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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
Hybrid event beds in the Miocene Cilento flysch and Cretaceous-Palaeocene Gottero fan : insights into short-length lateral facies variability in event bed make up
No full textCoalesced 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
Towards 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
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
Mud clast character and distribution in hybrid beds deposited in a ponded minibasin (Castagnola Fm, NW Italy)
No full textCryptic delamination beneath distal hybrid event beds: evidence for local substrate entrainment
No full textHybrid event beds (HEBS) are increasingly recognised as an important component of the stratigraphy of many deepsea fan and sheet systems. Previous models have attributed their origin to vertical and/or longitudinal segregation of co-genetic turbidite and debritic flows in a downcurrent direction, triggered by mud acquisition in up-dip channels or channel-lobe transition zones.
Sedimentological analysis of the deep-water Cretaceous-Paleocene Gottero Sandstone (NW Apennines, Italy) has revealed large substrate delamination features at the bases of mudclast-rich HEBs in the distal sector of the basin. The delamination features are generally bedding-parallel and can be shallow (15–20 cm deep and few 10s of meters wide) or deep (c. 1–2 m deep and several hundred meters wide). Such features can remain completely undetected without a careful sedimentological analysis and bed by bed correlations, and the minimum amount of stratigraphy removed can be estimated only in a few cases where at least part of the original stratigraphic succession is preserved laterally. The deepest features are overlain by thick hybrid event beds containing large rafts derived from the underlying substrate and composed of mudstone and intervening thin sandstone beds, passing rapidly along both downstream and lateral facies tracts into chaotic or mud-clast-rich HEB debrites. Similar bed types are present in other deep-water systems (Castagnola, Marnoso Arenacea, Ventimiglia flysch, Ross Formation), usually associated with more conventional HEB types. However, their significance and association with substrate delamination has not been demonstrated before. The field observations suggest that in the Gottero system incorporation of muddy substrate occurred when dense sandy flows were able to extend sand injections into the shallow substrate and detach large slabs, carrying them for short distances before they broke up due to sand injection and internal shearing. The substrate entrainment was therefore not due to turbulent processes that could not have picked up and carried such large rafts. Remnants of undetached sea floor ‘flaps’ are only occasionally preserved. Similar process at smaller scale may have accompanied the widening of tiered down-cutting surfaces seen elsewhere in the Gottero and these may have contributed smaller mudclasts to down-dip HEBs. An angular gradient change between the proximal fan-lobe area and the distal basin plain sectors, possibly accentuated by distal basin confinement, could have promoted substrate delamination and the development of raft-bearing and mudclast-rich hybrid event beds in this and other basins
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