ALBERT

Description: :  Backstepping cross-strata on steep foresets of a Gilbert-type proglacial fluviodeltaic system are ascribed to cyclic steps and other associated supercritical bedforms. They provide insight into how sandur river flows transition into the marine realm. These sedimentary structures are located on steep foresets (up to 17°) with corresponding top-lying, flat-based topsets in an upper Pleistocene delta on the North Shore of the Gulf of St. Lawrence, Québec, Canada. Packages of backstepping cross strata of sand and gravel, lying in the lower part of the delta front outcrop, are organized in 10–20 m spaced pseudo-foresets with a mean slope of 11–12° seawards. Backstepping strata include frequent internal erosion surfaces that onlap upslope on pseudo-foresets and are interpreted as cyclic steps. Narrow, deep, and asymmetrical scours and upslope-climbing cross beds are interpreted as chutes-and-pools and antidunes respectively. Very shallow (〈 15 m) depositional paleo-bathymetry is inferred from the preservation of the delta brink. The well-organized stratal pattern in cyclic step to antidune deposits indicates relatively steady and uniform flow patterns. There is insufficient distance for a headscarp large enough to transform to the volume of observed accreted sands or for a flow transformation from a gravitational collapse to net-depositional cyclic steps. These deposits are sandier than the topsets beds and are thus not derived from them, but rather correspond to topset erosional surfaces. The development of cyclic steps from hyperpycnal flows was likely enhanced by tidal drawdown processes. The resulting sediment-laden supercritical flows plunged inertially and evolved into an underflow that generated the cyclic steps on the upper foresets. The cyclic steps have a high aspect ratio and represent an end member of coarse-grained sediment deposited on steep slopes, in contrast to low-gradient, low-aspect-ratio muddy deposits.

Description: 〈span〉The western North Atlantic passive margin is considered relatively stable, with few slope instabilities recognized during the Holocene. However, new multibeam bathymetry mapping and sediment core acquisition off eastern Canada indicate that previously unidentified, large, submarine landslide events occurred during the Late Holocene, between 4 and 1.5 ka. The recognition of these new gravitational events, in addition to the well-known C.E. 1929 Grand Banks earthquake-induced landslide, indicates that approximately one large landslide event per 1000 years has occurred offshore eastern Canada within the past 4000 years, a much shorter recurrence interval than hitherto reported. This Late Holocene recurrence rate is also similar to active margins around the world and is likely due to the under-consolidation and resultant instability of Scotian Slope sediments attributable to high glacial sedimentation rates. The discovery of these new Late Holocene landslides was made possible through detailed examination of cores recovered from the lower slope. These results demonstrate that submarine landslide hazard has been underestimated on the western North Atlantic margin—home to significant submarine infrastructure and proximal to a large coastal population.〈/span〉

Description: 〈p〉The Laurentian Fan is one of the largest submarine fans on the western margin of the North Atlantic. Recently acquired high-resolution multibeam bathymetric data (60 m horizontal resolution) reveal a major mass-transport deposit (MTD) on the Western Levee of Western Valley (WLWV), covering 〉14 000 km〈sup〉2〈/sup〉 in water depths from 3900 to 〉5000 m. Typical submarine landslide features are observed such as headscarps that in places reach the crest of the levee, crown cracks, extensional ridges, blocky debris and flow lineations. Multiple headwalls are observed on 3.5 kHz sub-bottom profiles, indicating that the landslide retrogressed upslope. While the upper parts of the MTD consist of intact blocks that were displaced downslope as ridges and troughs, the lower parts exhibit a 〈i〉c.〈/i〉 30 m thick incoherent to transparent acoustic facies, typical of debris flows. Landslide geomorphology therefore suggests that it was generated as a retrogressive spread and that slide blocks disintegrated downslope to become a blocky landslide with a surficial debris flow. The blocky landslide/debris flow extends downslope 〈i〉c.〈/i〉 90 km and partially fills a submarine channel. The superposition of the MTD filling the channel and its location at the top of the stratigraphic succession in the levee suggests that it is late Quaternary in age, possibly Holocene. Deeper seismic reflection data also show that this is a rare event during the Quaternary; it is the largest MTD observed in the upper 〈i〉c.〈/i〉 375 m of the levee succession and among the largest and deepest in the western North Atlantic.〈/p〉

