Bengal Fan Research Articles

Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal

Sedimentological and provenance data from the lower Miocene–Pliocene Dumri Formation and Siwalik Group in western Nepal provide new information about the timing of thrust faulting and the links between erosional unroofing of the Himalaya and the Cenozoic 87 Sr/ 86 Sr record of the ocean. In western Nepal, the Dumri Formation is an ∼750–1300-m-thick fluvial sandstone and overbank mudstone unit. The Siwalik Group is >4200 m thick and consists of a lower member (>850 m) of 2–12-m-thick fluvial channel sandstones and oxidized calcareous paleosols, a middle member (>2400 m) of very thick (>20 m) channel sandstones and mainly organic-rich Histosols, and an upper member (>1000 m) composed of gravelly braided river deposits. Paleocurrent data indicate that middle Miocene–Pliocene rivers in western Nepal flowed southward, transverse to the thrust belt, throughout deposition of the Siwalik Group. No evidence was found for an axial fluvial trunk system (i.e., the paleo-Ganges River) in Siwalik Group sandstones. A major increase in fluvial channel size is recorded by the transition from lower to middle Siwalik members at ∼10.8 Ma, probably in response to an increase in seasonal discharge. Modal petrographic data from sandstones in the Dumri Formation and the Siwalik Group manifest an upsection enrichment in potassium feldspar, carbonate lithic fragments, and high-grade metamorphic minerals. Modal petrographic analyses of modern river sands provide some control on potential source terranes for the Miocene–Pliocene sandstones. The Dumri Formation was most likely derived from erosion of sedimentary and low-grade metasedimentary rocks in the Tibetan (Tethyan) Himalayan zone during early Miocene emplacement of the Main Central thrust. The presence in Dumri sandstones of plagioclase grains suggests exposure of crystalline rocks of the Greater Himalayan zone, perhaps in response to tectonic unroofing by extensional detachment faults of the South Tibetan detachment system. During deposition of the lower Siwalik Group (∼15–11 Ma), emplacement of the Dadeldhura thrust sheet (one of the synformal crystalline thrust sheets of the southern Himalaya) on top of the Dumri Formation supplied abundant metasedimentary lithic fragments to the foreland basin. A steady supply of plagioclase grains and high-grade minerals was maintained by deeper erosion into the Main Central thrust sheet. From ∼11 Ma to the present, K-feldspar sand increased steadily, suggesting that granitic source rocks became widely exposed during deposition of the upper part of the lower Siwalik Group. This provenance change was caused by erosion of passively uplifted granites and granitic orthogneisses in the Dadeldhura thrust sheet above a large duplex in the Lesser Himalayan rocks. Since the onset of deposition of the conglomeratic upper Siwalik Group (∼4–5 Ma), fault slip in this duplex has been fed updip and southward into the Main Boundary and Main Frontal thrust systems. We obtained 113 U-Pb ages on detrital zircons from modern rivers and Siwalik Group sandstones that cluster at 460–530 Ma, ∼850–1200 Ma, ∼1.8–2.0 Ga, and ∼2.5 Ga. An abundance of Cambrian–Ordovician grains in the Siwalik Group suggests sources of Siwalik detritus in the granites of the Dadeldhura thrust sheet and possibly the Greater Himalayan orthogneisses. The older ages are consistent with sources in the Greater and Lesser Himalayan zones. An overall upsection increase in zircons older than 1.7 Ga suggests increasing aerial exposure of Lesser Himalayan rocks. None of the detrital zircons (even in the modern river samples) yielded a Cenozoic age that might suggest derivation from the Cenozoic Greater Himalayan leucogranites, but this may be attributable to the inheritance problems that characterize the U-Pb geochronology of the leucogranites. When compared with recent studies of the 87 Sr/ 86 Sr composition of paleosol carbonate nodules and detrital carbonate in paleosols from the Siwalik Group, the provenance data suggest that erosion and weathering of metamorphosed carbonate rocks in the Lesser Himalayan zone and Cambrian–Ordovician granitic rocks of the crystalline thrust sheets in central and eastern Nepal may have played a significant role in elevating the 87 Sr/ 86 Sr ratio of middle Miocene synorogenic sediments in the Indo-Gangetic foreland basin and the Bengal fan, as well as global seawater.

