88 research outputs found
SEDIMENTOLOGICAL ANALYSIS OF DEEP WATER, UPSLOPEMIGRATING CROSS-BEDDED DEPOSITS IN A DISTALLY STEEPENED CARBONATE RAMP (MENORCA, BALEARIC ISLANDS, SPAIN)
No full textThe upper Miocene units cropping out along the southern coast of the Island of
Menorca (Balearic Islands, Spain), are mainly represented by two carbonate
depositional systems: an early Tortonian distally steepened ramp (Lower Bar Unit)
and an upper Tortonian – lower Messinian reef-rimmed platform prograding complex
(Reef Complex). Within the distally steepened ramp, Pomar et al. (2002)
distinguished four facies belts: fan-delta conglomerates passing upwards to
bioturbated packstones (inner ramp), cross-bedded grainstones (middle- ramp),
clinostratified rhodolithic rudstone (ramp slope) and fine-grained wackestonepackstone
with planktonic foraminifera (outer ramp).
The backset-bedded units analysed in this work are placed at the transition
between toe-of-slope and outer ramp sediments, below the wave-base-level. They
infill the axial depression of large slide/slump scars. These scars truncate the gently,
10°- 12° basinward dipping, slope-to-outer ramp clinoforms.
Backset beds are cross-bedded forms that dip against the direction of flow of the
depositing currents, therefore they present foresets migrating upcurrent (Gary et al.,
1972).
These sedimentary structures are well known and largely described on the foreset
and toeset of Gilbert-type fan delta (Postma, 1984; Massari, 1984, 1996; Nemec,
1990). In carbonate depositional systems these type of bedforms are rarely found
and only little described.
The backset-bedded units, here analysed, are channel-like, wedge-shaped, 10-12
m thick, pinching out landward and extend laterally for tens of meters. Each unit is
formed by several amalgamated set of backset beds, 40 cm to 2 m thick. These units
are mainly conglomerates composed by bioclastic coarse-grained grainstone to
rudstone. Large components are rhodoliths, bivalves, skeletal and ooid-rich pebbles
to boulders, gastropods and corals. Matrix is of a bioclastic coarse-grained sand to
fine gravel, made of fragments of bivalves, gastropods, rhodoliths, bryozoans, algae,
echinoids, loose ooids and planktonic and benthic foraminifera. Ooids are locally very
abundant both in matrix and as main components of pebbles. Pebbles are mainly
flattened, elongated, of average size 6-8 cm (a-axis) and sometimes have mollusc
borings on their surface: large (20-30 cm) rounded and spherical boulders are locally
present. Intergranular and intergranular porosity is very high, cementation low and
dolomitization patchy.
Foreset laminae dip upslope with varying angles ranging from almost horizontal to
30°; higher angles are mostly found in the basinward part of the unit. Lamination is
underline by the orientation along laminae of coarser components especially of
bivalves, pebbles and rhodoliths. Grain-size distribution has a particular trend that shows a progressive decrease in size landwards and upwards. Sorting may noticeably
vary being high or absent in different bodies.
The lower boundary of the backset-bedded units is represented by scour surfaces
which, on a parallel-to-flow section are almost concordant with the stratification
below, while on a perpendicular-to-flow section are concave-up shaped, presenting
the very steep walls.
The study of different outcrops along the coast evidenced some important
variation in components: moving northward composition changed from almost
completely rhodolithic-dominated to rhodolith-bivalve-ooid-pebble-dominated to
bivalve-ooid-pebble-dominated with first findings of corals.
Upslope bedform migration has been explained as forming when a supercritical
flow encounters a local obstruction or a local break on the slope, and a hydraulic
jump may occur within the flow, upcurrent from the obstruction. Sediment will be
therefore deposited at the obstruction forming an up-flow-dipping slipface that will
tend to accrete and migrate in the upflow direction (Nemec, 1990 and reference
therein).
The backset deposits of Menorca are found in deep-water settings but they are
composed of shallow-water sediment. The formation of these backset beds is
interpreted to be related to high energy storm-events able to remove sediment from
shallow water and to transport it into deeper position. The sediment-rich outgoing
flows channalized and accelerated along slide-scar axis, eroding and rapidly infilling
up-slope the scours. In this portion of the ramp preservation potential is higher
thanks to sediment deposition which buries and preserves these structures.
