Geologic Time Measurement GY111 Physical Geology Geologic Time Measurement GY111 Physical Geology lectures on Geologic Time measurement.

Geological “Clocks” Geologic time determination is subdivided into 2 categories: Relative Dating: simply determines whether or not an event occurs before or after another event (e.g. a granite dike is younger than the surrounding sandstone since it intrudes across the sandstone in an outcrop). Absolute Dating: assigns a date to an event in terms of years before present with an error bracket (e.g. 366 Ma +/- 5 Ma) Geological “Clocks”: Relative Dating: simply determines whether or not an event occurs before or after another event (e.g. a granite dike is younger than the surrounding sandstone since it intrudes across the sandstone in an outcrop). Absolute Dating: assigns a date to an event in terms of years before present with an error bracket (e.g. 366 Ma +/- 5 Ma)

Stratigraphic Principles Principle of Original Horizontality: layers of sedimentary strata are assumed to be deposited in a horizontal or near horizontal orientation. Principle of Superposition: in a sequence of undeformed strata the younger layers are uppermost in the sequence. Principle of Faunal Succession: life has evolved over time therefore more recent life forms occur only in young strata. Stratigraphic Principles: Principle of Original Horizontality: layers of sedimentary strata are assumed to be deposited in a horizontal or near horizontal orientation. Principle of Superposition: in a sequence of undeformed strata the younger layers are uppermost in the sequence. Principle of Faunal Succession: life has evolved over time therefore more recent life forms occur only in young strata.

Original Horizontality Sediments are deposited horizontally or nearly so in the natural environment. Principle of Original Horizontality: Sediments are deposited in an original (or nearly so) horizontal orientation. Strata that is no longer horizontal has been deformed.

Superposition Layers on top are younger than those below. Principle of Superposition: 1. In undeformed strata the layers on top are younger than those on the bottom of the sequence.

Faunal Succession Fossils display a progression of increased complexity over geologic time Strata deposited at the same time will display similarities in fossil content Principle of Faunal Succession: Fossils display a progression of increased complexity over geologic time. Strata deposited at the same time will display similarities in fossil content.

Cross-cutting Relationships If a body of rock cuts across another it must be younger than the rock it cuts. If an inclusion of one type of rock is found contained in another the inclusion must be older than the surrounding rock. Cross-Cutting Relationships: 1. If a body of rock cuts across another it must be younger than the rock it cuts. 2. If an inclusion of one type of rock is found contained in another the inclusion must be older than the surrounding rock.

Example: Cross-Cutting Granite dike intruding sandstone and shale. Country Rock Cross-Cutting Example: Dike cross-cutting horizontal strata. If the dike age was determined to be 100Ma by radiometric analysis then we could also say that the strata cut by the dike must be older than the dike. Dike

Example: Inclusion Granite contains pieces of country rock that are completely surrounded (xenoliths). Principle of Inclusion Example: 1. Xenoliths in the photo (darker material) must predate the intrusion of the granite magma.

Unconformities Represent erosional events where strata is removed, or is simply a time of non-deposition. Types of Unconformities: Angular: tilted layers of strata below unconformity Disconformity: layers above and below unconformity are parallel to the unconformity Nonconformity: crystalline rocks are found below the unconformity, sedimentary rocks above Unconformities: 1. Represent erosional events where strata is removed, or is simply a time of non-deposition. 2. Types of Unconformities: Angular: tilted layers of strata below unconformity. Disconformity: layers above and below unconformity are parallel to the unconformity. Nonconformity: crystalline rocks are found below the unconformity, sedimentary rocks above. Represents a large interval of time.

Angular Unconformity Requires a period of deformation before uplift causes erosion of unconformity surface. Angular Unconformity: 1. Requires a period of deformation before uplift causes erosion of unconformity surface. 2. Deformation tilts strata below the unconformity before erosion bevels the region. 3. Younger strata are deposited in horizontal layers on the unconformity.

Disconformity A disconformity usually represents a relatively short interval of erosion and/or non-deposition Disconformity: Layers above and below unconformity are parallel to unconformity. Most difficult to detect. Represent relatively short intervals of non-deposition.

Nonconformity Nonconformities indicate large time intervals of erosion and/or non-deposition Nonconformity: Crystalline rocks below the unconformity. Un-metamorphosed horizontal strata above the unconformity. Represent large intervals of time that have no corresponding rock record in that area.

Geologic Time Scale Geologic Time Scale: 1. Know the order and boundary times of the Eons, Eras, and Periods.

Radiometric Decay Isotopes are elements with different atomic mass numbers. Some isotopes decay into different elemental isotopes at a predictable rate. Using an instrument that can detect isotopic abundances (mass spectrometer) the age of a rock can be estimated. The starting isotope is the “parent” and the decay product is the “daughter” isotope. Radiometric Decay Systems: 1. Isotopes are elements with different atomic mass numbers. 2. Some isotopes decay into different elemental isotopes at a predictable rate. 3. Using an instrument that can detect isotopic abundances (mass spectrometer) the age of a rock can be estimated. 4. The starting isotope is the “parent” and the decay product is the “daughter” isotope.

Radiometric Age Calculation Half-life is the time required for half of the parent isotope to decay to daughter isotopes. Note that if you started with 100 parent atoms after 2 half-lives there would be 25 parent atoms left. Large half-lives are required to measure geological materials that may be billions of years old.

Isotopic Decay Systems Isotopic Decay Systems useful for Geological Materials: C14 > N14 (5730 years) K40 > Ar40 (1.4 Ga) U235 > Pb206 (4.468 Ga) Rb87 > Sr87 (48.8 Ga) Sm147 > Nd143 (106.0 Ga)

Problems with Radiometric Dating Most systems are restricted to felsic rocks. Ar is a volatile gas that does not fit into mineral lattices. Radiogenic Pb is easily contaminated by a variety of anthropogenic activities. C14 can date only 100,000 year or less events. U-Pb minerals are refractory. Cannot date sedimentary rocks. Metamorphic events reset mineral isotopic ratios. Problems with Radiometric Dating: 1. Most systems are restricted to felsic rocks. 2. Ar is a volatile gas that does not fit into mineral lattices. 3. Radiogenic Pb is easily contaminated by a variety of anthropogenic activities. 4. C14 can date only 100,000 year or less events. 5. U-Pb minerals are refractory. 6. Cannot date sedimentary rocks. 7. Metamorphic events reset mineral isotopic ratios.

Isochron Diagrams Isochron Diagram: Slope of isochron is proportional to the age of the material. The Y-intercept is the Sr87/86 initial ratio. Note that Sr 86 is not part of a decay sequence.

Exam Summary Know the various types of relative dating. Know the Eons, Eras, and Periods of the Geologic Time Scale. Know the important date boundaries on the geologic time scale. Know the types of radiometric isotopic systems. Be familiar with how a isochron diagram works. Know the various types of unconformities. Exam Summary: 1. Know the various types of relative dating. 2. Know the Eons, Eras, and Periods of the Geologic Time Scale. 3. Know the important date boundaries on the geologic time scale. 4. Know the types of radiometric isotopic systems. 5. Be familiar with how a isochron diagram works. 6. Know the various types of unconformities.