Tracking changes through time:Time Sequences and DatingLecture 7Chapter 6
Objectives:1.Be able to apply basic stratigraphic principlesin order to interpret and reconstruct a seriesof events in the geologic record.2.Understand how fossils are used to tell timein the geologic record.3.Understand method of absolute dating, theiruses and limitations
Sediments accumulate in discrete episodes each of whichforms a tabular layer known as astratum(pluralized:strata).Stratificationandbeddingdescribes the arrangement ofsedimentary rocks in discrete layers.Stratigraphyis the study of stratified rocks.Correlationis the procedure of demonstratingcorrespondence between strata (matching of units).Stratigraphy
Lithostratigraphyis the division of rock strata on the basis oflithology: physical and chemical characteristics of the rock.The hierarchy of lithostratigraphy is as follows:BedMemberFormationGroupRock Units
Stratigraphic units•Bed: lithologically distinct layer•Member: lithologically distinct part of aformation•Formation: primary units of a sequence,should be distinct from other formations•Group: two or more formations that sharelithologic characteristics•Supergroup: two or more associated groups
Member: Subdivisions of a formation are called members;e.g., Bradley Creek Member of the Nolichucky Formation.The process of naming rock units is very precise, and isgoverned by the North American and International Codesfor Stratigraphic Nomenclature.Bradley Creek MemberNolichucky Formation
Formation:This is the basic unit of lithostratigraphy,and it has a two-part name.The first is named after a geographic feature, or after atown near the type locality.The second part is named after the lithology of rock, but ifthe rock has variable lithology, it is simply called a"formation".
Examples are the Copper Ridge Dolomite andthe Nolichucky Formation (limestone and shale).Both names begin with upper case.Nolichucky Formation
Group:This simply refers to more than onelithologically related formations.For example, the Conasauga Group in southwest Virginia ismade up of the Maynardville Limestone, Pumpkin ValleyShale, Shady Dolomite, the Nolichucky, Maryville,Rogersville, Rome and Rutledge Formations.Maryville FormationNolichucky FormationConasauga Group (part)
Danish physician Nicolaus Steno formulatedprinciples for studying stratified rocks during theseventeenth century.1.Principle of Superposition2.Principle of Original Horizontality3.Principle of Original Lateral Continuity4.Principle of Cross Cutting Relationships5.Principle of ComponentsNicolaus StenoTime Sequences in rock and relative dating
Principle of Superposition.In an undisturbed sequence of rocks the older strata lie atthe bottom and successively higher strata are progressivelyyounger.
Principle of Original Horizontality.When originally formed almost all strata are initially morehorizontal than vertical.Principle of Original Lateral Continuity.When originally formed, strata were laterally continuousunless they terminated against another solid surface.