Description: 〈p〉Deglacial sedimentary sequences recording the decay and final demise of ice sheets result from intricate interactions between the pattern of ice margin retreat, inherited basin physiography and relative sea-level (RSL) changes. A specific emphasis is here given to the glacio-isostatic adjustment (GIA), which may force postglacial local RSL fall in spite of concomitant glacio-eustatic rise. In this contribution, we characterize a Quaternary deglacial succession emplaced in such a setting, subsequently used as an analogue to interpret an end-Ordovician deglacial record. The Quaternary deglacial succession, tens of metres thick, formed under condition of RSL fall forced by the GIA in 〈i〉c.〈/i〉 10 000 years in the aftermath of the deglaciation. This sedimentary succession consists of a lower, fining-upward sequence representing the backstepping of ice-contact depocentres following the retreat of the ice margin, and an upper, coarsening-upward sequence that relates to the subsequent progradation of a glaciofluvial delta system. A very similar stratigraphic stacking pattern characterizes the Ordovician analogue, suggesting a comparable deglacial sequence. By analogy with the Quaternary succession, this ancient deglacial record would have hence been emplaced under conditions of RSL fall forced by the GIA. Moreover, it must only represent a very short time interval that could be viewed as virtually instantaneous regarding the Late Ordovician glaciation. Such a vision is at odds with commonly accepted interpretations for such successions.〈/p〉

Description: 〈p〉Landslides are common in aquatic settings worldwide, from lakes and coastal environments to the deep sea. Fast-moving, large-volume landslides can potentially trigger destructive tsunamis. Landslides damage and disrupt global communication links and other critical marine infrastructure. Landslide deposits act as foci for localized, but important, deep-seafloor biological communities. Under burial, landslide deposits play an important role in a successful petroleum system. While the broad importance of understanding subaqueous landslide processes is evident, a number of important scientific questions have yet to receive the needed attention. Collecting quantitative data is a critical step to addressing questions surrounding subaqueous landslides.〈/p〉 〈p〉Quantitative metrics of subaqueous landslides are routinely recorded, but which ones, and how they are defined, depends on the end-user focus. Differences in focus can inhibit communication of knowledge between communities, and complicate comparative analysis. This study outlines an approach specifically for consistent measurement of subaqueous landslide morphometrics to be used in the design of a broader, global open-source, peer-curated database. Examples from different settings illustrate how the approach can be applied, as well as the difficulties encountered when analysing different landslides and data types. Standardizing data collection for subaqueous landslides should result in more accurate geohazard predictions and resource estimation.〈/p〉

Description: Deglacial sedimentary sequences recording the decay and final demise of ice sheets result from intricate interactions between the pattern of ice margin retreat, inherited basin physiography and relative sea-level (RSL) changes. A specific emphasis is here given to the glacio-isostatic adjustment (GIA), which may force postglacial local RSL fall in spite of concomitant glacio-eustatic rise. In this contribution, we characterize a Quaternary deglacial succession emplaced in such a setting, subsequently used as an analogue to interpret an end-Ordovician deglacial record. The Quaternary deglacial succession, tens of metres thick, formed under condition of RSL fall forced by the GIA in c. 10 000 years in the aftermath of the deglaciation. This sedimentary succession consists of a lower, fining-upward sequence representing the backstepping of ice-contact depocentres following the retreat of the ice margin, and an upper, coarsening-upward sequence that relates to the subsequent progradation of a glaciofluvial delta system. A very similar stratigraphic stacking pattern characterizes the Ordovician analogue, suggesting a comparable deglacial sequence. By analogy with the Quaternary succession, this ancient deglacial record would have hence been emplaced under conditions of RSL fall forced by the GIA. Moreover, it must only represent a very short time interval that could be viewed as virtually instantaneous regarding the Late Ordovician glaciation. Such a vision is at odds with commonly accepted interpretations for such successions.