Seamounts an additional tool to confirm the nature of the crust and to locate possible mineral resources for dredging

Study of bathymetric data for the distal Bengal Fan reveals the presence of positive topographic features resembling seamounts. These features exhibit a relief of about 800 m above the surrounding seafloor at 4,400 m and are associated with large‐amplitude magnetic and gravity anomalies. Magnetic model studies suggest that one of the seamounts evolved along with the oceanic crust represented by the magnetic isochron A32 (∼ 72 My), while the other seamount was emplaced over the Cretaceous Quiet Zone Crust with reversed polarity. The seamounts studied provide an environment for possible locations of polymetallic encrustations. Morphology of the seamounts depends on several factors, including physical and chemical composition of the magma, eruption style, sediment thickness, maturity of underlying crust, and time of origin. Studying the seamounts together with integrated geophysical and physical data gave useful indications of their evolution and the existence of possible mineral deposits.

Palynology of the Late Tertiary sediments (DSDP Site 218) in the Bengal Fan, Indian Ocean

Palynological assemblage from DSDP Site 218 in the Bengal Fan comprises spores, pollen, fungal fruiting bodies and dinoflagellate cysts. The palynofloral composition has minor variation in core no. 27 to 12. The microthyriaceous fungal remains show comparatively higher frequency than other fungal entities. The gymnosperms are represented mostly by the pollen grains of Podocarpaceae and Araucariaceae. Angiosperm pollen are sparsely observed in the assemblage. The pteridophytic spores occur mostly in the bottom and middle cores (core nos. 27-26 and 23-20). Recycled Cretaceous spores have also been found in most of the core samples. This palynoassemblage from Site 218 is comparable to that of the Miocene sediments of northeast India. The environmental relationship of the taxa shows similarity with the modem plants inhabited in the subtropical climate.

Subaqueous delta of the Ganges-Brahmaputra river system

The Ganges-Brahmaputra is among the world's three largest river systems in terms of sediment load, but, until now, no high-resolution seismic data have been obtained to document the nature of the sediment deposit seaward of the rivers' mouths. The other two (Amazon, Huanghe) discharge into energetic coastal environments and form subaqueous deltas with characteristic clinoform stratigraphy. High-resolution seismic reflection profiles of the Bengal shelf reveal similar stratigraphy: topset beds dip gently (0.036 °) and diverge offshore; more steeply dipping foreset beds (0.190 °) converge farther seaward; and relatively thin, gently dipping bottomset beds (0.022 °) extend across the outer shelf, overlying an erosional surface presumed to be of Late Pleistocene age. Sediment accumulation rates are highest in the foreset region (≥ 5 cm/year) and reduced in the bottomset region (< 0.3 cm/year), corroborating the relative thickening and thinning of strata observed in seismic profiles. Taken together, these data indicate a subaqueous delta is actively prograding across the Bengal shelf. Volume estimates for the Holocene subaqueous delta reveal that about one third of the total load of the Ganges-Brahmaputra has accumulated on the shelf. The remainder is likely partitioned between the river floodplain/delta plain and off-shelf transport via the submarine canyon, Swatch of No Ground. The canyon incises the shelf in the area of highest sedimentation rates (foreset), and growth faults and slumping of modern sediments near the head of the canyon support the idea that significant off-shelf transport of sediments to the Bengal Fan is occurring.

