1,721,323 research outputs found
Sedimentological and taphonomic characterization of low energy rudist-dominated Senonian carbonate shelves (Southern Apennines, Italy).
No full textStudies carried out on Upper Cretaceous rudist-bearing carbonate sequences in the central-southern Apennines highlighted two main depositional settings characterized by different hydrodynamic behaviour: high-energy vs. low-energy settings. The purpose of the present paper is to provide an overview of the lithofacies and of the depositional geometries of Upper Cretaceous rudist-bearing limestones pertaining to the centralsouthern Apenninic carbonate successions, in which low-energy to intermittently perturbed conditions can be recognized.
The depositional sites were characterized by a large-scale facies polarity. Under closer examination, a patchy distribution of the latter appears, often related to channelized systems or current pathways through rudist settlement areas. The resulting depositional successions are characterized by peloidal silty-muddy sediments with periodical intercalations of fundamentally porous, coarse bioclastic deposits, in a generally tabular stacking. These geometries appear of great interest in relation to the prediction of geometries and porosities of the coarse-grained sedimentary bodies that
intercalate and/or laterally pass to fine-grained deposits
Rudist lithosomes related to current pathways in Upper Cretaceous, temperate-type, inner shelves: a case study from the Cilento area, southern Italy
No full textShallow-water foramol limestones have been studied from a locality in the southern Apennines in which outcrop conditions show an excellent overview of the lateral and vertical evolution of rudist bodies and allow their geometry and the dynamic aspects to be reconstructed.
The lithofacies suggest open depositional settings characterized by peloidal silty-muddy sediments. Rudists inhabited well-defined sectors of these shelves, giving rise to wide biostromal bodies, and supplied most of the skeletal debris via bioerosion and minor physical breakdown. In particular, the characteristics of rudist lithosomes document the existence of a complex network of channel-like depressions. In such a depositional context, the evolution of rudist lithosomes seems to have been strongly controlled by the hydrodynamic conditions of the environment. The resulting composite rudist assemblages are characterized by individuals quite often found toppled and oriented parallel to bedding, suggesting continuous sediment movement and/or removal between the organisms. The good preservation of the shells and the common articulation of the valves, however, point to an absence of sustained transport but rather a slight sediment destabilization. The gross lenticular geometry of the shell beds could be related to the above-mentioned patterns of weak, maybe channelized, currents and/or pathways. In most cases, lithosomes may be considered as a multistorey growth in channel-like systems in a persistently subtidal setting.
The depositional setting may be considered as a low-energy system with low tidal range, such that facies transition is very gradual and facies belts are broad, while distinct shoal/barrier deposits are absent. Such a depositional system implies a very low bathymetric slope on which wave energy was minimal, dampened by friction along a broad expanse of shallow water hundreds of kilometres wide. The resulting sea bottoms, dominated by biogenic carbonate sedimentation, were separated by troughs and/or tidal passes.
In such a depositional context, rudist colonization on channel margins assumes particular importance as it documents the rudist ability to exploit a wide array of environments, comparable to that of oysters in Recent seas, and reflecting the probable opportunistic nature of rudists
Bauxites and related paleokarst in Southern Italy, Sicily and Sardinia
No full textA brief summarization of general geological and stratigraphical setting and genetical characteristics of bauxite deposits and related paleokarst in Peninsular Italy, Sicily and Sardinia is presented. The most important features and data are demonstrated in maps, diagrams and tables. -Author
Rudists and related sediments in Late Cretaceous open shelf settings. A case history from Matese area (central Apennines, Italy)
No full textLe successioni carbonatiche neritiche del Cretacico mostrano generalmente depositi associati a rudiste che hanno una evoluzione da ambienti differenziati fra di loro (da lagune fangose a margini bioclastici ad alta energia) del Cretacico inferiore ad ambienti più uniformi di piattaforma aperta (con solo locali restrizioni) del Cretacico superiore.
Qui presentiamo un esempio di questa evoluzione osservata nelle successioni cretaciche dei Monti del Matese (Appenino centro-meridionale).
