Chapter 8 Time and Geology / Geologic time basic principles

Sequence of events for nonconformities

(1) crystallization of igneous or metamorphic rock at depth; (2) erosion of at least several kilometers of overlying rock (the great amount of erosion further implies considerable uplift of this portion of Earth's crust) (3) deposition of new sediment, which eventually becomes sedimentary rock, on the ancient erosion surface

Sequence of events for angular unconformities

(1) deposition and lithification of sedimentary rock (or solidification of successive lava flows if the rock is volcanic) (2) uplift accompanied by folding or tilting of the layers (3) erosion (4) renewed deposition (usually preceded by subsidence) on top of the erosion surface & A particular sequence of events ( A-D ) producing an angular unconformity. Marine deposited sediments are uplifted and folded (probably during plate-tectonic convergence). Erosion removes the upper layers. The area drops below sea level (or sea level rises) and renewed sedimentation takes place. (An angular unconformity can also involve terrestrial sedimentation.)

Correlation

-In geology, correlation usually means determining time equivalency of rock units -Rock units may be correlated within a region, a continent, and even between continents -Correlation can also be pursued between rocks, but it will likely lose its temporal meaning -So we're trying to find rocks that are the same age

Albert Oppel

-Known for expanding the Geologic Time Scale -Thought of a more advanced method to divide geologic formations into different zones, based on the overlapping stratigraphic range of two fossil zones. -Oppel was able to trace fossil pattern throughout all of Europe, and break them down (for finer stratigraphic resolution) into overlapping range zones, bounded at the bottom by the appearance of certain taxa and at the top by the appearance of other taxa -The continuous, rapid establishment of biostratigraphic zones that followed these discoveries led to the development of a more and more refined relative time scale, which eventually allowed the construction of what we know today as the Geologic Time Scale with its subdivisions

James Hutton

-Regarded as the father of geology -The first to speak about Uniformitarianism -Coined Deep time -broke from conventional thinking that Earth is no more than a few thousand years old -Realized that geologic features could be explained through present-day processes -He recognized that our mountains are not permanent but have been carved into their present shapes and will be worn down by the slow agents of erosion now working on them. He realized that the great thicknesses of sedimentary rock we find on the continents are products of sediment removed from land and deposited as mud and sand in seas. The time required for these processes to take place had to be incredibly long.

Steno's Principles

1. Superposition 2. Original Horizontality 3. Lateral Continuity

4 types of unconformities

1.Paraconformities 2.Disconformities 3.Angular Unconformities 4.Nonconformities

Numerical ages for the Paleozoic / Mesozoic

252 mya

Numerical age for the Precambrian / Phanerozoic boundary

541 mya

Numerical age for the Mesozoic / Cenozoic boundaries

66 mya

contact

A contact is the boundary surface between two different rock types or ages of rocks. In sedimentary rock formations, the contacts are usually bedding planes. -Contacts are particularly useful for deciphering the geologic history of an area.(Contacts are the surfaces separating two different rock types or rocks of different ages.) -Types of contacts: intrusive contacts and sedimentary contacts. Faults are a third type of contact. The final important type of contact is an unconformity. -Each type of contact has a very different implication about what took place in the geologic past.

Nonconformity

A nonconformity is a contact in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock -Sedimentary rocks covering eroded plutonic and/or metamorphic rocks -An unconformity in which older metamorphic or intrusive igneous rocks are overlain by younger sedimentary strata. -A nonconformity implies deep erosion because metamorphic or plutonic rocks have been exposed and subsequently buried by younger rock -A nonconformity generally indicates deep or long-continued erosion before subsequent burial, because metamorphic or plutonic rocks form at considerable depths in Earth's crust.

