Bioclastic Deposits Research Articles

The Aptian–Albian of the Apulia Carbonate Platform (Gargano Promontory, southern Italy): evidence of palaeoceanographic and tectonic controls on the stratigraphic architecture of the platform margin

Impressive megabreccia levels that were deposited mainly on by-pass, steep margins of carbonate platforms have usually been linked to syn-sedimentary tectonics and, more recently, to other mechanisms including sea-level low-stands. The Apulia Carbonate Platform, a main palaeogeographic domain of the Mediterranean Tethys, is characterized by huge megabreccia bodies which were deposited along its flank at various intervals during the Cretaceous. This paper deals with the stratigraphy and the genetic meaning of a peculiar early Cretaceous megabreccia level occurring on the Apulia platform margin exposed in the southern Gargano Promontory (southern Italy). The stratigraphic implications of the megabreccias within the tectonic evolution of the Apulia platform are also discussed along with a comparison with previous models. A sub-vertical blind contact separates a tectonized Berriasian shallow-water succession (Lower S. Giovanni Rotondo Limestones) from a thick rock-fall megabreccia succession pertaining to the Late Aptian–Albian Posta Manganaro Megabreccias. The contact, which is interpreted as a syn-sedimentary fault plane, is sealed by undeformed late Albian slope bioclastic sediments (lower Monte S. Angelo Rudist Limestones). Tensile tectonics downfaulted an external sector of the platform generating an articulated, steep, retreating margin. Field analysis and stratigraphic reconstructions show that the syn-tectonic wedge-shaped megabreccias partly filled an asymmetric depocenter (half-graben) resulting from the activity of a normal fault system located behind the previous prograding, low-energy, leeward margin (Montagna degli Angeli Limestones). Nevertheless, syn-sedimentary tectonics severely conditioned the composition, depositional processes and dispersal pattern of sediments supplied towards the original platform-to-basin transition (Upper Mattinata Bioclastic Limestones). This stratigraphic framework is believed to reflect a local response of the sedimentation to a late Early Cretaceous syn-sedimentary tectonic phase representing an important turning point in the evolution of the Apulia Carbonate Platform. It is supposed here that the observed tectonics, which are assumed to be related to the onset of the Austrian orogenic phase at the distant Adria plate margin, could only control the extension of the down-faulted platform. More probably, its drowning was decisively caused by the coeval global-scale Selli palaeoceanographic event (OAE 1a), which reduced the healing power of the platform by poisoning the shallow-water, sediment-donor communities. As a major causal mechanism, the OAE Selli event controlled the development of a rapid facies evolution during the late Early Aptian within the shallow-water successions of the Apulia and many other Tethyan carbonate platforms through the onset of nutrient-enriched water conditions. In this light, a stratigraphic correlation between the bases of the late Early Aptian shallow-water Orbitolina Level and the basinal Fucoid Marls is proposed. Their deposition within distinctive tracts of many platform-to-basin transitions on the southern Tethyan margin is viewed as the response of a previously pure carbonate sedimentation to the onset of environmental control on a global scale during the late Early Aptian.

Skeletal growth rates of Upper Cretaceous rudist bivalves: implications for carbonate production and organism-environment feedbacks

Abstract The skeletal growth rates of late Cretaceous rudist bivalves have been inferred from cyclic variations of isotopic and chemical compositions which are found in sclerochronological profiles of outer shell layers. Annual shell accretion of 11 studied shells from different environmental settings was in the range of less than 10 to 54 mm. CaCO 3 production of individual rudists was calculated to range from 12 to 214 g a −1 , and estimates of annual production of rudist communities assuming dense growth fabrics range from 4.6 to 28.5 kg m −2 . These production rates are not significantly higher when compared to modern mussel or oyster beds but, in contrast to modern analogues, rudist associations were much more important for the sedimentary budget of low-latitude shallow-water depositional environments. Controlling factors for the formation of growth fabrics and carbonate production are evaluated in a case study of rudist formations from a single depositional sequence. From turbulent outer platform environments to lagoonal inner platform settings, a decrease both in carbonate production and size of shells was found among specimens of a single species. No differences in carbonate production are evident when carbonate production in siliciclastic and calcareous environments is compared. Potential feedbacks between rudists and their environment induced by vertical growth, large production of bioclastic sediment and ejected biodeposits are discussed.

