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Bed by bed correlations and lateral variability in hybrid event beds in the Ramaceto area (Gottero Sandstone, northern Apennines, Italy)
No full textBasal erosion and mudclast character and distribution in hybrid beds deposited in a ponded minibasin (Castagnola Fm, NW Italy)
No full textHybrid beds are commonplace in many deep-water clastic systems. They are characterised by a vertical association of sedimentary facies thought to be deposited under a turbulent regime (e.g. massive/laminated clean sand) and those deposited under a more transitional or cohesive flow regime (e.g. chaotic mudclast-rich and/or muddy sand), within a single event. Multiple models are proposed for their emplacement, focussing on the rheological changes within a flow, as well as their spatial and temporal occurrence. However our understanding of depositional processes, thus character and distribution of hybrid beds, remains limited. The Lower Miocene Castagnola Fm. (>1000m) records the deep-water infill of a small (a few km2 ) ponded piggyback sub-basin in the eastern part of the Tertiary Piedmont Basin (NW Italy). Six detailed sedimentary logs taken at similar distances along a 3.1 km proximal-to-distal transect encompass a low net-to-gross (0.2), 250 m thick interval. Generally, bed types comprise either thin (<1m), highly structured deposits, or thicker (1- 5m) beds. The latter are commonly hybrid bed-like in character with a poorly structured lower division, overlain by a recessive division frequently enriched in mudclasts whose size range is wide (from cm-sized up to 1-2m) and often bimodal. Erosional scouring and 'frozen' entrainment of mudstone substrate at bed bases allows at least some of the larger mudclasts to be directly related to local substrate rip-up. However, the occurrence of smaller mudclasts, with a vertically variable position within the bed, suggests that these were sourced further upstream and travelled greater distances prior to deposition. Such observations are consistent with other systems where hybrid beds contain fartravelled mudclasts (e.g. Braux system, Marnoso Arenacea system, etc.). Characterising the degree of local erosion below hybrid beds and the origin of mudclasts within might help to better understanding the depositional processes and the character and distribution of hybrid beds
Partial ponding of turbidity currents, Castagnola mini-basin, Early Miocene, Northwest Italy
No full textMany deep-water settings are characterized by complex sea floor topography (e.g. salt-withdrawal mini-basins or structurally confined basins). Turbidity currents traversing such areas are affected by confinement, with associated changes to both flow behaviour and deposits. When some or all of the flow is trapped into a topographic low, partial or full flow ponding, respectively, occurs. While some processes are common to both partial and full ponding scenarios (e.g. flow deflection/reflection), full ponding results in distinct deposits (e.g. development of thick mud caps). The degree of ponding is controlled by a) basin geometry; b) range of flow volumes; c) flow structure (grain size range and concentration profile), influencing flow climbing capacity. This study aims to characterise the stratigraphic persistence of the depositional signature of partial ponding, contrasting it with that expected in fully ponded scenarios. The Castagnola sub-basin (Early Miocene, Tertiary Piedmont Basin, NW Italy) records the fill of a ponded minibasin, characterised by tabular deposits with no evidence of significant tectonic deformation during deposition. In the upper part of the basin fill a clear switch to unconfined conditions is recorded, marked by the disappearance of thick mud caps and sandstone beds with reflected facies. However, more than 400 m of the preserved 800 m basin fill records a continuous trend of slow increase in net/gross and decrease in mud cap to sandstone bed thickness, interpreted to reflecting different degrees of partial ponding. The significant thickness of deposits accreted under partially ponded conditions indicates that aggradation onto basin margins, including the sill to the next basin, must have been ongoing synchronously with deposition in the basin. This is in agreement with the observed geometry of the southern margin, which is characterised by a feathered onlap with significant amount of sediment deposited above the confining topography. The unexpectedly high proportion of the stratigraphy laid down under partially ponded conditions has implication for classic fill-and-spill models and for techniques used in the subsurface to infer ponding conditions (such as bed thickness statistics). Understanding the factors controlling the degree of ponding, together with any associated diagnostic signatures in the resulting deposits, represents a key advance in subsurface predictive models for fill and spill minibasins
Mud clast character and distribution in hybrid beds deposited in a ponded minibasin (Castagnola Fm, NW Italy)
No full textTowards a classification of hybrid event beds
No full textHybrid event beds (HEBs) are a type of deep-water sediment gravity flow deposit comprising a basal clean (H1) and/or banded (H2) sandstone overlain by a muddier sandy facies (H3) emplaced during the same transport event. They generally have a tabular geometry but an internal complexity in terms of relative thickness and texture of the component divisions. HEBs are increasingly recognised in outcrop and in hydrocarbon reservoirs, requiring an improved understanding of their textural make-up, association, context and impact on reservoir properties.
