Bioclastic Deposits Research Articles

Influence of paleo-Trade Winds on facies patterns of the Cambrian Shanganning Carbonate Platform, North China

The Shanganning Carbonate Platform (SCP) of the North China Craton (NCC) was located in the Trade Winds belt during the Cambrian, which influenced its facies development. In recent years, numerous drillcores have been recovered from these units, but the depositional environments and facies architecture of this large (~2.0 × 105 km2) carbonate platform system have not yet been reported in detail. In this study, we integrate sedimentological and magnetic fabric data collected from outcrops and drillcores in order to document variation in microfacies and facies associations within the SCP, to infer paleowind directions and constrain the paleogeographic location of the SCP, and to evaluate eolian controls on platform development. This analysis documented numerous carbonate microfacies belonging to six fabric types (mudstone, wackestone, packstone, grainstone, framestone, and dolostone) along with rarer siliciclastic sediments. These microfacies belong to facies associations representing tidal flat, open platform, platform margin, slope, and deep basinal depositional environments. Anisotropy of magnetic susceptibility (AMS) measurements yielded mean paleocurrent directions for the Early, Middle, and Late Cambrian of 114°, 63°, and 83° (± ~30°) from paleo-north, respectively, that were relatively consistent for 12 out of 13 platform-margin sites, all vectors being corrected for a post-Cambrian ~150° clockwise rotation of the NCC. These wind orientations are consistent with the SCP shifting from south of the paleo-Equator in the Early Cambrian to north of it by the Middle Cambrian. Paleo-Trade Winds influenced facies patterns across the platform, producing a distinct windward-leeward zonation. The windward margin is characterized by well-sorted oolitic grainstone, as well as (favositids) coral and (dasycladacean) algal-sponge framestone, and the leeward margin by poorly-sorted oolitic wackestone-grainstone and fine bioclastic sediments. The present study is innovative in 1) integrating microfacies and AMS analyses in order to establish paleowind patterns and influences on an ancient carbonate platform, 2) documenting windward-leeward variation in quantifiable sediment properties across the SCP, and 3) constraining the paleo-hemispheric location of the NCC during the Cambrian.

Marine Habitats in Conservation Units on the Southeast Coast of Brazil

Continental shelves with diversified physiography, geomorphology marked by rocky reefs, channels, and marginal terraces, besides several terrestrial and marine sediments, have a rich mosaic of habitats ideal for the conservation and reproduction of marine life. This work, as part of the research project Detailed Study of the Ocean Bed inside the Santa Cruz WLR, Costa das Algas EPA and 2000m Immediate Environment, aims at mapping the marine habitats located in two Conservation Units, Costa das Algas EPA (Environmental Protection Area) and Santa Cruz WLR (Wildlife Refuge), located south of the Doce River mouth, on the southeast coast of Brazil. For this purpose, data on the type, distribution, and composition of seabed sediments, as well as information on the seabed geomorphology using bathymetric data, seabed records by ROV imaging, and benthic fauna at the phylum taxonomic level were determined in the studied areas. The results indicated the presence of five sedimentary facies defined by the terrigenous and marine sedimentation regimes, separated in the vicinity of the -35 m isobath. The sandy-clayey sediments deposited by the fluvial discharges from the region rivers predominate in the first case. On the other hand, the marine sedimentation consists of biolitoclastic and bioclastic sediments, with rhodolith pebbles as well. In addition, three hydrodynamic zones/regions were defined, according to the presence and type of seabed ripples formed by near-bottom waves. The bathymetric data revealed seabed features such as reef structures, or rocky reefs, as well as submerged channels originated by marine regression. This set of characteristics defined eleven marine habitats, and among them, those consisting of biolitoclastic, bioclastic and rhodolith sediments, as well as rocky reefs and submerged channels, which had the greatest abundance of marine fauna and flora organisms. On the other hand, the habitats defined by the terrigenous sedimentation had a lower abundance of organisms. This finding indicates that the marine sedimentation regime predominated over the terrigenous, especially in the Costa das Algas EPA, which generally represents an area with high potential for proliferation and preservation of marine benthic and demersal fauna and flora. In view of this, the mapping of the Conservation Units presented here for the first time on a detailed scale can help to understand the importance of these areas for the preservation and maintenance of marine life.

