Graphic Correlation Assignment
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 3, the data points are more scattered and lie further from the line of correlation. Data points such as the first occurrence of Kingstonia pontotocensis (diamond 4), and the last occurrence of Kormagnostus simplex (square 1), which lie far from the line of correlation, imply that these fossils in the James River section are not representative of their total known stratagraphic range. Data points like diamond 2 and square 4 are on the wrong side of the line of correlation, so they had to be adjusted using horizontal shifts to the line of correlation, in the same manner as in considering the Morgan Creek and White Creek correlations. It is not surprising that the James River stratigraphy was less similar to the other two sections considered, because by simply looking at the amount of rock that the six fossils are found in at the James River section, one notices that there is much less of it, which probably represents poor preservation of these sedimentary rocks and fossils at that location.
Using the graphic correlation method, our team was able to develop a scale that has definite boundaries. To establish this scale we designated equidistant points along the CSRS, called composite standard time units (CSTU). These CSTU were projected into the James River section and White Creek section and compared to the CSRS. The CSRS has been divided into units 20 meters