Frans J. Emmel

Structure, Tectonics, and Geological History of the Northeastern Indian Ocean

This paper is a summary and progress report of a study of the northeastern Indian Ocean, covering the areas of the Bengal and Nicobar Fans, the western Wharton Basin, the continental margins surrounding the Bay of Bengal, the Andaman Sea, the Andaman-Nicobar Ridge, the Sunda Arc off Sumatra and Java, and the adjacent land areas (Fig. 1). Bathymetry and topography of the study area are shown in Fig. 2. Combined, it is one structural province, extending from the Assam Valley on the northeast and the Ganges flood plain on the northwest, southward over oceanic crust to the distal ends of the Bengal and Nicobar Fans. This Mesozoic-Cenozoic structural basin will henceforth be referred to as the Bengal Geosyncline.

The Bengal Fan: morphology, geometry, stratigraphy, history and processes

Abstract The Bengal Fan is the largest submarine fan in the world, with a length of about 3000 km, a width of about 1000 km and a maximum thickness of 16.5 km. It has been formed as a direct result of the India–Asia collision and uplift of the Himalayas and the Tibetan Plateau. It is currently supplied mainly by the confluent Ganges and Brahmaputra Rivers, with smaller contributions of sediment from several other large rivers in Bangladesh and India. The sedimentary section of the fan is subdivided by seismic stratigraphy by two unconformities which have been tentatively dated as upper Miocene and lower Eocene by long correlations from DSDP Leg 22 and ODP Legs 116 and 121. The upper Miocene unconformity is the time of onset of the diffuse plate edge or intraplate deformation in the southern or lower fan. The lower Eocene unconformity, a hiatus which increases in duration down the fan, is postulated to be the time of first deposition of the fan, starting at the base of the Bangladesh slope shortly after the initial India–Asia collision. The Quaternary of the upper fan comprises a section of enormous channel-levee complexes which were built on top of the preexisting fan surface during lowered sea level by very large turbidity currents. The Quaternary section of the upper fan can be subdivided by seismic stratigraphy into four subfans, which show lateral shifting as a function of the location of the submarine canyon supplying the turbidity currents and sediments. There was probably more than one active canyon at times during the Quaternary, but each one had only one active fan valley system and subfan at any given time. The fan currently has one submarine canyon source and one active fan valley system which extends the length of the active subfan. Since the Holocene rise in sea level, however, the head of the submarine canyon lies in a mid-shelf location, and the supply of sediment to the canyon and fan valley is greatly reduced from the huge supply which had existed during Pleistocene lowered sea level. Holocene turbidity currents are small and infrequent, and the active channel is partially filled in about the middle of the fan by deposition from these small turbidity currents. Channel migration within the fan valley system occurs by avulsion only in the upper fan and in the upper middle fan in the area of highest rates of deposition. Abandoned fan valleys are filled rapidly in the upper fan, but many open abandoned fan valleys are found on the lower fan. A sequence of time of activity of the important open channels is proposed, culminating with formation of the one currently active channel at about 12,000 years BP.

Large submarine slide (olistostrome) associated with Sunda Arc subduction zone, northeast Indian Ocean☆

Reflection profiling in a region of anomalous topography and structure in the Bay of Bengal off Burma has revealed the presence of a large submarine slide (olistostrome) at the base of the continental slope off the Bassein River. The slide overlies a thick section of Bengal Deep-Sea Fan turbidites and has a complex internal structure consisting of two primary elements. The lower element is pervasively disturbed and is interpreted as a mudflow generated at the time of the slide which spread over a large area to as much as 35 km beyond the topographic toe. This mudflow poured into a distributary channel on the Bengal Fan and virtually filled it for 145 km along its length. The upper element comprises a series of relatively coherent blocks of stratified sediments (olistoliths) bounded by curved fault planes. The blocks have been transported as much as 55 km from the original Sunda Trench wall. Their dimensions, up to 360 m thick and 2.8 km between faults, are similar to olistoliths of the slide terrain in the Apennines. The blocks are blanketed by younger slope strata. The total area covered by the slide, including the mudflow, is almost 4,000 km2, and total volume of the slide is over 900 km3. Material of the slide consists of Bengal Fan turbidites offscraped above the Sunda Subduction zone and blanketed by rapidly deposited slope sediments from a western Irrawaddy River distributary (the Bassein River) during Late Quaternary glacial low sea level. This rapid loading, probably coupled with a large earthquake, triggered the slide.

