Quantitative compositional analysis of a Triassic carbonate platform (Southern Alps,Italy)

We have estimated abundance and distribution of automicrite, marine cements and skeletal grains in the Triassic Sella massif, an isolated platform flanked by steep (25–35°) clinoforms. 108 samples were taken at constant intervals from measured sections of the major zones of the platform edifice: the platform top, margin–upper slope, and lower slope. In a first step, carried out in the field and on hand specimen, purely detrital deposits were separated from automicrite facies, i.e. beds with automicrite, cement-filled, primary vugs and admixtures of skeletal carbonate and lithoclasts. In the second step, samples with automicrite facies were thin-sectioned and point counted. The categories used for point counting were (a) automicrite, (b) vugs and cement, (c) microspar or neomorphic spar, (d) skeletal grains and (e) internal sediments. At the platform top 46% of samples are pure detrital deposits, 27% consist of automicrite facies and 27% are too strongly altered by dolomitization to allow classification. At the margin–upper slope 68% of samples consist of automicrite facies, 22% are pure detrital sediments and 10% are strongly altered. At the lower slope 63% are detrital deposits, 10% automicrite facies and 27% are extensively dolomitized. The most important contributors to the automicrite facies are automicrite (41% on the platform top, 29% on the margin–upper slope, 28% on the lower slope) and early marine cement (35% on the platform top, 48% on the margin–upper slope, 27% on the lower slope). The amount of skeletal grains is less than 10%.The automicrite facies stabilized the platform margin and upper slope. Automicrite, abundant early marine cements and micro-organisms such as Tubiphytes, formed a rigid framework, thus substituting for the lack of a metazoan reef. On the upper slopes, the framework of automicrite facies stabilized the slope but intermittently. The automicrite layers are frequently dissected by sediment-filled fractures or are broken into clasts. We assume that they slid on the layers of loose detritus. Bigger slides turned into rubbly debris flows that formed metre-thick breccias at the lower slope and the proximal basin floor. The planar shape and steep angle of the clinoforms indicate that the large-scale geometry of the slope was not controlled by the automicrite but rather by non-cohesive layers of sand and rubble piled up to the angle of repose.The production mode of the Sella is comparable of that of a (mud) mound factory. This factory was highly productive: in 1 Ma, the platform aggraded over 300 m and prograded over 2000 m in all directions.     

Reef Fabrics, biotic crusts and syndepositional cements of the Latemar reef margin (Middle Triassic), northern Italy

The Latemar reef buildup of the central Dolomites (northern Italy) provides a rare opportunity to examine an in-place Middle Triassic (Upper Anisian to Lower Ladinian) platform margin that is not strongly deformed or dolomitized. The margin lies between the flat lying platform interior and steeply dipping foreslope clinoforms. Across this transition, the depositional profile relates directly to a consistent lateral facies pattern: (1) restricted-biota grainstone of the platform interior, (2) ‘Tubiphytes’-rich boundstone and (3) diverse-biota grainstone that grades into (4) foreslope breccia beds. The boundstone and diverse-biota grainstone facies comprise the platform margin. The boundstone facies consists of a framework of small (< 10 cm) skeletal remains (< 10% by volume) with associated biotic crusts, internal sediments and syndepositional cements. Crusts and cements constitute most of the rock volume and created the boundstone fabric. Biotic crusts exhibit gravity-defying geometries and range from a light grey, ‘structure grumeleuse’ rind to dark grey, micritic laminae. Both cements and biotic crusts occur as redeposited talus in the foreslope talus deposits, indicating a syndepositional origin. The diverse-biota grainstone facies primarily consists of skeletal-peloidal grainstone with a diverse open marine biotic assemblage, in contrast to the restricted biota grainstones of the platform interior that have a low diversity, restricted marine biota. Metre scale hexacoral boundstone and centimetre-scale sponge boundstone and microbial boundstone occur as isolated patches (tens to hundreds of metres apart) within the diverse-biota grainstone facies. The depositional profile, facies zonation and biotic constituents all indicate that the Latemar buildup had a shallow water reef margin, in contrast to previous interpretations that these were upper slope reefs. The syndepositional biotic crusts and inorganic cementation played key roles in stabilizing the boundstone fabric to form a wave-resistant reef fabric.     

