P. Van Rensbergen

Large deep-water coral banks in the Porcupine Basin, southwest of Ireland.

The Porcupine Basin, southwest of Ireland, was one of the earliest sites from where the deep-water corals Lophelia sp. and Madrepora sp. were recovered. These deep-water corals have since been found all along the Atlantic margins of Europe, in water depths ranging from 50 to more than 2000 m. Recent geophysical studies have demonstrated the mound-building potential of deep-water corals. Available data indicate that three major provinces of coral bank occurrences can be identified in the Porcupine Basin: (1) high-relief surface mounds which have a dimension of 1 by 5 km and a height up to 200 m (‘Hovland’ mounds), flanked to the north by (2) a swarm of buried mounds, somewhat smaller (up to 90 m), and with more irregular shapes than those recognised in area 1 (‘Magellan’ mounds), and (3) outcropping or buried, conical mounds (single or in elongated clusters, up to 150 m high) occurring on the southeastern slope of the basin (‘Belgica’ mounds). As far as can be inferred from shallow cores, the surface lithology predominantly consists of an upper layer rich in foraminiferal sand and terrigenous silty clay with intercalations of biogenic rubble. The banks host a remarkable number of colonies of living and dead Lophelia pertusa and Madrepora oculata. The living and dead assemblages are underlain by a significant layer of coral debris in a muddy matrix. Deep-water coral debris together with a living association of the same species covers the surface of the ‘Belgica’ and ‘Hovland’ mounds, which may suggest that these corals have played a significant role in the development of the mound structures. The capacity for mound formation by scleractinian corals in the aphotic zone has been known for some time. Examples are found at different locations along the shelves and the continental margins of the North Atlantic. The role of the corals in these deep-water build-ups is still a point of debate. Though the genesis and initial control of mound settings in this basin might be related to hydrocarbon seeps, it appears that the major development of the Porcupine coral banks in recent geological times has most likely been controlled by oceanic circulation and dynamics in water masses and nutrient supply.

Sublacustrine mud volcanoes and methane seeps caused by dissociation of gas hydrates in Lake Baikal

Four lake-floor seeps have been studied in the gas-hydrate area in Lake Baikals South Basin by using side-scan sonar, detailed bathymetry, measurements of near-bottom water properties, heat-flow measurements, and selected seismic profiles in relation to results from geochemical pore-water analysis. The seeps at the lake floor are identified as methane seeps and occur in an area of high heat flow, where the base of the gas-hydrate layer shallows rapidly toward the vent sites from ;400 m to ;150 m below the lake floor. At the site of the seep, a vertical fluid conduit disrupts the sedimentary stratification from the base of the hydrate layer to the lake floor. The seeps are interpreted to result from local destabilization of gas-hydrate caused by a pulse of hydrothermal fluid flow along an active fault segment. This is the first time that methane seeps and/or mud volcanoes as- sociated with gas-hydrate destabilization have been observed in a sublacustrine setting. The finding demonstrates the potential of tectonically controlled gas-hydrate destabiliza- tion to cause extreme pore-fluid overpressure and short-lived mud volcanism.

High-resolution seismic stratigraphy of glacial to interglacial fill of a deep glacigenic lake: Lake Le Bourget, Northwestern Alps, France

Abstract Lake Le Bourget is a deep, peri-alpine glacial lake originating from the last glacial, the Wurm. The lake has amassed over 250 m of sediment since the Last Glacial Maximum. Two seismic sparker surveys on the lake revealed the glacial and post-glacial sedimentary infill, the depth and the morphology of the Wurm erosion surface and penetrated the Riss–Wurm interglacial deposits as deep as the underlying Riss erosion surface. The density of the seismic grid and the high resolution of the data, even at greater depths, allowed a detailed study of the sedimentary processes and of their evolution throughout the deglaciation. Five main seismic facies characterise the basin fill. They are interpreted as glacial deposits, glacio-lacustrine sediments, proglacial lacustrine fans, alluvial fan deltas and authigenic lacustrine drape. These occur as a suite of deposits associated with the major lake tributaries and are the result of major changes in sediment supply and style of discharge into the lake. The three-dimensional facies associations within the basin fill document the history of the basin and its catchment area since the onset of the deglaciation.

