Erin Kalassay & Lisel Shoffner
December 18, 1997
Abstract
The Liberty Formation is part of the Richmondian Stage of the Cincinnatian Series of the Upper Ordovician stratigraphy. The Liberty Formation at the HWY 1 outcrop in Brookville, IN consists largely of shales with thin limestone beds. Fossils are good indicators of depositional environment. Fossils can point to the climate, type of area, and water conditions present during deposition. Fossil evidence at the HWY 1 outcrop suggests that the Liberty Formation resulted from a shoal environment that was affected by frequent storms.
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1. Introduction
The study of ancient geological processes is important because it provides clues as to how the earth functions in the present. Paleontology is an important aspect of geology for two reasons; it is used to define biostratigraphy and to determine depositional environment (Selley, 1996).
The depositional environment of a rock or formation is important to understand because even small changes in these environments can have wide spread impacts. Once the environment of deposition is known, factors that may have changed that environment can be deduced. Understanding the factors of environmental changes in ancient times can help researches predict how factors, such as human activities, will impact modern environments.
The HWY 1 study area in Brookville, Indiana has an exceptional outcropping of the fossiliferous Arnheim through Whitewater Formations of the Richmondian stage. The Liberty Formation was selected for this study because, during previous visits, it appeared a slightly higher abundance of fossils that were easily collected than the other formations. As the study was concerned with how fossils can indicate depositional environment, it was beneficial to use the formation with the most easily attainable data.
2. Methods
2.1 Methods of study
This research study was made by making two trips to the HWY 1 outcrop in Brookville, IN. For both visits, our sample area was a randomly selected, five foot wide section that extended vertically (approximately 17 feet), through the formation.
This area included several distinctly different types of strata. The first strata extended the first 13 feet of the formation. This was composed of a very fissile and broken apart layer of shale. This layer contained no limestone. The second strata was the next two feet of the of the formation. This layer included shale with very thin limestone interbedded in it. The third strata was strictly a limestone layer. It was one massive bed that contained no crossbedding.
2.2 Sampling Techniques
To collect our data, we did surface sampling throughout the sample area. We marked off six inch segments with rope to insure we had a precise sampling area. We examined six inch by six inch square samples that were randomly selected, making sure that we stayed within the boundaries of the five foot wide section.
At each location, we examined our sampling area and recorded the data on a data sheet. We focused predominantly on brachiopods. The different species of brachiopods that found, as well as the number of bryozoans, trilobites, horn corals, gastropods, and pelecypods were recorded. Abundances were also recorded by noting how many whole and fragmented pieces there were of each species present.
2.3 Statistical Tests
We performed a series of statistical tests of the data that we found at the HWY 1 roadcut. Each of these tests were done on Microsoft Excel to organize and compute the data more effectively.
First, we performed an index of dominance test. This test is used to compare the concentrations of species dominance within each strata we tested. Value you get from computing your data can range from 0.0- 1.0. High values indicate that there is community dominance by few species. A low value indicates that dominance is shared by every species in the community.
Second, We performed an index of similarity test. This test can be used to compare species composition of strata with consideration of numbers of species present and number of species in common. The values can range from 0.0 to 1.0. A high value indicates more similarity in species composition in the strata and a low value indicates less similarity.
Third, we performed a species richness test and diversity test. These will calculate the richness and diversity between each strata.
We also used a Spearman Statistics program that allowed us to calculate the Spearman Rank analysis. This test gave us p- values and z- values. A p- value less than .05 would indicate a significant difference between strata. A p- value greater than .05 would indicate no significance between the strata.
3. Setting & Background
During the Ordovician Period 475 million years ago, the Cincinnati region was located between 15 and 25 degrees south latitude, and was rotated 45 degrees north from its current orientation (Haneberg et al., 1992). Cincinnatian sediments resulted from a shallow sea of depths up to 300 to 400 feet, which covered the area. Overall, the Cincinnati area during the Upper Ordovician Period was characterized by a tropical, marine environment.
The shales and limestones that make up the Liberty Formation represent distinctive environments of deposition. Shales are terrigenous, extrabasinal rocks (Friedman et al., 1992). Shales are composed of clay or silt sized grains. For such grains to be deposited, the water must be relatively still, having low-intensity flow, so that fine particles may settle out. Such flow occurs in deeper waters, where surface currents and disturbances rarely affect the bottom of the body of water. Ohio black shales, such as those which occur at HWY 1, are representative of anoxic basins (Reading, 1986). Fissility is an important characteristic of shales because it indicates that burrowing organisms were not present during deposition, due to environmental restrictions (Friedman et al., 1992).
Limestones are intrabasinal, carbonate rocks. Particle size in limestones can range from clay or fine silt size mud grains to cobble size (60 to 250 mm) skeletal fragments. Limestones consist of four basic allochem types: fossils, ooids, peloids, and intraclasts (Boardman, 1997). As a result, deposition of a limestone can occur over a wide range of environments, from open marine-type to shoals. The support of a limestone (matrix/grain), structures within the rock, and the type of allochems present are the factors which indicate depositional environment of a limestone.
