Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic materials Relative dating – the space/time relationship between two or more geologic events or units Numerical dating – absolute time or measured dates through geochemical analysis

Principles of Relative Age Dating Uniformitarianism – James Hutton – The present is the key to the past. Horizontality/Superposition – Layers were originally laid down horizontally with oldest at bottom/youngest at top Cross-cutting relationships – any intrusive formation must be younger than the rocks it intrudes or cuts. Inclusions – must be older material picked up during intrusion Faunal succession – there is a direct link between the procession of fossils and time (e.g. once a plant/animal becomes extinct it is never seen again)

Unconformities Gaps in geologic record forming boundaries that separate rocks of different ages. Non-conformity – (pg. 251) boundary between unlayered intrusion and overlying sedimentary rocks Angular unconformity – (Fig. 9.6) nonconforming boundary between horizontal rocks that are rotated and overlying rocks that were deposited horizontally Disconformity – (pg. 251) boundary between parallel layers of sedimentary rocks

Great Unconformity of the GC (angular unconformity)

Great Unconformity of GC (nonconformity – Tapeats SS over crystalline basement)

Great Unconformity Box Canyon Ouray, Colorado

Disconformities in GC (gaps between sedimentary layers)

Correlation Rocks that are of the equivalent age but are separated spatially (Fig.9.9) Fossil evidence – biota of same age Key beds – short duration event over a wide area (Ft. Apache Lm. In Sedona) Facies change within a unit

Stratigraphic Sections

Correlating Stratigraphic Sections Vertical measurement of rock unit thickness Section is a hypothetical column of rock units for a particular area Multiple sections can be correlated by drawing lines between similar or identical beds or units Facies change can be identified if type of rock between correlated units changes between measured sections

Unit Correlation

Stratigraphic Unconformities

Numerical Dating Techniques Isotope dating – rate of decay for radioactive isotopes – uses known rates to measure the time elapsed since mineral formation Half life – time it takes half the atoms of the parent isotope to decay into daughter isotopes (Fig.9.15) Typically more useful for igneous rocks Most reliable in closed system Limited by time Uranium-thorium-lead – 10million to 4.6 billion only for oldest rocks Potassium-argon – 100,000 to 4.6 billion very old and very young dates Rubidium-strontium – 10 million to 4.6 billion half-life only rocks > 10my Carbon 14 – good for < 100,000 yrs old

Radioactive Parent Stable Daughter Half life Potassium 40 Argon 40 1.25 billion yrs Rubidium 87 Strontium 87 48.8 billion yrs Thorium 232 Lead 208 14 billion years Uranium 235 Lead 207 704 million years Uranium 238 Lead 206 4.47 billion years Carbon 14 Nitrogen 14 5730 years

Parent/Daughter Ratio

How does Carbon-14 dating work? Cosmic rays from the sun strike Nitrogen 1 4 atoms in the atmosphere and cause them to turn into radioactive Carbon 14, which combines with oxygen to form radioactive carbon dioxide. Living things are in equilibrium with the atmosphere, and the radioactive carbon dioxide is absorbed and used by plants. The radioactive carbon dioxide gets into the food chain and the carbon cycle. All living things contain a constant ratio of Carbon 14 to Carbon 12. (1 in a trillion). At death, Carbon 14 exchange ceases and any Carbon 14 in the tissues of the organism begins to decay to Nitrogen 14, and is not replenished by new C-14. The change in the Carbon 14 to Carbon 12 ratio is the basis for dating. The half-life is so short (5730 years) that this method can only be used on materials less than 70,000 years old. Archaeological dating uses this method.) Also useful for dating the Pleistocene Epoch (Ice Ages). Assumes that the rate of Carbon 14 production (and hence the amount of cosmic rays striking the Earth) has been constant (through the past 70,000 years).

Geologic Time Scale (Fig. 9.17) *Read Box 22.3 Eons Eras – Cenozoic – Age of mammals, early primates, Ice Ages, man (3.4-3.8mya), spread of modern humans last 10,000yrs. Mesozoic, age of dinosaurs, early flowering plants, formation of the Rocky Mountains Paleozoic, - Mt. Building in N.A. and Eur., early land plants, coal forming swamps Precambrian 4.6by Earth forms, ~1.5by Pangaea forms, .5by first multi-celled organisms

Periods – Breakdown of Eras Paleozoic Permian Pennsylvanian Mississippian Devonian Silurian Ordovician Cambrian