Principle of Faunal and Floral Succession

A principle or law stating that fossil species succeed one another in a definite and recognizable order; -In general, fossils in progressively older rock show increasingly greater differences from species living at present. Def2:Fossil faunas and floras follow each other in a sedimentary sequence according to a known and predictable order

Ranges

A range is the vertical distribution of a fossil species in rocks, from the moment of its first occurrence (FO) to the moment of its last occurrence (LO)

geologic time scale

A record of the geologic events and life forms in Earth's history. -Geologists can use fossils in rock to refer the age of the rock to the standard geologic time scale , a worldwide relative time scale. -Widespread use of fossils led to the development of the standard geologic time scale. -Based on fossil assemblages, the geologic time scale subdivides geologic time. On the basis of fossils found, a geologist can say, for instance, that the rocks of the lower portion of horizontal layers in the Grand Canyon formed during the Cambrian Period. -Originally based on relative age relationships, the subdivisions of the standard geologic time scale have now been assigned numerical ages in thousands, millions, and billions of years through isotopic dating -Think of the geologic time scale as a sort of calendar to which events and rock units can be referred

Dike

A slab of volcanic rock formed when magma forces itself across rock layers -A dike is a tabular (shaped like a tabletop), discordant, intrusive structure. Discordant means that the body is not parallel to any layering in the country rock. (Think of a dike as cutting across layers of country rock.)

Index fossils

An Index Fossil is the fossil of a species which: - lived for a very short amount of time - was widespread - lived in different environments - it is easy to find in the fossil record •Index Fossils, unfortunately, are rare Need them to tell time and correlation -Ideally, a geologist hopes to find an index fossil , a fossil from a very short-lived, geographically widespread species known to exist during a specific period of geologic time. A single index fossil allows the geologist to correlate the rock in which it is found with all other rock layers that contain that fossil

angular unconformity

An angular unconformity is a contact in which younger strata overlie an erosion surface on tilted or folded layered rock -The sequences of strata above and below the unconformity are at an angle -Tilted rocks are overlain by flat-lying rocks -An angular unconformity implies that folding or tilting of rocks took place before or around the time of erosion

Periods in the Geological Time Scale

Cambrian, Ordovician, Silurian, Devonian, Mississippian & Pennsylvanian (Carboniferous), Permian, Triassic, Jurassic, Cretaceous, Paleogene & Neogene (Tertiary), Quaternary

Paleozoic

Come over some day, maybe play poker. -Cambrian, Ordovician, Silurian, Devonian, Mississippian & Pennsylvanian (Carboniferous), Permian

Uniformitarianism

Def: The concept that geologic processes operating at present are the same processes that operated in the past Def2: Charles Lyell's idea that geologic processes have not changed throughout Earth's history. -The principle of uniformitarianism (or actualism, ) a fundamental concept of geology, states that the present is the key to the past.

fossil assemblage

Def: a group of fossil species found in a specific sequence of sedimentary rock -Finding several species of fossils (a fossil assemblage) in a layer of rock is generally more useful for dating rocks than a single fossil is, because the sediment must have been deposited at a time when all the species represented existed -Where various combinations of these fossils are found in three rocks, the time of formation of each rock can be assigned to a narrow span of time -Some fossils are restricted in geographic occurrence, representing organisms adapted to special environments. But many former organisms apparently lived over most of the Earth, and fossil assemblages from these may be used for worldwide correlation

Disconformity

Erosion & Parallel layers -In a disconformity , the contact representing missing rock strata separates beds that are parallel to one another -Probably what has happened is that older rocks were eroded away parallel to the horizontal bedding plane; renewed deposition later buried the erosion surface -The sequences of strata above and below the unconformity are parallel, and there was erosion -Beds above and below a disconformity are parallel, generally indicating less intense activity in Earth's crust -Because it often appears to be just another sedimentary contact (or bedding plane) in a sequence of sedimentary rock, a disconformity is the hardest type of unconformity to detect in the field -Usually, the disconformity can be detected only by studying fossils from the beds in a sequence of sedimentary rocks. -Although it is most likely that some rock layers are missing because erosion followed deposition, in some instances neither erosion nor deposition took place for a significant amount of geologic time

Basic use of fossils for correlation

Finding one single fossil per se is not enough - if an organism lived for a very long time, it might not be very useful for correlation -Many fossils are of little use in time determination because the species thrived during too large a portion of geologic time. -Ex:Sharks have been in the oceans for a long time, so discovering an ordinary shark's tooth in a rock is not very helpful in determining the rock's relative age. -Ex: Nautilus is considered to be a "living fossil," as the species has undergone little change in the last 400 million years. The nautilus first appeared about 265 million years before the first dinosaurs. Because of this, a Nautilus fossil will not be very indicative of any time period in Earth History

