Continental shelf architecture and sea-level cycles: Late Quaternary high-resolution stratigraphy of the Gulf of Cádiz, Spain

 The stratal architecture of the Gulf of Cádiz continental margin (SW Spain) has been analyzed by using single-channel, very high-resolution seismic reflection profiles. An evolutionary scheme of asymmetrical depositional sequences is proposed that was governed by the Late Pleistocene–Holocene sea-level fluctuations. Stratigraphic analysis defined 14 seismic units, that are configured into two major type-1 depositional sequences related to 4th-order eustatic sea level changes (100–110 ka). Within these sequences, minor asymmetrical depositional sequences have been recognized related to 5th-order eustatic cycles (22–23 ka) superimposed and modulated by the regressive trends of 4th-order cycles. In 5th-order depositional sequences, the forced regressive and lowstand deposits are volumetrically dominant. They cause the main progradation of the margin in such a way that they form the margin structure almost entirely. Received: 6 April 1995 / Revision received: 8 March 1996     

Seismic stratigraphy of the quaternary Yellow River delta,Bohai Sea,eastern China

The upper 40 m of stratigraphy of the Yellow River (Huang He) subaqueous delta has been well documented, but the nature of the underlying strata is currently unknown at high-resolution. To address this deficiency we used a Geopulse seismic system to image shallow sedimentary deposits up to 120 m deep on the Yellow River delta. High-resolution seismic reflection images were processed with a series of specific techniques (e.g. swelling attenuation, dynamic s/n filter; f-x deconvolution, predictive deconvolution dipscan stack), and used with borehole data to investigate the Quaternary offshore sequences in the Yellow River (Huang He) delta. Repetitive sequences were observed and interpreted as containing layers of transgressive and regressive deposits. Six seismic transgressive and regressive cycles are identified. Unit M₆F–C₆F correlates with a relative sea-level rise (173–157 ka) and fall (231–173 ka), while Unit M₅F–C₅F is associated with a relative sea-level rise (124–100 ka) and fall (157–124 ka). Unit M₄F–C₄F spans a period of sea-level fall at 100–87 ka, followed by a rise at 87–76 ka. Unit M₃F–C₃F is a transgressive–regressive cycle dated as 76–58 ka. Unit M₂F–C₂F correlates with relative sea level fall at 58.2–36 ka and subsequent rise at 36–22 ka. Unit M₁F–C₁F was deposited during relative sea level fall (22–18 ka), followed by a rise, especially since 8.5 ka.     

Fission track study: heating and denudation of marginal ridge of the Ivory Coast–Ghana transform margin

 Six sandstone blocks sampled during dives along the southern slope of the Ivory Coast–Ghana continental margin have been studied using fission tracks in apatite and zircon. Measurements demonstrate that the rocks were heated above 120°C but below 390°C and cooled quickly. The ages of cooling recorded by the apatite crystals are 90 Ma in the western part of the margin, and 80–70 Ma in the central and eastern part. Heating is interpreted by the heat liberation due to the friction along the active transform fault and by the vicinity of an oceanic spreading center, which slipped along the margin. Cooling is interpreted by two stages of denudation due to minor faults and landslides produced by the increasing of the bathymetric step between the continental margin and the oceanic crust. Received: 12 April 1995 / Revision received: 20 December 1995     

Paleoenvironmental changes of the Yellow Sea during the Late Quaternary

 Based on stable isotope data and carbon and sulfur elemental analyses, the sedimentary environment of the Yellow Sea was significantly influenced by the sea level changes during the Late Quaternary. At the low sea level stand when the sea level was lower by 56 m, the salinity of seawater was reduced to about 7.6‰, and the sedimentation rate in the central part of the Yellow Sea was three times higher than the present rate. The high C/S ratio during the low sea level stand is strong evidence that sedimentation took place in a lower salinity environment than exists at present. Received: 25 September 1997 / Revision reveived: 15 June 1998     

