Cross Lamination Research Articles

A new conceptual model for the deposition process of homogenite: Application to a cretaceous megaturbidite of the western Pyrenees (Basque region, SW France)

The north Pyrenean megaturbidite is a 19–63 m exceptional thick bed deposited during the Late Turonian and extending over more than 90 km in the Basque region Country (SW France). Its thickness varies from more than 63 m to about 19 m from east (Mauléon area) to west (Basque coast). It represents a total compacted volume of about 90 km 3 of carbonates. On the field, rare sedimentary structures are visible in the turbidite bed. They consist of laminar planar lamination and rare cross laminations in the eastern region and antidune-like structures in the western region. Five sites have been sampled with a vertical step of 50 cm to 1 m. Thin sections have been quantitatively analyzed for counting the terrigeneous fraction, the quartz grain size and the mineral orientation. The deposits fine westward, which suggests a source located in the east of the Mauléon Basin. This is consistent with the quartz grain orientation in the lower part of the megaturbidite. The deposits fine upward from medium sand to clayey-silt. This is consistent with the classical definition of homogenites and suggests that these kinds of deposits are turbidites. These observations also suggest that the term “megaturbidite” is appropriated for these deposits. The sedimentary analysis indicates that the deposit results from a single event. The volume of sediment involved in the process, as well as the quartz grain orientation indicating flow motion in the opposite direction of the initial flow and the antidune-like structure suggest the formation of reflected flows and the formation of standing waves over the complete water column in the Basque flysch sub-basins corresponding to a “Seiche effect”. The origin of the megaturbidite is probably an earthquake-generated collapse on the carbonate platform. This example allows providing a new conceptual model for the process of homogenite deposition. This model explains the deposition of thick, fine-grained, crudely-graded megaturbidites.

宮崎県日南海岸沿いに分布する新第三系宮崎層群青島層の重力流堆積物とその層序パターンの特異性

The Neogene Aoshima Formation of the Miyazaki Group, well exposed along the Nichinan coast, has been considered to be a turbidite succession deposited at a forearc basin, although the thickness-frequency distributions and the sedimentary facies of the formation are in disagreement with those of typical turbidite successions. In this study, we investigated the depositional system and the sedimentary environments of the formation, on the basis of the facies of sediment-gravity flow deposits and stratigraphic variations of bed-thickness distributions. From the results, we can classify the sediment-gravity flow deposits of the formation into the following types; Type G: sedimentary structures containing grading beds, whose wavelength is in the range of 0.5-5m; Type I: inverse grading structures composed of a finer lower unit with climbing-ripple cross lamination or HCS-mimics, and a coarser upper unit with massive sandstones; and Type M: massive structureless sandstones. Individual sediment-gravity flow deposits can be correlated over 10 km using key-tuff markers along palaeocurrent directions. Stratigraphic fluctuations of the hemipelagic-mudstone thickness and sediment-gravity flow thickness show that the formation along the Aoshima Island consists of a lower progradational system indicated by a thinning-upward trend of hemipelagic mudstones and an upper system dominated by lobate sand bodies characterized by the cyclic sedimentation of thick-sediment-gravity flow deposits. These results indicate that the depositional setting of the formation is a deep-offshore basin of a fandelta along the south-east coast of the Kyushu Island.

Earthquake-related soft-sediment deformation structures in Palaeogene on the continental shelf of the East China Sea

Earthquake, as disastrous events in geological history, can be recorded as soft-sediment deformation. In the Palaeogene of the East China Sea shelf, the soft-sediment deformation related to earthquake event is recognized as seismic micro-fractures, micro-corrugated laminations, liquefied veins, ‘vibrated liquefied layers’, deformed cross laminations and convolute laminations, load structures, flame structures, brecciation, slump structures and seismodisconformity. There exists a lateral continuum, the wide spatial distribution and the local vertical continuous sequences of seismites including slump, liquefaction and brecciation. In the Palaeogene of East China Sea shelf, where typical soft-sediment deformation structures were developed, clastic deposits of tidal-flat, delta and river facies are the main background deposits of Middle-Upper Eocene Pinghu Formation and Oligocene Huagang Formation. This succession also records diagnostic marks of event deposits and basinal tectonic activities in the form of seismites.

