Geology 1203 Earth History Assignment: stratigraphic correlation Understanding the history of the Earth involves determining the sequence of geologic events which occurred over immense spans of time. The geological time scale that we use today was first developed in the 1800’s and did not really extend much beyond the Cambrian Period; however it is constantly being revised as our understanding improves. Era Period Cenozoic Quaternary Teritary Mesozoic Cretaceous Jurassic Triassic Permian Geologists deal with time in two different ways: Relative Time - the occurrence of events is determined relative to one another (from oldest to most recent). This can be a local series of events or be applied to global events. The eras and periods of the geological time column were established using relative time (mostly based upon the fossil content). Carboniferous Paleozoic Devonian Silurian Ordovician Cambrian Precambrian Absolute Time - The discovery of radiometric dating allowed geologists to assign a numerical age to the formation of certain types of rocks; fixing how old they actually were in billions (Ga), millions (Ma) or thousands (Ka) of years before the present. Hence the term “absolute” time. These two ways of looking at time are not unique to geologists. You use them everyday. If you agree to meet a friend “after class” - that is a relative time. The time of day when you actually meet is dependent upon when your class is scheduled and when it is finished. If instead you arrange to meet at 1:00pm -that is absolute time. Part A: Creating A Personal Time Column In this exercise you are going to create a time column of your own. Begin by creating a relative time column with key events. Then you will assign years to these events fixing them in absolute time. Remember that when ordering events you must start with the oldest on the bottom (when listing vertical) or on the left (when listing horizontally). It is also good practice to indicate the oldest and youngest events (this will help you to avoid putting them in the wrong order). To give you an idea how to proceed I have provided a template with the starting and ending events you should use (on the last two pages of this handout). You may include whatever events you wish in between but you must have at least ten more events. Part B: Stratigraphic columns In the previous section you created a very simplified column - the list of events on the left were organized based upon relative time, the list of dates on the right fix the events in absolute time. Geologists use the principles of relative time to order a sequence of strata (or beds) to produce what is called a stratigraphic column for a particular area. Stratigraphic sections A stratigraphic section is a detailed record of the rocks exposed at one particularly location. Data about the type of rock, structures, fossils, and thicknesses are compiled by a stratigrapher. Information from several sections is added to make a stratigraphic column. This column represents a composite record of all the strata occurring in a particular area. According to The Principle of Superposition the oldest beds are at the base and the youngest at the top. 50 metres youngest a b cliff a c b p to d c f e h g A d e C f g a d b B f e c f d 1 h g oldest h stratigraphic column g 2 3 For the example above we have 3 sections from different locations or exposures along a cliff (Figure A ). For simplicity let us assume that they are relatively undeformed strata (or beds) and have only been tilted and not completely overturned. The upper surface of the beds (the “way up”) is indicated by the small arrow on bed a). The location of our stratigrpahic sectiosn is indicated by the red columns. Next look at Figure B In section 1 we can see that bed d is older than bed c which is in turn older than bed b. In section 2 we can see that bed e is older than bed d and bed f is older than e and so on. Similarly for section 3 we can deduce that bed h is older than bed g. So to construct our composite column we try to match similar beds across the sections. Lets say that we match bed d in section 1 with bed d in section 2. We can now say that bed g is older than f which is older than e and so on. If we then correlate bed g in section 2 with bed g in section 3 we can then say that bed h is older that bed g which is older than bed e. Putting it all together we have our composite stratigraphic column from oldest to youngest (Figure C ). Notice that the oldest is at the bottom. Stratigraphic columns come in many different styles from the very simple, like above, to very complex ones that display not only relative ages of the strata but also the thickness of the beds, type of rock, the size of the grains, and any structures or fossils. Part C: Correlation The Principle of Lateral Continuity states that rock beds can be traced across a wide area and matching strata at one locality with strata at another is the process of correlation. Several (or more) stratigraphic columns can therefore be correlated to give a picture of what changes occur in the rocks horizontally. All this information allows geologists to reconstruct the geologic history of an area. There are, however, several types of correlation. Lithological Correlation is the matching of the strata based upon their composition and other features such as structures, colour and thickness. This primarily matches strata that were deposited under the same conditions. Chronological Correlation is matching rocks that were deposited at the same time. Radiometric dating (this is absolute time) can only be used to a limited extent because it has a fairly large margin of error (several million years). Most geological events usually occur over a long period of time, but there are occasional events that occur almost instantaneously. Volcanic eruptions are one type, and volcanic ash is often a good time marker. Perhaps the best know time marker is the iridium layer that marks the Cretaceous-Tertiary boundary. This records the asteroid