Solution-collapse breccias of the Minkinfjellet and Wordiekammen Formations, Central Spitsbergen, Svalbard: a large gypsum palaeokarst system

Large volumes of carbonate breccia occur in the late syn-rift and early post-rift deposits of the Billefjorden Trough, Central Spitsbergen. Breccias are developed throughout the Moscovian Minkinfjellet Formation and in basal parts of the Kazimovian Wordiekammen Formation. Breccias can be divided into two categories: (i) thick, cross-cutting breccia-bodies up to 200 m thick that are associated with breccia pipes and large V-structures, and (ii) horizontal stratabound breccia beds interbedded with undeformed carbonate and siliciclastic rocks. The thick breccias occur in the central part of the basin, whereas the stratabound breccia beds have a much wider areal extent towards the basin margins. The breccias were formed by gravitational collapse into cavities formed by dissolution of gypsum and anhydrite beds in the Minkinfjellet Formation. Several dissolution fronts have been discovered, demonstrating the genetic relationship between dissolution of gypsum and brecciation. Textures and structures typical of collapse breccias such as inverse grading, a sharp flat base, breccia pipes (collapse dolines) and V-structures (cave roof collapse) are also observed. The breccias are cemented by calcite cements of pre-compaction, shallow burial origin. Primary fluid inclusions in the calcite are dominantly single phase containing fresh water (final melting points are ca 0 °C), suggesting that breccia diagenesis occurred in meteoric waters. Cathodoluminescence (CL) zoning of the cements shows a consistent pattern of three cement stages, but the abundance of each stage varies stratigraphically and laterally. δ¹⁸O values of breccia cements are more negative relative to marine limestones and meteoric cements developed in unbrecciated Minkinfjellet limestones. There is a clear relationship between δ¹⁸O values and the abundance of the different cement generations detected by CL. Paragenetically, later cements have lower δ¹⁸O values recording increased temperatures during their precipitation. Carbon isotope values of the cements are primarily rock-buffered although a weak trend towards more negative values with increasing burial depth is observed. The timing of gypsum dissolution and brecciation was most likely related to major intervals of exposure of the carbonate platform during Gzhelian and/or Asselian/Sakmarian times. These intervals of exposure occurred shortly after deposition of the brecciated units and before deep burial of the sediments.     

Thermal-baric structure and P–T history of the Brooks Range metamorphic core, Alaska

Documentation of pressure–temperature (P–T) histories across an epidote‐amphibolite facies culmination provides new insight into the tectono‐thermal evolution of the Brooks Range collisional orogen. Thermobarometry reveals that the highest grade rocks formed at peak temperatures of 560–600 °C and at pressures of 8–9.5 kbar. The thermal culmination coincides with the apex of a structural dome defined by oppositely dipping S2 crenulation cleavages suggesting post‐metamorphic doming. South of the thermal culmination, greenschist facies and lowermost epidote‐amphibolite facies rocks preserve widespread evidence for an early blueschist facies metamorphism. In contrast, no evidence for an early blueschist facies metamorphism was found in similar grade rocks of the northern flank, indicating that the southern flank underwent initial deeper burial during southward underthrusting of the continental margin. Thus, while the dome shows a symmetric distribution of peak temperatures, the P–T paths followed by the two flanks must have varied. This variation suggests that final thermal re‐equilibration to greenschist and epidote–amphibolite facies conditions did not result from a simple process of southward underthrusting followed by thermal re‐equilibration from the bottom upward. The new data are inconsistent with a previous model that invokes such re‐equilibration, along with northward thrusting of epidote–amphibolite facies rocks over lower grade rocks presently on the southern flank of the culmination, to produce an inverted metamorphic field gradient. Instead, it is suggested that following blueschist facies metamorphism, rocks of the southern and northern flanks were juxtaposed, during which time the more deeply buried south flank was partially emplaced above rocks to the north, where they escaped Albian epidote–amphibolite facies overprinting. Porphyroblast growth, which post‐dates the main fabric on the north flank of the culmination may be the result of Albian thermal re‐equilibration following this deformation. Post‐metamorphic doming resulted from a combination of Albian‐Cenomanian extension and Tertiary deformation.     

