Heavy mineral content and grain sizes of sediment samples from foreshore to dune around Skagen, north Denmark

Aufbau und Ausdehnung der Schwermineral-Anreicherungen (Ilmenit, Granat, Amphibol) am Strand südlich Skagens wurden in langen Schürfgräben untersucht. Die Seifenlagen ziehen durchgehend vom Kliff-Fuß bis zur mittleren Meereshöhe hin und liegen meist diskordant auf der alten Strandschichtung. Ihre strandparallele Ausdehnung beträgt bis zu 100 m. Aufgebaut werden sie aus dünnen Schwermineral-Lamellen, die in kleinerem Umfang überall in den Strandablagerungen zu finden sind und hier das Gefüge nachzeichnen (Rippeln, Strandwallschichtung, Schichtstörungen). Die Seifenbildung geht in einem Gebiet mit verstärktem Küstenabtrag vor sich (Lee-Erosion südlich der Hafenmolen von Skagen). Dieses deutet darauf hin, daß die Schwerminerale bei Aufarbeitung bereits vorhandener Sedimente infolge ihres unterschiedlichen hydraulischen Verhaltens Zurückbleiben und schließlich angereichert werden. Die Korngrößenverteilung der Minerale in verschiedenen Sedimentproben zeigen, daß mit steigender Schwermineral-Anreicherung eine Kornverfeinerung und Zunahme der spezifisch schwersten Minerale (opake Erzminerale und Zirkon) auftritt. In ähnlicher Weise werden die Sortierungswerte besser. Die Aufbereitung des Sedimentes wird, in Anlehnung an v. ENGELHARDT (1939), mit einem doppelten Sortierungsvorgang durch die Wasserbewegung am Strand erklärt. Beim Absinken des Sandes nach dem Brecherschwall tritt eine Vorsortierung ein, die den Abtransport der leichteren und größeren Minerale im Sog begünstigt. Verbindungen zu Vorstellungen der Aufbereitungstechnik (Rundherdverfahren) und Hydrodynamik ('laminare Unterschicht') werden hergestellt. Die Dünensande Skagens sind infolge ihres hohen Schwermineralgehaltes und günstiger Äquivalentgrößen der einzelnen Minerale besonders bedeutsam für die Seifenbildung am Strand.

Downhole variations in grain sizes at DSDP/ODP Hole 504B

The maximum grain sizes of plagioclase and magnetite in the groundmass of the sheeted dike complex drilled at Hole 504B have been measured. Downhole variations through a 440-m-long section show a crude zig-zag pattern consisting of a gradual decrease or increase followed by an abrupt jump. The gradual decrease or increase in grain size extends over many lithologic units, and hence, does not reflect variations in grain size within a single dike. Such a zig-zag pattern is well explained by grain-size variations through multiple dikes. By using the observed inclination of sheeted dikes of 81° ± 2.5°, thickness of the multiple dikes varies from 0.7 to 8.5 m and averages to 4 ± 1 m. The average thickness of individual dikes forming multiple dikes is 0.8 m. We expect such multiple dikes to be formed during rifting events beneath mid-oceanic spreading ridges. If the average expansion at rifting episodes is twice as wide as the average width of the multiple dike units, the full spreading rate of 7.2 cm/yr of Cocos Ridge gives 112 ± 33 yr for a time interval of the rifting. A simple one-dimensional conductive cooling model is applied to solidification of multiple dikes. Numerical simulations show that the grain-size variations observed through the drill hole are more consistent with a model where a new injection of a dike occurs periodically with a constant time interval rather than one where the next dike intrudes just after the solidification of the previous one. Grain-size variations within simple dikes from Iritono, Japan, and those for Makaopuhi lava lake, Hawaii, show that square root of crystallization time is linearly correlated with the logarithm of plagioclase size. By using an empirically derived relationship between these two variables, the variations of plagioclase size through Hole 504B are directly compared with the calculated times for crystallization. Each rifting episode at the Costa Rica Rift lasts for several years, and periodic injection of a new dike occurs into the center of a previously solidified multiple dike at time intervals varying from 1 to 12 months.

