Bioclastic Deposits Research Articles

Late Cretaceous heterozoan carbonates: palaeoenvironmental setting, relationships with rudist carbonates (Provence, south-east France)

During Late Cretaceous (Coniacian–Early Santonian) times, heterozoan-rich bioclastic carbonates prevailed in the Provence Platform area. They substituted for rudist-rich carbonates, which flourished during the Late Turonian in the same area. This article presents a detailed sedimentological study of these heterozoan-rich carbonates, carried out in the Méjean syncline, west of Marseille. At this locality, the thickness of the bioclastic series exceeds 500 m. The heterozoan carbonates are referred mainly to packstones and grainstones, organized in meter-thick, cross-bedded sedimentary strata. Microfacies analysis has shown that bryozoans, red algae and echinoderms are the main skeletal constituents mixed with reworked fragments of rudists and benthic foraminifera. Detritic elements are mainly represented by quartz and glauconite grains. In the study area, heterozoan carbonates underwent a complex diagenetic evolution. Iron precipitation occurred early in environments with high carbonate productivity and a high hydrodynamic level. Early-marine cements developed mainly in the upper portions of shallowing-upward parasequences during intervals with decreasing sedimentation rates. Late burial cements occurred sequentially as syntaxial rim cements, dentate calcite crystals and blocky calcite. During the mesogenetic burial history, microstylolitisation developed in the packstones. Deposition of the heterozoan carbonates began during the transgressive Coniacian cycle, following a relative sea-level fall recorded in the Late Turonian. Bioclastic sedimentation rates during the Coniacian vary between 74 and 88 m/Myr, a value similar to that recorded in the Holocene Australian and New Zealand shelves. Coniacian–Santonian heterozoan carbonates had a widespread palaeogeographic distribution in south-east France and Sardinia. The availability of hard substrates due to a high hydrodynamic level during the Coniacian transgression was a major cause for the development of the heterozoan association as a whole. Likewise, iron-rich terrigenous supply and upwelling patterns probably enhanced nutrients on the shelf and favoured in turn the development of suspension-feeders and mesotrophic organisms. Possible open marine, relatively deep cool-water conditions were responsible for the deposition of the heterozoan association versus the photozoan rudist association during the Coniacian. This latter association progressively recovered and thrived when shallow marine warm-water conditions re-established during the Santonian. Comparisons are made with modern analogue environments in which heterozoan dominated cool-water carbonate sedimentation occur.

愛媛県松山市沖の浅海砂堆群を構成する生砕物の特徴と供給源

愛媛県松山市沖の浅海砂堆群を構成する生砕物の特徴と供給源

Environmental history since 11,000 14c yr b.p. of the northeastern Pampas, Argentina, from alluvial sequences of the Luján River

Sedimentological, malacological, and pollen analyses from 14C-dated alluvial sections from the Luján River provide a detailed record of environmental changes during the Holocene in the northeastern Pampas of Argentina. From 11,200 to 9000 14C yr B.P., both sedimentary and biological components suggest that the depositional environment was eutrophic, alkaline, and freshwater to brackish shallow water bodies without significant water circulation. During this time, bioclastic sedimentation was dominant and the shallow water bodies reached maximum development as the climate became more humid, suggesting an increase in precipitation. Short-term fluctuations in climate during the last stage of this interval may have been sufficient to initiate changes in the water bodies, as reduction of the volume alternated with periods of flooding. The beginning of the evolution of shallow swamps in the wide floodplain or huge wetlands was contemporaneous with a sea level lower than the present one. From 9000 and 7000 14C yr B.P., mesotrophic, alkaline, brackish, probably anoxic swamps existed. Between 7000 and 3000 14C yr B.P., anoxic calcareous swamps were formed, with subaerial exposure and development of the Puesto Berrondo Soil (3500–2900 14C yr B.P.). A trend to a reduction of water bodies is recorded from 9000 to ca. 3000 14C yr B.P., with a significant reduction after ca. 7000 14C yr B.P. A shift to subhumid–dry climate after 7000 14C yr B.P. appears to be the main cause. During this time, an additional external forcing toward higher groundwater levels was caused by Holocene marine transgression causing changes in the water bodies levels. The climate became drier during the late Holocene (ca. 3000 yr B.P.), when clastic sedimentation increased, under subhumid–dry conditions. Flood events increased in frequency during this time. From ca. A.D. 1790 to present, the pollen record reflects widespread disturbance of the vegetation during the European settlement.