Bimodal Cross-Lamination in Wave-Ripple Form Sets: A Possible Origin

Abstract Wave-ripple cross-lamination formed in a sandy lacustrine shoreface in the Canadian Great Lakes is characterized by (i) small, randomly superimposed sets of angle-of-repose cross lamination, with strongly bimodal dips; (ii) small, superimposed sets of angle-of-repose cross-lamination, where the thickest and most prevalent sets have onshore dips, and the thinner, subsidiary sets dip offshore. (iii) small, superimposed sets of angle-of-repose cross-lamination, with near unimodal landward dips; (iv) supercritical, sinusoidal lamination indicative of high rates of ripple climb. Preserved surface ripples frequently reveal form discordant internal structures with a set of landward-dipping cross-lamination superimposed on a lakeward-dipping unit, both within a near-symmetrical ripple form. The origin of this internal lamination is discussed in the context of the second-order theory proposed for mass transport in turbulent wave boundary layers. The characteristic increase and subsequent decrease in wave ...

Sedimentary rocks at Meridiani Planum: Origin, diagenesis, and implications for life on Mars

Abstract The MER rover Opportunity has carried out the first outcrop-scale investigation of ancient sedimentary rocks on Mars. The rocks, exposed in craters and along fissures in Meridiani Planum, are sandstones formed via the erosion and re-deposition of fine grained siliciclastics and evaporites derived from the chemical weathering of olivine basalts by acidic waters. A stratigraphic section more than seven meters thick measured in Endurance crater is dominated by eolian dune and sand sheet facies; the uppermost half meter, however, exhibits festoon cross lamination at a length scale that indicates subaqueous deposition, likely in a playa-like interdune setting. Silicates and sulfate minerals dominate outcrop geochemistry, but hematite and Fe3D3 (another ferric iron phase) make up as much as 11% of the rocks by weight. Jarosite in the outcrop matrix indicates precipitation at low pH. Cements, hematitic concretions, and crystal molds attest to a complex history of early diagenesis, mediated by ambient ground waters. The depositional and early diagenetic paleoenvironment at Meridiani was arid, acidic, and oxidizing, a characterization that places strong constraints on astrobiologial inference.

New Stratigraphic Subdivision, Depositional Environment, and Age Estimate for the Upper Cretaceous Djadokhta Formation, Southern Ulan Nur Basin, Mongolia

Studies of key and newly discovered sections of the Upper Cretaceous Djadokhta Formation along the southern margin of the Ulan Nur Basin allow a new subdivision based on lithology. The formation and its members were mapped at both Bayn Dzak, an area that includes the Flaming Cliffs, and Tugrugyin Shireh, an area about 50 km to the northwest of Bayn Dzak. Stratigraphic sections at both localities were remeasured. The considerably enlarged formation comprises a lower Bayn Dzak Member, dominated by moderate reddish orange sands with subordinate mudstone units, and an upper Tugrugyin Member, composed of pale orange to light gray sands. Investigations of key sections at Tsonzh and Alag Teer demonstrate the presence of transitional mudstone lenses between these members within the Djadokhta Formation. Two distinct, sandy, sedimentologic facies are recognized in both members. Crossbedded intervals, occasionally exhibiting wind-ripple cross lamination, document the presence 1 The Geological Institute of the Mongolian Academy of Sciences, Peace Street 63, Ulaanbaatar-51, Mongolia

Evidence for ocean water invasion into the Chicxulub crater at the Cretaceous/Tertiary boundary

Abstract— The possibility of ocean water invasion into the Chicxulub crater following the impact at the Cretaceous/Tertiary boundary was investigated based on examination of an impactite between approximately 794.63 and 894.94 m in the Yaxcopoil‐1 (Yax‐1) core. The presence of cross lamination in the uppermost part of the impactite suggests the influence of an ocean current at least during the sedimentation of this interval. Abundant occurrence of nannofossils of late Campanian to early Maastrichtian age in the matrices of samples from the upper part of the impactite suggests that the carbonate sediments deposited on the inner rim margin and outside the crater were eroded and transported into the crater most likely by ocean water that invaded the crater after its formation. The maximum grain size of limestone lithics and vesicular melt fragments, and grain and bulk chemical compositions show a cyclic variation in the upper part of the impactite. The upward fining grain size and the absence of erosional contact at the base of each cycle suggest that the sediments were derived from resuspension of units elsewhere in the crater, most likely by high energy currents association with ocean water invasion.