Age determination and median grain size of sediment core EN31-PC2

Hemipelagic muds deposited during the past 5.3 cal kyr in the northern Gulf of Mexico (Orca Basin) contain seven intervals punctuated by relatively coarse siliciclastic grain-size peaks, planktonic faunal turnovers, and negative d13C excursions. We believe these episodes represent megaflood deposits reflecting historically unprecedented outfall of North American floodwater and terrigenous mud plumes into the gulf, resulting in collapse of the open-ocean pelagic ecosystem. The deposits record multidecadal episodes of high continental precipitation and large Mississippi River floods at ~4.7, 3.5, 3.0, 2.5, 2.0, 1.2, and 0.3 cal ka (500-1200-year recurrence interval). Variations in tropical plankton frequencies define submillenial warming intervals that culminate in these fluvial episodes. Strengthened tropical currents in the gulf at these times appear to have increased sea surface temperatures and associated flow of moist gulf air to the midwest. Terrestrial paleohydrologic records support the marine evidence for millennial-scale changes in recurrence of large midwest flood episodes.

Grain size distribution of Oligocene and Miocene sediments from the Indus Fan

The Owen Ridge south of Oman represents oceanic crust that was uplifted by compressional tectonic forces in the early Miocene. Build-out of the Indus Fan led to deposition of a thick sequence of turbidites over the site of the Ridge during the late Oligocene and early Miocene. Early Miocene uplift of the Ridge led to a pelagic cap of nannofossil chalks. Two short sequences of turbidites from the pre- and syn-uplift phases were chosen for detailed grain size analysis. The upper Oligocene section at Site 731 is composed of thin (centimeter-decimeter scale) graded mud turbidites separated by relatively thick (decimeter-meter scale) intervals of homogeneous, non-bioturbated clayey siltstones. These finer intervals are unusually silt-rich (about 60%) for ungraded material and were probably deposited as undifferentiated muds from a series of turbidity current tails. By contrast, the lower Miocene section at Site 722 is comprised of a sequence of interbedded turbidites and hemipelagic carbonates. Sharp-based silt turbidites are overlain by burrow-mottled marly nannofossil chalks. The Oligocene sequence may have accumulated in an overbank setting on the middle fan - the local topographic position favoring frequent deposition from turbidity current tails and occasional deposition from the body of a turbidity flow. Uplift of the Ridge in the early Miocene led to pelagic carbonate deposition interrupted only by turbidity currents capable of overcoming a topographic barrier. Further uplift eventually led to entirely pelagic carbonate deposition.

Grain size composition and age of ODP Hole 167-1017E sediments

Siliciclastic sedimentation at Ocean Drilling Program Site 1017 on the southern slope of the Santa Lucia Bank, central California margin, responded closely to oceanographic and climatic change over the past ~130 ka. Variation in mean grain-size and sediment sorting within the ~25-m-thick succession from Hole 1017E show Milankovitch-band to submillenial-scale variation. Mean grain size of the "sortable silt" fraction (10-63 µm) ranges from 17.6 to 33.9 µm (average 24.8 µm) and is inversely correlated with the degree of sorting. Much of the sediment has a bimodal or trimodal grain-size distribution that is composed of distinct fine silt, coarse silt to fine sand, and clay-size components. The position of the mode and the sorting of each component changes through the succession, but the primary variation is in the presence or abundance of the coarse silt fraction that controls the overall mean grain size and sorting of the sample. The occurrence of the best-sorted, finest grained sediment at high stands of sea level (Holocene, marine isotope Substages 5c and 5e) reflect the linkage between global climate and the sedimentary record at Site 1017 and suggest that the efficiency of off-shelf transport is a key control of sedimentation on the Santa Lucia Slope. It is not clear what proportion of the variation in grain size and sorting may also be caused by variations in bottom current strength and in situ hydrodynamic sorting.