Sequence surfaces and paleobathymetric trends in Albian to Maastrichtian sediments of Ariyalur area, Cauvery Basin, India

Exposed Late Cretaceous (Albian-Maastrichtian) marine rocks of the Ariyalur area in the Cauvery Basin have been extensively studied based on biostratigraphy and paleobathymetry with paleobathymetric interpretation carried out using vertical and lateral relationships of rock facies, macro- and microfossil assemblages, textural characteristics and diagenetic changes of the lithologic units. The integration of these data reveals four Transgressive-Regressive (T-R) cycles, viz. Dalmiapuram, Garudamangalam, Sillakkudi and Kallankurichchi (in stratigraphic order). These T-R cycles have been compared with global published relative sea level curves of the study area. The major sea level changes during the Late Turonian and Late Maastrichtian in the study area correlate well with global sea level changes of [Vail et?al., 1977] and [Haq et?al., 1987] and Miller et al. (2005). Based on biostratigraphy, stratal patterns and their relationship, the Late Cretaceous succession of the Ariyalur area is thus subdivided into four 2nd/3rd order sequences.     

Sea level variations in the regional seas around Taiwan

The patterns and trends of sea level rise in the regional seas around Taiwan have been investigated through the analyses of long-term tide-gauge and satellite altimetry data. Series of tide-gauge data extending over 50 years reveal decadal and interannual variations and spatially-inhomogeneous patterns of generally rising sea level. The East Asia tide-gauge stations around Taiwan show an average trend of +2.4 mm/yr from 1961–2003, which is larger than the reported global rate of +1.8 mm/yr for the same period. These stations also show significantly larger sea level rise rates (+5.7 mm/yr) than global values (+3.1 mm/yr) during the period from 1993–2003. Consistent with the coastal tide-gauge records, satellite altimetry data show similar increasing rates (+5.3 mm/yr) around Taiwan during the same period. Comparisons with temperature anomalies in the upper ocean suggest that thermal expansion and heat advection in the upper layer contribute significantly to the long-term sea level variations in this area with correlations >0.9 for observations after 1992. Thermosteric sea level variations may also explain the interannual and decadal variations of the observed sea level rises around Taiwan. Our analysis also indicates that the altimetry data are only part of a long-term, larger-scale signal. Finally, we have found that a non-linear smoother, LOESS, is more suitable for extracting long-term trends in sea level than the traditional linear regression approach.     

A geophysical survey of the De Gerlache Seamounts: preliminary results

 The De Gerlache Seamounts actually consist of two medium-sized guyots, with summits at depths of 350–600 m. Acoustic profiler data show no significant sediment on these guyots. Alkaline basalts dredged from the summit of the eastern guyot yield K/Ar ages between 20.1±1.0 and 23.2±1.2 Ma. Basement ridges and sediment-filled troughs between the guyots are associated with the prominent gravity anomaly extending north from the Antarctic margin. This structure possibly played a role in the guyot formation, however, the question of how the De Gerlache Seamounts are related to this gravity anomaly remains uncertain. Received: 13 February 1997 / Revision received: 30 September 1997     

Helium and oxygen isotope analyses of hydrothermal fluids from the East Pacific Rise between 17°S and 19°S

 Hydrothermal vent fields south of the Garret Fracture zone were sampled for the isotope composition of helium and oxygen ([¹⁸O]H₂O/[¹⁶OH₂O). The helium isotopes end-member (³He / ⁴He=8.3×R _a and [⁴He]≈1.2–2.4×10^-5 cm³ STP g^-1) is quite similar to other known hydrothermal sites pointing to the homogeneous helium composition of the upper mantle. The δ¹⁸O end-member value (δ¹⁸O≈0.5–0.6‰) confirms previous suggestions from other sites and from isotope modeling, that hydrothermal fluids are slightly enriched in ¹⁸O relative to the ocean as a result of water–rock interactions at high temperature. Received: 11 December 1995/Revision received: 20 December 1996     