A new roman fish-salting workshop in the Saltés Island (Tinto-Odiel Estuary, SW Spain): La Cascajera and its archaeological and geological context

The southwestern Atlantic coast of the Iberian Peninsula presents an important Roman heritage that includes numerous fish-salting workshops, with an industrial activity that went on for almost a millennium (1st century BC-7th century AD). Nevertheless, a future broad research is still necessary to determine the geologic substratum on which they are based, their palaeoenvironmental evolution, their main economic objectives and the byproducts derived from their activities. This paper is focused on the geology, dating and the archaeological record of La Cascajera, a new site located in the Tinto-Odiel estuary (SW Spain). This new cetaria occupied the northwestern end of La Cascajera ridge (Saltés Island), constituted by sandy, bioclastic deposits of previous washover fans (1st-2th centuries AD). During the main period of activity (middle of 4th century AD-5th century AD), the existence of a certain typology of amphorae as well as the documentation of a shell deposit formed mostly by Glycymeris suggest that this factory was oriented to the production of mixed fish sauces and the handling of edible bivalves. The main features of this deposit (texture, paleontology, taphonomy) could be used to differentiate middens from natural shelly ridges.

Composition and Distribution of Biodegradable Compounds in the Macrophyte Dominated Zone of Lake Taihu

In shallow eutrophic lakes, benthic bioclastic deposits accumulate abundant organic carbon derived from macrophyte detritus. Taking the typical macrophyte-dominated Xukou Bay as the study area, field investigations were performed using sediment cores to evaluate benthic phytodetritus accumulation. Specifically, nutrient contents, TOC/TN ratios, pigmentation, and biodegradable compounds were measured as descriptive parameters. The results show that the benthic bioclastic deposit had accumulated abundant pigments, nutrients, and biodegradable compounds derived from macrophytes detritus. These were mainly localized in the top 15 cm of sediments. Nitrogen loading in the sediments was significantly higher than phosphorous loading, with a distinct spatial difference; the total nitrogen content ranged from 127.2-2092.8 mg·kg-1 and total phosphorous content ranged from 222.1-528.4 mg·kg-1. Moreover, nitrogen loading (1033.6 mg·kg-1) in the vegetated zones were higher than in the unvegetated zones (325.2 mg·kg-1). In addition, carbohydrate (3.7 mg·g-1) was the dominant component of sedimentary bioclastic material, with lipids (2.8 mg·g-1) being the second most abundant of the biodegradable compounds. The major sources of nutrients and biodegradable compounds in the sediment were massive aggregates of macrophyte detritus. The pigment, nutrient, and biodegradable compound contents in the vegetated zones were significantly higher than in unvegetated zones (P<0.01). The benthic eutrophic state showed a trend from mesotrophic to eutrophic in Xukou Bay, which should be given more attention in the future management of freshwater lake ecosystems.

Threshold of motion of bivalve and gastropod shells under oscillatory flow in flume experiments

AbstractThe threshold of motion of non‐fragmented mollusc shells was studied for the first time under oscillatory flow. In this regard, flume experiments were used to investigate the threshold of motion of three bivalve and three gastropod species, two typical mollusc classes of coastal coquina deposits. The sieve diameters ranged from 2·0 to 15·9 mm. These experiments were performed on a flat‐bottom setup under regular non‐breaking waves (swell) produced by a flap‐type wave generator. The critical Shields values for each species of mollusc were plotted against the sieve and nominal diameter. Moreover, the dimensionless Corey shape factor of the shells was evaluated in order to investigate the effect of mollusc shell shapes on the threshold of motion. According to their critical Shields parameter, the mollusc threshold data under oscillatory flow present smaller values than the siliciclastic sediments when considering their sieve diameter. In addition, the mollusc datasets are below the empirical curves built from siliciclastic grain data under current and waves. When considering the nominal diameter, the critical Shields parameter increases and the mollusc data are closer to siliciclastic sediments. Bivalves, which have a flat‐concave shape (form factor: 0·27 to 0·37), have a higher critical Shields parameter for smaller particles and more uniform datasets than the gastropod scattered data, which have a rounded shape (form factor: 0·58 to 0·62) and have varied morphologies (ellipsoidal, conical and cubic). The comparison between previous current‐driven threshold data of bioclastic sediment motion and the data of mollusc whole shells under oscillatory flow shows a fair correlation on the Shields diagram, in which all datasets are below the mean empirical curves for siliciclastic sediments. These findings indicate that the shape effect on the transport initiation is predominant for smaller shells. The use of the nominal diameter is satisfactory to improve the bioclastic and siliciclastic data correlation.