Factors regulating the development of elevator rudist congregations

Abstract Upper Cretaceous carbonate platform deposits show a widespread development of distinctive lenticular to tabular bodies of rock (lithosomes) formed by congregations of upright tubular (‘elevator’) rudist bivalves. Here we discuss the factors that regulated the initiation, consolidation and termination of such lithosomes, based on examples in the Santonian of the southern Central Pyrenees. In the study area, rudist congregations developed between phases of sediment influx from neighbouring source areas. For the initiation of settlement, sedimentation rate evidently had to be very low or nil. The first rudist settlers both provided more hard substrates for subsequent recruitment, and fuelled the in situ formation of bioclastic sediment, leading to the embedding and consolidation of the congregation (both positive feedbacks to establishment). Thereafter, rudist density correlated with inferred sediment destabilization at the benthic boundary layer, which in turn affected rudist recruitment. Thus, successful rudist larval settlement declined with increasing numerical density of individuals in the congregation — a crucial negative feedback mechanism. The density of the rudist congregations could then have been maintained at more or less the same level through time by this stabilizing process. Finally, development of the rudist congregations ceased with progressive shallowing, usually involving reworking of their upper parts and/or burial by renewed influxes of sediment.

Response of triassic reef coral communities to sea-level fluctuations, storms and sedimentation: Evidence from a spectacular outcrop (Adnet, Austria)

The Upper Rhaetian coral limestone of Adnet, southeast of Salzburg Austria has been repeatedly referred to as one of the most spectacular examples of an ancient ‘autochthonous’ coral reef structure. The ‘Tropfbruch’ quarry is probably the best outcrop for interpreting the distributional patterns of biotic successions and communities of a late Triassic patch reef. Our study is based on the interpretation of a) outcrop photographs, b) reef maps resulting from quadrat transects, and c) the analysis of quantitative data describing the distribution and frequency of reef organisms and sediment. A new methodological approach (combination of reef mapping and photo-transects) is used to obtain quantitative field data which can be compared in greater detail with data from modern coral reefs investigated by corresponding quantitative surveys. Three unconformities and three well-defined ‘reef growth stages’ reflecting the vertical and lateral development of the reef structure were differrentiated using transects: Stage 1, representing the reef growth optimum, is characterized by laterally differentiated coral reef knobs with corals in growth position. Criteria supporting this interpretation are the extraordinary size of the corals, their preservation in situ and the great thickness of this interval. The massive coralPamiroseris grew under higher energy conditions at the rim of the reef knob, whereas branchingRetiophyllia colonies preferred less agitated water in the center. Vertical changes are reflected by an increase in frequency of the dasycladacean algaDiplopora adnetensis and by the decreasing size ofRetiophyllia. These sedimentological and biological criteria together with the unconformity above indicate a fall in the sea level as a major control mechanism. Stage 2, separated from stage 1 by an unconformity caused by partial subaerial exposure and karstification, is characterized by vertically stacked coral successions with diverse reef debris. Facies heterogeneity is reflected by differences in the diversity, taphonomy and packing density of reef-building organisms as well as by differences in sediment input from the platform. Water depths and accommodation space were lower, therefore minor sea level fluctuations had a stronger effect on the biotic composition. The high percentage of coral debris and corals reworked by storms and the increase in the input of platform sediment led to a reduction of reef growth. Stage 3, again separated at the base by an unconformity associated with karstification, is characterized by bioclastic sediments with isolated reefbuilders forming a level-bottom community. The distribution of different coral morphotypes suggests that sea level fluctuations were not the only controlling factor. Variations in the substrate were caused by differences in the input of platform sediment. The three-step development seen in Adnet documents the response of low-diverse coral associations to variations caused by small-scale sea level changes, storm activity and sedimentation. The vertical changes in reef community structures correspond to a sequence of ‘allogenic replacements’. The Adnet reef structure should not be regarded as a general model of Alpine Upper Rhaetian reefs, because of the particular setting of the patch reef. Only the ‘capping beds’ of the Upper Rhaetian Reef Limestone of the Steinplatte exhibit criteria similar to Adnet. Potential modern analogues of features seen in the coral communities of Adnet are the internal structure of theRetiophyllia thickets, the key role of branching corals within the communities, the scattered distribution and low and even diversity of corals subsequent to breaks in settlement, segration patterns of corals indicating ‘contact avoidance’, toppling of large coral colonies by intensive boring, and decreasing coral coverage from deeper and sheltered settings to more shallower water depths.

