Introduction to stratigraphyEstablishing relationships between rocks

Formation - a mappable group of rocks with characteristics that enable you distinguish it from units above and belowFormations can be subdivided into MembersFormations can be grouped into GroupsEx: Trenton Group Glens Falls Formation Larabee Member

A cross section of the rocks that comprise part of the Grand CanyonIf the Tapeats Ss represents a shoreline sand, then you can see that as sea level changed and the beach moved, the Tapeats is a different age at different locations in Arizona. Rock formations do not represent time.

How do we correlate rocks?correlation = establishing equivalencyPhysical Stratigraphy:1. Lithostratigraphy = establishing similarity of rock type = the same environment and resulting rock. NOT a matter of establishing age or age equivalency2. Magnetostratigraphy = using similar magnetic polarity to establish age equivalency (because the same rocks that record a strong magnetism are those that can be dated by radiometric decay)3. Sequence Stratigraphy = sea level curves

Biostratigraphy = using fossils to establish age equivalency1. zones or biozones - the time interval betweenthe first and last appearance of a fossil or fossils2. position within evolutionary lineages - fortraits that change gradually over time you can tellthe age of the fossil-bearing rock layer3. Index fossil - a fossil that existed for a short periodof time in earth history; its presence indicates an interval of time

Chronostratigraphy = establishing isochrons, or time lines.This is usually accomplished through the use of marker beds that represent a geologic instant in time, like a volcanic ash layer.

Lithostratigraphiccorrelation practice

The unconformity surface can be considered an isochron (anhorizon of equal time)

Magnetostratigraphy is based onthe idea of correlating rocks based on the sequence of polarityreversals they record. Shown hereis the polarity reversal record ofthe past 69 Ma. Each period ofnormal and reversed polarity is termed a chron. These chronswere determined by sampling longcontinuous records of sedimentthat are well dated by fossils.the ages of the various polarity reversals are then known. The very best use of magnetostrat-igraphy is that it enables you to correlate from marine to non-marine environments, something noother correlation technique permits.

an example of using magnetostratigraphy to correlate, in this case, cores of ocean floor sediment around Antarctica. Paired vertical columns show lithologies and polarity of the sediment. Greek letters stand for fossil range zones. Horizontal lines between cores show the correlation. Note that the correlation is NOT lithology.

An example of how range zones are used to establish the age ofrock layers. Here are the distributions of fossil graptolites inSilurian rocks. You can see that there is a horizon where the faunachanges its composition (arrows pointing downward are graptolite species that existed up until this time; arrows pointing upward are graptolite species that exist from that time onward) This is the zone boundary. Some species exist for long time periods. The two graptolite species for which the zones are named are shown.

Techniques of biostratigraphy:Range zones. In A the rangeis the interval of time when thespecies existed. In B therange = the interval of time when two species overlapped.In C range = the interval of time when one species existedbut the other did not.

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