During the last few decades submarine mass wasting and associated mass-transport deposits (MTDs) ... more During the last few decades submarine mass wasting and associated mass-transport deposits (MTDs) have been well documented via high-resolution imaging of both seafloors and their subsurface, thereby recognizing their role in sediment transfer to deep water and seafloor reshaping. A number of failure and transport mechanisms are reported in literature, including block and debris slides, slump and debris flow or any combination thereof, reflecting rheology and progress of disaggregation of the materials initially involved in the failure. Despite recent advancement in geophysical imaging techniques, the internal heterogeneity of MTDs is still poorly detailed, limiting our understanding of their process sedimentology. This study investigates a number of MTDs from the Ventimiglia sub-basin of the Alpine foreland basin, NW Italy. The basin was predominantly filled by turbidites (Ventimiglia Flysch Fm., VF; Upper Eocene) sourced from the south and deposited on top of the Marnes Bleue Fm. marlstones (MB; Middle Eocene) within a N-S elongate depocentre. MTDs of resedimented MB occur at different levels and locations within the basin fill, suggesting they resulted from collapses of basinal slopes via submarine slide block detachment. The best exposed MTD is up to 80 m-thick and spreads over a minimum area of 30 km2. Main constituents include, from bottom to top: i) a deep basal surface cutting into the substrate; ii) a zone where turbidites are deformed and locally detached; iii) a chaotic megabreccia composed of undeformed MB blocks (1-10s metres in size) and rare folded turbidite beds and iv) a crudely graded conglomerate of MB, infilling the top irregularities of the underlying megabreccia and becoming dominant at MTD peripheries. Outsized MB blocks (up to 10s of metres thick, and 100s of metres in length) occur either in the megabreccia or in association with the conglomerate. Proximal to distal and axial to lateral, rapid transitions (hundreds of metres) are observed from a thick megabreccia deeply eroding the substrate, to rafted MB floating in the conglomerate and sitting on deformed turbidites. Typically the MTD top has a rugosity of few to few tens of metres over lengths of hundreds to few thousands of metres, hosting confined accommodation space to post-mass wasting turbidites. The most plausible architectural interpretation is one of a focused mass-flow axis, with an erosional keel flanked by zones of deformed turbidites, which served to focus late-stage debris flow, that eventually overspilled laterally, and may have facilitated long-range transport of megaclasts
ABSTRACTThe sedimentary architecture of channelized turbidites can be highly complex as it reflec... more ABSTRACTThe sedimentary architecture of channelized turbidites can be highly complex as it reflects the response of submarine channels to several interplaying factors. Although intensively investigated through seismic imaging, turbidite channel fills are not convincingly calibrated for sedimentary facies at a sub‐seismic scale. This contribution addresses the sedimentary architecture and the controls on the evolution of a ca 20 m thick channel‐levee complex of the Tachrift turbidite subunit (Upper Miocene, the Melloulou Formation), which accumulated along the southern slope of the Neogene Taza‐Guercif Basin (Rifian Corridor of north‐east Morocco). Facies and architectural analyses indicate that the studied channel‐levee complex is the result of three‐fold evolution. From base to top, it is comprised of: (i) a ca 7 m thick lower mud‐prone interval containing relatively small and vertically stacked channel fills with poorly developed muddy levees; (ii) a ca 4 m thick and >1 km wide...
