Module A.pdf

39 Pages
Unlock Document

Earth and Ocean Sciences
EOSC 326
Michael Wheeler

1. GEOLOGICAL TIME a. Age of the Earth In the western tradition, one of the earliest and most influential figures in the interpretation of geological time was James Ussher (1581 - 1665), Archbishop of Armagh. An important historical figure in his own right, Ussher also published a chronology of Earth's history using all dates mentioned in the Bible to establish a timeline. Using this technique, he established the first day of creation to be October 22, 4004 BC. This date would make the Earth a little over 6,000 years old! Figure 1.1 James Ussher (1581 - 1665), Archbishop of Armagh. Image from Wikipedia. As science continued to develop during the 1700s, people started to become dissatisfied with Ussher's estimated age for the planet. One such scientist and notable natural historian was George Louis De Buffon (1707 - 1788). Believing the Earth to have been initially as a hot molten mass, Buffon heated iron spheres (which he thought was a reasonable model for the structure of the planet) and calculated the time they took to cool. Using this method Buffon believed the Earth to be around 75,000 years old. Figure 1.2 George Louis De Buffon (1707 - 1788). Image from Wikipedia. Irish Geologist John Joly published a paper in 1899 in which he estimated the Earth's oceans (which he believed to be the same age as the planet) to be about 90 million years old. He calculated this by estimating how long it would take for the oceans to reach their current salinity (from an original fresh water state) as salt is added via the erosion of minerals in rocks. (Today we understand that the Earth's oceans have not been getting increasingly salty with time. As such, present day salinity levels cannot be used as a gauge to estimate the passage of geological time.) Later in his career Joly was to work with Ernest Rutherford using radioactive decay in minerals to estimate the age of rocks. This technique provides us with the current age of the Earth at 4.6 Ga (Giga-anum: billions of years). Figure 1.3 Ocean water. Photo by S. Sutherland. b. Deep Time 4.6 Ga is a vast amount of time, especially when the oldest recorded human being was only 122 years old when he died. It is this vast amount of time that the geologist and paleontologist must consider when trying to understand the evolution of the Earth and its biological systems. This is often referred to as the concept of deep time. Analogy is often used to help people grasp the vast tracts of time that have passed since our planet formed. If you compress all of Earth's history into one year, these are some of the significant events in our planets history: January 1: Earth accretes out of Solar disk February 1: Formation of oldest rock (preserved until today) November 15: Creatures with shells first appear December 15: Dinosaurs evolve December 26: Dinosaurs become extinct December 31, 23:59:18: end of last Glaciation December 31, 23:59:46: Birth of Christ Figure 1.4 An artist's rendition of Geological Deep Time. Image from Wikipedia. 2. GEOLOGICAL CONCEPTS AND TERMINOLOGY Like any science, geology and paleontology has its own terminology and fundamental concepts. Before we can explore the evolution of Earth through deep time, you need to become familiar with these concepts and the language we use to describe them. Rocks and Minerals As cells and tissues are some of the fundamental units to a biologist, so rocks and minerals are the fundamental units to a geologist. A mineral can be defined as "a naturally occurring crystalline solid with a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties." Figure 1.5 Quartz (left) and feldspar (right) are the most common minerals in the Earth's crust. Image from Wikipedia. A rock on the other hand is an aggregate of minerals. Rocks fall into a basic three fold classification: a. Igneous; b. Metamorphic; and c. Sedimentary a. Igneous Rocks Igneous [audio] rocks (derived from the Latin "ignis" meaning fire) are rocks that have formed by the cooling of magma or lava. Igneous rocks are generally composed of interlocking crystals of varying sizes. Figure 1.6 An image of granite [audio], an igneous rock composed of the minerals quartz, feldspar, and mica, as seen from under a microscope. Image from Wikipedia. If a magma cools within the Earth's crust it is referred to as an intrusive [audio] igneous rock. This term is derived from the manner in which magmas physically work their way through or ‘intrude' into the surrounding rock as they rise from their point of generation. As they cool slowly they often develop large crystals (like you commonly see in kitchen counter tops). If a magma escapes via volcanic activity and forms a lava the igneous