Anisotropy and Modes of Deposition of Pelitic Mississippi Fan Deposits

Wetzel, Andreas

doi:10.2973/dsdp.proc.96.152.1986

Cited by 4 publications

(3 citation statements)

References 15 publications

(21 reference statements)

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“…One factor influencing anisotropy is the deposition mode: in fine‐grained sediments from the Mississippi fan (DSDP leg 96), vertical to horizontal anisotropy is very limited in current‐deposited sediments but larger in normally settled, hemipelagic sediments [ Wetzel , 1987]. Anisotropy also increases as a result of compaction [ Wetzel , 1986]. These trends show that the geometry of the pores and grain‐to‐grain contacts is essential to anisotropy, which should therefore be also dependent on effective pressure.…”

Section: Discussionmentioning

confidence: 99%

Acoustic and mechanical properties of Nankai accretionary prism core samples

Raimbourg¹,

Hamano

Saito

et al. 2011

Geochem Geophys Geosyst

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[1] We studied undeformed sediment and accreted strata recently recovered by Ocean Drilling Program/ Integrated Ocean Drilling Program (ODP/IODP) drilling in Nankai Trough convergent margin to unravel the changes in physical properties from initial deposition to incipient deformation. We have derived acoustic (V p ) and mechanical (uniaxial poroelastic compliance, compaction amplitude) properties of samples from various drill sites along the Muroto (ODP 1173) and Kii transects (IODP C0001, C0002, C0006, and C0007) from isotropic loading tests where confining and pore pressure were independently applied. We quantified the dependence of V p on both effective (P eff ) and confining (P c ) pressure, which can be used to correct atmospheric pressure measurements of V p . Experimental V p obtained on core samples extrapolated to in situ conditions are slightly higher than logging-derived velocities, which can be attributed either to velocity dispersion or to the effect of large-scale faults and weak zones on waves with longer wavelength. In the high-porosity (30%-60%) tested sediments, velocities are controlled at first order by porosity and not by lithology, which is in agreement with our static measurements of drained framework incompressibility, much smaller than fluid incompressibility. Rather than framework incompressibility, shear modulus is probably the second-order control on V p , accounting for most of the difference between actual V p and the prediction by Wood's (1941) suspension model. We also quantified the mechanical state of Nankai samples in terms of anisotropy, diagenesis, and consolidation. Both acoustic and mechanical parameters reveal similar values in vertical and horizontal directions, attesting to the very low anisotropy of the tested material. When considering the porous samples of the Upper Shikoku Basin sediments (Site 1173) as examples of diagenetically cemented material, several mechanical and acoustic attributes appeared as reliable experimental indicators of the presence of intergrain cementation. We also detected incipient cementation in samples from IODP Site C0001 (accretionary prism unit). In terms of consolidation, we distinguished two classes of material response (shallow, deformable samples and deep, hardly deformable ones) based on the amount of compaction upon application of a P eff large with respect to the inferred in situ value, with a transition that might be related to a critical porosity.Components: 15,300 words, 15 figures, 1 table.

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Section: Discussionmentioning

confidence: 99%

Acoustic and mechanical properties of Nankai accretionary prism core samples

Raimbourg¹,

Hamano

Saito

et al. 2011

Geochem Geophys Geosyst

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“…The downhole development of anisotropies of various physical properties in suites of sedimentary sequences has received a great deal of attention; namely, acoustic anisotropies have been investigated since the days of DSDP by a number of workers (e.g., Carlson and Christensen, 1977;Carlson et al, 1983;Fuji, 1981;Wetzel, 1986). The reason for this continuing interest is obvious: if the development of acoustic anisotropy in different types of sediment and depositional environments could be easily correlated with specific genetic processes or typical microfabrics, it would represent a very fast and effective analytical tool for these properties.…”

Section: Generalmentioning

confidence: 99%

Acoustic Anisotropy and Microfabric Development in Accreted Sediment from the Nankai Trough

Brückmann¹,

Moran²,

Taylor³

1993

Proceedings of the Ocean Drilling Program

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Complete penetration of frontal thrust and décollement of the Nankai Trough accretionary complex in Site 808 during ODP Leg 131 provided a wealth of structural observations and physical property data. In this paper possible mechanisms are discussed that could be responsible for the development of irregular downhole trends in acoustic anisotropy observed in Site 808. After various steps of data reduction and screening, a paleomagnetic reorientation procedure is applied to a selected group of physical property data sets. This facilitates the integration of the observed changes in physical properties with the geotectonic framework at the deformation front of the Nankai Trough accretionary complex. The paleomagnetic database was employed in the reconstruction of directional properties of acoustic velocities of the Lower Shikoku Basin sedimentary sequence, which is divided by a sharply defined décollement into an accreting and a subducting portion. P-wave velocity anisotropies derived from paleomagnetically oriented samples in the upper part of this 420-m-thick hemipelagic sequence show maximum values in the direction parallel and normal to the inferred vector of plate convergence (31O°-315°). No preferred orientation of P-wave velocity anisotropy is found in the subducting part of the sequence.The preferred direction of maximum anisotropy parallel to the convergence vector is also in accordance with the true direction of the observed macro-to mesoscale structural features in Site 808. Microfractures and microcracks forming as stress relief and tensile fractures in cores of semilithified sediment normal and perpendicular to the maximum horizontal stress are discussed as control mechanisms for the development of the observed anisotropy pattern.