Hybrid event beds (HEBs) are bi- or tripartite event beds with a lower clean sandstone division a... more Hybrid event beds (HEBs) are bi- or tripartite event beds with a lower clean sandstone division and an upper or central chaotic mud-rich division. Similarly to classic turbidites, the bed is usually overlain by a mudstone cap thought to represent deposition from a muddy cloud generated as part of the same event. In recent years research has been focussed on the character of the parent flow (turbulent, transitional or laminar) and on the origin of the mud deposited as part of the central chaotic and mud-rich division (updip vs. local mud acquisition). Ponded basins trap entire flow volumes and develop tabular bed geometries, making them effective natural laboratories to study the relationships between parent flows and the resulting event beds, as the thickness of the bed is thought to reflect the volume of the parent gravity flow. By contrasting the differences between turbidite and HEB bed thickness statistics in a ponded setting, this study aims to improve understanding of HEB depositional processes. The Castagnola Fm (Early Miocene, Tertiary Piedmont Basin, NW Italy) records the ponded to unconfined transition of a tectonically-controlled minibasin. Bed thickness analysis from turbidites allows recognition of a fully to partially ponded lower unit 650m thick. The ratio of sandstone thickness to event bed thickness (i.e. sandstone plus mudstone cap) for turbidites thicker than 30 cm ranges from 0.1 to 0.5 in the interval between 50 and 450m from the lower observed depocentre. Mudclast-rich hybrid event beds constitute around 10% of the basin fill thickness. Their bed thickness analysis reveals a number of key differences with turbidites: 1) the distribution of thicknesses for individual event beds is enriched in events between 1 and 3 metres thick and is depleted in thinner events; 2) the ratio of the sandstone thickness to event bed thickness is higher (e.g. 0.5 to 0.7 in the 50-450m interval). We infer that these differences stem from a lower parent flow magnitude threshold on HEB formation and incorporation of eroded muddy substrate and turbiditic mud into the central mud-rich division of HEBs, hence increasing the final sandstone bed thickness and decreasing that of its mudstone cap. The reported findings help improve our understanding of HEBs depositional processes (e.g., the loci of mud acquisition and deposition) and associated architectures; they can be applied to inform and interpret the results of bed thickness analysis in outcrop, core or wireline in HEB-prone confined basins
Flow direction in turbidites is related to hydrocarbon reservoir position; Lattice Preferred Orie... more Flow direction in turbidites is related to hydrocarbon reservoir position; Lattice Preferred Orientation of mica and quartz is known to be related to flow direction of turbidite, though quantitative data are still poorly known. In order to evaluate the Lattice Preferred Orientation of rock-forming mineral of sandstone samples we used the non-disruptive method of lattice preferred orientation by neutron diffraction texture analysis, or Quantitative Texture Analysis (QTA), successfully applied on various types of rocks, from simple monomineralic quartzites, marbles, limestones, dunite or glaucophanite to more complex poly-phase rocks, as amphibolite, quartz-feldspatic mylonitic orthogneiss, subacqueous lavas or sandstones. Results from experiment 1_02_202 show that chlorite, white mica and quartz LPO (Fig. 1) are closely and strongly related with flow directions which have been measured in the field. Interpretation Figure 1 List of pole figures for any sample. the attention is focused...
Confined depocentres (such as salt withdrawal minibasin or structurally confined basins in active... more Confined depocentres (such as salt withdrawal minibasin or structurally confined basins in active settings) represent important depositional sinks for deep-water sediments. Most classic models of turbidite bed statistics and onlap geometries are underpinned by the assumption that as a confined basin fills, the transition from a fully ponded to a non-confined condition is represented by only a relatively minor proportion of the basin fill. Although tectonic deformation can provide a mechanism to increase the proportion of the fill that records partially ponded conditions, i.e., when basin subsidence and sedimentation rates are similar, this condition might not be very common. However, even in a structurally quiescent basin, flows of different volumes will have different abilities to escape the confining topography. Hence, the range of the flow volume distribution will define the minimum duration of the situation when at least some flows are partially ponded. Moreover, aggradation on the basin margins/lips can occur while deposition in the depocentre takes place. This is probably most commonly occurring under partially ponded conditions (when the lower concentration and less erosive part of the flows will be able to escape the confinement and deposit material on the basin sill). This process will increase the thickness of basin fill deposited under partially ponded conditions. The Castagnola sub-basin (Early