Organic carbon burial forcing of the carbon cycle from Himalayan erosion

1-4 . Recent attention has focused on increased silicate weathering of tectoni- cally uplifted areas in the India-Asia collision zone as a possible cause for falling atmospheric CO2 levels in the Cenozoic era 5-7 . The chemistry of Neogene sediments from the main locus of sedimentary deposition for Himalayan detritus, the Bengal Fan, can be used to estimate the sinks of CO2 from silicate weathering and from the weathering and burial of organic carbon resulting from Himalayan uplift. Here we show that Neogene CO2 con- sumption from the net burial of organic carbon during Himala- yan sediment deposition was 2-3 times that resulting from the weathering of Himalayan silicates. Thus the dominant effect of Neogene Himalayan erosion on the carbon cycle is an increase in the amount of organic carbon in the sedimentary reservoir, not an increase in silicate weathering fluxes. Silicate weathering is typically incongruent, yielding both a solute and a secondary mineral phase, so direct evidence of chemical weathering can be found in the record of secondary minerals in sedimentary basins. The Bengal Fan and Ganges-Brahmaputra (GB) delta contain a huge volume of sediment derived from erosion of the India-Asia collision zone, with 6 3 10 6 km 3 deposited in the past 20 Myr (ref. 8). Isotopic data for Nd, Sr and O from Bengal Fan sediments show that the source for over 80% of the detritus since 20 Myr ago has been the high-grade metasedimentary rocks of the High Himalayan crystalline (HHC) sequence 9 . Clastic and carbo- nate sediments of the Precambrian Lesser Himalaya (LH) and Palaeozoic-Mesozoic Tethyan Himalaya (TH) are the other impor- tant sources of sediment to the Bengal Fan during the Neogene. Carbon dioxide consumption from silicate weathering can be represented schematically by: CaSiO3 ˛ 2CO2 ˛ 2H2O ! Ca 2˛ ˛ 2HCO 2 3 ˛ H4SiO4

Molasse-Delta-Flysch Continuum of the Himalayan Orogeny and Closure of the Paleogene Katawaz Remnant Ocean, Pakistan

Molasse strata on the Indo-Pakistan subcontinent and modern Indus delta-submarine fan sediments in the northern Indian Ocean were derived from the Himalaya. Earliest molasse sedimentation started around the late Paleocene, soon after formation of the early Himalayan orogenic highlands. However, turbidite sedimentation of the modern Indus fan did not begin until the early Miocene, and occurred even later on the Bengal fan in the modern Indian Ocean. Paleogene deltaic and flysch sediments that are missing from the Indian Ocean are preserved as the Khojak Formation in the Katawaz basin and the Makran area, Pakistan. This implies that the Katawaz remnant ocean was the main depocenter of early Himalayan marine sediments prior to deposition in the Indian Ocean, the Neogene depocenter. The Katawaz remnant ocean, a southwestern extension of the Neo-Tethys, was closed by the end of early Miocene time and the Paleogene delta-flysch sediments were incorporated into the Indo-Pakistan subcontinent.

Discovery of the palaeo-Indus delta-fan complex

The Indus River carries sediments from the western Himalaya and deposits some of these as channel and floodplain sediments or molasse. The rest of its load forms the Indus delta at the margin of the Indian Ocean. The Indus delta passes some of its sediments to be deposited as Indus submarine fan turbidites. Thus, as elsewhere, Himalayan molasse, delta, and fan deposition are related hi tune and space. However, when we examine fluvial and marine age-range data of the older Indus deposits, in terms of this sedimentary assemblage, a major portion of its marine record is missing. The oldest known molasse along the Indus suture zone, and hi the foredeep are middle Eocene and late Palaeocene hi age, respectively. A recent synthesis of sedimentation in the northern Indian Ocean, however, shows that turbidite sedimentation started around early Miocene in the modern Indus fan, and even later in the Bengal fan. Where are the Palaeogene Indus delta and fan sediments? We suggest herein that these are preserved as the Palaeogene siliciclastic Khojak Formation hi the Katawaz Basin and eastern Makran. The modern Indus River is 2900 km long (Coumes &amp; Kolla 1984) and has 30 000 m 3 s − 1 discharge (Milliman et al. 1984) in summer (Fig. 1), twice that of the Mississippi River. The annual suspended sediment load (Wells &amp; Coleman 1984) varies between 395 and 435 x 10 3 t, almost equal to that of the Mississippi. The existence of the middle Eocene to middle Miocene Indus molasse along the

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

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 world's 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.

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.

