- 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
Lees, A. and Miller, J. (1995) Waulsortian Banks, 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.ch7
- Published Online: 14 APR 2009
- Published Print: 17 JUL 1995
Print ISBN: 9780865429338
Online ISBN: 9781444304114
- Waulsortian banks (‘mud-mounds’);
- banks are classified- tabular, knoll, sheet forms based on their external form and internal structure;
- Depositional slopes and sea-floor relief;
- skeletal components;
- non-skeletal grains and fabrics;
- matrix wackestone or packstone;
- coarser sparry fabrics in masses of varied sizes
Deep-water mud-mounds are ecological reefs that lack a dominant metazoan component to their frameworks and are instead composed mostly of micrite or biomicrite. The composition and the temporal and environmental distribution of mud-mounds therefore reflects the evolution, extinction and ecological success of various groups of metazoan frame builders. Proterozoic and Middle Cambrian to Lower Ordovician reefs from both shallow and deeper water are entirely microbial in origin. Sessile, skeleton-secreting frame builders that competed successfully for substrate space appeared briefly in the Early Cambrian, and again in the Early Ordovician. Thus, the mud-mound type of framework was restricted to deeper-water settings after the Early Ordovician, although even then, varying levels of turbulence produced a variety of fabrics, ranging from tufa-like masses rich in synsedimentary cement to biomicrites with networks of large growth-framework cavities.
Mud-mounds have formed persistently in relatively deep water, between about 50 m and 100 m, since the early Proterozoic and contain a fine record of the evolution of the benthic microbial and metazoan frame-building and sediment-producing reef-dwelling community. Proterozoic examples achieved a variety of framework styles ranging from Conophyton stromatolites to highly porous microbial tufas. Biomicrite mud-mounds in the Lower Cambrian point to a spurt of metazoan diversification during the earliest Phanerozoic that ended with a mass extinction before the Middle Cambrian. Otherwise, most Cambrian to Lower Ordovician mud-mound frameworks are made of thin crusts of calcified cyanobacterial filaments and degraded filaments in which Girvanella is commonly preserved.
The biomicrite character of deep-water mud-mounds was restored in the Middle Ordovician when the benthos diversified again, but metazoan frame builders such as siliceous sponges, stromatoporoids and mainly tabulate corals were still only sporadic contributors. Mat-forming cyanobacteria and probably bacteria continued to play a dominant role in younger Palaeozoic mud-mounds, by producing particles, binding physically deposited sediment, and calcifying to form clotted thromboids and microbial crusts. They briefly dominated mud-mound construction immediately after the end-Frasnian mass extinction, but were ‘diluted’ when diversification was renewed, especially with the expansion of fenestrate bryozoans and crinoids in the Mississippian.
Biomicrite mud-mounds persisted into the latest Palaeozoic, but fenestrate bryozoan and crinoidal facies disappeared by the end of the Permian. The Permo-Triassic produced mud-mounds of different character: these frameworks are composed of intergrown thromboids or microbial crusts, newly evolved calcisponges and sessile foraminifera coated by microbial encrustations. The reef-dwelling community, however, seems to have been impoverished, and frame builders are commonly encased in synsedimentary cement rather than matrix sediment. With the decline of the calcisponge fauna, thromboid-siliceous sponge associations comprise some mud-mounds in the Upper Triassic and are widespread in the Jurassic. Cretaceous deep-water mud-mounds also have a dominant microbial component, but although this was reduced with the dramatic niche expansion by hermatypic corals and crustose coralline algae during the Cenozoic, it remains significant as a framework element in deep water.