Thin Turbidites Research Articles

Field expressions of the transformation of debris flows into turbidity currents, with examples from the Polish Carpathians and the French Maritime Alps

Various transformation mechanisms can generate turbidity currents from subaqueous debris flows. Different transformation mechanisms have been described and interpreted in the past from laboratory experiments and from deposits, but the two approaches have not generally been linked. This has made the genetic interpretation and comparison of deposits difficult. In this paper a generic classification scheme of debrite–turbidite couplets is proposed based on transformation mechanisms inferred from laboratory experiments. Five different flow types (called A–E herein) and their resulting deposits are detailed, but they are all part of a continuous spectrum, and a mixture of types is likely to be found in the field. Type A flows are strong, dense debris flows that undergo little transformation. Their deposit will be a debrite overlain by a thin turbidite, which is separated from it by a clear grain size break. Type B flows are weaker and can develop waves at the debris flow-turbidity current interface. The deposit will be a debrite with a wavy top overlain by a turbidite that is thicker than for type A flows. For type C flows, the interfacial waves will grow so much that the debris flow disintegrates into separate parts. The deposit will consist of debrite lenses encased in a turbidite. Type D flows will undergo even more mixing than type C flows so that the debrite parts will be mixed. Their deposit will be a turbidite with laterally varying areas of debrite characteristics near the bed. Type E flows will be so transformed that the debris flow character has disappeared and the flow is a turbidity current with high sediment concentration. The deposit will be largely turbiditic. The flow types and deposits will be illustrated with some examples from two field areas: the Polish Carpathians and the French Maritime Alps.

Sedimentary record from Lake Hovsgol, NW Mongolia: Results from the HDP-04 and HDP-06 drill cores

The Hovsgol Drilling Project retrieved Pleistocene sediment section with the basal age of ca. 1 Ma from the Hovsgol rift basin, NW Mongolia. Detailed lithologic data on drill cores is presented and compared with analogous sediment facies in the radiocarbon-dated records of the last glacial–interglacial transition. Drill cores from two sites, presently in 239 m and 235 m water depth, represent somewhat different depositional settings. The shorter HDP-06 drill core (26 m) at the base of the gentler SE slope of the rift basin contains lithologic evidence for several recent lake lowstands on the order of −200 m. The longer HDP-04 drill core (81 m) some 8 km away at the base of the steep NW underwater slope is composed of finer sediments and contains at least 10 characteristic transitions from calcareous to carbonate-free (diatomaceous) layers. These lithologic transitions are interpreted here as signals of repeated Pleistocene lake transgressions in the Hovsgol basin. Transgressions appear to have been associated with lower sedimentation rates and with the deposition of thin turbidite beds at the drill site. Comparison of drill core lithology with the available seismic data shows reasonable agreement in terms of the number of lowstand events and the general trends of changing lake level. HDP-04 drill core retrieved shallow-water facies containing sand and carbonate oolites deposited at the time of the most dramatic mid-Pleistocene regression of the lake. At ca. 24 m core depth, this interval corresponds to a major basinwide angular unconformity apparent in the seismic pattern. Lake Hovsgol, a smaller sister rift lake of the grand Lake Baikal, has a confined local catchment, which makes it very sensitive to regional variations in the effective moisture. Consisting primarily of calcareous mud, the sedimentary record of Lake Hovsgol provides a unique regional sedimentary archive. Future multi-proxy studies of the Hovsgol sedimentary records will allow constraint of the mid-late Pleistocene history of the hydrologic budget in the Baikal region of continental interior Asia.

Evolution of Turbidity Currents Deduced from Extensive Thin Turbidites: Marnoso Arenacea Formation (Miocene), Italian Apennines

Previous work has shown that large-volume (> 100 km3) turbidity currents are capable of depositing extensive (> 100 km) turbidites in non-channelized settings. This study of the Miocene Marnoso Arenacea Formation demonstrates that smaller-volume (< ~ 0.5 km3) turbidites can also be remarkably extensive. Some thin (< 40 cm) turbidites extend for 120 by 30 km within this non-channelized sequence. Flows that produced these thin turbidites were capable of traversing the basin plain in opposing directions, despite being too weak to cause significant substrate erosion. Some flows traveled upslope for at least 60 km. Long run-out distances result from the relatively large fraction of fine particles in these highly efficient flows. Sandstone intervals thin in an approximately exponential fashion, across the proximal 20 to 30 km of outcrop. Similar sandstone shapes are observed for flows that traversed the basin plain in opposite directions. This sandstone shape is a generic property of spatially depletive flows; it is consistently reproduced by laboratory and numerical models in which velocity, sediment concentration, and deposition rates decrease down-flow. Rates of sandstone thinning are orders of magnitude smaller than those observed in deposits from laboratory experiments. This discrepancy primarily reflects a low ratio of flow thickness to sediment settling velocity, and low rates of sediment suspension from the bed, for slower-moving laboratory flows. The distal part of the thin-bed sandstone interval is thinner (< 5 cm) and more tabular. Starved ripples suggest that this distal tabular sandstone shape results partly from bedload reworking. The thickest turbidite mudstone occurs in the same two locations, for flows that traveled across the basin in opposite directions. This suggests that substantial volumes of muddy fluid ponded within bathymetric lows. Abrupt lateral termination of two mudstone beds, which have a distinctly tabular shape, suggests that certain mudstone intervals were deposited by dense fluid mud layers rather than expanded turbulent suspensions.