Miocene, Tertiary Piedmont Basin, NW Italy) is a small (10-20 km2) minibasin filled by c. 1 km of southerly-sourced siliciclastic turbidites; there is no evidence of significant tectonic deformation during deposition. The northern (fully) and southern (partially) basin margins are preserved, allowing different onlap geometries (feathered vs abrupt) to be recognised. The succession is tabular at the km scale, with erosion limited to a small number of thicker beds; net-to-gross evolves from 0.2 at the base to 0.7 toward the top. Thick mud caps in the lower section, along with presence of reflected ripples and overall sheet-like architecture of the turbidite system are suggestive of ponded deposition. While the gross evolution of the basin is that of a transition from dominantly ponded to non-ponded conditions, marked by the disappearance of thick mud caps and sandstone beds with reflected facies, a significant thickness of the stratigraphy (c. 400 m) shows less dramatic but still resolvable trends of net to gross increase and mud cap to sandstone bed thickness decrease, interpreted as a signal of long-lasting partially ponded conditions. The high proportion of the stratigraphy laid down under partially ponded conditions has implication for classic fill-and-spill models, turbidite beds pinch-outs and onalp geometries 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 an advance in establishing predictive models for confined basins
During the last few decades submarine mass wasting and associated mass-transport deposits (MTDs) ... more During the last few decades submarine mass wasting and associated mass-transport deposits (MTDs) have been well documented via high-resolution imaging of both seafloors and their subsurface, thereby recognizing their role in sediment transfer to deep water and seafloor reshaping. A number of failure and transport mechanisms are reported in literature, including block and debris slides, slump and debris flow or any combination thereof, reflecting rheology and progress of disaggregation of the materials initially involved in the failure. Despite recent advancement in geophysical imaging techniques, the internal heterogeneity of MTDs is still poorly detailed, limiting our understanding of their process sedimentology. This study investigates a number of MTDs from the Ventimiglia sub-basin of the Alpine foreland basin, NW Italy. The basin was predominantly filled by turbidites (Ventimiglia Flysch Fm., VF; Upper Eocene) sourced from the south and deposited on top of the Marnes Bleue Fm. marlstones (MB; Middle Eocene) within a N-S elongate depocentre. MTDs of resedimented MB occur at different levels and locations within the basin fill, suggesting they resulted from collapses of basinal slopes via submarine slide block detachment. The best exposed MTD is up to 80 m-thick and spreads over a minimum area of 30 km2. Main constituents include, from bottom to top: i) a deep basal surface cutting into the substrate; ii) a zone where turbidites are deformed and locally detached; iii) a chaotic megabreccia composed of undeformed MB blocks (1-10s metres in size) and rare folded turbidite beds and iv) a crudely graded conglomerate of MB, infilling the top irregularities of the underlying megabreccia and becoming dominant at MTD peripheries. Outsized MB blocks (up to 10s of metres thick, and 100s of metres in length) occur either in the megabreccia or in association with the conglomerate. Proximal to distal and axial to lateral, rapid transitions (hundreds of metres) are observed from a thick megabreccia deeply eroding the substrate, to rafted MB floating in the conglomerate and sitting on deformed turbidites. Typically the MTD top has a rugosity of few to few tens of metres over lengths of hundreds to few thousands of metres, hosting confined accommodation space to post-mass wasting turbidites. The most plausible architectural interpretation is one of a focused mass-flow axis, with an erosional keel flanked by zones of deformed turbidites, which served to focus late-stage debris flow, that eventually overspilled laterally, and may have facilitated long-range transport of megaclasts
ABSTRACTThe sedimentary architecture of channelized turbidites can be highly complex as it reflec... more ABSTRACTThe sedimentary architecture of channelized turbidites can be highly complex as it reflects the response of submarine channels to several interplaying factors. Although intensively investigated through seismic imaging, turbidite channel fills are not convincingly calibrated for sedimentary facies at a sub‐seismic scale. This contribution addresses the sedimentary architecture and the controls on the evolution of a ca 20 m thick channel‐levee complex of the Tachrift turbidite subunit (Upper Miocene, the Melloulou Formation), which accumulated along the southern slope of the Neogene Taza‐Guercif Basin (Rifian Corridor of north‐east Morocco). Facies and architectural analyses indicate that the studied channel‐levee complex is the result of three‐fold evolution. From base to top, it is comprised of: (i) a ca 7 m thick lower mud‐prone interval containing relatively small and vertically stacked channel fills with poorly developed muddy levees; (ii) a ca 4 m thick and >1 km wide...
