Introduction to stratigraphy Establishing relationships between rocks

Formation - a mappable group of rocks with characteristics that enable you distinguish it from units above and below Formations can be subdivided into “Members” Formations can be grouped into “Groups” Ex: Trenton Group Glens Falls Formation Larabee Member

A cross section of the rocks that comprise part of the Grand Canyon If 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 equivalency Physical Stratigraphy: 1. Lithostratigraphy = establishing similarity of rock type = the same environment and resulting rock. NOT a matter of establishing age or age equivalency 2. 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 equivalency 1. zones or biozones - the time interval between the first and last appearance of a fossil or fossils 2. position within evolutionary lineages - for traits that change gradually over time you can tell the age of the fossil-bearing rock layer 3. Index fossil - a fossil that existed for a short period of 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.

Lithostratigraphic correlation practice

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

Magnetostratigraphy is based on the idea of correlating rocks based on the sequence of polarity reversals they record. Shown here is the polarity reversal record of the past 69 Ma. Each period of normal and reversed polarity is termed a “chron.” These chrons were determined by sampling long continuous records of sediment that 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 no other 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 of rock layers. Here are the distributions of fossil graptolites in Silurian rocks. You can see that there is a horizon where the fauna changes 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 range is the interval of time when the species existed. In “B” the range = the interval of time when two species overlapped. In “C” range = the interval of time when one species existed but the other did not.