Marine geological maps of the Campania Region (Southern Tyrrhenian sea, Italy): considerations and contributions to a different scale of geological survey

Marine geological maps of the Campania region have been constructed both to 1:25.000 and to a 1:10.000 scales in the frame of research projects financed by the Italian National Geological Survey, focusing, in particular, on the Gulf of Naples (Southern Tyrrhenian sea), a complex volcanic area where volcanic and sedimentary processes strongly interacted during the Late Quaternary and on the Cilento Promontory offshore. In this paper the examples of the geological sheets n. 464 “Isola di Ischia” and n. 502 “Agropoli” have been studied. The integration of the geological maps with the seismo-stratigraphic setting of the study areas has also been performed based on the realization of interpreted seismic profiles, providing interesting data on the geological setting of the subsurface. The coastal geological sedimentation has been studied in detail in the Ischia and Agropoli offshore. The mapped geological units are represented by: i) the rocky units of the acoustic basement (volcanic and/or sedimentary), ii) the deposits of the littoral environment, including the deposits of submerged beach and the deposits of toe of coastal cliff, iii) the deposits of the inner shelf environment, including the inner shelf deposits and the bioclastic deposits, iv) the deposits of the outer shelf environment, including the clastic deposits and the bioclastic deposits, v) the Lowstand System Tract, vi) the Pleistocene relict marine units, vii) different volcanic units, Pleistocene in age. The seismo-stratigraphic data, coupled with the sedimentological and environmental data provided by the geological maps, allow to give new insights on the geologic evolution of this area during the Late Quaternary.

Depositional model of early Cretaceous lacustrine carbonate reservoirs of the Coqueiros formation - Northern Campos Basin, southeastern Brazil

Abstract Recent discoveries of giant hydrocarbon accumulations in coquinas and other lacustrine carbonate rocks of Rift and Post-rift (pre-salt) super-sequences of the Campos and Santos Basins have motivated a series of studies focused on understanding the genesis of these unique reservoirs. An integrated petrologic-sedimentologic-stratigraphic-seismic study of the Coqueiros Formation delineated the depositional model and evolution of reworked bioclastic rocks and other associated rocks deposited on the external high in the northern Campos Basin during the transition between late-rift and sag stages. Bioclastic rudstones and grainstones with moderate to high shell reworking represent 65% of the samples described. The following pieces of evidence point to a predominantly shallow lacustrine depositional environment, dominated by storm currents: (i) presence of high-angle (>14°) longshore cross-bedded rudstones; (ii) limited occurrence of muddy sediments interbedded with thick packages of amalgamated bioclastic rudstones; (iii) great lateral continuity of rudstone deposits devoid of matrix on the structural high, consistent with processes of storm action on large expanses of shallow lakes; (iv) deposits of massive rudstones, with disorganized shells, similar to the modern coastal bioclastic deposits of California and Shark Bay. Three sets of facies association are proposed: (i) high-energy facies, composed of bioclastic rudstones and grainstones devoid of matrix deposited above the fair weather wave base (FWB); (ii) low energy facies, composed of hybrid rocks with muddy or peloidal matrix and other fine grained rocks deposited below the FWB or in protected shallow environments; (iii) an alkaline set, composed of mainly magnesian clay ooids deposited in a more alkaline composition of lake water. The evolution of the facies associations over time demonstrates an overall fall of the relative lake level. High proportion of low energy facies associated with a deeper lacustrine environment occurs at the Coqueiros Fm. base, while at the top predominate high energy facies associated with a shallow lacustrine environment, in addition to magnesian clay-rich facies associated to restricted and alkaline water bodies. Paleocurrents suggest the activity of two main wind direction (northwest and southeast) over the external high that result in clean bioclastic facies with good reservoir potential on both flanks. The knowledge of wind directions during the late Barremian and early Aptian in the northern Campos Basin is extremely important for targeting the best depositional facies for hydrocarbon accumulation in the Coqueiros Formation.