Contrasting models of temperate carbonate sedimentation in a small Mediterranean embayment: the Pliocene Carboneras Basin, SE Spain

Temperate carbonates consisting of bioclastic (bryozoan–bivalve–coralline algal) packstones to rudstones formed in the Carboneras Basin, a small embayment of the Mediterranean Sea in SE Spain, during the early Pliocene. Transgressive systems tract carbonate deposits exhibit three distinct sedimentary styles, with contrasting lithofacies patterns and stratal-geometry arrangements. Palaeotopography and local hydrodynamic conditions were the major factors controlling sedimentation. On the steep northern margin, affected by southeasterly wind-driven storms, a prograding platform (distally steepened ramp) with well-marked, platform-slope clinoforms developed. Bivalves extensively colonized the outer platform, and rhodolith pavements covered its edge. Coarse-grained, bioclastic sediments were frequently removed from the platform by storms and redeposited on the platform slope. On the southern margin of the basin, longshore currents driven by southeasterly winds hydrodynamically accumulated carbonate particles to form a spit-platform, on top of which some shoals developed. These shoals were dismantled from time to time by the northern storms, and the sediment was redeposited on the lee-side slope of the spit-platform. On the gentle and protected western margin, oyster banks and rhodolith pavements colonized the inner-ramp environment whereas coralline algal–bryozoan–bivalve bioconstructions formed on the outer ramp. This example shows that several sedimentary styles of temperate carbonate deposition can coexist in a single basin.

New, high resolution swath bathymetry of Gettysburg and Ormonde Seamounts (Gorringe Bank, eastern Atlantic) and first geological results

High resolution swath bathymetry of shallow water (< 200 m) oceanic seamounts is a relatively rare issue. During the recent Gorringe_2003 cruise over the Gorringe Bank (Eastern Atlantic) we collected multibeam bathymetry on the bank’s two shallow summits, Gettysburg and Ormonde in the –25/–400m depth range at a resolution rarely achieved over an oceanic seamount. We also carried out bottom samplings and ROV dives in the same bathymetric interval. The acquisition parameters and the characteristics of the echosounder employed allowed to generate a Digital Terrain Model (DTM) with metric spatial resolution upto 75–100 m depths. To ensure proper tidal corrections a tide-gauge was deployed at sea-bottom during the survey. DTM reveals for the Gettysburg Seamount an almost perfectly circular summit resulting from the blanket of bioclastic sediments over an igneous ‘core’ consisting of sheared and foliated serpentinites. The core is dissecated by N 10° W trending ridges elevating some tens of metres and filled in between by bioclastic sands. Both foliation and ridge patterns seem related to primary igneous fabric rather than later structural deformation. The overall circular shape confirms the origin of the seamount as a mantle serpentinite diapir in analogy with similar, but subduction-related, circular seamounts observed in the Bonin Trench (western Pacific). In contrast the Ormonde elongated summit follows the regional tectonic trend with a N 60° E active (seismogenic?) fault on its southeastern flank. Its basement morphology corresponds to the outcrops of igneous rocks chiefly consisting of gabbros, volcanic rocks and dyke intrusions. On both seamounts topographic profiles show that the ‘shelf’ area is somewhat convex rather than flat like that of ‘Pacific type’ guyots and is bordered by a depositional, locally erosional shelf break, located between –170 and –130 m. Various terraced surfaces and some geological evidence confirm previous observations and indicate relative sea-level oscillations with partial emersion of the two summits that seem occurred during the last glacial cycle (past 120 ka).