Correlation:physical continuity

Finding physical continuity —that is, being able to trace physically the course of a rock unit—is one way to correlate rocks between two different places -You can see or trace your layers directly (visually) -Limited to small areas

Significance of Fossils

Fossil give us two lines of information: • the Environment of Deposition - the physical place where organisms lived & • Relative Time (because of William Smith's Principle of Fossil Succession: faunas and flora follow each other in time according to a known and predictable order) - the 8me interval when said organisms lived -Fossils are used for worldwide correlation of rocks -Sedimentary rocks are assigned to the various subdivisions of the geologic time scale on the basis of fossils they contain, which are arranged according to the principle of faunal succession.

Correlation: Fossils

Fossils are remnants of ancient forms of life, or of their activity • Since fossils change (evolve) in time according to a known and predictable order, they can be used as a correlation tool -Best way to find correlation amongst rocks • Principle of Fossil Succession • Concept of Index Fossil • Concept of Fossil Assemblage (remember Albert Oppel's zones)

xenoliths

Fragments of rock that are distinct from the body of igneous rocks in which they are enclosed

Numerical age (also known as absolute age )

How many years ago a geologic event took place -age given in years or some other unit of time -Numerical age is generally obtained by using isotopic dating techniques. --Isotopic dating is accomplished by determining the ratio of the amount of a radioactive isotope presently in a rock or mineral being dated to the amount originally present.

Key beds

In some regions, a key bed, a very distinctive layer, can be used to correlate rocks over great distances -An example is a layer of volcanic ash produced from a very large eruption and distributed over a significant portion of a continent

How do horizontal layers get tilted?

In this example we see five different stages of deformation: A - undisturbed horizontal layers B - lateral compression causes mild folding (structures called synclines and anticlines) C - compression continues and causes a fold to develop a steep tilted side (limb) and a vertical side D - a rupture occurs (a thrust fault*) E - continued compression at this point causes almost horizontal sliding of one block over the other * a fault is a fracture along which movement occurs when the fault surface is nearly horizontal and allows for very old rocks to overlie very young rocks, it is called a thrust fault)

Actualism (Uniformitarianism)

Many geologists prefer actualism in place of uniformitarianism. -The term actualism comes closer to conveying the principle that the same processes and natural laws that operated in the past are those we can actually observe or infer from observation as operating at present. -It is based on the assumption, central to the sciences, that physical laws are independent of time and location. Under present usage, uniformitarianism has the same meaning as actualism for most geologists

Paraconformity

NO erosion and No sediments deposited; also parallel layers Def: The sequences of strata above and below the unconformity are parallel, and there was no erosion, just non-deposition -An unconformity for which there is no evidence but some geologists think should exist -Paraconformities and disconformities are often difficult to tell one from the other -Clues can be found in fossils, hardened surfaces, bioturbation, etc.

Cenozoic Periods

Paleogene, Neogene, Quaternary -Please No Questions

Charles Lyell

Popularized Hutton's idea of Uniformitarianism in his book Principles of Geology -Gradualism; Lyell's addition to Hutton's ideas which says that the rate (the velocity, or speed) at which these processes occurred was the same -We now know that this extreme approach to uniformitarianism is not true. We understand that, for instance, a change in climate could speed up or slow down weathering processes, thus increasing or decreasing the amount of sediment available during the same time interval. Consequently, a sedimentary sequence can appear thicker or thinner, display a different texture, such as different sorting and/or rounding, or different colors, because parameters have changed. The same concept applies, for instance, to mid-oceanic ridge spreading rates, or to other geological process

Eras

Precambrian, Paleozoic, Mesozoic, Cenozoic

Formations are based on _______?