Interplay between biotic and environmental conditions in pre-salt Messinian microbialites of the western Mediterranean (Upper Miocene, Mallorca, Spain)

Microbial buildups that predate the Messinian salinity crisis, including one of the few Phanerozoic examples of ‘giant’ microbialites, crop out upon the island of Mallorca (W Mediterranean). Sedimentological and geochemical data from microbialites and associated deposits, both essentially dolomitic, indicate that they grew in shallow marine conditions but relatively restricted from the open marine realm. Two microbialite-bearing sequences occur, both consisting of subtidal to supratidal deposits. Periods of restriction led to evaporative and hypersaline conditions that favored the development of microbialites and local precipitation of sulfates. By contrast, ephemeral periods marked by more open conditions were devoid of microbialites and allowed production of bioclastic deposits. Microbialites range from decameter- to decimeter-scale, reflecting differences in accommodation space. Despite contrasting sizes, all microbialite bodies record similar mesostructure evolution through time, from thrombolites to stromatolites, with a sharp transition between these endmembers. The change from subtidal to shallower, more restricted and saline intertidal environments triggered biotic substitution of thrombolite-generating microbial communities to stromatolite-generating ones. Furthermore, a wide variety of microstructures, from agglutinated to micritic with fossilized microbes, indicates that two main accretion processes occurred: microbially-influenced primary dolomite precipitation and grain trapping and binding, which were controlled by the interaction between microbes and changes in environmental conditions (e.g. grain supply, hydrodynamics and hydrochemistry). Therefore, the diversity of macro-, meso- and microstructures of these microbialites was caused by a complex interplay between depositional, biotic and hydrological parameters, which offers useful insights for the palaeoecological interpretation of other examples, at any scale and throughout geological time.

The seagrass skeletal assemblage from modern to fossil and from tropical to temperate: Insight from Maldivian and Mediterranean examples

AbstractSeagrasses are marine angiosperms that form extensive submarine meadows in the photic zone where carbonate producing biota dwell as epiphytes on the leaves or as infaunal forms, and act as prolific carbonate sediment factories. Because seagrasses have a low preservation potential and records of exceptionally well‐preserved and plant material from marine settings are rare, these palaeoenvironments are difficult to identify in the rock record. Consequently, sedimentological and palaeontological proxies are the main indicators of the presence of seagrass‐dominated ecosystems. This work investigates the skeletal assemblage of Modern (Maldivian and western Mediterranean) and fossil (Eocene; Apula and Oman carbonate platforms and Oligocene; Malta platform) seagrass examples to characterize the skeletal assemblage of modern and fossil seagrasses. Two main types of grains, calcareous algae and foraminifera, constitute around 50% of the bioclastic sediment in both tropical Maldivian and temperate Mediterranean scenarios. However, in the tropical setting they are represented by green algae (Halimeda), while in the Mediterranean they are represented by corallinacean red algae. In contrast, in the Eocene examples, the foraminifera are the most conspicuous group and the green algae are also abundant. The opposite occurs in the Maltese Chattian, which is dominated by coralline algae (mean 42%), although the foraminifera are still abundant. It is suggested to use the term foralgal to identify the seagrass skeletal assemblage. To discriminate between red algae and green algae dominance, the introduction of the prefixes ‘GA’ (green algae) and ‘RA’ (red algae) is proposed. The investigated examples provide evidence that the green algae–foralgal assemblage is typical of tropical, not excessively dense seagrass meadows, characterized by a well‐illuminated substrate to support the development and calcification of the Halimeda thallus. Contrarily, the red algae‐foralgal assemblage is typical of high density tropical to subtropical seagrass meadows which create very dense oligophotic conditions on the sea floor or in temperate settings where Halimeda cannot calcify.

