The influence of particle shape on the stress-strain and creep response of a fine silica sand

The influence of particle shape on the stress-strain response of fine silica sand is investigated experimentally. Two sands from the same source and with the same particle size distribution were examined using Fourier descriptor analysis for particle shape. Their grains were, on average, found to have similar angularity but different elongation. During triaxial stress path testing, the stress-strain behavior of the sands for both loading and creep stages were found to be influenced by particle elongation. In particular, the behavior of the sand with less elongated grains was more like that of rounded glass beads during creep. The results highlight the role of particle shape in stress transmission in granular packings and suggest that shape should be taken more rigorously into consideration in characterizing geomaterials. © 2005 Taylor & Francis Group.

A study on water film in saturated sand

Water film can serve as a sliding surface and cause landslides on gentle slopes. The development of "water film" in saturated sand is analyzed numerically and theoretically based on a quasi-three-phase model. It is shown that stable water films initiate and grow if the choking state (where the fluid velocity decreases to near zero) remains steady in a liquefied sand column. Discontinuity can occur in pore water velocity, grain velocity and pore pressure after the initiation of a water film. However, the discontinuity and water film can disappear once the choking state is changed. The key to the formation of water film is the choking in the sand column caused by eroded fine grains.

Lithic grain and mineral composition of sand and sandstones from the Japan and Mariana forearc and backarc (Table 3)

One hundred and twenty point counts of Oligocene to Recent sands and sandstones from DSDP sites in the Japan and Mariana intraoceanic forearc and backarc basins demonstrate that there is a clear compositional difference between the continentally influenced Japan forearc and backarc sediments, and the totally oceanic Mariana forearc and backarc sediments. Japan forearc sediments average 10 QFL%Q, 0.82 P/F, 2 Framework%Mica, 74 LmLvLst%Lv, and 19 LmLvLst%Lst. In contrast, the Mariana forearc and backarc sediments average 0 QFL%Q, 1.00 P/F, 0 Framework%Mica, 98 LmLvLst%Lv, and 1 LmLvLst%Lst. Sediment compositions in the Japan region are variable. The Honshu forearc sediments average 5 QFL%Q, 0.94 P/F, 1 Framework%Mica, 82 LmLvLst%Lv, and 15 LmLvLst%Lst. The Yamato Basin sediments (DSDP Site 299) average 13 QFL%Q, 0.70 P/F, 3 Framework%Mica, 78 LmLvLst%Lv, and 14 LmLvLst%Lst. The Japan Basin sediments (DSDP Site 301) average 24 QFL%Q, 0.54 P/F, 9 Framework%Mica, 58 LmLvLst%Lv, and 21 LmLvLst%Lst. P/F and Framework%Mica are higher in the Yamato Basin sediments than in the forearc sediments due to an increase in modal potassium content of volcanic rocks from east to west, on the island of Honshu. Site 301 possesses a higher QFL%Q and LmLvLst%Lst, and lower LmLvLst%Lv than Site 299 because it receives sediment from the Asian mainland as well as the island of Honshu. DSDP Site 293 sediments, in the Mariana region, average 0.97 P/F, 1 Framework%Mica, 13 LmLvLst%Lm and 83 LmLvLst%Lv, due to their proximity to the island of Luzon. The remaining Mariana forearc and backarc sediments show a uniform composition.

Composition of the sand fraction of sediment cores from the West-African continental margin