Rock type -A formation may be a single thick bed of rock such as sandstone. A sequence of several thin sandstone beds could also be called a formation, as could a sequence of alternating limestone and shale bed

truncated

Slice off -Ex: The sedimentary beds on either side of the valley appear to have been sliced off, or truncated, by the valley

Nicolaus Steno

Steno is famous because he has been the first to clearly outline the three basic principles of stratigraphy: superposition, original horizontality and lateral continuity: the principle of superposition (states that in a sequence of undisturbed sedimentary layer, the younger layer is found at the top and the older layer at the bottom), the principle of original horizontality (states that sedimentary layers form horizontally), and the principle of lateral continuity (states that sedimentary layers extend laterally to the edge of a sedimentary basin or taper out and change into other, different layers)

Hiatus

The geologic time not represented at an unconformity is defined as a hiatus

cross-cutting relationships

The principle of crosscutting relationships states that a disrupted pattern is older than the cause of disruption. Ex: A layer cake (the pattern) has to be baked (established) before it can be sliced (the disruption). def2: Principle stating that a fault or intrusion is younger than the rock it cuts across. def 3: What cuts, is younger than what has been cut

Lateral Continuity

The principle of lateral continuity states that an original sedimentary layer extends laterally until it tapers or thins at its edges -Sediment extends laterally, in all directions until it thins and pinches out, or terminates against the edge of the depositional basin -The principle of lateral continuity tells us that sedimentary beds normally become thinner toward the edges rather than stop abruptly

original horizontality

The principle of original horizontality states that beds of sediment deposited in water formed as horizontal or nearly horizontal layers • Because of the influence of gravity, sediment is deposited in horizontal layers (there are some exceptions, like cross-bedding) - So, when you see sequences where layers are not horizontal, those sequences have been tectonically deformed --some layers can be completely vertical -By applying the principle of original horizontality, we can determine if a geologic event—tilting of bedrock— has occurred

The age of Earth is estimated to be approximately

The scientifically determined age of Earth is 4.5 to 4.6 billion years.

Stratigraphy

The subdiscipline of geology that uses interrelationships between layered rock (mostly) or sediment to interpret the history of an area or region is known as stratigraphy (from the Latin word stratum, meaning a thing spread out, or a cover) -Four of stratigraphy's principles are used to determine the geologic history of a locality or a region. These are the principles of (1) original horizontality, (2) superposition, (3) lateral continuity, and (4) cross-cutting relationships.

Mesozoic

Triassic, Jurassic, Cretaceous

William Smith

While cutting canals, he realized that fossils found in rock sequences always follow each other in a specific order, and that this order was exactly the same even when rocks were found as isolated layers at different locations. It was then possible to correlate these layers with each other based on their fossil content. -He realized that different sedimentary layers are characterized by distinctive fossil species and that fossil species succeed one another through the layers in a predictable order. -Smith's discovery became known as the Principle of Faunal and Floral Succession: fossil faunas and floras follow each other in a sedimentary sequence according to a known and predictable order. -Smith's discovery of this principle of faunal succession allowed rock layers in different places to be correlated based on their fossils. We now understand that faunal succession works because there is an evolutionary history to life on Earth -Because the same species never evolves twice (extinction is forever), any period of time in Earth history can be identified by the species that lived at that time. -Therefore, sedimentary rocklayers anywhere in the world can be assigned to their correct place in geologic history by identifying the fossils they contain -Thanks to his observation and discoveries, Smith was also able to publish the first large-scale geologic map ever conceived (1815)

Formation

a body of rock of considerable thickness that is large enough to be mappable, and with characteristics that distinguish it from adjacent rock units -Although a formation is usually composed of one or more beds of sedimentary rock, units of metamorphic and igneous rock are also called formations. It is a convenient unit for mapping, describing, or interpreting the geology of a region -Formations , are bodies of rock of considerable thickness with recognizable characteristics that make each distinguishable from adjacent rock units. They are named after local geographic features, such as towns or landmarks

Charles Darwin

a copy of Lyell's book travelled with Charles Darwin during his scientific exploration trip aboard The Beagle -Darwin was very likely influenced in his thoughts by the work of Lyell, and he had chances to see Uniformitarianism at work (for instance, by being a witness to an earthquake and its consequences along the Pacific coast of Chile). -Despite his significant geological contributions, Darwin is mostly remembered for his Theory of Evolution through gradual variation and natural selection, exposed and discussed in his aptly titled book On the Origin of Species by Means of Natural Selection. -His evolutionary theory involving survival of the fittest, published in the mid-1800s, required the great amount of time that the works of Hutton and Lyell proposed -The theory of Evolution was also a good explanation for William Smith's Principle of Faunal and Floral Succession. When we apply the theory of Evolution and the Principle of Faunal and Floral Succession to Steno's principle of Superposition, we realize that we can actually use fossils for stratigraphic dating and correlation

