MISSISSIPPIAN MUD MOUNDS OF DERBYSHIRE (ENGLAND, UK): FACIES ARCHITECTURE AND BIOTIC EVOLUTION

Mud mounds are carbonate buildups, part of the reef spectrum, consisting dominantly of carbonate mud, with no or minor skeletal framework. The carbonate mud commonly shows texture (clotted peloidal, homogeneous with primary cavities), indicative of an origin through in-situ precipitation by biologically-induced and biologically-influenced processes mediated by a microbial-metazoan marine benthic community (automicrite). Mud mounds are the dominant reef type in the Mississippian but have no extant analogue, which hampers the understanding of their growth style, depositional environment, and palaeoecology. The factors influencing mud mound growth and the role of the metazoan dwellers still remain a matter of debate. In particular, the distribution and role of brachiopods in Mississippian mud mounds are scarcely constrained, despite brachiopods are among the most common marine benthic invertebrates in the Mississippian. The brachiopod-rich mud mounds occurring in the uppermost Monsal Dale Limestone For-mation, upper Visean (Brigantian), Mississippian, of the Derbyshire Carbonate Platform succession (UK), were selected as object of this thesis to address some of the open ques-tions on Mississippian mud mounds. Comparative investigation of mud mound lithofacies architecture, texture, and composition and brachiopod fauna in various depositional set-tings on the platform (north-eastern outer platform, intraplatform ramp, southern outer plat-form) allows investigating the factors influencing the growth of the mud mounds and bra-chiopod fauna colonization. Derbyshire mud mounds are decametre-scale lens-shaped buildups frequently coalesced into mud mound complexes, with a massive automicrite-rich core passing laterally outwards into skeletal packstone inclined flank beds transitional to off-mud mound, originally nearly horizontally bedded, skeletal packstone. Commonly bed-ded units occur below (basal unit) and on top (cap unit) of the mud mounds. Chapter 1 presents the introduction to this thesis. Chapter 2 is a literature review of the mud mound concept, the Mississippian record of mud mounds, and the associated brachiopod fauna. Chapter 3 contains a systematic study of a brachiopod fauna collected mainly from one of the Monsal Dale Limestone Fm mud mound complexes cropping out at Ricklow Quarry, developed in the intraplatform ramp. The brachiopod fauna consists of taxa of seven orders: Productida, Orthotetida, Orthida, Rhynchonellida, Spiriferida, Spiriferinida, Terabratulida. Substrate variety in the mud mound complex (soft allomicrite, stiff to hardened in-situ pre-cipitated automicrite, skeletal grains) sustained the brachiopod diversity. Seminfaunal productides were adapted to soft allomicrite substrates while pedicle-attached to ambitopic brachiopods (mainly orthides, rhynchonellides, spiriferides, terebratulides) were adapted to live on a wide range of substrates, often showing adaptations to increase the surface/vol-ume ratio to prevent sinking in soft muddy substrates. Food resources seem to have played the major control on brachiopod fauna composition and distribution: spinose, concavo–convex productides are dominant in the mud mound complex basal unit and core, both in terms of the number of specimens, species, and biovolume; their success was favoured by scattered food resources which they better exploited through their simple feeding appa-ratus, providing a comparative advantage over spiriferides. Chapter 4 provides a study on the lithofacies character and architecture and brachiopod distribution of the mud mound complex at Ricklow Quarry. The mud mound complex grew below fair-weather wave base in middle ramp environment following a relative sea level rise and accommodation creation, overlying inner ramp strata . The mud mound complex core is 10 m thick, 250 m wide, and dominated by clotted peloidal automicrite boundstone with early-marine fibrous calcite cement. The basal bedded unit and inclined flank beds consist of dense automicrite (leiolite, closely-packed clotted peloidal micrite) mixed with skeletal packstone, with no fibrous calcite cement. The mud mound complex is capped by the regional subaerial exposure marking the top of the Monsal Dale Limestone Fm, and overlain in onlap against the exposure surface by wackestone to packstone beds of the Eyam Limestone Fm. Major skeletal