Major Submarine Fan Research Articles

GLACIAL AND OCEANIC HISTORY OF THE POLAR NORTH ATLANTIC MARGINS: AN OVERVIEW

The five-year PONAM (Polar North Atlantic Margin: Late Cenozoic Evolution) programme was launched by the European Science Foundation in 1989. Its aim was to study the major climate-driven environmental variations in the Norwegian-Greenland (also Nordic) Sea and its continental margins over the last 5 milliion years. The programme has provided substantial new insights into the contrasting behaviour of the ice sheets covering the Svalbard-Barents Sea and East Greenland over the last glacial-interglacial cycle in particular. The highly dynamic Svalbard - Barents Sea Ice Sheet, after reaching the shelf edge during each stadial, almost vanished during subsequent interstadials. By contrast, the East Greenland Ice Sheet showed only minor advances confined to fjord basins or ending on the inner shelf. Although there is a striking correspondence in the timing and duration of the first post-Eemian ice advance in East Greenland and on Svalbard, their chronology and dynamics have been very different since about 65 ka. The Svalbard-Barents Sea Ice Sheet showed well-defined Middle and Late Weichselian ice advances, whereas the East Greenland Ice Sheet was characterised by a 55 kyr-long period with a relatively stable ice margin located in fjords or the inner shelf. The contrasting behaviour of the two ice sheets is probably linked to the palaeoceanographic circulation pattern in the Polar North Atlantic. East Greenland is under the influence of the cold East Greenland Current, whereas the development and behaviour of ice in the Barents Sea is influenced by the continuous, but highly variable. North Atlantic meridional current system that has resulted in a northward inflow of relatively warm waters of Atlantic origin on the eastern side of the Polar North Atlantic. Of particular interest are the so-called “Nordway events” in glacial stages 6 and 4 to 2. These represented periods of pronounced inflow of temperate waters from the south and an associated increase in seasonally open waters, providing moisture for ice-sheet growth. The largest of these events ended in major glaciations, which were reflected in terrestrial glacial sequences and in deep-sea records of ice-rafted debris. Differences in ice extent and dynamics around the Polar North Atlantic are expressed in the evolution and architecture of its east and west continental margins. The Svalbard-Barents Sea Ice Sheet developed much later than the East Greenland Ice Sheet, in the Late Pliocene as compared with the Middle/Late Miocene. The Svalbard-Barents Sea margin is characterised by major prograding fans, built mainly of stacked debris flows. These fans are interpreted as products of rapid sediment delivery from fast-flowing ice streams reaching the shelf break during full glacial conditions. Such major submarine fans are not found north of the Scoresby Sund Fan off East Greenland, where ice seldom reached the shelf break, sedimentation rates were relatively low and sediment transport appears to have been localised in several major deep-sea submarine channel systems. Few debris flows are present and more uniform, acoustically-stratified sediments predominate. In general, the Greenland Ice Sheet has been more stable than those on the European North Atlantic margin, which reflect greater variability in heat and moisture transfer at timescale varying from 100,000 year glacial cycles to millennial-scale fluctuations.

The glacier-influenced Scoresby Sund Fan, East Greenland continental margin: evidence from GLORIA and 3.5 kHz records

