The Rise and Nature of Carbonate Mud-Mounds: An Introductory Actualistic Approach
- C. L. V. Monty,
- D. W. J. Bosence,
- P. H. Bridges and
- B. R. Pratt
Published Online: 14 APR 2009
Copyright © 1995 The International Association of Sedimentologists
Carbonate Mud-Mounds: Their Origin and Evolution
How to Cite
Monty, C. L. V. (1995) The Rise and Nature of Carbonate Mud-Mounds: An Introductory Actualistic Approach, in Carbonate Mud-Mounds: Their Origin and Evolution (eds C. L. V. Monty, D. W. J. Bosence, P. H. Bridges and B. R. Pratt), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304114.ch2
- Published Online: 14 APR 2009
- Published Print: 17 JUL 1995
Print ISBN: 9780865429338
Online ISBN: 9781444304114
- rise and nature of carbonate mud-mounds;
- the rise of calcined cyanobacteria;
- The Middle and Upper Cambria;
- micrite precipitation may be related to algal life processes and decay';
- types of cavity- stromatac-tis, zebra, reticulated cavities;
- intense periods of calcification of cyanobacteria - related to warm climatic episodes
Late Precambrian sequences record a continuing decline of stromatolites both in size and diversity. But the late Precambrian is also marked by an important event, namely the rise of calcified cyanobacteria which, in collaboration with bacteria, played an important role in the development of mud-mounds.
The earliest Cambrian sequences show a rapid evolution of carbonate buildups ranging from small decimetric bioherms built by microbes, to decametric bioherms 90 m thick or more incorporating archaeocyaths and calcimicrobes. By Atdabanian times true reefs and incipient stromatactoid mud-mounds were already forming. After a long interval, mud-mounds reappeared in mid-Ordovician times.
Mud-mounds are made predominantly of massive carbonate mudstones and wackestones; however, they incorporate various metazoans and algae. The lime mud, which is generally very pure, was consolidated by cyanobacterial filaments and had a viscoplastic behaviour susceptible to slumps and injections. Many carbonate mud-mounds were essentially microbial in nature and included phototrophic and heterotrophic bacteria which degraded sponges and mucilages. Iron bacteria may have intervened in some cases. Traces of microbial life are present and show calcified filaments, moulds or casts of filaments, peloids, traces of unicells, and particular microbial fabrics. Preservation is highly variable. Fossil cyanobacterial filaments had an extensive bathymetric range, from surficial to deep or very deep waters. This parallels observations in the Recent period. Mud-mounds are not necessarily monolithic buildups but may show facies changes during their development. These are generally related to shallowing-up events, although deepening-up events also occurred. The rise and development of mud-mounds and reefs coincided in part with the decline of stromatolites. However, their increasing size through the Phanerozoic, together with persistent small buildups, does not accord with the idea that the decline of the stromatolites was related to decreasing sea-water saturation and cropping by metazoans. These processes should also have affected mud-mounds because many of them were built by bacteria and cyanobacteria. The presence of biofilms enabled metazoan debris and skeletons to colonize and contribute to carbonate sedimentation. Biotic traffic and regular disappearance of biofilms accounts for the absence of widespread lamination in mud-mounds (except for clay seams, superposition of platy skeleton etc.) and the absence of significant bioturbation. Mud-mounds also harboured cavity-dwelling stromatolites (or endostromatolites; Monty 1982b) living in fractures.
Fibrous calcitic cements (fascicular radial calcite, fascicular optic calcite) are shown to have nucleated on bundles of microbial filaments which supported the mosaic; this is another argument in favour of the view that these cements were not recrystallized.
Biological processes controlled active and passive precipitation. The former relied on properties of the membranes ensuring carbonate precipitation and binding. The latter involved metabolical activities and increase in alkalinization. Photosynthesis was not essential for precipitation of carbonate. Microbial micrites contrast with chemical ones which have another diagenetic story. Biomicrites can be identified by the morphology of grains and the presence of fossilized bacteria in their centre.
Cavities (stromatactis, zebra, sills and fractures) are discussed and the redefinition of stromatactis by Bourque & Boulvain (1993) is considered as inadequate.
The role of cyanobacteria in the formation of micritic to microsparitic matrix was essential and related to the large amounts of mucilage secreted by these microbes. The polysaccharides were heavily browsed by heterotrophic bacteria. Microbial mucilage was normally in the state of sol, but it could turn to gel. It may induce injections of slimy micrites and microsparites in the sediments, resulting in mélanges of lithologies. Degradation of organic matter was a very important process in mud-mounds: it resulted in carbonate precipitation and erased many important features, including filaments and cells.