Although HEBs share the described common characters that allow them to be differentiated from ‘classic’ turbidites, observations from a range of sedimentary basins show great variability in their sedimentological character. The texture of the relatively mud-rich H3 division and the size and shape of substrate clasts within it are key feature for classification and process interpretation. Two important and recurring bed associations are identified: (1) a range of commonly thick beds in which the H3 division can include very large substrate slabs and blocks, evidence of extensive autoinjection and clast break-up, and dense mudclast concentrations, all set in a sandstone with elevated interstitial clay. This association typically is found in outer fan and confined sheet systems in a downdip position. (2) beds in which H3 divisions are characterised by high levels of dispersed clay, floating mudstone clasts and matrices that are enriched in hydraulically-fractionated components (mica, organic matter, clay flocs). Beds with thin H3 divisions typically pass down-dip to those in which H3 is expanded. This association is found in fan lobe successions where it can alternate with turbidites. The two associations are interpreted to reflect different modes of flow transformation. In the first case, the rafts and chaotic textures are related to local substrate delamination processes that culminated in the formation of a linked cohesive debris flow because of intense internal shearing and clast disaggregation. The second association formed by mud entrainment at channel mouths, proximal lobe locations or flow expansion points and developed through progressive longitudinal flow transformation and rapid deceleration and may include deposition from transitional flows
Coalesced 3D scours on the base of a hybrid event bed from the Gottero Sandstone, NW Italy
No full textErosion beneath sediment gravity flows can determine patterns of bed amalgamation and hence sand connectivity. Where significant clay is incorporated in the flow, this may impact on subsequent flow dynamics down-dip. Evidence for scouring beneath turbidites is commonly seen in outcrops, mainly in vertical profile or on limited bedding plane exposures where small-scale (cm to m-scale) sole structures are well documented. The presence of broader scour features is also often inferred on the basis of erosive steps although the 3D geometry of such features is generally unclear as it is rare to be able to inspect large-scale (10-100s m scale) bedding plane exposures. The Cretaceous-Palaeocene Gottero Sandstone on Mount Ramaceto, NW Italy provides one such example. Here an overturned syncline provides a spectacular inverted bed base on which a shallow (up to 15 cm) but extensive (100s m2) scour field is preserved. This was formed beneath a flow that emplaced a 2.3 m thick sheet-like hybrid event bed that can be correlated laterally for up to four kilometres in what was a relatively distal setting.
The scour field comprises a composite erosional feature up to 150 m wide and at least 40 m long. It is made up of a terraced surface reflecting three levels of substrate erosion. The shallowest level is covered by small-scale grooves and flutes and is cut by larger elongate scours that coalesced to excavate the sea floor in patches down to a mid-level bed-parallel surface. The latter is then further incised by the deepest scours. Individual scours (40-150 cm across and 1-5m long) have distinctive asymmetric cross-sections with both inclined and undercut lateral margins. The undercut margins are associated with mudclast detachment.
The extent of erosion given the distal setting is curious and implies hybrid flow development could be triggered or enhanced by clay entrainment in relatively distal settings. Distal erosion may be promoted by enhanced turbulence during the early stages of flow transformation, as suggested by some experimental work (see Baas et al. 2011, Sedimentology 58 1953-1987). As scour coalescence results in mainly bedding parallel terraces, it can easily be overlooked in vertical sections
How sampling procedures and sedimentary processes of confined basins can modify the observed frequency distribution of turbidite thickness? : examples from Tertiary confined basins of central and northern Apennine (Italy)
No full textThe assessment of turbidite thickness statistics, along with variability of bed geometry, represents an open research question for both applied and pure sedimentology. Empirical frequency distributions of turbidite thickness reported in the scientific literature are diverse (e.g. truncated Gaussian, lognormal, exponential and power law) and variously interpreted as resulting from modification of the input signal (i.e. distribution of parent flow magnitude) by depositional/erosional processes. Yet biases toward or against certain thickness classes due to bed geometry, erosion and data collection are commonly overlooked, casting doubts on thickness statistics as a tool for answering some of the key research questions of turbidite sedimentology. However, in situations where turbidites are confined by basin topography and erosion is negligible, some of the variables of the ‘bed thickness equation’ can be relaxed, making easier to investigate what the primary controls on turbidite thickness statistics are.