rock is called an extrusive [audio] igneous rock. These rocks cool quickly and so the crystals that form are very small and often invisible to the human eye. Another form of extrusive igneous rock, volcanic ash, often forms as magma fragments explosively during a volcanic eruption. Figure 1.7 Igneous rock types and examples. Images from Wikipedia and the US Geological Survey. b. Metamorphic Rocks Metamorphic [audio] rocks form as the result of the transformation of an existing rock via heat, pressure and/or the action of fluids. Such conditions commonly occur deep in the Earth's crust. The original rock could be igneous, sedimentary, or an older metamorphic rock. The study of metamorphic rocks can help provide information about the processes that occur deep in the sub surface. Figure 1.8 A gneiss [audio], such as this outcrop found near Geirangerfjord, Norway, is a metamorphic rock that forms at high temperatures and pressures deep in the Earth's crust. It is exposed here at the surface following millions of years of erosion of the rock that originally covered it. Image from Wikipedia. c. Sedimentary Rocks Sedimentary [audio] rocks are probably the most useful rocks for the determination of Earth's ancient climate and biology. Sediments form in response to particular environmental conditions and as such, provide clues to Earth's past including climate, ancient geography, and life forms. Sedimentary rocks form via the sedimentation (or deposition) of materials at the Earth's surface and within water bodies, commonly in layers sometimes called strata[audio] or sedimentary beds. Sedimentary rocks can be generated by physical, chemical, and/or biological processes. Diagenesis [audio] is a term used to refer to the transformation of sediments into a sedimentary rock via a collective variety of chemical, physical, and sometimes biological processes. Diagenesis commonly occurs as sediments are buried below successively younger strata. Figure 1.9 Layers of red sandstone, a sedimentary rock, found in Lower Antelope Canyon, Arizona. Image from Wikipedia. d. Clastic Sedimentary Rocks Most sedimentary rocks are produced by the erosion of pre-existing rocks producing fragments or grains that are transported and deposited at various distances from the site of erosion. Such rocks are called clastic [audio] sedimentary rocks. Clastic sediments can be deposited anywhere they can settle out of by gravity from the media that is transporting them (water or air). The largest, but by no means exclusive, depositional area (or basin) for sediments are the oceans. Figure 1.10 Examples of environments where sediments are being deposited. Click on 'Zoom-in' for a more detailed view. Background image by G. Lascu, foreground image from the US Geological Survey. Part of the aim of this course is to provide some insight into how we can interpret ancient processes and environments. Clastic sedimentary rocks provide a simple tool that can be used to determine the source, origin, and length of transport of a particular sediment prior to it becoming a sedimentary rock. The composition of the grains or the larger fragments (clasts [ audio]) of a sediment can help in the determination of the original source of that sediment and perhaps the pattern of ancient drainage and river systems. If a sediment is deposited close to the rocks from which it was originally eroded, it will have number of characteristics that will uniquely identify it as such. It would probably contain a large number of coarse/angular grains and clasts (greater than 4 mm) as the process of transport has not had sufficient time to fragment and round off the particles in the sediment. The sediment would also tend to have a higher proportion of unstable minerals and fragments of rock. Quartz is the most stable mineral at surface temperatures and pressures. Other minerals eroded from older rocks are destroyed with long-term transport. In addition, the rock would generally exhibit poor sorting. Sorting refers to the variation in clast/grain sizes in a sediment, which is often large in sediments that have not traveled far from their original source. A sediment with characteristics as described above is called an immature sediment. Given this, a clastic sediment that has undergone extensive transport (for example down a long river system or blown around a desert for many years or washed continually back and forth across a shallow ocean) will tend to be fine grained and composed mostly of well-rounded and well-sorted quartz grains. A sediment with these characteristics is called a mature sediment. Figure 1.11 Correlating sediment maturity and transport distance. Images from Wikipedia and Northern Virginia Community College. e. Calcium Carbonate Although clastic sedimentary