Seven cores from the West African continental margin in 12-18° N have been investigated by means of a coarse fraction analysis. Four of the seven cores contain allochthonous material: turbidites and debris flow deposits. The source of the allochthonous material is in about 300-600 m water depth. The age of the slide induced debris flow deposits is at the end of oxygen isotope stage 2. One debris flow deposit is covered by a turbidite (core GIK13211-1). The turbidites in the deep-sea core GIK13207-3 originate from river-influenced sediments from the West-African continental margin, whereas the autochthonous sequences are influenced by volcanic material from the Cape Verde Islands. Particle by particle supply from upper slope areas has been found in all four cores from the continental slope. Current sorting occurs on the submarine diapir (core GIK13289-3), whereas core GIK13291-1 on the NW-flanc, 200 m below core GIK13289-3, has no current sorting, except for stage 1 and parts of stage 5. The current sorting is reflected by parallel variations of median diameters of whole tests and of fragments of planktonic foraminifers, by higher median diameters of foraminifers on top of the diapir, by reduced accumulation rates and increased sand fraction percentages in core GIK13289-3 compared to core GIK13291-1. The Late Quarternary climatic history of the West-African near coastal area (12-18° N) has been redrawn: - in oxygen isotope stage 1 a humid climate is found in 12-18° N (This "humid impression" in 18° N, which is actually an arid area, is due to the poleward directed undercurrent, which transports Senegal river material to the north). - in oxygen isotope stage 2 an arid climate existed in 14-18° N, whereas in 12° N river discharfe persisted. But within stage 2 dune formation occured in 12° N on the (dry) shelf, additionally to fluviatile sediment input. - Older periods are preserved in autochthonous sediments of core GIK13289-3 and GIK13291-1, where oxygen stage 3,5 and 7 (the latter only in core GIK13289-3 present) show a humid climate (as well as in stage 5 of core GIK13255-3), interrupted by short arid intervals in core GIK12389-3, and stage 4 and 6 show an arid climate, interrupted by short humid periods The allochthonous stage 5 sediment in core GIK13211-1 also reflects a humid climate. The dissolution of planktonic foraminifers is strongest in th eLate Holocene and shows a minimum in the early Holocene, where also pteropods are preserved. The degree of carbonate dissolution is related mainly to the fine matter content (< 63 µm) whereas water depth is a less decisvive factor.

Pb isotopes in individual sand-sized (>150 µm) ice-rafted feldspars deposited at subpolar North Atlantic Ocean Deep Sea Drilling Project Site 611 during the late Pliocene (Marine Isotope Stages, MIS, G6 through 100)

The onset of abundant ice-rafted debris (IRD) deposition in the Nordic Seas and subpolar North Atlantic Ocean 2.72 millions of years ago (Ma) is thought to record the Pliocene onset of major northern hemisphere glaciation (NHG) due to a synchronous advance of North American Laurentide, Scandinavian and Greenland ice-sheets to their marine calving margins during marine isotope stage (MIS) G6. Numerous marine and terrestrial records from the Nordic Seas region indicate that extensive ice sheets on Greenland and Scandinavia increased IRD inputs to these seas from 2.72 Ma. The timing of ice-sheet expansion on North America as tracked by IRD deposition in the subpolar North Atlantic Ocean, however, is less clear because both Europe and North America are potential sources for icebergs in this region. Moreover, cosmogenic-dating of terrestrial tills on North America indicate that the Laurentide Ice Sheet did not extend to ~39°N until 2.4 ±0.14 Ma, at least 180 ka after the onset of major IRD deposition at 2.72 Ma. To address this problem,we present the first detailed analysis of the geochemical provenance of individual sand-sized IRD deposited in the subpolar North Atlantic Ocean between MIS G6 and 100 (~2.72-2.52 Ma). IRD provenance is assessed using laser ablation lead (Pb) isotope analyses of single ice-rafted (>150 mm) feldspar grains. To track when an ice-rafting setting consistent with major NHG first occurred in the North Atlantic Ocean during the Pliocene intensification of NHG (iNHG), we investigate when the Pb-isotope composition (206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb) of feldspars deposited at DSDP Site 611 first resembles that determined for IRD deposited at this site during MIS 100, the oldest glacial for which there exists convincing evidence for widespread glaciation of North America. Whilst Quaternary-magnitude IRD fluxes exist at Site 611 during glacials from 2.72 Ma, we find that the provenance of this IRD is not constant. Instead, we find that the Pb isotope composition of IRD at our study site is not consistent with major NHG until MIS G2 (2.64 Ma). We hypothesise that IRD deposition in the North Atlantic Ocean prior to MIS G2 was dominated by iceberg calving from Greenland and Scandinavia. We further suggest that the grounding line of continental ice on Northeast America may not have extended onto the continental shelf and calved significant numbers of icebergs to the North Atlantic Ocean during glacials until 2.64 Ma.