Carbonate platform flanks: slope angle and sediment fabric

More than 20 examples of fossil carbonate platform systems were compared for slope angle and sediment fabric. Plots of slope angle versus sediment fabric show that grainy, non-cohesive, mud-free sediments build steeper slopes than muddy, cohesive, sediments. Examples near the end-members of grainy and muddy carbonate platform flanks are found in the Triassic of the Dolomites in northern Italy and in the Bahamas, respectively. They document the flank geometry and the processes readjusting the slope profile once the limiting slope angle is exceeded. The grainy flank sediments in the Dolomites, modified by shearing and avalanching, produce straight slope profiles with declivities up to 35°, whereas the muddy Bahamian flank sediments, modified by large-scale creep and rotational to translational sliding and slumping, produce a concave-upwards slope profile, inclined at less than 4°. The comparison between slope angle and sediment fabric indicates that the physical behaviour of sediments in the gravity field, angle of shearing and mode of readjustment processes, is linked to the composition of the slope sediment. Among the variables such as sea-level, subsidence, climate, plate motion and oceanographic setting (windward-leeward), sediment fabric is suggested to be a major, if not the major control on slope angle and slope curvature of carbonate platform flanks. Besides the recently documented tendency of carbonate sediments to build steeper slopes than siliciclastics, this proposed relation sheds new light on the analysis and quantification of the variables influencing the geometry and depositional evolution of carbonate systems. Furthermore, it provides an opportunity to deduce sediment composition from seismic lines and predict lithology prior to drilling.     

Mid-Cretaceous basin margin carbonates, east-central Mexico

Isolated, high relief carbonate platforms developed in the intracratonic basin of east-central Mexico during Albian-Cenomanian time. Relief on the platforms was of the order of 1000 m and slopes were as steep as 20–43°. Basin-margin debris aprons adjacent to the platforms comprise the Tamabra Formation. In the Sierra Madre Oriental, at the eastern margin of the Valles-San Luis Potosi Platform, an exceptionally thick (1380m) progradational basin to platform sequence of the Tamabra Formation can be divided into six lithological units. Basinal carbonate deposition that preceded deposition of the Tamabra Formation was emphatically punctuated by an allochthonous reef block 1 km long by 0·5 km wide with a stratigraphic thickness of 95 m. It is encased in Tamabra Formation unit A, approximately 360 m of peloidal-skeletal wackestone and lithoclastic-skeletal packstone that includes some graded beds. Unit B is 73 m of massive dolomite with sparse skeletal fragments and intraclasts. Unit C, 114m thick, consists of structureless skeletal wackestone passing upward into graded skeletal packstone. Interlaminated lime mudstone and fine grained bioclastic packstone with prominent horizontal burrows are interspersed near the top. Unit D is 126 m of breccia with finely interbedded skeletal grainstone and burrowed or laminated mudstone. The breccias contain a spectrum of platform-derived lithoclasts and basinal intraclasts, up to 10 m in size. The breccias are typically grain supported (rudstone) with a matrix of lightly to completely dolomitized mudstone or skeletal debris. Beds are up to several metres thick. Unit E is 206 m of massive, sucrosic dolomite that replaced breccias. Unit F is approximately 500 m of thick bedded to massive skeletal packstone with abundant rudists and a few mudstone intraclasts. Metre scale laminated lime mudstone beds are interspersed. The section is capped by El Abra Formation platform margin limestone, consisting of massive beds of caprinid packstone and grainstone with many whole valves. Depositional processes within this sequence shift from basinal pelagic or peri-platform sedimentation to distal, platform-derived, muddy turbidity currents with a large slump block (Unit A); through more proximal (coarser and cleaner) turbidity currents (Unit B?, C); to debris flows incorporating platform margin and slope debris (Units D, E). Finally, a talus of coarse, reef-derived bioclasts (Unit F) accumulated as the platform margin prograded over the slope sequence. Interspersed basinal deposits evolved gradually from largely pelagic to include influxes of dilute turbidity currents. Units containing turbidites with platform-derived bioclasts reflect flooding of the adjacent platform. Breccia blocks and lithoclasts were probably generated by erosion and collapse of the platform during lowstands. Laminated, black, pelagic carbonates, locally cherty, are interbedded with both breccias and turbidites. At least those interbedded with turbidites may have been deposited within an expanded mid-water oxygen minimum zone during relative highstands of sea level. They are in part coeval with mid-Cretaceous black shales of the Atlantic Ocean.     

Isolated stationary carbonate platforms: the Middle Triassic (Ladinian) of the Marmolada area, Dolomites, Italy