This study reviews the bed thickness statistics of four confined to unconfined turbidite units (namely, the Castagnola Fm. and the Cengio-Bric la Croce-Castelnuovo turbidite systems of Tertiary Piedmont Basin and the Laga and Cellino Fms. of the Apennine Foreland Basin System) from the tertiary of Central-Northern Apennines (Italy), where bed geometry and sedimentary character have been previously assessed. Based on comparison of thickness subsets from turbidite systems with different degree of confinement and diverse locations within the basin fill, this contribution aims at answering the following questions: i) how data collection choices and field operational constraints (e.g. location, outcrop quality, use of thickness from single vs. multiple correlative sections, length of the stratigraphic section from which thicknesses were retrieved) can affect statistics of an empirical distribution of turbidite thicknesses so to induce biased interpretations? ii) how depositional controls of confined vs. unconfined basins can modify the initial thicknesses distribution of turbidites?; iii) is there in turbidite thickness statistics a ‘flow confinement’ signature which can be used to distinguish between confined and unconfined depositional settings?
Results indicate that: i) best practices of data collection are crucial to a meaningful interpretation of turbidite thickness data; ii) a systematic bias against cm-thick beds deposited by small volume low density flows exists, which modifies the thin-bedded tail of turbidite thickness distributions and, iii) the thick-bedded tail of thickness distributions of the case studies bears some relationship to the transition from confined and unconfined depositional settings, suggesting its statistics can represent a proxy for the degree of flow confinement
Lateral facies changes and architecture of hybrid-like beds deposited in a ponded minibasin (Castagnola Fm, NW Italy)
No full textHybrid-like beds (HEBs) are a class of gravity flow deposit including a basal sandy division followed by a chaotic division made up of both sand and clay in different proportions sometimes capped by an upper structured clean sandstone division. Although their existence has been documented nearly from the onset of the turbidite concept, HEBs have been the object of renewed attention in the last ten years, mainly as they can occur within hydrocarbon reservoirs. A range of generation mechanisms has been suggested, partly reflecting the wide variety of such deposits. Most models focus on the occurrence of rheological changes within the flow, with the chaotic mud-enriched division being deposited under transitional or laminar conditions. HEBs are thought to be more common at fan fringes or in close proximity to confining topography. However their depositional processes remain an object of on-going research, and thus our understanding of their character and distribution continues to evolve.