Evidence for ocean water invasion into the Chicxulub crater at the Cretaceous/Tertiary boundary

Abstract The possibility of ocean water invasion into the Chicxulub crater following the impact at the Cretaceous/Tertiary boundary was investigated based on examination of an impactite between approximately 794.63 and 894.94 m in the Yaxcopoil‐1 (Yax‐1) core. The presence of cross lamination in the uppermost part of the impactite suggests the influence of an ocean current at least during the sedimentation of this interval. Abundant occurrence of nannofossils of late Campanian to early Maastrichtian age in the matrices of samples from the upper part of the impactite suggests that the carbonate sediments deposited on the inner rim margin and outside the crater were eroded and transported into the crater most likely by ocean water that invaded the crater after its formation. The maximum grain size of limestone lithics and vesicular melt fragments, and grain and bulk chemical compositions show a cyclic variation in the upper part of the impactite. The upward fining grain size and the absence of erosional contact at the base of each cycle suggest that the sediments were derived from resuspension of units elsewhere in the crater, most likely by high energy currents association with ocean water invasion.

A Completion Solution for the Paddy Reservoir in Northeast British Columbia: A Case Study

Abstract The Paddy reservoir, located near the Foothills of the Rocky Mountains in northeast British Columbia, Canada, is a fluvioestuarine valley-filled sandstone that has moderate to high permeability. Evidence of tectonic stress has been observed during hydraulic fracture treatments in the form of excessive wellbore friction and net pressures. Historically, attempts to fracture-stimulate these wells has been difficult due to the inability to generate sufficient fracture width necessary for proppant placement. This paper presents a completion solution for the Paddy reservoir in northeast British Columbia that enabled successful placement of propped fracture treatments. Pressure response of past fracture treatments were analysed to understand the cause of past failures, and to quantify the influence of tectonicallyinduced stress. A recommended completion solution is presented which includes orientated perforating to reduce near-wellbore friction and a process for designing fracture treatments to enable successful placement of proppant. Field examples of the new completion technique are presented which show that near-wellbore friction was reduced by 50%, and that up to four times the amount of proppant was placed in the formation compared to previous treatments. Pressure match simulation results from the fracture treatments are also presented. Geologic Summary The Paddy Member forms the upper portion of the Peace River Formation and is distributed over the Peace River area of northwest Alberta and northeast British Columbia. It is underlain by dominantly coarse-grained shoreface deposits of the Cadotte Member and overlain by marine shales of the Shaftesbury Formation. The Paddy Member is made up of thinly bedded sandstones and shales with occasional thin coals. In the study area, the Paddy is approximately 40 m thick. Reservoir quality, incised valley sandstones located in the upper part of the unit are up to 18 m thick. They are dominantly fine to medium and occasionally coarse grained, fairly quartz-rich sandstones with 10 - 30% chert and other lithic grains. Beds are from centimetre to metre scale in thickness and show small to large-scale trough cross lamination. Where bedding thickness is reduced, millimetre scale wavy shale laminae are common and are often incorporated into stylolitic concentrations, which greatly reduce vertical permeability. The cleaner sandstones were probably deposited under fluvial conditions with some tidal influence where thin shale laminae are common. The dominant cement in these sandstones is quartz, which substantially occludes intergranular porosity in the study area. Small amounts of carbonate cements also occur but are rarely volumetrically important. In addition to intergranular porosity, a significant amount of microporosity has been generated through the dissolution of detrital grains, primarily chert. The abundance of microporosity in these sandstones, together with low water saturations indicated by wireline log analysis, may be the origin of the widely held concern that the Paddy reservoir is susceptible to formation damage through water blocking. Core from the study area indicates that porosity in the Paddy reservoir ranges up to 20%, with associated permeability to air at minimal confining pressure of several hundred millidarcies. A minimum porosity cutoff for pay is taken at approximately 8 - 9%, which is typically associated with 1 md permeability.