As a result of a phase of extensional tectonics in the western Tethyan region, a horst and graben topography formed during the Middle Triassic (Ladinian) in northern Italy. Horsts were sites of shallow water carbonate sedimentation, while pelagic and volcaniclastic sediments were deposited in the grabens. Two carbonate platforms approximately 500 m thick can be distinguished in the Marmolada area of the Dolomites: the Marmolada platform proper, which covered an area of 6 km², and the Costabella platform, which extended for about 12 km in a NW-SE direction and was about 3 km across. The facies of these isolated platforms reflect the influence of storms from the SW. Windward platform margins were characterized by a marine sand belt of skeletal and aggregate grainstones with a dominant platform directed cross-stratification. The central portions of the platforms were occupied by supratidal sand cays which are made up of storm washovers. Leeward parts of the platforms are composed of shallow subtidal sand flat deposits. Laterally discontinuous reefs chiefly composed of various calcareous algae are developed at the outer margins of the platforms. Along windward margins, reefs may form a belt several hundred metres wide; along leeward margins their width is commonly reduced to some tens of metres. Foreslope talus breccias surround the platforms. Clinoform bedding showing basinward dips of 30°-40° is typical of this facies belt, which is approximately 2 km wide. Basinal sediments, only some tens of metres thick, are radiolarian micrites. Abundant sediment-gravity-flow deposits expand the basinal sequence at the toes of windward margins and were probably triggered by storm return flows. Synsedimentary faults striking both NNE-SSW and NW-SE separate the bedded platform limestones from flank deposits and reefs. They account for the stationary nature of the platforms. Neptunian dykes show preferred NNE-SSW and E-W trends. Sinistral displacements are associated with NW-SE trending faults. Depressions in the basins, filled with red, turbiditic pelagic sediments, show N-S trends and are probably compressional in origin. The structural pattern may have resulted from oblique, NW-SE oriented extension of the E-W trending Middle Triassic graben zone of the Dolomites. In the Ladinian of the Dolomites, the stationary platform type can be distinguished from a retrograding type, whereas continuously prograding platforms apparently did not develop.     

The carbonate factory of Middle Triassic buildups in the Dolomites, Italy: a quantitative analysis

Middle Triassic carbonate buildups of the Dolomites were high in relief (500–1000m) and small in size (one to a few square kilometres in area). A paradox results from the carbonate platform model that invokes the platform top, including reef rims, as the carbonate factory and flanking beds as talus deposits. Most buildups consist largely of clinoforms (inclined at 10-50°) whereas massive reef rocks and stratified buildup interiors are poorly developed or absent. Facies and modal analysis of 323 thin sections from buildups of the Marmolada indicate that clinoforms are: (i) predominantly composed of in situ boundstones (56% of all samples); (ii) primarily made up of early cements (37 vol.%), microbial crusts (17 vol.%), micritic intraclasts (10 vol.%) and Tubiphytes (8 vol.%); and (iii) contain diagnostic shallow water grains (dasyclads, coated grains) that are less abundant by 1-2 orders of magnitude compared with buildup interior facies. These data suggest that the clinoforms themselves were the main carbonate factory of the Triassic buildups. Stratified buildup interior rocks and massive reef rocks were apparently not a prerequisite for buildup growth and clinoform progradation.     

Turonian Radiolarians from Karnezeika,Argolis Peninsula,Peloponnesus (Greece)

Near Karnezeika a roughly 140 m thick Upper Cretaceous section consists of interbedded pelagic limestones, cherts and coarse polymict breccias including ophiolites and shallow water limestones. At the base, pink pelagic limestones rest on deeply altered and fractured Lower Jurassic Pantokrator Limestone. This first pelagic facies is dated as middle Turonian, based on planktonic Foraminifera. Over 100 m of coarse ophiolite-carbonate breccias, interpreted as a channel or canyon fill in a pelagic environment, document the erosion of the Late Jurassic nappe edifice along the Cretaceous Pelagonian margin. Above these breccias, we mesured 16 m of principally pink and red pelagic limestones and radiolarian cherts, in which we recovered well-preserved radiolarians discussed here. In this interval, the presence of planktonic Foraminfera allows to state a late Turonian to Coniacian age. More than 40 radiolarian species are described and figured in this work. The radiolarian chronostratigraphy established by 10 different authors in 11 publications was compared for this study and used to establish radiolarian ranges. This exercise shows major discrepancies between authors for the radiolarian ranges of the studied assemblage. Nevertheless, a Turonian age can be stated based on a synthesis of cited radiolarian ranges. This age is consistent with the age based on planktonic foraminifera. In combining the ages of both Radiolaria and planktonic Foraminifera, the studied samples can be restricted to the late Turonian. However, the discrepancies of published radiolarian ranges call for an urgent, major revision of the Late Cretaceous radiolarian biochronology. The integration of planktonic foraminifera with radiolarians may greatly enhance biochronologic resolution in sections where both groups occur.     