Palaeoenvironmental evolution of the Late Cretaceous sequence in the Tarfaya Basin (southwest of Morocco)

Abstract The Late Cretaceous sedimentary evolution of the Tarfaya Basin is reviewed based on late Albian-Campanian outcrops and two cored exploration wells (S13 and S75) with a stratigraphic range from the early Cenomanian to the late Turonian. Standard sedimentological and micropalaeontological methods were used, together with clay mineralogy and organic geochemistry. Two distinct depositional environments were found. The first is late Albian-early/middle Cenomanian in age and dominated by terrigenous silici- and bioclastic sediments containing primarily terrestrial organic matter with a low organic content. Abundant illite and chlorite, eroded from crystalline rocks outcropping in the Anti-Atlas mountain and Mauritanides is present, as is abundant kaolinite reworked from soils surrounding the Tarfaya Basin. The second environment comprises late Cenomanian-Campanian sediments characterized by pelagic marls and limestones with a high organic content and a abundant pelagic macro- and microfauna. These sediments, which were probably deposited in an upwelling system, are characterized by higher contents of smectite.

Upper Ordovician facies in the Lac Saint‐Jean outlier, Québec (eastern Canada): palaeoenvironmental significance for Late Ordovician oceanography

Upper Ordovician (Caradocian) carbonates of eastern North America were deposited along the Iapetus continental margin and record a transition from warm‐ to cool‐water settings despite this margin having been within the southern hemisphere tropical belt. This event has been documented from Virginia (USA) to southern Québec (Canada) although, not previously from areas close to the palaeoequator.Field, petrographic and major element geochemistry data have been gathered from the poorly‐known Upper Ordovician carbonate succession outcropping in the Lac Saint‐Jean outlier in central Québec. The succession consists of a lower siliciclastic formation (Tremblay) overlain by three limestone formations (Simard, Shipshaw and Galets) and capped by shales (Pointe‐Bleue Shale). From macro‐ and microfaunal evidence, carbonate sedimentation occurred during the late Caradoc and is younger than the early‐ to mid‐Caradoc carbonate succession present farther south. Relative sea level fluctuations recorded in the sediments suggest an overall sea level rise briefly halted by a minor end‐Caradocian sea level fall.The lower limestone formation (Simard) consists of muddy sediments with algal‐coral‐stromatoporoid boundstones; green algae are abundant. This unit reflects low energy sedimentation on a shallow warm‐water carbonate ramp colonized by a diverse chlorozoan fauna. The upper limestone formation (Galets) is typified by coarse‐grained bioclastic sediments punctuated by numerous phosphate‐rich hardgrounds with evidence for high energy shallow marine conditions. Faunas were dominated by crinoids and bryozoans. This unit represents high energy sedimentation on a cool shallow water carbonate ramp colonized by a brynoderm faunal association. Between both units, a deeper marine (outer shelf) limestone formation (Shipshaw) was developed.In the Lac Saint‐Jean area, a transition from warm‐ to cool‐water carbonate ramps occurred in latest Caradoc times and is litho‐ and biofacies‐wise, similar to what is documented for lower Caradocian limestones present farther south. Upwelling of nutrient‐rich cool bottom oceanic waters was a probable cause for this transition.

Bioclastic sediment lobes on a supply dominated Upper Cretaceous carbonate platform margin, Montagna della Maiella, Italy

A thick bioclastic sediment wedge was deposited on the slope of the Maiella carbonate platform margin in the Late Campanian to Late Maastrichtian. The wedge consists of lobate depositional units (laterally and vertically convex structures). The complex internal geometries of the lobes combine characteristics of unidirectional sandwaves and the alternating point‐sources of deltas. Excellent outcrop permits a detailed documentation and discussion of the depositional processes.The sediment wedge constitutes a supersequence, which prograded along the platform margin as a result of high sediment supply and forced regression. Within the supersequence, a hierarchy of higher‐order stratigraphic units (sequence sets and sequences) are developed. The individual bioclastic sediment lobes are interpreted as systems tracts and parasequences of the sequences within the sequence sets.