Seismic sequence analysis and reservoir potential of drowned Miocene carbonate platforms in the Madura Strait, East Java, Indonesia

Seismic analysis of four Miocene carbonate buildups (the Porong, KE-11C, KD-11E, and BD buildups) in the Madura Strait region, Indonesia, shows that they are constructed of four seismic units (1-4). All the buildups are located atop an east-west-trending Oligocene fault block, which, through its uplift, favored the deposition and aggradation of shoal-water carbonates. Aggradation, platform narrowing, and backstepping of leeward reefs into the upper parts of individual buildups typify the growth profiles. Adjacent off-buildup sedimentary rocks, deposited at the same time as the buildups, show clear horizontal onlap relationships. In combination, these observations imply that all the platforms were drowned. The uppermost parts (unit 4) of each of four major drowned buildups in the trend have been drilled to test for the presence of hydrocarbons. An analysis of the drilling results, in combination with our seismic analysis, shows that those buildups that have relatively high levels of hydrocarbons have suitable trap configurations but lack seal integrity. For example, compactionally induced strain and gravitational gliding created leaky collapse-graben faults (extensional structures) atop some buildups (Porong, KE-11C), which allowed hydrocarbons to drain. Likewise, the updip inclination of some potential seal beds facilitated leakage (KE-11E). In terms of potential targets in similar drowned buildups in the region, higher priority should be given to targets that (1) show evidence of ongoing platform backstepping and so-called give-up reef growth profiles associated with incipient drowning, (2) contain all four seismic units (units 1-4) in the buildup, (3) lack an extensional collapse graben over the crest of the drowned structure, and (4) demonstrate bed inclinations in the sealing beds that indicate likely closure over the crest of the buildup. (Begin page 214)

High-resolution seismic stratigraphy of late quaternary fill of Lake Annecy (northwestern Alps): evolution from glacial to interglacial sedimentary processes

Abstract Lake Annecy is a peri-alpine lake of glacial origin, remnant of the last glaciation. The lake basin accumulated over 150 m of sediment since the Last Glacial Maximum. A dense grid of high-resolution seismic data, acquired during a 1993 sparker survey, revealed the glacial and post-glacial sedimentary infill, the depth and morphology of the Wurm erosion surface and penetrated the Riss-Wurm interglacial deposits as deep as the underlying Riss erosion surface. This paper focuses on the interpretation of the high-resolution seismic facies of the infilling sediments of the lake basin in correlation with the results of the Annecy core. The quality and density of the seismic data allow, in this case, detailed analysis of the seismic facies in their three-dimensional organisation and permit reliable interpretation in terms of depositional process and palaeo-environmental setting. The study reveals that the geometry and seismic facies of the infilling deposits are controlled essentially by the basin morphology and the characteristics of the sediment supply. The sediment supply to the lake changes throughout the deglaciation from subglacial drainage, to proglacial meltwater streams, to alluvial streams and finally to the present-day interglacial condition where authigenic sedimentation prevails. The depositional patterns change accordingly from chaotic outwash deposits and glacio-lacustrine muds at the base, over axially aggrading lacustrine fans, prograding alluvial fan deltas at the lake margins to, finally, a drape of lacustrine chalk and marl. The interpreted sedimentary environment and deduced character of the sediment source are used to reconstruct the changing palaeo-environment in the catchment area.

TECTONICALLY CONTROLLED METHANE ESCAPE IN LAKE BAIKAL

Methane, which is at least partly stored in the bottom sediments of Lake Baikal as gas hydrates, is released on the lake floor in the deeper parts of the basin along major faults, forming venting structures similar to small mud volcanoes. The CH4 venting structures are considered to be the surface expression of escape pathways for excess CH4 generated by the dissociation of pre-existing hydrates. The existence of a local heat flow anomaly associated with the seep area is most likely due to a heat pulse causing the dissociation of the underlying gas hydrates. The heat pulse may be caused by upward flow of geothermal fluids along segments of active faults, possibly accelerated by seismic pumping. It is assumed that this fluid flow is tectonically triggered, considering that left-lateral strike-slip movements along the border faults act as a major factor in fluid accumulation: even a reduced lateral displacement is able to generate fluid flow in the compressional direction, resulting in fluid escape along faults directed along the main direction of extension. The tectonic effect may be coupled to the sediment compaction due to a high sedimentation rate in the area of mud volcanism. Both processes may generate a large-scale convective fluid loop within the basin-fill sediments which advects deeper gases and fluids to the shallow sub-surface. Even in the extensional tectonic environment of Lake Baikal, local compressional forces related to a strike-slip component, may play a role in fluid flow, accumulation and gas escape along active faults. The mechanisms that result in the expulsion of the CH4 in the Lake Baikal sediments are considered as an analogue of what could happen during CO2 sequestration in a similar tectonic environment.