Signatures of hydrocarbon venting in a Middle Devonian Carbonate Mound (Hollard Mound) at the Hamar Laghdad (Antiatlas, Morocco)

Summary The Middle Devonian Hollard Mud Mound is situated in the eastern Hamar Laghdad, which is a small mountain range in the Tafilalt in SE Morocco. In contrast to the well known Lower Devonian Kess-Kess mounds, the Hollard Mound is of Middle Devonian age. The facies in the core of this mud mound differs from that of the other parts of the mound, and exhibits signatures of ancient hydrocarbon venting. The carbonate phases of the core facies are derived from the oxidation of vent fluids and consist of clotted micrite, a cryptocrystalline carbonate associated with spheres of uncertain origin, and a calcitic rim cement (rim cement B). These vent carbonates show δ¹³C values in the range of −11 to −20% PDB indicating that some of their carbon is derived from isotopically light hydrocarbons. Fossiliferous micrite has been affected by hydrocarbon venting in the proximity of the vent site, which is indicated by intermediate δ¹³C values between vent carbonates and not affected sediments. Bivalves occur in dense populations within the core facies. They form autochthonous shell accumulations and are almost exclusively articulated. it is likely that these bivalves were dependent on chemosynthesis similar to their counterparts at modern vents. The vent deposits also exhibit an unusual prasinophyte assemblage, which might have been linked to the specific nutrient availability at the vent site. The ancient vent site is characterized by an enhanced carbonate precipitation and rapid lithification. The latter is corroborated by the three-dimensional preservation of phytoplankton (prasinophytes and acritarchs) and the occurrence of stromatactoid pores. An early phase of carbonate corrosion predating the formation of vent carbonates affected the fossiliferous micrite of the core facies and is thought to be related to a phase of H₂S-rich venting.     

Paleoecology of Middle Ordovician stromatoporoid mounds in Vermont

Fossiliferous mounds of carbonate mud are a distinctive facies in the middle Chazy Group (Crown Point Formation) at Isle La Motte, Lake Champlain. The mounds are surrounded by bedded calcarenite of spar-cemented pelmatozoan debris. Channels which cut into the mounds during mound growth are filled with the same calcarenite. The mud-free intermound rocks and the mound biota suggest agitated, normal marine shallow-water environments. The principal lime-secreting organisms within the mounds are stromatoporoids, calcareous algae, tabulate corals, sponges, and bryozoans. Each mound is dominated in terms of biomass by one of three groups: stromatoporoids, calcareous algae, and bryozoans. Most of the mound biota first appear at the base of the Crown Point Formation. In the lower Crown Point Formation the organisms increase in number and species. Both changes in the biota are related to periods of shallowing of the Chazy sea which are also reflected in the character of the carbonate sands.     

Mud mounds: A polygenetic spectrum of fine-grained carbonate buildups

Summary This research report contains nine case studies (part II to X) dealing with Palaeozoic and Mesozoic mud mounds, microbial reefs, and modern zones of active micrite production, and two parts (I and XI) summarizing the major questions and results. The formation of different types ofin situ formed micrites (automicrites) in close association with siliceous sponges is documented in Devonian, Carboniferous, Triassic, Jurassic and Cretaceous mounds and suggests a common origin with a modern facies found within reef caves. Processes involved in the formation of autochthonous micrites comprise: (i) calcifying mucus enriched in Asp and Glu, this type presumably is linked to the formation of stromatolites, thrombolites and massive fabrics; (ii) protein-rich substances within confined spaces (e.g. microcavities) result in peloidal pockets, peloidal coatings and peloidal stromatolites, and (iii) decay of sponge soft tissues, presumably enriched with symbiotic bacteria, lead to the micropeloidal preservation of parts of former sponge bodies. As a consequence, there is strong evidence that the primary production of micrite in place represents the initial cause for buildup development. The mode of precipitation corresponds to biologically-induced, matrix-mediated mineralization which results in high-Mg-calcites, isotopically balanced with inorganic cements or equilibrium skeletal carbonates, respectively. If distinct automicritic fabrics are absent, the source or origin of micrite remains questionable. However, the co-occurring identifiable components are inadequate, by quantity and physiology, to explain the enhanced accumulation of fine-grained calcium carbonate. The stromatolite reefs from the Permian Zechstein Basin are regarded as reminiscent of ancestral (Precambrian) reef facies, considered the precursor of automicrite/sponge buildups. Automicrite/sponge buildups represent the basic Phanerozoic reef type. Analogous facies are still present within modern cryptic reef habitats, where the biocalcifying carbonate factory is restricted in space.     