A currents of removal approach for interpreting carbonate sedimentary processes

This study develops a currents of removal methodology to examine and quantify the nature of physical transport processes affecting the formation of bioclastic deposits in the Cocos (Keeling) Islands, an Indian Ocean atoll. This approach is based on the hydraulic settling and threshold of entrainment characteristics of bioclastic deposits and measured current energy. Potential mobility (PM) analysis of 157 samples quantifies the proportion of deposits able to be transported in each geomorphic zone. Results show that under mean energy conditions wave-induced currents entrain and transport sediment within the atoll and show that transport of sediment is not solely reliant on storm energy conditions. Gradients of PM values are used to reconstruct sediment transport pathways from the reef flat (85–100% PM), through shallow passages (75–100% PM) to sand aprons (50–75% PM) and shallow (0–20% PM) and deep lagoon (0% PM). Comparison of settling velocity distributions of bed material and sediment retained in sediment trap experiments show that actual mobility levels correspond with PM estimates. Potential mobility analysis also identifies the immobile portions of deposits which increase lagoonward. Constituent analysis of immobile fractions and analysis of settling frequency distributions are used to differentiate the importance of physical and biological processes in the formation of deposits throughout Cocos and identify the role of each geomorphic zone in the transport system. The pattern of sediment is controlled by physical processes between the reef flat, the primary sediment production zone, with sediment transported through the shallow passage conduits to the sand aprons. The formation of shallow and deep lagoon deposits is controlled by autochthonous sediment production and storm deposition. Potential mobility analysis is a powerful tool enabling physical transport processes within bioclastic sedimentary environments to be quantified. The ability to examine an individual deposit's hydraulic behaviour and development also enables depositional processes to be examined at a much finer resolution than previously attempted using conventional textural approaches.

Contemporary sedimentation in the Cocos (Keeling) Islands, Indian Ocean: interpretation using settling velocity analysis

Textural parameters of grain-size distributions derived from sieve analysis and component analysis have been used to the exclusion of other methods to examine sedimentary processes in modern reef environments. However, because these methods do not reflect the hydraulic behaviour of bioclastic sediment this study uses settling velocity analysis (accounting for grain size, density, and shape variability), and component analysis of separated settling velocity fractions, to describe and differentiate bioclastic deposits and infer transport pathways in the Cocos (Keeling) Islands, an Indian Ocean atoll. Principal Component and Cluster analysis of settling (251 samples), and component (90 samples) characteristics, discriminated eight settling velocity and five component classes. A derived settling velocity classification, reflecting the hydraulic properties of the sediments, describes a lagoonward gradient from those with fast settling characteristics on the outer reef flat to those with moderate settling character on sand aprons. The active sediment transport system is constrained between these zones. Component analysis of bulk samples identifies two broad sediment types: (1) those with reef-derived components, found throughout the active transport zone (reef flat to sand aprons); and (2) those characterised by greater proportions of gastropod and Halimeda fragments which are produced and deposited in situ in the lagoon. Component analysis of settling fractions further resolves details of sediment movement in the active transport zone. Sediment is transported from the productive reef flat to passage entrances which form initial depositional environments. Slower settling material is selectively transported from the entrances, through the passage conduits, to passage exit environments. These form the main depositional areas under mean energy conditions. During moderate- to high-energy events sediment is flushed from the passage to sand apron zones, with slower settling grains deposited in the deep lagoon sinks. Results show that settling velocity analysis and component analysis of settling fractions can resolve the nature of sediment transport patterns in reef environments at a much finer scale than studies in which analysis is based on grain-size distributions.