Plio-Quaternary sedimentation on the Wilkes land continental rise: preliminary results

Abstract The Wilkes Land continental rise is characterised by mounds and channels with approximately a north–south elongation, perpendicular to the margin. Proximal mound relief is up to 1000 m, decreasing to about 300 m in the central part. During the geophysical and geological survey conducted on in February–March 2000 by the joint Italian and Australian WEGA Project, onboard R/V Tangaroa, 11 piston cores were collected along two transects crossing the channel-mound system. All cores were logged for physical (magnetic susceptibility and density) and acoustic properties (P-wave velocity). Split cores were X-rayed and samples were analysed for clay mineral assemblages, chemical and micropaleontological (diatoms, foraminifera and nannofossils) content. Glacial and interglacial intervals have been recognised in the sediment cores. The interglacial facies consists of massive mud, distinguished by: (a) bioturbated massive mud occasionally with fine-grained ice-rafted debris (IRD), and (b) structureless massive mud with abundant fine to coarse-grained IRD, containing well-preserved open-ocean diatoms. The glacial facies is represented by laminated mud with planar and/or cross laminations, with occasional isolated dropstones, and rare, poorly preserved, sea-ice diatoms. The sharp boundary, characterizing the limit between massive to laminated facies, is interpreted to indicate a fast glacial onset. In contrast, the smooth passage presents from laminated to massive sediments, indicates a gradual glacial waning. Recent down-slope gravity flows have been identified in a turbidite, a normally graded coarse-grained sand, recovered in the thalweg of Jussie Canyon, and probably also in the massive debris facies from the steep side of Mound A. The massive debris is characterised by structureless and unsorted gravel and pebbles within a muddy matrix. Clay mineral assemblages and grain lithologies indicate a hinterland Wilkes Basin and continental shelf provenance for the terrigenous fraction.

Sedimentary facies and Holocene progradation rates of the Changjiang (Yangtze) delta, China

The Changjiang (Yangtze) River, one of the largest rivers in the world, has formed a broad tide-dominated delta at its mouth during the Holocene sea-level highstand. Three boreholes (CM97, JS98, and HQ98) were obtained from the Changjiang delta plain in 1997–1998 to clarify the characteristics of tide-dominated delta sediments and architecture. Based on sediment composition and texture, and faunal content, core sediments were divided into six depositional units. In ascending order, they were interpreted as tidal sand ridge, prodelta, delta-front, subtidal to lower intertidal flat, upper intertidal flat, and surface soil deposits. The deltaic sequence from the prodelta deposits to the delta front deposits showed an upward-coarsening succession, overlain by an upward-fining succession from the uppermost part of the delta front deposits to the surface soil. Thinly interlaminated to thinly interbedded sand and mud (sand–mud couplets), and bidirectional cross laminations in these deposits show that tide is the key factor affecting the formation of Changjiang deltaic facies. Sediment facies and their succession combined with AMS 14C dating revealed that isochron lines cross unit boundaries clearly, and delta progradation has occurred since about 6000 to 7000 years BP, when the rising sea level neared or reached its present position. The average progradation rate of the delta front was approximately 50 km/kyear over the last 5000 years. The progradation rate, however, increased abruptly ca. 2000 years BP, going from 38 to 80 km/kyear. The possible causes for this active progradation could have been an increase in sediment production in the drainage basin due to widespread human interference and/or decrease in deposition in the middle reaches related to the channel stability caused by human activity and climatic cooling after the mid-Holocene.

