Slump Folds Research Articles

Discussion on the North Sea Silverpit Crater: impact structure or pull-apart basin?

Ken Thomson writes: The suggestion that the Silverpit Crater is a pull-apart basin (Smith 2004) and not an impact structure (Stewart & Allen 2002) demonstrates that its origin requires further investigation. As noted by Underhill (2004) the coincidence of the structure with a salt withdrawal basin suggests that halokinesis may provide an alternative mechanism. Although the model provided by Smith involves salt movement it is overly complex as strike-slip detachment tectonics are not required. Instead gravitational collapse of the post-salt cover provides a simpler mechanism. The Southern North Sea is dominated by the development of large linear salt swells (Jenyon 1988; Stewart & Coward 1995). Elevation and tilting of the swell flanks results in gravitational instability and a range of deformation styles can develop (Jenyon 1988; Stewart & Coward 1995; Thomson 2004). The instability commonly leads to listric faulting of the post-salt cover down slope, towards the adjacent salt low (Jenyon 1988; Stewart & Coward 1995; Thomson 2004). Detachment levels for the listric faults occur at a number of stratigraphic levels. The top of the Upper Permian Zechstein Group salt, which forms the swells, provides a detachment at the base of the post-salt cover whilst detachment within the post-salt cover in both the Triassic Rot Halite and within the Upper Cretaceous chalk have also been recognized (Jenyon 1988; Stewart & Coward 1995; Thomson 2004). The majority of the flank collapse faults dip in the same direction as the cover rocks they displace (i.e. towards the salt withdrawal basin and away from the swell), but occasionally listric faults can be found which dip away from the salt withdrawal basin, towards the swell (Jenyon 1988). Associated with the listric faulting rollover anticlines can develop in the hanging walls (Stewart & Coward 1995) whilst slump folds …

Facies and evaporite genesis of the Kuşcular Formation (Lower Paleocene) saline playa complex, Eastern Turkey

The Kuşcular Formation (Lower Paleocene), located in Eastern Turkey, is an alluvial and playa foreland basin succession deposited in a semi-arid climate. The playa deposits of the Kuşcular Formation, composed of interbedded green sandstone–siltstone–mudrock, gypsiferous sandstone, red-green gypsiferous mudrock, and banded nodular secondary gypsum, are found among the distal alluvial fan deposits. Interbedded green sandstone–siltstone–mudrock with desiccation cracks are interpreted as dry mudflat–sandflat deposits. The banded nodular secondary gypsum and gypsiferous mudrocks, which contain desiccation cracks, are interpreted as a saline playa and saline mudflat. Distal alluvial fan deposits are represented by stratified conglomerate and stratified pebbly sandstone. The sulfur in the evaporites was very likely derived from the oxidation of sulfides in Upper Cretaceous magmatic rocks comprising the basement of the basin. Soft sediment deformation structures (slump folds), which indicate seismically active areas, were formed within the playa and dry mudflad–sandflat complex. The facies aspects of playa deposits would suggest that the climate in Turkey was semi-arid during the Early Paleocene.

Stratigraphy, paleoecology, and origin of Lower Devonian (Emsian) carbonate mud buildups, Hamar Laghdad, eastern Anti-Atlas, Morocco, Africa

