Flow behaviour of ponded turbidity currents

Sea floor topography can constrict, deflect or reflect turbidity currents resulting in a range of distinctive deposits. Where flows rebound off slopes and a suspension cloud collects in an enclosed basin, ponded or contained turbidites are deposited. Ponded turbidites have been widely recognised in slope mini-basins and on small, structurally-confined basin floors in strike-slip and foreland basin settings. They can have a variable internal structure the significance of which remains poorly understood in terms of flow behaviour. New experiments demonstrate that the ponding process can comprise up to four phases: 1) cloud establishment, 2) inflation, 3) steady-state maintenance, and 4) collapse. The experiments explored the behaviour of sustained turbidity currents draining into small basins and show that the ponded suspensions that form are characterised by an important internal interface; this divides a lower outbound-moving layer from an upper return layer. The basal layer evolves to constant concentration and grain size, whereas the upper layer is graded (concentration and grain size decrease upward). During the cloud inflation stage, the concentration and velocity profiles within the ponded suspension evolve and this phase can dominate the resulting deposit. Outbound internal waves can travel along the interface between the outbound and return layers and impinge against the confining slope and their amplitude is highest when the inter-layer density contrast is greatest, e.g., when the input flows are thin and dense. The experiments show that flow reversals can arise in several ways (initial rebound, episodic collapse of the wedge of fluid above the counter slope, ‘grounding’ of the internal velocity interface) and that despite steady input, velocities decay and the deposit grades upwards. Internal waves emanate from the input point, i.e., do not form as reflections off the counter slope. The internal grain size interface within the suspension may dictate textural trends in sands onlapping the confining slopes. Where flows are partially ponded, internal waves can generate pulsing overspill to basins down dip

Hybrid Event Beds Generated By Local Substrate Delamination On A Confined-Basin Floor

The outer parts of deep-water fans, and the basin plains into which they pass, are often described as areas where erosion is negligible and turbidite systems have net aggradation. Nevertheless sedimentological and stratigraphic analysis of outer fan lobe and confined basin plain deposits in Cretaceous-Paleocene Gottero Sandstone (NW of Italy) has revealed extensive but cryptic bedding-parallel substrate-delamination features at the base of many sheet-like event beds. These comprise a variety of shallow but wide scour structures showing evidence of lateral expansion by sand-injection. The scours commonly occur at the base of beds made up of a basal clean sandstone overlain by argillaceous sandstone containing abundant mudstone clasts and locally large substrate rafts (up to 20 meters long). These strata are interpreted as a type of hybrid event bed. Field observations suggest that mud-clast entrainment occurred by delamination at the base of dense sandy flows. The large rafts, in some cases only partly detached, were incorporated in the flows locally and then carried for short distances (100s m to a few km) before partly disaggregating and undergoing deformation due to internal shearing. The development of such features may be common in flat and/or confined basin settings where high-volume flows interact with a cohesive and well layered substrate (e.g. muddy outer fans or confined or ponded basins with thick mudstone caps). Delamination is therefore suggested as an alternative mechanism leading to the formation of hybrid event beds following local substrate entrainment on the basin floor as opposed to on more remote slopes and at channel-lobe transition zones

Inflation of ponded, particulate laden, density currents.

Field-based, physical modeling and analytical research approaches currently suggest that topographically confined particle-laden density currents commonly inflate to produce suspension clouds that generate tabular and texturally homogeneous sedimentary deposits. Here, a novel three-dimensional theoretical model details a phase space of the criteria for inflation as a function of flow duration, basin size and geometry, total mass transport, sediment concentration, and particle grain size. It shows that under most circumstances cloud inflation is unlikely at real-world scales. Even where inflation is possible, inflation relative to initial flow height is small except for suspensions of silt or finer-grained sediment. Tabular deposits therefore either arise from processes other than flow ponding, or deposits in confined settings may be significantly more complex than are currently understood, due to processes of autogenic compensation and channelization, with associated implications for reservoir characterization in applied contexts. This study illustrates the potential of analytical flow modeling as a powerful complement to other research approaches

