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EESA06H3 (234)
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Chapter 19

Chapter 19 Textbook Notes.odt

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Department
Environmental Science
Course
EESA06H3
Professor
Nick Eyles
Semester
Winter

Description
Chapter 19 Time and Geology ➢ Introduction  Geoscientists working the field are usually concerned with relative aging  E.g. Glacial sediments on top are younger than the surface that they cover  Isotopic dating allows us to find absolute age  Through Isotopic dating we have discovered that rocks of the NorthAmerican craton range in between 4 billion and 1 billion years old  SoALMOST close to the age of the earth (4.56 Ma)  Determining age relationships between geographically widely separated rock units is necessary for understanding the geologic history of a region, continent, or the whole Earth  Proving the plate tectonics theory requires evidence that the continents were once one great body – this can come from correlation of rock units  Discovery of fossils in strata – this helped geologists identify stratigraphic successions and eventually allows the creation of the standard geologic timescale  Isotopic dating has allowed for the addition of specific ages (million, billion) to each time period ➢ WHAT IS UNIFORMITARIANISM?  In 17th/18 century Christianity had large influences over what was believed about Earth  Earth was believed to be only around 6000 years old.  Noah's flood is believed to have produced fossil in sediments on mountain tops  Mountains and all other geological structures are beleived to have been created as they are now = divine intervention  James Hutton came around and changed this  He is the “father of modern geology”  Suggested that present processes are probably what took place in the past  “We find no sign of a beginning – no prospect for an end”.  Uniformitarianism: Charles Lyell popularized his idea of present operations being a reflection of the past = principle of uniformitarianism  “The present is the key to the past”  Problem with term  Suggests that changes take place at a uniform rate.  Actualism is a better term; now interchangeable with uniformitarianism  Actualism: The same processes & natural laws that operated in the past are those we can actually observe or infer from observation as operating at present.  Absolute/numerical age  The clock that started working when rocks were formed = radioactivity  Numerical age is given in years or some other unit of time  Does not have to be “exact” - can be an estimate “1 and a ½ centuries ago”  Relative Time  Of concern to geologists working in the field or lab with maps, cross-sections & photographs  They are concerned with the sequence in which events took place, rather than the number of years exactly.  Relative Time is more commonly used ➢ HOW CAN THE SEQUENCE OF PAST GEOLOGICAL EVENTS BE DETERMINED?  Relative Time  Deriving Relative Time of Grand Canyons by analyzing • a) Horizontal layers of rock • b) Inclined Layers • c) Rocks underlying inclined layers (plutonic & metamorphic) • d) Canyon itself  Principles Used to Determine RelativeAge  We have to look at each of these individual parts in order to solve the bigger problem  Contacts: • Are the surfaces separating two different rock types or rocks of different ages • Contacts representing buried erosion surfaces are Unconformities! • They are particularly useful for deciphering the geologic history of an area  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.  Stratigraphy • Subdiscipline of geology that uses interrelationships between layered rock or sediment to interpret the history of an area or region • Uses 4 principles • Original horizontality • Superposition • Lateral continuity • Cross-cutting relationships  Original Horizontality (“Cold Canyon” Figure) • This principle states that beds of sediment deposited in water formed as horizontal or nearly horizontal layers • The bottom layers of the figure (Copper Creek, Kamloops, Revelstoke) must have been tilted after they were deposited as horizontal layers. • These layers may also have been subjected to uplift and erosion before new layers settled  Superposition • The principle of superposition states that within a sequence of undisturbed sedimentary or volcanic rocks, the layer gets younger going from bottom to top • Applies to sediments and lava flows • So top layer (Osoyoos Lake Limestone) is the youngest • The youngest of the tilted bedrock (Copper Creek)  Lateral Continuity • States that an original sedimentary layer extends laterally until it tapers or thins at its edge • Pentiction Formation tapers • We do not see the others either because we're not seeing their full extent or because they have been truncated (cut off abruptly) due to later events  Cross-cutting Relationships • Principle states that a disrupted pattern is older than the cause of disruption • Alayer cake (the pattern) has to be baked (established) before it can be sliced (the disruption) • The event that caused the “valley” probably occurred much after the all layers were deposited = since original horizontality states that they are horizontal and lateral continuity states that they taper off rather than end abruptly • Event occurred after Osoyoos Lake Limestone deposited  CORRECT ORDER • Kamloops Formation – bottommost layer is the oldest • Granite intruded afterAbottsford Formation • Granite is an “intrusive contact” • It is only through cross-cutting relationships that you can determine whether granite intruded before or after tilting of the bottom rocks. • Tilting & intrusion of the granite occurred before Thompson River formation • There must have been addition layer/rock above Thompson River Formation that is not present in the figure – erosion of these rocks suggested the way in which the dike is cut of by Okanagan Formation • So Thompson River Formation is the erosion surface = erosion took place during its deposit • THISALLTAKES PLACE UNDERWATER! • At some point, the area was raised above sea level and the stream began to carve the canyon • Last event is the carving of the valley since they truncate the youngest layers of rock • Limitations on estimating granite's age • Not sure if it's older or younger than Copper Creek = no “contact” between them • Not sure if it formed before, during, or after the titlting of the bottom layers • Principles do not apply when... • Intense deformation by tectonic forces overturn or disrupt beds so much that the principle of superposition cannot be used • Geoscientists should not be dogmatic in applying principles  Extra principle: Inclusion • Fragments included in a host rock are older than the host rock • Granite contains inclusion (xenoliths) of the tilted sedimenary rock so granite is younger than tilted rock • Rock overlying granite has granite pebbles in it, thereofore younger than granite  Unconformities  Different types of contacts: intrusive, sedimentary, and faults  Another type: unconformity  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  3 types • Disconformities • In a disconformity, the contact representing missing rock strata separateds beds that are parallel to one another • Hardest type of unconformity to detect in the field: appears to be just another sedimentary contact/bedding plane • Usually detected only by studying fossils from the beds in a sequence of sedimentary rocks • If certain fossil beds are absent, indicating that a portion of geologic time is missing from the sedimentary record, it can be inferred that a disconformity is present in the sequence • Although it's possible that rock layers are missing due to erosion, in some instances neither erosion nor deposition took place for a significant amount of geologic time. • Angular Unconformities • Is a contact in which younger strata overlie an erosion surface on tilted or folded layered rock • Usually older rocks have been subjected to folding/tilting prior to new deposition • Nonconformities • Between sedimentary and metamorphic/igenous rocks! • Occurs when sedimentary rock has been deposited above pre-existing (eroded) metamorphic or igneous rock, indicating and environmental alteration in mode of strata formation • Generally indicates deep or long-continued erosion before subsequent burial • ➢ HOW CAN ROCK UNITS BE TRACED FROM ONEAREATOANOTHER?  Correlation usually means determining time equivalency of rock units  Rock units may be correlated within a region, a continent, and even between continents  Various methods of correlation  Determining whether rocks in one area are older or younger than rocks in another area  1) Physical Continuity  Finding physical continuity is being able to trace physically the course of a rock unit  This is one way to correlate rocks between 2 diff places  FOLLOWING ROCKS UNTIL IT DISAPPEARS  This can be done at the Grand Canyon  But it is NOT GOOD if you want to correlate rock units BETWEEN REGIONS  2) Similarity of Rock Types  Correlation between two regions can be made by assuming that similar rock types in 2 regions formed at the same time  Use this with caution  Example: • Comparing two white sandstone units (Grand Canyon and Zion National Park) • Cross-bedding shows that both were once a series of sand dunes • BUT superposition shows that sandstone in Zion is younger than the Grand Canyon sandstone  Might be useful when... • You find a similarity in rock and in sequence (example: a 3 layer sequence that is exactly the same for both rock units) • Ex: Green shale on top of a red sandstone, above basalt of former lava flow that is also present in another area = conclude that the two might have formed at the same time  Evidence of these successions is what gave proof to continental drift = similar successions found in parts of SouthAmerica, Australia,Africa,Antarctica, and India.  Akey bed • a distinctive layer than 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  3) Correlation by Fossils  Fossil remains are preserved in sedimentary rock  Most of the fossil species found in rock layers are now extinct (99% are extinct)  Fossils nearer the bottom of a sequence (in older rock) are more unlike today's plants and animals than those at the top  Faunal Succession • William Smith in 19 century 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  This allowed rock layers in different places to be correlated based on their fossils  It works because there's an evolutionary history to life on earth  Any period of time in Earth's history can be identified by the species that lived at that time  Paleontologists have gone through the trouble
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