Lower Triassic peritidal cyclic limestone: an example of anachronistic carbonate facies from the Great Bank of Guizhou, Nanpanjiang Basin, Guizhou province, South China

Most shallow marine sections described from the Lower Triassic formed in mixed carbonate-siliciclastic systems. Shallow marine limestones from an isolated carbonate platform, the Great Bank of Guizhou, in the Nanpanjiang Basin, South China, provide a unique opportunity to examine a pure peritidal carbonate depositional system. The platform had a low-relief bank profile in the Early Triassic with oolite shoals at the margin, shallow-subtidal and peritidal deposits in the interior, and pelagics, debris-flow deposits and turbidites on gentle basin-margin slopes. Interior strata are 265 m thick beginning in the lowermost Triassic with Renalcis biostromes, followed by lime mudstone, oolite, and cyclic peritidal limestone. The cyclic limestone is Olenekian in age, 164 m thick, and contains up to 83 cycles (parasequences). Parasequences are 0.2–7.4 m thick and typically shallow upward from subtidal oolite grainstone and skeletal packstone facies followed by Renalcis reef mounds or biostromes and capped by intertidal flaser-bedded ribbon rock. The skeletal packstone and reef mounds contain a low-diversity biota of cyanobacteria, echinoderms, bivalves, gastropods, lingulid brachiopods, spirorbids, and ostracodes. Ribbon rock contains alternating lime mud and fine peloidal packstone and grainstone laminae, scour surfaces, ripple cross lamination, lime-mud drapes, and minor prism cracks. Parasequence stacking patterns exhibit gradual increase and decrease of parasequence thickness, defining a third-order sequence boundary and several 4th-order parasequence sets that correlate between interior sections. Parasequence stacking patterns similar to those of Lower Palaeozoic green-house sequences and different from ice-house sequences. The Lower Triassic platform-interior parasequences represent an anachronistic facies in that they are strikingly similar to the common Renalcis-mound, ribbon rock parasequences of the Lower Palaeozoic and different from other Permo–Triassic parasequences (e.g. Capitan shelf parasequences or Loferites). Facies similarities such as Renalcis mounds and flaser-bedded intertidal ribbon rock reflect anomalous oceanic conditions resulting in low biodiversity and low intensity of bioturbation after the end-Permian extinction. Similarities in parasequence stacking patterns reflect low-amplitude, high-frequency sea-level fluctuations resulting from green-house conditions common to the Early Palaeozoic and Early Triassic.

Effects of Element Size and Orientation in the Production of High Strength Resin Impregnated Wood Based Materials

Summary The effects of low molecular weight phenolic resin impregnation and high pressure hot pressing (150°C, 30–100 MPa) on the physical and mechanical properties of wood were first investigated by using sawn wood prepared from a block of Japanese birch (Betula meximowicziana). Subsequently, the effects of element size and orientation were examined by using sliced veneers, particles and powder prepared from the remaining portion of the block. Due to the combination of resin impregnation and hot pressing under high pressure, the specific bending strength at 20°C and 65% RH of sawn wood increased 50% accompanied by enormous decreases in moisture content, and bending strength reached around 400 MPa. The mechanical properties of veneer laminated product did not differ significantly from those of sawn wood product. Among isotropic products, the highest bending strength at 20°C and 65% RH of plywood, particleboard and powderboard was 242 MPa, 166 MPa and 175 MPa, respectively. The difference of bending strength between plywood and other isotropic products could be explained by the difference in element orientation, cross lamination and random distribution. Furthermore, the results for the particleboard and powderboard showed that when the resin impregnated elements were hot pressed under high pressure, a decrease in element size did not result in a decrease in bending strength.

An experiment on a sand-dune environment in Southern Venetian coast based on GPR, VES and documentary evidence

The internal structures of some surviving sand dunes and the ancient shore-lines along the coast south of Venice have been investigated integrating Ground Probing Radar (GPR) profiles, Vertical Electrical Soundings (VES) and water conductivity measurements in some boreholes. The GPR penetration depth has been limited (4-5 m,using a 400 MHz antenna) by the high conductivity of salt water saturating pores of the shallow sediments. On the other hand, the excellent spatial resolution of the radar survey provided an estimate of internal dune bedding features, such as cross lamination and forwarding ancient covered coast-lines dated in the Thirties. The interpretation of the data, in particular along one line 360 m long intercepting a sizable sand-dune bank, seems to offer clues to the evolutional history of the coast line and the depth of transition from fresh-water to brackish-salt water. The water table was detected with electrical measurements and direct observations in boreholes, whereas the transition between fresh and salt water (brackish water) was pointed out indirectly by the high energy absorption and total back-reflection of the EM waves, encountered at this boundary, and directly by the strong decrease in VES resistivity values.