A partire dal Cenomaniano sono presenti facies marginali in cui le rudiste, diffuse e diversificate, colonizzarono ambienti di piattaforma aperta molto poveri in coralli.
Durante il post-Cenomaniano le rudiste raggiunsero la loro più ampia diffusione e colonizzarono un largo spettro di ambienti che nel complesso furono caratterizzati da coperture bioclastiche grossolane; i coralli e le alghe verdi erano molto subordinati. Nei settori marginali le rudiste erano a volte associate con coralli ermatipici, che avevano perso comunque il loro ruolo primario di costruttori di scogliere.
La grande diffusione di rudiste coincise quindi con la riduzione di coralli come organismi dominanti nei settori marginali di piattaforma, probabilmente in seguito a condizioni di stress sfavorevoli alle comunità costruttrici di scogliera (associazione chlorozoan) e favorevoli allo sviluppo di associazioni biologiche differenti ricche in molluschi, foraminiferi bentonici, alghe rosse e briozoi (associazione foramol s.l.) impedendo così lo sviluppo di vere scogliere. La maggiore caratteristica di questi depositi a rudiste è la presenza di corpi organogeni in cui piccoli addensamenti di rudiste (bouquets, clusters) e grossolani frammenti bioclastici si rinvengono complessamente arrangiati con minore contributo di una componente siltitica derivata da bioerosione. Questi corpi distribuiti sparsamente in un'area di piattaforma aperta si interdigitavano con coltri bioclastiche essenzialmente costituite da frammenti di rudiste con un piccolo contributo di foraminiferi bentonici, alghe rosse, briozoi ed echinidi.
Questi sedimenti sembrano corrispondere ai depositi di tipo 'foramol" s.l. che caratterizzano molte piattaforme aperte attuali, in cui sparse e limitate costruzioni con basso rilievo, parzialmente assimilabili a piccoli patch reefs, sono associate ad abbondanti sedimenti bioclastici. A causa dell'intensa bioerosione queste costruzioni danno una grande quantità di detriti scheletrici, che sono accumulati con una limitata frazione fangosa, essendo la frazione più sottile, derivante da una profonda bioerosione, portata via a causa della mancanza di margini ben sviluppati.Research on the relationships between sediment and rudist-rich bodies and the different growth modalities of rudists shows that the carbonate sedimentation areas of Late Cretaceous time were characterized by bioclastic covers produced in situ by bioerosive phenomena acting on limitated and scattered organic communities, mostly made by rudists. In the rudist-rich bodies the undamaged shells, both those in growth position and those only slightly reworked, are subordinated compared to the great amount of coarse skeletal debris and even to its finer part. Up to 90% of such materials is made up of variously altered rudist shell fragments. As a consequence, rudist communities can be considered as "sediment givers" rather than "reef builders".
Late Cretaceous outcrops showing such characteristics have been studied in the Matese Mounts (Central -Southern Apennines, Italy). They show marginal facies in which rudists, widespread and diversified, colonized shelf coral-poor environments. The gathered rudist areas were randomly arranged on an open shelf, interfingering with bioclastic talus mostly made of rudist debris and small amount of benthic forams, bryozoans, red algae and echinoids.
The large spreading of the rudists coincided with a reduction of the corals as dominant organisms in shelf margin sectors, probably following stress conditions, unfavourable to the reef building communities (chlorozoan assemblages) and inclined to the development of different biological associations, rich in molluscs, benthic forams, red algae and bryozoans (foramol s.l. assemblages), thus preventing the development of real reefs.
In this view, a depositional model, for the Late Cretaceous rudist associated deposits, can be found neither in the recent coralgal (chlorozoan assemblages) buildups nor in the related bioclastic bars and shoals which generally do support nothing more than limitated sediment givers communities. These sediments seem to correspond to the recent open shelf, "foramol" sensu lato, deposits in which scattered and limited areas dwelled by a luxuriant life are associated to abundant loose bioclastic sediments. Due to the intensive bioerosion, these assemblages give a large amount of skeletal debris, which is accumulated with a limited muddy fraction. The thinner fraction, only deriving from deep bioerosion, seem to be swept off because of the lack of well developed rims
Tectonically Controlled Carbonate Channelized Slope Complexes.A Cretaceous-Miocene Case-History From The Matese Mountains (Central-Southern Apennines, Italy).