Towards a Southern European Tethyan Palaeomargin provenance signature: sandstone detrital modes and detrital zircon U–Pb age distribution of the Upper Cretaceous–Paleocene Monte Bignone Sandstones (Ligurian Alps, NW Italy)

Constraining the source terranes of Alpine siliciclastic flysch sequences is crucial for building a clearer picture of the palaeogeography and geodynamic evolution of the Western Tethys in the framework of impending continental collision. This paper presents an integrated study that involves palaeocurrent dispersal analysis, sandstone petrography and detrital zircon geochronology of the Upper Cretaceous–Paleocene Monte Bignone Sandstones, a siliciclastic turbidite system deposited during the pre-collisional evolution of the Ligurian Alps. Palaeocurrent analysis illustrates an overall eastward transport of the proximal sediments in the present configuration. Considering the ca. 45°–50° counter-clockwise rotation of the Tertiary Piedmont Basin and of the Corsica–Sardinia block in the late Paleogene, this indicates the derivation of the sediments from the northern margin of the Piedmont–Ligurian Ocean. Sandstone petrography records a stratigraphic evolution from quartzose sandstones towards lithic and then to lithic sub-arkosic composition. This trend is interpreted to reflect the gradual unroofing of the provenance terrane. The lithotypes of the recycled sedimentary rock fragments and the up-section increase in dolostone and carbonate clast proportions suggest the erosion of the sedimentary cover of the southern European palaeo-margin. New geochronological data (U–Pb detrital zircon ages) correspond to the pre-Alpine stages of crustal growth recorded in the Variscan Maures-Tanneron Massif, and therewith confirm the derivation of the sediments from the passive palaeo-European margin. This conclusion highlights the importance of the lower plate in providing the source of coarse-clastic deep-water successions during pre-collisional convergent steps. Results from this multi-proxy provenance analysis contribute to better defining the detrital signatures associated to the continental micro-fragments that constituted the palaeo-European plate as it supplied deep-sea siliciclastic sediments into the Piedmont–Ligurian Ocean prior to continental collision

Hybrid event beds in the proximal to distal extensive lobe domain of the coarse-grained and sand-rich Bordighera turbidite system (NW Italy)

The Upper Cretaceous Bordighera Sandstone of NW Italy is a coarse-grained, sand-rich elongated turbidite system (ca. 15 × 45 km in outcrop) up to 250 m thick, interpreted to have been deposited in a trench setting. The siliciclastic succession interfingers with muddy calcareous turbidites, which become more abundant toward the lateral and distal domains. Bed type associations allow the distinction of a proximal channelized domain which transitions to a more distal lobe domain, characterized by abundant mudclast-rich sandstones and by bipartite and tripartite beds with a mud-rich middle or upper division (hybrid event beds). The transition between the proximal and distal domains occurs over a relatively limited spatial extent (ca. 5 km). The presence of lenticular bed-sets made up of coarse grained and mud-poor sandstones throughout the distal domain suggests that distributary channels were present, indicating sediment bypass further down-dip toward the most distal and not preserved parts of the system. Hybrid event beds - commonly associated with distal and marginal fan environments such as fan fringes - are present throughout the lobe domain and extend for up to ca. 30 km in down-dip distance. They are more abundant in the proximal and axial depositional lobe domain and their appearance occurs within a short basin-ward distance from the inferred channel-lobe transition zone. Flow expansion at the termination of the channelized domain and the enhanced availability of cohesive substrate due to the presence of intra-basinal muddy calcareous beds are interpreted as the key controls on pronounced argillaceous sandstone distribution. The abrupt appearance and the persistent occurrence of such beds across an extensive domain have implications for characterizing bed-scale (sub-seismic) heterogeneity of deep-water clastic hydrocarbon reservoirs

Digital Reproduction of Clastic Sedimentary Architecture by Means of Relational Databases