A new stratigraphy for the islands of the Sumatran Forearc, Indonesia

The Sumatran Forearc of western Indonesia contains a number of islands where extensive exposures of basement rocks and their sedimentary cover may be examined. The islands, such as Nias and the Batu Islands, are located along the outer-edge of the Sumatran Forearc, whilst others, including the Banyak Island Group and Pini Island, lie within the forearc basin. Detailed sedimentological, palaeontological and palaeobathymetric data from the Tertiary strata from the forearc region require a new stratigraphy, as previous stratigraphic schemes have not explained the variations across the region adequately. This stratigraphy, developed initially from detailed data collected on Nias and the Banyak Islands, can fully account for the successions of sedimentary rocks on the Banyak and Batu Islands and Siberut and explains many of the apparent inconsistencies between previous stratigraphies. A basement complex and six new formations are formally defined in this paper; important sedimentological differences between these formations represent key stages in the evolution of the outer part of the Sumatran Forearc. Studies of the basement rocks across the forearc area suggest the basement is inhomogeneous; large intact sections of ophiolitic material occur in some areas, whilst there is evidence for both oceanic and continental basement in others. Such heterogeneity is to be expected in a long lived obliquely convergent margin. In Oligocene-earliest Miocene times extension of the heterogeneous basement is inferred through indirect evidence. Palaeobathymetric data from the Oyo Formation indicates that the initial deposition in the newly formed extensional sub-basins on Nias was, in most areas, deep marine, in many cases below the CCD. Detailed biostratigraphic analyses and structural and geochronological studies indicate a major Early Miocene unconformity in the western (Lahewa Sub-basin) and parts of central Nias (Mujoi Sub-basin). This unconformity was developed as a direct result of a period of basin inversion that affected western parts of Nias. Whilst sub-aerial erosion occurred in parts of western Nias, conformable deposition of the Gawo and Olodano Formation continued in the Gomo and eastern parts of the Mujoi Sub-basins. The shallow marine sedimentary rocks of the Olodano Formation tended to accumulate on intra sub-basinal highs whose position was controlled by active faults that transected the sub-basins. The sedimentary record reveals that the Lower and Middle Miocene phases of differential uplift and subsidence had ceased by the Late Miocene. A massive influx of Himalayan derived Bengal Fan sediments reached the Sunda Trench in the Sumatra area in the late Middle Miocene. Continued addition of Bengal Fan material to the accretionary wedge south-west of Nias resulted in steady plate deflection and subsidence through flexural processes in forearc basin areas. The flexural consequence of increased load added to the prism, and associated subsidence history is documented by the sedimentary record on Nias where the shallow marine Olodano Formation passes up into the neritic to upper bathyal Lahomie Formation. The Pliocene unconformity which is observed over all areas studied in the forearc is well constrained by structural, biostratigraphic and sedimentological studies. The unconformity represents the initiation of a major phase of uplift and deformation that continues to the present day. Rapid uplift of the outer arc ridge and deformation of the prism during the Pliocene led to increased subsidence landward of the deformation. The rapid subsidence of the forearc basin landward of the outer-arc ridge has contributed greatly to the “apparent” differences between the forearc basin and the outer-arc ridge at the present day: two areas with remarkably similar pre-Pliocene histories now have remarkably different physiographies.