The Lower Miocene Castagnola Turbidite System (800-1000m thick) records the deep-water infill of a small (a few to tens of km2) ponded piggyback sub-basin in the eastern part of the Tertiary Piedmont Basin (NW Italy). Six detailed sedimentary logs taken along a 3.1 km proximal-to-distal transect encompass a low net-to-gross (0.2), 250m thick interval. Generally, bed types comprise either thin (<1m), highly structured deposits, or thicker (1-5m) beds. The latter are commonly hybrid-like, with a poorly structured lower sandy division, overlain by a recessive division frequently enriched in mudclasts and capped by a sandstone that is usually highly structured. Thanks to the tabular nature of the bedding, all of these beds can be confidently correlated along the entire transect. The relative proportion of mudclast-rich intervals is highly variable, changing from over 75% of the bed to less than 25% within some tens of meters. The hybrid character does not appear to vary consistently with distality or proximity to the confining slope but it commonly occurs throughout the basin. This areal pattern challenges current models of hybrid-like bed distribution and has implications for better understanding the character and distribution of hybrid-like beds in similar settings in the subsurface
Assessing turbidite depositional architecture and ponding through bed and facies statistics: an example from the Castagnola mini-basin (Early Miocene, NW Italy)
No full textIn turbidity current mechanics, ponding is the ability of topography to fully trap flow, thereby producing a flat topped suspension spreading over the whole basin. Conversely, spilling occurs when turbidity currents are able to partially run up the bounding topography so that part of their suspension cloud is subject to flow stripping. Among the many sedimentary signatures, thick, basin-wide mud caps have been since long recognized as a proxy for inferring flow ponding in confined basins. However, whether ponding or flow stripping dominate mostly depends on physical structure of incoming turbidity currents (i.e. density stratification, thickness, flow duration and discharge rate etc.) and height of bounding topography which may be time-variant due to changes in sedimentary input and basin topography. Examples of ponded turbidite systems are present in different geodynamic contexts (e.g. salt-withdrawal mini-basins of passive margins, structurally confined basins of rifted margins and foreland basin systems) both on outcrop and in the subsurface where they can constitute prolific target in hydrocarbon exploration. Fostered by the need of oil industry for hard data to be used in stochastic modeling, geoscience statistics have previously been applied to assess ponding in turbidites from statistical distribution of bed and facies thickness datasets either from outcrop or well log.
This study focuses on the sheet-like turbidite systems of the Castagnola Basin (Early Miocene), which were deposited in a small, structurally-confined depocentre of the Tertiary Piedmont Basin (Ligurian Alps, Northern Italy). Based on event bed thickness and facies statistics on a base-to-top sedimentary section logged in the field in great detail and estimation of basin size, this study aims at portraying the response of depositional architecture to progressive infilling and subsequent widening of the Castagnola Basin (from few km2 to few tens Km2). Data analysis shows an upward increase in net/gross, sandstone bed frequency and ratio of sandstone bed to mud cap thickness, along with an overall decrease in mud caps thickness. Yet, sandstone bed/mud cap thickness shows distinctive statistical distributions for different thickness classes which possibly relate to interplay of turbidity current physical structure and basin size.
Collectively, these results suggest switching of the Castagnola turbidite systems from a ponded- to a spill-dominated depositional setting in which stripping of the finer-grained fraction of turbidity currents becomes progressively more efficient as the host basin is filled up. Further development of this research may contribute to evolutionary models of fill and spill mini-basin and provide a wealth of quantitative data useful in stochastic and forward modeling of ponded turbidite systems
Influence of basin physiography upon the character and distribution of hybrid event beds
No full textConfinement of sedimentary gravity currents and the deep-water depositional systems they emplace by sea-floor topography is commonplace in a number of settings (e.g., salt mini-basins, intra cratonic basins, passive margins with gravity-driven tectonic systems). Distinct sedimentary gravity flow deposits, containing co-genetic matrix- mud-clastrich and matrix-poor sandstone, are recognised in a range of deep-water depositional systems, including topographically complex settings where gravity flows can be confined and or contained (ponded) by sea floor topography. In the latter settings HEBs can exhibit systematic development and variation in their depositional character with increasing proximity towards their confining topography. New research from the Castagnola Basin (Miocene, Northern Italy) and the Pennine Basin (Namurian, Northern England) has begun to highlight contrasts between sedimentary systems that are confined and contained (CC) and those which are confined but uncontained (CU). In CC settings HEBs are less likely to be localised to their topographic confinement and exhibit no systematic variation in depositional character in respect to palaeoflow direction or proximity towards the confining slope. Compared to CC settings, HEBs in CU settings are more likely to be localised to confining topography and exhibit systematic variation in depositional character over short length-scales; however systematic variation in depositional character can occur over longer length-scales where HEB extend further upstream and are not locally restricted to the confining basin margin. These contrasts are considered to arise due to flow containment which restricts flow expansion and promotes a number of factors. Specifically significant erosion of muddy substrate, high sedimentation rates and the extensive complex 3D flow dynamics following interaction with multiple basin margins.
This work highlights a range of boundary conditions which influence the character and distribution of HEBs, and thus that of depositional reservoir quality, in topographically complex settings. Awareness of these boundary conditions may provide insight when attempting to reconstruct basin geometries and evolutions using the character of their sedimentary infill
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