Geoacoustic and physical properties of carbonate sediments of the Lower Florida Keys

 Near-surface sediment geoacoustic and physical properties were measured from a variety of unconsolidated carbonate sediments in the Lower Florida Keys. Surficial values of compressional and shear speed correlate with sediment physical properties and near-surface acoustic reflectivity. Highest speeds (shear 125–150 m s^-1; compressional 1670–1725 m s^-1) are from sandy sediments near Rebecca Shoal and lowest speeds (shear 40–65 m s^-1; compressional 1520–1570 m s^-1) are found in soft, silty sediments which collect in sediment ponds in the Southeast Channel of the Dry Tortugas. High compressional wave attenuation is attributed to scattering of acoustic waves from heterogeneity caused by accumulation of abundant shell material and other impedance discontinuities rather than high intrinsic attenuation. Compared to siliciclastic sediments, carbonate sediment shear wave speed is high for comparable values of sediment physical properties. Sediment fabric, rather than changes due to the effects of biogeochemical processes, is responsible for these differences.     

Ground-truthing 11- to 12-kHz side-scan sonar imagery in the Norwegia–Greenland Sea: Part I: Pockmarks on the Vestnesa Ridge and Storegga slide margin

 Numerous small (50- to 300-m-diameter) strong-backscatter objects were imaged on the 1200- to 1350-m deep crest of Vestnesa Ridge (Fram Strait) and along the 900- to 1000-m deep northeast margin of the Storegga slide valley. Ground-truthing identified most of these objects as 2- to 10-m-deep pockmarks, developed within soft, acoustically stratified silty clays (typical wet bulk density: 1400–1600 kg m^-3; sound speed: 1480– 1505 m s^-1; porosity, 65–75%; shear strength: 5–10 kPa; water content: 80–120%; and thermal conductivity: 0.8–0.9 W m^-1 deg C^-1 in the top 3 m). Gas wipeouts, enhanced reflectors, and reflector discontinuities indicate recent or ongoing activity, but the absence of local heat flow anomalies suggests that any upward fluid flows are modest and/or local.     

南海周边相对海平面变化特征及2004年苏门答腊地震影响分析

利用南海周边1989-2014年的潮汐资料和GPS长期观测资料,分析了南海周边相对海平面变化特征,以及2004年苏门答腊地震对该区域相对海平面变化的影响。研究结果表明,南海周边的相对海平面变化以上升为主,平均上升速率（4.53±0.20） mm/a,高于全球平均速率,且2004年后上升趋势加剧;南海周边相对海平面呈现6类较典型的变化特征,并存在与板块构造相对应的分区聚集现象,形成了中国东南和越南沿海、马来半岛、加里曼丹岛北部、菲律宾群岛等4个变化特征区。受2004年苏门答腊大地震的影响,马来半岛、南沙和西沙海域的地壳形变由上升趋势转为下沉,加剧了相对海平面的上升;中国东南沿海和菲律宾群岛受地震影响较小;越南沿海和加里曼丹岛北部区域的地震影响还有待进一步研究。     

Long-term trends in the variability of deep-water thermohaline characteristics of the Black Sea

By using the archival hydrological data for 1955–1998, we analyze the trends of deep-water thermohaline characteristics of the Black Sea and their interannual and decadal variability. It was discovered that the level of salinity increased at depths greater than 1000 m in the west part of the sea from the mid-1950-s till the early 1980s and the opposite trend was observed for the next 15–20 yr. The average rate of increase in the deep-water salinity between 1960 and 1980 and its decrease after 1980 was equal to 0.05‰ per 20 yr. These facts demonstrate that the water exchange through Bosporus was intensified for the first 25 yr of the analyzed period and weakened for the next 20 yr. The interannual variability with a typical period of 6.5 yr and a quasi-20-yr periodicity are detected against the background of the parabolic trend. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 4, pp. 18–30, July–August, 2006.     