The oldest record of a tyreophoran track in Gondwana: Geological implications of subaerial exposure in the lower part of the Lajas Formation at the Covunco section (Neuquén Basin), Patagonia, Argentina

The outcrops of the Lajas Formation at the Covunco section expose a succession that contrasts with the classical localities for the unit, due to subaerial exposure confirmed by a vertebrate track documented in the basal section of the unit. The stacking pattern, facies distribution, subaerial exposure in the previously interpreted prodelta facies and the documented unconformity in the middle part of the unit are incompatible with the continuous deltaic evolution suggested previously for the studied locality. Regardless of the absence of a precise age for the succession, the regional literature of subsurface data, the paleontological remains and the intra-Callovian unconformity at the top of the unit, suggest a Bathonian (late?) up to Early Callovian age for the unit in the area. It is younger than the classical deltaic deposits extensively studied in outcrops or subsurface to the north-east and south of the Huincul High and confirms the diachronism between the south and north of the high and it sheds light on the late evolution of the unit. The younger age in Covunco section is compatible with the suggested age differences. The sedimentary record in the logged section was divided in three parts: a lower section containing marginal-marine deposits with tidal features like sigmoidal structures and muddy and coaly drapes, marine trace fossils (Asteriacites quinquefolius) and bioclastic deposits that are transitionally overlying the Los Molles Formation and end with fluvial deposits containing the vertebrate track. It is documentedat the top of an interval with3D dunes. The track is tridactyl and assigned to cf. Deltapodus. It constitutes the oldest record in Gondwana of a track attributed to tyreophorans. Isaberrysaura (Neoornitischia/Tyreophora) is the candidate producer, taking into account the stratigraphic and geographical proximity of the underlying Los Molles Formation that contains the skeletal remains. The track has some morphological features that suggest a possible swimming-wading behaviour as was previously proposed for stegosaurs. Overlying the fluvial interval, the middle section is composed of transgressive deposits containing abundant bioclastic conglomerates and channels with trunks. The upper section was deposited after a facies dislocation overlying a previously interpreted unconformity that suggests an abrupt change in the sedimentary record, containing pebbly sandstones intercalated with sandstone beds with Skolithos resembling “pipe rocks”. They are replaced upward by fine-grained sandstones with HCS, suggesting open marine conditions over the storm wave base level and a deepening upward trend. Then, the succession becomes sandier and highly bioturbated with abundant Rhizocorallium specimens and some lenticular channels with lateral accretion and abundant symmetrical wave ripple levels developed during a regressive phase.

MIDDLE MIOCENE DROWNED RAMP IN THE VICINITY OF MARIJA BISTRICA (NORTHERN CROATIA)

Fossiliferous Middle Miocene deposits from the surroundings of Marija Bistrica (north-east of Zagreb) transgressively overly older pre-Cenozoic bedrocks. Fossils from shallow marine environments are in most cases preserved as bioclasts, while deep marine calcareous oozes characterize the pelagic marls. The age of the transgressive sequence is estimated on the basis of planktic biota from marls (foraminifera, nannoplankton and pteropods) as the Badenian NN5 Nannozone. The following palaeoenvironments can be distinguished or presumed on the basis of biota and sedimentary features: (1) beach characterized by polymictic conglomerates with rhodolith-rich carbonate matrix ; (2) oyster banks, recognized from secondarily found oyster clusters ; (3) lagoons marked with compact bioclastic deposits and rhodolith-halimeda assemblage ; (4) patch-reefs recognized from the surrounding bioclastic deposits ; (5) shallow subtidal maerl beds preserved as loose bioclastic deposits and (6) distal slope argillaceous marls with pelagic biota. Palaeoenvironmental analyses indicate rapid drowning, most probably corresponding to the transgression during the Middle Badenian TB 2.4 3rd order transgressive-regressive sequence.

Recent evaporite deposition associated with microbial mats, Al-Kharrar supratidal–intertidal sabkha, Rabigh area, Red Sea coastal plain of Saudi Arabia