rocks form the majority of the sedimentary rocks on the Earth's surface, biologically precipitated sediments are also an important source of sedimentary rocks. The most obvious is the precipitation of calcium carbonate by various creatures including corals and mollusks to form thick deposits of limestone. Figure 1.12 Satellite picture of the Atafu coral atoll in Tokelau in the Pacific Ocean and a fossil coral (inset). Images from Wikipedia. Equally important are microplankton calcium carbonate producers such as coccolithophores. During the Cretaceous (145 - 65 Mya, million years ago), warm shallow tropical oceans covered much of the Earth's continents providing perfect conditions for the proliferation of coccolithophores and ultimately the generation of vast thicknesses of the fine chalk (a fine grained limestone). Figure 1.13 The famed White Cliffs of Dover in SE England is almost 100% coccolithophore-produced chalk, another type of calcium carbonate rock. Inset: a coccolithophore, about 2 µm (micrometres or microns) in diameter. Images from Wikipedia. f. Evaporites In addition to clastic and biologically precipitated sediments, the intense evaporation of water can also precipitate sediments in the form of salt crystals. Rocks formed in this manner are called evaporites [audio]. Evaporites can form in a number of ways including the evaporation of an inland sea or part of an ocean with restricted contact to the wider ocean. This may occur in arid areas with limited freshwater input. As the salinity of the water increases, crystals of evaporite minerals such as halite [ audio] (table salt) and gypsum [audio] crystallize out of the water body and settle in layers on the ocean floor. Evaporite formation can occur on a vast scale. The Mediterranean Sea for example, has on occasion been closed off at the Strait of Gibraltar and the entire sea has evaporated away. This accounts for the thick deposits of salt that can be found just below the ocean floor of the Mediterranean. Figure 1.14 (top) The process of evaporite formation in a shallow ocean basin, such as the Mediterranean Sea. (bottom) Salts precipitating around the edge of the Dead Sea, the deepest hypersaline lake in the world. Images from Wikipedia. 3. STRATIGRAPHY Now that we have a basic understanding of rocks and minerals we can now consider how geological features (and in particular sedimentary rocks) are arranged in the real world. This is the science of Stratigraphy [audio]. Stratigraphy studies how rock layers (beds or strata) are arranged and is a key concept in the interpretation of the passage of geological time. We will examine 4 Basic Principles of Stratigraphy: a. Principle of Superposition b. Principle of Original Horizontality c. Principle of Lateral Continuity d. Cross-Cutting Relationships The first three of these principles were outlined by Nicholas Steno, a Danish anatomist and geologist, in the late 1600s. Figure 1.15 Nicholas Steno (1638 - 1686), a Danish pioneer in anatomy and geology. Image from Wikipedia. a. Principle of Superposition The Principle of Superposition states that in layered strata (sedimentary rocks or lava flows), the oldest layer will be at the bottom of the exposed strata and the youngest at the top. Effectively " what's on top is youngest”. Although this might appear to be somewhat obvious it is important to remember that this principle was being introduced to a world that believed that everything around us had been created over a period of days. To suggest that a vast pile of strata had not simply been called into existence but had developed over a very long period of time was a new paradigm. Figure 1.16 Strata in Isfjorden, Svalbard, Norway. Image from Wikipedia. b. Principle of Original Horizontality According to the Principle of Original Horizontality , sediments are deposited horizontally. After they have been transformed into rock it is possible for these strata to become tilted by various tectonic movements. Basically, " tilted or folded layers used to be flat”. Figure 1.17 Dipping (tilted) chalk strata, Cyprus. Image from Wikipedia. c. Principle of Lateral Continuity The Principle of Lateral Continuity states that a stratum (layer) of sedimentary rock will continue in all directions until it thins, grades into another type of sediment, or comes against the edge of the depression into which it is being deposited. Figure 1.18 A schematic drawing that shows an application of the Principle of Lateral Continuity in assessing the evolution of the present (bottom) from its origin (top). d. Cross-cutting Relationships The principle of Cross-cutting Relationshipswas developed by James Hutton, a Scottish geologist, physician, chemist, and experimental farmer. Hutton believed that the relationships