Sedimentation in the Sunda Trench and forearc region

Summary Sedimentation in the Sunda Trench and forearc region is dominated by the transport of terrigenous detritus from orogenic and volcanic terranes around the margins of the north-eastern Indian Ocean. Quartzose Himalayan detritus is transported into the Bay of Bengal and down the trench axis as far south as the Sunda Strait. Prior to development of the outer-arc ridge, the majority of arc-derived sediment bypassed the forearc basin and was deposited in the trench. Nearly all sediment derived from the arc terranes of Java and Sumatra during the Neogene has been trapped in the forearc basin and has not reached the trench. Hemipelagic sedimentation dominates on the lower part of the inner trench slope. Calcareous microfossils and volcanic ash are the dominant constituents of these hemipelagic deposits. Higher on the slope, terrigenous and hemipelagic sediments accumulate in large trench-slope basins. The Sunda forearc basin is a series of smaller individual basins separated from each other by transverse highs, isolating the respective sedimentary sequences. Off north Sumatra, quartzose detritus accumulates, whereas off Java, volcaniclastic sediments predominate. Long-distance transport of sediment down the trench axis, damming of sediments behind the outer-arc ridge, and the segmented nature of the forearc basin leads to the juxtaposition of sediment bodies with different provenances.

The Bengal Submarine Fan, Northeastern Indian ocean

Bengal Submarine Fan, with or without its eastern lobe, the Nicobar Fan, is the largest submarine fan known. Most of its sediment has been supplied by the Ganges and Brahmaputra Rivers, probably since the Early Eocene. The “Swatch-of-No-Ground” submarine canyon connects to only one active fan valley system at a time, without apprent bifurcation over its 2500-km length. The upper fan is comprised of a complex of huge channel-levee wedges of abandoned and buried older systems. A reduction of channel size and morphology occurs at the top of the middle, fan, where meandering and sheet flow become more important.

Bengal Fan, Indian Ocean

Bengal Submarine Fan, with or without its eastern lobe, the Nicobar Fan, is the largest submarine fan known. Most of its sediment has been supplied by the Ganges and Brahmaputra Rivers, probably since the Early Eocene. The “Swatch-of-No-Ground” submarine canyon connects to only one active fan valley system at a time, without apparent bifurcation over its 2500- km length. The upper fan is comprised of a complex of huge channel- levee wedges of abandoned and buried older systems. A reduction of channel size and morphology occurs at the top of the middle fan, where meandering and sheet flow become more important.

A submerged late Pleistocene delta and other features related to sea level changes in the Malacca Strait

Abstract Bathymetry, sea floor echo characteristics, and shallow subbottom reflectors in the Malacca Strait and the slope of the Mergui—North Sumatra Basin of the southern Andaman Sea show a varied and locally complex character, which we interpret as a Pleistocene lowered sea level alluvial-delta-fan system. Sediment waves and elongate banks in the narrow and shoal southern part of the Strait indicate strong bottom currents, and a rugged subbottom topography demonstrates erosion during low sea level of the late Quaternary. Shallow subbottom reflectors at the northwest approaches to the Strait show progradation and cut and fill valleys and the presence of a compound late Pleistocene delta of the confluent Sumatran and Malayan rivers. The younger prograding layers are probably of mid-Wisconsin (Wurm) age (ca. 40− 20 × 10 3 yr B.P. ), and the deeper foresets may be early Illinoian glacial stage (Riss, ca. 150 × 10 3 yr B.P. ). Farther seaward, a small fan progrades from the slope a short distance into the shallow northeastern part of the Mergui—North Sumatra Basin.

Dynamic events near the upper and mid-fan boundary of the Bengal Fan

Bathymetric sections across the most recent valley just above and below the transition of upper to middle fan show a northward shift of the thalweg between the confines of the levees. This small-scale migration was followed by a levec break on the higher north side. Part of the bottom currents, which normally would have been contained within the levees, spilled over and spread laterally in a northward direction. This levee break caused a canyonward shift of the boundary between the upper and mid fan, and the diverted flow croded smaller valleys in the western part of the fan.

Channel piracy on the lower Bengal Fan

Differences in morphology between the most recently active valley and older valleys suggest that channel piracy occurred on the lower Bengal Fan. Cutoff of two older valleys took place in the late Wisconsinan fan lobe between Sri Lanka and the 85°E Ridge at about 5°N and 83°E.

Demise of the Diamantina Dent

Abstract A new satellite-navigated survey line has shown that the most pronounced indentation in the outline of the western Sunda Trench off Sumatra, frequently the subject of tectonic speculation, was only an artifact of a single pre-satellite navigation survey line. This note is intended as a reminder that tectonic patterns can be no better than the data on which they are based.