THE LANGUAGE OF THE
EARTH – PART VII
Geochronology
Methods for Determining the
Relative and Absolute Age of
Rocks
Relative Age Dating - Superposition
Up isn’t always
Younging
Up Indicators
Graded Bedding
Pillowed Lavas
Amygdaloidal Zones in Lavas
Ripple Marks
Cross-bedding
Principles of Faunal
Succession
and Faunal
Assemblage
Index Fossils
- Widespread distribution
- Restricted, known time of
existence
- Common to many
environments
Principle of Cross-cutting Relationships
Chill
Zones
Vesicular/
Amygdaloid
Zone
Global Time Markers
Volcanic Ash Eruptions
Long Valley
Caldera
700,000 yr
Mt Mazama
Eruption
6,500 yr
Global Time Markers
Meteor Impacts
K-T Boundary
mudstone-impact layer (Ir anomaly)
K-T impact site
The 1.85 Ga Sudbury Impact
The First Major Extinction Event ??
Iron Formation
Breccia
Accretionary
Lapilli
And you thought you were
having a bad day…
CALCULATED ARRIVAL TIMES
FOR EFFECTS AT GUNFLINT LAKE
(480 miles from Sudbury Impact)
Meteorite.org, Pangea International, Inc
1) ~13 seconds—Fireball (thermal radiation=3rd degree burns; 50 minutes)
2) ~2-3 minutes—Earthquake (magnitude >10 at Sudbury, 1000X Haiti)
(New data estimates magnitude 13 at Chicxulub)
3) ~5-10 minutes—Airborne ejecta arrives (~1-3 m thick , fragments < 1 cm)
4) ~40 minutes—Air blast (compression wave, wind speeds >1400 mph)
5) ~1-2 hours—Tsunami (the first of several?)
6) Post-impact environmental changes (duration and magnitude? Global?)
www.lpl.arizona.edu/impacteffects
Gaps in the Geological Record
Unconformities
Siccar Point, Scotland
“The mind seemed to grow giddy
looking so far into the abyss of
time”
John Playfair – Illustrations of
Huttonian Theory (1809)
Creating
Unconformities
Nonconformity
Angular Unconformity
Grand Canyon
Stratigraphy
Sedimentary Facies – Time Trangressive Units
Paleozoic Formations of the Upper Midwest
The Jordan Sandstone
Disconformity
Missing Fossils
Gaps in the Minnesota Timescale
Absolute Age Determinations
Radiometric Dating
Radioactive Isotopes
Radioactive Decay
Radioactive Decay
Half-life
Zircon
ZrSiO4
U substitutes
for Zr, but Pb
does not.
Rb-Sr Dating of a Granite
 87Sr; 86Sr is stable
Rb – chemically substitutes for K
Sr – chemically substitutes for Ca
87Rb
Isochron
t=2
t=1
87Sr/86Sr
Metamorphism
at t=1
87Sr/86Sr°(m)
t=0
87Sr/86Sr°
87Rb/86Sr
Ca-plagioclase
Granite whole rock
K-feldspar
Biotite (K-rich)
THE LANGUAGE OF THE
EARTH – PART VIII
Geologic Maps
1918
Documenting the Story
2001
Base – Topographic Map
clustering
of contour
lines
indicates
steep slopes
contour
interval
–
10’elevat
ion
contour lines in valleys “V” upstream contour lines encircle h
Features of Topographic Maps
Meadow
Forest
Township/Range/Se
Township = 36 Sec
Section = 1 sq. mile
Railroad
Benchmark
1:100,000
1:24,000
The Map
Outcrops = the “facts”
The Map – Structural Measurements
more “facts”
Planar Features - Strike and Dip
Sedimentary Bedding
Lava flow contacts
Intrusive contacts
Foliation (metamorphic and igneous)
Cleavage
Gneissic banding
Layering (modal, textural, phase)
Laminations
Dikes
Veins
Joints
Shear zones
Fault planes...
Linear Features – Trend and Plunge
Mineral lineations
Cleavage-Bedding intersections
Slickensides
Fold axes
Glacial striations...
The Map – Map Units
Definition: includes rocks or unconsolidated materials at the
earth’s surface that generally share a common mode of origin
and age. Rarely composed of a single rock type.
“Lumping & Splitting” – the iterative process of deciding what
geologic attributes and variables to include in a particular map
unit
The Map - Geologic Lines
Contacts, Faults, Fold axes, Structural fabric, Geophysical lin
Gradations in
Confidence:
• Certain
• Approximately located
• Inferred
• Speculative
• Concealed
Jirsa and Boerboom, 2003
Geologic Cross Sections
Projections into the 3rd Dimension
Correlation Diagrams
showing relative ages and genetic relationships of map units
TIME
youngest
oldest
Description of Map Units
Written for other geologists intimately familiar with the language; Seek out the basic information
GEOPUZZLE