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Geol 102

Graphic Correlation Assignment

Tim Simmons

The first step toward correlating the three sections was selecting an initial standard reference section. The Morgan Creek section was chosen for this because it had, on average, a higher number of fossil occurrences for the six species that were examined. The first and last occurrences of the six species from this section (x axis) were plotted against the first and last occurrences of the same species from the White Creek section (y axis). The product was figure 1.

Figure 2 is the same graph as figure 1, but it has a best fit line through the data, called the line of correlation. The diamond shaped points are first occurrences, while the squares are last occurrences. First occurrences which plot to the right of the line of correlation indicate that this species is found in older rocks in the White Creek section than in the Morgan Creek section. An example of this is the fossil of species Kingstonia pontotocensis (diamond #4). In order to time correlate this fossil to the Morgan Creek section, the point is carried over horizontally to the line of correlation from which the appropriate x value (point in the Morgan Creek section) was found. In other words, the first occurrence of this fossil was at a point in time that correlates to 133 m from the base of the Morgan Creek section, but due to poor preservation conditions, erosion, or some other geologic phenomena, the fossil is actually found at 138 m from the base of the Morgan Creek section. When a last occurrence plots to the left of the line of correlation, this means that the fossil occurs in younger rocks in the White creek section than in the Morgan Creek section. Kingstonia pontotocensis is the example again. To correlate this to the appropriate height in the Morgan Creek section, the data point is moved horizontally to the line of correlation, and from a vertical line to the x axis one can see that the point need be adjusted from 145m to 153m in the Morgan Creek section. By using this correlation method, a theoretical section is created, by combining first and last occurrences from both sections. It is important to note that this section is not found in nature; it is simply an idea which represents what a section would look like if fossil preservation problems did not exist, and all six fossil ranges were accurately represented in the rock record.

Next, this theoretical section, now called a composite standard reference section, was used to correlate the James River section to the Morgan Creek and White Creek sections. Another graph was created in a similar fashion. The composite standard reference section was used as the x axis, and the James River section as the y axis. As can be seen from figure