David G. Moore

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.

Growth of the Bengal Deep-Sea Fan and Denudation in the Himalayas

A geological and geophysical survey in 1968 has shown that the Bengal Deep-Sea Fan is almost 3000 km long, and 1000 km wide. We estimate that it may exceed 12 km in thickness. The sediments of the fan have been transported by turbidity currents from the Ganges-Brahmaputra River delta, through the “Swatch of No Ground” submarine canyon and into an extensive, complex, meandering, and braided net of fan valleys. Present rate of sediment influx suggests a regional rate of denudation in the Himalayan source area of over 70 cm/10 3 years. The sediment section in reflection profiles of the fan has been subdivided into three units separated by prominent unconformities. Volumes of the upper two units compared with the sediment influx rate extrapolated into the past suggest that the unconformities may be late Miocene and earliest Pleistocene. These times correspond to periods of orogeny in the Himalayas and suggest contemporaneity between plate-edge orogeny and mid-plate tectonic activity.

Sedimentary and Tectonic Processes in the Bengal Deep-Sea Fan and Geosyncline

Extensive geophysical studies have shown that the modern Bengal Deep-Sea Fan is the uppermost 4 km of the geosynclinal pile of sediments filling the Bay of Bengal, northeast Indian Ocean. The fan postdates the first collision of India and Asia and uplift of the ancestral Himalayas at the end of the Paleocene. Underlying the fan are continental rise sediments up to 12 km thick, which extend into the Bengal and Assam valleys, deposited off the margin of India following its separation from Antarctica and Australia in the Cretaceous. Deposits of the modern fan are structurally complex, particularly in the proximal part, where overlapping and interleaving natural levees and channel deposits make up the bulk of the section. Modern and buried channels in the proximal fan are tens of kilometers wide and hundreds of meters deep and are bounded by extensive natural levees, which terminate abruptly about 450 km downslope from the canyon mouth. Channel size and levee development are significantly less beyond this point. Turbidity currents which top the large channels of the proximal fan spread largely as sheet flow. Thus, in contrast to the proximal fan, most of the central and distal fan is sheet-flow-derived and shows lateral continuity in section, with isolated channels and channel deposits. Deformation is occurring simultaneously with deposition, as the fan and geosyncline pass obliquely northeast into the subduction zone of the Sunda Arc and Indoburman Ranges.

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.

Plate-Edge Deformation and Crustal Growth, Gulf of California Structural Province

Fracture zones in the Gulf of California are charted from data of reflection profiling and bathymetry. Positions of active spreading centers are located through magnetic anomalies at the mouth, positions of deep troughs in the central gulf, and locations of oceanic ridge-type earthquake swarms in the northern gulf and Salton Trough. Assuming the 6-cm-per-yr spreading rate determined by earlier magnetic anomaly studies has been constant and that spreading has been symmetrical and has not involved transpeninsular faulting, areas of new and old crust within the province are delineated. With the same assumptions, translation of Pacific Plate terrain 240 km back along the 306° azimuth of fracture zones shows the initial Pacific-North American Plate boundary and results in geography clearly requiring a protogulf prior to the current episode of plate separation. Interpretations of reflection profiles and faunal studies of dredged rocks imply that sediments of the central protogulf were deposited in depths significantly greater than 1,000 m and that, in early stages of rifting, they may have been uplifted and subsided as much as 1,200 m. Poorly developed, semi-coherent stratification of postrifting new crustal area is contrasted to well-developed strata of the protogulf areas. This contrast, plus the general absence of ponded turbidites and recording of normal growth-faults at a trailing plate-edge trough, lead to the hypothesis of “clastic compensation.” As continental, or intermediate, crusts are pulled apart by plate motions in the presence of high or moderate clastic sediment sources, crustal growth and resulting structure are controlled by the balance between separation and the flow of sediment into the gaps. Partial isostatic compensation is achieved by supraplate injection of elastics and subplate rising of mantle-derived basalts. Very high supplies of elastics, as in the northern Gulf of California province fed by the Colorado River, preclude the formation of oceanic-type crust as a result of plate separation and lead instead to the formation of intermediate crust, typical neither of oceanic nor continental realms.