between stratigraphic units indicated that Earth was very old. The famous geological section that Hutton used to demonstrate this is Siccar Point in Scotland. We will discuss this next. e. James Hutton at Siccar Point, Scotland At Siccar Point, Scotland you can see vertical sediments cross-cutby an erosion surface called an unconformity [audio]. On top of this surface younger sediments have been deposited. See figure below. Hutton realized that this relationship of stratigraphic units could have only developed as a result of Earth processes operating over many millions of years. The character of the rocks on either side of the unconformity also speak to the vast changes in the environment that can occur over any one given point on the Earth's surface. At Siccar Point, the rocks below the unconformity were originally deposited in a deep ocean but the rocks now on top of them were deposited in a hot desert on land! Figure 1.20 Siccar Point, Scotland, where James Hutton and his colleagues found strong evidence that confirmed his ideas about unconformities and the relationship between the rock layers. Image from Wikipedia , annotations by S. Sutherland. With knowledge of the 4 basic stratigraphic principles, we can now unravel the history of a geological section such as Siccar Point (refer to figure below): i. Sediments below the unconformity originally deposited horizontally in a deep ocean (Principle of Original Horizontality); ii. Sediments buried, transformed into rock via diagenesis, tilted, and folded by tectonic activity; iii. Rocks uplifted by tectonic activity and exposed at the surface by erosion producing an erosion surface; iv. Desert sediments deposited horizontally on top of the erosion surface producing the unconformity (Cross-cutting Relationships and Principle of Superposition); v. Section buried again and desert sediments transformed into rock via diagenesis; vi. Section tilted again by tectonic activity; vii. Section uplifted by tectonic activity and exposed at the surface by erosion. Figure 1.21 The "geological history" of Siccar Point, Scotland. f. Unconformities Unconformities represent periods of non-deposition of sediment or active erosion of strata. They help us appreciate that the geological record in any one location is NOT complete but contains gaps. Unconformities may represent important periods of activity in Earth history such as mountain building events where strata are being actively uplifted and eroded. Figure 1.22 Schematic of selected types of unconformity. Several types of unconformity are recognized (refer to figure above): i. Disconformity: exists where the layers above and below an erosional boundary have the same orientation ii. Nonconformity: develops where sediments are deposited on top of an eroded surface of igneous or metamorphic rocks iii. Paraconformity: strata on either side of the unconformity are parallel, there is little apparent erosion iv. Angular unconformity: strata is deposited on tilted and eroded layers (such as at Siccar Point) Here are some examples of unconformities from around the world. Figure 1.23 Taum Sauk Precambrian - Cambrian nonconformity indicating a 1 Ga gap in the geological record. Image fromWikipedia, annotations by S. Sutherland. Figure 1.24 Permian - Cretaceous disconformity, Texas showing a 165 Ma (million years) gap in the geological record. Image from Wikipedia, annotations by S. Sutherland. Figure 1.25 Angular unconformity between the folded Entrada Formation (Middle Jurassic) and overlying Morrison Formation (Late Jurassic) strata, Utah. Image copyright © by Thomas McGuire, made available via the Earth Science World Image Bank. Unconformities are just one type of cross-cutting relationship. Intrusive igneous rocks may also demonstrate cross-cutting relationships in a geological outcrop indicating how they physically intruded into pre-existing strata. Figure 1.26 Igneous dike cross-cutting horizontal strata, Grand Canyon, Arizona. Faults (planes along which movement have occurred) also demonstrate cross-cutting relationships with the fault disrupting the strata it cuts through. Figure 1.27 Faulted Rocks, Guatemala. Photo courtesy of the Geology and Geophysics Science Centre, US Geological Survey . h. Relative Dating Working out the history of a geological section in the manner above is called relative dating. Relative dating in this instance does not provide us with any information of the actual age of this section (for example, in millions of years). Rather it provides us with the relative sequence of events that led to its formation. Relative dating can be used to provide a roughguide as to the amount of geological time that may have passed. For example: if in general, muddy sediments accumulate at a rate of 10 