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“…However, in instances where the turbidites are similar in color and bioturbation of the sediments is low, it can be difficult to determine the boundaries between e and/division muds. Many authors (e.g., Kelts and Arthur, 1981;Wetzel, 1986;Shipboard Scientific Party, 1987;Pilkey, 1987;Brunner and Normark, 1985;Brunner and Ledbetter, 1987; and many others) discussed or noted difficulty in identifying turbidite muds and/or in establishing the true thicknesses of turbidites (and, hence, the proportion that turbidite muds make up of particular sediment columns). Biogenic turbidites can be particularly difficult to distinguish, as their composition, color, and sometimes the biostratigraphic age of the contained microfossils may be similar to the enclosing hemipelagic sediments.…”

Section: Difficulties In Distinguishing Betweenmentioning

confidence: 99%

Clayey Nannofossil Ooze Turbidites and Hemipelagites at Sites 834 and 835 (Lau Basin, Southwest Pacific)

Rothwell¹,

Weaver²,

Hodkinson³

et al. 1994

Proceedings of the Ocean Drilling Program

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The western Lau Basin, between the Central and Eastern Lau Spreading Centers and the Lau Ridge, contains several small, elongate, fault-bounded, partially sediment-filled sub-basins. Sites 834 and 835 were drilled in the oldest part of the Lau Basin in two of these small extensional basins close to the Lau Ridge, formed on late Miocene to early Pliocene oceanic crust. Both sites show a similar sediment sequence that consists of clayey nannofossil oozes and mixed sediments interbedded with epiclastic vitric sands and silts. The vitric sands and silts are largely restricted to the deeper part of the sediment column (early Pliocene-late Pliocene), and the upper part of the sediment column at both sites consists of a distinctive sequence of brown clayey nannofossil ooze, stained by iron and manganese oxyhydroxides (late Pliocene-Holocene). However, the clayey nannofossil ooze sequence at Site 835 is anomalously thick and contains several medium-to very thick beds of matrix-supported, mud-clast conglomerate (interpreted as muddy debris-flow deposits), together with large amounts of redeposited clayey nannofossil ooze and coherent rafted blocks of older hemipelagic material. Redeposited clayey nannofossil oozes can be distinguished from hemipelagic nannofossil oozes using several sedimentological criteria. These include variation in color hue and chroma, presence or absence of bioturbation, presence or absence of scattered foraminifers, grain-size characteristics, variability in calcium carbonate content, presence or absence of pumice clasts, and micropaleontology. Clayey nannofossil ooze turbidites and hemipelagites are also geochemically distinct, with the turbidites being commonly enriched in Mn, Ni, Pb, Zn, Cr, and P. The sediment sequence at Site 835 is dominated by allochthonous sediments, either muddy debris-flow deposits, coherent rafted blocks, or thick clayey nannofossil ooze turbidites. Since 2.9 Ma, only 25% of the 133 m of sediments deposited represents hemipelagic deposition, with an average sedimentation rate of 1.5 cm/k.y.~1. Allochthonous sediments were the main sediment type deposited during the Brunhes geomagnetic Epoch and make up 80% of the thickness of sediment deposited during this period. Short intervals of mainly hemipelagic deposition occurred from 0.4 to 0.9 Ma, 1.0 to 1.4 Ma, and 1.7 to 2.1 Ma. However, allochthonous sediments were again the dominant sediment type deposited between 2.1 and 2.5 Ma, with a large slide complex emplaced around 2.5 Ma. We conclude that the adjacent high ground, surrounding the basin in which Site 835 was drilled, was affected by marked instability throughout the late Pliocene and Pleistocene. In contrast, sedimentation at Site 834 during this period has been dominated by hemipelagic deposition, with redeposited sediments making up slightly less than 17% of the total thickness of sediment deposited since 2.3 Ma. However, there was a marked increase in frequency and magnitude of redeposited sediments at around 0.2 Ma at Site 834, which broadly corresponds to the on...

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Anisotropy and Modes of Deposition of Pelitic Mississippi Fan Deposits

Cited by 4 publications

References 15 publications

Acoustic and mechanical properties of Nankai accretionary prism core samples

Acoustic and mechanical properties of Nankai accretionary prism core samples

Acoustic Anisotropy and Microfabric Development in Accreted Sediment from the Nankai Trough

Clayey Nannofossil Ooze Turbidites and Hemipelagites at Sites 834 and 835 (Lau Basin, Southwest Pacific)

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