Cross‐stratified calcarenites from New Zealand: subaqueous dunes in a cool‐water, Oligo‐Miocene seaway

ABSTRACTCross‐bedded, cool‐water, bioclastic limestones of the Te Kuiti Group on the North Island of New Zealand are composed primarily of bryozoans, echinoderms, and benthic foraminifers. Their prominent, large‐scale, unidirectional cross‐stratification is interpreted as produced by migrating subaqueous dunes on the floor of a 50–100 km wide, north‐east‐trending seaway in water depths of 40–60 m. These dunes are thought to have developed in response to strong, seaway‐parallel, tidal currents combined with a north‐east‐directed, set‐up or oceanic current.Cross‐stratification is organized into four hierarchical levels: (1) cross‐lamination; (2) first‐order sets; (3) second‐order sets; and (4) cross‐stratified successions. The levels are based on increasing degrees of internal complexity. Distinct attributes such as internal organization, cross‐set thickness, foreset shape, and lower bounding‐surface shape are used to describe and interpret the cross‐stratification. All these attributes are here integrated in a new and expanded classification of unidirectional cross‐stratification that emphasizes flow and bedform dynamics rather than overall set shape.Individual cross‐stratified successions are interpreted to have formed by dunes with varying sinuosity, superposition, and flow history, under conditions of different current strength but constant sediment production. Horizontally bedded successions are the result of robust, active dune fields that grew during times of vigorous sediment transport. Formset successions were produced from large compound dunes and are the expression of languid and decaying dune fields that developed during times of decreasing sediment transport. These decaying dunes were gradually smothered by continuously and locally produced bioclastic sediment. Formset cross‐stratified successions are most likely to develop in carbonates, where the sediment is produced in place, than in terrigenous clastics where the sediment is imported.

A comparison of settling and sieve techniques for the analysis of bioclastic sediments

This paper examines differences in the use of sieve and settling techniques for the determination of textural parameters of grain-size distributions for 30 bulk bioclastic (reefal) deposits and examines the interpretive use of the results. A comparison of results of the key textural parameters shows that sieve-size estimates deviate from settled-size determinations of mean grain-size and dispersion values by up to 2 and up to 1.5 phi units, respectively. These differences are explained by the different grain properties measured by the two techniques exacerbated by the heterogeneous character of bioclastic deposits. Changes in sample components which control grain shape, and grain bulk-sample density, also influence the magnitude of size and dispersion differences. Observed differences are discussed in relation to interpretation of the depositional environment and hydraulic regime, leading to the conclusion that sieve results can grossly distort depositional and energy processes operating within bioclastic environments. In contrast, settled-size estimates (an index of deposit hydraulic behaviour) show that bioclastic sediment transport systems operate within a much narrower energy regime than sieve-sizes suggest. Hydraulic-size estimates are considered to provide a better tool with which to interpret sedimentary processes in bioclastic environments.

Hydraulic characteristics of bioclastic deposits: new possibilities for environmental interpretation using settling velocity fractions

ABSTRACTThis paper examines the hydraulic behaviour of heterogeneous bioclastic sediments using settling velocity fractions. Bioclastic deposits are divided into fractions by splitting hydraulically sorted samples. Compositional analysis of fractions shows that grains exhibit marked variation in composition, shape, size and density which control the hydraulic behaviour of bioclastic deposits. Despite the heterogeneous grain properties of fractions (analogous to sieve fractions) sedimentation analysis shows that they possess a narrow range of settling velocity values. Flume experiments also show that the settling velocity fractions possess a narrow range of threshold velocity values. A threshold relationship is identified in which the velocity required to entrain mixed settling fractions increases from 20.5 to 65‐0cm s −1 for settling fractions with mean settling values of 5.5 chi (2‐2 cm s −1) to 1.4 chi (36.0 cm s −1), respectively. Heterogeneous settling fractions, due to intergrain effects, were found to possess larger threshold velocities than individual components showing that the composition of bioclastic deposits controls hydraulic behaviour. Thus, caution must be exercised in using the threshold relationship for homogeneous sediments or deposits of markedly different composition.As settling velocity fractions reflect narrow hydraulic (settling and threshold) properties of sediments the mean settling velocity of fractions is considered a good indicator with which to interpret transport and depositional processes.Comparison of settling and sieve‐size distributions shows that size distributions do not reflect the hydraulic behaviour of bioclastic deposits and should not be used to interpret environmental processes. The study indicates that examination of settling velocity fractions (for which hydraulic settling and threshold properties are known) coupled with compositional analysis of these fractions will allow much greater environmental interpretation of deposit‐forming and energy processes in reef environments.