Clinoform geometry,geomorphology, facies character and stratigraphic architecture of a sand‐rich subaqueous delta: Jurassic Sognefjord Formation,offshore Norway

The integration of core sedimentology, seismic stratigraphy and seismic geomorphology has enabled interpretation of delta‐scale (i.e. tens of metres high) subaqueous clinoforms in the upper Jurassic Sognefjord Formation of the Troll Field. Mud‐prone subaqueous deltas characterized by a compound clinoform morphology and sandy delta‐scale subaqueous clinoforms are common in recent tide‐influenced, wave‐influenced and current‐influenced settings, but ancient examples are virtually unknown. The data presented help to fully comprehend the criteria for the recognition of other ancient delta‐scale subaqueous clinoforms, as well as refining the depositional model of the reservoir in the super‐giant Troll hydrocarbon field. Two 10 to 60 m thick, overall coarsening‐upward packages are distinguished in the lower Sognefjord Formation. Progressively higher energy, wave‐dominated or current‐dominated facies occur from the base to the top of each package. Each package corresponds to a set of seismically resolved, westerly dipping clinoforms, the bounding surfaces of which form the seismic ‘envelope’ of a clinoform set and the major marine flooding surfaces recognized in cores. The packages thicken westwards, until they reach a maximum where the clinoform ‘envelope’ rolls over to define a topset–foreset–toeset geometry. All clinoforms are consistently oriented sub‐parallel to the edge of the Horda Platform (N005–N030). In the eastern half of the field, individual foresets are relatively gently dipping (1° to 6°) and bound thin (10 to 30 m) clinothems. Core data indicate that these proximal clinothems are dominated by fine‐grained, hummocky cross‐stratified sandstones. Towards the west, clinoforms gradually become steeper (5° to 14°) and bound thicker (15 to 60 m) clinothems that comprise medium‐grained, cross‐bedded sandstones. Topsets are consistently well‐developed, except in the westernmost area. No seismic or sedimentological evidence of subaerial exposure is observed. Deposition created fully subaqueous, near‐linear clinoforms that prograded westwards across the Horda Platform. Subaqueous clinoforms were probably fed by a river outlet in the north‐east and sculpted by the action of currents sub‐parallel to the clinoform strike.     

Seismological constraints on the 2018 Mt. Etna (Italy) flank eruption and implications for the flank dynamics of the volcano

3D earthquake locations, focal mechanisms and stress tensor distribution in a 16‐month interval covering the 2018 Mt. Etna flank eruption, enabled us to investigate the relationship between magma intrusion and structural response of the volcano and shed light on the dynamic processes affecting the instability of Mt. Etna. The magma intrusion likely caused tension in the flanks of the volcano, leading to significant ground deformation and redistribution of stress on the neighbouring faults at the edge of Mt. Etna's unstable sector, encouraging the ESE sliding of the eastern flank of the volcano. Accordingly, FPSs of the post‐eruptive events show strike slip faulting mechanisms, under a stress regime characterized by a maximum compressive σ₁, NE‐SW oriented. In this perspective, any flank eruption could temporarily enhance the sliding process of both the southern and eastern flanks of the volcano.     

陕西富平中-上奥陶统深水碳酸盐重力流沉积模式

陕西富平地区,在中-晚奥陶世位于华北地台南侧的弧后盆地的大陆边缘。其中完好地保存了一套以半远洋的泥晶灰岩夹重力流的角砾灰岩和砂屑灰岩为特征的深水沉积岩系。一般认为,与半远洋的石灰岩共生的碳酸盐重力流沉积的复杂岩相,是大陆坡的标志,而相应的陆源碎屑沉积,则被作为海底扇的产物。然而,富平地区的碳酸盐重力流沉积,却是在弧后盆地伸进浅水台地之间的深海前槽中,沿海槽轴向呈席状流搬运、沉积的。     

Structural controls and genesis of epithermal gold-bearing breccias at the Lebong Tandai mine, Western Sumatra, Indonesia