Integrated biostratigraphy of foraminifers, radiolarians and conodonts in shallow and deep water Middle Permian (Capitanian) deposits of the "Rader slide", Guadalupe Mountains, West Texas

A diverse assemblage of microfossils is present in a 6m thick sequence of three debris flow deposits interbedded with thin turbidite limestone beds and fine grained siliciclastics exposed above the megaconglomerate in a section (known as the “Rader Slide" in numerous guidebook stops) of the Rader Limestone Member of the Bell Canyon Formation of Capitanian age (Middle Permian) in the Guadalupe Mountains of West Texas. Each debris flow, derived from nearby Capitan Reef shelf-margin and slope deposits, contains a distinct microfossil assemblage. Small foraminifers and fusulinaceans, conodonts, radiolarians, sponge spicules, fish dermal plates and teeth, and other fragmental fossils are present in this sequence. Conodonts are relatively scarce in the first (or lowest) debris flow, except in its upper part, but they are common to abundant in the other two debris flows, and very abundant in several of the thin turbidite limestone beds. All of the conodonts present appear to be morphotypes of one population of the species Jinogondolella postserrata, except for one new conodont species, and the Jinogondolella postserrata Zone is clearly documented in this sequence. The debris flows contain the fusulinaceans Rauserella, rare Codonofusiella, Polydiexodina, Leella? and various species of the small foraminifers Globivalvulina, Hemigordius, Baisalina, Abadehella, Deckerella, Neoendothyranella, Vachardella, Geinitzina, and Polarisella. Some of the thin turbidite limestone beds contain a foraminiferal assemblage similar to that found in the debris flows, but with lower diversity. Many small foraminiferal species appear to be endemic, although a few are closely related to species known in Permian age strata in Italy, Greenland, the Russian Far East, northeastern part of Russia (Omolon massif), and the Zechstein of Germany and the Baltic area. Two thin limestone beds above the second debris flow contain primarily radiolarian species known from the Follicucullus japonicus Zone of Japan. Nine new species of small foraminifers (Bisphaera? improvisa, Vissariotaxis? nativus, Multidiscus raderensis, Baisalina miscella, Agathammina minuscula, Polarisella globosa, Geinitzina jucunda, Robustopachyphloia texana and Spireitlina capitanensis) and one new conodont species Jinogondolella gladirobusta are described.

Modeling seismic reflection patterns from Ocean Drilling Program Leg 168 core density logs: Insight into lateral variations in physical properties and sediment input at the eastern flank of the Juan de Fuca Ridge

At the eastern flank of the Juan de Fuca Ridge, single‐channel and multichannel very high resolution seismic data were recorded with a water gun and two Generator‐Injector Guns, which provided different source frequencies. To ground truth the seismic records, traces were visually correlated with reflection patterns of synthetic seismograms calculated from closely spaced density measurements on Ocean Drilling Program Leg 168 drill cores. The quality of both core logging and seismic data is sufficiently high to reveal a relation between density variations and seismic reflections at higher source frequencies. This suggests that density approximates subbottom variations in seismic impedance. The core density data are also sufficient to model a frequency‐dependent decrease in seismic reflection amplitudes, which is observed above a buried basement ridge. This indicates that low reflectivity in recorded traces is independent of processing artifacts, rough topography, or seafloor inclination and can be related to locally low in situ sediment density and high porosity, which appear to be associated with fluid flow. Further, the part of the amplitude decrease, which is due to an overall reduction of density contrasts, appears to be frequency‐dependent. In addition, the comparison between seismic records and core data reveals distinct depositional environments, which are divided by the buried basement ridge. In the western part of the study area a transition from an older to a younger part of the sediment section is observed. The former is characterized by focused turbiditic input in the vicinity of ODP Site 1024 and a gradual decrease in grain size toward ODP Site 1025. In the younger part, however, much thinner turbidites were deposited more evenly between both sites. Only one seismic reflector can be traced throughout the study area, suggesting that only major turbiditic events are capable of affecting both sides of the ridge. The sediment thickness above this reflector indicates that average accumulation rates for the youngest part of the sediments east and west of the ridge are rather similar.

