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Laterally accreted deposits in low efficiency turbidites associated with a structurally-induced topography (oligocene molare group, tertiary piedmont basin, nw italy)
Full text linkThe origin of laterally accreted deposits in ancient deep marine successions is often controversial. Indeed, not always do these features imply the occurrence of meanders or high-sinuosity turbidite channels, but they can be generated by other causes, such as sediment-gravity-flow dynamics controlled by the morphology of tectonically confined mini-basins. This work discusses laterally accreted deposits composed of sharp-based, normally graded beds in a very small tectonically controlled mini-basin. These beds, characterized by a well-defined asymmetrical crosscurrent facies tract, form well-developed lateral-accretion surfaces dipping in directions ranging between W and SW, and perpendicular to the paleocurrents directed towards the N. For this reason, these deposits have always been interpreted as point bars related to meandering channels. A new detailed stratigraphic framework and facies analysis have led to an alternative interpretation, namely that these deposits record lateral deflections of small volume, longitudinally segregated turbidite dense flows against a structurally controlled morphological high. This interpretation is also supported by a comparison to other tectonically controlled turbidite systems that are characterized by higher degrees of efficiency but show similar laterally accreted deposits and cross-current facies tracts
Slope fans and aprons dominated by supercritical bedforms: topographic and feeding system controls (Southeastern Tyrrhenian Sea)
No full textIn the marine environment, turbidite supercritical bedforms have been widely reported from channel-axis and overbank wedges. On the contrary, their dominance in the make-up of fans and apron, apart from local areas such as channel mouths, is at present not recognized. However, since it has been postulated that turbidity currents reach the supercritical conditions for slope > 0.5°, submarine slopes should contain abundant supercritical flow deposits. Here, we provide a review of different types of slope fans and aprons dominated by supercritical bedforms, based on examples from the modern seafloor. We compare depositional elements located in different intraslope basins of the Tyrrhenian Sea, through high-resolution bathymetry, chirp subbottom section and, where available cores. The variable geological context results in axial and transvers slope fans with highly variable sizes (few to tens of kilometres) and geometries, dependent upon the erosive and/or depositional processes involved, as well as the seafloor topography of the area. In particular, we have recognized two types of lobe-shaped deposits characterized by supercritical bedforms: channel-attached fans and detached aprons. The first ones are connected to a canyon-channel system and develop on slope gradients of 0.5° to 1.2°, display small-scale bedforms (wavelength of about 150 m and height < 10 m), with upslope asymmetric or symmetric cross-sections, interpreted as cyclic steps and antidunes. According to the amplitude of the reflections, cores, and to the bedform aspect ratio, the channel-attached fans are interpreted to be composed of coarse-grained sediments. Our examples highlight that cyclic steps and antidunes dominate the channel-attached fans both in axial and lateral portion while scours mark topographic changes such as breaks in slope or laterally confined areas. Detached aprons develop from the un-incised shelf edge on steep slopes of about 1.2° to 3° and are composed by large-scale bedforms (wavelength of about 500 m and height of about 5 m) mainly upslope asymmetric, associated with cyclic steps. The low amplitude of the seismic reflections suggests the fine-grained nature of the aprons. This study shows that there are significant differences in the distribution and character of supercritical bedforms in slope settings according to the type of feeding system, the degree of flow confinement and the seafloor topography. The analysis of the downslope evolution of turbidity currents, and of the character of associated bedforms in deep-water systems can contribute new perspectives to refine our models of deep-sea depositions
Asymmetrical cross-current turbidite facies tract in a structurally-confined mini-basin (Priabonian-Rupelian, Ranzano Sandstone, northern Apennines, Italy)
No full textThis work discusses the stratigraphy and facies analysis of the Ranzano Sandstone, in the northern Apennines (Italy), a confined low-efficiency turbidite system deposited in a series of small piggy-back basins, which show strong analogies with intraslope minibasins commonly observed in divergent margins. The detailed physical stratigraphy of these deposits, which are the counterpart of the Annot Sandstone of the classic “Trilogie Priabonienne” cropping out in the western Alps, shows a cross-current asymmetrical distribution of the facies related to basin morphology. In an E-W oriented transect, roughly perpendicular to the general paleocurrents directed towards SSW, coarse-grained amalgamated massive sandstones that onlap against the eastern basin margin, grade towards the west, over very short distances, into a stratigraphic succession dominated by coarse-grained sandstones reworked in megaripples. This westward lateral facies change is also associated with: a) progressive deviation of the megaripple paleocurrents towards the west, b) progressive eastward increase in the sandstone/mudstone ratio, impact flow structures and beds with mudstone clasts, and c) progressive westward decrease in bed thickness characterizing massive facies. This cross-current transition is interpreted as related to bipartite turbidity currents experiencing deceleration against a relatively steeper eastern margin, producing flow decoupling and consequent bypass of upper turbulent flows that can spread out towards the less steep western bounding slope. The lateral spreading of turbulent flows causes reworking of the coarse-grained massive facies, producing tractive structures that indicate westward flow divergence. The lack of medium, fine-grained sands and mud testifies that these grain sizes must have been transported by turbulent flows into another adjacent basin, allowing the described deposits to be interpreted as recording a “flow stripping” phase as found in the ponded intraslope basins of the Gulf of Mexico and other confined settings, such as wedge-top basins and inner foredeeps