Lebong Tandai is a low-sulphidation, volcanic-hosted epithermal gold deposit of Neogene age, located within the foothills of the Barisan Mountains, Sumatra. To date, the mine has produced approximately 40 tonnes of gold and 400 tonnes of silver. The mineralisation is exclusively in the form of tabular quartz-cemented breccias bodies which are localised along faults. The breccias comprise angular to sub-rounded clasts of the wallrocks and earlier barren breccias cemented by banded or massive quartz, and in many instances, the clasts are supported within the quartz cement.The sulphide minerals occur as either a single cockade band around the clasts in the breccia, or as polymineralic aggregates disseminated throughout the breccia cement. The main precious-metal-bearing phase is electrum, with silver-sulphosalts and silver-tellurides also present. Highly variable concentrations of pyrite, sphalerite, galena and chalcopyrite are associated with the precious-metal phases.With the exception of two minor lodes, the mineralised breccias are localised along strike-slip faults which display changes in orientation indicative of D-, R- and P-shears and T-fractures, with individual segments ranging from a few metres to a few hundred metres in length. Two strike-slip fault systems are recognised, one sinistral, trending east-west and the other dextral, trending northwest, the latter of which is parallel to the Sumatran Fault System. The majority of gold and silver production is from breccias localised along faults formed during the sinistral tectonism. The breccias are believed to have been generated during compressional reactivation of the east-west sinistral strike-slip faults in response to the subduction of the Indian-Australian plate beneath Sumatra. Supralithostatic fluid pressures are a necessary pre-requisite for such reactivation, and the sudden drop in fluid pressure during reactivation is thought to have resulted in both the formation of the breccias by hydraulic fracturing, and the deposition of amorphous silica, precious metals and base metal sulphides. High rates of fluid flow subsequent to fracturing are thought to have led to fluidisation of the breccia clasts and abrasion to their current morphologies.Microthermometry of fluid inclusions in sphalerite indicates that the mineralising fluids were of low salinity, less than 3 wt% NaCl_equivalent, and that mineralisation took place at temperatures of 260–280°C. Variations of salinity and homogenisation temperature due to boiling are poorly developed, although if boiling occurred, the metalliferous minerals would have been deposited early in the boiling process before the fluid had cooled appreciably.     

Questioning the microbial origin of automicrite in Ordovician calathid–demosponge carbonate mounds

Calathid–demosponge carbonate mounds are a feature of Early to Middle Ordovician shallow‐marine carbonate depositional environments of tropical to subtropical palaeolatitudes. These mounds contain an important amount of autochthonous non‐skeletal microcrystalline calcium‐carbonate (automicrite) conventionally considered microbial in origin. Here, the automicrite of calathid–demosponge carbonate mounds (Tarim Basin, north‐west China) is broken down into five distinct fabrics: an in situ peloidal–spiculiferous fabric (AM‐1), an in situ peloidal fabric (AM‐2), an aphanitic–microtubular fabric (AM‐3), a minipeloidal fabric (AM‐4) and a laminoid–cerebroid fabric (AM‐5). Type AM‐1 occurs with AM‐2 being succeeded by an assemblage of AM‐3 and AM‐4. Types AM‐4 and AM‐5 are separated by an erosional disconformity. A good correlation of fluorescence and cathodoluminescence of automicrites indicates that induced and supported organomineralization produced automicrite, probably via the permineralization of non‐living organic substrates adsorbing dissolved metal–humate complexes. Using a spreadsheet with six parameters and 17 characters, AM‐1 to AM‐4 turn out to be non‐microbial in origin. Instead, these automicrites represent relics of calcified metazoan tissues, such as siliceous sponges, non‐spiculate sponges or the basal attachment structures of stalked invertebrates. Fabric AM‐5 is a microbial carbonate but is post‐mound in origin forming a drape within a reefal framework established by AM‐4. The five automicritic fabrics, individually or as an assemblage, are a common element of Ordovician calathid–demosponge carbonate mounds in general. The reassessment of the origins of these automicritic fabrics holds consequences for understanding of the Great Ordovician Biodiversification Event in terms of community structure, reef ecology and reef evolution. Episodically, these fabrics are also present in other carbonate build‐ups stretching from the Neoproterozoic over the entire Phanerozoic Eon. The massive calcification of metazoan soft tissue (AM‐1 to AM‐4) characterizes episodes and conditions of enhanced marine calcification and might be of value to refine secular trends of p CO₂, Ca concentration and Mg/Ca ratio at the scale of individual sedimentary basins.     

Submarine reworking of exhumed subcontinental mantle rocks: field evidence from the Lherz peridotites,French Pyrenees

In the Pyrenees, the lherzolites nowhere occur as continuous units. Rather, they always outcrop as restricted bodies, never more than 3 km wide, scattered across Mesozoic sedimentary units along the North Pyrenean Fault. We report the results of a detailed analysis of the geological setting of the Lherz massif (central Pyrenees), the type‐locality of lherzolites and one of the most studied occurrences of mantle rocks worldwide. The Lherz body is only 1.5 km long and belongs to a series of ultramafic bodies of restricted size (a few metres to some hundreds of metres), occurring within sedimentary formations composed mostly of carbonate breccias originating from the reworking of Mesozoic platform limestones and dolomites. The clastic formations also include numerous layers of polymictic breccias reworking lherzolitic clasts. These layers are found far from any lherzolitic body, implying that lherzolitic clasts cannot derive from the in situ fragmentation of an ultramafic body alone, but might also have been transported far away from their sources by sedimentary processes. A detailed analysis of the contacts between the Lherz ultramafic body and the surrounding limestones confirms that there is no fault contact and that sediments composed of ultramafic material have been emplaced into fissures within the brecciated carapace of the peridotites. These observations bear important constraints for the mode of emplacement of the lherzolite bodies. We infer that mantle exhumation may have occurred during Albian strike‐slip deformation linked to the rotation of Iberia along the proto‐North Pyrenean Fault.     