ABSTRACT Recent debates concerning the origin of Emsian carbonate mud buildups exposed at Hamar Laghdad in the eastern Anti-Atlas of Morocco centre around geochemical studies that suggest authigenic or chemosynthetic processes induced precipitation of mud from hydrothermal fluids. Geochemical data alone cannot be used to demonstrate conclusively the origin of the mud comprising the buildups, but an in situ source and early cementation can be inferred from morphology, internal architecture, and sedimentary structures. Morphological types vary from symmetrical mounds up to 12 m high to asymmetrical pinnacles that locally coalesced, forming elongate multicored complexes up to 56 m high. Their steep-sided nature (margins dipping up to 54°) and sporadic occurrence of erect invertebrates suggest localized production of mud rather than hydrodynamic piling or baffling from suspension. Irregular nodular beds that thicken internally imply centralized production and a local source for the mud. Displaced nodules and slump folds indicate early cementation of mud to at least a firm consistency, followed by downslope movement. Abrupt termination of carbonate production during late transgression resulted in drowning of the mud buildups and adjacent platform. Except for a thin condensed bed deposited during maximum flooding, the buildups and platform (transgressive systems tract deposits) remained uncovered for an extended period (upper inversus-laticostatus to lower serotinus zones) before being buried beneath progradational clinoforms of nodular limestone and marlstone (regressive systems tract deposits). Overlying Middle Devonian carbonates contain evidence of hydrothermal seepage, based on the occurrence of solemyid and anomalodesmatan bivalves, which are thought to be chemosymbiotic. These bivalves occur in a mud buildup rooted on an Emsian counterpart, suggesting periodic hydrothermal activity beneath the buildups. A hydrothermal origin for the Hamar Laghdad mud buildups appears to be consistent with other mud buildups of similar age in other parts of northwest Africa. Vast deposits of coeval Minette-type ooidal ironstones, which also have a seep origin in the Tindouf Basin, south of the Anti-Atlas, testify to occasional widespread hydrothermal activity along the northwest margin of Gondwana during much of the Devonian. End_Page 217------------------------

Slump‐initiated and controlled syndepositional sandstone remobilization: an example from the Namurian of County Clare, Ireland

Remobilization of sandstones can dramatically reconfigure original depositional geometries and results in very unusually shaped sandstones, which resemble little, if any, of the original geometry. A number of deformational sandstone bodies, dykes and volcanoes from the upper part of the Carboniferous Ross Formation are described, which offer the opportunity to examine a suite of field‐scale reconfigured sandstones. These structures are located in close proximity to the Ross Slide, which outcrops along a 2‐km section on the northern coast of the Loop Head Peninsula, County Clare, Ireland. Dome‐ and ridge‐shaped deformational sandstone bodies, dykes and volcanoes are interpreted to be the product of remobilization of a turbiditic sandstone. Liquification and remobilization were triggered by translation, cessation and loading of the underlying turbiditic sandstone by the Ross Slide. Deformational sand body, dyke and volcano development occurred in an asynchronous fashion with deformational sand bodies formed during slump translation. Sand dykes and volcanoes developed after the cessation of slump movement. During slump translation, the minimum principal stress (σ3) was orientated vertically and the slump behaved in a `ductile' manner. After slump arrest, the minimum principal stress was oriented horizontally, and the unit regained shear strength to behave in a `brittle' manner. The relative change in rheological states with changing applied shear stress is indicative of thixotropic‐like behaviour within the slump mass. Ridge‐shaped deformational sand bodies are aligned parallel to slump folds, and their morphology is inferred to be controlled by compressional slump deformation associated with heterogeneous cessation of slump movement that was initiated by frontal arrest of the translating mass.

Geodynamical interpretation of a major unconformity in the Taconic Foredeep: slide scar or onlap unconformity?

The Middle Ordovician ribbon limestones of the Dolgeville Formation (Trenton Group) and the overlying black shales of the Utica Group were deposited as the Laurentian continental plate impinged upon a Taconic trench. A prominent unconformity separates the two units in the central Mohawk Valley region (the “Thruway unconformity”), and this unconformity locally bevels the crests of soft-sediment folds. The Thruway unconformity has been described both as a locally developed feature and as a major boundary underlying a transgressive systems tract. Paleoflow measurements in the central Mohawk Valley region demonstrate that in the Dolgeville Formation the paleoflow was not directed easterly; rather, it was directed primarily southwesterly, parallel to syndepositional faults. Paleoflow directions in the overlying Utica Group remain the same as those in the Dolgeville Formation in locations removed from the syndepositional faults, but change direction at several sites close to faults. These prominent changes in paleoflow direction can be ascribed to (1) fault block rotation that redirected the regional paleoflow, and (2) fault scarps that redirected the regional paleoflow. Slump folds occur immediately below the unconformity at most sites in the central Mohawk Valley region. The different slump trajectories across the region and the different relative timing of the slumps suggest that they represent several discrete slumps. Regional chronostratigraphy also demonstrates that the hiatus at the unconformity differs in duration among the fault blocks. That the unconformity represents different discrete time intervals at different sites, and that at most sites slump folding lies directly beneath the unconformity, strongly suggests that the unconformity in the central part of the Mohawk Valley first developed as a series of slide (slump) scars, rather than as a direct response to changes in the eustatic sea level. The sediment slides (or slumps) probably developed in response to changes in slope from fault block rotations and to increased seismic activity along the nearby faults. These effects were, in turn, results of the plate bending as it entered the trench. The local reversal in fault motion, as documented by growth fault geometries and inferred paleoslopes, as well as the ubiquity of slumps, suggests that the Thruway unconformity records a time of unusual fault activity. Such activity may have been related to a changing stress field in response to plate motion (e.g., changing convergence rates or plate location relative to the peripheral bulge).