Numerical simulations of subaqueous bipartite gravity flows
No full textLarge mass gravity flows are characterized by bipartite flows: a basal very dense laminar flow having Bingham fluid characteristics and an overlaying turbulent flow. In a first phase the turbulent flow is generated and dragged by the basal dense flow, then it continues to flow independently over very long distances after the basal flow has come to rest. The physical modelling of such complex phenomenon has been approached with the development of two coupled models, one for the basal dense flow, the other for the turbulent flow. The present paper briefly describes the coupled models and illustrates the results of their application to simulate outcropping ancient events
Contained-Reflected Megaturbidites of the Marnoso-arenacea Formation (Contessa Key Bed) and Helminthoid Flysches (Northern Apennines, Italy) and Hecho Group (South-Western Pyrenees)
Full text linkContained-reflected beds deposited by fully-ponded or partially-reflected turbidity currents are important because their correct evaluation can give important indications on the degree of basin confinement and on the type, size and orientation of the morphological obstacle. Through a detailed facies analysis of various significant megabeds in the Marnoso-arenacea Formation, including the Contessa key bed, in the helminthoid flysches in the northern Apennines (Italy) and in the Pyrenees (megaturbidite MT5), this work proposes a depositional model that is well consistent with the recent experimental data available in the literature, discussing their strengths and limits. The Contessa and flysch megabeds fit very well with the experimental conditions because they are deposited in narrow and elongated confined basins characterized by axial flows. Indeed, in the proposed model, it is possible to recognize facies deposited by: 1) a basal underflow directed towards the bounding slope (Facies A), 2) an intermediate part of the flow characterized by lateral deflections (facies B1), 3) an upper well-developed reversing flow (facies B2) and 4) an uppermost residual reversing flow recording the final collapse of the fine-grained suspended load forming a poorly-sorted slurry facies C and a very thick mudstone unit D. Facies A, B1 and B2 are usually separated by very thin fine-grained muddy drapes rich in carbonaceous matter, which can be traced throughout the basin. These drapes - very common in contained and confined beds in these settings - can be related to internal density surfaces, along which decoupling processes, separating underflows from reversing overflows, can easily occur. Conversely, as the MT5 is characterized by a source transversal to an elongated narrow basin, the large flow volume versus basin capacity hinders the generation of reversing flows and rebound layers favoring the formation of fully-ponded pulsating overflows able to deposit alternations of laminated and massive units. This facies type can be observed in the basins that are characterized by axial flows only near the basin margins where the pulsating collapse of the reversing flow can dominate. This study shows that the integration of detailed field studies are essential to validate experimental data from an applicative point of view
Downslope evolution of supercritical bedforms in a confined deep-sea fan lobe, Amantea Fan, Paola Basin (Southeastern Tyrrhenian Sea)
Full text linkThe sedimentology of upper flow regime bedforms represents an important research topic at the present. Deposits interpreted as those of supercritical flows are widely recognized in modern fan systems, but their recovery is challenging. Most of the sedimentological information has come from channel thalwegs but supercritical bedforms are also frequently downslope from the channel mouths. Such an environment has been identified in the Paola basin, where erosive and depositional cyclic steps have been imaged and identified in a sandy submarine lobe of the Amantea Fan. High-resolution sub-bottom profiles provide insight into the bedform internal architecture and their relationships with a frontally-confining ridge. For the first time, supercritical bedforms in a submarine lobe have been interpreted in two distinct positions: in the scour of an erosional cyclic step and in the stoss side of a depositional cyclic step. Coarse to medium-grained massive sand with flame structures, indicating rapid sediment fall-out and frequently associated with the occurrence of hydraulic jumps, has been identified in the scour and at the toe of the ridge. The latter represents an example of topographically induced hydraulic jumps driven by a frontal confinement. Top-cut-out medium to fine sands with tractive structures have been interpreted as the deposits related to the stoss side of a cyclic step or small-scale antidune superimposed on the cyclic step surface. The presented data broaden the understanding of the range of processes that are driven by the interaction between turbidity currents and seafloor topography and the dip of the slope. The recognition that topography influences the density structure and the degree of criticality of the flow and, consequently, the morphodynamics and facies of the relative deposits may help to explain sediment distribution and improve depositional models of fan lobes in confined settings
Depositional record of confined turbidites in syn-subduction intraslope basin: insight from the Tufiti di Tusa Formation (Southern Apennines, Italy)
Full text linkA detailed lithostratigraphy and facies analysis of a type section of the Tufiti di Tusa Formation, including deepmarine
clastic successions with syn-orogenic volcanic detritus and deposited in the late Eocene - early Miocene
basin system at the front of the growing Maghrebian - Southern Apennines orogen, is discussed in the paper.