The supratidal–intertidal sabkha of the Al-Kharrar area, Red Sea coast, Saudi Arabia, contains the evaporite minerals gypsum, anhydrite, and halite. Microbial mats flourish adjacent to the sabkha evaporites in tidal flats and pools of the Al Kharrar lagoon. Desiccation and decay of some microbial mats in tidal flat areas have led to precipitation of gypsum and halite there. The evaporite minerals have been precipitated through displacive, inclusive, and replacive growth within mud, sand, gravelly sand, and bioclastic sediment of the sabkha. Gypsum occurs as lenticular and tabular crystals whereas anhydrite occurs as nodular (individual, mosaic, and enterolithic) and pseudomorphs of lenticular gypsum crystals that grew displacively and replacively near the surface of the sabkha. Halite exists as a diagenetic cement within the sabkha sediment, or as primary rafts and skeletal crystals in desiccated tidal pools with salinity over 220‰. Microbial mats are growing on the surface of the upper tidal flat areas and in pools at a salinity range of 80–110‰, and they lead to biostabilization of the sediment. They have induced a range of sedimentary surface structures (MISS) including gas domes, reticulate patterns, tufts, pinnacles, wrinkles, and microbial shrinkage cracks. The occurrence, abundance, and association of evaporite minerals and MISS are controlled by local environmental factors such topography of the sabkha, emergence or submergence of tidal areas, surface area of the evaporite basin, contribution of meteoric water from floods from the adjoining Red Sea Mountains, and water salinity. These factors promote the growth of the microbial mats in the winter months, and deposition of evaporite minerals during summer months. Field and petrographic data indicate that the main recharge to the sabkha area is from tidal flow and water seepage from the Al-Kharrar lagoon. The results of this study indicate that within a small sabkha area of Al-Kharrar (3 × 17 km), a large variation in evaporite mineral types and morphologies grade into and are associated with MISS due to local environmental parameters. The interpretation of this association of evaporite minerals and MISS provides useful data for understanding the mechanisms responsible for precipitation of evaporite minerals and formation of MISS.

A late Holocene palaeoenvironmental ‘snapshot’ of the Angamma Delta, Lake Megachad at the end of the African Humid Period

During the African Humid Period (AHP) there was a large increase in the area of lakes and wetlands. Lake Megachad, one of several huge lakes, underwent dramatic fluctuations during the AHP prior to regression in the mid Holocene. However, the timing and nature of AHP termination has been disputed. We present evidence from sediments of the Angamma Delta, from the northern end of the palaeolake, for Lake Megachad lake-level fluctuations at the end of the AHP. Delta slope deposits were deposited around 7000 cal BP at the height of the AHP. Overlying bioclastic sediments, from 4300 to 4800 cal BP at an elevation of 285–290 m, lie below the palaeolake highstand (339 m) but close to the elevation of the Bahr el Ghazal sill, which divided the lake's two sub-basins. Ostracod δ18O values indicate that the waters of the northern sub-basin were evaporated to levels similar to modern Lake Chad. Palaeoecological evidence suggests that the lake was perennial and evaporative enrichment is attributed to restricted circulation of lake waters as the sill emerged. The age and elevation of the bioclastic sediment, coupled with published lake level reconstructions, suggest a complex lake-level history with a major regression at the end of the AHP, followed by a short lived, lake level rise and then a further regression. This new evidence for changes in lake level provides support for other geological records and some modelling experiments that suggest rapid fluctuations in hydroclimate at the end of the AHP.

Assessing the sediment quality of the Laje de Santos marine state park and other marine protected areas of the central coast of São Paulo (Brazil)

Abstract In this study, the quality of sediments from three Marine Protected Areas (MPAs) located on the coast of São Paulo (Laje de Santos Marine State Park - PEMLS; Xixova-Japui State Park - XJSP; and Central Coast Marine Protection Area - APAMLC) was assessed. Four sampling surveys were conducted (September/October 2013; January 2014; July 2014; January 2015). Sediment samples were collected at10 sites, distributed along the 3 MPAs. Samples were analyzed for sediment texture, total organic carbon, CaCO3, metals, aliphatic and polycyclic aromatic hydrocarbons, and for whole-sediment and sediment-water interface toxicities. Principal component analysis (PCA) was used to integrate data. Most of the sediments exhibited low concentrations of chemicals, with the exception of those from P2 (APAMLC) where moderate levels of contaminants were detected. Sediments from P7 and P9 (PEMLS) occasionally showed signs of petroleum hydrocarbons. The other sediments showed no relevant contamination but presented variable toxicity, especially those of bioclastic composition. The PCA indicated a contribution of the sediment properties to the toxicities, especially the CaCO3. In bioclastic sediments, toxicity might be due to physical causes by or any indirect factor such as the presence of ammonia. It was concluded that both natural and anthropic factors are causing toxicity in sediments from the MPAs studied.