m (metres) per million years, then it can be surmised that 100 m of mudstone will take 10 million years to accumulate. Obviously there are inherent errors with this estimate: • The rate of accumulation of sediment might not be constant; • Periods of non-deposition or active erosion may have occurred; • During diagenesis, sediments are compressed and compacted so that the vertical extent of sedimentary rocks may not represent the original vertical extent of sediment. Although this method of calculating geological time would be useless for relatively most outcrops of stratigraphy, the errors tend to average themselves out with large packages of sediment and may provide a very rough estimate of geological time. Although more accurate or "absolute" dates for Earth time would have to wait for the development of our understanding of radioactive [audio] or radiometric [audio] dating, the stratigraphic principles detailed in this lesson can be used extensively in the unraveling of the Earth's geological history. Figure 1.29 The process of deducing the history of a succession of sediments, such as that shown above, using relative dating, would be a very complex task. Photo courtesy of Ian West, as published in "Geology of the Wessex Coast of Southern England". i. Uniformitarianism With their understanding of stratigraphy, geologists in the late 1700s started to get the impression that the Earth was much older than Ussher's estimate of 6,000 years. Geologists started to see the world through the eyes of James Hutton who believed that the Earth's history as recorded in the rocks could be interpreted by observing the slow methodical process of erosion and deposition that we see occurring around us every day. In short, the statement " The present is the key to the (understanding of the) past", a summary of Hutton's Uniformitarianism as it became known, formed the basis for the understanding of the Earth and of the science of modern geology. Uniformitarianism [audio] is neatly summarized by the rock cycle (see below), which describes the slow dynamic transitions through geological time of the three main categories of rock. Note however, that the rock cycle does not imply that every rock must pass through every stage represented in the diagram. Figure 1.30 The Rock Cycle describes the slow dynamic transitions through geological time of the three main categories of rock. Figure by G. Lascu. Today the term actualism is commonly used, a term that acknowledges that although most geological processes (like the uplift or erosion of mountains) are very slow, some geological processes (such as volcanic eruptions or the impacts of meteorites) can cause changes that are relatively geologically instantaneous. Figure 1.31 Charles Lyell (1797 - 1875 ), an influential geologist and close friend of Charles Darwin. His insights into cross-cutting relationships and geological time were extremely important in our modern understanding of Earth history. Image from Wikipedia. 1. SEDIMENTARY FACIES Figure 2.1 The Badlands-Drumheller, Alberta. Image from Wikipedia. Different types of sediments are being deposited in different areas due to differing conditions of deposition. Areas where sediments are being deposited are often called sedimentary environments. Sedimentary environments can be found all over the surface of the planet including in high mountains, along river plains and lakes, in hot arid deserts, along coast lines, and in the deep ocean. Each of these sedimentary environments will be characterized in the geological record by particular groups and association of sediments. a. Near Shore Sedimentary Environment Figure 2.2 A simplified diagrammatic representation of a near shore sedimentary environment. Figure by S. Sutherland. Consider an example of a near shore sedimentary environment where a river meets the ocean and starts to deposit its sediment load. The coarsest (most dense) portion of the sediment from the river is deposited close to the shoreline where the water is the most turbulent. The finer (least dense) material stays in suspension until it settles out of the water where conditions are less turbulent ("quieter"). Beyond the area where all the material from the river has been deposited, biological processes by calcium carbonate-secreting organisms provide the main sediment type: carbonate mud. This accounts for the changing lateral character of the sediments in this environment from coarse sandy material close to the shore line passing into mud/clay and ultimately carbonate mud further off shore. Figure 2.3 Figure by S. Sutherland. Inset image of coccolithophore from Wikipedia. In the cross section of a near shore environment (see figure above) we see the same pattern of deposition from coarse to fine clastic