The upper Ordovician shales and limestones of the Cincinnatian Series are rich in fossils. The fossils of the Richmondian Stage are especially good in terms of size, abundance, and preservation. The fossils represent a wide range of species, from bryozoa to trilobites. These fossils are useful in the study of the Richmondian Stage formations, because they are indicators of depositional environment.
From his data, R.H. Osbourne concludes that brachiopods and bryozoans lived in similar types of environments (1966). The habitat includes all marine environments except smooth sand or muddy bottoms because they need a firm, clean surface to attach themselves to. Evidence has also shown that some species of brachiopods and bryozoans require clear water in which the sediments are unstirred. Most modern brachiopods live in areas were the bottoms are not disrupted by strong currents because they are not physiologically adapted to handle large amounts of sediment accumulation (Osbourne, 1966).
Before the late stages of the Richmondian regression, the Rafinesquina- Zygospira community, which lived in a turbulent shoal environment, was not very diversified. Because organisms that lived in this turbulent shoal environment were subjected to higher amounts of physiological stress than those offshore, the abundance of species capable of living in this environment was considerably less. Other factors such as fluctuations in temperature, frequent environmental irritations, and salinity changes may have also contributed to this lack of diversity. Therefore, Rafinesquina and bryozoa joined Zygospira to form the dominant group of animals in this unpredictable shoal environment.
Brachiopod distribution in the Cincinnatian Series was easily patterned due to depth-related faunal arrays. ³Despite variations in sedimentation rate and turbulence, Rafinesquina and Zygospira were always more abundant farther offshore than Platystrophia and Herbertella. Invariably Omniella [Dalmanella] and Sowerbyella occurred farther offshore than any of the other brachiopods² (MacDaniel, 1976). At later dates, new taxa were introduced into the shallower water of Richmondian environments. Lepiodocyclus, Leptaena, and Strophomena are examples of this new genera of brachiopods.
According to the book Fossils of Ohio, Ordovician brachiopods are generally abundant in thinly bedded Upper Ordovician limestones and shales that can be located throughout the tri- state area (1996). These brachiopods are well preserved and their shell material is commonly intact.
Because invertebrate body fossils are principally controlled by salinity and salinity variations, the provide the best means of identifying marine environments (Reading, 1986). Cepholopods, articulate brachiopods, and bryozoa are stenohaline species. As a result they occupy narrow, normal salinity ranges (Reading, 1986). Life assemblages of stenohaline fossils indicate a shelf environment.
Benthic communities, those occurring on the bottom underlying a body of water, are usually preserved in or near their life habitat, displaying only minimal transport after death (Reading, 1996). However, the bottoms of shallow seas, like that which existed during Ordovician, can be greatly impacted by the occurrence of currents or storms (Haneberg, et al., 1992). Such disturbances could result in the post-mortem transport and subsequent mixing of benthic communities. Storms cause a winnowing of fine materials from limestones near a shore. When storms cease, the fine materials are deposited as silts and clays offshore. Skeletal fragments will build-up on the sea floor when organisms die. The storm-related winnowing process will result in a consolidated shell layer. A transgression of the sea will then result in deposition of mud on the shell layer (Haneberg, et al., 1992). Thus, storm-dominated deposition is evidenced by the occurrence of alternating shales and limestones.
4. Hypothesis
We have developed three hypotheses based on our background information. First, we believe that within each strata, there will be very little difference in diversity and abundance of species. Second, we believe that there will be a difference in diversity and abundance of species between different layers of shale strata. For example, there will be a significant difference when comparing strata 1 and strata 2 at the HWY 1 outcrop. Third, we believe there will also be a difference in diversity and abundance of species between the limestone and shale strata. For example, there will be a difference between the shales layers of strata 1 and 2 and the limestone layer of strata 3 at the HWY 1 outcrop.
The reason we formulated these hypotheses is three-fold. First, the very nature of a depositional environment dictates what type of fossils will appear in the sediments of that environment. Second, different strata represent different phases of deposition. Finally, different phases of deposition may indicate changes in the depositional environment.
5. Data
5.1 Statistical Analysis
Appendix 1 represents the actual data that was collected during the two trips to the HWY 1 outcrop. The fossil data is divided into whole fossils and fragments of fossils because the degree of desiccation can provide useful information about the depositional environment. It should be noted that a large amount of fossil fragments were not recorded because they were too small to be identified in the field.