Unravelling the origin of a Paleogene unconformity in the Latium-Abruzzi carbonate succession: A shaved platform

The Latium-Abruzzi carbonate platform constitutes the sedimentary record from the Triassic to the late Miocene of the southern Tethyan margin outcropping in the Apennines. This sedimentary succession is punctuated by an important hiatus between the end of the Cretaceous and the early Miocene. This Paleogene hiatus has long been interpreted as the result of prolonged subaerial exposure. However, no evidence of such exposure has ever been documented on the Latium-Abruzzi platform. This is contrary to the coeval and adjacent Apulia and Lessini carbonate platforms where a broadly-developed paleokarst system formed between the Eocene and the early Miocene. The assumption that the Miocene marine transgression deeply eroded a karstified Cretaceous substrate on the Latium-Abruzzi platform is not supported because this surface appears in the field as a perfectly flat, bioeroded paraconformable surface. In this work, a marine origin is proposed for this unconformity. From late Paleocene to early Miocene, the Latium-Abruzzi carbonate platform was a shaved isolated platform exposed to wave action in the middle of the proto-Mediterranean area. Bioclastic sediment accumulated during transgressive and sea-level highstand phases, whereas in the following falling and lowstand stages, sediment was eroded as the seafloor came into the zone of wave abrasion. The sediment eroded from the platform was shed into the basin where it produced coarse bioclastic intercalations in the hemipelagites of the Scaglia Formation. At the end of the Oligocene, when the adjacent basin was filled, a small ramp was established in the transitional zone between the platform and the basin. There, cross-bedded carbonates accumulated in middle to outer ramp environments. The inner ramp remained within the zone of wave abrasion such that sediment generated there was washed out into the middle and outer ramp environments. Finally, in the early Burdigalian, the Mediterranean progressively evolved into closed sea during the initial stages of the closure of Indo-Pacific connection. The reduction of the wave-base depth, typical of an enclosed sea, created increased accommodation space allowing sediments to accumulate and form the complete inner to outer ramp facies belts of the Miocene Latium-Abruzzi ramp.

Seabed sediments (grain size) of Nordland VI, offshore north Norway

ABSTRACTThis paper presents a seabed sediments (grain size) map of the Nordland VI area (25,000 km2) off the Lofoten islands, north Norway. The map is based on multibeam echosounder data (bathymetry and backscatter), visual analysis of 215 video transects (each 700-m long), and visual and grain-size analysis of seabed sediment samples from 40 sampling stations acquired by grabs, boxcores and multicores. A total of 14 sediment classes were identified, with sediments varying in grain size from mud to boulders. Seabed types also include bedrock and bioclastic sediments from degrading cold-water coral reefs. The continental shelf is mostly characterised by coarse-grained sediments such as gravelly sand and sandy gravel, especially in till areas. In basins and glacial troughs, finer-grained sediments such as sandy mud and muddy sand dominate. The upper continental slope (300–600-m water depth) is characterised by coarse-grained sediments related to the influence of the strong north-east flowing Norwegian Atlantic Current. In deeper areas, finer-grained sediments are prominent. Below 1000-m depth, mostly mud and mud with sediment blocks occur.

Biogenic sediments from coastal ecosystems to beach–dune systems: implications for the adaptation of mixed and carbonate beaches to future sea level rise