Integrated chitinozoan, conodont, and graptolite biostratigraphy from the upper part of the Cape Cormorant Formation (Middle Ordovician), western Newfoundland

The Cape Cormorant Formation of the Table Head Group exposed on the Port au Port Peninsula, western Newfoundland, is composed of dark-brown to black shales with interbeds of thin calcareous silty and sandy distal turbidites. Distinctive carbonate conglomerates and breccias derived from the foundering shelf are occasionally found in the formation. The sediments accumulated in the foreland basin formed during the early stage of the Taconic orogeny. The faunas from the upper part of the Cape Cormorant Formation include graptolites, conodonts, and chitinozoans. The graptolites are well preserved, but are of low diversity and are referred to the Darriwil Pterograptus elegans Zone. Conodonts recorded from the distal turbidites are rare and fragmented. The faunas include taxa that are known from the St. George and Table Head groups. The conodont fauna is tentatively assigned to the Histiodella kristinae Phylozone and to the younger, unzoned interval. The chitinozoans are well preserved and the yield is high. The fauna is assigned to the Cyathochitina jenkinsi Zone and to an undefined interval. The abundance and diversity of the chitinozoan assemblages display a cyclic pattern, which is related to changes of the oceanic watermass in the foreland basin. The new chitinozoan species Belonechitina nevillensis n. sp., Belonechitina uniformipunctata n. sp., and Cyathochitina cormorani n. sp. are described.

The Andrew Formation and ‘biosteering’ — different reservoirs, different approaches

Abstract Biosteering, the provision of real-time biostratigraphic data for the drilling of horizontal wells, has developed and evolved a wide range of approaches and applications in recent years. Its value to the success of a development project has grown beyond its core role as an effective and rapid method by which to define the relative stratigraphic position of the drill bit for steering purposes. High-resolution biostratigraphy and biosteering are now being used as part of an integrated service which is being incorprated in the planning of a horizontal drilling programme and individual well paths. This paper illustrates some of this diversity of applications by describing two very different styles of biosteering from age-equivalent turbidite sands of the Andrew Formation, on the United Kingdom continental shelf (UKCS). The first example describes biosteering through thin (10–15 ft) turbidite sands from the Joanne Field. The reservoir is one of several thin allochthonous sand and limestone subunits, requiring a very detailed biostratigraphic framework to provide sufficient resolution to identify each of the subunits. The framework utilizes both in situ and derived/reworked assemblages and associations (microfacies) to identify up to 10 discrete and correlative lithostratigraphic subunits. Local dip variations, faults of subseismic resolution and thinness of sand hampered efforts to continuously remain within the reservoir during horizontal drilling operations. Well-site micropalaeontological interpretation correctly identified the position of the drill bit in relation to the reservoir sand, allowing the correct well-path deviation to be enacted. In the absence of this technique, hundreds of feet of additional non-reservoir section would have been drilled. The second example describes the Andrew Field, again comprising deep-marine turbidite sandstones of the Andrew Formation. Development of the Andrew Field has incorporated both the biosteering function of well-site biostratigraphy as well as pre-drill, high-resolution facies analysis of the microfaunas to assist in depositional modelling of the field. Microfacies analysis provides data relating to the potential sealing qualities and lateral continuity of adjacent silt and claystones, and how these may act as barriers or baffles during production. This information is utilized in the planning of the well path to optimize production through well placement and stand-off from oil-water and gas-oil contacts, and at the well site during horizontal drilling to confirm, through micropalaeontological analysis, key horizons at which to geosteer.

Upper Jurassic basin axial turbidites within the Gertrud Graben, Danish Central Graben

For more than twenty years, the Jurassic succession in the Danish Central Graben has been subject to intense exploration for hydrocarbons. Approximately 43 wild-cats have been drilled and most of these tested structural traps located on footwall crests. The reservoirs encountered were Middle and Upper Jurassic sandstones deposited mainly in nearshore depositional environments. Some of these wells penetrated thin turbidites of Late Jurassic to Early Cretaceous age. Within the Gertrud Graben distinct seismic anomalies indicate the presence of basin floor turbidites, which can be correlated to fan fringe turbidites encountered in the Jeppe-1, Gwen-2 and Mona-1 wells. Seismic inversion provides additional evidence of sandstone deposits in the zone characterized by high acoustic impedance. The turbidites are located within the axial zone of the NW–SE trending Gertrud Graben. Southward the turbidites pinch out gradually; the northern limit of the deposits (in Norwegian territory) is unknown. The deposition of the turbidites is interpreted to be associated with a lowstand of sea-level when conditions were favourable for deposition of coarse-grained deposits in basinal areas. The sediments were probably sourced from an area to the north. Basin axial turbidites are well-known phenomena in various basins. They are not always associated with structural closures and consequently constitute a high risk exploration target. However, in mature basins such as the Danish Central Graben this type of play should be considered in future exploration.