Laminaset geometries in fine grained shallow marine sequences: core data from the Rannoch Formation (North Sea) and outcrop data from the Kennilworth Member (Utah, USA) and the Bencliff Grit (Dorset, UK)

ABSTRACTSandbodies from storm‐dominated marine and marginal marine environments commonly contain intervals of laminated fine sandstones. A characteristic of such lamination is the presence of low angle cross lamination. In order to model correctly the effects of such lamination on a waterflood of an oil‐bearing shoreface sequence it was necessary to quantify the geometry of the laminaset elements.This challenge has been greatly complicated by the lack of outcrop of the formation of interest. The Middle Jurassic Rannoch Formation of the North Sea only occurs in the subsurface where it is not possible in core to measure the aspect ratio of laminasets directly. In this study, the laminaset geometry data that can be obtained from core (e.g. apparent set thicknesses) were collected for the Rannoch Formation. These data were compared with similar data from potential outcrop analogues in (1) the Cretaceous Kennilworth Member of the Blackhawk Formation in Utah, USA and (2) the Upper Jurassic Bencliff Grit from the Dorset coast, UK.A quantitative analysis of laminaset geometries has been used to compare subsurface core with potential outcrop analogues. The Rannoch Formation core is characterized by numerous low angle truncations. We have measured these features in two wells (means of 7.2° and 12.1°). Mean apparent set thicknesses were 0.24 and 0.19 m.In the outcrop sections studied, truncation angles ranged from 9.6° to 13.4° and mean set thicknesses from 0.24 to 0.34 m. Mean bounding surface dips of 5.8° and 8.6°, and mean laminaset lengths of 2.3 and 4.1 m were also measured. directly in the field and by using photomosaics.On the basis of this comparison, the Kennilworth Member in Utah was found to be the most suitable and the geometries (i.e. aspect ratios) measured there were used to generate an appropriate geometry of Rannoch laminaset geometries for use in engineering studies: laminaset length, 2.0 m; laminaset thickness, 0.2 m.

Improved Well Productivity Realized By Fracturing With Frac Oil As Compared To Diesel

Abstract Increased AOF potentials averaging 24% have been realized on tight Basal Quartz sandstone reservoirs stimulated using frac oil as the carrier fluid when compared to reservoirs similarly stimulated using diesel. This statistical study of post-stimulation results examines 36 wells in the Caroline/Carrington area of southwestern Alberta. In consideration of the over-all economics, the improved well productivity provides earlier payout which more than offsets the slight cost premium for frac oil over diesel. Initial laboratory core studies showed that the Basal Quartz zone was immune co damage by completion fluids, but early attempts to fracture the zone using water as the base carrier fluid resulted in very discouraging well performance. This brought about a shift to the use of hydrocarbon-based fracturing fluids. Fra oil and diesel emerged as the prime completion fluids because of their low RVP (Reid vapour pressure) which allows safe handling under high pumping pressures. Frac oil is a refined derivative of condensate. Compared co diesel, frac oil is higher in the preferred C7 C8 and C9 constituents and lower in less desirable C11 and heavier components. In addition, it has a significantly higher percentage of solvents in the form of toluene, methylcyclohexane and xylene. Frac oil is produced by the distillation of condensate at gas plants where the condensate stream contains an appropriate mix of C5 + constituents. Introduction The Caroline/Garrington fields are located 120 km northwest of Calgary in the foothills of Alberta. The Basal Quartz sands in this area are typically unproductive unless stimulated by hydraulic fractures. In 1982, it was determined that wells fractured using gelled hydrocarbon carrier fluids yielded much greater productivity than those fractured using water as a carrier. Although industry wide the majority of fracture treatments are performed using water as a base carrier fluid, water-based stimulations on the Caroline tight sandstone proved to be disappointing. Since that time Dome Petroleum has completed 72 Basal Quartz gas wells using either diesel, reformate or frac oil. This study was undertaken to evaluate the effectiveness of stimulations with gelled frac oil versus gelled diesel fuel as the carrier fluids. Discussion Geology In the Caroline/Garrington area, the main reservoirs that were studied were deposited during Lower Cretaceous time within a laterally extensive tidal flat complex. The most extensive reservoir unit in the Caroline/Garrington area, using Dome Petroleum's terminology, is the Basal Quartz ‘A’ sand (Fig. 1). This gas-prone sand occurs within the uppermost 20 m of Lower Mannville "clean sand " development. At various stratigraphic levels within the tidal flat complex, there are two types of reservoirs:Thick tidal channel deposits.Thin but a really extensive regional sands. A thick occurrence of the Basal Quartz ‘A’ sand (10 m to 20 m) covers a narrow arcuate strip near the western limits of the Caroline Field and consists of fine-to medium-grained quartz, rounded, moderately to well sorted with a minor amount of detrital clay matrix. Basal contacts of the thick sands are generally sharp and erosive in nature. In cores, the sands display bidirectional and trough cross laminations.

