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Carbonate Buildups and Waulsortian Mud-Mounds |
Institute for Biology and Geology, University of Tromsø, Dramsveien 210, N-9037 Tromsø, Norway
Following a series of mass extinction events towards the end of the Devonian (Frasnian), true skeletal reef-building organisms were effectively lost from latest Devonian (Famennian) and Lower Carboniferous (Dinantian) marine ecosystems. Nevertheless, nonskeletal, mud-dominated buildups (mud-mounds) are common throughout this geological interval, and microbial activity (fungi, algae and/or bacteria) is considered to have played a significant role in the development of these buildups. Evidence of microbial activity in Dinantian buildups may be divided into two broad categories: (a) direct evidence, such as the development of stromatolitic and thrombolitic macrostructures, presence of cryptic incrustations and preservation of calcimicrobes such as Renalcis; (b) cryptic evidence, in the form of peloidal ‘mud’ which forms the matrix of many Dinantian buildups. The matrix peloidal micrites closely resemble modern microbially mediated (algal and bacterial) carbonate precipitates. Precipitation of the peloidal fabrics is considered to have occurred within a microbial mat that bound the accreting buildup surface, thereby allowing steep depositional slopes to develop in deep water buildups and protecting shallow water buildups from erosion. Extensive cavity systems, often developed in Dinantian buildups, may also contain stromatolitic mats and micritized cavity walls. These fabrics indicate the additional presence of internal heterotrophic microbial communities and suggest that a diverse range of microbes were present in Dinantian buildups.
Controls on the development of Dinantian buildups are likely to have been multiple. Nutrient-rich (mesotrophic) water masses, in combination with relatively low sedimentation rates, may have promoted buildup initiation, while regional palaeogeography and tectonic events played a role in their location. Tournaisian buildups commonly developed in a wide range of water depths on carbonate ramps, whereas Viséan buildups, particularly in northwestern Europe, were often restricted to relatively shallow water over tectonically controlled topographic highs.
Evolution of new organisms during mid-Carboniferous time led to a diversification of buildup types. However, microbial fabrics identical to those developed in Dinantian buildups are still present in many late Carboniferous and Permian phylloid algal, Palaeoaplysina and bryozoan buildups. The role of microbial activity in the development of these late Carboniferous and Permian buildups may have been greatly underestimated.