A major submarine fan (gradient about 2 °) offshore of the Scoresby Sund fjord system is indicated by the crescentic shape of the shelf break. GLORIA long-range side-scan sonar imagery was obtained over about 20,000 km 2 of the fan along with 1000 km of 3.5 kHz records. Three acoustic facies were defined from GLORIA backscatter signatures and sea-floor morphology and sediment acoustic character on 3.5 kHz records. Facies 1 includes a series of acoustically transparent features (0.5–2 km in width), elongate downslope, with irregular surface topography, which are interpreted as debris flows. This makes up the bulk of relatively recent sedimentation on the upper fan. Diamictic sediments in a core support a debris-flow origin. Facies 2 is featureless on GLORIA images. 3.5 kHz profiles reveal irregular former sea-floor morphology, above which is a draping unit (< 15 m thick). This northern region of the fan is a less active area where hemipelagic sediments and limited ice-rafted debris overlie older material formed by past debris-flow activity. The more distal area of the adjacent ocean basin has a flat floor with parallel sub-bottom reflectors of Facies 3. This facies is probably an area of low-energy hemipelagic sedimentation, punctuated by occasional ice rafting and turbidity current activity. The debris flows interpreted from GLORIA and 3.5 kHz data are basic building blocks in the long-term development of the Scoresby Sund Fan. Glacier-influenced fan volume is about 15,000 ± 5000 km 3, based on seismic reflection studies. During full glacials in East Greenland, the inland ice sheet advances to fill the Scoresby Sund fjord system and extends across the shelf to reach the shelf break in some glacial cycles. Debris flows form in areas of most rapid sediment flux. The Scoresby Sund Fan is relatively similar to the Storfjorden Fan on the eastern Polar North Atlantic margin, but differs from the larger Bear Island Fan in having a steeper fan gradient, much smaller debris flows and no large-scale slides.

Mesozoic and late tertiary submarine fan sequences and their tectonic significance, Sumba, Indonesia

The Indonesian island of Sumba occupies a critical zone which separates a “normal” subduction complex (Java Trench System) from a continent-island arc collision zone (Banda Arc). Sumba is considered by some authors to be a microcontinental fragment, although in this context its pre-fragmentation origin is uncertain. Detailed studies of the stratigraphy of Sumba, however, throw light on the origin and subsequent geological history of the island and surrounding areas. On available evidence, the oldest outcropping rocks are of Cretaceous age and locally contain a tropical Tethyan marine fauna. They comprise dark-coloured, sometimes carbonaceous and often volcanogenic mudstones, sandstones, gravels and diamictites, pervasively intruded by andesitic and dacitic dykes and locally intruded by granodiorite plutons. These sediments are siliclastic and appear to have had a continental provenance. The Cretaceous rocks in many areas compromise part of a major submarine fan complex which, at least in south-central Sumba, prograded towards the southwest and south. Shallow marine and non-marine Paleogene sediments and volcanic agglomerates lie unconformably above the uplifted, tilted and eroded Cretaceous strata. These are overlain in turn by the widely outcropping Miocene Sumba Formation, an oceanic sequence with island-arc affinities. Accessible outcrops of the Sumba Formation comprise platform carbonates with abundant reef developments in west Sumba, and a major submarine fan—pelagic chalk complex in east Sumba. The fan complex, which extends beneath the southern Savu Sea north of Sumba, is dominated by large-scale slumps, olistostromes and thin- and thick-bedded turbidites. Turbidite sands and gravels invariably are volcaniclastic, containing components derived from a typical inner (volcanic) arc complex. Paleocurrent indicators are difficult to find in the Sumba Formation, thus it is uncertain at this stage whether the volcaniclastic turbidites were derived from the present volcanic arc or from an undetermined source which may since have subsided. The portion of the fan complex which underlies east Sumba, however, has undergone vertical uplift since its emplacement of between one and several kilometres. Uplift is actively in progress at present, possibly due to underthrusting by northward-moving Australian continental crust. In this context the island of Sumba comprises an uplifted, north-tilted and subaerially exposed part of the forearc basin of the Indonesian Arc. This uplift can be related to initial stages of continental collision.