Mudstones and Thin-bedded Turbidites Associated with the Upper Cretaceous Wheeler Gorge Conglomerates, California: A Possible Channel-Levee Complex

ABSTRACT Three conglomeratic thinning- and fining-upward sequences in the Upper Cretaceous at Wheeler Gorge, California have previously been assigned to submarine-channel environments. There is continuing controversy as to whether the channels represent inner to midfan environments, or whether they are incised into basin-plain facies. To shed further light on the problem, the mudstones and turbidites below and above the conglomerates are described here in detail. Below the conglomerates, the sequence is dominated by mudstones interbedded with very fine grained turbidite sandstones in beds up to 5 cm thick. There is only one slumped horizon, about 240 m below the conglomerates. By contrast, the mudstones above the conglomerates contain more and thicker turbidites (beds up to 47 cm), which occur in at least three thinning- and fining-upward sequences. Bouma sequences (A, AC, BC) are common, whereas below the conglomerates, the thin turbidites show only graded bedding (very fine sandstone to mudstone) or ripple cross lamination. Slumping is much more common above the conglomerates; at least eight distinct slumped horizons are scattered through about 33 m. The slumped turbidites are interpreted as channel-margin or levee facies, and the thinning- and fining-upward sequences of relatively thin, unchannelized turbidites suggest a levee environment with gradual lateral migration of the channel away from the depositional site, yielding the thinning and fining upward. Below the conglomerate complex, the thinness of the turbidites (average 1 cm, maximum 5 cm), the absence of thinning- or thickening-upward sequences, and the dominantly muddy nature of the rocks, suggest a depositional environment on a basin floor far from any channel influence. The overall sequence of units records the incision of a channel complex (the conglomerates) into quiet basin-floor mudstones. Following deposition of all of the conglomerates, there was lateral shifting of the channel, with concurrent deposition of a channel-levee complex. The channel-levee complex is a variant of the more general lobate submarine-fan model. In the fan model, the fan is fed by a single channel: the channel-levee complex replaces that feeder channel by a group of channels. Similarly, the few branching channels on the lobate midfan may be replaced by extensive channel-levee complexes with little development of smooth lobes.

Geological features and origin of the chromite deposits of sukinda valley, Orissa, India

The chromiferous ultramafic rocks of Sukinda Valley (21°0'–21°5'N:85°43'–86°0'E) of Orissa are intrusive into the Iron-Ore Supergroup (2,950–3,200 Ma) at the eastern periphery of the Indian Precambrian shield. Both laterally and vertically, chromite occurs as persistent layers, lenses or pockets in the serpentinized and silicified dunite-peridotite extending over a strike length of 25 km. The ultramafic rocks and the chromitite layers are cofolded with the Iron-Ore Supergroup into a plunging syncline. Primary layering, ball and pillow structures, cross laminations, graded bedding etc. can also be detected. The different varieties of chromite ore present in the area are massive, banded and spotted, laminated and friable. The grain size of chromite varies between 0.25 and 4 mm, and the fineness of the grain increases from the bottom to the top layers. The cell dimension of chromite (8.23–8.32 A) decreases with the increase of Al2O3.