Microbialite morphology, structure and growth: a model of the Upper Jurassic reefs of the Chay Peninsula (Western France)

During the Early Kimmeridgian, the northern margin of the Aquitaine Basin (Western France) is characterised by a significant development of coral reefs. The reef formation of the Chay Peninsula comprises two main reefal units, in which the microbial structures can contribute up to 70% of framework. The microbial crusts, which played an important role in the stabilisation and growth of the reef body, show the characteristic clotted aspect of thrombolitic microbialites. Corals are the main skeletal components of the build-ups. The bioconstructions of the Chay area are thus classified as coral-thrombolite reefs. Four main morpho-structural types of microbial crusts are distinguished: (1) pseudostalactitic microbialites on the roof of intra-reef palaeocaves; (2) mamillated microbialites, found either on the undersides or on the flanks of the bioherms; (3) reticular microbialites in marginal parts of the reefs and between adjacent bioconstructed units; and (4) interstitial microbialites in voids of bioclastic deposits. Thrombolitic crusts developed on various substrates such as corals, bivalves, or bioclasts. The thrombolites formed a dense, clotted and/or micropeloidal microbial framework, in which macro- and micro-encrusters also occur. Variations in accumulation rate strongly influenced the reef morphology, in particular its relief above the sediment surface. The coalescence of the coral-microbialite patches created numerous intra-reef cavities of metre-scale dimensions. The direction of microbial growth, which defined the macroscopic microbialite forms, strongly depended on the position within the reef framework but was also controlled by water energy, accumulation rate and light availability.

Coral dominance at the reef - sediment interface in marginal coral communities at Sodwana Bay, South Africa

Sedimentation is an important factor that influences the composition of coral communities. The high-latitude, marginal coral community in the central reef complex at Sodwana Bay, South Africa, is subjected to sedimentation at the reef–sediment interface. A lens of bioclastic sediment surrounds the sandstone reefs in the area and acts as a scouring and smothering agent. This affects the composition of the subcommunity on the reef margin at the reef–sediment interface. The interface was studied on two reefs in the complex and their subcommunities did not differ. Alcyoniidae and Scleractinia (34.4% and 34.4% cover, respectively) are the dominant taxa in the reef–sediment subcommunity, which includes genera such as Sinularia, Lobophytum, Sarcophyton, Montipora, Favia and Astreopora (in decreasing order of abundance). Sediment-tolerant species are prevalent, the Alcyoniidae being the most prominent and tolerant by virtue of their morphology. Corals in this environment are generally flat and fairly rigid colonies of Lobophytum and Sinularia spp. (14% and 10% cover, respectively), some with lobes to channel the movement of surge-driven sediment, or are erect, soft and pliable forms such as Sarcophyton spp. (10% cover) that easily shed sediment. Their dominance in this constantly disturbed environment may also be attributable to the low mortality, slow growth and overall persistence of the Alcyoniidae described in the literature.

Microstratigraphy and taphonomy of rudist shell concentrations in Upper Cretaceous limestones, Cilento area (central-southern Italy)

Rudist bed type and distribution has been investigated in Upper Cretaceous limestones cropping out in the northern Cilento area (southern Italy). These limestones are dominated by fine-grained, peloidal, silty packstone in which rudist-rich beds are intercalated. An inner shelf environment may be inferred on the basis of the recognized sedimentary and taphonomic features. The rudist shell beds are characterized by low species diversity, with slight differences in abundance of a few species belonging to the Durania, Bournonia, Sauvagesia, Gorjanovicia and Biradiolites genera, which usually form oligo- or monospecific congregations. The internal fabric of these levels (i.e. orientation, arrangement, packing and sorting of the skeletal elements; internal microstratigraphy) has permitted us to distinguish two broad shell bed categories: (a) shell beds considered as “Primary Shell Concentration”, in which the shell concentration is essentially created by the behaviour of local shell producers, preserved in situ and in growth position; (b) shell beds considered as “Hydraulic Shell Concentration”, which were deposited under the influence of hydraulic processes and/or input of surrounding bioclastic sediments. The taphonomic analyses allowed us to highlight the role of some of the biotic and abiotic factors that controlled the distribution of the rudists in the various habitats. The increase of physical disturbance (especially hydrodynamism) is the primary difference between these shell bed categories. The establishment and development of the densest rudist congregations appear to be related to the accommodation space made available by means of relative sea level rise. The lowering of the sea level was often accompanied by the increased influence of waves and/or currents on the seabed and the consequent sediment disturbance and demise of the rudist lithosome, although other factors cannot be excluded.