Geometric and Sedimentologic Characteristics of Mid-Miocene Lowstand Reservoir Sandstones, Offshore Northwest Java, Indonesia: ABSTRACT

Numerous reservoirs in the Upper Cibulakan Formation (Mid-Miocene) of the Offshore Northwest Java shelf occur in sharp-based sandbodies that range from less than 1 m up to 10 m in thickness. Well-log derived net-sand isopach and seismic amplitude maps of these sandbodies depict elongate features, that are 1-2 km wide and 5-8 km long. The orientation of the longest axis of these sandbodies is predominantly north-south. Conventional cores reveal that these sandbodies are burrowed to completely bioturbated sandstones. Common trace fossils associated with these sandbodies include Ophiomorpha, Teichichnus and Thalassinoides. The lower contact of these sands is typically sharp and is commonly associated with a Glossifungites surface and siderite mud clasts. Overlying and underlying mudstones are relatively devoid of burrowing. Benthonic foraminifera assemblages within these mudstones indicate inner to outer neritic conditions in a relatively restricted marine setting. The upper contact of these sandstones is gradational over a 0.5 to 1m interval. Sandbodies of the same age and similar facies were observed in outcrops in onshore west Java. Here, they can be observed to pinch out over a distance of 500 m. The lower bounding contact appears discordant with underlying interbedded sandstones and mudstones. Several of the sandstones contain abundantmore » accumulations of the large, open marine, benthonic foraminifera Cycloclypeus and Lepidocyclina. Occasionally the concentration of these large foraminifera form limestones within the sharp-based sandbodies. These bioclastic deposits commonly exhibit planar-tabular and trough cross-stratification. The sandbodies are interpreted as having been emplaced during relative falls in sea-level within a large Mid-Miocene embayment. Our understanding of their geometry and sedimentologic characteristics is leading to a more effective exploitation strategy for these sandbodies in the Offshore Northwest Java area.« less

Hauterivian shallow marine calcareous biogenic mounds: S.E. Spain

Abstract Hauterivian biogenic deposits from the Prebetic northern margin near Caudete (Albacete Province, southeastern Spain) are represented by low domed bodies, less than 10 m thick, surrounded by bioclastic sediments and capped by siliciclastics. They consist of a coral, stromatoporoid and microbial framework with cavities filled by mud, rapidly lithified. Intermound bioclastics, derived from the mound organic community, reflect high energy conditions and shallow water settings. Mound growth and architecture were controlled by nutrient fluxes and internal light gradients. Comparisons with closely related structures from the Palaeozoic and the Recent show some similarities but also significant differences; the example documented here also departs from other Cretaceous occurrences.

The growth fabric of gregarious rudist elevators (hippuritids) in a Santonian carbonate platform in the southern Central Pyrenees

Abstract The growth fabrics of living congregations of erect sessile epifauna range between two extremes. With superstratal growth, the growing skeletons project well (∼ metres) above the ambient sedimentary surface, creating an upstanding framework, which may be secondarily filled by sediment, as in modern tropical coral reefs. With constratal growth, by contrast, the growing tips of the skeletons project little (∼ centimetres) from the seafloor, such that the bulk of the skeletal fabric is embedded in, and supported by, the accumulating interstitial sediment, as in Pinna, and oyster beds. Here we investigate the original growth fabrics and palaeorelief of hippuritid congregations in the Santonian rudist formations around the Sant Corneli anticline, near Tremp in the southern Central Pyrenees. The study particularly concentrates on a hippuritid lithosome, exposed over some 0.25 km2, on the northern flank of the anticline. Four aspects are analysed. (1), The lithosome is shown to have a tabular (biostromal) geometry, and, like others in the area, is covered by a laterally accreted bioclastic blanket. (2) Specimens in upright life position show that the pioneers of the paucispecific congregations grew in bouquets, but later recruits settled on the flanks of established, and fallen shells. Adults were only loosely clustered and supported by the accumulating interstitial sediment. (3) Most shells, however, are now horizontal with respect to the bedding and densely stacked (“dense horizontal fabric”, or “d.h.f.”). This fabric is shown to be a result of current-induced removal of sediment and toppling of shells. (4) In contrast to the open marine aspect of the overlying bioclastic sediments, the fine matrix sediment of the hippuritid lithosomes is faunally restricted, and appears largely derived from the in situ biodegradation of shells. We conclude that the hippuritid congregations grew constratally (i.e. without any supporting biogenic framework), and that they formed biostromal accumulations lacking relief. So there is no justification for calling them reefs as some authors have done. Rather, these rudists grew as gregarious sediment-dwellers in the restricted waters of the platform top, where they were sporadically disturbed by storms, and eventually blanketed by bioclastic material swept in from the platform margin.