A qualitative assessment of seismic risk along the Peninsular coast of India, south of 19°N

Many earthquakes have been recorded from the coastal margin of the Indian peninsular shield during the last 200 years. Largely made up of Precambrian assemblages with variable cover of Jurassic to Quaternary sedimentary rocks and Cretaceous-Eocene volcanics, the peninsular shield was long held to be aseismic. Recent measurements, however, show that this continental fragment is being pushed northeastward by the Carlsberg and Central Indian ridges; and the Indo-Myanmar subduction zone is exerting vigorous slab pull towards the east. Repeated cycles of sea level change during the Quaternary have also induced continuing hydro-isostatic adjustment due to variable melt water loading in the Bay of Bengal and the Arabian sea. All these forces produce space-time fluctuations of strain around many small to large faults, which occur in the upper crust of the shield. Some of the faults have been intermittently active (during the past 100 kyr); others were active earlier. Although the Shillong plateau and the associated hill ranges of northeastern India and Myanmar are subject to the maximum seismic hazard, the peninsular coast is also vulnerable to intermittent seismicity. We present illustrative evidence of some active faults, which are recognisable (a) on coastal land by displaced Pleistocene weathered cover, hot springs, leakages of native mercury and allochthonous geochemical anomalies of base metals and (b) offshore below the inner shelf by horst-shaped uplifted segments and intra-formational slump folds on and below the top shallow seismic (3.5 kHz) reflector. On the other hand, there are long stretches of the east coast at Vishakhapatnam and Manappad Point, which do not show active faults. Step-like marine terraces, which occur up to+6 m above the low tide level (LTL) preserve records of relative sea level fluctuations during the Holocene and the Last Interglacial. In such sectors, absence of tectonic disturbance during the last 100 ka is also corroborated by lateral continuity of shallow seismic reflectors below the inner shelf over many kilometers. Since authentic historical (200–1000 years B.P.) records of seismicity along the Peninsular coast are virtually unavailable, the likely recurrence interval between earthquakes in each sector cannot be gauged. We, therefore, propose a scale of seismic risk, based on geometry of the mappable faults and available seismic records of the last two centuries. These could be used in combination to rank the densely populated coastal tracts sector-wise.

Large-scale slumping deduced from structural and sedimentary features in the Lower Palaeozoic Anglo-Brabant fold belt, Belgium

Distinguishing slump folds from tectonic folds in poorly exposed areas can be difficult, especially when the scale of the slump folds exceeds outcrop scale. In the southeastern part of the single-phase deformed, Lower Palaeozoic Anglo-Brabant fold belt a comparison of cleavage/fold relationships and stratigraphic polarity shows that a 200 m thick interval of middle Caradoc fine-grained turbidites in the core of a large synform was overturned prior to tectonic deformation. This overturning is attributed to large-scale slumping, which was most likely a result of middle Caradoc seismic activity. The exposed portion of the large slump sheet contains only a few small slump folds and intraformational breccias, making up less than 5% of the exposed thickness. If the beds were not overturned, large-scale slumping would never be suspected and the small slump folds would probably be interpreted as localized features in an overall ‘stable’ sedimentary pile. This may explain why so few ancient large-scale slides and slumps have been reported: the small amount of internal deformation makes them very difficult to recognize, especially when dealing with poorly exposed areas. As such, large ancient slides and slumps may be more common than suggested by the geological literature.