Grains size and components distribution in turbidite layesr of ODP Leg 101

Textural and compositional differences were found between gravity-flow sheets in an open-ocean environment on the northern slope of Little Bahama Bank (Site 628, Pliocene turbidite sequence) and in a closed-basin depositional setting (Site 632, Quaternary turbidite sequence). Mud-supported debris-flow sheets were cored at Site 628. Average mean grain size of the turbidite samples was lower, mud content was higher, and sorting was poorer than in comparable samples from Site 632. This reflects the deposition of proximal, low-energy turbidity currents and debris flows on a base-ofslope carbonate apron. No mud-supported debris-flow sheets were deposited in the investigated sediment sequence of Hole 632A. Many larger turbidity currents from around the margins of Exuma Sound may have reached this central basin setting, depositing sediments that had been transported over longer distances. Planktonic components dominate in the grain-sized fraction (500-1000 µm) of turbidite samples from Hole 628A, while platform detritus is rare. We interpreted this as resulting from the erosion and reworking of a large area of open-ocean slope sediments by gravity flows. In contrast, large amounts of benthic and platform components were found in the turbidite samples of Hole 632A. This may be explained by the fact that the slopes of the enclosed Exuma Sound are steep, and turbidity currents bypassed much of these slopes through pronounced channels, delivering more shallow-water detritus to the deep basin. Erosion of slope sediments, a possible source area of planktonic detritus, is assumed to be low. The small slope area in relation to the larger surrounding platform areas and lower production of planktonic components in the enclosed waters of Exuma Sound may also explain the observed low number of planktonic components at Hole 632A. Turbidite material from both open-ocean and enclosed-basin environments was deposited at Site 635.

Grain counts and XRD mineralogy of turbidite sand and hemipelagic mud from ODP Leg 168 sites

Sequences of late Pliocene to Holocene sediment lap onto juvenile igneous crust within 20 km of the Juan de Fuca Ridge in northwestern Cascadia Basin, Pacific Ocean. The detrital modes of turbidite sands do not vary significantly within or among sites drilled during Leg 168 of the Ocean Drilling Program. Average values of total quartz, total feldspar, and unstable lithic fragments are Q = 35, F = 35, and L = 30. Average values of monocrystalline quartz, plagioclase, and K-feldspar are Qm = 46, P = 49, and K = 5, and the average detrital modes of polycrystalline quartz, volcanic-rock fragments, and sedimentary-rock plus metamorphic-rock fragments are Qp = 16, Lv = 43, and Lsm = 41. Likely source areas include the Olympic Peninsula and Vancouver Island; sediment transport was focused primarily through the Strait of Juan de Fuca, Juan de Fuca Channel, Vancouver Valley, and Nitinat Valley. Relative abundance of clay minerals (<2-µm-size fraction) fluctuate erratically with depth, stratigraphic age, and sediment type (mud vs. turbidite matrix). Mineral abundance in mud samples are 0%-35% smectite (mean = 8%), 18%-59% illite (mean = 40%), and 29%-78% chlorite + kaolinite (mean = 52%). We attribute the relatively low content of smectite to rapid mechanical weathering of polymictic source terrains, with little or no input of volcanic detritus from the Columbia River. The scatter in clay mineralogy probably was caused by converging of surface currents, turbidity currents, and near-bottom nepheloid clouds from several directions, as well as subtle changes in glacial vs. interglacial weathering products.

Mud, sand, heavy and light minerals composition in lower Cretaceous at DSDP Hole 93-603B

During the drilling of Hole 603B on Deep Sea Drilling Project Leg 93, an unexpected series of sand-, silt-, and claystone turbidites was encountered from Cores 603B-45 through -76 (1224-1512 m sub-bottom depth). Complete and truncated Bouma sequences were observed, some indicating deposition by debris flows. Sand emplacement culminated with the deposition of a 30-m-thick, unconsolidated sand unit (Cores 603B-48 through -45). The purpose of this preliminary study is to determine the nature of the heavy mineral suites of this sediment in order to make tentative correlations with onshore equivalents. The heavy mineralogy of Lower Cretaceous North American mid-Atlantic coastal plain sediment has been extensively studied. This sediment is classified as the Potomac Group, which has a varied heavy mineral suite in its lower part (Patuxent Formation), and a limited suite in its upper part (Patapsco Formation). The results of this study reveal a similar trend in the heavy mineral suites of sediment in Hole 603B. Hauterivian through lower Barremian sediment has a heavy mineral suite that is dominated by zircon, apatite, and garnet, with minor amounts of staurolite and kyanite. Beginning in the mid-Barremian, a new source of sediment becomes dominant, one which supplies an epidote-rich heavy mineral suite. The results of the textural analyses show that average grain size of the light mineral fraction increases upsection, whereas sorting decreases. The epidote-rich source may have delivered sediment with a slightly coarser mean grain size. This sediment may represent a more direct continental input at times of maximum turbidite activity (mid-Barremian) and during deposition of the upper, unconsolidated sand unit.