A genetic model for the H.Y.C. Deposit, Australia; based on regional sedimentology, geochemistry, and sulfide-sediment relationships

The fine-grained, laminated, stratiform zinc-lead ores of the McArthur River (HYC) ore deposit have been the subject of continued debate in regard to the timing of zinc-lead mineralization. Early workers (e.g., Croxford and Jephcott, 1972; Lambert, 1976) considered the ores to be sedimentary-exhalative (sedex), while more recent researchers (e.g., Williams, 1978a; Eldridge et al., 1993; Hinman, 1996) have argued for a syndiagenetic replacement origin. Several lines of evidence, including studies on the regional Sedimentology, sediment geochemistry, nature of sulfide layering, paragenesis, and chemistry of base metal transport and deposition, are used here to demonstrate that a synsedimentary origin is likely for the HYC deposit.Studies on the regional Sedimentology and geochemistry of the Barney Creek Formation have shown that the host rocks to the ore deposit, the HYC Pyritic Shale, represent a period of water deepening. Water depth was probably in excess of several hundred meters, with the deepest water euxinic facies corresponding to the ore position at the base of the HYC Pyritic Shale. A widespread manganese halo, developed in the W-Fold Shale Member below the ore horizon, is considered to relate to the start of the basin-deepening event, with manganese-bearing connate brines released into the basin along marginal rift faults. Manganese deposition occurred in the transitional dolomitic shale facies, where shallow-water, oxidized basin fluids intermittently mixed with deeper-water, reduced fluids along a redox interface. Sedimentary manganese deposition was immediately followed by zinc-lead mineralization in the deep-water, reduced facies of the HYC Pyritic Shale Member.The intimate relationship between the eight stratiform ore lenses and inter-ore sedimentary breccias indicates that sedimentary breccia deposition and ore formation are genetically linked. It is suggested that deep seismic events led to the following ore cycle: fault movement; sediment breccia deposition sourced from faults; episodic release of hot metalliferous brines along faults accompanied by sedimentary turbidites; and metal sulfide deposition in muds and turbidites adjacent to faults.The three modes of base metal sulfide occurrence in the ores, each considered by Eldridge et al. (1993) to have formed by replacement, are shown to have separate origins. The fine-grained sphalerite-galena bands (Mode 1) are intricately interlayered with pelagic organic-rich beds and thin turbidite beds. They exhibit classic sedimentary textures, such as load casts and flames, indicative of a synsedimentary origin. Coarser-grained Mode 2 base metal sulfides show complex intergrowths with nodular dolomite bands, and are considered to form by replacement during diagenesis. The discrete patches of coarse, Mode 3 base metal sulfides are interpreted to represent clastic input from turbidity currents sourced partly by the erosion of vein-style mineralization from the faulted margins of the basin.The detailed nature of the layering in the ores suggests that the introduction of metalliferous brines into the basin was not continuous but occurred as a series of pulses interspersed with pelagic sedimentation and turbidite deposition. Preliminary calculations suggest that On the order of 10,000 discrete pulses of brine were required to form each ore lens, and it is suggested these pulses were related to microseismic activity following each major seismic event in the basin.Solubility considerations indicate that the mineralized brines were most probably sulfate-rich (SO (super 2-) 4 > H 2 S) and capable of transporting up to several thousand parts-per-million zinc and lead at moderate temperatures (150 degrees -250 degrees C), and near-neutral pH. Zinc-lead deposition was the result of two possible processes: (1) thermochemical reduction of brine sulfate by organic matter in anoxic basin water, and (2) diffusion of H 2 S from the anoxic water column into the metalliferous brine layer. Sulfur isotope data are best explained if both processes operated together to deposit galena and sphalerite.

Near-Bottom Sedimentation Processes Revealed by Echo-Character Mapping Studies, Northwestern Mediterranean Basin

Subbottom echoes recorded on 3.5 kHz echo sounders in the northwestern Mediterranean Basin from 1978 to 1991 can be classified into five main categories of echo character: (1) Continental shelf (CS) echo character characterizes the continental platform and areas of shallow relief and commonly is related to consolidated sediments. (2) Hyperbolic (H) echo character is recorded from areas with rough bottom morphology. Echo type H1 corresponds to steep slopes; echo type H2 reflects very coarse grained sediments deposited by energetic turbidity currents. (3) Bedded (B) echo character is widely observed throughout the study area and corresponds to terrigenous deposits distributed by turbidity currents. Echo type B1 is related to thin turbidites deposited on overbank portions of channel levees by lateral overflow of turbiditic currents moving down the channels; echo type B2 corresponds to coarser turbiditic materials. (4) Rugged (R) echo character is observed in the axes of submarine canyons and channels and characterizes hard sea floor with coarse heterogeneous turbiditic deposits subjected to energetic gravity-flow processes. (5) Transparent (T) echo character is ubiquitous in the study area and is attributed to mass-movement deposits. The widespread occurrence of those deposits testifies to the general instability of the study area. Types and distribution of echo character throughout the study area show the importance of gravity-dominated depositional processes (turbidity currents and mass wasting) in this region. This remobilization of siliciclastic sediments, induced by high sediment rates, steep slopes, and halotectonics, theoretically could create potential hydrocarbon reservoirs within the deep-water portions of the continental margin.