Overburden deformation patterns and mechanisms of salt diapir penetration in the Central Graben, North Sea

Active and passive diapirism control the deformation and geometry of hydrocarbon traps in the overburden, and a more detailed understanding of this process will help reservoir prediction and hydrocarbon recovery. Cores studies of seven Central Graben diapirs indicate Zechstein salt penetrated Late Cretaceous chalk by extreme tectonic thinning with high-angle (>70° to bedding) normal faulting, tensile fracturing and pressure solution. Attenuation of the chalk significantly weakens the overburden, allowing buoyancy forces to dome up the overburden. Doming created enough topography for downslope sliding of chalk slabs on slip planes parallel to bedding or, in the case of the Kyle diapir, for chaotic debris flows of lithified chalk. Significant extensional bedding-parallel faults and slump folds are developed within Palaeocene shale on the diapiric flanks. Inter-granular slip in unconsolidated clastic material was probably the dominant deformation mechanism. Diapirs have penetrated the Palaeocene clastic sediments by maintaining topographic relief, so that unlithified sediment continually slid off the crest, producing translated intact rafts up to several tens of metres in thickness.

Flow pattern within a Permian submarine slump recorded by oblique folds and deformed fossils, Ulladulla, south‐eastern Australia

The flow pattern within a slump in Permian marine rocks of the southern Sydney Basin, Australia, is recorded by folds and deformed fossils. Abundant brachiopod and bryzoan fossils in the slumped rocks are relatively undeformed, but fossil crinoid stems have been deformed by relative rotation of individual ossicles. Measurement of the strain indicates that the deformation of the crinoids is consistent with flexural flow folding within the slump. Previous models assume that curved slump fold axes remain parallel to the enveloping bedding surface of a slump sheet. Detailed measurements of the orientation of slump folds in this study found fold axes to be oblique to bedding, which is interpreted as a result of folds plunging downward towards the flanks of the slump or slump lobes. In the present model, fold axes are not generally parallel to the strike of the fold axial surface, and this can explain differences between the orientations of slump fold axes and axial surfaces when these are used as directional indicators of slump movement.

Speckled beds; distinctive gravity-flow deposits in finely laminated diatomaceous sediments, Miocene Monterey Formation, California

Distinctive nonlaminated intervals we term speckled beds (SBs) are common in finely laminated diatomites of the Monterey Formation at Celite Quarry, Lompoc, California. SBs have sharp and bedding-parallel upper and lower contacts, tabular geometry, and are less than or equal to 10 cm thick. SBs are characterized by a bedding-parallel arrangement of sand- to granule-size diatomaceous and detrital aggregates that are uniformly distributed in a fine-grained matrix of macerated biosilica and detrital mud. The unique physical properties of diatomaceous sediments provided cohesive substrates and high fluid content that were prerequisite for the formation of SBs. Abundant hybrid fault/vein structures, slump folds, and complexly deformed laminae in the quarry attest to syndepositional deformation of laminated diatomaceous sediments on a subaqueous slope within the oxygen-minimum zone. The unique features of SBs permit an interpretation of stepwise rheological transformation during mass movement, via passage through critical-point transformations, from Bingham plastic (slump and/or slide block) to turbulent fluid (turbidity current) to viscous fluid exhibiting laminar flow (debris flow). The recognition of SBs clarifies the origin of these distinctive yet problematic deposits, and clearly differentiates them from other nonlaminated intervals. SB composition indicates provenance from an oxygen-minimum zone and further illustrates that a hiatus is not required to form cohesive firmgrounds in diatomaceous sediments. The recognition of SBs as recorders of slope failure events complements paleoseismic studies of Holocene diatomaceous laminites from Saanich Inlet, British Columbia (Ocean Drilling Program Sites 1033 and 1034).

