Earth History GEOL 2110 Lecture 8 Fundamentals of Stratigraphy II Biostratigraphy, Time Markers, and Unconformities
Dec 23, 2015
Earth History GEOL 2110
Lecture 8Fundamentals of Stratigraphy II
Biostratigraphy, Time Markers, and Unconformities
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)
Biostratigraphy
Index Fossils and Sedimentary Facies
Poor Index FossilBrachiopods – Slow evolving
Sand burrowers
Good Index FossilCephlapods – Rapid evolving
Floaters
Index Fossils and Sedimentary Facies
Facies-dependent
Facies- independent
Best Ever! Index FossilsConodontsEel-like creatures with hard “teeth and jaw” parts; Existed Late Cambrian (495 Ma) to Late Triassic (200 Ma)
GraptolitesPlanktonic colonial zooids that floated
in the oceans (“ocean beehives”)
Existed from Ordovician (490
Ma) to Devonian (419 Ma)
Biozones – Formations of Biostratigraphy
Regional Time MarkersVolcanic Ash Eruptions
Long Valley Caldera 700,000 yr
Mt Mazama Eruption 6,500 yr
Global Time MarkersMeteor Impacts
K-T impact site
K-T Boundary
mudstone-impact layer (Ir anomaly)
The 1.85 Ga Sudbury ImpactThe First Major Extinction Event ??
Iron FormationBreccia
Accretionary Lapilli
CALCULATED ARRIVAL TIMES FOR EFFECTS AT GUNFLINT LAKE(480 miles from Sudbury Impact)
www.lpl.arizona.edu/impacteffects
And you thought you were having a bad day…
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?)
Meteorite.org, Pangea International, Inc
6) Post-impact environmental changes (duration and magnitude? Global?)
Global Time MarkersMagnetic Reversals
ODP site 1149
Global Time MarkersClimate Change
A typical deep sea sedimentcore record of ∂18O inforaminifera 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
Angular Unconformity
Disconformity
Nonconformity Grand Canyon Stratigraphy
Creating Unconformities
Unconformities related to Regression - Transgression
Disconformity
Paleozoic Formations of the Upper Midwest
The Jordan SandstoneDisconformity
Missing Fossils
Oneota Dolomite
Jordan Sandstone Ordovician
Cambrian
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