Appendix 2 lists the results of our statistical analysis of the data. The first analysis performed was and index of dominance (IOD). The index of dominance for each strata varied by only about .05 between whole fossils and fragments. For example, the IOD of strata 4 was .21 for whole fossils and .16 for fragments. This indicates that dominance of species within the strata was independent whether the fossil occurred as a whole or as a fragment. With the exception of strata 2, the IOD of the Liberty strata was extremely low less than .3. This indicates that the Liberty strata do not show evidence of single species community dominance. This suggests that strata represent either an environment in which a wide range of species can survive, such as a normal salinity near shore environment, or that the strata represents an offshore environment to which skeletal fragments were transported, post-mortem, from an environment like that first mentioned. Strata 2 displays an index of dominance of .5 for both whole and fragmented fossils. This comparatively high IOD is misleading, however, as it is due to the fact that few fossils, thus few species, were found in the strata.
Because dominance is not high, species richness is. Our analyses demonstrates that, even in strata 2 which had a very limited number of fossils, the amount of different species is high compared to the number of fossils collected. This supports the idea that deposition occurred in a near shore community or as a result of post-mortem transportation.
A diversity analysis further supported the findings of the index of dominance and the species richness analyses. With the exception of strata 2, the Liberty strata had rather high diversity rankings of averaging around .75 for both whole and fragmented fossils. These findings again confirm that deposition had to occur either in a location where a wide range of species could survive, or in a location where a Rafinesquina-Zygospira community would live but to where other organisms would be transported post-mortem.
Appendix 3 shows the results of the Spearman Rank tests we performed. A p-value of less than .05 indicates that there is a correlation between the species ranking of different strata. The whole fossil rankings suggest that there is a correlation between strata 2 and 3 and strata 2 and 4. This would suggest that the same order of abundance exists between these strata because of some inherent similarity, such as they are of the same lithology or depositional environment. This does not seem to make sense for the case of strata 2 and 3, however, because strata 2 is a shale while strata 3 is a limestone. It is possible that strata 2 and 4 are similar as they are both shales.
The ranking indicates that there is no correlation between strata 1 and 4. This does not seem logical as they are both the same shale unit, but from different sides of the street. One possible explanation is that the fossil assemblages are not uniform throughout the strata because the strata¹s depositional environment is not laterally uniform.
The Spearman Rank for the fossil fragments suggests the only correlation between strata exists between strata 2 and 3. This supports the rank correlation of the whole fragments. An explanation for this phenomenon is that the number of fossils found within strata 2 was so small that it created false similarities between the ranks of strata 2 and 3. The fragment ranking also supports the indication in the whole ranking that there is no correlation between strata 1 and 4. This gives further evidence that the depositional environment may not have been uniform throughout this strata unit.
5.2 Raw Data
The index of similarity (IOS) analysis showed higher similarity between fragmented fossils than whole fossils of different strata. Fossil fragment similarities were well above .5 for all strata comparisons. This lends support to the theory that the fossil fragments were transported, post-mortem, to the area of deposition. Whole fossil fragment similarities were high for all comparisons except that of strata 1 and 2. This is probably due to the very small number of fossils found in strata 2 compared to strata 1, such that it misrepresents the similarity of the strata. The high similarity between strata suggests that deposition occurred in deeper offshore waters, as extreme differences in fauna occur only in shallow coastal environments (MacDaniel, 1976).
The data shows that significantly more fossil fragments than whole fossils were found. Although this did not impact our statistical analyses, it does suggest that the majority of fossils were not deposited where they died. The majority of fossils were probably transported the depositional area, thus, they were broken down as they were transported. This suggests that the fossils within the Liberty Formation represent a death assemblage.
Rafinesquina, Zygospira, and Dalmanella are known to have survived in deeper waters where shales could form. The occurrence of whole fragments of these species, suggests that, at least the first shale strata may represent a benthic life assemblage. If this is the case, then the first Liberty strata represents a post-mortem mixing of benthic communities. Such mixing can occur in storm-dominated depositional environments (Haneberg et al., 1992).
6. Conclusions
Statistical analysis suggests that deposition of Liberty strata could have occurred in one of two possible environments. Species dominance, richness, diversity, and similarity suggest that deposition had to occur either where all the species could survive, or where the remains of the species were transported. This suggests deposition occurred either near shore in a normal salinity shallow water environment, or offshore in a deeper water environment to which skeletal fragments were transported, post-mortem.
The statistical analyses seem to better support the post-mortem deposition theory. The notion that the fossil assemblages in the Liberty Formation are death assemblages is further supported by the raw data. The significant abundance of fragmented fossils compared to whole fossils, suggests that the skeletal remains of the organisms were transported and desiccated before being deposited.
The presence of whole fossils of Rafinesquina, Zygospira, and Dalmanella suggests that the environment may have supported a Rafinesquina-Zygospira benthic community. This idea combined with the death assemblage theory suggests that post-mortem storm driven mixing of benthic communities occurred during Liberty deposition (Frey, 1976). Thus, fossil evidence indicates that the environment of deposition of the shale and limestone strata of the Liberty Formation was one of storm-dominated disturbances, which caused the unique alternating sequence of these lithologies.
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