components in the mound basal unit and core are fenestellid bryozoans, brachiopods, and siliceous sponge remains (calcified spicules, few sponge bodies), whereas crinoids are common in the flank and off-mud mound complex beds. Contrary to previous interpretations, brachiopods are not confined to storm-scoured 'pockets' in the core, but are abundant and widespread in all mud mound complex lithofa-cies. Thus, the upper Visean mud mounds of Derbyshire are identified as a newly-defined fenestellid bryozoan–brachiopod–siliceous sponge mud mound sub-category, a sub-type of the fenestellid bryozoan–crinoid–brachiopod Type 3 buildups of Bridges et al. (1995). Chapter 5 expands the investigation on lithofacies architecture and brachiopod fauna to other mud mounds in the uppermost Monsal Dale Limestone Fm developed in the north-eastern outer platform (Linen Dale, Bradwell Dale), intraplatform middle-outer ramp (Bradford Dale), and southern outer platform (National Stone Centre). All mud mounds developed below fair-weather wave base following a relative sea-level rise, overlying units deposited in shallower protected to open marine settings above fair-weather wave base. The core lithofacies variability was influenced by the depositional setting (hydrodynamic energy, water depth); clotted peloidal automicrite boundstone dominated in the in-traplatform ramp and in the lower part of mud mounds on the southern outer platform, while fenestellid bryozoan boundstone/skeletal grainstone with early-marine fibrous calcite cement dominated in the north-eastern and southern outer platform. Flank beds always consist of automicrite boundstone passing downslope to skeletal packstone. The cap unit, absent at Ricklow Quarry, consists of mixed automicrite boundstone and skeletal wack-estone, packstone to grainstone/floatstone to rudstone, and was deposited in all settings from below to above the fair-weather wave base. Deposition of the cap unit reflects pro-gressive accommodation filling during mud mound growth and relative sea-level fall lead-ing to the capping regional subaerial exposure at the top of the Monsal Dale Limestone Fm. The brachiopod fauna is widespread and abundant in all the mud mounds but in the lower mud mound core of Bradford Dale, possibly for limited larval transport in the deeper (outer) part of the intraplatform ramp. Diverse substrates sustained seven orders of brachiopods, with dominant seminifaunal productides, suggesting a mesotrophic environment. The main control on brachiopod distribution and diversity was exerted by the food resources and competition with crinoids, able to access a higher tiering level. Higher relative abundance of seminfaunal productides in outer platform than in intraplatform ramp mud mounds is possibly due to less abundant food resources than in the restricted intraplatform ramp set-ting. In synthesis, the comparative analysis of mud mounds in the uppermost Monsal Dale Lime-stone Fm of Derbyshire in a wide spectrum of depositional environments allows to point out that: -Derbyshire upper Visean mud mounds are dominated by precipitated automicrite associ-ated with siliceous sponges, previously underestimated, whereas the contribution of detrital micrite, allomicrite, is minor. -Brachiopods are abundant and diverse in all mud mounds in Derbyshire and indicative of mound growth in mesotrophic conditions. Brachiopod shells contributed to the growth and stability of the mud mounds. Brachiopod distribution was mainly controlled by the availa-bility and distribution of food resources and competition with crinoids, though substrate variety played a role in promoting brachiopod diversity. -Derbyshire upper Visean mud mounds grew following a relative sea-level rise on the Der-byshire Carbonate Platform, with decreased sedimentation rate. Accommodation creation was probably linked to tectonic activity. Mud mound growth was halted by forced regres-sion driven by a, possibly eustatic, sea-level fall leading to regional subaerial exposure. During the following transgressive phase only limited mud mound occurrences are just few, in intraplatform inner ramp environment. -Derbyshire mud mounds are part of a spectrum of upper Visean (Brigantian) automicrite-dominated mud mounds differing from older upper Visean (Asbian) reefs (e.g., Cracoean reef of Yorkshire) and mud mounds in the absence of calcareous algae, corals, less skeletal diversity, and possibly deeper depositional setting.