Earth History
GEOL 2110
Lecture 8
Fundamentals of Stratigraphy II
Biostratigraphy, Time Markers, and
Unconformities
Biostratigraphy
A biostratigraphic unit is a
body of rock that is defined or
characterized by its fossil
content.
A fossil zone (or biozone) is an
interval of strata characterized
by a particular index fossil.
The best index fossils are
those that evolve rapidly and
were not sensitive to the
sedimentary environment
(flyers and floaters)
Index Fossils and Sedimentary Facies
Good Index Fossil
Cephlapods – Rapid evolving
Floaters
Poor Index Fossil
Brachiopods – Slow evolving
Sand burrowers
Index Fossils and Sedimentary Facies
Faciesdependent
Faciesindependent
Best Ever! Index Fossils
Conodonts
Eel-like creatures with hard “teeth and
jaw” parts; Existed Late Cambrian (495
Ma) to Late Triassic (200 Ma)
Graptolites
Planktonic colonial
zooids that floated
in the oceans
(“ocean beehives”)
Existed from
Ordovician (490
Ma) to Devonian
(419 Ma)
Biozones – Formations of Biostratigraphy
Regional 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
Global Time Markers
Magnetic Reversals
ODP site 1149
Global Time Markers
Climate Change
A typical deep sea sediment
core record of ∂18O in
foraminifera shells
Unconformities
Gaps in the Geological Record
Siccar Point, Scotland
Utah
Types of Unconformities
Nonconformity – Sedimentary/volcanic strata resting on intrusive or metamorphic rocks
Angular Unconformity – Relatively flat-lying strata resting on steeply-dipping strata
Disconformity – Strata resting conformably on other strata across a significant time gap
Creating
Unconformities
Nonconformity
Angular Unconformity
Grand Canyon
Stratigraphy
Unconformities related to Regression - Transgression
Disconformity
Paleozoic Formations of the Upper Midwest
The Jordan Sandstone
Disconformity
Oneota Dolomite
Jordan Sandstone
Ordovician
Cambrian
Missing
Fossils
Gaps in the Minnesota Timescale
Global Unconformities
Ordovician
Global Unconformities at the
Edges of the Continents
Reading Time in Strata
Next Lecture
Absolute Dating of the Earth
Quiz – Chapters 4 & 5