Review of Irish Lower Carboniferous (Mississippian) Mud Mounds: Depositional Setting, Biota, Facies, and Evolution
Ian D. Somerville, 2003. "Review of Irish Lower Carboniferous (Mississippian) Mud Mounds: Depositional Setting, Biota, Facies, and Evolution", Permo-Carboniferous Carbonate Platforms and Reefs, Wayne M. Ahr, Paul M. (Mitch) Harris, William A. Morgan, Ian D. Somerville
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Lower Carboniferous (Mississippian) mud mounds in Ireland developed in two main depositional settings: on the distal part of ramps and on the outer shelf margins. They formed predominantly during the Late Tournaisian and Late Viséan, associated with major transgressive episodes. The majority of the massive mounds have peloidal mud-matrix textures and stromatactoid cavities.
Waulsortian mud mound complexes reached maximum development (geographical extent and total thickness) in Late Tournaisian times. These mounds initiated in relatively deep water (150-300 m) on the distal parts of ramps in low-energy, aphotic environments. Their characteristic biota comprises crinoids, fenestellid bryozoans, and sponge spicules (Type 1 mounds). Apart from stromatactoid cavities, they may also exhibit sheet spars, fissure fills, and dewatering structures associated with slumping. Waulsortian complexes contain laterally extensive mounds and banks ranging typically from 20 to 100 m thick, which may coalesce and when stacked can be over 900 m in total thickness, as in the Shannon Trough. Individual lensoid mounds are often enclosed by thin-bedded, crinoidal, argillaceous limestones. Except from the upper parts of mounds, calcareous algae are normally absent. “Waulsortian-type” mud mounds (Type 1A mounds) extend through the Viséan and into the Upper Carboniferous sections as a continuum. They are recorded from relatively deep- water ramps and basins in Arundian and Asbian rocks in NW and SW Ireland. These younger mounds have a biota similar to the Waulsortian, but they often have colonial corals and dasycladacean green algae towards the top, indicative of growth up into the dysphotic or lower euphotic zone.
In Viséan time, massive shallow-water mud mounds developed at outer shelf margins facing deep-water basins. These carbonate shelves resulted from active tectonism in Early Viséan time. In the northern margin of the Dublin Basin, isolated domical mounds occur, ranging in thickness from 10 to 100 m and enclosed by bedded, skeletal grainstones. These mounds have a much more diverse and richer biota than Waulsortian mounds, most notably including locally abundant red and green calcareous algae, foraminifers, and cyanophytes (Type 2 mounds). Unlike most Waulsortian mounds, they have encrusting foraminifers (Tetrataxis and Aphralysia), encrusting bryozoans, Problematica (Fasciella), and calcimicrobes (Renalcis and Ortonella). In the Upper Viséan sections the upper parts of some large mounds contain in situ colonial rugose corals (Siphonodendron and Lithostrotion), which, together with the microbiota, can form a framework or meshwork in the upper parts of mounds (Type 2A mounds). Oncoids, pillars, and stromatolitic microbial laminae (microbialites) occur within the cores of the mounds.
Some of the upper Viséan (Asbian-Brigantian) mud mound complexes show intervals of peloid-rich lime mudstone and wackestone (“core” facies), and coarser-grained foraminiferal-algal packstone and grainstone (“flank”, “crest”, or “intermound” facies). The latter are rich in green algae (Koninckopora, Kamaena) and red algae (Ungdarella, Stacheoides). Rare phylloid algae (?Archaeolithophyllum) at the tops of some mounds, together with colonial rugose coral thickets and encrusting cyanophytes, may have formed an effective wave-resistant bindstone (Type 2B mounds). These phylloid algae became the dominant biota of buildups (Type 3 mounds) in the Late Carboniferous.