A new stratigraphy for the islands of the Sumatran Forearc, Indonesia

Abstract The Sumatran Forearc of western Indonesia contains a number of islands where extensive exposures of basement rocks and their sedimentary cover may be examined. The islands, such as Nias and the Batu Islands, are located along the outer-edge of the Sumatran Forearc, whilst others, including the Banyak Island Group and Pini Island, lie within the forearc basin. Detailed sedimentological, palaeontological and palaeobathymetric data from the Tertiary strata from the forearc region require a new stratigraphy, as previous stratigraphic schemes have not explained the variations across the region adequately. This stratigraphy, developed initially from detailed data collected on Nias and the Banyak Islands, can fully account for the successions of sedimentary rocks on the Banyak and Batu Islands and Siberut and explains many of the apparent inconsistencies between previous stratigraphies. A basement complex and six new formations are formally defined in this paper; important sedimentological differences between these formations represent key stages in the evolution of the outer part of the Sumatran Forearc. Studies of the basement rocks across the forearc area suggest the basement is inhomogeneous; large intact sections of ophiolitic material occur in some areas, whilst there is evidence for both oceanic and continental basement in others. Such heterogeneity is to be expected in a long lived obliquely convergent margin. In Oligocene-earliest Miocene times extension of the heterogeneous basement is inferred through indirect evidence. Palaeobathymetric data from the Oyo Formation indicates that the initial deposition in the newly formed extensional sub-basins on Nias was, in most areas, deep marine, in many cases below the CCD. Detailed biostratigraphic analyses and structural and geochronological studies indicate a major Early Miocene unconformity in the western (Lahewa Sub-basin) and parts of central Nias (Mujoi Sub-basin). This unconformity was developed as a direct result of a period of basin inversion that affected western parts of Nias. Whilst sub-aerial erosion occurred in parts of western Nias, conformable deposition of the Gawo and Olodano Formation continued in the Gomo and eastern parts of the Mujoi Sub-basins. The shallow marine sedimentary rocks of the Olodano Formation tended to accumulate on intra sub-basinal highs whose position was controlled by active faults that transected the sub-basins. The sedimentary record reveals that the Lower and Middle Miocene phases of differential uplift and subsidence had ceased by the Late Miocene. A massive influx of Himalayan derived Bengal Fan sediments reached the Sunda Trench in the Sumatra area in the late Middle Miocene. Continued addition of Bengal Fan material to the accretionary wedge south-west of Nias resulted in steady plate deflection and subsidence through flexural processes in forearc basin areas. The flexural consequence of increased load added to the prism, and associated subsidence history is documented by the sedimentary record on Nias where the shallow marine Olodano Formation passes up into the neritic to upper bathyal Lahomie Formation. The Pliocene unconformity which is observed over all areas studied in the forearc is well constrained by structural, biostratigraphic and sedimentological studies. The unconformity represents the initiation of a major phase of uplift and deformation that continues to the present day. Rapid uplift of the outer arc ridge and deformation of the prism during the Pliocene led to increased subsidence landward of the deformation. The rapid subsidence of the forearc basin landward of the outer-arc ridge has contributed greatly to the “apparent” differences between the forearc basin and the outer-arc ridge at the present day: two areas with remarkably similar pre-Pliocene histories now have remarkably different physiographies.

Os isotopic compositions of leachates and bulk sediments from the Bengal Fan

Os isotopic compositions and Os and Re concentrations were measured in H 2O 2—H 2SO 4 leachates and bulk sediment samples from Holes 717C and 718C of ODP Leg 116 in the Bengal Fan. Os isotopic results indicate that, at the sediment surface, the leachable Os fraction is derived from seawater. In contrast, leachable Os from Ganges River sediments has 187Os/ 188Os ratios [1] much higher than the marine value. This difference suggests that the leachable radiogenic Os carried by the river sediments is completely released to the oceans prior to sediment deposition in the Fan. A simple calculation, assuming these sediments to be typical of those delivered by the Ganges-Brahmaputra river system, suggests that this process can account for a substantial part of the rise in the seawater Os isotopic ratio observed over the past 16 m.y. Bengal Fan leachate 187Os/ 188Os ratios increase with increasing depositional age, in contrast to the seawater Os isotopic ratio, which decreases with increasing age. Several lines of evidence suggest that, at the time of sediment burial, the leachate Os compositions most likely reflected the seawater values. Thus, the current divergence is probably the result of post-depositional processes. One such process, in situ radiogenic ingrowth of 187Os, can be excluded because the measured Re concentrations of these sediments are too low. Similarly, since most of the bulk rock Os isotopic ratios were lower than those of the associated leachates, the high leachate 187Os/ 188Os values cannot be explained by in situ sediment alteration. Instead, it is proposed that the increase with age results from radiogenic Os brought in by thermoconvective circulation from further upslope in the Fan. The ultimate source of this 187Os would then be alteration of radiogenic sediments or post-depositional radioactive decay of Re in sediments rich in organic carbon. Finally, the divergence between the results obtained on Bengal Fan sediments and those obtained in the open ocean [2] by the same leaching technique suggest that Os sediment leachate data must be interpreted with caution.