A microbial model for the Lower Devonian stromatactis mud mounds of the Montagne Noire (France)

Summary Lower Devonian mud mounds and stromatactis fabrics are exceptionally well exposed in quarry walls and industrially sawed blocks in the Montagne Noire in southern France. Interlayered red biomicrites and white to grey sparitic calcites form mounds up to 70 m high. The red biomicrites contain predominantly bryozoans, sponges and echinoderms. The sparitic layers show typical features of stromatactis fabrics, as outlined byBathurst (1982). We recognize two types of stromatactis fabrics: (1) Stromatactis type A: exentsive cavity systems filled by multiple cement generations, which are interpreted to be related to microbial mats, and (2) Stromatactis type B: smaller patches of blocky spar which are mainly diagenetic in origin, but show characteristic features of stromatactis. Type A is far more important in terms of rock volume. The cyclic interlayering of red biomicrites and sparitic layers is supposed to result from frequent changes in the composition of the mound biota. The bryozoan/sponge community was displaced by short term propagations of microbial mats during times of extremely low sedimentation. Sedimentation and thus the biotic community was probably determined by high frequency (6th order) sea level changes. Despite these changes, mound growth continued, because once established the ecological advantage over the surroundings was maintained by both communities alternating with each other. The microbial mats and the cavities they left after their decay were important for the stabilization of the mounds, the latter allowing for enormous quantities of dissolved carbonate to be transported and precipitated. We anticipate a close interrelation between mound formation and stromatactis formation, and we believe that it is not incidential that both, mud mounds and stromatactis, are mainly restricted to the same interval, namely the Paleozoic.     

Reconstructing atoll-like mounds from the Frasnian of Belgium

A succession of Frasnian mounds on the southern border of the Dinant Synclinorium (Belgium) was investigated for their facies architecture, sedimentary dynamics and palaeogeographic evolution. Seven mound facies were defined from the Arche (A) and Lion (L) members, each characterized by a specific range of textures and association of organisms (A2/L2: red or pink limestone with stromatactis, corals and crinoids; A3/L3: grey, pink or green limestone with stromatactis, corals and stromatoporoids; A4/L4: grey limestone with corals, peloids and dasycladaceens; A5/L5: grey microbial limestone; A6/L6: grey limestone with dendroid stromatoporoids; A7/L7: grey laminated limestone with fenestrae; and A8/L8: grey bioturbated limestone). Laterally equivalent sediments include substantial reworked material from the buildups and background sedimentation. Textures and fossils suggest that A2/L2 and A3/L3 facies developed close to storm wave base, in a subphotic environment. Facies A4/L4, occurring near fair weather wave base in the euphotic zone, includes lenses of A5/L5 with stromatolitic coatings and thrombolithes. A6/L6 corresponds to a slightly restricted environment and shows a progressive transition to fenestral limestone of A7/L7. This facies was deposited in a moderately restricted intertidal area. A8/L8 developed in a quiet lagoonal subtidal environment. The mounds started with A2/L2 or A3/L3 in which microbial lenses and algal facies A4/L4 became progressively more abundant upwards. Following 20 m of laterally undifferentiated facies, more restricted facies occur in the central part of the buildups. This geometry suggests the initiation of restricted sedimentation, sheltered by bindstone or floatstone facies. The facies interpretation shows that after construction of the lower part of the mounds during a transgression and a sea-level highstand, a lowstand forced reef growth to the margin of the buildups, initiating the development of atoll-like crowns during the subsequent transgressive stage. The persistence of restricted facies results from the balance between sea-level rise and reef growth.     

Franken Mound: facies and biocoenoses on a newly-discovered “carbonate mound” on the western Rockall Bank,NE Atlantic

Cold-water coral carbonate mounds are widespread along the Irish continental margin. Whereas the Porcupine Seabight and the Rockall Trough are relatively well studied with regard to mound topography, coral coverage, and benthic life diversity, the situation on the western Rockall Bank is rather unknown. Detailed facies and biocoenoses mapping based on video footage analyses was conducted on the newly-discovered Franken Mound. Facies were identified ranging between cliff-like to planar hardgrounds and soft sediments that are partly rippled. A variety of biocoenoses are associated with these facies comprising discrete live coral colonies, dense live and dead coral framework coverage, abundant to scattered coral debris, and a soft sediment faunal community, whereas the latter is three times less speciose as biocoenoses containing live framework-building corals. The facies and biocoenosis classes are supplemented by exposed dropstones, lost fishery nets, and rubbish. The distribution of the classes clearly indicates a close relationship with local current effects and current intensification. Due to the dominance of dead coral framework and the partially exposed internal sediment sequences on the mound flanks, it is assumed that Franken Mound is approaching the “mound retirement” mound growth state.     

