title: Synecology and Faunal Succession of the Upper Mississippian Great Blue Limestone, Bear River Range and Wellsville Mountain, North-Central Utah
Creators: name: Sweide, Alan P.
Publisher: name: Utah State University
Year: 1977

description: The late Mississippian Great Blue Limestone was studied at four locations around Cache Valley, Utah. One location was at Dry Lake in the Pisgah Hills to the west of Cache Valley. Three locations were in the Bear River Range to the east of Cache Valley. From north to south the locations are: 1) Beirdneau Peak, 2) Logan Peak, and 3) the ridge to the north of East Fork Canyon. The Dry Lake location represents the upper, middle, and some of the lower part of the section. The base of the section is covered. Locations in the Bear River Range represent the lower and middle parts of the Great Blue Limestone. The upper part of the section has been removed by erosion. The sections at the four locations are correlated with one another by the Lithostrotion-Turbophyllum Zone present at all four locations at the top of the lower part.
Field work included measuring each section and collecting samples, for insoluble-residue analysis, at regularly spaced intervals at each location. Slab samples representative of the faunal assemblages were collected for laboratory analysis. Orientations of recumbent rugose coralla on exposed bedding planes of the Lithostrotion-Turbophyllum Zone at the four locations were measured for paleocurrent analysis.
Laboratory work included: 1) flume experiments, 2) analysis of slabs for faunal composition, preserved orientations, species associations, density, diversity, and species morphology, 3) insoluble-residue analysis, and, 4) X-ray diffraction to determine the characteristic mineralogy. Statistical analysis of the data collected in the field and from slabs included calculations of: 1) index of affinity and generation of faunal association dendograms, 2) mean, mode and angular deviation of the coral orientational data, 3) mean, skewness, and kurtosis of size-frequency distributions of rugose corals and the opportunistic species Rugosochonetes-loganensis and, 4) species equitability-diversity.
The Great Blue has been divided into three units on the basis of lithology. The lower Great Blue is a massive, thick-bedded, darkgray, microcrystalline limestone. The Long Trail Shale is a thinbedded, light-brown shale. The upper Great Blue is a massive, thickbedded, dark-gray, microcrystalline limestone. These units correspond to units 2, 3, and 4 of the Brazer Formation.
Seven faunal zones were identified in the Great Blue. In the lower Great Blue the Turbophyllum Zone and the Lithostrotion-Turbophyllum Zone are present. The Brachiopod-Pelecypod Zone is present in the Long Trail Shale. In the upper Great Blue the Coral-Cephalopod, Caninia, Rugosochonetes-Orthotetes, Rugosochonetes-Paladin, and Crinoid Bryozoan Zones are present.
Orientations of the apical ends of recumbent rugose conals in the Lithostrotion-Turbophyllum Zone of the lower Great Blue were used to reconstruct paleocurrent directions and intensities. Results of the survey indicate a preferred orientation of the apical ends of the rugose coralla in the direction which is now southeast. Results of the flume experiment indicate that rugose coral coralla subjected to unidirectional currents became oriented with the apical end pointed into the current. The modal orientations of the long axis of the coralla exposed on the bedding-plane surfaces suggest the existence of currents, predominantly from the south. The paleocurrent analysis suggests gentle tidal currents, with a stronger ebb component, operating for long periods of time.
Micritic lithologies within the Turbophyllum Zone were deposited during a transgression. The zone is populated by high and very-high filter-feeders, but species diversity is low. The faunal and lithologic evidence from the Lithostrotion-Turbophyllum Zone suggest an offshore environment, at or near effective wave base, inhabited by a few species which were low, high, or very-high filter-feeders. The Brachiopod-Pelecypod Zone contains a faunal assemblage associated with terrigenous lithologies suggestive of a nearshore environment that resulted from a regression. Species diversity in this zone is high. The Coral-Cephalopod Zone contains a sparse fauna and includes dark, micritic lithologies interpreted to have been deposited under conditions of almost no current activity, in possibly the deepest water, relative to the other zones. A rapid transgression and poorly oxygenated conditions above the substrate did not allow the development of a coral-crinoid community similar to the Lithostrotion-Turbophyllum Zone. Lithologic and faunal evidence in the Caninia Zone suggest a gradual regression that resulted in a shoaled environment similar to that of the Lithostrotion-Turbophyllum Zone. The zone is habited by low, high, and very-high level filter-feeders. Species diversity is high. The Rugosochonetes-Orthotetes Zone contains a fauna similar to the Brachiopod-Pelecypod Zone, but the fauna is not as abundant or diverse. Terrigenous lithologies and faunal evidence suggest a shallower environment that resulted from a continuation of the regression initiated during deposition of the lithologies within the Coral-Cephalopod and Caninia Zones. The succeeding calcilutites of the Rugosochonetes-Paladin Zone were deposited at a water depth greater than the underlying terrigenous muds of the Rugosochonetes-orthotetes Zone. The zone displays high dominance and low diversity. The biospartic lithology of the Crinoid-Bryozoan Zone represents an environment situated in deeper water than the Rugosochonetes-Paladin Zone. Species diversity is very low and only the very-high filter-feeding niches are occupied.
The succession of the communities appears to be controlled by the nature of the substrate and the associated turbidity, current agitation, and sedimentation rate during deposition. The insoluble-residue survey indicates zones of terrigenous influx which influenced the nature of the substrate and permits inferences concerning proximity to the source area. Asymmetrical cycles of rapid transgression and gradual regression over shallow shelf habitats controlled the faunal successions and retrogressions.