Foraminifera from the Vectis Formation (Wealden Group, Lower Cretaceous) of the Wessex Sub-basin, southern England: a preliminary account

Foraminifera are documented from the uppermost part of the essentially lagoonal Vectis Formation (Barremian to possibly early Aptian) of the Isle of Wight, southern England. Monospecific and low-diversity influxes of indigenous agglutinated taxa are identified from claystones of mudflat or subaqueous origin, and their palaeoecological significance is assessed. In contrast, reworked Upper Jurassic foraminifera are associated with coarser-grained bioclastic deposits and indicate the proximity of the Wessex Sub-basin margin, immediately to the north.

Sedimentology of the Upper Triassic reef complex at the Hochkönig Massif (Northern Calcareous Alps, Austria)

The Upper Triassic Dachsteinkalk of the Hochkonig Massif, situated 50 km south of Salzburg in the Northern Calcareous Alps, corresponds to a platform margin reef complex of exceptional thickness. The platform interior limestones form equally thick sequences of the well known cyclic Lofer facies. Sedimentation in the reef complex was not so strongly controlled by low-amplitude sea-level oscillations as was the Lofer facies. The westernmost of the 8 facies of the reef complex is an oncolite-dominated lagoon, in which wave-resistant stromatolite mounds with a relief of a few metres were periodically developed. The transition to the central reef area is accomplished across the back-reef facies. In the back-reef facies patch reefs and calcisponges appear. The proportion of coarse bioclastic sediment increases rapidly over a few hundred metres before the central reef area is encountered. The central reef area consists of relatively widely spaced small patch reefs that did not develop wave-resistant reef framework structures. The bulk of the sediment in the central reef area is coarse bioclastic material, provided by the dense growth of reef organisms and the wave-induced disintegration of patch reefs. Collapse of the reef margin is recorded by the supply of large blocks of patch reef material to the upper reef slope. Additionally, coarse, loose bioclastic debris was supplied to the upper reef slope and this was incorporated into debris flows on the reef slope and turbidites found at the base of the slope and in the off-reef facies. Partially lithified packstones and wackestones of the lower to middle reef slope were modified by mass movement to form breccia and rudstone sheets. The latter reach out hundreds of metres into the off-reef facies environment. A reef profile is presented which was derived by the restoration of strike and dip information. In conjunction with constraints imposed by sedimentary facies related to slope processes, the angle of slope in the reef margin area ranged from 11° to 5°, forming a concave (dished downwards) slope. Water depth estimations require that the central reef area did not develop in water of less than 10 metres depth. At the reef margin water depths were about 30 metres, at the base of the reef slope 200 metres and deepening in the off-reef facies to 250 metres. While previous work on reef complexes from this type of setting suggests growth in a heavily storm-dominated environment, the present author finds little evidence for the storm generation of the fore reef breccias, although there is good evidence for storm-influenced sedimentation and reworking in the central reef area.

Architecture of an unconformity-based tidal sandstone unit in the Ametlla Formation, Spanish Pyrenees

This study is concerned with the sedimentary architecture of a shallow-marine sandstone unit in the Lower Eocene Ametlla Formation, Ager Basin, Spanish Pyrenees. The sandstone unit was formed in a tide-dominated setting, and is continuously exposed in a 10.5 km long, mainly strike-parallel, cutcrop. Its base is a prominent unconformity (sequence boundary), and it consists of two sub-units separated by a tidal ravinement surface. Sub-unit 1 (lower sub-unit) formed in an estuarine setting that was created by flooding of an incised valley system. Outside the incised valleys, an oyster bed is present as a lateral equivalent to the estuarine deposits. The sequence bounding unconformity was generated during relative sea-level lowstand, whereas the overlying estuarine and bioclastic deposits of sub-unit 1 formed in the early stages of the transgressive systems tract. In sub-unit 2, above the tidal ravinement surface, estuary mouth-bar and tide-dominated delta deposits are the main facies associations, interfingering with offshore to inner shelf sediments westwards. These deposits formed in later stages of the transgressive systems tract. The large-scale model of the sandstone unit served as a framework within which the internal arrangement of architectural elements (AE) was recorded. Twelve AE were recognized. They mostly have a lensoid geometry, and their lateral transitions are abrupt or occur by interfingering over distances of 50–250 m. The AE distribution shows that the tide-influenced setting was characterized by a high degree of spatial and temporal variability with regard to sedimentary processes. This, together with the prominent thickness variation of the unit studied, makes correlation between vertical profiles difficult, even over distances of only a few hundred metres. Stratal surfaces such as the sequence-bounding unconformity, the tidal ravinement surface, and the flooding surfaces, represent the only reliable correlation markers in this kind of environment.