Norian serpulid and microbial bioconstructions: Implication for the platform evolution in the Lombardy Basin (Southern Alps,Italy)

Summary The development of peculiar margin facies and abundant talus breccias within the Dolomia Principale inner platform is commonly observed in the Lombardy Basin during the Norian. The organisms building these margins are mainly serpulids, benthic microbes, subordinate porostomata and other encrusting forms; typical margin organisms, as sponges or corals, are extremely rare or absent. The build-ups form narrow rims along the borders of tectonic-controlled intraplatform basins. Regional back-stepping and progradation of the margin facies on the talus breccias produced by the erosion of the reef is commonly observed in the uppermost Dolomia Principale depositional system. Widespread occurrence of serpulids and microbial margins in middle-late Norian times is indicative of stressed environmental conditions—fluctuation of salinity and temperature on the inner platform and in the intraplatform basins—controlled by palaeogeographic setting. Physical characteristics allowed the bloom of forms able to develop in a wide range of environmental conditions, such as serpulids. In the Late Norian, major input of fine-grained clastics is recorded; close to the Norian-Rhaetian boundary, carbonate ramps were regionally restored. Locally, small serpulid and microbial bioconstructions still persist in the lowermost part of the shaly succession, even if they are less abundant with respect to the Dolomia Principale. Patch-reefs generally do not build a platform margin, but represent isolated mounds within shaly deposits. These build-ups occur on the edge of former structural highs; the communities survived the environmental change responsible for the siliciclastic input and locally managed to produce mounds during the deposition of the lower part of the upper depositional system (Riva di Solto Shale).     

Iron microbial communities in Belgian Frasnians carbonate mounds

Summary The Belgian Frasnian carbonate mounds occur in three stratigraphic levels in an overall backstepping succession. Petit-Mont and Arche Members form the famous red and grey “marble” exploited for ornamental stone since Roman times. The evolution and distribution of the facies in the mounds is thought to be associated with ecologic evolution and relative sea-level fluctuations. Iron oxides exist in five forms in the Frasnian mounds; four are undoubtedly endobiotic organized structures: (1) microstromatolites and associated forms (blisters, veils...), possibly organized in “endostromatolites”; (2) hematitic coccoids and (3) non dichotomic filaments. The filaments resemble iron bacteria of theSphaerotilus-Leptothrix “group”; (4) networks of dichotomic filaments ascribable to fungi; (5) a red ferruginous pigment dispersed in the calcareous matrix whose distribution is related to the mound facies type. The endobiotic forms developed during the edification of the mounds, before cementation by fibrous calcite. The microbial precipitation of iron took place as long as the developing mounds were bathed by water impoverished in oxygen.     

From sediment to rock: diagenetic processes of hardground formation in deep-water carbonate mounds of the NE Atlantic

Modern cool-water carbonate mounds topped by corals form an extended reef belt along the NW European continental margin at 200–1200 m water depth. An essential element of mound growth are hardgrounds which provide a stable substratum for mound-building invertebrate colonisation and stabilise the inclined mound flanks. Evaluating the degree of lithification and the slope stability against erosion represents an important task within the ESF programme MOUNDFORCE under the umbrella of EUROMARGINS. Sampling of hardgrounds during RV Meteor cruises M61-1 and -3 in 2004 by means of the IFM-GEOMAR TV-grab and the Bremen ROV QUEST focused on carbonate mounds of the Porcupine Seabight and northwestern Rockall Bank off Ireland. Lithified carbonates of mid-Pleistocene age were exhumed during the Holocene and are now exposed on the top and flanks of numerous carbonate mounds showing a patchy to dense colonisation by living corals and associated invertebrates. The sediments, composed of foraminiferal–nannoplankton oozes and admixed mound-derived invertebrate skeletons, range from partly lithified chalks to dense micritic limestones. These wackestones to packstones clearly differ from bacterially induced authigenic carbonate crusts typical of hydrocarbon seep settings by showing current-induced sedimentary structures, a non-luminescing matrix indicating oxic pore fluids, and a marine isotopic signature lacking any depleted carbon regime which is typical of anaerobic methane oxidation. The carbonate lithification is driven by carbonate ion diffusion from supersaturated seawater into the pore fluids in the studied areas. Vigorous bottom currents were the ultimate control not only of carbonate cementation by enhancing the diffusion process and supporting a pumping mechanism, but also of hardground formation and mound shaping by exhuming lithified carbonates and preventing fine-grained sediment accumulation at the downslope mound flanks.     