Seismites in a Proterozoic tidal succession, Singhbhum, Bihar, India

Early Proterozoic metasediments of the Chaibasa Formation (Galudih–Ghatsila–Dhalbhumgarh region, Singhbhum, Bihar, India) comprise a number of cyclic fining-upward prograding successions of tidalites. The tidalites show indications for earthquakes in the form of synsedimentary deformation features, apart from the structures due to high-energy wave action. Deformed cross-bedding, convolute laminations, synsedimentary faults, graben-like structures, sandstone dykes, pseudonodules and slump folds record the seismic activity. A gradual decline in the frequency of seismites and tsunami-related depositional features, in combination with an upward increase in thickness of the tidal cycles, are attributed to gradual diminishing of tectonic activity within the basin.

Paleoslope Analysis of Slump Folds in the Devonian Flysch of Maine

Ancient submarine slide and slump deposits in the Devonian flysch of central and northern Maine show considerable variation in fold style, from symmetric to asymmetric to sheath geometries. Building on earlier work by Farrell and Eaton, we suggest that the spectrum of fold styles reflects the degree of simple shear within each slump deposit. We present a stereographic approach to paleoslope analysis that exploits fold hinge attitudes, axial surface attitudes, sheath axes, vergence, S‐ and Z‐asymmetry—depending on the style of slump folding. Our paleoslope determinations from widely scattered locations across the Devonian foreland basin in Maine show a regionally consistent pattern of westerly to northwesterly slopes.

Basin-Floor Fans in the North Sea: Sequence Stratigraphic Models vs. Sedimentary Facies: Reply

Examination of nearly 12,000 ft (3658 m) of conventional core from Paleogene and Cretaceous deep-water sandstone reservoirs cored in 50 wells in 10 different areas or fields in the North Sea and adjacent regions reveals that these reservoirs are predominantly composed of mass-transport deposits, mainly sandy slumps and sandy debris flows. Classic turbidites are extremely rare and comprise less than 1% of all cores. Sedimentary features indicating slump and debris-flow origin include sand units with sharp upper contacts; slump folds; discordant, steeply dipping layers (up to 60°); glide planes; shear zones; brecciated clasts; clastic injections; floating mudstone clasts; planar clast fabric; inverse grading of clasts; and moderate-to-high matrix content (5-30%). Many f the cored reservoirs either have been previously interpreted as basin-floor fans or exhibit seismic (e.g., mounded forms) and wireline-log signatures (e.g., blocky motif) and stratal relationships (e.g., downlap onto sequence boundary) indicating basin-floor fans within a sequence stratigraphic framework. This model predicts that basin-floor fans are predominantly composed of sand-rich turbidites with laterally extensive, sheetlike geometries. However, calibration of sedimentary facies in our long (400-700 ft) cores with seismic and wireline-log signatures through several of these basin-floor fans (including the Gryphon-Forth, Frigg, and Faeroe areas) shows that these features are actually composed almost exclusively of mass-transport deposits consisting mainly of slumps and debris flo s. Distinguishing deposits of mass-transport processes, such as debris flows, from those of turbidity currents has important implications for predicting reservoir geometry. Debris flows, which have plastic flow rheology, can form discontinuous, disconnected sand bodies that are harder to delineate and less economical to develop than deposits of fluidal turbidity currents, which potentially produce more laterally continuous, interconnected sand bodies. Our core studies thus underscore the complexities of deep-water depositional systems and indicate that model-driven interpretation of remotely sensed data (i.e., seismic and wireline logs) to predict specific sedimentary facies and depositional features should proceed with caution. Process sedimentological interpretation of conventional cor is commonly critical for determining the true origin and distribution of reservoir sands.

Slope-controlled seismic deformation and tectonic framework of deposition: Koldaha Shale, India

