Earth History GEOL 2110

Earth History GEOL 2110Lecture 7Fundamentals of Stratigraphy IGeologic Time Scale, Stratigraphic Units, Sedimentary FaciesMajor ConceptsIn the early 1800s, the geologic rock record and time scale was deciphered from careful mapping and observations of fossil assemblages throughout EuropeIn studying modern sedimentary environments it became obvious that different types of sediment were deposited in transitional geological enviornments simultaneously this gave rise to the idea of sedimentary faciesAs a correlary to the recognition of sedimentary facies, it is clear that rock units (formations) are not perfectly time equivalent along their lateral extentAlthough time is evident in a vertical succession of strata, one must use fossil correlation to establish time equivalence laterally

Early Mapping and Correlation of Strata

Armed with the principles of superposition, original horizontality, cross cutting relationships, correlation of faunal assemblages, and index fossils, the relative ages of rocks throughout Europe were mapped out in the early 1800sRelative Geologic Time

The Modern (relative) Geologic Time Scale

The study of layered sequences of sedimentary and/or volcanic rock that conform to the principles of superpositionStratigraphy

A geologic rock unit is a naturally occurring body of rock or rock material distinguished from adjoining bodies of rock on the basis of some stated property or properties. Commonly used properties include composition, texture, included fossils, magnetic signature, radioactivity, seismic velocity, and age.

A lithostratigraphic unit is a geologic rock unit that conform to the principles of superposition.

Biostratigraphy is the branch of stratigraphy that deals with the distribution of fossils in the stratigraphic record and the classification of bodies of rock or rock material into biostratigraphic units based on their contained fossils.6The Birth of StratigraphyGreat BritainRock Units vs. Time Units

From A. HallamGreat Geological ControversiesOxford Press, 1989

7Units based on content or physical limits

Lithostratigraphic

Lithodemic

Magnetopolarity

Biostratigraphic

Pedostratigraphic

AllostratigraphicCategories of Stratigraphic Units

A lithodemic unit is a defined body of predominantly intrusive, highly deformed, and/or highly metamorphosed rock, distinguished and delimited on the basis of rock characteristics. In contrast to lithostratigraphic units, a lithodemic unit generally does not conform to the Law of Superposition.8

Lithostratigraphic

Lithodemic

Magnetopolarity -alternating normal and reversed polarityBiostratigraphic

Pedostratigraphic

AllostratigraphicA magnetostratigraphic unit is a body of rock unified by specified remanent-magnetic properties and is distinct from underlying and overlying magnetostratigraphic units having different magnetic properties.Categories of Stratigraphic Units9

A biostratigraphic unit is a body of rock that is defined or characterized by its fossil content.Lithostratigraphic

Lithodemic

Magnetopolarity

Biostratigraphic based on the principles of faunal succession and superposition

Pedostratigraphic

AllostratigraphicCategories of Stratigraphic Units10

Lithostratigraphic

Lithodemic

Magnetopolarity

Biostratigraphic

Pedostratigraphic Soil horizons (regolith)

AllostratigraphicA pedostratigraphic unit is a body of rock that consists of one or more pedologic horizonsCategories of Stratigraphic Units11

Lithostratigraphic

Lithodemic

Magnetopolarity

Biostratigraphic

Pedostratigraphic

AllostratigraphicAn allostratigraphic unit is a mappable body of rock that is defined and identified on the basis of its bounding discontinuitiesCategories of Stratigraphic Units12Heirarchy of Lithostratigraphic UnitsA lithostratigraphic unit is a defined body of sedimentary, extrusive igneous, metasedimentary, or metavolcanic strata which is distinguished and delimited on the basis of lithology and stratigraphic position. It generally conforms to the Law of Superposition and commonly is stratified and tabular in form.Supergroup- a formal assemblage of related or superposed groups, or of groups and formations. Such units have proved useful in regional and provincial syntheses. Supergroups should be named only where their recognition serves a clear purpose. Group - the lithostratigraphic unit next higher in rank to formation; a group may consist entirely of named formations, or alternatively, need not be composed entirely of named formations. Formation - the fundamental unit in lithostratigraphic classification. A formation is a body of rock identified by lithic characteristics and stratigraphic position; it is prevailingly but not necessarily tabular and is mappable at the Earth's surface or traceable in the subsurface.Member - formal lithostratigraphic unit next in rank below a formation and is always a part of some formation. It is recognized as a named entity within a formation because it possesses characteristics distinguishing it from adjacent parts of the formation. A formation need not be divided into members unless a useful purpose is served by doing so. Some formations may be divided completely into members; others may have only certain parts designated as members; still others may have no members.Bed(s) - the smallest formal lithostratigraphic unit of sedimentary rocks.Flow - the smallest formal lithostratigraphic unit of volcanic flow rocks. A flow is a discrete, extrusive, volcanic body distinguishable by texture, composition, order of superposition, paleomagnetism, or other objective criteria.From NASC, 200513Categories of Geologic Time UnitsA chronostratigraphic unit is a body of rock established to serve as the material reference for all rocks formed during the same span of time. Example: Devonian SystemA geochronologic unit is a division of time distinguished on the basis of the rock and fossil record preserved in a chrono-stratigraphic unit. Example: Devonian Period.