The repetitive occurrence of backset bedded units within the outer-ramp
sediments and the progressive variation in composition suggest that those processes
where probably active at the transition between the ramp and the reef systems.
Therefore the formation of these sedimentary structures is interpreted to be strictly
link to concurrence of peculiar morphological features, hydrodynamic energy and
grain-size availability.
Computational fluid dynamic (CFD) numerical simulation have been performed as
an integrated part of this work to improve the understanding of the development of
hydraulic jumps within concentrated density flows. The simulated parameters do not
refer to the example of Menorca but to turbidity currents for which finer-grain size
(sand-size) have been used in a smaller-scale topography compared to the one
studied in outcrop. The work presented proposes some new stating points for further
simulations to constrain more precisely the main parameters controlling and
determining the occurrence of a hydraulic jump and the consequent deposition of
sediment with backset bedding
NOMINATION OF THE DOLOMITES FOR INSCRIPTION ON THE WORLD NATURAL HERITAGE LIST UNESCO: THE GEOLOGICAL CONTRIBUTION.
No full textSedimentological features of coarse-grained deep water backset beds in a distally steepened carbonate ramp (Late Miocene, Menorca, Balearic Islands, Spain)
No full textEvolution and organization of the Sciliar/Schlern carbonate buildup (Triassic Dolomites - Italy).
No full textThe Sciliar/Schlern area is the type area of the Dolomia dello Sciliar. Here it’s where the biological origin of ancient carbonate platforms has been recognized for the first time in the Dolomites. This area has been widely studied since the 19th century because of the outstanding outcropping of carbonate platforms: here lateral facies transition and depositional geometries are very well preserved. This allowed us to study composition, architecture and depositional environments of a fossilized reef. In this report we present an integrated stratigraphical and sedimentological study combined with a facies analysis of the area. Our data lead us to better understand the evolution and the growing mode of several different generations of superimposed carbonate buildups from Anisian to Carnian p.p. (Triassic), and the relationships between the inner organization of the carbonate platforms, the margin rim and slope and the correlated basinal facies. The stratigraphic correlations resulted from the lithological data and the facies analysis have been integrated with known and new biostratigraphic data (mostly ammonoids and palynomorphs) in order to build a high resolution bio-chronostratigraphic framework. The result is the remodeling of the evolution of the Sciliar/ Schlern buildup which is the consequence of the growing of different generations of carbonate platforms. The three-dimensional development of each growing phase of the platform seems to be controlled mainly by the rate of accommodation , by the inherited morphology of the previous body and by the variation in basinal sedimentation rate that forces the decrease of the clinoforms’ angle. Another peculiarity of the last phases of the evolution of this platform is the huge number of megabreccias bodies and carbonatic olistoliths (“Cipit boulders”) found in the Alpe di Siusi/Seiser Alp volcanoclastic basinal sediments. These bodies are probably related to frequent displacements which involved both the slope and the reef, testified by several scallops features recognized along the paleo-slope
NEW BIO AND CHRONOSTRATIGRAPHIC CONSTRAINTS TO THE UPPERMOST LADINIAN EVOLUTION OF CARBONATE PLATFORMS FROM THE WESTERN DOLOMITES (SOUTHERN ALPS, ITALY)
No full text
Characterization of upslope migrating cross-bedded deposits in a carbonate ramp (Upper Miocene, Menorca, Balearic Islands, Spain)
No full textMenorca is subdivided in main regions with different stratigraphic and tectonic characteristics. To the north, folded, thrusted and faulted Paleozoic, Mesozoic and Paleogene rocks compose the Tramuntana region. To the south, upper-Miocene carbonate and, locally, siliciclastic rocks compose the Migjorn region. Upper Miocene rocks unconformably overlie Paleozoic to middle Miocene basement. In the Migjorn, three Miocene depositional units were recognized (Pomar et al., 2002 and references herein): the Basal Conglomeratic Unit (Burdigalian p.p.), interpreted as near-shore to fan-delta deposits; the Lower Bar Unit (early Tortonian), interpreted as a distally steepened carbonate ramp; the Reef Complex (upper Tortonian - lower Messinian), a reef-rimmed platform prograding complex. Within the ramp, Pomar et al. (2002) distinguished four facies belts: fan-delta conglomerates and bioturbated packstones (inner ramp), crossbedded grainstones (middle- ramp), rhodolithic rudstone clinobeds (ramp slope) and fine-grained wackestone-packstone with planktonic foraminifers (outer ramp). The cross-bedded units here analysed occur at the transition lower slope-outer ramp. They infill the axial depression of large slide/slump scars. These scars truncate the gently, 10°-12° basinward dipping, slope-to-outer ramp succession.