impact that is now believed to be associated with the extinction of the dinosaurs. Biological Correlation uses fossils to establish age equivalency in strata. Fossils are not more accurate that volcanic ash, but they are more common. Fossils used to correlate strata are called index fossils, but not just any fossil will do; an index fossil has to be distinctive, abundant, with a wide geographic distribution, and a short time range. Trilobites are a good example. Correlating strata A stratigrapher will place several stratigraphic columns side by side and draw lines between the columns indicating the lateral continuation of the different strata. The lines are drawn indicating those points in the columns that are similar - this is usually at the boundary where there is a change in the type of rock, or perhaps the size of the grains, or some other structure. In the example below the solid lines mark the boundaries between rock types (as indicate by colour and patterns). Look closely at the columns - make sure you understand what the correlation is showing you. The very top and bottom of the columns are not connected with lines because usually a column will only show what was exposed, or was recorded stratigraphic column a stratigraphic column b solid lines indicate that the boundary between beds are similar in both columns unconformities are indicated with a wavy line where the bed is missing the lines automatically point to an unconformity a dashed line indicates that there is not enough information to determine if there is a corresponding bed in the other column The figure above shows two simple columns for which I have drawn lines correlating the different beds. You can see in one column where erosion has occurred creating an unconformity (indicated by the wavy line). You cannot tell by looking at this column alone how much has been removed. We can tell that the limestone (grey) unit is missing, and so might be some of the older bed below. Remember when drawing the lines you are connecting surfaces (often bedding planes) and so these lines should connect strata that are similar. Done correctly you should be able to colour the area between the lines so that you can see the lateral continuation of the strata. Biological Correlation Strata will normally vary in thickness horizontally and may naturally thin or pinch as they reach the edges of their depositional area, or there may be changes in the composition or texture (for example an increase in the size of sand grains). So some beds may not appear in all columns or sometimes there will not be enough similarities to match the beds across column - this is where fossils come in handy. Biological Correlation uses fossils to establish age equivalency (the sediment was deposited around the same time) in strata. Index fossils have a short time range and a wide geographic distribution. Even though the beds may not be identical if they have the same index fossils it means they were deposited around the same time. Biological correlation uses index fossils and the Principle of Faunal Succession, along with lithological to correlate beds. Look a the example below and make sure you understand how the presence of fossils are used to match the strata in each column with the equivalent stratum in the second column. stratigraphic column a the presence of this index fossil indicates that these beds were deposited around the time, and so are of the same age rather than the bed higher up in the column with a different index fossil stratigraphic column b same age where the bed is missing the lines automatically point to an unconformity same age notice that there is no line at the bottom - because you do not know if there are more strata below this point What do I submit? Print out the last two pages of the assignment and complete the exercises on them. Make sure you put your name and student number on the sheets. Photograph or scan the completed exercises. Submit the images to the dropbox folder on the course site. You should check that the images are clear and readable. Name: Student # Exercise 1 Create a time column of your own. Begin by creating a relative time column with key events. Then you will assign years to these events fixing them in absolute time. Remember that when ordering events you must start with the oldest on the bottom (when listing vertical) or on the left (when listing horizontally). Year you enrolled in GEOL 1203 2018 Youngest relative time absolute time Event Ý Oldest you were born ? Exercise 2 Using the principle of superposition, look at the sections below and compose a stratigraphic column. List the units by their letter, from oldest to youngest (be sure to indicate in this in your answer). Assume that the Priciple of Superpostion applies. Remember you are working with the units with the letters ( which represent different types of rocks) and not the sections. YOUNGEST C K K J T M P S J C T J D P M T S M D D OLDEST Name: Student # Exercise 3 The figure below is a set of three fairly simple columns. Draw lines between the columns indicating the lateral continuation of the different strata. Remember to use solid line where the strata are similar and dashed lines where there is not enough information to determine if there is a similar stratum. Mark any location where a bed is missing (an unconformity) with a wavy line. In this correlation we are applying the Principles of Superposition and Lateral Continuity. a b c lithology limestone shale sandstone mudstone unconformity Exercise 4 In the figure below you are give three columns. Using lithology and index fossils to correlate the columns. Remember to use solid lines where the strata are similar and dashed lines where there is not enough information to determine if there is a similar stratum. Mark any location where a strata is missing (an unconformity) with a wavy line. column b column a column c lithology index fossils limestone shale sandstone conglomerate unconformity 
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