Based on facies analysis and composition, the study section was subdivided into the following units, from bottom
to top: Unit I, mostly formed by contained-reflected beds (including a bed similar to the Contessa megabed of the
Marnoso-arenacea Formation in the Northern Apennines), with the ratio of sandstone intervals to mudstone
intervals (S/M) of 0.6 and with mostly-calciclastic sandstone to siltstone fraction; Unit II, recording a moderate
decrease in contained-reflected beds and a moderate increase in slurry beds, with S/M ratio of 0.9, and with
mostly-siliciclastic sandstone to siltstone fraction; Unit III, recording a further decrease in contained-reflected
beds and an evident increase in slurry beds and very-thick beds with a basal massive very coarse to coarsegrained
sandstone, with S/M ratio of 2.5, and with mostly-volcaniclastic sandstone to siltstone fraction.
In accordance with the depositional models for the infill of confined turbidite basins, Units I and II are here
interpreted as representing a flow ponding depositional phase, while Unit III as iconic of a flow stripping
depositional phase. The compositional variation from Unit I to Unit II records cutoff of calciclastic supply from
underplate sources, possibly tied to tectonic uplift of the external basin margin; while that from Unit II to Unit III
records sudden availability of volcaniclastic sediment possible due to burial of morphological high(s) between
the internal volcanic arc (source of the volcaniclastic sediment) and the depositional basin, and/or establishment
of tectonically-controlled conduits cutting the above high(s). This study may improve the knowledge not only of
infilling evolution of confined turbidite basins, but also of the depositional setting of the late Paleogene Southern
Apennines subduction margin in the Central Mediterranean
Downslope evolution of supercritical bedforms in a confined deep-sea fan lobe, Amantea Fan, Paola Basin (Southeastern Tyrrhenian Sea)
No full textThe sedimentology of upper flow regime bedforms represents an important research topic at the present. Deposits interpreted as those of supercritical flows are widely recognized in modern fan systems, but their recovery is challenging. Most of the sedimentological information has come from channel thalwegs but supercritical bedforms are also frequently downslope from the channel mouths. Such an environment has been identified in the Paola basin, where erosive and depositional cyclic steps have been imaged and identified in a sandy submarine lobe of the Amantea Fan. High-resolution sub-bottom profiles provide insight into the bedform internal architecture and their relationships with a frontally-confining ridge. For the first time, supercritical bedforms in a submarine lobe have been interpreted in two distinct positions: in the scour of an erosional cyclic step and in the stoss side of a depositional cyclic step. Coarse to medium-grained massive sand with flame structures, indicating rapid sediment fall-out and frequently associated with the occurrence of hydraulic jumps, has been identified in the scour and at the toe of the ridge. The latter represents an example of topographically induced hydraulic jumps driven by a frontal confinement. Top-cut-out medium to fine sands with tractive structures have been interpreted as the deposits related to the stoss side of a cyclic step or small-scale antidune superimposed on the cyclic step surface. The presented data broaden the understanding of the range of processes that are driven by the interaction between turbidity currents and seafloor topography and the dip of the slope. The recognition that topography influences the density structure and the degree of criticality of the flow and, consequently, the morphodynamics and facies of the relative deposits may help to explain sediment distribution and improve depositional models of fan lobes in confined settings
The specchio unit (northern apennines, Italy): An ancient mass transport complex originated from near-coastal areas in an intra-slope setting
No full textWithin the Eocene-Oligocene syn-orogenic deposits of the Epiligurian succession (Northern Apennines of Italy), a field-based study of the Specchio Unit (lower Rupelian) reveals that this complex is made up of three distinct but amalgamated mass- Transport deposits (MTDs), the largest of which reaches a maximum volume of ca. 150 km3. These bodies were deposited inside the complex system of intraslope basin systems, developed atop the submerged Ligurian accretionary prism at the collision with the Adria continental plate. The MTDs originated from catastrophic retrogressive collapses starting from the upper slope and involving progressively shallow-water environments, from distal shelfal pro-delta and deltafront sediments up to proximal coastal fan-delta deposits. These recurrent and close in time, catastrophic slope failures were probably caused by tectonic and climatic triggers, such as the enhanced tectonic activity due to incipient Apenninic continental collision and the onset of harsh climatic conditions, as suggested by oxygen isotopic maxima (e.g., Oi-1a event). Although the wedge toe/foredeep systems are generally considered the principal loci of such, usually located in deep-water settings, here we stress the importance of catastrophic mass transport events also atop the wedge, in shallow-water depositional domains. Mass transport processes also have a fundamental role in reshaping the upper physiographic profile of an evolving accretionary wedge. The correct interpretation of such mass transport processes has also important implications for geohazard forecasting in modern active continental margins, for example in terms of tsunamigenic potential. © Springer Science+Business Media B.V. 2012
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