Christian Hübscher

Active growth of the Bengal Fan during sea-level rise and highstand

New stratigraphic and high-resolution seismic data from the Bengal Fan indicate that the world9s largest fan shows active growth during the most recent sea-level rise and the recent highstand. This unique phenomenon contradicts common sequence-stratigraphic models, and the sediment preserved provides new insight into the sedimentological response of a fan system to sea-level rise, climatic terminations, and monsoon intensity during the past climatic cycle. We present a detailed dated sequence of turbidite sedimentation based on a core transect perpendicular to the active channel-levee system in the upper mid-fan area. Between the two major terminations 1a (12 800 14 C yr B.P.) and 1b (9700 14 C yr B.P.), and especially at the end of the Younger Dryas, a 13-km-wide channel built up levees 50 m high. With decreasing sediment supply, continued sea-level rise, and increasing monsoon intensity during the early Holocene, turbidity currents were confined to the channel and gradually filled it. The canyon “Swatch of No Ground,” a shelf depocenter that serves as the source for frequent turbidity currents, and the channel-levee system provide the unique opportunity for studying an active highstand system. Many fans showed this behavior only during lowered sea-level.

The submarine delta of the Ganges–Brahmaputra: cyclone-dominated sedimentation patterns

The Ganges–Brahmaputra belongs to the worlds largest rivers in terms of sediment discharge, but little is known yet about the processes controlling sediment distribution on the shelf and the documentation of these processes in large- and small-scale sedimentary structures. For this study the submarine delta of the river system was investigated by high-resolution seismic reflection profiles, and piston and gravity coring. Logged physical properties of the cores were combined with visual core description, grain-size analysis, and radiography. Sedimentation rates were estimated by 210Pb and 137Cs gamma spectrometry and cross-checked with evaluation of seismic profiles. The data reveal that the broad delta topset (<20 m waterdepth) is formed by a thick layer of sand + silt. Highest sedimentation rates (about 10 cm a−1) occur at the centre of clinoform delta foreset beds where about 20% of the total riverine sediment load is deposited and thus cause an annual progradation of the subaqueous delta front by about 15 m. The foreset beds consist of graded sand + silt layers which are thought to be deposited by sediment-laden flows generated during the surge of tropical cyclones (tempestites) interbedded with silty clay layers. The head of a canyon, the Swatch of No Ground, that deeply incises into the shelf, forms a sediment trap along the westward cyclonic transport path where sedimentation rate increases to about 50 cm a−1. Episodically earthquakes or storms destabilize especially the rapidly accumulated foreset beds in the eastern delta creating up to 8 m thick mass flows. On the outer shelf in water depth beyond 80 m no Holocene sediment is deposited.

The youngest channel-levee system of the Bengal Fan: results from digital sediment echosounder data

Abstract A high-resolution seismic survey focused on the youngest channel-levee system of the Bengal Fan was carried out during January/February 1994 from the RV Sonne . Acoustic strata patterns and the downslope development of the channel-levee system were examined with the parametric sediment echosounder parasound . Determination of the age of the sedimentary strata shows turbiditic activity during sea level rise and highstand. The initial formation of the system in the middle fan occurred in the late glacial and outer levee growth stopped with glacial termination. Several vertical, aggradational segments constitute the inner levees created in the Holocene. The formation of the inner levee segments indicates the constriction of a formerly 14 km wide channel in discrete phases. The top of the segments form topographic pinnacles, explaining the morphology of other channel-levee systems from other fans in the worlds oceans. Some cross-sections from the lower fan reveal lenticular channel-levee systems with common reflection characteristics. Prograding distinct reflections on the outer side of the upper levees terminate with a downlap against an unconformity, which separates the upper part of the overbank deposits from a mostly diffusely reflecting lower part. Channel constriction has also been observed in the lower fan. Two models are introduced to explain the generation of the most common acoustic strata patterns by sea level fluctuations or downslope shifting of the channel mouth.

