M. De Batist

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.

Seafloor evidence of a subglacial sedimentary system off the northern Antarctic Peninsula

Swath-bathymetry data and high-resolution seismic reflection profiles allow us to portray a subglacial sedimentary system off the northern tip of the Antarctic Peninsula, in the Central Bransfield Basin, during the Last Glacial Maximum with unprecedented detail. Postglacial reworking and sedimentation are weak enough for the subglacial morphology of the Last Glacial Maximum to be preserved on the present seafloor. The studied sedimentary system extends 250 km, from ∼1000 m above sea level to ∼2000 m water depth. The data set supports a model for subglacial sedimentary systems that consists of: (1) an upper ice catchment or erosional zone on the innermost continental shelf, extending onshore; (2) a transitional erosional-depositional zone on the inner shelf with drumlinized seafloor; (3) a depositional outer shelf zone with mega-scale bundle glacial lineations; and (4) a debris apron on the continental slope and base of slope formed under floating ice shelves with debris delivery linked to grounding lines along the shelf break.

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.

Seafloor imagery from the BIG'95 debris flow, western Mediterranean

Seafloor backscatter data are used to image the product of one of the youngest major mass-wasting events in the northwestern Mediterranean Sea: a 26 km3 debris-flow deposit that covers 2000 km2 of the Ebro continental slope and base of slope, offshore Spain. Backscatter images provide unprecedented insights on debris-flow dynamics in the deep sea. A pattern of low-backscatter patches represents large sediment blocks that moved while keeping their internal coherence. High-backscatter alignments restricted to topographic lows that represent coarse sediment pathways separate the blocks. The results presented prove the occurrence of large catastrophic sediment failures near heavily populated coastal areas even in continental margins considered to be geodynamically quiet, such as those of the northwestern Mediterranean.

The Gebra Slide: a submarine slide on the Trinity Peninsula Margin, Antarctica

A large submarine slide – the Gebra Slide – has been discovered on the continental margin of Trinity Peninsula, Central Bransfield Basin, Antarctic Peninsula. The slide scar is clearly expressed in the bathymetry and is cut into the toe of the glacial-period slope-prograding strata on the lower continental slope. Seismic data give evidence of an associated debris-flow deposit embedded in the interglacial-period basin-fill strata of the basin floor. The total volume of sediment involved in the mass movement is about 20 km3. Indirect dating of the mass-wasting event, based on seismic–stratigraphic relationships of the slide scar and associated debris-flow deposit with underlying glacial-period slope units and the overlying interglacial-period basin-floor units, suggests that it took place at the transition between the last glacial period and the present-day interglacial. The initiation of the Gebra Slide is attributed to a combination of several factors, such as high sedimentation rates during the last glacial period, the unloading effect of a retreating ice sheet during deglaciation, pre-existing tectonic fabric and high seismicity in the area. This is the first recent submarine slide of this size identified on the glacial, continental margins of Antarctica. In morphology and general characteristics it is quite similar to the well-known large-scale submarine slides from the northern hemisphere glacial margins, although it is smaller. Its most striking characteristic is its lower-slope position (at 1500–2000 m of water depth), which remains up to now difficult to explain.

Architecture and long term evolution of a tidal sandbank: The Middelkerke Bank (southern North Sea)

Abstract The internal structure of the Middelkerke Bank (one of the Flemish Banks located in the southern North Sea off the coast of Oostende, Belgium) has been studied in the framework of the Marine Science and Technology (MAST) program co-funded by the European Community. A dense grid of high and very high resolution seismic profiles has been used, as well as several vibrocorings. Seven major seismic units can be identified in the Quaternary sediments, bounded by major discontinuities correlated across the whole study area. The lower units clearly appear as being deposited during periods of relative low sea level (channel infillings, shoreface, estuarine and/or ebb-tidal delta deposits). The present shape of the bank results partly from recent erosional processes, reworking the underlying deposits. Thus, the lower part of the bank as a morphological feature does not consist of “offshore tidal sands”. The master bedding of the upper part of the bank consists of inclined reflectors, dipping at an angle of about 5° in the same direction as the banks “steep” face. These reflectors, very similar to those described by Houbolt (1968), are interpreted as being the result of alternating periods of deposition and erosion related to the episodic combination of tidal currents and storms.