Neogene non-tropical carbonate sedimentation in a warm temperate biogeographic province (Rethymnon Formation, Eastern Crete, Greece)

The Apostoli Basin, in the central-west part of Crete, was formed as a fore-arc type basin related to the convergent plate boundary between the African and the Eurasian plates. Most of the Neogene sediments filling the basin were deposited in a terrestrial to shallow marine environment. The succession is a transgressive cycle, which culminates in the alternation of Rethymnon bioclastic limestones with marls, documenting the important Tortonian marine transgression. The Rethymnon limestones are classified as a typical non-tropical carbonate lithofacies. Two particular lithofacies have been recognized: (a) a rhodalgal-type lithofacies, characterized by predominance of encrusting coralline algae and bryozoans, and (b) an echinofor-type lithofacies, characterized by predominance of echinoderms and/or benthic foraminifera. The coralline algae occur mostly as in situ spheroidal or branched rhodoliths, whereas benthic foraminifera are mainly represented by larger foraminifera. In both lithofacies, typical tropical carbonate elements are lacking. Skeletal elements consisted of low- and high-Mg calcite. Although the observed lithofacies possess many similarities with facies of non-tropical carbonates, the presence of large benthic foraminifera suggests development in a warm temperate biogeographic province. The depositional environment corresponds to a shallow ramp, the sediments being deposited in a nearshore environment and under conditions analogous to those prevailing in the present-day circalittoral bottoms of the Mediterranean Sea. The main carbonate accumulation area is located at the factory area itself (rhodalgal-type sediments), and downslope from the factory area (echinofor-type sediments). In the classic zonation of Mediterranean benthic assemblages of Peres and Picard [Rec. Trav. Stn. Mar. (1964)], the rhodalgal-type sediments of the Rethymnon Fm correspond to the “Facies a Pralines”, developed in areas strongly controlled by currents (e.g., tops of plateaus), and the echinofor-type sediments to the “Detritique Cotier” bioclastic deposits, derived mostly by bioerosion and fragmentation of local and adjacent calcareous benthic communities.

The Parona chaotic complex: a puzzling record of the Messinian (Late Miocene) events in Monferrato (NW Italy)

The Parona chaotic complex (PCC) from Monferrato (NW Italy) is a mélange composed of carbonate-cemented blocks floating in a matrix of weakly consolidated mud breccias. It forms a minor (1 km 2) chaotic sedimentary body enclosed in a larger Messinian mélange that rests unconformably on older deposits and is followed, again unconformably, by Pliocene sediments. Three main facies have been recognized in the blocks. (a) Bioclastic rudstones and coquinoid grainstones of early Messinian age, composed of oligotypic assemblages of gastropods and the brackish-water bivalve Cerastoderma, mixed with shallow-water, marine biota (mollusks, corallinae algae, benthic foraminifers, echinoid fragments, rare ahermatypic corals). These sediments record shallow-water carbonate sedimentation with episodic brackish incursions and show a peculiar diagenetic overprint (selective dissolution of aragonitic grains and early dolomitic cements). (b) Monogenic breccias, composed of clasts of coquinoid grainstones floating in a dolomitic microcrystalline matrix. (c) Polygenic conglomerates, composed of rounded clasts of pre-Messinian pelagic sediments, of lower Messinian bioclastic carbonates and of Messinian evaporitic carbonates. Finally, a few blocks of strongly cemented micritic limestones, interpreted as methane-derived carbonates for the geochemical signature of the carbonate phases (strongly depleted in 13C), are present. The matrix enclosing the blocks is a mud breccia containing centimeter-sized clasts of marly sediments. Micropaleontological analyses have shown that lower Messinian and upper Tortonian planktic foraminifers are mixed with lower Miocene, Oligocene and Eocene forms. The PCC results from complex processes involving: (1) deposition of the peculiar sediments preserved in the blocks and (2) their dismemberment and mixing. As for the depositional history, composition of the blocks has allowed to sketch the following evolution. (a) Deposition of bioclastic rudstones and coquinoid grainstones that are interpreted as lower Messinian carbonate platform deposits for their strong compositional and diagenetical similarity with the lower Messinian carbonates cropping out in the Mediterranean region. The Monferrato bioclastic sediments are the northernmost example of Messinian shallow-water carbonates yet known. (b) Deposition of monogenic breccias. They formed during the evaporitic phase of the Messinian that was accompanied by a relative sea-level drop and by erosion of the previously deposited coquinas. Depositional and diagenetical processes during the evaporitic phase are also recorded by clasts of evaporitic carbonates in the polygenic conglomerates. (c) Deposition of polygenic conglomerates that are interpreted as upper Messinian post-evaporitic fan delta facies resulting from an important erosional phase. After deposition, this succession was dismembered in the blocks forming the present-day mélange. Geometry, stratigraphic relationships, and internal characteristics of the PCC point to an origin related to gravity-driven phenomena, triggered by tectonics. However, the faunal mixing in the mud breccias and the occurrence of blocks of methane-derived carbonates suggest that mud diapirism also played a role for the genesis of the PCC. This latter mechanism, which has been poorly considered until now, could also have been effective in the genesis of Messinian chaotic deposits that extensively crop out in NW Italy.