Submarine Fan Sedimentation, Ouachita Mountains, Arkansas and Oklahoma: ABSTRACT

More than 10,000 m (33,000 ft) of interbedded sandstones and shales comprise the Upper Mississippian and Lower Pennsylvanian flysch succession (Stanley, Jackford, Johns Valley, Atoka) in the Ouachita Mountains of Arkansas and Oklahoma. Deposited primarily by turbidity current and hemipelagic processes in bathyal and abyssal water depths, these strata form major submarine fan complexes that prograded in a westerly direction along the axis of an elongate remnant ocean basin that was associated with the collision and suturing of the North American and African plates. A longitudinal fan system is visualized as the depositional framework for these strata which were deposited in a setting analagous to the modern Bengal fan of the Indian Ocean. Facies analysis of the Jackfork Sandstone indicates that inner fan deposits are present in the vicinity of Little Rock, Arkansas; middle fan distributary channel and crevasse splay deposits occur at DeGray Dam, Arkansas; and outer fan depositional lobe deposits are present in southeastern Oklahoma. Basin plain equivalents are postulated to exist as far away as the Marathon region in west Texas. Boulder-bearing units (olistostromes) with exotic clasts were shed laterally into the Ouachita basin, primarily from its northern margin. These olistostromes occur throughout the fan succession in all facies (i.e., inner, middle, and outer fan). This relationship may serve as a useful criterion for recognizing similar fan systems in the rock record. End_of_Article - Last_Page 517------------

Channel Sands and Petroleum Potential of Monterey Deep-Sea Fan, California

The possibility of substantial petroleum resources in deep water adjacent to the continents in areas of hemipelagic sedimentation has been suggested by numerous authors. One such area adjacent to the United States is the continental rise off California between 33 and 40° N lat. The rise covers 200,000 sq km and consists of three major submarine fans, the Arguello, Monterey, and Delgada, with sediment thicknesses up to 3 km in water depths of 3,000 to 4,500 m. The fans are composed primarily of continental debris carried down submarine canyons and deposited on the fan through a system of branching and meandering submarine valleys and channels. During the development of the fan, the active channelway shifts periodically in response to locally increased gradients, produ ing abandoned channels. The coarser sediment naturally confined to the channel becomes covered, after abandonment, by finer grained material because of slumping, reworking, and subsequent fan deposition. Such channels, after sufficient burial, would make excellent stratigraphic traps for hydrocarbons, especially if abandonment occurs on the upper fan where the grain size of channel-fill sediments is coarser and the channel width and relief are greater. On the Monterey fan, potential stratigraphic traps would be seaward of the mouths of the Monterey-Carmel, Ascension, and Pioneer Canyons as shown by subbottom profiling. The proximity of such potential reserves to the United States and to existing refineries and markets in coastal areas makes buried channels on large deep-sea fans such as he Monterey fan particularly attractive prospects.

Petroleum Potential on Continental Rise Off Central California: ABSTRACT

The JOIDES program has demonstrated the technical feasibility of drilling in the deep ocean. However, except for petroleum shows in the Sigsbee Knolls in the Gulf of Mexico, there is no firm evidence of petroleum reservoirs in the deep ocean beyond the limits of the continental slope to require economic use of such technology. Emery suggested the possibility of vast petroleum resources in deep water adjacent to the continents in the areas of hemipelagic sedimentation. One such area adjacent to the United States is the continental rise off California between Point Conception and Cape Mendocino, which covers 200,000 sq km and consists of three major submarine fans, the Arguello, Monterey, and Delgada, at depths of about 3,000 m to 4,500 m. The fans are composed of continental debris carried down submarine canyons and deposited on the fan through a system of branching and meandering submarine End_Page 1461------------------------------ channels. During the depositional growth of the fan, the active channelway shifted periodically, producing abandoned channels. The coarser sediment naturally confined to the channel became covered, after abandonment, by finer grained material. The interchannel deposits, composed chiefly of green hemipelagic muds, are potential petroleum source beds because of their significant carbon content. Thus abandoned channels after sufficient burial and invasion from source beds make excellent stratigraphic traps, especially near the mouth of tributary submarine canyons where the grain size would be larger and the channel width and depth greater. Off central California, potential stratigraphic traps would be seaward of the mouths of the Arguello, Sur-Partington-Lucia, Monterey-Carmel, Ascension, Pioneer, Farallon, Bodega, and Delgada canyons as shown by subbottom profiling. The proximity of such potential reserves to the United States makes buried channels on large-scale deep-sea fans particularly attractive prospects similar to prospects in turbidite basins of the California borderland. End_of_Article - Last_Page 1462------------