Deep‐sea avulsion and morphosedimentary evolution of the Rhône Fan Valley and Neofan during the Late Quaternary (north‐western Mediterranean Sea)

ABSTRACTThe Petit‐Rhône Fan Valley (north‐western Mediterranean) is a broad, sinuous, filled valley that is deeply incised by a narrow, sinuous thalweg. The valley fill is differentiated into three seismic subunits on high‐resolution seismic‐reflection profiles. The lower chaotic subunit probably consists of channel lag deposits that seem to be in lateral continuity with high‐amplitude reflections representing levee facies. The intermediate transparent subunit, which has an erosional base and clearly truncates levee deposits, is interpreted to be mass‐flow deposits resulting from the disintegration of the fan‐valley flanks. The upper bedded subunit shows an overall lens‐shaped geometry and the seismic reflections onlap either onto the top of the underlying transparent subunit or onto the Rhône levees. Piston core data show that the upper few meters of this upper subunit consist of thin turbidites, probably deposited by overflow processes. The few available 14C ages suggest that the upper stratified subunit filled the Petit‐Rhône Fan Valley between 21 and 11 kyr BP.The upper bedded subunit is deposited within the Petit‐Rhône Fan Valley downslope of a major decrease in slope gradient. This upper subunit and the thalweg are genetically related and represent a small channel/levee system confined within the fan valley. Previous studies interpreted this thalweg to be an erosional feature resulting from a recent avulsion of the major channel course. Our interpretation implies that the thalweg is not a purely erosional feature but a depositional/erosional channel. This small channel/levee system is superimposed on a large muddy channel/levee system after the sediment supply changed from thick muddy flows during the main phase of aggradation of the Rhône Fan levees, to thin, mixed (sand and mud) flows at the end of Isotope Stage 2 (∼16–18 ka BP). The pre‐existing morphology of the Petit‐Rhône Fan Valley played a determinant role in the sediment dispersal leading to the creation of this small and confined channel/levee system. These mixed flows have undergone flow stripping resulting from the changes in the slope gradient along the thalweg course. The finer sediment overflowed from the thalweg and were deposited in the Petit‐Rhône Fan Valley. Coarser channelled sediment remaining in the thalweg were deposited as a ‘sandy’lobe (Neofan). As indicated by 14C dating, sedimentation on this lobe continued until very recently, suggesting a further evolution of the turbidity flows from small mixed flows to small sandy flows. the deposition of this study lobe and the sedimentary fill of the Petit‐Rhône Fan Valley may be related to widespread shelf edge and canyon wall failures with a resulting downslope evolution of failed sediment into turbidity currents.

Late Quaternary Turbidite Emplacement on the Horseshoe Abyssal Plain (Iberian Margin)

ABSTRACT The Horseshoe abyssal plain (HAP) is located on the Iberian margin at approximately 4800 m water depth, and is confined within topographic elevations with a relief of about 3000 m, with the exception of Gorringe Bank, which rises to only 20 m below sealevel. The plain is built up by an alternation of turbidites of the order of meter-thick beds, numbered H2, H3, H8, and H13, contrasting with sets of thinner beds of the order of decimeters thick (H4,5,6,7 and H9,10,11,12), and pelagites centimeters in thickness. An intensive study of eight piston cores, using visual observation (color and thickness), relative stratigraphic position of units, magnetic susceptibility logs, calcium carbonate content, and mineralogy of turbid te bases, concludes in a bed-by-bed correlation of all individual turbidites. The major source of terrigenous material feeding the HAP is the Sao Vicente canyon, which incises the Portuguese shelf, while minor sources are the surrounding seamounts. The elongate geometry of the abyssal plain with its single dominant source of sediments produced laterally continuous deposits from both large and small flows. These covered the entire 356 km length of the plain. Grain-size analysis of the four thickest turbidites (H2, H3, H8, and H9) demonstrates only slight downcurrent fining and vertical grading, expressed best in the ratio of coarse silt to fine silt plus clay. A small amount of sand is carried the length of the plain. The thicker units all show the same pattern of thickness, with a maximum in the middle of the plain around a topographic constriction and bend. This is most plausibly explained as due to reduction in flow speed of an initially supercritical flow causing enhanced deposition Some of the beds appear to have a double coarse layer in the base, which may indicate partial reflection of flows from the side of the basin. It is suggested by application of equations for flow behavior that both thick ( 3 m) and thin ( 0.3 m) beds are due to supercritical flows a few tens of meters high. However, the thick beds resulted from high-concentration flows (Cv 4% by volume) whereas thinner beds require low concentration (Cv < 1%) to run out over the full length of the basin. The stratigraphy is tied into the dated oceanic pelagic record by analysis of foraminifera in the pelagic layers above the turbidites and through recognition of two Heinrich layers (H-1 and H-2, ages 14.3 and 21 ka). The resulting age framework shows higher turbidite frequency in the glacial (2.7/kyr) than interglacial (Holocene) (1.0/kyr). This also gives higher mass flux during the glacial. Emplacement of turbidites cannot be clearly related to sea-level changes but may well be due to seismic activity. However, one of the largest earthquakes in human experience (Lisbon in 1755) triggered only a thin turbidite, invalidating the term seismite for thick turbidites.