Palynofacies of marginal marine deposits: the rhaetian of the east margin of the Massif Central (France)

The methodology presented concerns the definition of palynofacies. It develops 3 procedure stages based on multivariate analyses: 1 — define the palynomaceral facies, 2 — define and compare palynomorph facies with palynomaceral facies and 3 — exploit the two precedent categories of facies. The study deals with Rhaetian sedimentary cycles corresponding to the western limit boundary of the southeastern France intracratonic basin. Rhaetian strata are representative of paralic depositional environments, overlying Triassic fluvial deposits. On the palynomaceral sequence, that is revealed by multivariate analyses, the most buoyant particles are the needle-like phytoclasts. Crevasse splays, dewatering, drowned soils and restricted wave-influenced deposit facies yield them. The less buoyant particles are the black massive or polygonal phytoclasts. They were produced by crevasses, deltaic shales, distributary channels, palaeosoils, sheet-floods, shoals and shoreface facies. The bisaccate pollen Ovalipollis and the nonsaccate pollen Circumpolles are associated with needle-like particles. They are linked to deltaic shales, restricted deposits sheet-floods, shoal, shoreface and storm facies. The marine element Dapcodinium is not related to a particular palynomaceral type but could be compared to the lowest buoyant phytoclast. Deltaic plain and back-shoal lagoonal facies favour the concentration of Dapcodinium. The distribution of the small size dinoflagellate cysts ( Dapcodinium) and large size dinoflagellate cysts ( Rhaetogonyaulax) emphasizes the role of the size in the floatation competence. Base level-rises and highs are marked by amalgamated bioclastic deposits, fluvial channel fills, mixed siliciclastic-bioclastic amalgamated shoals, storm deposits or ravinement surfaces. Needle-like particles, Ovalipollis and Circumpolles are associated to these deposits. Base level-falls are represented by crevasse splay deposits, fluvial stream incisions, non-incising fluvial channels, sheet flood and stacked palaeosol facies. Black roundish or equidimensional translucent phytoclasts, Rhaetogonyaulax, short-spine Micrhystridium (acritarch) and smooth-walled spores are associated with these facies. In addition to to the energy factor, the water depth constrains both phytoclast and palynomorph sedimentation.

Rhodalgal-bryomol temperate carbonates from the Apulian shelf (Southeastern Italy), relict and modern deposits on a current dominated shelf

Mound-shaped algal banks and associated bioclastic deposits have been investigated on the Apulian shelf, SE Italy, off Capo d’Otranto and Santa Maria di Leuca within a bathymetric range of 53–60m water depth. The bioclastic deposits form different sand-gravel skeletal assemblages; each one characterised by different associations and occurrence of prevailing organisms in relation to substratum types. These deposits lie on a lee shore and form rhodalgal and bryomol relict-modern deposits which originate from the banks’ erosion by means of bottom currents and storms. These play a significant role in reworking bottom preventing colonization by a sessile stabilizing bottom community, and mobile deposits prevent large encrustations. Sediments are thus characterised by a variable taphonomical preservation state and time-averaging as shown by the occurrence of abraded bryozoan growth forms. The banks, sub-circular in shape, display a maximum diameter of 70 m and 1–2m relief. Their position on the shelf coincides with the present day 50 m isobath and it is assumed to correspond to a paleoshore such as that which formed in the Holocene during the transgression following the last ice age. Living counterparts of these banks occur today north of the study area at 10–30 m water depth.