sediments and eventually carbonates, once all the clastic material has settled out of the water column. As you can see, changing environmental conditions (particularly the turbidity of water in this case) across this sedimentary environment are controlling the character of the sediments that are being deposited laterally. By understanding the controls on sediment deposition, a geologist examining sedimentary rocks can interpret and reconstruct ancient sedimentary environments and so build a picture of the planet as it existed many millions of years ago. These environmentally controlled sediment differences are called facies [audio]. A facies refers to all of the characteristics that can be used to define a particular sedimentary unit. These characteristics are: sediment type (e.g. sandstone, mudstone, limestone), presence of any sedimentary structures (example: ripples, cross bedding), and sometimes the fossil content. Since depositional environments grade laterally into other environment, facies changes are gradual. Facies are given names that attempt to represent the defining characteristics of a particular unit. For example, the facies pictured below could be described as fossiliferous mudstone facies (1), cross-bedded sandstone facies (2), and rippled siltstone facies (3). Figure 2.4 Examples of sedimentary facies. Images from Wikipedia. b. The Temporal (time) - Sea Level Component The naming model discussed previously however, gets a little more complicated when we have to consider geological time. This is because over time, the distribution of sedimentary facies is controlled by changes in sea level. As shown in the plot below, sea level has changed globally many times – it is not just a recent phenomenon. It is also important to remember that sea level can change locally as well as globally. Figure 2.5 Global sea level change over the past 542 million years. Image created by Robert A. Rohde, Global Warming Art. Scenario 1: No Sea Level Change Let's take the same simplified model of a nearshore environment shown earlier, and try to determine what would happen to the pattern of facies we would record as sea level changes over time. Consider a first case scenario where the rate that sediments are deposited keeps pace with changes in sea level. This means that facies get built on top of each other, producing a pattern where facies boundaries and the location of the shoreline will not change location over time. Figure 2.6 Stacking of sediments with no relative change in sea level. Figure by S. Sutherland. If we were to extract a core of sediments from the area at A' – A (see figure above), a sedimentary core would contain the same facies from the bottom to the top of the core. As the sedimentary facies are the same over time from the bottom of the core (oldest) to the top of the core (youngest), then the environmental conditions (sea level) must have been the same over time. A diagram of the core is shown below. Figure 2.7 Core of sediment taken from A' – A in previous diagram for the case where the rate that sediments are deposited keeps pace with changes in sea level. Figure by S. Sutherland. Scenario 2: Rising Sea Level - Transgression Now, let us consider what would happen if a near shore sedimentary environment experiences a rise in sea level. At a certain Time T1, sediments are deposited into the near shore as shown in the figure below. Figure 2.8 Facies changes with increasing sea level at Time 1 (T1). Figure by S. Sutherland. Now with a rise in sea level between Time 1 and Time 2, the shore line moves back towards the land. A new package of sediment (between T1 and T2 on the diagram below) has been deposited. Note that the boundaries between the facies also get pushed back towards the land as they follow the changing environmental conditions. This movement of the ocean onto the land in this manner is called a marine transgression [audio]; facies move towards the land. Figure 2.9 Facies changes with increasing sea level at Time 2 (T2). Figure by S. Sutherland. Sea level continues to rise with the deposition of the sediment between T2 and T3 on the diagram below. The shore line and the boundaries between facies continue to move towards the land. Figure 2.10 Facies changes with increasing sea level at Time 3 (T3). Figure by S. Sutherland. Sea level continues to rise with the deposition of sediments between T3 and T4 (figure below). We can now see that facies are describing a particular pattern in response to sea level change. Figure 2.11 Facies changes with increasing sea level at Time 4 (T4). Figure by S. Sutherland. NOTE: if we were to map the same sediment type (i.e. facies; yellow line in figure above) we would becross-cuttingthe Time lines (red lines at T1, T2, T3, T4) and would