VIZUALIZING EARTH HISTORY By Loren E. Babcock Chapter 3 Geologic Time

Compare relative age dating and numerical age dating. Geologic Time Scale Compare relative age dating and numerical age dating. Relative age dating – (Older vs. younger) Relative age dating is a technique of establishing a chronology of events arranged in sequential order. Relative dating puts events in sequence using Nicholas Steno’s principles of superposition, original horizontality, and original lateral continuity; applying Charles Lyell’s principles of cross-cutting relationships and included fragments; and also applying William Smith’s principle of biotic succession.

Compare relative age dating and numerical age dating. Geologic Time Scale Compare relative age dating and numerical age dating. Numerical age dating — Numerical age dating is a technique of establishing when events occurred according to how much time has elapsed since their occurrence. Numerical age will involve in calculating many thousands, millions, or billions of years ago an event occurred. Numerical age dating techniques include counting tree rings, counting layers of ice drilled from glaciers, and counting sediment layers in ancient lakes.

Understand the various techniques of stratigraphic correlation. Geologic Time Scale Understand the various techniques of stratigraphic correlation. Stratigraphy — Stratigraphy is the study of layered rocks, including their compositions, origins, geometric relationships, and ages. Correlation — Correlation is defined as matching of strata from one location to another. The goal of studying stratigraphy and correlation is to try to match rock units from one place to another to determine their environment of formation and deposition.

Understand the various techniques of stratigraphic correlation. Geologic Time Scale Understand the various techniques of stratigraphic correlation.

Understand the various techniques of stratigraphic correlation. Geologic Time Scale Understand the various techniques of stratigraphic correlation.

Understand the various techniques of stratigraphic correlation. Geologic Time Scale Understand the various techniques of stratigraphic correlation.

Understand the various techniques of stratigraphic correlation. Geologic Time Scale Understand the various techniques of stratigraphic correlation.

Understand the various techniques of stratigraphic correlation. Geologic Time Scale Understand the various techniques of stratigraphic correlation. Correlating Rock Units Rock units can be correlated, using relative age dating techniques, on the basis of their lithology (lithostratigraphy) or on the basis of criteria that are proxies for geologic time (chronostratigraphy). Fossils, variations in chemical isotopes, variations in physical properties (such as radioactivity levels), and sequences of unconformity-bounded strata are some of the variables that provide an understanding of the relative timing of events in geologic history, and which can be used to correlate strata from one place to another.

Geologic Time Scale LITHOSTRATIGRAPHY Lithostratigraphy involves the correlation of rocks on the basis of lithology, or rock type. The lithology of a rock usually involves some combination of the rock composition, grain or crystal size, and color. Lithostratigraphic units can be correlated according if they are composed of sandstone, shale, conglomerate, limestone, or granite, for instance, but commonly additional qualifiers are needed to help distinguish one unit from another. Color of a rock is often used as a guide to correlation.

Geologic Time Scale LITHOSTRATIGRAPHY Lithostratigraphy — Stratigraphic correlation on the basis of rock type. Formation — Fundamental unit of lithostratigraphy; it has a definable top and bottom, and is mappable across geographic space.

Geologic Time Scale BIOSTRATIGRAPHY Biostratigraphy — Zoning of stratigraphic layers, and arrangement of those layers according to relative time of deposition, using the ranges of fossils. Zone (biozone) — A stratigraphic interval defined by its fossil content, and usually given the name of a characteristic fossil present in that interval.

Geologic Time Scale BIOSTRATIGRAPHY

Geologic Time Scale CHEMOSTRATIGRAPHY Chemostratigraphy — Correlation of strata using ratios of chemical isotopes. Isotopic excursion — A positive or negative shift in the isotopic ratio of an element as recorded through a succession of stratigraphic layers.

SEQUENCE STRATIGRAPHY Geologic Time Scale SEQUENCE STRATIGRAPHY Unconformity - A surface of erosion or non-deposition. Sequence stratigraphy - Correlation of strata on the basis of erosion surfaces that separate packages of sediments or sedimentary rocks called depositional sequences (sequences).

SEQUENCE STRATIGRAPHY Geologic Time Scale SEQUENCE STRATIGRAPHY Unconformities fall into three basic kinds: nonconformities, angular unconformities, and disconformities

SEQUENCE STRATIGRAPHY Geologic Time Scale SEQUENCE STRATIGRAPHY Distinguish the three types of unconformities. Nonconformities occur where sedimentary strata have been deposited over top of crystalline (igneous or metamorphic) rock. Angular unconformities occur when horizontally parallel strata are deposited on tilted and eroded layers that may be either vertical or at angle to the overlying horizontal layers. Disconformities are unconformities between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition.

Geologic Time Scale SEQUENCE STRATIGRAPHY

SEQUENCE STRATIGRAPHY Geologic Time Scale SEQUENCE STRATIGRAPHY Understand how events in Earth history are dated. Eustatic (sea level) — Global sea level. Sequence (depositional sequence) — A relatively conformable package of sedimentary strata that are bounded below and above by unconformities or their equivalent conformities.

