Date of Award:

5-1977

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geosciences

Department name when degree awarded

Geology

Committee Chair(s)

Richard R. Alexander

Committee

Richard R. Alexander

Committee

Peter T. Kolesar

Committee

Robert Q. Oaks, Jr.

Abstract

The Mississippian (Kinderhookian-Osagean) Lodgepole Formation contains a diverse fossil assemblage. Taxa present include brachiopods, crinoids, gastropods, cephalopods, trilobites and corals. Corals and associated fauna were collected from four localities within the Bear River Range. These are, from north to south, Beirdneau Hollow, Spring Hollow, Leatham Hollow and Porcupine Dam. The well-preserved tabulate and rugose (compound and solitary) corals exhibit a high degree of morphologic variability. The colonial corals of the Lodgepole Formation (particularly Lithostrotionella, Syringopora) exhibit a morphologic gradient from platy to hemispherical forms. The six morphologic categories of colonial corals discussed in this study are identified by mean corallus diameter/corallum height ratios, by the corallite growth direction, and by the shape of the base of the colony. Type I corals have an average mean diameter/height ratio of 3.4; corallites are directed laterally away from the flat base. Type I corals are interpreted to have been adapted to offshore, quiet-water conditions. Type II corals are flattened hemispheres; they have an average mean diameter/height ratio of 4.1. Corallites are directed radially (i.e., with vertical as well as a lateral component) away from the flat colony base. Type II corals are interpreted in this study to have been adapted to shallow, moderately-turbulent environments in which vertical growth was inhibited. Type III corals have an average mean diameter/height ratio of 3.9 and are similar to Type II corals in all respects but one, namely that there is an absence of corallites on the crown of the corallum. This feature is called balding and is interpreted in this study to have been the result of desiccation and subsequent death of coral polyps. Type III corals are thus interpreted to have inhabited very shallow water wherein subaerial exposure of the crown of the corallum occurred during periods of exceptionally low tides. Type IV corals are dome-shaped or slightly-flattened hemispheres; they have an average mean diameter/height ratio of 2.3. Corallites are directed radially away from the flat base. Type IV corals are interpreted to have inhabited a depth zone intermediate between that of Type II corals (within or barely below tidal range) and Type I corals (near or below wave base). The average mean diameter/height ratio of Type V corals is 1.7. Corallites are directed almost entirely vertically away from the rounded-to-conical colony base. Type V corals are interpreted to have inhabited areas where sedimentation rates were sufficiently high to encourage vertical growth to the virtual exclusion of lateral growth. Type VI corals are composite corals, consisting of combinations of hemispherical forms and platy forms. This morphologic type is characterized by a change in the direction of growth during the astogenetic development of the colony. The combinations of varying growth forms presumably reflect fluctuations in sedimentation rate.1

Type I corals display a significantly higher degree of intermural budding than Types II, III, IV or V corals. A high frequency of intermural budding accompanied slow growth, according to Jones2 and Philcox3. In rather deep, quiet water environments, skeletal accretion may have been retarded.4 Thus the high budding frequency of Type I corals is consistent with the bathymetric assignment of this growth-form.

Among the solitary rugose corals, five morphologic types were identified. The coralla of Type 1 corals are straight. This feature is interpreted to be the result of vertical growth after an initial stable attachment (i.e., the coral exhibited a negative geotropic response). Another possibility is that the coral, although initially obliquely oriented in the substrate, inhabited an area of slow sedimentation and/or low current velocities. Type 2 rugose corals are geniculate and have been interpreted to have formed as a result of penecontemporaneous erosion of the surrounding matrix. Scouring caused the coral to topple to a horizontal position. Vertical growth away from the substrate would result in the geniculation. Increasing rates of sedimentation in the immediate vicinity of obliquely-attached corals would necessitate rapid vertical growth in order that the polyp maintain a position above the substrate.5 Repetition of these processes accounts for the Type 3 (multiply geniculate) morphology. Either sedimentation rates were variable, and/or current velocities sufficiently high to cause toppling of rugose corals. Type 4 rugose corals have prominant constrictions of their epithecae, a feature referred to as rejuvenescence. Changing environmental conditions that may have proven traumatic to the coral (i.e., storm), resulting in temporary growth-stoppage are interpreted to have caused this feature. Type 5 corals include coiled and grooved protocoralla. These features are interpreted to be adaptations which facilitated post-larval attachment of juvenile corals to objects at or above the sediment-water interface.6

Detailed study of internal and external calical features allowed for interpretation of the relative abilities of the Lodgepole corals in terms of their efficiency at rejecting sediment from the calice. Each coral genus was rated on the basis of calical relief, calical shape, septal density and septal length. In general, the corals categorized as inefficient sediment rejectors, such as Caninia, had low calical relief, horizontal calical floors, low septal densities and short septae. The inefficient corals (i.e., those inferred to have possessed comparatively passive polyps) were most often found in mudstones and wackestones. In contrast, corals categorized as efficient sediment rejectors, such as Zaphrentoides and Lithostrotionella, had high calical relief, v-shaped calical floors, high septal densities and long septae. The more efficient corals (i.e., those inferred to have possessed active polyps) are comparatively ubiquitous in their lithologic associations and are often found in impure mudstones or wackestones, or in grainstones and packstones.

At least one insoluble residue sample was prepared for the lithologic matrix associated with each taxon. In general, the lithologic matrix associated with deposit-feeding organisms (trilobites, gastropods), trace fossils and algal structures yielded the highest weight percent insoluble residue values. Crinoids, bryozoans and corals yielded intermediate values, and the lowest values were obtained for the brachiopods.

A similarity matrix based on indices of affinity as calculated by Faber and McGowan,7 facilitated an analysis of faunal associations. Two assemblages, one dominated by suspension-feeders and one a mixed assemblage of suspension and deposit feeders, are apparent. Although some biological interrelationships were inferred to have operated between the organisms, the physical factors (i.e., substrate, turbulence, sedimentation rate depth) are interpreted to have been the dominant controlling factors in the distribution of the Lodgepole fauna.

The construction of a hypothetical bathymetric profile showing the inferred bathymetric distribution of the fossil corals, associated faunal elements, abiotic sedimentary structures, trace fossils and algal structures was based upon a model of carbonate deposition in clear-water epeiric seas.8 According to the model, three marine, inter-grading hydraulic zones characterized epeiric seas. Zone X is a low-energy, quiet water zone located offshore. The pelletal, fossiliferous mudstones and wackestones of the Lodgepole Formation are interpreted to have formed in this zone. Types I, IV, V colonial corals and Type 1 solitary corals probably inhabited Zone X. Zone Y is located landward of Zone X and is a high-energy zone, located above wave base and subject to both wave and strong current action. Packstones, bioclastic grainstones, boundstones and fossiliferous wackestones may have formed in this zone. Types II and III colonial corals and Types 2 and 3 solitary corals probably formed in Zone Y. Zone Z, the nearshore quiet-water zone, is the zone in which impure pelletal mudstones and wackestones, and laminated calcilutites were deposited. No colonial corals other than the Aulopora were found in these rocks. Zaphrentoides, an efficient sediment-rejector, and (rarely) Vesiculophyllum are the only solitary corals found in Zone Z. On the basis of their functional morphology and sedimentary and biological associations, the corals of the Lodgepole Formation are interpreted to have inhabited the shallow subtidal to intertidal regime.

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