Benthic palaeoecology of Danian deep-shelf bryozoan mounds in the Danish Basin

Early Danian cool-water bryozoan mounds exposed in the coastal cliff Stevns Klint in Denmark were formed shortly after the Cretaceous–Tertiary mass extinction. They represent a relatively deep-water, highly diverse benthic ecosystem within the epeiric seaway that covered the Danish Basin. The mounds are 50–110 m long and reached a height of about 5–10 m above the seafloor; they are asymmetrical with a steep southern and a gentle northern flank, and were dominated by small suspension feeders. The benthic elements generally occur as fragments set in a carbonate mud matrix. The main skeletal contributors are delicate branching bryozoans with minor contributions of bryozoan sheets, and nodular/arborescent bryozoans. Locally abundant octocorals occur on the mound crests and upper parts of the steep flanks. Echinoids are present in minor amounts, but are locally abundant. Serpulids, crinoids, asteroids, brachiopods, bivalves, massive calcareous sponges, and benthic foraminifers are generally minor contributors to the benthic mound fauna. Influx of planktonic foraminifers, coccoliths and other planktonic organisms was high and was probably a major source of nutrient supply to the mainly suspension-feeding benthic fauna.

The faunal association reflects a relatively low energy environment with a high, possibly seasonal influx of particulate nutrients. The best growth conditions with respect to nutrient influx were on the mound crest and upper steep flank reflected by the diverse and relatively largest benthic faunal elements. Periodic reworking and winnowing occurred across the entire mound structure but most prominent on the gentle northern flanks limiting the benthic growth and notably the colony density and size of delicate branching bryozoans. Vagile benthic faunas were also adapted to different areas on the mound. Irregular echinoids preferred the intermound areas within fine-grained wackestone–packstone facies where they ploughed through the sediment, whereas regular echinoids were epifaunal and preferred the upper parts of the mounds, possibly feeding mainly on bryozoans. Skeletons of both groups became concentrated at the toe of the steep flanks and in the intermound areas by physical reworking during major storms.

Changes in faunal composition on the mound crests occurred rhythmically on both small and large scale during mound growth. Rhythmically recurring faunal assemblages reflect alternating hydrodynamic conditions on the seafloor with respect to nutrient influx and energy, which probably were linked to short-term seasonal and long-term climatic variations; the long-term alternation may be within the Milankovitch frequency band. Blooming events of bryozoan sheets resulted from relatively short periods with large amounts of available food and suitable substrate. Successful colonisation by octocorals on the other hand reflected longer-term favourable conditions on the mounds possibly associated with overall higher energy levels.

A possible Pleistocene analogue to the bryozoan-dominated Danian mounds occurs at the shelf-slope break of the Great Australian Bight. Both of these cool-water mound systems deviate from most other biogenic mounds known from the fossil record in their non-cemented nature, regular geometry and a lack of core and flank facies.     

塔里木板块塔中区上奥陶统良里塔格组的生物礁类型

塔里木板块塔中Ⅰ号坡折带附近上奥陶统良里塔格组取芯井段中可识别多种生物礁灰岩类型,包括珊瑚骨架/障积岩、海绵骨架/绑结岩、苔藓虫绑结岩、钙藻障积岩、钙质菌藻障积/绑结岩等礁灰岩类,藉此可归纳出珊瑚礁、珊瑚-钙藻礁、层孔虫礁、层孔虫-钙藻礁、珊瑚-层孔虫-钙藻礁、苔藓虫礁丘、钙藻礁丘、灰泥丘和微生物礁等生物建造单元。这些礁体的时空分布模式与古环境分异相关联,纵向上具有灰泥丘向珊瑚-层孔虫-钙藻礁至苔藓虫礁丘和钙藻礁的群落结构更替趋势;空间分布则向台地北缘,即I号坡折带延伸显示由低能带灰泥丘向高能带珊瑚-层孔虫-钙藻礁的相变,而且高能带珊瑚-层孔虫-钙藻主体礁和环其周缘相对低能带的钙藻礁丘、灰泥丘等在一定范围内构成造礁群落结构分异。     

A bryozoan buildup from the Lower Carboniferous of North Wales

A carbonate buildup dominated by trepostome Bryozoa is described from Dinantian (Asbian) strata near Llandudno in North Wales. A three-phase ecological succession is recognised within the buildup: (i) a basal diverse community with fenestrate, ramose, encrusting trepostome and cystoporate bryozoans in a mud rich wackestone; (ii) a bulk facies, dominated by encrusting and foliaceous, trepostome bryozoans in a fine packstone, and (iii) a thin capping phase, dominated by unilaminar, encrusting trepostome bryozoans in a slightly coarser lithology, including skeletal debris derived from the mound top and possible flanking beds. The buildup probably had topographic relief and developed in a shallow marine environment. The internal tripartite zonation reflects the growth of the structure into a shallower, higher energy regime, with the capping beds being deposited just below wave base. The buildup developed to the north of St. George's Land, on a carbonate shelf edge bordering the deeper basinal facies of the Irish Sea Basin. Dinantian, Asbian, buildup, trepostome Bryozoa, Foraminifera, corals, calcareous algae, Carboniferous, North Wales .     