Late Quaternary calcareous clayey-silty muds in the Obock Trough (Gulf of Aden): hemipelagites or fine-grained turbidites?

Summary Three cores have allowed us to survey Holocene deposits in a trough of the Gulf of Aden ridge. Within this particular environment (steep slopes, tectonic and volcanic activity) sediments are fine-grained, apparently homogeneous calcareous muds, rich in clay and planktonic microfossils: these features suggest that they are hemipelagic deposits. However, high sedimentation rates (20 to 50 cm/1000 yrs), lateral variation, and the occurrence of dynamic sedimentary structures (visible on X-radiographs and in thin sections), suggest that they have been emplaced mainly by gravity currents. These reworked hemipelagic materials are deposited on the shelf edge and on the trough slopes. This is confirmed by the sequential arrangement of sedimentary characteristics, as shown by analysis of microstructures, microfacies, carbonates, clay minerals, grain-size and other components. Two types of sequences have been observed; the first we call the ‘type sequence’. This commonly overlies an erosional surface and comprises three divisions from bottom to the top: Division I (equivalent to Piper’s E1 division); mud with 8–10% sand fraction, rich in biogenic tests which occur as well-sorted horizontal laminae. Division II (equivalent to Piper’s E2 and E3 divisions); mud with low sand fraction (1%), very poor in biogenic material, no lamination, but horizontal orientation of silt-sized bioclasts. Division III (equivalent to Piper’s E2 and E3 divisions); mud with low sand fraction (1%), very poor in biogenic material, no lamination, but horizontal orientation of silt-sized bioclasts. Division III (equivalent to Piper’s F division); mud with low sandy fraction, poorly-sorted biogenic material, no dynamic sedimentary structures. The second type of sequence is less common and comprises two divisions: Division I’a ; very thin layer (millimetric), rich in terrigenous silt, which scours underlying sediment and shows horizontal, oblique and cross lamination. Division I’b ; mud with low biogenic sand content, showing horizontal more or less clear and irregularly-distributed lamination. These two sequences correspond to divisions E and F defined by Bouma (1962) and Piper (1978) in terrigenous turbidite sequences. The ‘type sequence’ is comparable to turbiditic sequences described in pelagic or hemipelagic material, deposited in similar morpho-tectonic environments (e.g. the eastern Mediterranean Sea or Mid Atlantic Ridge fracture zone).

Reservoir Characterization of Mesaverde (Campanian) Bedload Fluvial Meanderbelt Sandstones, Northwestern Colorado: ABSTRACT

Reservoir characterization of Mesaverde meanderbelt sandstones is used to determine directional continuity of permeable zones. A 500-m (1,600-ft) wide fluvial meanderbelt in the Mesaverde Group is exposed as laterally continuous 3-10-m (10-33-ft) high sandstone cliffs north of Rangely, Colorado. Forty-eight detailed measured sections through 3 point bar complexes oriented at right angles to the long axis of deposition and 1 complex oriented parallel to deposition were prepared. Sections were tied together by detailed sketches delineating and tracing major bounding surfaces such as scours and clay drapes. These complexes contain 3 to 8 multilateral sandstone packages separated by 5-20 cm (2-8 in.) interbedded siltstone and shale beds. Component facies are point bars, crevasse splays, chute bars, and floodplain/overbank deposits. Two types of lateral accretion surfaces are recognized in the point bar facies. (1) Gently dipping lateral accretions containing fining-upward sandstone packages. Large scale trough cross-bedding at the base grades upward into ripples and plane beds. (2) Steeply dipping lateral accretion surfaces enclose beds characterized by climbing ripple cross laminations. Bounding surfaces draped by shale lags can seal vertically stacked point bars from reservoir communication. Scoured boundaries allow communication in some stacked point bars. Crevasse splays showing climbing ripples form tongues of very fine-grained sandstone which flank point bars. Chute channels commonly cut upper point bar surfaces at their downstream end. Chute facies are upward-fining with small scale troughs and common dewatering structures. Siltstones and shales underlie the point bar complexes and completely encase the meanderbelt system. Bounding surfaces at the base of the complexes are erosional and contain large shale rip-up clasts. End_of_Article - Last_Page 493------------