1,721,002 research outputs found
A high-precision Jacob’s staff with improved spatial accuracy and laser sighting capability
Full text linkA new Jacob’s staff design incorporating a 3D positioning stage and a laser sighting stage is described. The first combines a compass and a circular spirit level on a movable bracket and the second introduces a laser able to slide vertically and rotate on a plane parallel to bedding. The new design allows greater precision in stratigraphic thickness measurement while restricting the cost and maintaining speed of measurement to levels similar to those of a traditional Jacob’s staff. Greater precision is achieved as a result of: a) improved 3D positioning of the rod through the use of the integrated compass and spirit level holder; b) more accurate sighting of geological surfaces by tracing with height adjustable rotatable laser; c) reduced error when shifting the trace of the log laterally (i.e. away from the dip direction) within the trace of the laser plane, and d) improved measurement of bedding dip and direction necessary to orientate the Jacob’s staff, using the rotatable laser. The new laser holder design can also be used to verify parallelism of a geological surface with structural dip by creating a visual planar datum in the field and thus allowing determination of surfaces which cut the bedding at an angle (e.g., clinoforms, levees, erosion surfaces, amalgamation surfaces, etc.). Stratigraphic thickness measurements and estimates of measurement uncertainty are valuable to many applications of sedimentology and stratigraphy at different scales (e.g., bed statistics, reconstruction of palaeotopographies, depositional processes at bed scale, architectural element analysis), especially when a quantitative approach is applied to the analysis of the data; the ability to collect larger data sets with improved precision will increase the quality of such studies
Quantifying tabularity of turbidite beds and its relationship to the inferred degree of basin confinement
No full textTabular beds and sheet-like deposits in deep-water systems have been the subject of much research attention; they can form high quality hydrocarbon reservoirs, owing to their excellent lateral continuity and predictable geometry. Additionally, deposit tabularity is a piece of evidence used to infer flow confinement in ancient systems and thus to evaluate the suitability of outcrop datasets as reservoir analogues. However, the quantification of tabularity is rarely attempted and a consistent definition on how to describe it quantitatively is lacking. For this study, published data from eighteen well-constrained ancient turbidite systems in outcrop were analysed. A simple and novel methodology for the quantitative calculation of tabularity along a transect from log panels and photo panels was devised, based on: a) subdividing beds into two groups based on their thickness, b) calculating the percentage of beds continuous across a fixed window (500 m) and c) calculating the rate of thinning for the continuous beds within the same window. Calculations obtained from multiple locations within individual systems enable the investigation of proximal to distal, and axial to lateral changes in tabularity to be captured, and therefore permits the evaluation of tabularity in three-dimensions. A comparison between tabularity of the considered systems and their inferred degree of basin confinement shows that in the confined systems >= 90% of beds are continuous over 500 m compared to <= 40% for the two unconfined systems studied. In addition, different bed types were compared: hybrid event bed thinning rates are shown to be up to three times those of classical turbidites. This methodology provides a new tool to compare tabularity within and between systems quantitatively. It is hoped that the quantitative determination of tabularity will become a common workflow when describing ancient turbidite systems. It is suggested that this approach will enhance the value of outcrop data to inform models capturing the architecture of systems analogous to subsurface hydrocarbon reservoirs
FLOW BEHAVIOR OF PONDED TURBIDITY CURRENTS
No full textSea-floor topography can constrict, deflect, or reflect turbidity currents resulting in a range of distinctive deposits. Where flows rebound off slopes and a suspension cloud collects in an enclosed basin, ponded or contained turbidites are deposited. Ponded turbidites have been widely recognized in slope mini-basins and on small, structurally confined basin floors in strike-slip and foreland-basin settings. They can have a variable internal structure the significance of which remains poorly understood in terms of flow behavior. New experiments demonstrate that the ponding process can comprise up to four phases: 1) cloud establishment, 2) inflation, 3) steady-state maintenance, and 4) collapse. The experiments explored the behavior of sustained turbidity currents draining into small basins and show that the ponded suspensions that formare characterized by an important internal interface; this divides a lower outbound-moving layer froman upper return layer. The basal layer evolves to constant concentration and grain size, whereas the upper layer is graded (concentration and grain size decrease upward). During the cloud inflation stage, the concentration and velocity profiles of the ponded suspension evolve, and this phase can dominate the resulting deposit. Outbound internal waves can travel along the interface between the outbound and return layers and impinge against the confining slope, and their amplitude is highest when the density contrast between layers is greatest, e.g., when the input flows are thin and dense. The experiments show that flow reversals can arise in several ways (initial rebound, episodic collapse of the wedge of fluid above the counter slope, "grounding" of the internal velocity interface) and that despite steady input, velocities decay and the deposit grades upwards. Internal waves emanate from the input point, i.e., do not form as reflections off the counter slope. The internal grain-size interface within the suspension may dictate textural trends in sands onlapping the confining slopes. Where flows are partially ponded, internal waves can generate pulsing overspill to basins down dip