A close look at late carboniferous algal mounds: Schulterkofel,Carnic Alps,Austria

Summary During the uppermost Carboniferous and lowermost Permian algal mounds were formed in inner shelf settings of the Carnic Alps (Austria/Italy). A specific mound type, characterized by the dominance of the dasyclad green alga Anthracoporella was studied in detail with regard to geometry, relationship between mound and intermound rocks, composition of the sediment, biota and diagenetic criteria. The two meter-sized mounds studied, occur within depositional sequences of transgressive systems tracts in the Lower Pseudoschwagerina Limestones (uppermost Gzhelian) at the flank of the Schulterkofel. The mounds consist of an Anthracoporella core facies with a spongecrust boundstone facies at the base and at the top. The massive limestones of the Anthracoporella core facies exhibit abundant algal tufts and bushes, frequently in life position. The limestones of the intermound facies represented by thin-bedded bioclastic wackestones and packstones with abundant phylloid algae underlie and overlie the mounds. Intercalations of intermound beds within the mound facies indicate sporadic disruption of mound growth. Onlapping of intermound beds on steep mound flanks indicate rapid stabilization and lithification of mound flanks and the existence of a positive paleorelief. Asymmetrical shape of the mounds may be current controlled. Mound and intermound biota differ in the prevailing algae but are relatively similar with regard to associated foraminifera. Conspicuous differences concern bioerosion and biogenic encrustations. Bothare, high in intermound areas but low in the Anthracoporella core facies. The mounds show no ecological zonation. The mounds grew by in-place accumulation of disintegrated algal material and trapped bioclastic material between erect algal thalli. The comparison of the various Anthracoporella mounds demonstrates that almost each mound had ist own history. Establishing a general model for these mounds is a hazardous venture.     

Reefal and mud mound facies development in the Lower Devonian La Vid Group at the Colle outcrops (León province, Cantabrian Zone, NW Spain)

In the locality of Colle (Cantabrian Zone, NW Spain), the upper part of the Valporquero Shale Formation (Emsian, La Vid Group) contains an interval of shales and marlstones (barren, greenish-grey shales and fossiliferous, greenish-grey or reddish shales/marlstones) with beds and packages of homogeneous and cross-bedded skeletal limestones. Metre-scale mud mounds and coral biostromes occur encased in the fossiliferous reddish and greenish-grey shale/marlstones, respectively, with the coral biostromes overlying conspicuous skeletal limestone bodies. These rocks were deposited on a carbonate ramp, ranging from above storm wave base for the cross-bedded skeletal limestones to below the storm wave base for the remaining deposits, organic buildups included. The vertical stacking of these facies and the occurrence of the two types of buildups are interpreted to reflect the interplay among several (possibly 4th and 5th) orders of relative sea-level variations, during a 3rd-order highstand. Coral biostromes occur in early 5th-order transgressive system tracts developed within late 4th-order highstand, and are interpreted to have thrived on a stable granular substrate (skeletal limestones) in non-turbid waters, being later aborted by the onset of muddy sedimentation. Biostrome features suggest that they developed under environmental conditions essentially different from those related to the sedimentation of their granular substrate. Mud mounds occur in 5th-order transgressive and early highstand system tracts tied to early 4th-order sea-level rise. Field relationships suggest that mud mounds grew coevally with muddy sedimentation, with high-frequency variations in carbonate vs. terrigenous mud sedimentation influencing their development.An erratum to this article can be found at     

Turbulence-controlled succession in Middle Devonian reefs of eastern New York State

Wolosz. T. H. 1992 07 15: Turbulence-controlled succession in Middle Devonian reefs of eastern New York State.
The Edgecliff Member ol the Middle Devonian Onondaga Formation contains numerous reefs comprised of two distinct facies. The Phaceloid Colonial Rugosan Facies consists of thickets and mounds, while the Favositid/Crinoidal Sand facies occurs as flank beds surrounding rugosan mounds and as low shield-shaped banks interbedded with thickets of the colonial rugosan facics. Three of these reefs - the North Coxsackie. Albrights and Roberts Hill reefs - have been studied in order to determine the factors that controlled their development and their preserved paleocommunity succession. Both the Roberts Hill and Albrights reefs display well-developed rugosan mounds with an internal succession of rugosan genera. The North Coxsackie reef is a crinoidal sand bank with rugosan thickets and a back-reef satellite mound. Based on the lithology of the underlying limestone in which the reefs are rooted, the North Coxsackie reef is considered to have grown in a shallow-water environment, landwards of the two other reefs. Successional sequences or partial sequences are common to the three reefs, and are found to be reversible - a response attributed to changes in sea-level. As a result, the successions preserved in these reefs are interpreted as having been controlled by degree ol water turbulence.     