ABSTRACT: Carbonate Production, Relative Sea Level Fluctuations and Slope Geometry: Case Histories from the Middle-Triassic of the Italian Dolomites

The understanding of carbonate platform reservoirs can be enhanced through the analysis of outcropping counterparts, such as the ones provided by the study platforms (Cernera and Latemar), particularly rich in both marine cements and microbial boundstones. The birth of both coeval platforms was matched with an anoxic event and with the deposition of a potential source horizon, rich in marine organic matter. The evolution of the short lived Cernera Platform was controlled by a very fast subsidence, badly compensated by its aggrading evolution. The lengthening slopes rapidly steeped up, while the cementation intensity increased. The carbonate production was unable to support the fast platform volume increase and the slope deposits therefore became thinner and thinner, forcing the margin to retrograde; the platform eventually drowned and it was covered by condensed pelagic facies and deep water stromatolite-like structures. The Latemar slope experienced a similar lengthening and steeping up evolution; this carbonate system however grew in a comparatively less subsiding area and it was therefore able to keep and eventually catch up the relative sea level increase. While the platform core shallowed from subtidal environments to cyclic emersions, in the adjacent slopes loose bioclastic and micritic sediments gave place to breccia rich in carbonate cements and microbial boundstones. The subsidence then slowed down considerably enabling the platform to laterally prograde, but no margin are presently preserved from this phase. Both platforms were then at least partially sealed by volcanic and terrigenous deposits and incompletely affected by a permeability enhancing dolomitization.

Messinian events: new constraints from sedimentological investigations and new 40Ar/39Ar ages in the Melilla–Nador Basin (Morocco)

Abstract New 40Ar/39Ar ages obtained both from within a platform and from the adjacent basin allow, through the extension of a previous argon dataset, the establishment of a chronological framework for the Messinian carbonate complex of Melilla (Morocco). In the platform, prograding bioclastic deposits began around 6.87±0.02 Ma, whereas the youngest preserved deposits are 5.77±0.04 Ma old. The southern part of the basin was uplifted and emergent prior to 6.23±0.03 Ma. The Tortonian/Messinian and the Messinian/Pliocene boundaries have not yet been identified in the Melilla region. Durations of the prograding bioclastic deposits (ca. 410 ka), the prograding Porites reefs (ca. 360 ka) and the preserved part of the terminal carbonate complex (TCC) (ca. 330 ka) can be estimated. Sedimentological and geochronological investigations do not indicate any major subaerial unconformity nor any major time gap. If a major sea level fall did exist during Messinian times, it developed after 5.77±0.04 Ma. Chronological correlations show that the onset of the Sorbas gypsum deposition (Southern Spain), considered as the marker of the Messinian Salinity Crisis onset, is contemporaneous with aggrading Porites reef building of the terminal carbonate complex of Melilla. A major paleoceanographic change in the marginal areas of the Mediterranean, with the cessation of upwelling systems, could be partly coeval with the onset of the Messinian Salinity Crisis.

Cenomanian carbonate facies and rudists along shallow intraplatform basin margin-the island of Cres (Adriatic Sea, Croatia)

The island of Cres is located in the northern part of the Adriatic Sea. The island is built up of predominantly Cretaceous carbonates deposited in north-western part of extensive and long-lasting Adriatic Carbonate Platform. Owing to the influence of synsedimentary tectonics supported by eustatic changes during the latest Albian/Early Cenomanian, different sedimentary environments were established: from shallow intraplatform basin and related slope, across basin margin to protected shallow-platform. During the Early to Middle Cenomanian rudist communities (ichthyosarcolitid/caprinid/radiolitid)flourished along a relatively high-energy intraplatform basin margin. Fair amounts of coarse-grained bioclasts, derived almost exclusively from broken rudist shells, were deposited over a marginal depocenter. Contemporaneously, pithonellid wackestone-packstones containing microbioclasts and planktonic foraminifera were deposited basinward while marginal bioclastic sediments and limestone blocks of the basin margin origin were sporadically deposited within the basin. The opening of the Cres intraplatform basin was aborted and the basin was finally filled up during the Late Cenomanian. Since the Cres intraplatform basin was established at the beginning of the Cenomanian it probably represented the initiation phase in the north-western extension of the later Adriatic Trough development.