not be following the same time plane. In effect, we would be mapping the change in environmental conditions over time as demonstrated by the facies. Because of this characteristic, facies are often referred to as diachronous (or time crossing). If we were to follow the same time horizon (for example the package of sediment deposited between times T3 and T4), we would not remain in the same facies as we move laterally from the shore line but would pass into differing facies representing sediments deposited at the same time but under differing environmental conditions. These concepts are extremely important to consider when attempting to correlate rocks of the same geological age across large distances. This will be covered in the next lesson. Figure 2.12 Core of sediment taken from A' – A in previous diagram for the case where the near shore sedimentary environment experiences a rise in sea level. Figure by S. Sutherland. Now if we were to extract a core of sediment from A' – A in the previous diagram (see figure above), it would have recorded changes in facies vertically. This is because over time, facies have been migrating in space over this location in response to sea level change. Note that the bottom part of the core (older part) has near shore sediments while the top part (younger part) has offshore carbonate muds. This means that over time, sea level must have beenincreasing and a transgression must have occurred. Scenario 3: Falling Sea Level - Regression In the case when sea level falls over time, a different pattern of facies will occur. Shoreline and facies move away from the land – this is called a marineregression [audio]. Figure 2.13 Facies changes recording sea level fall. Figure by S. Sutherland. If we were to extract a core of sediment from A' – A in the previous diagram, it would have recorded a vertical pattern of sediments that is the opposite of that recorded for a marine transgression. The core (see figure below) goes from offshore carbonate muds at the bottom to near shore sediments at the top, which means that over time,sea level must have been decreasing and a regression must have occurred. Figure 2.14 Core of sediment taken from A' – A in previous diagram for the case where the near shore sedimentary environment experiences a fall in sea level. Figure by S. Sutherland. d. Summary Figure 2.16 Facies changes during marine transgression and regression. The figure above summarizes the concepts we just learned in this section: i. Facies are distributed based on the changing conditions of deposition over the surface of our planet. In the example discussed here, shallow high energy, near shore conditions are characterized by sand while deeper, quieter conditions are characterized by finer grained sediments. ii. As conditions change (such as sea level) facies will appear to migrate, "following" the particular environmental conditions under which they are deposited. iii. Over time this leads to patterns of facies as one facies migrates over another. This means that when we recover a vertical succession of rock like the one we used in our example we can deduce a couple of things... • As we go from coarse, near-shore facies at the bottom of the core to off-shore, deeper water carbonate sediments at the top of the core, sea level (in this example) must have been rising over time. • The facies that we are looking at in a vertical sense have lateral equivalents that were deposited in different environments but at the same time as the observed core. With this information (and if we have access to more than one core) we can predict lateral changes in facies from a vertical core of rock and test our prediction by finding their lateral equivalents. Figure 2.17 Lateral changes in facies. The uneven boundaries between the facies are an attempt to demonstrate that facies gradeinto each other just as environments grade into each other. Figure by S. Sutherland. All this is summed up in Walther's Law: "Facies that occur in a conformable (i.e. demonstrating no unconformities) vertical succession of strata were deposited in laterally adjacent depositional environments." in other words, "Adjacent sedimentary environments (facies) will end up overlapping one another over time". Figure 2.18 Facies changes in a near shore sedimentary environment. It is important to realize that changing sea level will not just control the distribution of sediments in a near shore environment. Changing sea level and the changing position of shore lines will have a major effect on sedimentary facies from the tops of the mountains to the deepest oceans. Consider times of very high sea level as existed during the Cretace
More Less

Related notes for EOSC 326

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.