Danian cold‐water corals from the Baunekule facies,Faxe Formation,Denmark: a rare taphonomic window of a coral mound flank habitat

Well‐preserved cold‐water corals are comparatively rare in the fossil record. This is partly due to the very low fossilization potential of the predominantly aragonitic corals but also due to the fact that coral ecosystems of deep water are a geologically young development. A Middle Danian cold‐water coral mound complex is well exposed in Faxe Quarry, Denmark. The coral mounds are intercalated with bryozoan mounds of various sizes and form the Faxe Formation. The coral limestone displays large variations in diagenesis, and this complicates the palaeoecological reconstructions. However, the Baunekule facies from the Faxe Formation contain a well‐preserved originally aragonitic and calcitic fauna. The aragonitic skeletons have been recrystallized to calcite during early diagenesis and the excellent preservation makes taxonomic identifications straightforward. A diverse fauna of ten scleractinian coral species, nine stylasterine coral species and seven octocoral species has been described from the Baunekule facies. The fossil fauna represents an ecological niche between the dead coral framework and coral rubble on a flank of a growing Dendrophyllia coral mound with multiple colonization events. The diversity and relative abundance of the fossil scleractinian corals are comparable to the modern settings in the NE Atlantic and Mediterranean. The distribution and diversity of the octocorals and the stylasterine corals are suggested to represent coral gardens as described from modern setting in the NE Pacific. The presence of a diverse and abundant stylasterine fauna suggests a stable palaeoenvironment, probably in a bathymetric depth range of 200–400 metre.     

Microbial boundstone dominated carbonate slope (Upper Carboniferous,N Spain): Microfacies,lithofacies distribution and stratal geometry

Summary The Carboniferous, particularly during the Serpukhovian and Bashkirian time, was a period of scarce shallow-water calcimicrobial-microbialite reef growth. Organic frameworks developed on high-rising platforms are, however, recorded in the Precaspian Basin subsurface, Kazakhstan, Russia, Japan and Spain and represent uncommon occurrences within the general trend of low accumulation rates and scarcity of shallow-water reefs. Sierra del Cuera (Cantabrian Mountains, N Spain) is a well-exposed high-rising carbonate platform of Late Carboniferous (Bashkirian-Moscovian) age with a microbial boundstone-dominated slope dipping from 20° up to 45°. Kilometer-scale continuous exposures allow the detailed documentation of slope geometry and lithofacies spatial distribution. This study aims to develop a depositional model of steep-margined Late Paleozoic platforms built by microbial carbonates and to contribute to the understanding of the controlling factors on lithofacies characteristics, stacking patterns, accumulation rates and evolution of the depositional architecture of systems, which differ from light-dependent coralgal platform margins. From the platform break to depths of nearly 300 m, the slope is dominated by massive cement-rich boundstone, which accumulated through the biologically induced precipitation of micrite. Boundstone facies (type A) with peloidal carbonate mud, fenestellid and fistuliporid bryozoans, sponge-like molds and primary cavities filled by radiaxial fibrous cement occurs all over the slope but dominates the deeper settings. Type B boundstone consists of globose centimeter-scale laminated accretionary structures, which commonly host botryoidal cement in growth cavities. The laminae nucleate around fenestellid bryozoans, sponges, Renalcis and Girvanella-like filaments. Type B boundstone typically occurs at depths between 20–150 m to locally more than 300 m and forms the bulk of the Bashkirian prograding slope. The uppermost slope boundstone (type C; between 0 and 20–100 m depth) includes peloidal micrite, radiaxial fibrous cement, bryozoans, sponge molds, Donezella, Renalcis, Girvanella, Ortonella, calcareous algae and calcitornellid foraminifers. From depths of 80–200 m to 450 m, 1–30 m thick lenses of crinoidal packstone, spiculitic wackestone, and bryozoan biocementstone with red-stained micrite matrix are episodically intercalated with boundstone and breccias. These layers increase in number from the uppermost Bashkirian to the Moscovian in parallel with the change from a rapidly prograding to an aggrading architecture. The red-stained strata share comparable features with Lower Carboniferous deeper-water mud-mound facies and were deposited during relative rises of sea level and pauses in boundstone production. Rapid relative sea-level rises might have been associated with changes in oceanographic conditions not favourable for thecalcimicrobial boundstone growth, such as upwelling of colder, nutrient-rich waters lifting the thermocline to depths of 80–200 m. Downslope of 150–300 m, boundstones interfinger with layers of matrix-free breccias, lenses of matrix-rich breccias, platform- and slope-derived grainstone and crinoidal packstone. Clast-supported breccias bound by radiaxial cement are produced by rock falls and avalanches coeval to boundstone growth. Matrix-rich breccias are debris flow deposits triggered by the accumulation of red-stained layers. Debris flows develop following the relative sea-level rises, which favour the deposition of micrite-rich lithofacies on the slope rather than being related to relative sea-level falls and subaerial exposures. The steep slope angles are the result of in situ growth and rapid stabilization by marine cement in the uppermost part, passing into a detrital talus, which rests at the angle of repose of noncohesive material. In the Moscovian, the aggradational architecture and steeper clinoforms are the result of increased accommodation space due to tectonic subsidence and due to a reduction of slope accumulation rates (from 240±45−605±35 m/My to 130±5 m/My). The increasing number of red-stained layers and the decrease of boundstone productivity are attributed to environmental changes in the adjacent basin, in particular during relative rises of sea level and to possible cooling due to icehouse conditions. The geometry of the depositional system appears to be controlled by boundstone growth rates. During the Bashkirian, the boundstone growth potential is at least 10 times greater than average values for ancient carbonate systems. The slope progradation rates (nearly 400–1000 m/My) are similar to the highest values deduced for the Holocene Bahamian prograding platform margin. The fundamental differences with modern systems are that progradation of the microbial-boundstone dominated steep slope is primarily controlled by boundstone growth rates rather than by highstand shedding from the platform top and that boundstone growth is largely independent from light and controlled by the physicochemical characteristics of seawater.     