Turbidite and contourite sediment waves in the northern Rockall Trough, North Atlantic Ocean

ABSTRACTTwo sediment wave fields have been identified in the northern Rockall Trough; one from the north‐eastern part of the trough, developed on the flank of an elongate sediment drift, and the other at the distal edge of the Barra Fan. Wavelengths in both areas vary from 1 to 2 km and wave heights from 5 to 20 m. The seismic character of both the wave fields is similar with a lower package of well‐layered, medium‐ to high‐intensity reflectors migrating upslope, overlain by a dominantly acoustically transparent unit containing irregular, semicontinuous reflectors. Eight cores have been recovered from the two wave fields, seven from the crest‐trough areas of the distal Barra Fan wave field, and a single core from the crest of the sediment‐drift waves. Lithologically, the cores show that different processes have been active across the two wave fields. Cores from the Barra Fan field contain thin turbidites with thicker, draped hemipelagites and hemiturbidites, corresponding to the well‐layered, reflective seismic units and transparent seismic unit, respectively. These waves have been maintained by turbidity currents, perhaps over older, originally bottom‐current‐formed waves. The single core from the small sediment‐drift wave field recovered hemipelagites and glaciomarine sediments grading upwards into muddy‐silty contourite deposits, topped by a sandy contourite. These waves were constructed by contour currents. Dating of cores from these two small wave fields revealed that the sequences of thin turbidite and hemipelagite sediments from the Barra Fan correspond to the Late Glacial‐Allerød/Bölling Interstadial with the overlying hemipelagite of Younger Dryas‐Holocene age. The contourite deposits from the north‐east Rockall Trough wave field have been dated as early Holocene, reflecting increasing bottom‐current activity at the changeover from a glacial to an interglacial regime.

Taphonomy and ecology of deep-water Ediacaran organisms from northwestern Canada

Although most occurrences of Ediacaran fossils are from shallow-shelf deposits, taxonomically-similar assemblages have recently been described from a 2.5 km-thick succession of dark mudstones and turbiditic sandstones in the Windermere Supergroup of the Mackenzie Mountains, northwestern Canada. The paleogeographic position (20-40 km seaward of the shelf edge), abundant evidence of mass flow, and the complete absence of in situ shallow-water features imply that deposition took place on a slope considerably below storm wave-base. Ediacaran fossils were not observed in axial trough deposits (lower parts of the Twitya and Sheepbed formations), but megafossils occur sporadically in lower to middle slope deposits higher in the same formations. Megafossils and trace fossils are present in upper slope settings (Blueflower Formation) at the top of the Ediacaran succession. The megafossil assemblage varies stratigraphically, but in all formations is dominated by discoid forms (e.g. Cyclomedusa, Ediacaria, Nimbia); frondose forms and vendomiids are very rare.Megafossils are preserved mainly as positive features on the soles of thin turbidite beds. Most fossiliferous beds begin with the rippled layer of the turbidite (Tc), but a few begin with the graded (Ta) or parallel-laminated (Tb) layer. Consistent orientation and high relief of individuals, evidence of mutual deformation during growth of adjacent organisms, and other taphonomic features imply that virtually all of the taxa represent benthic polypoid and frond-like organisms (not jellyfish). Slump structures occur commonly in the sandstone fill of fossils, suggesting that many of the organisms were buried alive by the turbidite and later decomposed. Other individuals, even on the same bedding plane, exhibit graded to laminated fill identical to that of the overlying turbidite bed, indicating that the depressions on the sea bottom produced by these individuals were empty at the time of turbidite deposition. Escape structures are absent, suggesting that the Ediacaran organisms were not capable of burrowing up through even thin layers of sand.Ediacaran megafossils are invariably preserved on black, wrinkled surfaces similar to those elsewhere interpreted as microbial mats. Molding of delicate features (including tentacles), preservation of open molds as negative epireliefs, and sedimentological evidence of considerable cohesion of these surfaces relative to the underlying turbiditic muds (Td,e) supports this interpretation, and suggests that microbial mats were as important in the preservation of these deep-water Ediacara faunas as they were in their shallow-water equivalents. The presence of the wrinkled mats and their associated Ediacaran fossils almost exclusively in the pyritic intervals of the succession suggests that both may have lived under exaerobic conditions in this deep-water setting.