INFLATION OF PONDED, PARTICULATE LADEN DENSITY CURRENTS
No full textField-based, physical modeling and analytical research approaches currently suggest that topographically confined particle-laden density currents commonly inflate to produce suspension clouds that generate tabular and texturally homogeneous sedimentary deposits. Here, a novel three-dimensional theoretical model details a phase space of the criteria for inflation as a function of flow duration, basin size and geometry, total mass transport, sediment concentration, and particle grain size. It shows that under most circumstances cloud inflation is unlikely at real-world scales. Even where inflation is possible, inflation relative to initial flow height is small except for suspensions of silt or finer-grained sediment. Tabular deposits therefore either arise from processes other than flow ponding, or deposits in confined settings may be significantly more complex than are currently understood, due to processes of autogenic compensation and channelization, with associated implications for reservoir characterization in applied contexts. This study illustrates the potential of analytical flow modeling as a powerful complement to other research approaches
RHEOLOGICAL COMPLEXITY IN SEDIMENT GRAVITY FLOWS FORCED TO DECELERATE AGAINST A CONFINING SLOPE, BRAUX, SE FRANCE
No full textHybrid event beds are now recognized as an important component of many deep-sea fan and sheet systems. They are interpreted to record the passage of rheologically complex sediment gravity currents (hybrid flows) that comprise turbulent, transitional, and/or laminar zones. Hitherto, the development of hybrid flow character has mainly been recognized in system fringes and attributed to distal and lateral flow transformations and/or declining turbulence energy expressed over lateral scales of several kilometers or more. However, new field data show that deposition from hybrid flows can occur relatively proximally, where flows meet confining topography. Turbidity currents primed to transform to hybrid flows by up-dip erosion and incorporation of clay may be forced to do so by rapid, slope-induced decelerations within 1 km of the slope. Local flow transformation and deposition of hybrid event-beds offer an alternative explanation for unusual facies developed at the foot of flow-confining seafloor slopes
Detrital signatures of impending collision: The deep-water record of the Upper cretaceous Bordighera Sandstone and its basal complex (Ligurian Alps, Italy)
Full text linkDespite intensive research efforts and significant advances in the understanding of subduction and obduction processes that affected several units which at the present day compose the Western Alps, the paleogeographic evolution of the Alpine Tethys represents a debated topic in Alpine geology. The role of the opposing continental margins (passive European margin and active Adriatic margin) as source regions for Cretaceous siliciclastic turbidites bordering the convergent system remains disputed. To address this question along the Ligurian Alps transect, a multi-proxy provenance analysis is applied to the two terrigenous superimposed units (Hauterivian-Campanian San Bartolomeo Fm. and Campanian-Maastrichtian Bordighera Sandstone) of the San Remo-Monte Saccarello Unit of the Western Ligurian Flysch complex. Petrographic analyses characterize the basal San Bartolomeo Fm. as quartz-rich mature sandstones. By contrast, the overlying Bordighera Sandstone represents texturally and compositionally immature first-cycle arkosic arenites. This change records the evolution of the sediment provenance from a stable craton into a continental basement uplift setting, reflecting erosion of granitoid plutons and the low-grade metamorphic basement. Geochronological data (U Pb detrital zircon ages) indicate that virtually the same source terranes provided the source for both formations. The detrital age spectra display age peaks are compatible with well-documented magmatic and metamorphic pulses that affected the Southern Variscides in the Paleo-European margin. The strong affinity of clastic detritus with the Paleo-European margin basement rocks underlines the importance of the lower plate passive continental margin in supplying sand-rich turbidite systems prior to the arrival of the passive margin in the subduction zone
Depositional evolution of a tectonically-confined proximal-foredeep deep-marine system: Miocene Serra Palazzo Formation (Southern Apennines, Italy)