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Upper Cretaceous ramp limestones from the Sorrento Peninsula (southern Apennines, Italy): micro- and macrofossil associations and their significance in the depositional sequences
No full textThe Upper Cretaceous limestones of the Sorrento Peninsula are primarily characterized by wackestone/packstone with benthic foraminifers, thaumatoporellaceans and Aeolisaccus sp. and by rudist floatstones. Rudists in growth position are rare; most of them appear toppled and locally oriented but not reworked. In some cases they form small bouquets no more than 30 cm thick, The successions analyzed in the Sorrento Peninsula encompass a variety of facies representative of inner shelf environments in a ramp-like depositional setting. On the basis of lithologic and sedimentologic characteristics and on a qualitative analysis of the biofacies, three major facies associations have been recognized corresponding to: (a) intertidal silty-sand flats and shallow lagoons; (b) subtidal mobile foraminiferal sand sheets; (c) subtidal rudist dwelt sand plain. In the lower part of the successions periodic (more or less prolonged) emersions (subaqueous with fresh/brackish waters or subaerial exposure) are documented. The upper part of each succession lacks any emersion evidence; submarine exposure surfaces are testified by firmground-related features. The periodic influence of high-energy regimes (some storm-related events) is documented by wave- and cross-laminations, HCS and the lack of fine: sediments. Lithofacies are arranged in depositional cycles that may correspond to individual beds. The boundaries of the depositional cycles show evidence of subaerial or submarine exposures. The nature and position of these discontinuity surfaces in the successions provide an important clue to interpretation in terms of both depositional environment and vertical evolution. The increase in thickness of the depositional cycles and the gradual change from peritidal/shallow-subtidal cycles to dominantly subtidal cycles, document an increase in the amount of accommodation space which resulted from a long-term rise in relative sea-level. A detailed study of benthic associations (micro- and macrofauna) has been carried out in order to identify accurately further signs of periodicity contained within the successions as a whole. These Upper Cretaceous limestones are abundantly fossiliferous with only a moderately diverse fauna. There are no significant differences in terms of species, the assemblages simply reflecting differences in abundance of the same species. Taphonomic studies of rudist-rich beds permit seven major shell-bed types to be distinguished. Six main foraminiferal assemblages have been detected, and the distribution and relative abundance of the species have been examined within the environments inferred through facies analysis. The taphonomic data and the foraminiferal assemblage abundance and diversity, have been compared with lithologic and sedimentologic data in order to elucidate their distribution within depositional cycles and throughout the successions. The successions show little up-section changes in lithology and in taxonomic composition of the foraminiferal assemblages, It is therefore difficult to establish a paleoenvironmental trend independent of the rudist shell concentrations, which do change up-section. We noticed an overall increase in thickness and abundance of the shell beds and a shift in types of shell concentration and in taxonomic composition. These characteristics. associated with the upward disappearance of emersion surfaces, the higher frequency of storm and/or wave intercalations, the increase in thickness of the depositional sequences and the gradual change from peritidal/shallow-subtidal cycles to dominantly subtidal cycles, document a general deepening-upward trend.More open water conditions with a depth between fairweather and storm wave-base became established as a result of relative sea-level rise and to a consequent increase in the available accommodation space. (C) 2000 Elsevier Science B.V. All rights reserved
Sedimentary setting and rhodolith deposits in the chanelized carbonate margins: case histories from the Early Miocene of the peri-Mediterranean area.
No full textShallow water carbonate rhodolith deposits constitute thick channelized sedimentary sequences in Early Miocene platforms from Southern Apennines and southern Sardinia. Sedimentology and palaeontological characters are described
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