RAPID RETREAT OF THE SOUTHWESTERN LAURENTIDE ICE-SHEET DURING THE BØLLING-ALLERØD INTERVAL
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Fluvial fans represent one of the dominant sedimentary systems at the active margins of non-marine foreland basins. The Puig-reig anticline at the north-eastern margin of the Ebro Foreland Basin (SE Pyrenees, Spain) exposes continuous outcrops of Late Eocene-Early Oligocene fluvial deposits, from proximal to medial fluvial fan environments. The proximal deposits are found in the north limb of the anticline, especially in the northwest zone. These deposits are characterised by conglomerates with minor interbedded sandstones, with thick and wide sheet-like geometries with unscoured or variably scoured basal surfaces. These are interpreted to be the deposits of unconfined flash floods and wide-shallow channel streams. The medial deposits, covering the rest of the anticline, consist of interbedded conglomerates, sandstones and claystones. These are interpreted to have been deposited from braided to meandering channel streams and overbank areas. Distal deposits are found towards the south, beyond the anticline, and are characterised by sandstone and clay deposits of terminal lobes and lacustrine deltas. This study assesses the impact of the primary depositional characteristics, diagenesis and deformation of the most heterolithic portion of the system, with implications for increasing our understanding of folded fluvial reservoirs. Diagenetic processes, mainly mechanical compaction and calcite cementation, resulted in overall low intergranular porosity, with limited relatively high porosity developed in sandstone lithofacies in the medial deposits. Deformation associated with thrusting and fold growth resulted in the formation of abundant fractures, with relatively high fracture intensities observed in sandstone lithofacies in the anticline crest. This study shows that post-depositional processes can both improve and diminish the reservoir potential of basin proximal fluvial deposits, through the development of fracture networks and by compaction-cementation. The comparison of the Puig-reig anticline with other similar settings worldwide indicates that foreland basin margin locations may be potential areas for effective reservoirs, even in the case of low intergranular porosity.
Anticline
Arenite
Syncline
Outcrop
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Metamorphic core complex
Lineation
Massif
Detachment fault
Transpression
Mylonite
Extensional tectonics
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Pennsylvanian foreland deformation associated with the Ouachita orogene reactivated a west-northwest-east-southeast Cambrian basement trend, the southern Oklahoma aulacogen, to form the Wichita uplift, southwest Oklahoma. The 30-km-wide subsurface Frontal fault zone separates the uplift from the Anadarko basin to the north. Horizontal shortening across this fault zone is estimated at 7-15 km (20-40%), vertical displacement totals 9-10 km from the uplift to the basin. Folds are mapped on an interformational scale within the Frontal fault zone, and on an intraformational scale (Cambro-Ordovician Arbuckle Group) in the Slick Hills, southwest Oklahoma. Additional shortening occurred along southwest dipping mountain flank thrusts and on bedding plane thrusts, respectively. Hanging wall blocks of major faults contain the shallow dipping limb and anticlinal hinge zone of the interformational scale folds. Oil and gas production is generally restricted to these anticlinal crests within Paleozoic rocks. Deep wells (> 6000 m) that have penetrated footwall imbricates of the mountain flank thrusts have drilled through steep-overturned beds and tight recumbent folds before passing through faults into a normal stratigraphic sequence. Basement thrust loading of the southern margin of the Anadarko basin controlled the trend (west-northwest-east-southeast) of the axis of maximum deposition within the basin during the Pennsylvanian.
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Thrust fault
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Bedrock
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Accretionary wedge
Anticline
Forearc
Mass wasting
Thrust fault
Seafloor Spreading
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Abstract Emplacement of submarine landslides, or mass‐transport deposits, can radically reshape the physiography of continental margins, and strongly influence subsequent sedimentary processes and dispersal patterns. Typically, progressive healing of the complicated relief generated by the submarine landslide occurs prior to progradation of sedimentary systems. However, subsurface and seabed examples show that submarine channels can incise directly into submarine landslides. Here, the evolution of a unique exhumed example of two adjacent, and partially contemporaneous, submarine channel‐fills is documented. The channels show deep incision (>75 m), and steep lateral margins (up to 70°), cut into a >200 m thick submarine landslide. The stepped basal erosion surface, and multiple terrace surfaces, are mantled by clasts (gravels to cobbles) reflecting periods of bedload‐derived sedimentation, punctuated by phases of downcutting and sediment bypass. The formation of multiple terrace surfaces in a low aspect ratio confinement is consistent with the episodic migration of knickpoints during entrenchment on the dip slope of the underlying submarine landslide. Overlying sandstone‐rich channel‐fills mark a change to aggradation. Laterally stacked channel bodies coincide with steps in the original large‐scale erosion surface, recording widening of the conduit; this is followed by tabular, highly aggradational fill. The upper fill, above a younger erosional surface, shows an abrupt change to partially confined tabular sandstones with normally graded caps, interpreted as lobe fringe deposits, which formed due to down‐dip confinement, followed by prograding lobe deposits. Overlying this, an up‐dip avulsion induced lobe switching and back‐stepping, and subsequent failure of a sandstone body up‐dip led to emplacement of a sandstone‐rich submarine landslide within the conduit. Collectively, this outcrop represents episodic knickpoint‐generated incision, and later infill, of a slope adjusting to equilibrium. The depositional signature of knickpoints is very different from existing models, but is probably reflective of other highly erosional settings undergoing large‐scale slope adjustment.
Submarine landslide
Aggradation
Turbidity current
Mass wasting
Progradation
Passive margin
Terrace (agriculture)
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Sequence (biology)
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The Oak Ridges Moraine in southern Ontario is a poly- genetic moraine constructed of a number of coalesced deposits of gla- cifluvial and glacilacustrine origin. A detailed study of the facies ar- chitecture has been completed on a series of pit sections extending ; 300 m subparallel to the paleoflow direction. Eight major lithofacies and five facies associations have been described. These data have been interpreted to be upper-flow-regime hyperconcentrated-flood-flow de- posits emplaced under a regime of rapid flow expansion and loss of transport capacity within a plane-wall jet with an associated hydraulic jump. Deposition from the plane-wall jet with jump occurred in three zones of flow transformation: zone of flow establishment, transition zone, and zone of established flow. Massive gravels with unconsolidated sand intraclasts and open-work gravel / gravel-sand couplets were de- posited in the zone of flow establishment by hyperconcentrated and supercritical flows, respectively. Immediately downflow low-angle cross-stratified sand incised by steep-walled scours infilled by diffusely graded sand define the transition zone, the zone of maximum vortex erosion, and the distal limit of deposits emplaced under upper-flow- regime conditions. These strata record rapid bed aggradation from sediment-laden supercritical flows that episodically were scoured by large vortices generated within migrating hydraulic jumps. Strati- graphically upward and downflow strata consist only of lower-flow- regime sedimentary structures. Medium-scale, planar cross-strata and small-scale cross-lamination related to migrating 2-D dunes and cur- rent ripples, respectively, characterize the zone of established flow. The facies and sediment architecture suggest that this fan was deposited during a relatively short period of time (days, weeks) by energetic sed- iment-laden floods.
Hyperconcentrated flow
Hydraulic jump
Debris flow
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Caldera
Alluvial fan
Bedrock
Aggradation
Landform
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Outwash plain
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