On the origin and flow behavior of submarine slides on deep-sea fans along the Norwegian–Barents Sea continental margin

 Debris lobes with characteristic lengths, widths, and thickness of 30–200 km, 2–10 km, and 10–50 m, respectively, represent the main building blocks of deep-sea fans along the Norwegian–Barents Sea continental margin. Their formation is closely related to the input of clay-rich sediments to the upper continental slope by glaciers during periods of maximum ice advance. It is likely that slide release was a consequence of an instability arising from high sedimentation rates on the upper continental slope. The flow behavior of the debris lobes can be described by a Bingham flow model. Received: 17 November 1995 / Revision received: 24 June 1996     

210Pb-derived sediment accumulation rates in the southwestern East Sea (Sea of Japan)

 Recent sediment accumulation rates are 18–230 mg cm^-2 yr^-1 (0.02–0.2 cm yr^-1) based on excess ²¹⁰Pb activity profiles in the southwestern part of the East Sea (Sea of Japan). Assuming no mixing beneath surface mixed layers, ²¹⁰Pb-derived sediment accumulation rates are 18–32 mg cm^-2 yr^-1 in the northern part of the Yamato Ridge and the Ulleung Basin, 29–136 mg cm^-2 yr^-1 in the Korea Plateau, and 230 mg cm^-2 yr^-1 in the southern shelf. These values generally agree with long-term sedimentation rates estimated from dated ash layers. Received: 6 October 1995 / Revision received: 31 May 1996     

Sequence model and its application to a Miocene shelf–slope system in the tectonically active Ulleung Basin margin, East Sea (Sea of Japan)

In a broader application of sequence stratigraphic concept to a tectonically active margin setting, this study presents a sequence model that considers all three controls on sequence development (i.e. eustasy, tectonic movement and sediment supply) as independent variables. The model introduces six sequence types (A to F) including type 1 and type 2 sequences defined in the original Exxon scheme. Each sequence shows a variety in number and stacking pattern of its constituent parasequence sets reflecting combined effects of accommodation change and sediment supply. This model is applied to a seismic sequence analysis of the shelf–slope system (middle to upper Miocene) in the southwestern margin of Ulleung Basin which has experienced significant crustal deformation during the Tertiary back-arc opening and subsequent closing of the East Sea (Sea of Japan). The model application delineates four sequence types whose development is closely associated with the tectonic evolution of the Ulleung Basin margin. During the back-arc opening (early to middle Miocene), type A and B sequences were emplaced as a result of steady creation of accommodation space due to a rapid subsidence combined with a tectonic-controlled high to moderate rate of sediment supply. The sequences associated with the extensional tectonism are characterized by active progradation and aggradation without forced regressive phases. In the initiation stage of back-arc closure (middle to late Miocene), subsidence rates were significantly reduced because of a widespread contractional deformation, while subaerial erosion of the uplifted thrust belt resulted in an increase in sedimentation rate. As a result, steady prograding type-E sequences were formed by alternating normal and forced regressions. During the quiescent phase of back-arc closure in the late Miocene, rise-dominant fluctuating relative sea-level change and moderate to low sediment supply gave rise to type-F sequences (similar to type-1 sequences of the Exxon group) reflecting a major control of eustatic sea-level change.     

The eustatic and tectonic origin of Neogene unconformities from the Great Australian Bight