Abstract. Coastal ecosystems produce and store carbonate particles, which play a significant role in the carbonate dynamics of coastal areas and may contribute to the sediment budget of adjacent beaches. In the nearshore seabed of temperate zones (e.g. Mediterranean Sea and South Australia), marine biogenic carbonates are mainly produced inside seagrass meadows. This study quantifies the contribution of biogenic sediments, mainly produced in Posidonia oceanica seagrass meadows and secondarily in photophilic algal communities, to the sediment budget of a Mediterranean beach–dune system (San Giovanni beach, western Sardinia, western Mediterranean Sea). A set of geophysical, petrographic and sedimentological data was used to estimate the sediment volume and composition of the beach–dune system as a whole. The San Giovanni beach–dune system contains 3 797 000 ± 404 000 t of sediment, 83 % (3 137 000 ± 404 000 t) of which is located in the coastal wedge, 16 % (619 000 ± 88 000 t) in the dune fields and 1 % (41 000 ± 15 000 t) in the subaerial beach. The sediments are composed of mixed modern bioclastic and relict bioclastic and non-bioclastic grains from various sources. The system receives a large input of modern bioclastic grains, mainly composed of rhodophytes, molluscs and bryozoans, which derive from sediment production of present-day carbonate factories, particularly P. oceanica seagrass meadows. Radiocarbon dating of modern bioclastic grains indicated that they were produced during the last 4.37 kyr. This value was used to estimate the long-term deposition rates of modern bioclastic sediments in the various beach compartments. The total deposition rate of modern bioclastic grains is 46 000 ± 5000 t century−1, mainly deposited in the coastal wedge (39 000 ± 4 000 t century−1) and dunes (7000 ± 1000 t century−1), and 46 000 t represents ∼ 1.2 % of the total beach–dune sediment mass. Carbonate production from coastal ecosystems was estimated to be 132 000∕307 000 t century−1, 28 % (15 % ∕ 34 %) of which is transported to the beach–dune system, thus significantly contributing to the beach sediment budget. The contribution to the beach sediment budget represents a further ecosystem service, which our data can help quantify, provided by P. oceanica. The value of this sediment-supply service is in addition to the other important ecological services provided by seagrass meadows. The dependence of the beach sediment budget on carbonate production associated with coastal ecosystems has several implications for the adaptation of mixed and carbonate beaches to the loss of seagrass meadows due to local impacts and the changes expected to occur over the next few decades in coastal ecosystems following sea level rise.

Caracterização Sedimentar e Tafonômica de Depósitos Bioclásticos na Reserva Tauá e Marina Búzios (Holoceno do Estado do Rio de Janeiro)

O estudo sobre acumulações bioclásticas tem aumentado nos últimos anos, devido à sua grande fonte de informações sedimentar, tafonômica e paleoambiental. Entretanto o conhecimento sobre as características de feições associadas com ambiente meteórico em acumulações bioclásticas não é completamente compreendido. O objetivo deste estudo é o reconhecimento de feições de alteração superficial, integrando a identificação taxonômica com análises sedimentológicas e tafonômicas, que ocorrem em conchas de moluscos de depósitos holocênicos da planície costeira do estado do Rio de Janeiro. O depósito Reserva Tauá é composto por uma matriz lamosa com biotrama densa e com 54% das conchas com concavidades voltadas para baixo. A Marina Búzios é composta por uma matriz de areia com biotrama densa a frouxa e 65% das concavidades voltadas para cima. As análises tafonômicas mostraram que os depósitos possuem fracas alterações tanto físicas e químicas, entretanto o pavimento exposto da Reserva Tauá propiciou uma alteração de caráter microbiana, observado pelas microperfurações nas conchas. Apesar da bioestabilização do depósito da Reserva Tauá já ocorre há algumas centenas de anos, a evolução dos processos pedogenéticos é limitada pela proximidade com o lençol freático.

Cloudina-Corumbella-Namacalathus association from the Itapucumi Group, Paraguay: Increasing ecosystem complexity and tiering at the end of the Ediacaran

The intriguing Ediacaran fossil Namacalathus is described from limestones of the Tagatiya Guazú Formation, Itapucumi Group, Paraguay. This is the fifth occurrence of the genus in the Ediacaran geological record. The Paraguayan Namacalathus specimens are preserved as partially complete spheroidal cups with an opening at the top and thin walled stems. The remains of this soft-calcified globe-shaped organism occur as sparse disarticulated parautochthonous fragments within bioclastic deposits dominated by Cloudina shells with subordinate Corumbella fragments. The co-occurrence of these three skeletal metazoan species in the same environmental context attests that the diversity of the Paraguayan accumulations is ecologically comparable to the typical skeletal assemblage of the Nama Group. The discovery of new samples of Namacalathus in the Itapucumi Group also indicates that this genus presented a broader paleobiogeographic distribution than previously thought and, in the same way as Cloudina, it represents a low latitude, shallow water metazoan of the Ediacaran Gondwana.

The stratigraphic architecture and depositional environments of non-marine carbonates from barremian-aptian pre-salt strata of the Brazilian continental margin

The stratigraphic architecture and depositional environments of non-marine carbonates from barremian-aptian pre-salt strata of the Brazilian continental margin