Structure and origin of Cambro-Ordovician thrombolites, western Newfoundland

• Main Author: Kennard, John Michael
• Format: Thesis
• Language: English
• Published: Memorial University of Newfoundland 1989
• Subjects: Newfoundland
• Online Access: https://research.library.mun.ca/6936/; https://research.library.mun.ca/6936/1/JohnMichaelKennard.pdf; https://research.library.mun.ca/6936/3/JohnMichaelKennard.pdf

An integrated scheme to systematically analyse the structure and interpret the origin of thrombolites is proposed on the basis of detailed field and petrographic analysis of Cambro-Ordovician thrombolites in western Newfoundland. This scheme utilizes a three-tiered analysis of microbial buildups: 1) megastructure, the overall bed form, 2) mesostructure, the internal fabric, and 3) microstructure, the microscopic fabric. This scheme has proved equally applicable to Cambro-Ordovician thrombolites, stromatolites, Epiphyton-Renalcis-Girvanella "microfossil" boundstones, and mixed microbial-metazoan buildups in western Newfoundland, elsewhere in North America and central Australia, and highlights differences between these types of buildups. -- Megastructure records the growth relationship between a buildup and its enclosing strata, is independent of mesostructure and microstructure, and reflects the sum of environmental factors acting on the buildup. Mesostructure records the spatial relationship between framework and inter-framework components, is governed by the shape and lateral continuity of the formative microbial community, and reflects a balance between biological and environmental factors. Microstructure is directly controlled by biological factors, and commonly yields clear evidence of specific sediment-forming processes (biologically influenced calcification, trapping and binding of detritus), and the gross morphologic composition (coccoid or filamentous) of the formative microbial community. -- Microstructural analysis indicates that Cambro-Ordovician thrombolites were most commonly constructed by relatively complex coccoid or coccoid-dominated microbial communities, and that the dominant process involved in their formation was in situ calcification of the microbial community. This calcified community formed a rigid framework between which autochthonous and allochthonous sediment accumulated. In contrast, co-occurring stromatolites were most commonly constructed by internally well differentiated ...