The Neogene succession recovered during ODP Leg 182 from the Great Australian Bight comprises unconformity-bounded, cool-water carbonates. A total of 15 hiatuses, each lasting ∼0.5 Myr or more, are identified or inferred primarily on the basis of planktonic foraminifer biostratigraphy from the basal Miocene to early Pleistocene. They are interpreted as local manifestations of major third-order boundaries at about 23.8 (H1), 22.3 (H2), 20.5 (H3), 18.7 (H4), 16.4 (H5), 14.8 (H6), 13.5 (H7), 11.5 (H8), 9.3 (H9), 7.0 (H10), 6.0 (H11), 4.5 (H12), 3.5 (H13), 2.5 (H14), and 1. 5 Ma (H15). The coincidence of these hiatuses with third-order global sequence boundaries suggests a eustatic control complicated by local tectonics. Three mega-hiatuses at about 15–16 Ma, 8–9 Ma and 1.5–2.5 Ma, each lasting >5 Myr at selected sites, are interpreted as being caused by large-scale slope failure during times of differential uplift/subsidence and sudden changes in relative sea level. These results provide evidence that the evolution of the southern Australian margin during the Neogene occurred in steps, controlled by a W–E stress field in the course of Australia’s northward drift and by changes in relative sea level that triggered sedimentation as well as sediment packaging by unconformities.     

Magnetic expression of some major lineaments and Cretaceous quiet zone in the Bay of Bengal

 Magnetic data over the eastern continental margin of India and adjacent Bengal fan demarcate two major lineaments. A high amplitude N–S-trending lineation of the Cauvery offshore Basin corresponds to the offshore fragment of the 80°E lineament recorded onland. A N–S lineation of very high amplitude anomaly off Chilka lake considered as the possible northward extension of the 85°E ridge delineated, hitherto in Bengal Fan. A subdued magnetic anomaly zone is demarcated seaward of the continent–ocean boundary (COB) in the Bengal Fan. Over the northern Bengal Fan this zone is delineated east of 85°E lineation. This quiet zone might have evolved during the Early Cretaceous period of normal magnetic polarity between M0 and 34 (120–84 Ma) anomalies. Received: 6 April 1995 / Revision received: 3 September 1996     

Sheared continent–ocean margins: an overview

 Continent–ocean fracture zones are the fossil transform offsets located along passive rifted continental margins. Kinematic models identify at least two principal stages in their evolution. During the first stage as rifting proceeds, continent–continent shearing dominates a narrow region in which the transform fault will eventually rupture. High-standing continental marginal ridges 50–100 km wide and bounding deep sedimentary basins, are derived in such settings. In stage two as sea-floor spreading proceeds, the younger oceanic block slides along the active transform, heating the older continental block, and possibly induces thermal uplift and accompanying denudation. Magnetic injection into the continental block at depth may also induce an isostatic uplift. After ridge–transform intersection time, mechanical coupling between the continental and oceanic blocks may influence the stratigraphy and structure of these margins. Received: 12 March 1996 / Revision received: 23 April 1996     

Peculiarities of the kinematics of the west scotia mid-oceanic ridge

The first map of the acoustic basement and a new map of the C⁴-C¹² chrons are made for the area of the paleospreading West Scotia Ridge. The analysis of the constructed maps and the calculations showed that the bottom growth in the ridge axes began in the southeast in the interval of chron C^12r (31.116–33.266 Ma B.P.). In the period of chron C11r (30.217–30.627 Ma), a hundred-kilometer northwestward jump of the spreading axis occurred. The maximal values of the bottom growth (about 6.3 cm/yr) were in the interval of chrons C6–C6B (18.748–22.546 Ma); then, the spreading began to fade. In the time of chron C3n.1r (3.300–4.493 Ma B.P.), the axis of the paleo-mid-oceanic ridge died. The spreading was accompanied with northeastward propagating of the axes, and the propagating proper had an impulse character.     

Fluorescent sands for measurements of longshore transport rates: a case study from Praia de Faro in southern Portugal

 A field experiment was carried out on a mesotidal reflective beach to relate longshore sand transport rates to wave measurements in medium-energy conditions (H _rms =0.6 m; T _m =6.5 sec; α=20°). Dispersion of fluorescent sand tracers was analyzed using the spatial integration method (SIM). The measured sand transport rate was three times larger than the prediction computed using the longshore wave power approach, while the K factor was determined empirically as 2.32. The study concluded that there is a need to calibrate bulk longshore transport predictors before applying them to steep gradient slopes under plunging waves. Received: 16 May 1997 / Revision received: 20 November 1997