Penecontemporaneous deformations with remarkable lateral continuity at selected stratigraphic levels record frequent seismicity in the Meso–Proterozoic Koldaha Shale, Vindhyan Supergroup, central India. Their consistent bipolar orientation reflects the master control of basinal tectonics. Boudinage, small-scale conjugate faults and bidirectional joint planes indicate east-northeast–west-southwest basinal extension. Detailed study of a 27-km strike-parallel stretch between Chorhat and Shikarganj against a broader perspective reveals existence of multiple NW–SE-elongated subbasins. Stratigraphic and sedimentologic criteria point to inheritance and subsequent sustenance of steep western and gentler eastern flanks of the sub-basins. It is also indicated that the perennial supply of sediment took place from the east, while the western slope contributed only during tectonic seiches. The relatively large-scale faults are correlatable with the steeper and the smaller deformation structures to the gentler flank. The NNW-oriented half graben structures that were reactivated time and again, is elicited.Previous geophysical studies inferred roughly E–W-elongated rift valley in the granitic basement and the present observations indicate that the rifting continued at least up to the time of Koldaha Shale deposition. The half graben could then be guided by cross-faults resulted from dextral shear. It is imperative then, the basin-interior sedimentation and deformation patterns were more closely controlled by the cross-faults, rather than by the basin-margin faults. Only a set of slump folds in a 10-m-thick interval indicates that the northern margin of the main basin was relatively steeper. This is further corroborated by concentration of large fans in mineralogically immature sediments and southerly paleocurrent at the northern fringe of the basin. In contrast, the Vindhyan sediments commonly belong to orthoquartzite–carbonate association. A regional northerly slope of the Vindhyan basin floor towards the relatively steeper flank provides an explanation for the general northwesterly paleocurrent, even though the eastern flank of the sub-basins sloped WSW. This integrated study of the physical aspects of the sedimentary pile against the background information of basement tectonics recognize rifting with a dextral shear component as the overriding factor controlling the early part of evolution of the Vindhyan basin.

Slump and debris-flow dominated basin-floor fans in the North Sea: an evaluation of conceptual sequence-stratigraphical models based on conventional core data

Abstract The sequence-stratigraphical concept of the basin-floor fan within the Vail/Exxon conceptual sea-level model is popular in hydrocarbon exploration because this model predicts that basin-floor fans are composed of sand-rich turbidites with laterally extensive reservoir geometries. Many producing deep-marine facies in the Palaeogene and Cretaceous of the North Sea have been interpreted as basin-floor fans based on their seismic (e.g. mounded forms) and wire-line log signatures (e.g. blocky log motifs). We have examined nearly 3700 m (12000 feet) of conventional core from features interpreted as basin-floor fans in the North Sea and adjacent regions to determine the sedimentary facies, which actually comprise these features. Our study reveals that basin-floor fans are predominantly composed of mass-transport deposits (mainly slumps and debris flows); turbidites are extremely rare. Sedimentary features indicative of slump and debris-flow origin include sand units with sharp upper contacts, slump folds, discordant, steeply dipping layers (up to 60°), glide planes, shear zones, brecciated clasts, clastic injections, floating mudstone clasts, planar clast fabric, inverse size grading, and moderate to high matrix content (5–30%). Calibration of these facies in long cores (120–210 m or 400–700 ft) with seismic and wire-line log signatures suggests that seismic mounds and blocky log motifs are actually a manifestation of amalgamated sand units emplaced by these mass-transport processes. The cores show that in some cases, sequence boundaries on seismic profiles actually correspond to primary glide planes (decollement) between slump sheets in cores examined. Unlike laterally continuous turbidites, slump- and debris-flow-emplaced sands are often laterally discontinuous, thus making reservoir geometry and continuity much harder to predict. Nevertheless, these mass-transport deposits still form important hydrocarbon-producing reservoirs (e.g. the Frigg Field). Because both sands and muds deposited by slumps and debris flows form features that are routinely interpreted as basin-floor fans on seismic and log data, process-sedimentological interpretation of conventional core is critical to the establishment of the true origin and distribution of sands contained in such features and thereby evaluate their hydrocarbon potential.

Late Triassic Radiolaria from the Kodiang Limestone, northwest Peninsular Malaysia

Two different cherty packstone-wackestone facies are exposed at an abandoned limestone quarry at Bukit Kodiang, Kedah. Chert occurs as discontinuous layers and nodules in the cherty packstone-wackestone. The thickness of individual chert layers varies from 2 to 10 cm. The facies exhibit slump folds. Eighteen species of Radiolaria were identified— Capnuchosphaera triassica De Wever, Triassocampe sulovensis Kozur and Mock, Tetraporobrachia asymmetrica Kozur and Mostler, Xenorum flexum Blome, Canoptum laxum Blome, Palaeosaturnalis triassicus Kozur and Mostler, Canoptum cf. farawayensis Blome, Pseudostylosphaera cf. spinulosa Nakaseko and Nishimura, Hagiastrum cf. augustum Pessagno, Perispongidium sp., Capnuchosphaera sp., Sarla sp., Kahlerosphaera sp., Canoptum sp., Latium sp., Paronaella sp., Sontonaella sp., and Spongostylus sp. The assemblage indicates that the age of the chert bearing strata ranges from late Carnian to middle Norian, Late Traissic.