As the amount of architectural data collected in sedimentological studies, and typically rendered available in published form, has increased over time, so a fundamental issue has become ever more important: the need to ensure that different datasets collected in different ways by different geologists (e.g. 2D architectural panels, 3D seismic surveys) are stored in a format such that analysis or synthesis of fundamentally different types of data can be made in a sensible and informative manner, without requiring extensive literature search and re-processing. Database systems are here proposed as a means for achieving the convergence of datasets in a common medium. The proposed database approach permits the digital reproduction of sedimentary architecture in tabulated form: hard and soft data referring to depositional products are assigned to standardized genetic units belonging to different scales of observation, which are themselves contained within stratigraphic volumes classified on deposystem parameters (e.g. subsidence rate, physiographic setting). Although the approach has general applicability, two different databases have been independently developed to capture the peculiarities associated with fluvial and deep-marine depositional systems. Through interrogation, the two database systems return output that – being in quantitative form and referring to standardized sedimentary units – is suitable for both synthesis and analysis. Deposystem classification permits data to be filtered on the parameters on which the systems are classified, allowing the exclusive selection of data associated with systems deemed to be analogous to a given subsurface succession in terms of deposystem boundary conditions and environmental setting. Alternatively, the quantification of architectural properties permits users to identify analogy in terms of sedimentary architecture. Outputs from the two databases are here presented in forms suitable for highlighting differences in the way fluvial and deep-water architecture is conceptualized and implemented, and for presenting ways in which analog information can be employed for the characterization and prediction of fluvial and deep-water reservoirs. Specific example applications include the use of database output to (i) generate quantitative facies models with which to guide core interpretation, (ii) to constrain stochastic reservoir models, and (iii) to guide well correlation of fluvial or deep-marine sandstones

Detrital signatures of impending collision: The deep-water record of the Upper cretaceous Bordighera Sandstone and its basal complex (Ligurian Alps, Italy)

Despite intensive research efforts and significant advances in the understanding of subduction and obduction processes that affected several units which at the present day compose the Western Alps, the paleogeographic evolution of the Alpine Tethys represents a debated topic in Alpine geology. The role of the opposing continental margins (passive European margin and active Adriatic margin) as source regions for Cretaceous siliciclastic turbidites bordering the convergent system remains disputed. To address this question along the Ligurian Alps transect, a multi-proxy provenance analysis is applied to the two terrigenous superimposed units (Hauterivian-Campanian San Bartolomeo Fm. and Campanian-Maastrichtian Bordighera Sandstone) of the San Remo-Monte Saccarello Unit of the Western Ligurian Flysch complex. Petrographic analyses characterize the basal San Bartolomeo Fm. as quartz-rich mature sandstones. By contrast, the overlying Bordighera Sandstone represents texturally and compositionally immature first-cycle arkosic arenites. This change records the evolution of the sediment provenance from a stable craton into a continental basement uplift setting, reflecting erosion of granitoid plutons and the low-grade metamorphic basement. Geochronological data (U Pb detrital zircon ages) indicate that virtually the same source terranes provided the source for both formations. The detrital age spectra display age peaks are compatible with well-documented magmatic and metamorphic pulses that affected the Southern Variscides in the Paleo-European margin. The strong affinity of clastic detritus with the Paleo-European margin basement rocks underlines the importance of the lower plate passive continental margin in supplying sand-rich turbidite systems prior to the arrival of the passive margin in the subduction zone

Tectonic Influence on the Geomorphology of Submarine Canyons: Implications for Deep-Water Sedimentary Systems