14Subdivisions of Geologic Units

From NASC, 2005Fundamental Units15Boundaries of lithostratigraphic units are placed at positions of lithologic change. Boundaries are placed at distinct contacts or may be selected at some arbitrary level within zones of gradation. Both vertical and lateral boundaries are based on the lithic criteria that provide the greatest unity and utility. i.e. lithostratigraphic boundaries are rarely time equivalent.Boundaries between Lithostratigraphic Units

From NASC, 200516Depositional Environments and Sedimentary Facies

Depositional Environments and Sedimentary Facies

Depositional Environments and Sedimentary Facies

Transgression/Regression20

Walthers LawThe vertical progression of facies will be the same as the corresponding lateral facies changes21

Tran

Transgression/Regression in the Midcontinent during the Early Paleozoic22

Advanced TransgressionRegression23

Paleozoic Formations of the Upper Midwest

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Earth History GEOL 2110Lecture 8Fundamentals of Stratigraphy IIBiostratigraphy, 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)Biostratigraphy26

Index Fossils and Sedimentary FaciesPoor Index FossilBrachiopods Slow evolving Sand burrowersGood Index FossilCephlapods Rapid evolving FloatersIndex Fossils and Sedimentary Facies

Facies-dependentFacies- independentBest Ever! Index Fossils

ConodontsEel-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)29

Biozones Formations of BiostratigraphyRegional Time MarkersVolcanic Ash Eruptions

Long Valley Caldera 700,000 yrMt Mazama Eruption 6,500 yr31Global Time MarkersMeteor Impacts

K-T impact site

K-T Boundarymudstone-impact layer (Ir anomaly)32

The 1.85 Ga Sudbury ImpactThe First Major Extinction Event ??

Iron FormationBrecciaAccretionary Lapilli33CALCULATED 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 secondsFireball (thermal radiation=3rd degree burns; 50 minutes) 2) ~2-3 minutesEarthquake (magnitude >10 at Sudbury, 1000X Haiti) (New data estimates magnitude 13 at Chicxulub) 3) ~5-10 minutesAirborne ejecta arrives (~1-3 m thick , fragments < 1 cm) 4) ~40 minutesAir blast (compression wave, wind speeds >1400 mph) 5) ~1-2 hoursTsunami (the first of several?)Meteorite.org, Pangea International, Inc 6) Post-impact environmental changes (duration and magnitude? Global?)34Chile EQ 2/2010=8.8, near largest in world @9.5 in 1960Website allows entry of known observationssize of crater, impacter, wet-dry, distance---then calculates effects.Simms, 2003 (Geology) references magn 13 for Chixculub; could cause soft-sed def up to 2800 km awayGlobal 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

Utah37Types of Unconformities

Nonconformity Sedimentary/volcanic strata resting on intrusive or metamorphic rocksAngular Unconformity Relatively flat-lying strata resting on steeply-dipping strataDisconformity Strata resting conformably on other strata across a significant time gap

Angular UnconformityDisconformityNonconformityGrand Canyon StratigraphyCreating Unconformities

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Unconformities related to Regression - TransgressionDisconformity40

Paleozoic Formations of the Upper Midwest

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The Jordan SandstoneDisconformityMissing Fossils42

Gaps in the Minnesota Timescale43Global Unconformities

OrdovicianGlobal Unconformities at the Edges of the Continents

Reading Time in Strata

Next LectureAbsolute Dating of the Earth

Quiz Chapters 4 & 5