Basin-Floor Fans in the North Sea: Sequence Stratigraphic Models vs. Sedimentary Facies

Examination of nearly 12,000 ft (3658 m) of conventional core from Paleogene and Cretaceous deep-water sandstone reservoirs cored in 50 wells in 10 different areas or fields in the North Sea and adjacent regions reveals that these reservoirs are predominantly composed of mass-transport deposits, mainly sandy slumps and sandy debris flows. Classic turbidites are extremely rare and comprise less than 1% of all cores. Sedimentary features indicating slump and debris-flow origin include sand units with sharp upper contacts; slump folds; discordant, steeply dipping layers (up to 60°); glide planes; shear zones; brecciated clasts; clastic injections; floating mudstone clasts; planar clast fabric; inverse grading of clasts; and moderate-to-high matrix content (5-30%). Many f the cored reservoirs either have been previously interpreted as basin-floor fans or exhibit seismic (e.g., mounded forms) and wireline-log signatures (e.g., blocky motif) and stratal relationships (e.g., downlap onto sequence boundary) indicating basin-floor fans within a sequence stratigraphic framework. This model predicts that basin-floor fans are predominantly composed of sand-rich turbidites with laterally extensive, sheetlike geometries. However, calibration of sedimentary facies in our long (400-700 ft) cores with seismic and wireline-log signatures through several of these basin-floor fans (including the Gryphon-Forth, Frigg, and Faeroe areas) shows that these features are actually composed almost exclusively of mass-transport deposits consisting mainly of slumps and debris flo s. Distinguishing deposits of mass-transport processes, such as debris flows, from those of turbidity currents has important implications for predicting reservoir geometry. Debris flows, which have plastic flow rheology, can form discontinuous, disconnected sand bodies that are harder to delineate and less economical to develop than deposits of fluidal turbidity currents, which potentially produce more laterally continuous, interconnected sand bodies. Our core studies thus underscore the complexities of deep-water depositional systems and indicate that model-driven interpretation of remotely sensed data (i.e., seismic and wireline logs) to predict specific sedimentary facies and depositional features should proceed with caution. Process sedimentological interpretation of conventional cor is commonly critical for determining the true origin and distribution of reservoir sands.

Slump-fold and palaeoslope orientations in Upper Silurian rocks, North Wales

AbstractSilurian mudstones and siltstones in the region south of Colwyn Bay, North Wales, contain numerous ‘disturbed beds’, with folds, fabrics, and other features resulting from pre-lithification deformation at or near to the sea floor. Orientation data are presented for slump folds in four areas of Ludlow rocks where the exposure allows three-dimensional measurement. The data show considerable scatter, perhaps reflecting an irregular slope morphology, and the palaeoslope inferred for each of the areas is slightly differently orientated. In general, however, the palaeoslope directions indicate that the slumps moved not eastwards, along the axis of the Denbigh Trough, as most recently suggested, but down the southern flanks of a gentle, intra-basinal swell.

Late ordovician—early silurian glaciofluvial deposits preserved in palaeovalleys in South Jordan