Facies Implications of Trichichnus and Chondrites in Turbidites and Hemipelagites, Marnoso-Arenacea Formation (Miocene), Northern Apennines, Italy

Trichichnus, a thread-like burrow possibly the work of sipunculan worms, is widespread in Bouma E (turbidite claystone) and H (hemipelagic claystone) layers and some thin turbidite sandstone beds in basin-plain and outer fan-lobe deposits of the Marnoso-arenacea Formation. Trichichnus closely resembles that described elsewhere in chalk and marlstone, although the burrows are smaller (mean diameter=0.13 mm) and burrow curvature is tighter. Trichichnus fills are darker than the host claystone because of high organic content and disseminated pyrite. Any distinctive internal structure of the burrow wall or fill was destroyed by pyrite growth. Trichichnus is most abundant in Bouma E layers, where burrow spacing ranges from 0.4 to 50 cm and burrows extend to depths of 160 cm. The Trichichnus maker commonly burrowed vertically down through E and D layers (clay and silt) and then horizontally on top of C layers (sand). The burrows also vertically penetrate Bouma C sandstone layers up to 6 cm thick, but they are rare in beds thicker than 3 cm. They are less abundant or not well displayed in H layers where Chondrites predominates. Trichichnus overprints Chondrites. Following deposition of the Contessa megaturbidite, which exceeds 9 m in thickness, the sediment surface was repopulated chieflymore » by Trichichnus and Chondrites producers. Trichichnus extends 160 cm below the top of the Contessa, whereas Chondrites extend only to 64 cm. This suggests a greater tolerance of Trichichnus makers to low in situ oxygen levels. The near absence of laminations in the E division of the Contessa megabed may be the result of Trichichnus burrows that were not pyritized and which lack textural identity.« less

Detailed Layer Studies in the East Wilmington Oil Field

Recent detailed studies have been made of the Pliocene and upper Miocene sequences in the East Wilmington oil field. Roughly 1,200 vertical ft of section (Ranger Zone) were evaluated in fault blocks VI and VII. The sequence was broken into 40 to 50 correlatable units (sub-subzones) based on electric logs. Twelve horizons (subzones) were used to guide the correlation work and aid in the construction of true stratigraphic logs. The TST logs were used to make many north-south and east-west cross sections. This provided good loop control for correlations. Over eight hundred wells were evaluated in this way. Structure maps, isochore maps, and percent sand maps were constructed. The primary purpose of this work was to determine the net pay for the Ranger zone. This work has found excellent secondary applications including definition of flow units, clarification of stratigraphic sequencing, and depositional history. The depositional history involved the sedimentation and transport of lobed deposits and sheet or draped deposits. The Upper Miocene sands are intercalated with shales and siltstone. These thin turbidites are reasonably wide spread and show some lenticularity but not the lobate lenticularity that is common in the Pliocene sands. The Pliocene section is dominated by large lobatemore » fans. These lenticular depositional units show much about sediment transport. The lobes lie laterally between each other with each successive lob filling the low area adjacent to or between the earlier lobes. Balancing of the deposited sand results. Knowledge of the lobed nature of the sands and specific localized characteristics helps in the design of injection programs that will greatly improve the sweep efficiencies.« less

Ordovician graptolite biostratigraphy in North Greenland

Up to 500 m of black-bedded cherts and mudstones with thin turbidites and thick local beds of redeposited limestone and chert conglomerates were deposited during the Ordovician in North Greenland, along the southern margin of an east-west trending deep-water trough forming a continuation of the Frank Jinian Basin of Arctic Canada. A large collection of Ordovician graptolites has recently been obtained from this clastic sequence. The graptolite fauna, not collected in continuous sections, compares particularly well with the fauna from the Canadian Cordillera and for the most part is interpreted in terms of the established biozones from that area. The faunas are also correlated with the Australian zonal sequences. In North Greenland neither the Cambrian-Ordovician boundary nor the Ordovieian-Silurian boundary ean be precisely demarcated by graptolites. The folIowing graptolite biozones are represented: Anisograptus, Adelograptus &amp; Clonograptus, T. approximalus, P. frulicosus, D. bifidus, I. victoriae lunatus, ? T. victoriae victoriae, ? l. victoriae maximus, Oncograptus, P. tentaculatus, ? 'D.' decoratus, ? H. lereliusculus, N. gracilis, C. bicornis, ? O. amplexicaulis, O. quadrimucronatus, D. ornatus and? P. pacificus. The relationship of the North Grecnland graptolites to the Ordovician 'Pacific faunal realm' and oceanic graptolite biofacies is briefly discussed.

Lacustrine sedimentation in a semiarid alpine setting: An example from Ladakh, northwestern Himalaya

The Lamayuru lacustrine strata in Ladakh typify many of the carbonate-rich Pleistocene alpine lakes found in the semiarid environment of the northern Himalaya. Created by a 200-m-thick landslide, the lake was in existence by at least 35,000 yr ago, and may have persisted until 500–1000 yr ago. Represented in the center by thin turbidites and laminated muds, the lacustrine sedimentation along the lake margins and low-relief deltas characteristically displays a marked contrast between (1) clastic lenses representing rapid, sporadic, matrix-poor debris flows and periglacial inputs from the alpine slopes and (2) abundant, diverse, shallow-water, biologically dominated carbonate strata, among which organism-rich, chalky beds and oncolithic and encrusted stem-rich strata predominate. Resemblances of the Lamayuru lacustrine strata and their setting to those of former lakes throughout areas north of the Greater Himalayan crest suggest that the alpine, semi-arid environment would favor diversified, spacially restricted carbonate sedimentation punctuated by occasional clastic influxes. Such a depositional regime contrasts strongly with that found immediately south of the Himalayan crest where more humid conditions promote a more continuous clastic influx into intramontane lakes.