Cavity‐filling dolomite speleothems and submarine cements in the Ediacaran Dengying microbialites,South China: Responses to high‐frequency sea‐level fluctuations in an ‘aragonite–dolomite sea’

Speleothems, mostly composed of calcium carbonate, are widely present in modern karst‐originated caves, but have rarely been reported in palaeokarst systems. This study presents a novel type of dolomite speleothem and subsequent submarine dolomite cement, which are widely present in the upper Ediacaran Dengying Formation in the upper Yangtze area. These precipitated materials occur in the cavity system that cuts across several peritidal cycles. The interconnected cavity networks with irregular shapes, embayed walls, internal breccias on cavity floor and their preferential development in the shallower cycle tops (for example, tepee‐deformed beds) suggest that they were initially generated by subaerial dissolution. As the earliest infills, the hemispherical protrusions, icicle‐like pendants and ground‐up columns show similar morphological features and occurrence patterns to the cave popcorn, stalactites and stalagmites, respectively. Thus, these earliest infills are speleothems resulting from associated meteoric precipitation during subaerial exposure. The isopachous growth pattern of subsequent more extensive fibrous dolomite cements points to a submarine diagenetic environment in which they were precipitated. Microscopically, the micritic to micro‐crystalline dolomite, acicular dolomite in speleothems and the subsequent fibrous dolomite share similar crystal fabrics to metastable precursors (for example, Mg‐calcite). Meanwhile, the carbon‐oxygen isotope compositions of the speleothem and fibrous dolomite, although partly altered by burial diagenesis, share a large overlap with host rock and coeval marine carbonates all over the Yangtze Platform. For these reasons, these speleothems and fibrous cements are considered to have been initially precipitated as metastable carbonate precursors in meteoric and submarine environments, respectively, and stabilized during submarine mimetic dolomitization. The cyclic occurrence of cavity systems filled with speleothems and submarine cements reflects periodic subaerial exposure and marine flooding of broad tidal flat in the upper Yangtze area, driven by high‐frequency sea‐level fluctuations. Furthermore, the Neoproterozoic seawater chemistry that favoured early dolomitization of carbonate precursor mineralogies was an advantage for the preservation of fabrics from metastable precursor minerals.     

'Linked' debrites in sand-rich turbidite systems – origin and significance

Abstract The outer parts of a number of small Late Jurassic sandy deep‐water fans in the northern North Sea are dominated by the stacked deposits of co‐genetic sandy and muddy gravity flows. Sharp‐based, structureless and dewatered sandstone beds are directly overlain by mudclast breccias that are often rich in terrestrial plant fragments and capped by thin laminated sandstones, pseudonodular siltstones and mudstones. The contacts between the clast‐rich breccias and the underlying sandstones are typically highly irregular with evidence for liquefaction and upward sand injection. The breccias contain fragments (up to metre scale) of exotic lithologies surrounded by a matrix that is extremely heterogeneous and strewn with multiphase and variably sheared sand injections and scattered coarse and very coarse sand grains (often coarser than in the immediately underlying sand bed). Markov chain analysis establishes that the breccias consistently overlie sandstones, and the character of the breccias and their external contacts rule out a post‐depositional origin via in situ liquefaction, intrastratal flowage or bed amalgamation and disruption. The breccias are interpreted as debrites that rode on a water‐rich sand bed just deposited by a co‐genetic concentrated gravity current. As such, they are referred to as ‘linked debrites’ to distinguish them from debrites emplaced in the absence of a precursor sand bed. The distinction is important, because these linked debris flows can achieve significant mobility through entrainment of both water and sediment from beneath, and they ride on a low‐friction carpet of liquefied sand. This explains the paradox presented by fan fringes in which there are common debrites, when conventional thinking might predict that deposits of low‐concentration gravity currents should be more important here. In fact, evidence for transport by low‐concentration turbidity currents is rare in these systems. Several possible mechanisms might explain the formation of linked flows, but the ultimate source of both sandy and clast‐rich flow components must be in shallower water on the basin margin (the debrites are not triggered from distal slopes). Flow partitioning may have occurred by upslope erosion and retardation of the mudclast‐charged portion of an erosional sandy density current, partial flow transformation of a precursor debris flow and/or hydraulic segregation and reconcentration of the flaky clasts and carbonaceous matter during transport. Linked debrites are not restricted to small sand‐rich fans, and similar mechanisms may be responsible for the long runout of debris flows in other systems. The recognition of a distinct class of linked debrites is of wider importance for facies prediction, reservoir heterogeneity and even carbon fluxes and sequestration on continental margins.     

