1,721,220 research outputs found
Palaeogeographic implications and relations between tectonics and sedimentation in satellite basins (Northern Apennines): insights from Neogene Epiligurian arcs.
No full textStarting from late Oligocene the migration of the Northern Apennine thrust belt was coupled with the counterclockwise rotation of the Corsica-Sardinia block, as shown by various palaeogeographic reconstructions. A further support to the palaeogeographic reconstructions comes from the tectonic transport direction registered by oroclinal bendings occurring in the Val Marecchia thrust sheet, cropping out between the Tuscan and Romagna-Marche Apennines. In the allochthonous sheet some antitaxial arcs are characterized by different directions of tectonic transport, reflecting a change of about 40 degrees (from N-S to NW-SE) in the direction of the Apennine deformational front during early Serravallian-early Tortonian. The timing of this change can be desumed both from the age of deposits underlying the allochthonus sheet and from arcuate Epiligurian deposits located on top of arcuate thrust faults.The unconformities separating the depositional sequences of these particular syntectonic deposits registered different Neogene tectonic events, related to the step-wise advancements of the Ligurian thrust sheet. These characteristics make them an excellent natural laboratory for gaining insight into arc-development processes and tectonic control on sequence boundaries in orogenic wedges. Following these data, a palaeogeographic reconstruction of the timing of the various stages of Neogene rotations of the Northern Apennine deformational front is proposed
Gli intervalli pelitici della Formazione Marnoso-arenacea del Mugello (Miocene medio): relazioni fra tettonica, fluidi freddi e instabilità sedimentaria
No full textMarine gas hydrates (clathrates) are ice-like solids composed predominantly of methane and water derived from methanogenic and thermogenic sources. Clathrates are readily destabilized by changes in pressure and /or temperature, which, in turn, are mainly due to tectonics, sea level change, sea-floor mass wasting and shallow geotherm raise (TREHU et al. 1999; KENNETT & FACKLER-ADAMS, 2000). During clathrate dissociation, the resultant methane and water become trapped by impermeable stratigraphic intervals (marlstones, mudstones and siltstones), producing overpressurization and associated low shear strength in pelitic sediments, making them highly susceptible to soft sediment deformations, and triggering slumps, slope failures and sediment gravity flows (sediment instability). Methane-based chemosynthetic communities and authigenic methane-related carbonates (chemoherms) are frequently associated with pelitic sediments bearing gas hydrates, confirming a relationship between carbonate formation and gas hydrate decomposition (PAULL et al. 1995; SUESS et al. 1999). These carbonates show peculiar brecciated structures, irregular vuggy fabrics and dense networks of non-systematic fractures, induced by a process similar to hydrofracturing of semi-consolidated sediment, due to ascending gas bubbles and the deposition of irregular gas hydrate layers (BOHRMANN et al. 1998). The occurrence of pelitic intervals enclosing chemoherms, and intraformational and extraformational slides within the middle Miocene foredeep turbidites (Marnoso-arenacea Formation) of the northern Apennines rises many questions about relationships between tectonics and sedimentation. In this paper, the classical gravitational hypothesis is integrated with a more complex syndepositional model. Pelitic intervals are probably linked to thrust activity; structural highs affected the middle Miocene sedimentation, with pelitic sediments overlapping structural reliefs, which could be related to blind thrusts. Thrust faults may have interrupted the gas hydrates stability zone, promoting hydrate destabilization, upward flow and entrapment of methane and water in impermeable pelitic sediments. Local inhomogeneity related to fractures contributed to seeping of fluids, random precipitation of authigenic carbonate, and chemosynthetic faunas. Development of fluid overpressures beneath pelitic structural reliefs may eventually undermine their stability causing deformation processes, sliding and slumping. Gas outburst is not only testified by chemoherm autobrecciated structures but also by the scraping off and mixing of the enclosing sediments (pelites and sandstones) during the rapid fluid rise along conduits or fractures (CONTI & FONTANA, 1999a, 1999b). Multiple phases of explosive breakage of micritic carbonates, slumping, sliding and reworking processes as debris flows or local turbiditic events are responsible of the chaoticization of wide portions of pelitic intervals. In this way, pelitic intervals with chemoherms reveal relationships between tectonics, cold seeps and sediment instability. In conclusion, pelitic intervals enclosing chemoherms are useful indicators of the advancing deformational front of the northern Apennines, thus heralding tectonic phases, as also demonstrated by the presence of extraformational slides of internal provenance (BERTI et al. 1994)
L'accertamento di paternità: analisi e generazione del software per l'esecuzione del calcolo biostatistico.
No full textBiomineralizzazione e fosfato di calcio: considerazioni geologico-paleontologiche.