Evolution models of two flysch basins: The northern carpathians and the southern apennines—reply

This study focused on the Cretaceous black shale successions, followed by red shales that crop out at the outer regions of the Romanian Carpathians, in the Moldavids. The oldest parts of the black shale units deposited in an abyssal plain during Late Valanginian–Late Barremian time; they are mainly characterized by hemipelagic and pelagic muddy siliciclastic rocks and carbonates, commonly intercalated with fine-grained turbidites.During the sedimentation of the middle part of the black shale units in the Late Barremian–Early Albian interval, the depth of the basin increased, as the carbonate hemipelagic sedimentation was replaced by a mainly siliceous one. Only a few thin turbidite intercalations are present.The youngest part (Albian pro parte) of the black shale units is characterized by a turbiditic sedimentation, with mainly sandy sequences of middle and lower deep-water fans. We may assume that the depth of the basin continuously decreased. The presence of authigenic glauconite in the Albian sandstones suggests a palaeoenvironmental change, linked to the occurrence of oxygenated turbidity current circulation.A significant shift in the sedimentation regime in the Eastern Carpathian Moldavids took place in the Late Albian, when Cretaceous Oceanic Red Beds (CORB) occurred. This type of sedimentation lasted up to the Coniacian. The lower part of the CORBs that contains radiolarites intercalated with variegated shales, pyroclastic tuffs and thin sandstones is interpreted as a hemipelagic and pelagic sedimentation in the abyssal plain environment, where rarely turbidites occurred. Upwards, there are mainly burrowed variegated red and green shales. The youngest parts of CORBs are characterized by increased thickness and frequency of the turbidites. While the main part of the CORB is carbonate free or has very low carbonate content, the upper part of these strata becomes rich in marl and mudstone strata, indicating a decrease of the basin-depth.The accumulation of black shales in the Eastern Carpathians during the Late Valanginian–Late Albian interval is linked to the existence of a small, silled basin of the Moldavian Trough, in which restricted circulation led to the density stratification of the water column, resulting in the deposition of anoxic Lower Cretaceous sediments (i.e., the black shales). Because of the tectonic deformation that took place within the Lower–Upper Cretaceous boundary interval, the restricted circulation had changed to an open circulation regime in the Moldavian Trough. Hence, the anoxic regime was progressively replaced by an oxic one, across the Albian–Cenomanian boundary interval. The beginning and the end of the CORBs in the Moldavid units depend thus on various palaeogeographic and palaeoenvironmental settings, and it was controlled by the regional tectonic activity.

Facies-related diagenesis in the Main Claymore Oilfield sandstones

AbstractFour major sedimentary sequences of the Triassic and Upper Jurassic of the Main Claymore Oilfield of the North Sea each contain a characteristic suite of diagenetic minerals and fabrics. (1) Triassic Skagerrak Formation fluvial sandstones contain early authigenic pore-lining smectite, together with kaolinite and chlorite which form grain replacements and pore fills. Quartz and feldspar overgrowths are minor. Ferroan dolomite forms a late diagenetic patchy poikilotopic cement. Smectite is converted to illite-smectite in a 5 m thick zone beneath the sub-Jurassic unconformity. Smectite formed early in diagenesis prior to oil migration and destroyed permeability. Thus oil is not found in these sandstones although they occur in the oilzone. (2) The Piper Formation (late Oxfordian/early Kimmeridgian) paralic deposits mainly contain authigenic, pore-lining illite-smectite, vermicular kaolinite grain replacements and pore fills. Quartz overgrowths are generally well developed. (3) The Kimmeridge Clay Formation (early Kimmeridgian/early Volgian) comprises thin marine sandstone turbidites, contained within a thick siltstone/shale sequence. In the sandstones (the ‘Ten Foot Sandstone’) discrete double-ended quartz crystals (1–20 µm) developed prior to quartz, K- and Na-feldspar overgrowths. Only minor kaolinite and lllite-smectite are present. Late diagenetic dolomitic occurs as a patchy poikilotopic cement and as clusters of pore-filling rhombs. (4) The Claymore Sandstone Member (early to middle Volgian) thick marine sandstone turbidites are interbedded with thin siltstones/shales. Sandstones have well-developed quartz, K and Na-feldspar overgrowths, and kaolinite and illite-smectite occur as grain replacements and rarely as pore fills. Late-diagenetic dolomite and ferroan dolomite form poikilotopic cement and clusters of pore-filling rhombs. The major factors which control diagenetic features are depositional environment and associated porewater together with original mineralogy. Burial history and textural features of the sandstones also have important influences. Reservoir quality is controlled by a complex interplay of these features.