Doctoral Dissertations

Date of Award

8-1988

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Thomas W. Broadhead

Committee Members

Kenneth R. Walker, Michael L. McKinney, Edward C. Clebsch

Abstract

The Rockhouse Limestone and Birdsong Shale members of the Ross Formation (Lower Devonian, Lockhov) of western Tennessee are a mixed limestone - shale sequence, approximately 30 m thick, that accumulated on a shallow, predominantly carbonate, platform, termed the Ross Shelf. The Ross Shelf was situated 25° to 30° south latitude during the Devonian and was structurally bounded by the Illinois Basin to the north, Reelfoot Rift and Ozark Uplift to the west, Nashville Dome to the east, and the continental margin to the south. Terrigenous mud was supplied by storm systems crossing the shelf from offshore.

The Rockhouse Limestone Member consists of well-washed bioclastic limestone and shale with locally developed bryozoan-dominated mudmounds. Four vertical phases of development are recognized in the mudmounds: (1) introduction of terrigenous mud containing allochthonous skeletal debris, (2) colonization and stabilization of patches of the muddy substrate, (3) a Buskopora-dominated phase characterized by an irregular surface with open cavities, and (4) interruption of the bryozoan dominated phase coinciding with a marked increase in terrigenous influx.

The overlying Birdsong Shale Member was dominated by muddy bottoms which were frequently scoured by storms to produce graded limestone - shale couplets. Although the seafloor consisted of soft substrata much of the time, the substrate sometimes underwent compaction and dewatering to foinn firmgrounds of fine-grained bioclastic debris. Early marine cementation is noticably absent in these rocks.

Biotic assemblages of the Birdsong Shale developed along different pathways depending upon the consistency of the substrate. When the substrate was soft mud, the fauna was characterized by adaptations that retarded sinking (e.g.,Favosites foerstei) or by adaptations for avoiding the sediment altogether (e.g., epibiosis). Some of the fauna (e.g., Favosites conicus) were adapted for living between major influxes of mud.

Although epizoans are abundant in the Birdsong, they do not show abundant obvious interactions with each other or with their hosts. Most associations with hosts were epitaphic in response to substrate conditions. Facultative associations occured between Macropleura macropleura and Spirorbis laxus in which S. laxus preferentially lived in the troughs between the plications of the host brachiopod and avoided the fold and sulcus region, probably due to the poorer quality of the excurrent water flow of the host. Colonial epibionts show little preference for location or orientation; however, preferred directions of growth toward the commissure are exhibited in some species. Two recurrent associations are present. The first is a symbiotic mutualism between Aulopora schohariae and a fistuliporid bryozoan to form either a massive intergrowth or a ramose growth form. Both members benefited from the structural support the mutualism offers. The second association is an epitaphic relationship between A. schohariae and brachiopod host for avoiding the substrate or a commensal relationship in which the coral benefited from feeding currents generated by the host.

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