No full textAt the onset of hard-part mineralization, during the Precambrian-Cam¬brian transition, most of the organisms formed phosphatic skele¬tons. This event was probably related to a global phosphogenic episode re¬sulted from a prolonged period of anoxia during which high-phosphorus waters were formed in the deep ocean. At the end of the Proterozoic a phase of continental drift produced and in¬creased the rate of oceanic overturn and waters rich of phosphorus were upwelled to continental shelfs. High-phosphorus contents and the formation of extensive shal¬low epicontinental seas, which were available for colonization, caused the change from soft bodied to shelly faunas and the Cambrian radia¬tion event. The present status of knowledge regarding fossil records of the Precambrian-Cambrian transition is rewieved and a trend in the de¬position of hard parts is proposed
Carta geologico-strutturale del Montefeltro e dell'alta valle dei fiumi Savio e Tevere alla scala 1:100.000 (Appennino tosco-romagnolo e marchigiano)
No full textCarta Geologica dell'Appennino Tosco-romagnolo-marchigian
Conclusioni. In: R. Gelmini “Le fasi tettoniche: tipologie e loro propagazione spazio-temporale”.
No full textIn una recente nota (GELMINI, 1991) richiamavo l'attenzione sul fatto che l'uso del termine "fase tettonica" ha subito una qual sorta di inflazione; esso viene spesso usato sia per indicare l'insieme dei fenomeni tettonici "omogenei" a scala regionale, sia per singoli eventi, talvolta a carattere molto locale, tanto da ingenerare una notevole confusione nella comprensione dei fatti descritti, della loro valenza e della loro corretta reciproca collocazione spazio-temporale.Al fine di chiarire il concetto di fase tettonica mi sembra opportuno richiamare alcuni concetti general
Oroclinal bendings in allochthonous units: influence on Epiligurian sedimentation and implications for a palaeogeographic reconstruction of the Northern Apennines during the Neogene.
No full textMost fold-and-thrust belts show pronounced plan-view geometrical curves defined by lateral variations in structural trends. According to the transport direction, they can be subdivided in antitaxial curves (convex in the direction of the vergence) and syntaxial curves (concave in the direction of the vergence). Two types of bends are recognized: the first, non rotational arc, is a bend where the segments do not change strike during its development. The second, rotational arc or orocline, is a bend where the segments change strike during its development. In this way, the recognition of arcuate faults belonging to allochthonous thrust sheets, as oroclines, permits their formation to be linked to the thrusting phase of the allochthonous unit. At the same time the trasport direction of a bowed fault can be determined by: 1) the bisetrix of the chord connecting its terminal ends, 2) the strike of the tear faults bounding it, 3) the internal geometry of syntectonic deposits located atop the fault (piggy-back basins). In this context, the recognition of Neogene antitaxial oroclinal bendings in the Ligurian thrust sheet of the Northern Apennines, is of outstanding importance in understanding the relationships among thrusting direction, tectonic control on sequence boundaries and rotation of the Italian peninsula.Starting from late Oligocene the migration of the Northern Apennine thrust belt was coupled with the counterclockwise rotation of the Corsica-Sardinia block and of the northern Italian peninsula around a pivot north of Genoa. Following this opinion, various paleogeographic reconstructions indicated a NE-SW direction of the deformational front during late Oligocene, a N-S direction during early-middle Miocene and a NW-SE direction, similar to the recent position, during late Miocene-early Pliocene. Palinspastic restorations are essentially based on paleomagnetic measurements, absolute dating of the rotation of the Corsica-Sardinia block and distribution of sedimentary facies. A further support to the paleogeographic reconstructions comes from the tectonic transport direction registered by oroclinal bendings occurring in the Val Marecchia thrust sheet, cropping out between Tuscan and Romagna-Marche Apennines. In this allochthonous sheet some antitaxial arcs are characterized by different transport directions, reflecting a change of about 40 degrees in the direction of the Apennine deformational front during early Serravallian-early Tortonian. The timing of this change can be desumed both from the age of deposits underlying the allochthonous sheet and from arcuate Epiligurian deposits located on top of arcuate thrust faults.In a section view arcuate Epiligurian basins describe a wedge-shape geometry, with the maximum thickness localised in the back portion of the basin, which is confined by tear faults and couples of reverse faults or couples of frontal reverse faults and rear normal or vertical faults. These basins are filled by thick late Oligocene-early Pliocene deposits, that can be grouped in several depositional sequences. Each depositional sequence crops out always parallel to the arc curvature, with convex shape boundaries. Every younger sequence tends to occupy a more internal position within the arc, with a decreasing inclination of bedding planes up to a sub-horizontal attitude. The unconformities separating depositional sequences registered different Neogene tectonic events, related to step-wise advancements of the Ligurian thrust sheet. In conclusion, these peculiar syntectonic deposits are an excellent natural laboratory for gaining insight into arc-development processes and tectonic control on sequence boundaries in orogenic wedges. Following these data an accurate paleogeographic reconstruction of the timing of the various stages of Neogene rotations of the Northern Apennine deformational front is proposed
Carta Geologica dell'alta Val Marecchia (Appennino tosco-marchigiano). S.EL.CA., Firenze
No full textS.EL.CA. Firenz
Un esempio di fossilizzazione eccezionale in alcuni Briozoi dell'Ordoviciano della Sardegna
No full textviene descritto per la prima volta la fossilizzazione di un polipide in uno zoeci
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