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