Sarah Beasley, Jim Foley and Pete Rivizzigno
Geology 405
Dr. Hays Cummins
18 December 1997
Abstract
Oceanic currents are a major influence on marine life in the world today as well as a factor in influencing life in the past. Currents have an effect on where and how a species lives and can have effects on the climate. During the Ordovician, Ohio was covered by a shallow sea that left an abundance of marine fossils. By collecting slabs of fossiliferous rocks and noting the orientation of the shells one can determine the type of current found in the depostional environment. During the Liberty formation at the Brookville Lake Causeway Roadcut there was no significant current recorded in the rock record, yet this data still gave significant information about the time period.
Introduction
During the Ordovician, Ohio was covered by a shallow sea, and was located a little South of the equator. This period left behind fossiliferous limestone and shale rocks in Southwestern Ohio and surrounding areas. The sedimentary environment during the time was a shallow shelf occupied by diverse and abundant fauna, with associated terrigenous input from the Appalachian Mountains(Hay et al. 1981). In shallow water areas, paleocurrents may be influenced by runoff from land, by wind, by tides, and by ocean currents(Schopf 1980). Benthic communities at this time would have been effected by prehistoric currents and tides. According to research done by Ross(1975), currents had an effect on trilobite distribution. Krawinkel(1995) also found that, Fossil concentrations can be grouped into fossil concentrations produced by non-turbulent shelf currents. The currents were also shaped by the benthic communities themselves. In denser communities where growth was abundant, a wave-current baffle was produced(Harris and Martin, 1979). The relationships among current strength, particle size, and sediment deposition and erosion have been widely studied. For any given particle size, there is a characteristic settling velocity(Schopf 1980). Obviously, if the settling velocity of a particle through water is faster than the velocity of the water itself, deposition should occur, and once it does occur, material is much more difficult to erode than it is to transport.
Methods
The site that we sampled was part of the Liberty formation at the Brookville Lake causeway roadcut. We selected a point at random along the road and designated this to be location A. We determined with a car’s odometer that point A was 483m (.3 of a mile) from the parking lot at the causeway. From point A we set up 3 more points, B,C, and D along the road at 30m intervals. From each of these points we set up transects within the formation. We collected the closest slabs to points 1m, 15m and 30m up each (A,B,C, or D) transect. We designated the slabs at the 1m interval to be "lower", slabs at the 15m interval were designated as "middle" and slabs at 30m were designated as "upper." We selected slabs that we still in their original strata and before removal we labeled their orientation to North using a compass and an indelible marker. We then carefully removed the slabs and returned them to the lab for further analysis. After cleaning each slab we selected a 15cm x 15cm area on each slab to sample from. Within the 15 by 15cm square we set up a grid at 5 cm intervals within the square to ease our data collection. For each brachiopod we measured it’s size, orientation from North, angle of repose, and noted weather the sample was whole or a fragment, and weather the shell was concave up or down. For size, we measured the longest part of the specimen. To determine the orientation from North, we designated a line that extended along the dorsal ventral axis to be the 000 or North orientation. If this axis was not aligned with north, we measured the angle it was displaced from north, from 0 to 360 degrees. We measured angle of repose in 9 categories, from 1 (0 to 10 degrees) being flat to 9 (81 to 90 degrees) being vertical. We noted if the specimen was whole or a fragment as well as if it was concave up or down. All of are observations were entered into a single data sheet which we then analyzed using the programs Statview and SuperANOVA. We used these programs to test the statistical significance of the data with the P-test and to create graphical representations of what we found.
Discussion
In doing this experiment we expected to find discernible patterns in size, angle of repose, direction from north, and concavity among the brachiopod specimens we sampled throughout the liberty formation. Our data, however, did not support any such conclusions. By plotting the slab and location by size and computing its statistical summary in figures 1 and 1A we found our P-value to be .0001. This indicates a significant decrease in the size of the specimens throughout the time period, with the middle liberty formation containing the smallest specimens and the upper liberty containing the widest range of specimen size. This result may suggest the current velocity decreased throughout the time period and was at its slowest in the middle liberty formation, allowing the smallest brachiopods to settle to the bottom of the warm, shallow ocean that existed in the area during the Ordovician time period.
In figures 2 and 2A we see the lower liberty was the only part of the formation which contained larger specimens oriented in a concave up position and the smaller specimens oriented in a concave down position. In the middle and upper liberty the concave down orientation was exhibited by the much larger species. Because the concave down position is a more stable position than the concave up position, these results suggest a very slow current. It would have allowed much smaller brachiopods to settle in a concave up position because there would have been little force to flip them over.
The graph and statistical summary of angle of repose versus size in figure 3 gave a P-value of .4921 indicating no significant difference in size of the specimens found at the various ranges of angle of repose. The frequency histogram in figure 3A shows how many specimens were found in the ten degree ranges from 0 degrees (horizontal) and 90 degrees (vertical). Far more were found near horizontal than at any other angle which suggests there was not much force acting on these brachiopods to tilt them to some other angle.
Figure 4 is a plot of fragment and location by size and does reveal a significant difference in the size of the fragments compared to the size of the whole specimens. The fragments of brachiopod valves we found were indeed small compared to the whole brachiopods, which may seem obvious but the presence of small fragments of valves among large whole valves indicates a very slow current. A current that was not strong enough to carry small fragments away.
We expected to see a common orientation in the direction the specimens were facing. That is, if there were a current moving in a particular direction it would cause most of the brachiopods to settle on the bottom in a similar manner. Figure 5 tells us there was no significant correlation between the size of the brachiopod and the direction it was facing when it settled on the bottom of the ocean among the lower, middle and upper liberty formation. We can again assume there was no particular current direction with enough magnitude to force the brachiopods into a common alignment.
As our data showed, there was no significant current affecting the brachiopods in our study area. This information still gave us some insight into the history of the area. Since there were no currents effecting the area, we can conclude that the area was probably not near any large land masses where tidal currents would be evident. Another possibility of the location the study area is that it was protected from current effects by a reef or another structure that blocked the currents. In order to determine these hypothesis other studies need to be conducted to see if currents had any effects on other locations throughout the Liberty formation. For now the only conclusion that we can draw is that Ordovician currents had no significant effects on the Brachiopods at our study location.
Works Cited
Harris, Frank W. and Martin, Wayne D., 1979 "Benthic
Community Development in Limestone Beds of the Waynesville Formation of Southeastern Indiana." Journal of Sedimentary Petrology, Vol. 49, No. 4. P. 1295-1306.
Hay, Helen B., Pope, John K., and Frey, Robert C., 1981. "Lithostratigraphy, Cyclic Sedimentation and
Paleoecology of the Cincinnatian Series in Southwestern Ohio and Southeastern Indiana." GSA Cincinnati-Field Trip No. 1.
Krawinkle, Hannelore, and Seyfried Hartmut. 1996 "Sedimentologic, palaeoecologic, taphonomic and ichnologic criteria for high-resolution sequence analysis: a practical guide for the identification and interpretation of discontinuities in shelf deposits." Sedimentary Geology, 102, 79-110.
Ross, Reuben James Jr. 1975 "Early Paleozoic Trilobites, Sedimentary Faceis, lithospheric plates and ocean currents." Fossils and Strata, n.4 307-329.
Schopf, Thomas J.M. Paleoceanography. Harvard University
Press, 1980
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