Frequent channel avulsions within the active channel-levee system of the middle Bengal Fan - an exceptional channel-levee development derived from Parasound and Hydrosweep data

The active channel–levee system of the middle Bengal Fan was studied by a combined analysis of Parasound echosounder and Hydrosweep swathsounder data. The channel is characterized by highly variable sinuosities. Compared to other mud-rich submarine fans, an exceptionally low channel slope is found. The system can be subdivided into inner and outer zones of significantly different depositional architecture. The inner zone consists of the active channel and sharply separated vertical blocks, which are characterized by parallel, distinct reflectors and planforms of bends. These blocks are interpreted as abandoned channel segments (cut-off loops). The outer zones represent undisturbed levees, which are constructed of parallel and wedge-shaped sedimentary units. The wedge-shaped units, varying significantly in thickness and lateral extent, are found at the outer convex arcs of active and abandoned channel loops caused by overspilling of channelized turbidity currents at sharp bends. The parallel units are the deposits of turbidity currents, which spread their sediments over wide areas as their size significantly exceeds the cross-section of the channel. The complex vertical and horizontal distribution of partially small sedimentary units suggests a more complicated deposition in time and space as hitherto reported from other submarine fans. Within the inner zone, more than 20 cut-off loops were identified over a channel length of 90 km. In contrast to most other large mud-rich submarine fans, channel avulsion within the active channel–levee system is a frequent process. In particular, a temporal succession of at least 4 cut-off loops was reconstructed in the southern study area, indicating channel avulsion on average every 750 years. Channel avulsion seems to be a repetitious process caused by erosion through turbidite currents in a highly sinuous channel. Compared to other submarine fans, no morphological parameter shows a remarkable difference except the channel slope, which is significantly smaller than, for example, on Amazon, Congo and Mississippi fans. The interaction between this low channel slope and the flow parameter of the turbidity currents is most likely the reason for the instability of the active channel planform, leading to an exceptionally large number of meander loop breaches and cut-off loops.

Monsoon-induced partial carbonate platform drowning (Maldives, Indian Ocean)

Multibeam maps and high-resolution seismic images from the Maldives reveal that a late Miocene to early Pliocene partial drowning of the platform was linked to strong sea-bottom currents. In the upper Miocene to Holocene, currents shaped the drowned banks, the current moats along the bank edges, and the submarine dune fields. Bottom currents in the Maldives are driven by the monsoon. It is proposed that the onset and the intensification of the monsoon during the Neogene provoked platform drowning through injection of nutrients into surface waters. Since the late Miocene, topographically triggered nutrient upwelling and vigorous currents switched the Maldives atolls into an aggradational to backstepping mode, which is a growth pattern usually attributed to episodes of rising sea level.

Periplatform drift: The combined result of contour current and off-bank transport along carbonate platforms

Hydroacoustic and sedimentological data from the western leeward flank of the Great Bahama Bank document the interplay of off-bank sediment export, along-slope transport, and erosion, which together shape facies and thickness distribution of slope carbonates. The integrated data set depicts the combined product of these processes and allows formulation of a comprehensive model of a periplatform drift that significantly amends established models of carbonate platform slope facies distribution and geometry. The basinward-thinning wedge of the periplatform drift at the foot of the bank escarpment displays along-slope and downslope variations in sedimentary architecture. Sediments are muddy carbonate sands that coarsen basinward. The drift wedge has a pervasive cover of cyclic steps. In zones of lower contour current speed, depth-related facies belts develop, whereas strike-discontinuous sediment lobes, scarps, and gullies characterize areas with higher current speed. This understanding of the impact of currents on carbonate-slope sedimentation has wider implications for seismic and sequence stratigraphic interpretation of carbonate platforms and for applied aspects such as hydrocarbon exploration.

