Sedimentology and paleoecology of two Upper Devonian (Frasnian) carbonate mud-mounds, southern Rocky Mountains, west-central Alberta, Canada
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Stromatactis-bearing mud-mound carbonate buildups remain an enigmatic reef type despite being common in Paleozoic ramp settings. Two Upper Devonian (Frasnian) mud-mounds in the Mount Hawk Formation provide an opportunity to develop a new case study that can be compared with the only other well-known coeval examples, in southern Belgium, as well as evaluate competing hypotheses for mud-mound formation. They crop out side-by-side in the southern Rocky Mountains of west-central Alberta. The southern mud-mound is 46.2 m thick and 38.6 m wide at the base, whereas the northern one is 53.3 m thick and 72.2 m wide at the base, and they exhibit three or four growth stages indicated by interfingering and onlapping geometries with flanking strata. The biota is diverse, but fossils only occupy 10.7% by volume, among which sponge spicules, brachiopods, ostracodes and calcimicrobes belonging to Girvanella and Rothpletzella are the most common. Five microfacies are discriminated in the mud-mounds: biomicrite, clotted micrite, spiculite, stromatolite and laminate, with clotted micrite comprising the largest portion. With respect to specific peloidal sediments, five types of peloids are differentiated: cyanobacterial, intraclastic, pseudo-, bioclastic, and microbial peloids. They are important components in those microfacies, especially microbial peloids making up the clotted micrite. The presence of calcimicrobes and calcareous algae throughout the mud-mounds indicate accretion within the photic zone. Flanking strata suggest the mounds grew in a deeper ramp setting seaward of a large carbonate platform to the east. Mud-mounds accreted above storm wave base but below the fair weather wave base. Formation of the carbonate mud involved a combination of multiple organisms, mechanisms and processes, but bacteria were dominant in precipitating clotted micrite and forming rigid masses through metabolism. Cyanobacteria were integral to the frameworks of mud-mounds and their accretion. The same microbial components, invertebrate biota and clotted micrite occur in underlying strata, suggesting there was a protracted period of potential mud-mound initiation before the exact conditions arose to trigger it. The ramp setting, antecedent sea floor topography and relative sea level likely contributed together to control the development of the mud-mounds. They grew during deposition of the transgressive systems tract and perhaps also the early highstand systems tract. Stromatactis is abundant and the cavities responsible were mostly due to excavation by currents rather than collapse of sponges. This study indicates that mud-mound formation was controlled by a combination of processes but they are essentially a microbial buildup and reflected a dynamic balance between constructional versus destructive processes. As the main components of microbial carbonates in general, evolution of microbial peloids through the Phanerozoic was influenced by two secular factors, metazoan diversity and saturation state with respect to CaCO3, similar to their roles in governing microbial carbonates. However, metazoan decline is not always linked to microbial resurgence or the other way around. This may be because: (1) metazoan diversity had no or too little effect on microbial carbonates; (2) a balance arose between metazoan diversity and other factors influencing microbial carbonates, like carbonate saturation state; or (3) the mechanism responsible for the decline of metazoans was also inhibitory to extensive microbial development. Mud-mound abundance through time shows a similar pattern in general reflecting the influence by fluctuations in metazoan diversity and CaCO3 saturation state. However, discrepancies between specific intervals suggest there may have been variation in the amount of CaCO3 available for permineralization of various co-existing microbial communities, influence of their specific environmental conditions and key importance of synsedimentary cementation.
DegreeMaster of Science (M.Sc.)
CommitteeBuatois, Luis; Mángano, Gabriela; Aitken, Alec; Lehmkuhl, Dennis; Morozov, Igor
Copyright DateMay 2018
mud-mounds, stromatactis, microbial, Upper Devonian