Echo characters of the continental margin, western Bay of Bengal, India

An echo character study of about 35,000 km length of 3.5 kHz echograms, recorded from the continental shelf, slope and rise domains in the western Bay of Bengal, reveals twenty echo types. Based on their distribution, the analysis of sediment cores, and comparison with the similar echo types reported from Atlantic continental margin, the echoes are interpreted to reflect variation in sediments produced by different processes. The distribution of various echo types are significantly different within each physiographic province. Shallow marine processes have influenced the continental shelf off non-deltaic areas whereas fluvio-deltaic processes dominate areas off deltas. Deposits formed by mass wasting dominate the continental slope off Puri-Machilipatnam and off Pondicherry where the sea floor is eroded and gullied by slides, slumps and debris flows. The deposits developed under the influence of combined processes are most widespread on the continental rise south of Kakinada. The rest of the continental rise, the continental slope off Nizampatnam Bay and Cauvery River, and the Bengal Fan are influenced by turbidity currents. The Bengal Fan is also affected by mass wasting.

Multiple seawater-derived geochemical signatures in Indian oceanic pelagic clays

This paper reports a geochemical study of oceanic clays. Major and trace elements were analyzed on smectite-rich, clay size (<2 μm) samples, bulk sediments, and leachate residues from the Central Indian Basin. SrNd isotopes were also studied to investigate their geochemical evolution during transport in the water column, sedimentation, and diagenesis. The region is of special interest because the sedimentation records the interaction between the detrital supply from the Bengal Fan in the north and the biosiliceous input associated with the equatorial divergence in the south. The clay size fractions display extremely variable trace element contents, e.g., [Ba] = 100–5000 ppm, [Sr] = 20–200 ppm, Ce/Ce ∗ = 0.9–3.3, [Nd] = 10–50 ppm. Although in the argillaceous samples, clay size fractions have a similar trace element imprint to the bulk sediment, some major fractionations occur in the biosiliceous samples between the clay and the bulk sediment, especially for Sr and rare earth elements (REE). Three major components may account for the variable geochemical signatures of these pelagic clays. The first component (component A), already identified by Fagel et al. (1994), is characterized by a homogeneous geochemical signature (La N/Yb N = 1.03–1.05; Th/Ta = 12.8–21.1; Ba/Th ∼ 28) and a nonradiogenic Nd isotopic composition ( 143Nd/ 144Nd ∼ 0.511880): it traces a detrital Himalayan-derived origin. The two other components display a seawater-derived isotopic composition with global Sr ( 87Sr/ 86Sr ∼ 0.709060) and regional Indian Ocean Nd ( 143Nd/ 144Nd ∼ 0.512200) signatures. Both components are enriched in Sr and Ba (Sr ∼ 150 ppm, Ba/Th ∼ 500), and they are either enriched in light rare earth elements (LREE, e.g., Nd ∼ 50 ppm) in the argillaceous sediments (component B) or LREE-depleted (Nd < 20 ppm) in the biosiliceous sediments (component C). The frequent occurrence of micrometric (<5 μm) Sr-REE-Th enriched barite grains showing three major habits (rhombic, rounded, dendritic) suggests that these biologically-derived mineral phases had a major role in the genesis of components B and C. A strong clay-barite equilibration is deduced from the Post Archean Australian Shales PAAS-like REE patterns of these barites and the Ba enrichment of the clays. We suggest that it results from two successive mechanisms of exchange. First, at the top of the oxygen minimum zone, the microbial-induced decay of organic matter is proposed to trigger a series of trace element transfers between the various particulate-forming components (clays, barites, and decaying organic coatings). This is proposed as the origin of the clay component B: the barite-derived components (Ba, Sr) and the organic-derived positive Ce anomaly are imported to the clay particles while the PAAS signature of the clays is retained by the remaining barite crystals. Second, after settling, the barites are believed to partly dissolve and recrystallize, especially in the anoxic part of the sedimentary column. This diagenetic barite dissolution is proposed as the origin of the clay component C.