Late Cenozoic subduction and continental margin truncation along the northern Middle America Trench

The narrow inner trench slope and the truncated igneous and metamorphic terrane along the west coast of Mexico between Cabo Corrientes and the Gulf of Tehuantepec indicate that part of the continental margin has in some way been removed during the process of subduction. However, a detailed marine geophysical survey of the inner trench slope near Acapulco indicates that this removal is not occurring now. South-southwest–trending magnetic anomalies produced by the Xolapa metamorphic complex extend seaward only 20 to 30 km. Oceanic magnetic anomalies that trend N50°W extend as much as 30 km landward of the trench. The boundary between these two magnetic patterns lies landward of the trench-slope break and beneath the upper-slope sediment pile. The nonmagnetic material forming the acoustic basement trenchward of the metamorphic rocks is interpreted to consist of late Miocene to Holocene deformed trench-floor turbidites. Deformation associated with subduction has reversed the gradients of several submarine canyons and tilted the seaward edge of the upper-slope sediment pile away from the trench. The morphology and structure of the inner trench slope is typical of accreting trench-arc systems, although the morphotectonic units in this system are smaller than usual. Accretion since late Miocene time is suggested by the age of dredged slope sediments and by analysis of offshore magnetic anomalies, which indicate a change from right-lateral oblique to perpendicular subduction at that time. Removal of the continental margin probably occurred intermittently between Late Cretaceous and late Miocene time. Possible mechanisms include subduction of continental crust (tectonic erosion), left-lateral translation associated with the Caribbean–North American plate boundary, and right-lateral translation associated with oblique subduction between the Farallon or Cocos plates and the North American plate. Geological data favor right-lateral offset and suggest that some of the missing margin may be the slivers of subduction complex found along the west coast of Baja California and possibly even farther north.

Minor Internal Structures of Some Recent Unconsolidated Sediments

Widespread occurrence of several different minor sedimentary structures in shallow-water Gulf of Mexico sediments has been shown by more than 2,000 cores and bottom samples. Minor internal structures in two areas, one off the east side of the Mississippi River Delta and the other along the central Texas coast, were found to be similar. Deposits in these two areas were subdivided into those containing: (1) regular layers (thin beds or laminations), (2) irregular layers (rough or crude layers and lenses), (3) mottles (discontinuous lumps, tubes, and pockets), and (4) structureless homogeneous sediments, depending on the principal type that occurs. Areal distributions of these different structures in the surficial sediments are mappable. Similar sedimentary structures and sequences of structures occur in the open gulf and in the bays and sounds. In the open gulf the structures occur in wide bands, and the sequences are developed through wide ranges of water depths; in the bays and sounds the bands generally are narrower, and the sequences are compressed into much smaller ranges of water depths. The different types of structures are formed on or near the depositional surface, contemporaneous or nearly contemporaneous with deposition. The differences between depositional environments which produce the various structures are differences in: (1) sediment sources, (2) in physical processes and their intensities, and (3) rates of deposition. Regular layers are characteristic of areas of rapid deposition and/or relatively few bottom-living animals; they are either primary--formed by fluctuations of sediment, or secondary--formed by wave or current winnowing. Irregular layers and mottles are mostly secondary features, formed mainly by bottom-living animals altering existing sediments. Homogeneous deposits are either primary or secondary; they form by extremely rapid deposition, unif rm deposition, or by complete secondary reworking. The appearance of many normal marine sediments is due in large measure to burrowing and crawling organisms. Structures similar to those in presently forming sediments of the Gulf of Mexico apparently were developed in rocks of all ages when environmental conditions were proper. Knowledge of how these structures form, why they vary, and the relationships between them furnishes a valuable key for interpreting deposition of ancient rocks.

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.

Recent bottom-current activity in the deep western Bay of Bengal☆

Abstract We present several types of data which show that strong geostrophic bottom currents are present in a broad valley in the deep western Bay of Bengal adjacent to the Indian margin. Sea-floor photographs show well-developed current lineations with scour marks on the northern sides and sediment deposition tails on the southern sides of some objects (such as fecal pellets) suggesting strong southward-flowing bottom currents. A direct current measurement made in the region confirms this inferred flow direction. The nepheloid layer is much stronger in the western Bay of Bengal than in any other region of the northern Indian Ocean and indicates strong turbulence and a high concentration of suspended sediment at or near the sea floor. Additional data which do not provide unequivocal evidence for, but may also be indicative of, the existence of the bottom currents are as follows: the dispersal of the peninsular Indian rivers-derived smectite-rich sediments all along the valley to as far as south of Sri Lanka; a zone of sediment waves (as recorded on 3.5-kHz echograms) parallel to the regional trend of bathymetric contours along the Indian margin; and the frequent occurrence of thin, sharp and uniform layers of fine sand and silt beds rather than thick graded turbidite beds in the cores from the broad valley in the deep western Bay of Bengal.

Laminated Diatomaceous Sediments from the Guaymas Basin Slope (Central Gulf of California): 250,000-Year Climate Record

During Deep Sea Drilling Project-International Program of Ocean Drilling leg 64, December 1978 to January 1979, the initial test of the Deep Sea Drilling Projects hydraulic piston corer obtained an almost undisturbed section from a 152-meter hole into the sediments of the oxygen minimum zone at a depth of 655 meters along the Guaymas slope in the central Gulf of California. The section records variations in climate, productivity, and circulation for more than 250,000 years of Late Pleistocene to Holocene history with recordings of seasonal variations in these parameters in the laminated sections.