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.

The 600 yr eruptive history of Villarrica Volcano (Chile) revealed by annually laminated lake sediments

Lake sediments contain valuable information about past volcanic and seismic events that have affected the lake catchment, and they provide unique records of the recurrence interval and magnitude of such events. This study uses a multilake and multiproxy analytical approach to obtain reliable and high-resolution records of past natural catastrophes from ~600-yr-old annually laminated (varved) lake sediment sequences extracted from two lakes, Villarrica and Calafquen, in the volcanically and seismically active Chilean Lake District. Using a combination of micro–X-ray fl uorescence (µXRF) scanning, microfacies analysis, grain-size analysis, color analysis, and magnetic-susceptibility measurements, we detect and characterize four different types of event deposits (lacustrine turbidites, tephra-fall layers , runoff cryptotephras, and lahar deposits) and produce a revised eruption record for Villarrica Volcano, which is unprecedented in its continuity and temporal resolution. Glass geochemistry and mineralogy also reveal deposits of eruptions from the more remote Carran–Los Venados volcanic complex, Quetrupillan Volcano, and the Huanquihue Group in the studied lake sediments. Time-series analysis shows 112 eruptions with a volcanic explosivity index (VEI) ≥2 from Villarrica Volcano in the last ~600 yr, of which at least 22 also produced lahars. This signifi cantly expands our knowledge of the eruptive frequency of the volcano in this time window, compared to the previously known eruptive history from historical records. The last VEI ≥2 eruption of Villarrica Volcano occurred in 1991. Based on the last ~500 yr, for which we have a complete record from both lakes, we estimate the probability of the occurrence of future eruptions from Villarrica Volcano and statistically demonstrate that the probability of a 22 yr repose period (anno 2013) without VEI ≥2 eruptions is ≤1.7%. This new perspective on the recurrence interval of eruptions and historical lahar activity will help improve volcanic hazard assessments for this rapidly expanding tourist region, and it highlights how lake records can be used to signifi cantly improve historical eruption records in areas that were previously uninhabited.

The Celtic Sea banks: an example of sand body analysis from very high- resolution seismic data

Very high-resolution seismic data from the Kaiser-I-Hind sand bank (southern Celtic Sea) recently highlighted the internal structure of the enigmatic Celtic Banks, which are among the deepest and largest shelf sand ridges. The main body of the bank is made up of 4 seismic/depositional units which reflect a transgressive evolution. New data on the detailed architecture of two of these units allow discussion of bank growth in terms of either (1) a channel–levee system preserved both by lateral migration and aggradation of the channels, or (2) a package of large offshore tidal sediment bodies (bar chains and/or giant dunes). Careful geometrical observations of seismic discontinuities make the second hypothesis more likely. The unit architecture is analysed in terms of long- to short-term processes of build-up. Long-term processes are evinced by the landward stacking of erosive sub-units in response of the last post-glacial sea-level rise, whereas short-term processes control the seaward progradation of sand bodies and fills due to the ebb predominance of the Western Channel Approaches.

Evidence for high-frequency cyclic fault activity from high-resolution seismic reflection survey, Rukwa Rift, Tanzania

A high-resolution seismic survey of the Rukwa Rift has imaged the upper 300 m of the Pliocene–Holocene section. The sediments are characterized by aggradational deposition in shallow lacustrine conditions that episodically dried out. Overall the reflection package expands northeastwards towards the Lupa Fault (half-graben bounding fault). Isopach maps for nine intervals (6 m to 65 m thick) show four periods of marked expansion of the section towards the Lupa Fault, interpreted to represent times of fault activity or infilling of fault-created topography during the early stage of fault quiescence. There are three periods of little or much reduced expansion interpreted to represent infilling of the accommodation space between the lake floor and the lake surface during periods of fault inactivity. Cyclicity of such high frequency has not been widely reported for faults which penetrate the crust. The periods of activity show remarkably similar displacement patterns with no lateral migration of the main depocentre, implying that seismic and aseismic slip has persistently clustered in the same parts of the fault. Identification of high-frequency fault cyclicity has implications for sequence stratigraphy in tectonically active areas, since there is a tendency to assume faults do not have cyclic effects on the scale of tens of thousands of years.