Marine geology of the Sodwana Bay shelf, southeast Africa

The geostrophic current-controlled northern Zululand shelf displays a unique assemblage of interesting physical, sedimentological and biological phenomena. The shelf in this area is extremely narrow (3 km) and is characterised by submarine canyons, coral reefs, and steep gradients on the continental slope. Three submarine canyons occur in the study area and are classified as mature- or youthful-phase canyons depending on the degree to which they breach the shelf. These canyons originated as mass-wasting features which were exploited by palaeo-drainage during sea-level regressions. Shelf lithology is dominated by a series of coast-parallel patch coral reefs which have colonised beachrock and aeolianite sequences that extend semi-continuously from −5 to −95 m, and delineate late Pleistocene palaeocoastline events. The unconsolidated sediment on the shelf is either shelf sand (mainly terrigenous quartz grains) or bioclastic sediment. Large-scale subaqueous dunes commonly form in the unconsolidated sediment on the outer-shelf due to the Agulhas Current flow. These dunes occur as two distinct fields at depths of −35 to −70 m; the major sediment transport direction is towards the south, but occasional bedload parting zones exist where the bedform migration direction changes from south to north.

LATE PALEOZOIC STRATIGRAPHY AND PETROGRAPHY OF THE THINI CHU GROUP (MANANG, CENTRAL NEPAL): SEDIMENTARY RECORD OF GONDWANA GLACIATION AND RIFTING OF NEOTETHYS

In the Manang area (north Annapurna Range; Nepal Himalaya), the Permo-Carboniferous succession is 1000 to 1500 m thick. Crinoidal biocalcarenites (Tilicho Lake Fm.) pass upward to alternating black shales and sharp-based white quartzose sandstones (Thini Chu Group). Detailed stratigraphic analysis of this unit allowed us to recognize and establish 5 new formations and 8 new members. The Marsyandi Fm., of Visean age, records an increase of subarkosic terrigenous detritus during the initial stage of Neotethyan rifting, and is capped by two sequences Syringotbyris beds) characterized by transgressive sandstones rich in Serpukhovian brachiopods. The overlying black shales with subordinate quartzarenites (Col Noir Shale) are followed in the more proximal Bangba section by diamictites yielding dolostone rock fragments, documenting the first advance of glacial ice on rift shoulders, actively uplifted since the Bashkirian-Moscovian (Bangba Fm.). Next, glaciomarine to transgressive shelfal deposits are enriched first in igneous detritus and then in arenaceous rock fragments (Braga Fm.). Mafic to felsic magmatism during the climax of rifting was thus followed by active erosion of sedimentary successions, probably around the Carboniferous/Permian boundary. A major Early Permian transgression, coinciding with ameliorating climates at the end of the Gondwana glaciation, was followed by mainly estuarine chert-bearing quartzose pebbly sandstones capped by richly bioclastic shelfal deposits (Puchenpra Fm.). This second major transgression, associated with quartzose sandstones documenting subdued rift reliefs, is dated as Bolorian at Bangba, as Murgabian-Midian at Col Noirand as Djulfian at Tilicho. The base of the condensed outer shelf/upper slope carbonates capping the Thini Chu Group (topmost biocalcarenites) is also strongly heterochronous, being dated as Bolorian to Kubergandian-Murgabian at Bangba and as Djulfian-Dorashamian at Col Noir and Tilicho. Thermal subsidence associated with the opening of Neotethys thus began as early as the Early Permian.