Periglacial, fluvial strata of Late Ordovician age, infilling incised palaeovalleys in the lower part of the shallow-marine, siliciclastic Khreim Group (Late Ordovician to Silurian) are described from outcrops in south Jordan. The palaeovalley fill predominantly comprises poorly sorted, medium- to coarse-grained, sub-arkose with subordinate beds of pebble- to cobble-size diamictite. Clasts include subangular rip-up sandstone and sparse shelly fossils derived from the local bedrock, together with exotic, rounded pebbles and cobbles derived from the Cambro-Ordovician sandstones and granitoid Precambrian rocks in the palaeohinterland, situated to the south. The palaeovalley fill is locally characterised by synsedimentary slump folds, micro-faulting and contorted bedding. These are attributed either to gravitational slumping at palaeovalley margins, or collapse due to melting of stagnant supporting ice. At some localities, the sandstones are typically trough cross-bedded, and exhibit upward-fining trends terminating in current ripples, suggesting episodic infill of the palaeovalleys. The few palaeocurrent measurements available from these sedimentary structures indicate a general palaeoflow to the north, coincident with the general orientation of the exposed palaeovalleys. The palaeovalleys may have been formed by glacial and/or fluvial processes; their predominantly sandstone fill is interpreted as fluvial and/or glaciofluvial in origin, and was probably deposited as proximal proglacial outwash at some distance from the ice-sheet which lay to the south, in present-day Saudi Arabia. The Batra Mudstone, which overlies coeval, non-channelised, glaciofluvial sandstones yields an Early Llandovery fauna, thus constraining the upper age of the palaeovalley infilling. By analogy with similar strata in Saudi Arabia, erosion of the palaeovalleys and infilling is likely to have taken place during Ashgill to Early Llandovery times. Erosion of the palaeovalleys and their subsequent infilling is associated with the Late Ordovician-Early Silurian glaciation of Gondwanaland, widely reported from central Saudi Arabia and, in the subsurface, from Jordan.

Ore metamorphism in sulfide mineralizations from the Cinco Villas Massif (Western Pyrenees, Spain)

Massive stratiform and vein-type Zn-Pb-Fe-F-Ba mineralizations, associated with Namuro-Westfalian metasediments, occur in the Cinco Villas Paleozoic massif. These mineralizations were previously considered to be the result of the precipitation of hydrothermal fluids liberated by the Aya granitic intrusion. However, the orebodies show several metamorphic features typical of similar mineralizations described in the geologic literature, suggesting a preintrusive genesis of probably exhalative sedimentary nature.The sedimentary sequence consists of two formations: (1) the Aranaz limestones, dated as Namurian, and (2) a Westphalian schistose formation, composed of pelitic, semipelitic, and psammitic rocks, with some conglomeratic tuffs, albite-rich laminae, tourmalinites, and interlayered basic sills. These materials have been affected by the Hercynian deformation and a low-pressure regional metamorphism ranging from anchizone to biotite zone. Mesoscale structural data and microscopical observations suggest three periods (D 1 , D 2 , and D 3 ) of progressive deformation, accompanied by fracturing and shearing phenomena. D 2 , the main deformation, developed isoclinal to recumbent folds and a regional foliation (S (sub 0-2) ). The more important metamorphic traits are produced during this deformation, and at the same time, the Aya granitoid intrusion was emplaced.The orebodies exhibit a lenticular to tabular morphology concordant with host rocks or forming a variable angle with S . Sphalerite, galena, and different proportions of pyrite, pyrrhotite, and chalcopyrite represent the major sulfide minerals. Quartz, carbonates, fluorite, and barite are the main gangue minerals. Regional deformation and metamorphism have caused uneven but important effects on the geometry and primary textures of the orebodies. However, there is evidence of some premetamorphic textures, such as debris-flow breccias, slump folds, and particularly in pyrite, framboidal relicts and colloform textures. Folding, foliation, transposition, shearing, durchbewegung, and vein-type ores are common metamorphic features in ore layers where the sulfides and gangue minerals have responded differently to deformation. Thus, textures in pyrite denote a fundamentally brittle behavior, but in some cases there is evidence of cataclastic flow, kinking, deformation bands, and recrystallization. In contrast, sphalerite, galena, chalcopyrite, and pyrrhotite exhibit different textures indicating that dislocation glide and creep, mechanical twinning, and recrystallization have generally been important mechanisms. These processes suggest deformation under temperatures <400 degrees C which is consistent with the degree of metamorphismon in the host rocks. Micro- and mesoscale remobilization of gangue and ductile sulfides due to a mixture of both mechanical and chemical processes is a generalized phenomenon in all deposits; probably a major-scale remobilization has been operative. This would explain the existence of important vein-type mineralizations in this region.