Lower Cambrian shelf and shelf margin buildups,Flinders Ranges,South Australia1

Carbonate buildups in the Flinders Ranges of mid-Early Cambrian age grew during a period of high archaeocyath diversity and are of two types: (1) low-energy, archaeocyath-sponge-spicule mud mounds, and (2) high-energy, archaeocyath-calcimicrobe (calcified microbial microfossil) bioherms. Mud mounds are composed of red carbonate mudstone and sparse to abundant archaeocyath floatstone, have a fenestral fabric, display distinct stromatactis, contain abundant sponge spicules and form structures up to 150m wide and 80 m thick. Bioherms are either red or dark grey limestone and occur as isolated small structures 2–20 m in size surrounded by cross-bedded calcarenites and calcirudites or as complexes of mounds and carbonate sands several hundreds of metres across. Red bioherms comprise masses of white Epiphyton with scattered archaeocyaths and intervening areas of archaeocyath-rich lime mudstone. Grey bioherms are complex intergrowths of archaeocyaths, encrusting dark grey Renalcis and thick rinds of fibrous calcite cement. The bioherms were prone to synsedimentary fracturing and exhibit large irregular cavities, up to 1.5 m across, lined with fibrous calcite. The buildups are isolated or in contiguous vertical succession. Mud mounds occur alone in low-energy, frequently nodular, limestone facies. Individual bioherms and bioherm complexes occur in high-energy on-shelf and shelf-margin facies. The two types also form large-scale, shallowing-upward sequences composed of basal (deep water) mud mounds grading upward into archaeocyath-calcimicrobe bioherm complexes and bioherms in cross-bedded carbonate sands. The uppermost sequence is capped by ooid grainstone and/ or fenestral to stromatolitic mudstone. The calcimicrobe and metazoan associations form the two major biotic elements which were to dominate reefs throughout much of subsequent Phanerozoic time.     

Controlling factors of volumetrically important marine carbonate cementation in deep slope settings

The controlling parameters of early marine carbonate cementation in shoal water and hemipelagic to pelagic domains are well‐studied. In contrast, the mechanisms driving the precipitation of early marine carbonate cements at deeper slope settings have received less attention, despite the fact that considerable volumes of early marine cement are present at recent and fossil carbonate slopes in water depths of several hundreds of metres. In order to better understand the controlling factors of pervasive early marine cementation at greater water depths, marine carbonate cements observed along time‐parallel platform to basin transects of two intact Pennsylvanian carbonate slopes are compared with those present in the slope deposits of the Permian Capitan Reef and Neogene Mururoa Atoll. In all four settings, significant amounts of marine cements occlude primary pore spaces downslope into thermoclinal water depths, i.e. in a bathymetric range between some tens and several hundreds of metres. Radial, radiaxial and fascicular optic fibrous calcites, and radiaxial prismatic calcites are associated with re‐deposited facies, boundstones and rudstones. Botryoidal (formerly) aragonitic precipitates are common in microbially induced limestones. From these case studies, it is tentatively concluded that sea water circulation in an extensive, near‐sea floor pore system is a first‐order control on carbonate ion supply and marine cementation. Coastal upwelling and internal or tidal currents are the most probable mechanisms driving pore water circulation at these depths. Carbonate cements precipitated under conditions of normal to elevated alkalinity, locally elevated nutrient levels and variable sea water temperatures. The implications of these findings and suggestions for future work are discussed.     

海南岛全新世海滩岩的胶结作用与成岩环境的关系

我国海南岛及南海诸岛沿岸,广泛发育海滩岩。1980年我队赴海南岛考察现代沉积时,对海滩岩的分布、岩性特征及其与周围环境的关系进行了观察和采样。样品采自崖县鹿回头三亚湾水尾岭海蚀崖、西洲岛、小东海、东瑁岛、西瑁岛、天涯海角,乐东县莺歌海,文昌县渔业等地(图1)。有关的地质、地貌及岩性特征等,已有许多描述,对海滩岩的岩石学及成岩作用也有许多研究。本文侧重探讨海滩岩的胶结作用及与成岩环境的关系。