No full textThe Miocene Serra Palazzo Formation (SPF) was deposited in a foredeep basin in front of the growing Southern Apennines orogenic wedge. A new detailed facies analysis, based on high-resolution physical stratigraphy, for a 570 m-thick interval, has been undertaken in order to reconstruct the depositional history. Two phases of development can be recognized. The older one was characterized by the sedimentation of sandy channel-lobe transition zone (cltz) deposits with various degrees of amalgamation; roughly in the middle, a 26 m-thick sandy megaturbidite was deposited. At this time, basin topography, including the inbound slope base and topography due to the orogen-related flexural subsidence, caused significant flow confinement illustrated by a number of sedimentary features, such as syn-sedimentary deformation structures, hummocky laminations, and differences in the palaeocurrent directions recorded in the same event bed. The younger phase was characterized by the sedimentation of more-proximal amalgamated sandy channel deposits, followed upwards by an even more-proximal sequence of thick levees with thin-bedded and fine-grained facies. Tectonics was one of the main controls on the stacking patterns of the submarine fans of the SPF basin. Particularly, the outward thrust migration governed the evolution from the cltz deposits to the levee deposits and it might have caused the repeated waxing-waning flow cycles recorded in the channel deposits and the changes in bed dip angles in the levee deposits
Submarine channel network evolution above an extensive mass-transport complex: A 3D seismic case study from the Niger delta continental slope
No full textA submarine channel network, named Abalama Channel System (ACS), has been recognised in the subsurface of the Niger Delta continental slope. It overlies a mass-transport complex (MTC) and consists of six channel segments, delimited by five avulsion points and one confluence point. High-resolution 3D seismic data are used to investigate the development of the ACS and to describe the interaction between the channels and the underlying MTC. The MTC mainly consists of highly disaggregated materials (MTC matrixes) and in plan-view has a very complex fingered geometry, characterised by the presence of erosional remnants (remnant blocks). The different character of the MTC matrixes compared to that of the remnant blocks likely resulted in a bathymetry characterised by negative and positive relief, which provided the initial confinement for the channels of the ACS. In areas where the MTC-induced confinement was weak or decreased abruptly, channels tended to develop higher sinuosity, increasing channels instability and ultimately causing avulsions. Three ideal categories of submarine channel avulsions are observed. Type 1 is characterised by parent and avulsion channel having similar size and maturity; Type 2 is characterised by a large, high-maturity parent channel and a small, low-maturity avulsion channel; Type 3 emphasizes the larger scale and higher maturity of the avulsion channel compared to the parent channel. In the distal part of the study area, topography related to mud diapirs provided lateral confinement that captured flows avulsed at different times resulting in a channel confluence phenomenon. Submarine channel network evolution recorded by avulsion and confluence points represents an important research theme in deep water sedimentology, as it controls the final distribution of sediments and the extension of sands in the whole deep-water depositional system; hence this study can be used to guide hydrocarbon exploration in analogue systems
Distribution of Hybrid Event Beds (HEBs within a mixed siliciclastic-calcareous turbidite sequence: the Bordighera deep-sea fan (Late Cretaceous,Western Alps, Italy)
No full textTectonic Influence on the Geomorphology of Submarine Canyons: Implications for Deep-Water Sedimentary Systems
Full text linkA database-informed metastudy of 294 globally distributed submarine canyons has been conducted with the aim of elucidating the role of tectonic setting on submarine-canyon geomorphology. To achieve this, data from seafloor and subsurface studies derived from 136 peer-reviewed publications and from open-source worldwide bathymetry datasets have been statistically analyzed. In particular, relationships between margin type (active vs. passive) or plate-boundary type (convergent vs. transform vs. complex) have been assessed for key morphometric parameters of submarine canyons, including: streamwise length, maximum and average width and depth, canyon sinuosity, average canyon thalweg gradient, and maximum canyon sidewall steepness. In addition, possible scaling relationships between canyon morphometric parameters and characteristics of the associated terrestrial catchment, continental shelf and slope, and of the broader physiographic setting for canyons along both active and passive margins have been evaluated. The following principal findings arise: 1) overall canyon geomorphology is not markedly different across tectonic settings; 2) slope failure might be more important in passive-margin canyons compared to active ones, possibly due to seismic strengthening in the latter; 3) some aspects of canyon geomorphology scale with attributes of the source-to-sink system and environmental setting, but the strength and sign in scaling might differ between active and passive margins, suggesting that the extent to which canyon geomorphology can be predicted depends on the tectonic setting. Insights from our analysis augment and improve conceptual, experimental and numerical models of slope systems at the scale of individual canyons and source-to-sink systems, and increase our understanding of the complex role played by tectonic setting in shaping deep-water systems
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