Correlation: Similarity of Rock type

correlation between two regions can be made by assuming that similar rock types in two regions formed at the same time. This method must be used with extreme caution, especially if the rocks being correlated are common ones. -This is tricky and you should be absolutely certain that you are looking at the same rock -Similar rock could be just because there is a similar environment/ temperature causing the same type of rock -It can't be the same rock if its not in the same layer -Correlation by similarity of rock types is more reliable if a very unusual sequence of rocks is involved -When the hypothesis of continental drift was first proposed, important evidence was provided by correlating a sequence of rocks

Superposition

def1:The principle of superposition states that within a sequence of undisturbed sedimentary or volcanic rocks, the oldest layer is at the bottom and layers are progressively younger upward in the stack def2:In an undisturbed succession of sedimentary rocks layers: - the oldest layer is at the bottom - the youngest layer is at the top • Or, the sequence gets younger from the bottom to the top ( this can happen if tectonic forces flip the rock) -The principle of superposition also applies to layers formed by multiple lava flows, where one lava flow is superposed on a previously solidified flow

Principle of Inclusions

def: INCLUSIONS, or fragments of one rock contained in another, are OLDER than the rock they are found in def2: An inclusion (or fragment) inside a body of rock is always older than the rock itself -def 3: fragments included in a host rock are older than the host rock

Relative time

def: the sequence in which geologic events occur in an area -Geologists working in the field or in a laboratory with maps, cross sections, and photographs are more often concerned with relative time, the sequence in which events took place, rather than the number of years involved -Because most geologic problems are concerned with the sequence of events, we discuss relative age first • In order to establish a sequence of events in time we use five different principles • Three of them are known as Steno's Principles (from Nicolaus Steno): superposition, original horizontality, and lateral continuity • Two more were added later: the principle of cross-cutting relationships and the principle of inclusions

Unconformities

gaps in the rock record during which either erosion occurred or deposition was absent -First recognized by James Hutton at Siccar Point -In this image, the unconformity is in the squiggly line that separates the horizontal and slanted layers -if layers are not continuous that means something cut it, and whatever it is, its younger than the rock -Unconformities are buried erosion surfaces that help geologists determine the relative sequence of events in the geologic past. -An unconformity is a surface (or contact) that represents a gap in the geologic record, with the rock unit immediately above the contact being considerably younger than the rock beneath -Most unconformities are buried erosion surfaces.

At unconformities, time is not recorded, and the geologic record is __________?

incomplete -The gap could be short (less than 1 million years) or very long (hundreds of millions to billions of years) -Unconformities are surfaces, and not layers

There are three main ways to correlate between rock sequences:

physical continuity, similarity of rock types, and use of fossils

The main criterion for distinguishing and naming a formation is _______?

some visible characteristic that makes it a recognizable unit -This characteristic may be rock type or sedimentary structures or both. For example, a thick sequence of shale may be overlain by basalt flows and underlain by sandstone. The shale, the basalt, and the sandstone are each a different formation. Or a sequence of thin limestone beds, with a total thickness of many tens of meters, may have recognizable fossils in the lower half and distinctly different fossils in the upper half. The limestone sequence is divided into two formations on the basis of its fossil content

Formations are given proper names:

the first name is often a geographic location where the rock is well exposed, and the second is the name of a rock type, such as Navajo Sandstone, Austin Chalk, Baltimore Gneiss, Onondaga Limestone, or Chattanooga Shale. If the formation has a mixture of rock types, so that one rock name does not accurately describe it, it is called simply "formation," as in the Morrison Formation or the Martinsburg Formation.

Eons

the largest division of geologic time -Hadean, archean, Proterozoic, Phanerozoic -Precambrian= Hadean, archean, Proterozoic