristics related to the depositional
  environment.

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Shoreline to Continental Shelf facies
distribution - Fig. 5.7
Decreasing Energy
As sea level rises or falls facies boundaries
migrate. Sea level rises landward migration.
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• Marine Transgressions and Regressions
• World-wide Sea Level affected by size of Polar
  Ice Sheets Oceanic Rift (Divergent Zone)
  activity.
• Local rises and falls of the land affect the
  geologic record also.
• Tapeats Sandstone records the flooding of the
  continent in Early/Mid Cambrian time. Sand
  facies migrated inland Bliss SS El Paso
  Late Cambrian/Early Ord.

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• In the Grand Canyon, as the Transgression (Sea
  Level Rise) continued, deposition of the Bright
  Angel Shale and then the Muav Limestone followed
  (Figure 5.8), as the water progressively
  deepened. Figure 5.9 (left column) shows
  Transgressive Sequence
• When conditions change and a Regression (Sea
  Level Decline) occurs, the facies boundaries
  migrate seaward and exposed sediments are subject
  to erosion. Figure 5.9 (right column) shows
  Regressive Sequence.

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• When a significant enough change in sea level
  facies shift occurs, different litho-logies
  stack up.
• When sea level is stable for a long period of
  time, continued sedimentation causes a
  progradation of facies into the basin, similar to
  regression.
• Lateral changes are generally gradual, e.g., the
  transition between a silt and clay facies would
  progress from a clayey silt to a silty clay. The
  transition from clay to limestone would be a
  limey shale to a clayey limestone.

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At the top of the Muav Limestone, there is a
significant disconformity of approx. 100 m.y..
The entire Ordovician and Silurian Periods and
most of the Devonian Period are missing.
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• Fossils and Fossilization
• Remains or traces of past life. Fossils useful
  for determination of relative ages of rocks
  ash, environments of deposition, and evidence of
  evolution.
• Types of Fossils Body and Trace
• Body Fossils Remnants of organic parts (hard or
  soft) or sediment impressions of hard parts.
• Trace Fossils Evidence of life activities, even
  if no tangible evidence of fossil exists.

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Types of Fossilization Body Fossils Unaltered -
composition structure preserved Freezing
Mummification/Dessication Preservation in amber
or tar Altered composition and/or structure
changed Permineralization - Addition of minerals
to pores cavities Recrystallization change in
crystal structure Replacement one compound
replaces another
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• Altered (continued)
• Carbonization preserved plant fossils in shale
• Molds and Casts shell left impression in
  sediment, shell material dissolved, void refilled
  with mud or other minerals.
• Trace fossils ichnofossils - preservation of
  life activity footprints, trails, burrows,
  droppings (copralites), nests.
• Univ. of Calif. Berkeley Museum of Paleontology

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Having a durable, hard exoskeleton
(invertebrates) provides best opportunity for
preservation. Vertebrate skeletons often become
disarticulated after death. Soft bodied
organism and soft parts are generally consumed,
unless rapidly buried. Environment may affect
possibility of preservation Ex cave sediments
not often preserved bat skeletons are fragile,
metamorphism, dissolution by groundwater, etc.
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• Fossils and Time estimates
• William Smith (1769 1839) independently
  observed superposition (but didnt name it) and
  reasoned that the same would occur with fossils,
  i.e., noticed succession of species and
  relationships between strata and fossils, i.e.,
  certain fossils were restricted to certain
  strata.
• Rock types (from similar environments) may be
  repeated through time, but particular fossils are
  not.

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Example of early efforts at Relative Geologic
Time Scale (pp. 86-87). Adam Sedgewick (1835)
identified Cambrian System in northern Wales,
little fossil identification. Roderick Murchison
(1835) identified Silurian System in southern
Wales, better identification of fossils. Overlap
of fossils in-between (stratigraphically and
geographically) caused a dispute. Settled by
Charles Lapworth identification of Ordovician
System (1879).