Sedimentology and stromatoporoid palaeoecology of Frasnian (Upper Devonian) carbonate mounds in southern Belgium

Da Silva, A.‐C., Kershaw, S. & Boulvain, F. 2011: Sedimentology and stromatoporoid palaeoecology of Frasnian (Upper Devonian) carbonate mounds in southern Belgium. Lethaia, Vol. 44, pp. 255–274. Stromatoporoids are the most abundant large skeletal organisms in middle Frasnian carbonate mound environments of southern Belgium. They occur in environments ranging from flank and off‐mound, mound core, shallow mound and restricted mound. A detailed log and comprehensive sampling of stromatoporoids in a single section cutting through all middle Frasnian mound levels in La Boverie–Rochefort Quarry, near Rochefort and Dinant reveals a stromatoporoid assemblage comprising 10 genera; 472 samples, containing an overall total of 3079 stromatoporoids (including complete and fragmented specimens) have been studied. The following list gives abundance using numbers of specimens and areas of total stromatoporoid area on outcrop surfaces (% number; % area in cm²): Actinostroma (0.4; 9.2), Amphipora (15.5; 1.7), Atelodictyon (0.2; 4.4), Clathrocoilona (0.3; 0.5), Euryamphipora (13.7; 0.7), Idiostroma (2; 1.9), Salairella (1.2; 9.6), branching Stachyodes (43.2; 59.1), laminar Stachyodes australe (1.9; 1.3), Stictostroma (4.8; 13.1) and Trupetostroma (0.2; 0.8), showing that Stachyodes is approximately half of the total assemblage. Deeper environments contain more abundant low profile forms, shallow water facies contain more domical and bulbous forms; branching forms are ubiquitous. Low profile stromatoporoids are likely to have been important sediment stabilizers that may have led to expansion of the carbonate factory, and they may have therefore contributed to the structural building of the mounds. Stromatoporoid‐coral intergrowths are observed in only Stictostroma suggesting that there is a close biological relationship between them; however, stromatoporoid skeletons in almost all cases appear to be unaffected by the presence of intergrown corals, suggesting they were commensals. □Frasnian, Late Devonian, mounds, palaeoecology, stromatoporoid.     