Mineralogy and geochemistry of surface sediments from the bengal fan, Indian ocean

The mineralogy and composition of 75 sediment samples from the Bengal Fan. Andaman Sea and the eastern flanks of the upper Nicobar Fan have been determined. Sediments from the Upper and Middle Bengal Fan are mainly terrigenous and derived from the Ganges Brahmaputra river system, whereas sediments from the Andaman Sea and the eastern flank of the upper Nicobar Fan vary from terrigenous to largely biogenic. For the terrigenous sediments, the continental influence is illustrated by the distribution of the major elements (excluding Ca) which decrease away from the source. The Ti/Al and Fe/Al ratios of the sediments are higher in samples from the cast coast of India than from the fans, indicating the influence of material from the Deccan basalts as well as from mafic rocks of the Indian Peninsula. In the clay mineral fraction of Upper Bengal Fan sediments, illite and chlorite are the dominant constituents, with minor amounts of smectite and traces of kaolinite. Illite is mainly a weathering product of lithic material from the Himalaya and is transported by the Ganges Brahmaputra river system. Smectite is the dominant constituent of the sediments adjacent to the Indian Peninsula.

Mineralogy and geochemistry of surface sediments from the Bengal Fan, Indian Ocean

Mineralogy and geochemistry of surface sediments from the Bengal Fan, Indian Ocean

Mineralogy and geochemistry of surface sediments from the Bengal Fan, Indian Ocean

Mineralogy and geochemistry of surface sediments from the Bengal Fan, Indian Ocean

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.

Calibration and application of marine sedimentary physical properties using a multi-sensor core logger

We examined geophysical data from a Multi-Sensor Core Logger (MSCL), a logging device providing continuous measurements of gamma-ray attenuation, p-wave travel time, and magnetic susceptibility on marine sediment cores. In the first part we focused on the gamma-ray system and compared two different calibration methods. From the gamma-ray attenuation, we calculated densities and porosities by incorporating mass weighted attenuation coefficients. The application of an iteration method reduces the error of the density and porosity estimates compared to GRAPE data. In addition, we derived equations to calculate water content and dry bulk density from gamma-ray attenuation measurements. Comparison with physical properties determined on discrete samples revealed a very good correlation of both data sets ( r = 0.99). This correlation is valid for sediments from substantially different geological settings (e.g., turbidites, hemipelagic muds, and opal-rich sediments). In the second part we applied our data to marine geological questions. For sediments from the Antarctic Polar Frontal Zone, there is indication that the content of biogenic opal can be assessed using a correlation of density and p-wave velocity. For sediments from the Bengal Fan, the relationship between the MSCL acoustic impedance (the product of density and p-wave velocity) and the grain-size distribution in discrete samples can be used to predict clay and sand/silt ratios for sediment cores from the shelf and upper continental slope.

The Late Oligocene-Early Miocene Himalayan belt Constraints deduced from isotopic compositions of Early Miocene turbidites in the Bengal Fan

We present a mineralogical and isotopic study of Early Miocene turbidites from the Bengal Fan, which are the oldest sediments recovered during ODP Leg 116. Clay fractions are dominated by illite and chlorite and have low δ18O (11–16‰). The clays are dominantly derived from a metamorphic precursor by physical erosion, and have undergone only minor low-T water-rock exchange and chemical weathering. Sr and Nd isotope analysis of silt and clay fractions and coarse muscovite, biotite and feldspar fractions are remarkably similar to those of the presently exposed High Himalaya Crystalline (HHC) sequence. The data show that a close analogue of the HHC was already subaerially exposed to active erosion during Early Miocene. Rb-Sr data on the biotite separates has been used to evaluate the time elapsed between cooling through the closure temperature for Sr in biotite in the HHC (ca. 325°C) and final sedimentation in the Bengal Fan. Despite the large uncertainties in the initial 87Sr86Sr values of sedimentary mixtures, our estimates of the elapsed time between cooling and deposition are short (4–14 Ma). Assuming a transport time scale (≤1 Ma), deduced exhumation rates are in the same range of modern values (1–3 mm/yr) since ca. 30 Ma. Given that (a) the rocks exposed at the surface, (b) the style of weathering, and (c) rates of exhumation were similar to the present, we infer that the factors controlling erosion of the Early Miocene Himalayan belt were also similar to the present. The data suggest that the Early Miocene Himalayan belt had significant relief and induced orographic effects on precipitation similar to the modern mountain range. It appears that the thrusting along the Main Central Thrust (MCT) or similar structures has uplifted metamorphic basement analogous to the present HHC at sufficiently high rate to maintain steep relief throughout Miocene time, despite high average erosion rates.