The wedge-shaped cross-bedded units, 10-12m thick, pinch out basinward and extend laterally for tens of meters. Several amalgamated cross-laminated bodies, 40cm- to 2m wide each with erosive base compose each unit.
They are composed by rhodolithic-mollusk rudstone with coarse grain/packstone matrix. Porosity is very high, cementation low and dolomitization patchy. Matrix consists of red-algal-, bivalve-, echinoderm, coral fragments, and benthic foraminifers. Ooids are, locally abundant along with limestone pebbles. Pebbles are flat, 5- to 10-cm wide, of oolitic grainstone, sometimes with mollusk borings. Cross bedding is underlined by the abundance and orientation of coarser components. Cross-laminae within units dip upslope (8° to 35°) forming backstepping backsets. Upslope bedform migration has been explained as forming when a supercritical flow encounters a local obstruction (or a local break on the slope), and a hydraulic jump may occur within the flow, upcurrent from the obstruction. Sediment will be therefore deposited at the obstruction forming an up-flow-dipping slipface that will tend to accrete and migrate in the upflow direction. In Menorca, these sedimentary structures have been interpreted by Pomar et al. (2002), as backset beds related to up-stream migrating hydraulic jump where slide scars acted as slope channels funneling platform debris down the slope.
The occurrence of aragonitic components (oolites, corals) within these units, absent in the ramp system, and the scars development at the end of the ramp progradation, suggest the involvement of Reef Complex sediment. Therefore, those events, involved both sediment of the upper Tortonian distally steepened ramp and part of the Reef Complex.
These structures have been mostly described in different environments of terrigenous systems. In carbonate systems are scarcely described and none from carbonate ramps. The architecture characterizing these upslope cross-bedded beds seams to call for a scour-and-fill process, therefore initially the action of a strong scouring event immediately followed by depositional events. The presence of these bodies in various intervals suggests the repetitive occurrence of this process.
In the studied system, deep-scouring events on the platform could be represented by slope failures that induced tsunami events or by eustatic variations, able to create strong downslope flows directed offshore. Scours have been rapidly infilled by shallow-water sediment
Late Anisian to Late Carnian subsidence depocenter shift recorded by shallow water carbonates (Dolomites, Southern Alps, Italy)
No full textUno studio condotto su numerose piattaforme carbonatiche fossili, svoltosi nell’ambito del progetto CARG, ha consentito di evidenziare la differente evoluzione dei tassi di subsidenza nell’area Dolomitica. Nuovi dati biostratigrafici consentono di tarare l’evoluzione su una scala cronostratigrafica di dettaglio e di riconoscere una sincronicità nelle fasi di arresto della subsidenza tra aree sensibilmente distanti. L’evoluzione dell’area di studio viene descritta in dettaglio con particolare attenzione al contesto geodinamico nella quale si inserisce
A genetic approach applied to Triassic Carbonate Platforms of the Dolomites (Triassic, Italy)
No full textLe cantigas di Roi Martinz d\u27Ulveira: edizione critica con commento
No full textIl saggio propone l’edizione critica delle quattro cantigas, una de amor e tre de amigo, di Roi Martinz d’Ulveira, trovatore portoghese attivo presumibilmente nell’ultimo quarto del XIII secolo. L’edizione è corredata da un’agile introduzione sull’autore e sulla tradizione manoscritta del suo ridotto canzoniere, dalla traduzione dei componimenti e da un puntuale commento, suddiviso in scheda metrica, cappello introduttivo e note esplicative ai versiThe essay proposes the critical edition of four cantigas, one de amor and three de amigo, by Roi Martinz d’Ulveira, a Portuguese troubadour presumably active in the last quarter of the 13th century. The edition is accompanied by an introduction on the author and the manuscript tradition of his work, the translation of the poems and a detailed commentary, divided into metrical analysis, an introductory presentation and explanatory notes to the verses
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