A database-informed metastudy of 294 globally distributed submarine canyons has been conducted with the aim of elucidating the role of tectonic setting on submarine-canyon geomorphology. To achieve this, data from seafloor and subsurface studies derived from 136 peer-reviewed publications and from open-source worldwide bathymetry datasets have been statistically analyzed. In particular, relationships between margin type (active vs. passive) or plate-boundary type (convergent vs. transform vs. complex) have been assessed for key morphometric parameters of submarine canyons, including: streamwise length, maximum and average width and depth, canyon sinuosity, average canyon thalweg gradient, and maximum canyon sidewall steepness. In addition, possible scaling relationships between canyon morphometric parameters and characteristics of the associated terrestrial catchment, continental shelf and slope, and of the broader physiographic setting for canyons along both active and passive margins have been evaluated. The following principal findings arise: 1) overall canyon geomorphology is not markedly different across tectonic settings; 2) slope failure might be more important in passive-margin canyons compared to active ones, possibly due to seismic strengthening in the latter; 3) some aspects of canyon geomorphology scale with attributes of the source-to-sink system and environmental setting, but the strength and sign in scaling might differ between active and passive margins, suggesting that the extent to which canyon geomorphology can be predicted depends on the tectonic setting. Insights from our analysis augment and improve conceptual, experimental and numerical models of slope systems at the scale of individual canyons and source-to-sink systems, and increase our understanding of the complex role played by tectonic setting in shaping deep-water systems

A global analysis of controls on submarine-canyon geomorphology

The role of possible controlling factors in influencing the geomorphology of submarine canyons has been investigated using a database of 282 globally distributed modern examples collated from the literature and open-source worldwide bathymetry. Canyon geomorphology has been characterised quantitatively in terms of maximum and average canyon dimensions, canyon sinuosity, average canyon thalweg gradient, and maximum canyon sidewall steepness. An assessment is made of how geomorphological characteristics vary with respect to the position of the canyon apex relative to the shelf break, continental-margin type, terrestrial source-to-sink system setting, oceanographic environment, and latitude. Scaling relationships between canyon morphometric parameters, and correlations between these and attributes of the canyon physiographic settings, terrestrial catchments, and continental shelves and slopes, have been quantified. Key findings are as follows: (i) a number of scaling relationships describing canyon morphometry (e.g. scaling between maximum canyon dimensions, relationships of maximum canyon sidewall steepness with maximum canyon width and depth) can be recognised globally, suggesting their general predictive value; (ii) possible causal links are identified between hydrodynamic processes (e.g. upwelling, longshore- and along-slope currents) and canyon morphology; (iii) potential predictors of aspects of canyon geomorphology include whether a canyon is incised into the shelf or confined to the slope, the continental-margin type, the oceanographic environment, latitude, and shelf-break depth; (iv) similarity in the distributions of maximum width-to-maximum depth ratios across all settings suggests that the relative magnitudes of canyon-margin erosion and intra-canyon deposition do not vary greatly depending on setting or canyon size. The relationships between canyon geomorphology and environmental variables identified in this study may be incorporated into conceptual models describing canyon geomorphology and its relationship both to other elements of deep-water systems, and to its broader source-to-sink context. The results provide a framework for future experimental and numerical studies of canyon geomorphology

Origin of mud in turbidites and hybrid event beds: Insight from ponded mudstone caps of the Castagnola turbidite system (north‐west Italy)

The partitioning of different grain‐size classes in gravity flow deposits is one of the key characteristics used to infer depositional processes. Turbidites have relatively clean sandstones with most of their clay deposited as part of a mudstone cap or as a distal mudstone layer, whereas sand‐bearing debrites commonly comprise mixtures of sand grains and interstitial clay; hybrid event beds develop alternations of clean and dirty (clay‐rich) sandstones in varying proportions. Analysis of co‐genetic mudstone caps in terms of thickness and composition is a novel approach that can provide new insight into gravity flow depositional processes. Bed thickness data from the ponded Castagnola system show that turbidites contain more clay overall than do hybrid event beds. The Castagnola system is characterized by deposits of two very different petrographic types. Thanks to this duality, analyses of sandstone and mudstones composition allow inference of which proportion of the clay in each of the deposit types was acquired en route. In combination with standard sedimentological observations the new data allow insight into the likely characteristics of their parent flows. Clean turbidites were deposited by lower concentration, long duration, erosive, muddy turbidity currents which were more efficient at fractionating clay particles away from their basal layer. Hybrid event beds were deposited by shorter duration, higher‐concentration, less‐erosive sandier flows which were less efficient at clay fractionation. The results are consistent with data from other turbidite systems (for example, Marnoso‐arenacea). The approach represents a new method to infer the controls on the degree of clay partitioning in gravity flow deposits