Structure and origin of southern Weddell Sea crust: results and implications

Abstract The investigation of the crustal structure in the southern Weddell Sea was one of the scientific targets of two refraction seismic experiments carried out by the Alfred Wegener Institute in 1984 and 1992. Two profiles were shot in front of the Ronne Ice Shelf between 50°W and the Antarctic Peninsula. The data reveal different crustal structures east and west of 60°W. In the east, sediments up to 13 km thick overlie basement about 20 km thick, which is of continental origin. The lower crust is characterized by velocities between 7.1 and 7.5 km s−1 which may indicate magmatic underplating and/or intrusions. From these data, it is estimated that the crust is stretched by a factor between 1.5 and 3.0. In the west, the sedimentary basin pinches out, whereas the upper crustal layers exhibit higher velocities (>5 km s−1) than in the east (<4 km s−1). The existence of continental crust in the southern Weddell Sea excludes geodynamic models which propose a palaeoposition of the Ellsworth-Whitmore Mountains between South Africa and Coats Land, East Antarctica. Based on recently published potential field data, a new model for the location of the continent-ocean boundary is proposed.

Global look at salt giants

Water basins with narrow outlets can preserve vast thicknesses of evaporites, known as salt giants, which are thought to form when marginal basins or subbasins are cut off from the main oceanic basin and the water evaporates. Sea salt precipitates and is deposited on the basin floor. They are potential frontiers for challenging research on structural, tectonic, biogenic, and fluid dynamic evolution. These structures are of global importance because they strongly affect the structural, chemical, and biological evolution of sedimentary basins.

Seismic reflectivity anomalies in sediments at the eastern flank of the Juan de Fuca Ridge: Evidence for fluid migration?

A high-resolution seismic survey was carried out in the vicinity of Ocean Drilling Program (ODP) Leg 168 drill sites at the eastern flank of the Juan de Fuca Ridge. Three seismic systems with different source frequencies up to 4000 Hz were used simultaneously along each seismic line. The data sets were combined to provide the best possible resolution at any given depth level. An integrated interpretation of narrow zones of low reflection amplitudes is presented, which are related to basement highs but are independent of seafloor topography. The effect is most pronounced above a buried basement ridge in the vicinity of ODP Sites 1030 and 1031, where higher porosity values are present. Higher porosities can reduce seismic impedance contrasts and may therefore cause the observed low reflection amplitudes. In addition, Biot-Stolls [Biot, 1956a, b; Stoll, 1989] model suggests that attenuation coefficients are porosity dependent and reflection amplitudes are further decreased at higher seismic frequencies when porosity increases. Since zones of higher porosities are potential pathways for fluids, they may be associated with hydrothermally driven fluid discharge, which was previously predicted above the buried basement ridge. It is therefore proposed that the observed seismic amplitude reduction in the vicinity of ODP Sites 1030 and 1031 indicates zones of upward fluid migration.

The continental margin off East Antarctica between 10°W and 30°W

Abstract The geodynamic understanding of the Gondwana break-up between Africa, South America and Antarctica depends strongly upon the quality and coverage of geological and geophysical data. In this area, the tectonics of the Weddell Sea embayment plays an important role in constraining plate movements during the early opening of the Southern Ocean. Geophysical data collected in 1990 and 1992, including multichannel seismic data and gravity measurements as well as altimeter data from ERS-1, give new insights into the shape and structure of the continental margin off East Antarctica between 10°W and 30°W. With the seismic network a new geological structure, the Polarstern Bank, could be mapped at 71°15′S, 25°W. We interpret the feature to consist of several seamounts, striking almost N-S. The location of this prominent structure between the Andenes and Explora escarpments makes the presence of a continent-ocean boundary in this area as proposed by other models unlikely. Our interpretation is supported by all data sets presented in this paper. Based on our results, the two escarpments must have different geological origins and may also have different ages. The data indicate that the continent-ocean boundary runs parallel to the present coastline of the East Antarctic marked by a pronounced positive free air gravity anomaly and a positive magnetic anomaly.