Relationship between sediment distribution and Posidonia oceanica seagrass

Relationships between sedimentary features and the distribution and structure of Posidonia oceanica meadows were investigated in the Gulf of Oristano (Sardinia) in a study site stretching from the mouth of the Tirso River in a more sheltered, inner sector of the gulf to an outer, more exposed sector near the rocky Cape San Marco. The results demonstrate that gradients in sediment texture and composition can be related to meadow distribution and areal coverage. We suggest that this involves the mechanical trapping of fine particles by the plants, enhanced mud deposition due to dampening of wave action in the meadows, and the admixture of bioclastic sediments linked to increased production by epiphytes and invertebrates. Independent evidence of seagrass erosion was recorded in an unvegetated breach in the meadow. The mass of fine sediment which would be resuspended in the wake of meadow destruction was estimated at 30–90 × 103 tonnes km−2, an amount which could substantially influence water quality and ecosystem stability in the gulf.

40Ar/ 39Ar dating of the pre-evaporitic Messinian marine sequences of the Melilla basin (Morocco): a proposal for some biosedimentary events as isochrons around the Alboran Sea

In the Mediterranean region, the Melilla basin (NE Morocco) represents a key area that recorded biosedimentary events and environmental changes relative to the pre-evaporitic Messinian times. 40Ar/ 39Ar dating of volcanic tuffs interbedded all along the Melilla Messinian shelf carbonates and coeval basin deposits has been performed in order to date accurately three main pre-evaporitic biosedimentary events: the prograding bioclastic deposition and the oligospecific prograding Porites coral reef buildings with coeval Halimeda blooms, both within the platform, and the diatomite deposition basinward. The new age data allow a precise chronological framework to be established for the pre-evaporitic sequence of Melilla basin demonstrating that diatomitic deposits are coeval with both prograding bioclastic and reefal units. The prograding bioclastic carbonate unit related to boreal influences in coeval basinal diatomites, began at least at 6.73±0.02 Ma and ended at 6.46±0.03 Ma. The oligospecific prograding Porites coral-reefs and Halimeda beds and coeval warm-water diatomites began at least at 6.46±0.03 Ma. Previous 40Ar/ 39Ar ages indicate that they ended prior to 6.0±0.1 Ma. Both biosedimentological similarities and chronological accordance within several platforms and adjacent basins all around the Alboran Sea show that these main bioevents, dated in Melilla, are synchronous over the Alboran realm. This accurate time scale for these pre-evaporitic biosedimentary events (6.9–6.0 Ma) is in accordance with the most recent work on the latter period corresponding to the Messinian Salinity Crisis.

Pedosedimentary and palaeoenvironmental significance of a Holocene alluvial sequence in the southern Pampas, Argentina

A representative section of the Holocene alluvial record of the southern Pampas (Argentina) com prises a sequence of clastic sediments, a diatomite and two palaeosols. Detailed macro- and micromorphological descriptions provide the basis for the reconstruction of its pedosedimentary history. Combining the pedosedi mentary reconstruction with palaeoecological data allows a detailed overview of alluvial landscape development and associated palaeoenvironmental change in the region during the Holocene. The early-Holocene shift from a subhumid dry to humid climate was marked initially by a diminution in clastic sedimentation, the development of the Puesto Callejón Viejo Soil and then re-establishment of fluvial aggradation with a dominance of bioclastic sedimentation. Loess inputs increased during the mid-Holocene as the climate reverted to subhumid dry, and temporary subaerial exposure of the sediments led to the formation of the Puesto Berrondo Soil. More variable environmental conditions followed with natural fluvial aggradation ceasing in the late Holocene, though aeolian sedimentation has continued until the present day. The resultant surface accretionary soil has been modified by recent alluvial inputs from flood events linked to agricultural disturbance in adjacent catchments.

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.