Hybrid event beds (HEBs) are bi- or tripartite event beds with a lower clean sandstone division a... more Hybrid event beds (HEBs) are bi- or tripartite event beds with a lower clean sandstone division and an upper or central chaotic mud-rich division. Similarly to classic turbidites, the bed is usually overlain by a mudstone cap thought to represent deposition from a muddy cloud generated as part of the same event. In recent years research has been focussed on the character of the parent flow (turbulent, transitional or laminar) and on the origin of the mud deposited as part of the central chaotic and mud-rich division (updip vs. local mud acquisition). Ponded basins trap entire flow volumes and develop tabular bed geometries, making them effective natural laboratories to study the relationships between parent flows and the resulting event beds, as the thickness of the bed is thought to reflect the volume of the parent gravity flow. By contrasting the differences between turbidite and HEB bed thickness statistics in a ponded setting, this study aims to improve understanding of HEB depositional processes. The Castagnola Fm (Early Miocene, Tertiary Piedmont Basin, NW Italy) records the ponded to unconfined transition of a tectonically-controlled minibasin. Bed thickness analysis from turbidites allows recognition of a fully to partially ponded lower unit 650m thick. The ratio of sandstone thickness to event bed thickness (i.e. sandstone plus mudstone cap) for turbidites thicker than 30 cm ranges from 0.1 to 0.5 in the interval between 50 and 450m from the lower observed depocentre. Mudclast-rich hybrid event beds constitute around 10% of the basin fill thickness. Their bed thickness analysis reveals a number of key differences with turbidites: 1) the distribution of thicknesses for individual event beds is enriched in events between 1 and 3 metres thick and is depleted in thinner events; 2) the ratio of the sandstone thickness to event bed thickness is higher (e.g. 0.5 to 0.7 in the 50-450m interval). We infer that these differences stem from a lower parent flow magnitude threshold on HEB formation and incorporation of eroded muddy substrate and turbiditic mud into the central mud-rich division of HEBs, hence increasing the final sandstone bed thickness and decreasing that of its mudstone cap. The reported findings help improve our understanding of HEBs depositional processes (e.g., the loci of mud acquisition and deposition) and associated architectures; they can be applied to inform and interpret the results of bed thickness analysis in outcrop, core or wireline in HEB-prone confined basins
Flow direction in turbidites is related to hydrocarbon reservoir position; Lattice Preferred Orie... more Flow direction in turbidites is related to hydrocarbon reservoir position; Lattice Preferred Orientation of mica and quartz is known to be related to flow direction of turbidite, though quantitative data are still poorly known. In order to evaluate the Lattice Preferred Orientation of rock-forming mineral of sandstone samples we used the non-disruptive method of lattice preferred orientation by neutron diffraction texture analysis, or Quantitative Texture Analysis (QTA), successfully applied on various types of rocks, from simple monomineralic quartzites, marbles, limestones, dunite or glaucophanite to more complex poly-phase rocks, as amphibolite, quartz-feldspatic mylonitic orthogneiss, subacqueous lavas or sandstones. Results from experiment 1_02_202 show that chlorite, white mica and quartz LPO (Fig. 1) are closely and strongly related with flow directions which have been measured in the field. Interpretation Figure 1 List of pole figures for any sample. the attention is focused...
Confined depocentres (such as salt withdrawal minibasin or structurally confined basins in active... more Confined depocentres (such as salt withdrawal minibasin or structurally confined basins in active settings) represent important depositional sinks for deep-water sediments. Most classic models of turbidite bed statistics and onlap geometries are underpinned by the assumption that as a confined basin fills, the transition from a fully ponded to a non-confined condition is represented by only a relatively minor proportion of the basin fill. Although tectonic deformation can provide a mechanism to increase the proportion of the fill that records partially ponded conditions, i.e., when basin subsidence and sedimentation rates are similar, this condition might not be very common. However, even in a structurally quiescent basin, flows of different volumes will have different abilities to escape the confining topography. Hence, the range of the flow volume distribution will define the minimum duration of the situation when at least some flows are partially ponded. Moreover, aggradation on the basin margins/lips can occur while deposition in the depocentre takes place. This is probably most commonly occurring under partially ponded conditions (when the lower concentration and less erosive part of the flows will be able to escape the confinement and deposit material on the basin sill). This process will increase the thickness of basin fill deposited under partially ponded conditions. The Castagnola sub-basin (Early Miocene, Tertiary Piedmont Basin, NW Italy) is a small (10-20 km2) minibasin filled by c. 1 km of southerly-sourced siliciclastic turbidites; there is no evidence of significant tectonic deformation during deposition. The northern (fully) and southern (partially) basin margins are preserved, allowing different onlap geometries (feathered vs abrupt) to be recognised. The succession is tabular at the km scale, with erosion limited to a small number of thicker beds; net-to-gross evolves from 0.2 at the base to 0.7 toward the top. Thick mud caps in the lower section, along with presence of reflected ripples and overall sheet-like architecture of the turbidite system are suggestive of ponded deposition. While the gross evolution of the basin is that of a transition from dominantly ponded to non-ponded conditions, marked by the disappearance of thick mud caps and sandstone beds with reflected facies, a significant thickness of the stratigraphy (c. 400 m) shows less dramatic but still resolvable trends of net to gross increase and mud cap to sandstone bed thickness decrease, interpreted as a signal of long-lasting partially ponded conditions. The high proportion of the stratigraphy laid down under partially ponded conditions has implication for classic fill-and-spill models, turbidite beds pinch-outs and onalp geometries 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 an advance in establishing predictive models for confined basins
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