During the Mississippian "mud mounds" emerged as a key geological feature, consisting of carbonate buildups with significant depositional relief and a composition primarily made up of carbonate mud (micrite). These structures typically lack a well-developed skeletal framework. A classification developed in the 1990s grouped mud mounds based on the dominant skeletal elements (e.g., bryozoans, crinoids, and brachiopods) and their depositional environments. However, this system fails to address the nature of the carbonate mud, a crucial component that requires reassessment.
Micrite found in mud mounds exhibits considerable variation in terms of grain size, color, and texture, reflecting the incorporation of different types of mud, each with distinct origins. Broadly, micrite can be divided into two categories: (1) allochthonous mud, transported by water currents and later trapped by benthic organisms, and (2) autochthonous mud, produced by biogenic processes occurring locally. The autochthonous fraction is generated through microbial activity or the presence of organic substrates. Identifying and distinguishing between these two types is vital for understanding the processes behind mud mound formation.
In this study, we present new data on the carbonate mud from upper Visean mud mounds located in the Derbyshire Carbonate Platform (UK). These large-scale mounds, characterized by a thick muddy core, host a range of bioclastic materials, including fenestellid bryozoans, brachiopods, and calcified sponge spicules. The majority of the mud is interpreted as allochthonous, enveloping bioclastic debris and exhibiting transport-related textures. A smaller portion of the mud is autochthonous, showing clotted peloidal and leiolite textures. The clotted peloidal material is likely microbially induced and is mainly found in small cavities between skeletal components, while the leiolite fraction, rich in sponge spicules, is probably the result of taphonomic processes involving decaying sponge tissues in interskeletal spaces. Fluorescence, geochemical, and SEM analyses further confirm the distinction between the two fractions.
These findings provide valuable insights into micrite composition within the Derbyshire mud mounds and contribute to refining the classification system for Mississippian mud mounds. This revision is particularly timely as recent studies have identified a variety of mud mounds across regions such as Western and Eastern Europe, North America, North Africa, South China, and Australia. A revised classification should focus on the origin and role of micrite, in addition to skeletal components, mound shape, and depositional environment.
 

mud mounds