Coral-rudist bioconstructions in the Upper Cretaceous Haidach section (Gosau Group; Northern Calcareous Alps, Austria)

Summary In the area of Haidach (Northern Calcareous Alps, Austria), coral-rudist mounds, rudist biostromes, and bioclastic limestones and marls constitute an Upper Cretaceous shelf succession approximately 100 meters thick. The succession is part of the mixed siliciclasticcarbonate Gosau Group that was deposited at the northern margin of the Austroalpine microplate. In its lower part, the carbonate succession at Haidach comprises two stratal packages that each consists, from bottom to top, of a coral-rudist mound capped by a rudist biostrome which, in turn, is overlain by bioclastic limestones and, locally, marls. The coral-rudist mounds consist mainly of floatstones. The coral assemblage is dominated by Fungiina, Astreoina, Heterocoeniina andAgathelia asperella (stylinina). From the rudists, elevators (Vaccinites spp., radiolitids) and recumbents (Plagioptychus) are present. Calcareous sponges, sclerosponges, and octocorals are subordinate. The elevator rudists commonly are small; they settled on branched corals, coral heads, on rudists, and on biolastic debris. The rudists, in turn, provided settlement sites for corals. Predominantly plocoid and thamnasteroid coral growth forms indicate soft substrata and high sedimentation rates. The mounds were episodically smothered by carbonate mud. Many corals and rudists are coated by thick and diverse encrustations that indicate high nutrient level and/or turbid waters. The coral-rudist mounds are capped byVaccinites biostromes up to 5 m thick. The establishment of these biostromes may result from unfavourable environmental conditions for corals, coupled with the potential of the elevator rudists for effective substrate colonization. TheVaccinites biostromes are locally topped by a thin radiolitid biostrome. The biostromes, in turn, are overlain by bioclastic limestones; these are arranged in stratal packages that were deposited from carbonate sand bodies. Approximately midsection, an interval of marls with abundantPhelopteria is present. These marls were deposited in a quiet lagoonal area where meadows of sea grass or algae, coupled with an elevated nutrient level, triggered the mass occurrence ofPhelopteria. The upper part of the Haidach section consists of stratal packages that each is composed of a rudist biostrome overlain by bioclastic wackestones to packstones with diverse smaller benthic foraminifera and calcareous green algae. The biostromes are either built by radiolitids,Vaccinites, andPleurocora, or consist exclusively of radiolitids (mainlyRadiolites). Both the biostromes and the bioclastic limestones were deposited in a low-energy lagoonal environment that was punctuated by high-energy events.In situ-rudist fabrics typically have a matrix of mudstone to rudistclastic wackestone; other biogens (incl. smaller benthic foraminifera) are absent or very rare. The matrix of rudist fabrics that indicate episodic destruction by high-energy events contain a fossil assemblage similar to the vertically associated bioclastic limestones. Substrata colonized by rudists thus were unfavourable at least for smaller benthic foraminifera. The described succession was deposited on a gently inclined shelf segment, where coral-rudist mounds and hippuritid biostromes were separated by a belt of bioclastic sand bodies from a lagoon with radiolitid biostromes. The mounds document that corals and Late Cretaceous elevator rudists may co-occur in close association. On the scale of the entire succession, however, mainly as a result of the wide ecologic range of the rudists relative to corals, the coral-dominated mounds and the rudist biostromes are vertically separated.     

Anthracoporella mounds in the Late Carboniferous Auernig Group,Carnic Alps (Austria)

Summary A heretofore undocumented example of skeletal mounds formed by the dasycladacean algaAnthracoporella spectabilis is described from mixed carbonate-clastic cycles (Auernig cyclothems) of the Late Carboniferous (Gzhelian) Auernig Group of the central Carnic Alps in southern Austria. The massive mound facies forms biostromal reef mounds that are up to several m thick and extend laterally over more than 100 m. The mound facies is developed in the middle of bedded limestones, which are up to 16 m thick. These limestones formed during relative sea-level highstands when clastic influx was near zero. The mound facies is characterized by well developed baffler and binder guilds and does not show any horizontal or vertical zonation. Within the massive mound faciesAnthracoporella is frequently found in growth position forming bafflestones and wackestones composed of abundantAnthracoporella skeletons which toppled in situ or drifted slightly.Anthracoporella grew in such profusion that it dominated the available sea bottom living space, forming ‘algal meadows’ which acted as efficient sediment producers and bafflers. BecauseAnthracoporella could not provide a substantial reef framework, and could not withstand high water turbulence, the biostromal skeletal mounds accumulated in shallow, quiet water below the active wave base in water depths less than 30 m. The massive mound facies is under- and overlain by, and laterally grades into bedded, fossiliferous limestones of the intermound facies, composed mainly of different types of wackestones and packstones. Individual beds containAnthracoporella andArchaeolithophyllum missouriense in growth position, forming “micromounds’. Two stages of mound formation are recognized: (1) the stabilization stage when bioclastic wackestones accumulated, and (2) the skeletal mound stage when the sea-bottom was colonized byAnthracoporella and other members of the baffler and binder guilds, formingAnthracoporella bafflestones and wackestones of the mound facies. A slight drop in sea-level led to the termination of the mound growth and accumulation of organic debris, particularly calcareous algae, fusulinids, crinoids and bryozoans, forming well bedded limestones, which overlie the mound facies