Description: The nature of glaciomarine sediments deposited during ice margin retreat can vary according to physiographic setting and relative sea level fluctuations. To understand the effects of these two parameters on sedimentation, we analyzed the sediment records of four lakes located within former isolated glaciomarine embayments of the northern Champlain Sea basin. These lakes were initially inundated by marine water of the Champlain Sea, following deglaciation, and have subsequently experienced basin isolation owing to glacio-isostatic rebound. Three of these lakes reveal a common litho- and acoustic stratigraphic succession, characterized by an IRD-free glaciomarine to marine facies consisting of homogeneous to faintly laminated clayey silts grading into well-laminated silts with rapidly deposited layers. These two units recorded the transitional environment from glaciomarine sedimentation below multiyear shorefast ice to increased terrestrial runoff and rapid glacio-isostatic rebound once the ice margin retreated inland. During ice margin retreat, relative sea level fell concomitantly resulting in the deposition of coarser sediments in marine embayments. Upon the complete retreat of the ice margin, the supply of terrestrial sediments diminished and lake isolation, driven by relative sea level fall, led to higher biogenic content and increased bioturbation. This study provides a framework for sedimentation in isolated glaciomarine embayments which differs from deep-water sedimentation owing to the presence of shorefast sea-ice and their protected location from major ice-stream outlets.

Description: Landslides are common in aquatic settings worldwide, from lakes and coastal environments to the deep sea. Fast-moving, large-volume landslides can potentially trigger destructive tsunamis. Landslides damage and disrupt global communication links and other critical marine infrastructure. Landslide deposits act as foci for localized, but important, deep-seafloor biological communities. Under burial, landslide deposits play an important role in a successful petroleum system. While the broad importance of understanding subaqueous landslide processes is evident, a number of important scientific questions have yet to receive the needed attention. Collecting quantitative data is a critical step to addressing questions surrounding subaqueous landslides. Quantitative metrics of subaqueous landslides are routinely recorded, but which ones, and how they are defined, depends on the end-user focus. Differences in focus can inhibit communication of knowledge between communities, and complicate comparative analysis. This study outlines an approach specifically for consistent measurement of subaqueous landslide morphometrics to be used in the design of a broader, global open-source, peer-curated database. Examples from different settings illustrate how the approach can be applied, as well as the difficulties encountered when analysing different landslides and data types. Standardizing data collection for subaqueous landslides should result in more accurate geohazard predictions and resource estimation.

Description: The Laurentian Fan is one of the largest submarine fans on the western margin of the North Atlantic. Recently acquired high-resolution multibeam bathymetric data (60 m horizontal resolution) reveal a major mass-transport deposit (MTD) on the Western Levee of Western Valley (WLWV), covering 〉14 000 km 2 in water depths from 3900 to 〉5000 m. Typical submarine landslide features are observed such as headscarps that in places reach the crest of the levee, crown cracks, extensional ridges, blocky debris and flow lineations. Multiple headwalls are observed on 3.5 kHz sub-bottom profiles, indicating that the landslide retrogressed upslope. While the upper parts of the MTD consist of intact blocks that were displaced downslope as ridges and troughs, the lower parts exhibit a c. 30 m thick incoherent to transparent acoustic facies, typical of debris flows. Landslide geomorphology therefore suggests that it was generated as a retrogressive spread and that slide blocks disintegrated downslope to become a blocky landslide with a surficial debris flow. The blocky landslide/debris flow extends downslope c. 90 km and partially fills a submarine channel. The superposition of the MTD filling the channel and its location at the top of the stratigraphic succession in the levee suggests that it is late Quaternary in age, possibly Holocene. Deeper seismic reflection data also show that this is a rare event during the Quaternary; it is the largest MTD observed in the upper c. 375 m of the levee succession and among the largest and deepest in the western North Atlantic.

Description: 〈span〉〈div〉Abstract〈/div〉The western North Atlantic passive margin is considered relatively stable, with few slope instabilities recognized during the Holocene. However, new multibeam bathymetry mapping and sediment core acquisition off eastern Canada indicate that previously unidentified, large, submarine landslide events occurred during the Late Holocene, between 4 and 1.5 ka. The recognition of these new gravitational events, in addition to the well-known C.E. 1929 Grand Banks earthquake-induced landslide, indicates that approximately one large landslide event per 1000 years has occurred offshore eastern Canada within the past 4000 years, a much shorter recurrence interval than hitherto reported. This Late Holocene recurrence rate is also similar to active margins around the world and is likely due to the under-consolidation and resultant instability of Scotian Slope sediments attributable to high glacial sedimentation rates. The discovery of these new Late Holocene landslides was made possible through detailed examination of cores recovered from the lower slope. These results demonstrate that submarine landslide hazard has been underestimated on the western North Atlantic margin—home to significant submarine infrastructure and proximal to a large coastal population.〈/span〉