Date of Award:
Master of Science (MS)
Richard R. Alexander
Community succession, the orderly changes a community experiences with time, is considered to be the result of either autogenic or allogenic factors working singularly or in combination. It has been further suggested that, as communities undergo succession, certain biological parameters change in a predictable manner. Examination of members 2 through 4 of the Mississippian (KinderhookOsage) Lodgepole Formation of Samaria Mountain, Idaho provides a means to evaluate these concepts.
Within the study section comprised of 125 beds (61.5 m thick), four rock types are recognized. These include: (1) fossiliferous wackestone (78% of beds), which ranges in color from dark-gray (N3) to medium-light gray (N6), is fine to coarse crystalline, has an average insoluble content of 3.1 percent by weight, and an average organic content of 0.25 percent by weight; (2) fossiliferous mudstone (1S% of beds), which ranges in color from dark-gray (N3) to medium-gray (NS), is very fine to fine crystalline, has an average insoluble content of 2.3 percent and an average organic content of 0.30 percent by weight; (3) fossiliferous packstone (6% of beds), which ranges in color from medium-gray (NS) to light-gray (N7), is medium to coarse crystalline, has an average insoluble content of 3.3 percent by weight and an average organic content of O. 17 percent by weight; and (4) crystalline carbonate (1% of beds), which is light-gray (N7) in color, is coarse crystalline, has an insoluble content of 9.3 percent by weight, and an organic content of 0.2 percent by weight. However, at ninety-five percent confidence level, no statistical relationship could be seen between the rock types and either the insoluble contents or the organic content.
Twenty-one taxa, including corals, brachiopods, crinoids, blastoids, gastropods, echinoids, and sharks were recognized, with most taxa ranging throughout the study section. Well-preserved fossils are generally rare. However, this deficiency seems to be due to weathering of the containing strata rather than to currents acting on the skeletons prior to burial. Orientational data support this conclusion. Measurements of the direction (vector) from the apical end to the calical end of the horn coral Zaphrentis show that the orientations of toppled corals is random.
Two biological parameters are calculated for each of the bedding surfaces examined, i.e., diversity and calcified biovolume. Diversity is calculated two ways, i.e., in terms of equitability, and in terms of richness. For richness, values range from 0.0 to 7.41, with a mean of 4.12, and for dominance diversity, values range from 0.0 to 1.95, with a mean value of 0.87. Calcified biovolume, which is used as the basis for inferences involving biomass, ranges from 0 to 30,015 cubic centimeters per bedding surface, with a mean value of 744.1 cubic centimeters per bedding surface.
Three faunal associations, as well as several sub-associations within the three major associations, are indicated by the clustering of indices of affinity. These associations include: (1) Zaphrentis-Crinoid- Syringopora-Echinoid-Cleiothryridina-Unispirifer-Spirifer-Orthotetes- Flexaria-Camarotoechia-Schizophoria-Lithostrotionella; (2) Cruziana-Dwelling tube; (3) Shark-Blastoid-Helminthopsis. Trophic relationships within the three associations suggest that they were the result of two factors: (1) the tendency towards a vertical stratification of the association's members, which resulted in a more efficient use of the water column; (2) direct physical interaction among association members, in the form of predator-prey or symbiotic relationships. The former type of interaction may have been operative between the sharks and blastoids of faunal association 3.
Environmental reconstruction involved the determination of four parameters: (1) paleocurrent direction and intensity; (2) sedimentation rate; (3) bathymetry; and (4) substrate. From the study of both physical and biological evidence, it is concluded that, during the time of Lodgepole deposition a very weak, unidirectional current, or multidirectional currents of similar competency operated over a carbonate-mud substrate. Evidence suggests that the sedimentati on rate was extremely low, and that the Eh= 0 line was just below the sediment-water interface. The water depth at this location was probably below normal effective wave base, but above the zone of oxygen depletion.
Study of fossils on bedding surfaces overlying barren bedding surfaces or surfaces containing fossil hash, suggests that four successional stages can be recognized. Because changes in the faunal composition between the various successional stages appear to take place both with (10 times), and without lithologic changes (16 times), it is concluded that succession may result from either biological modifications of the environment or physical changes. Therefore, succession was both autogenically or allogenically controlled. Additionally, the comnrunities were retrograded to an "earlier" successional stage 24 times within the 125 bed succession.
As succession proceeded through the successional sequence, values for equability-diversity and calcified biovolume generally increase. This trend is in agreement with previously predicted trends. With succession the overall trend observed in the trophic structure appears to be one of an increase in the proportion of filter feeders in the community relative to the proportion of deposit feeders in the community.
Hines, Gary Keith, "Faunal Succession and Depositional Environments within the Lodgepole Limestone (Early Mississippian) of Samaria Mountain, Idaho" (1981). All Graduate Theses and Dissertations. 6671.
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