OTHER CORRELATION TECHNIQUES Geologic Time Scale OTHER CORRELATION TECHNIQUES Measurements of the geophysical properties of rocks are also indicators of lithology. Gamma-ray profile - Sedimentary rocks emit varying amounts of gamma rays depending on the amount of radioactive uranium, thorium, and potassium they contain. Gamma-ray profile can be used to correlate strata exposed at the surface, or ones penetrated by drilling into the subsurface.

Geologic Time Scale

Geologic Time Scale

OTHER CORRELATION TECHNIQUES Geologic Time Scale OTHER CORRELATION TECHNIQUES Polarity directions — Measuring polarity directions recorded in rocks from the time the rocks were formed. Earth’s polarity has switched many times during this planet’s long history, and the sequence of normal and reversed polarity episodes provides a good basis for the correlation of strata.

Geologic Time Scale OTHER CORRELATION TECHNIQUES

Understand the geologic time scale. The geologic time scale is a chronology, or “calendar” of Earth history. Units of geologic time are not “natural” or followed a preset plan (or “chapter outline”) that Earth adhered to. Geologic time units have been defined and named by geologists who recognized that Earth has passed through a series of stages, each having its own distinct characteristics. The switchover from one stage to the next, or the boundary point between two time units, must be arbitrary but based on sound scientific reasoning.

Geologic time scale — A chronology of Earth history.

Geologic time scale — A chronology of Earth history. GSSP - Acronym for Global Standard boundary Section and Point, an internationally ratified point in strata marking the boundary between two time-rock (chronostratigraphic) units, and their equivalent time units. Chronostratigraphic unit - A time-rock unit; the tangible representation of a geologic time (geochronologic) unit.

Distinguish geologic time units from chronostratigraphic units. Geologic Time Scale Distinguish geologic time units from chronostratigraphic units. CHRONOSTRATIGRAPHY System - The fundamental unit of chronostratigraphy. Period - The unit of geologic time equivalent to a system. Eons, eras, periods, and epochs are formally defined units of geologic time. They are abstract entities, just like minutes and hours. The geologic time scale shows abstract time units in their correct order, and hierarchically arranged.

Geologic Time Scale CHRONOSTRATIGRAPHY Correlation techniques used to establish chronostratigraphic units must be time-parallel, meaning that they must define units representing the same “instant” (in geological terms) everywhere in the world. Biostratigraphic units (biozones or zones) qualify as time-parallel units because biological evolution and extinction is a non-repeating process. Each species has had only one time of appearance on Earth, and each has gone extinct or eventually will do so only once.

Geologic Time Scale CHRONOSTRATIGRAPHY Chemostratigraphic profiles record variations in chemical isotopic ratios locked into sediments through geologic time. Details of the sine-wave pattern of positive and negative excursions provide a virtual “fingerprint” for time units. Sequence-stratigraphic units, or sequences, are bounded by disconformities of short duration (geologically speaking). They are commonly the result of rapid global sea level changes, which means disconformities of equivalent age appear in stratigraphic sections around the world.

Geologic Time Scale CHRONOSTRATIGRAPHY Lithostratigraphic units, especially formations, are, for the most part, not time-parallel. Formations usually represent a sedimentary facies, the type of sediment deposited in one area (say a beach, a reef, a lake, or a stream), and are not directly tied to time intervals. In the world today, many different sedimentary facies coexist in time, and this was also true in the geologic past.

Geologic Time Scale GEOCHRONOLOGY Parent - In a radioactive decay series, an unstable isotope that decays, or transforms, into a daughter product. Daughter - Isotope formed from the radioactive decay of a parent isotope. Geochronology refers to the dating of rocks according to their numerical ages. Geochronologic dating involves laboratory analyses of radiometric isotopes. In radioactive decay, a radiogenic parent isotope changes through one or more steps, each referred to as a daughter product, until becoming a stable end product.

Geologic Time Scale GEOCHRONOLOGY Radioactive decay occurs in a number of ways. Three processes useful in geochronology are called alpha decay, beta decay, and electron capture. Alpha decay is a type of nuclear fission, in which the parent splits into two daughter products. It tends to occur in isotopes that have large atomic numbers. Beta decay is of two forms. In neither one does the atomic mass of a nucleus change. Emission of a beta particle (an electron, β-), and the emission of an electron’s antiparticle, which is called a positron (β+).

Geologic Time Scale GEOCHRONOLOGY In electron capture, a parent nucleus captures one of its own electrons and then emits a neutrino. The atomic mass of a nucleus does not change. Potassium-40 (40K) transforms to argon-40 (40Ar) by means of electron capture. Half-life - the amount of time it takes for one-half of the parent to decay to a daughter product is called a half-life.

Geologic Time Scale GEOCHRONOLOGY

Geologic Time Scale GEOCHRONOLOGY After one half-life, 50% of the parent remains, and 50% of the atoms have become a daughter product. After two half-lives, 25% (or half of the 50% amount) of the parent remains, and 75% of the atoms have become a daughter product. After three half-lives, 12.5% (or half of the 25%) of the parent remains, and 87.5% of Further decay continues in the same way.