“…) following the procedure published by Mena et al . (). Each core was scanned using a high‐resolution Hitachi ECLOS medical scanner (Hitachi, Tokyo, Japan).…”
This work presents a detailed study of CONTOURIBER and Integrated Ocean Drilling Program 339 sediment data targeting sand-rich contourites in the Eastern Gulf of Cadiz. All of the collected sediments are interpreted as contourites (deposited or reworked by bottom currents) on the basis of oceanographic setting, seismic and morphometric features, and facies characteristics. A variety of sandy and associated facies are found across the study area including: (i) bioturbated muddy contourites; (ii) mottled silty contourites; (iii) very fine mottled and fine-grained bioturbated sandy contourites; (iv) massive and laminated sandy contourites; and (v) coarse sandy/ gravel contourites. The thickest sands occur within contourite channels and there is a marked reduction in sand content laterally away from channels. Complementary to the facies descriptions, grain-size analysis of 675 samples reveals distinctive trends in textural properties linked to depositional processes under the action of bottom currents. The finest muddy contourites (<20 lm) show normal grain-size distributions, poor to very poor sorting, and zero or low skewness. These are deposited by settling from weak bottom currents with a fine suspension load. Muddy to fine sandy contourites (20 to 200 lm) trend towards better sorting and initially finer and then coarser skew. These are typical depositional trends for contourites. As current velocity and carrying capacity increase, more of the finest fraction remains in suspension and bedload transport becomes more important. Clean sandy contourites (>200 lm) are better sorted. They result from the action of dominant bedload transport and winnowing at high current speeds. The results highlight the importance of bottom current velocity, sediment supply and bioturbational mixing in controlling contourite facies. Despite growing interest in their hydrocarbon exploration potential, contourite sands have remained poorly understood. This research therefore has important implications for developing current understanding of these deposits and aiding the correct interpretation of deep marine sands and depositional processes.
2223
“…) following the procedure published by Mena et al . (). Each core was scanned using a high‐resolution Hitachi ECLOS medical scanner (Hitachi, Tokyo, Japan).…”
This work presents a detailed study of CONTOURIBER and Integrated Ocean Drilling Program 339 sediment data targeting sand-rich contourites in the Eastern Gulf of Cadiz. All of the collected sediments are interpreted as contourites (deposited or reworked by bottom currents) on the basis of oceanographic setting, seismic and morphometric features, and facies characteristics. A variety of sandy and associated facies are found across the study area including: (i) bioturbated muddy contourites; (ii) mottled silty contourites; (iii) very fine mottled and fine-grained bioturbated sandy contourites; (iv) massive and laminated sandy contourites; and (v) coarse sandy/ gravel contourites. The thickest sands occur within contourite channels and there is a marked reduction in sand content laterally away from channels. Complementary to the facies descriptions, grain-size analysis of 675 samples reveals distinctive trends in textural properties linked to depositional processes under the action of bottom currents. The finest muddy contourites (<20 lm) show normal grain-size distributions, poor to very poor sorting, and zero or low skewness. These are deposited by settling from weak bottom currents with a fine suspension load. Muddy to fine sandy contourites (20 to 200 lm) trend towards better sorting and initially finer and then coarser skew. These are typical depositional trends for contourites. As current velocity and carrying capacity increase, more of the finest fraction remains in suspension and bedload transport becomes more important. Clean sandy contourites (>200 lm) are better sorted. They result from the action of dominant bedload transport and winnowing at high current speeds. The results highlight the importance of bottom current velocity, sediment supply and bioturbational mixing in controlling contourite facies. Despite growing interest in their hydrocarbon exploration potential, contourite sands have remained poorly understood. This research therefore has important implications for developing current understanding of these deposits and aiding the correct interpretation of deep marine sands and depositional processes.
2223
“…Five gravity cores were collected during ForSaGal 09 in 2009 from the GIB, a narrow basin (around 100 km width; 2500–3000 m water depth) along the NW Iberian margin 17 . Sedimentary facies (i.e., Pelagic, Hemipelagic, Contouritic, Turbitidic and IRD layers) were previously characterized based on grain size, composition, sedimentary features and Computed Tomography (CT) scanning 12,17 . Contourite deposits, concretely muddy contourites, were identified in three cores.…”
Section: Methodsmentioning
confidence: 99%
“…This setting provides a detailed record of Quaternary contourite deposits 13–16 . Contouritic facies appear as massive to coarsely laminated silt to very fine silty sand and, show varying degrees of bioturbation 17 .Figure 1Location of cores analysed in this study. GB, Galicia Bank; GIB, Galicia Interior Basin; TZ, Transitional Zone.…”
Although bioturbation is commonly recognized in contourites, only a few studies have analyzed the ichnological content of these deposits in detail. These studies have mainly focused on meso-scale bigradational sequence (a coarsening upward followed by a fining-upward sequence resulting from variations in current velocity). Here we present data from gravitational cores collected along the NW Iberian Margin showing systematic variation in ichnological content across proximal to distal depocenters within a large-scale elongated contourite drift. Data demonstrate that tracemakers’ behavior varies depending on the distance relative to the bottom current core. Trace fossils are already known to be a useful tool for studying of contouritic deposits and are even used as criterion for differentiating associated facies (e.g., turbidites, debrites), though not without controversy. We propose a mechanism by which the distance to the bottom current core exerts tangible influence on specific macro-benthic tracemaker communities in contourite deposits. This parameter itself reflects other bottom current features, such as hydrodynamic energy, grain size, nutrient transport, etc. Ichnological analysis can thus resolve cryptic features of contourite drift depositional settings.
“…In recent years, medical computed tomography (CT) scanners have become common in the nondestructive analysis of sediment cores [St-Onge et al, 2007], including high-resolution reconstructions of relative or calibrated density for paleoenvironmental interpretation [Boespflug et al, 1995;Ashi, 1997;Lis e-Pronovost et al, 2009;St-Onge and Long, 2009;Støren et al, 2010;Davies et al, 2011;Dorfman et al, 2015], assessment of coring deformation [Ashi, 1997;Barletta et al, 2010;Walczak et al, 2017], and variations in sedimentological structures or components [Michaud et al, 2003;Gagnoud et al, 2009;Goldfinger et al, 2013;Mena et al, 2015;Patton et al, 2015]. Medical CT scanners measure the attenuation of X-rays, a function of density and atomic number, and store these values in pixels as relative gray scale values or Hounsfield units (HU), achieving resolution around 0.5 3 0.5 3 0.5 mm [Hounsfield, 1973].…”
Computed tomography (CT) of sediment cores allows for high‐resolution images, three‐dimensional volumes, and down core profiles. These quantitative data are generated through the attenuation of X‐rays, which are sensitive to sediment density and atomic number, and are stored in pixels as relative gray scale values or Hounsfield units (HU). We present a suite of MATLAB™ tools specifically designed for routine sediment core analysis as a means to standardize and better quantify the products of CT data collected on medical CT scanners. SedCT uses a graphical interface to process Digital Imaging and Communications in Medicine (DICOM) files, stitch overlapping scanned intervals, and create down core HU profiles in a manner robust to normal coring imperfections. Utilizing a random sampling technique, SedCT reduces data size and allows for quick processing on typical laptop computers. SedCTimage uses a graphical interface to create quality tiff files of CT slices that are scaled to a user‐defined HU range, preserving the quantitative nature of CT images and easily allowing for comparison between sediment cores with different HU means and variance. These tools are presented along with examples from lacustrine and marine sediment cores to highlight the robustness and quantitative nature of this method.
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