Patrick Lajeunesse

Sedimentology of an ice-contact glaciomarine fan complex, Nastapoka Hills, eastern Hudson Bay, northern Québec

Abstract Emerged Holocene coalescing glaciomarine fans are exposed along the Nastapoka Hills, eastern Hudson Bay. The fans form part of a regional, stratified drift belt associated with terrestrial and submarine frontal moraines and other grounding lines deposits. These thick, extensive and, in many cases, coalescent sediment bodies were deposited during a stillstand of the Quebec–Labrador Sector (QLS) of the Laurentide Ice Sheet (LIS) that pinned on the hills during eastward retreat from Hudson Bay. The fans are composed of basal interbedded and interlaminated sand, silt and silty clay (Facies association I, FA I) deposited by suspension settling from overflow plumes, sediment gravity flows and ice rafting. Towards the ice-contact zone, FA I interfingers with diamict layers (FA II) deposited by gravity flows and ice rafting. These two sedimentary units are overlain by interbedded sand and gravel (FA III), formed by reworking during subsequent emergence. Parts of FA I and FA II were remobilized by submarine gravity flows, which prevailed long after ice retreat. The sedimentology of the Nastapoka Hills provides information on the processes operating near the grounding line of a temperate ice sheet reaching tidewater.

New Evidence of Holocene Mass Wasting Events in Recent Volcanic Lakes from the French Massif Central (Lakes Pavin, Montcineyre and Chauvet) and Implications for Natural Hazards

High-resolution seismic profiling (12 kHz) surveys combined with sediment cores, radiocarbon dating, tephrochronology and multibeam bathymetry (when available) allow documentation of a range of Holocene mass wasting events in nearby contrasting lakes of volcanic origin in the French Massif Central (45°N, 2°E): two deep maar lakes (Pavin and Chauvet) and a shallow lake (Montcineyre) dammed by the growth of a volcano. In these lacustrine environments dominated by authigenic sedimentation, recent slide scars, acoustically transparent to chaotic lens-shaped bodies, slump deposits or reworked regional tephra layers suggest that subaqueous mass wasting processes may have been favoured by gas content in the sediments and lake level changes. While these events may have had a limited impact in both lakes Chauvet and Montcineyre, they apparently favoured the development of lacustrine meromicticity in maar Lake Pavin along with possible subaerial debris flows resulting from crater outburst events.

Buried preglacial fluvial gorges and valleys preserved through Quaternary glaciations beneath the eastern Laurentide Ice Sheet

The geomorphology, geometry, and sedimentary infill of buried gorges and V-shaped valleys observed at the base of major river valleys in the formerly glaciated southeastern Canadian Shield region have been revealed from excavation and drilling data acquired during the construction of hydro-electric dams and seismic data collected on lakes and offshore. Compilation of these previously published and unpublished data provided an exceptional opportunity to examine the morphology and spatial distribution of buried bedrock gorges and the mechanical processes responsible for their erosion. In some valleys, detailed observations of deep gorges have been allowed by their exhumation over large areas. Archive photographs show deep and large potholes, natural pillars, furrows, flutes, and scallops on the well-polished bedrock walls of the gorges. They also reveal that gorges and valleys have a sharp-ending V shape and very narrow base and are superimposed by a U-shaped valley, forming a buried valley-within-valley topography. The narrow and deep cross-profile of these gorges, their well-polished slopes, and the type of bedforms observed within them are typical features of fluvial bedrock channels. Drilling operations at many sites have also provided data on bedrock topography of these gorges within valleys and on the nature of their sedimentary infill. The different lines of evidence presented in this paper indicate that gorges and V-shaped valleys of the region were not eroded by the Laurentide Ice Sheet during Quaternary glaciations but are relics of a preserved preglacial fluvial system eroded during a lower base level. This paleofluvial system is interpreted to be linked to fluvially cut channels observed on the seafloor of the Estuary and Gulf of St. Lawrence.

Multibeam Bathymetry Investigations of Mass Movements in Lake Le Bourget (NW Alps, France) Using a Portable Platform

Here we report on a recent survey undertaken on Lake Le Bourget (NW Alps, France) using a multibeam echosounder mounted on a portable and shallow-draft platform. The 3.6 × 2.8 m survey platform can be transported from one lake to another and deployed within a few hours. Its shallow draft allows surveying in areas as shallow as 3 m, while the multibeam echosounder allows maximum sounding depths of 500 m. The collected data have allowed identifying two main areas of the lake affected by mass movements. In the northwest sector of the lake, a large mass wasting deposit of 1.8 km2 is found at the base of a very steep slope. This large deposit may be correlated with a major late-glacial mass-wasting event that triggered a seiche in Lake Le Bourget. To the north of this deposit a mass-wasting deposit is interpreted to have been caused by the AD 1822 earthquake. In the southeast sector of the lake, many lobes and a large mass wasting deposit are reported for the first time. The presence of gas-rich sediments in this area has prevented acoustic penetration and thus their identification during previous seismic surveys. The occurrence of mass movements is correlated with interflow deposition zones. In addition to the mass movements, a series of collapse craters are identified in the northern sector of the lake. These collapse craters are located near the head scars of an incipient slide. Considering that earthquakes are common in the region and can cause liquefaction within the lake sediments, there is a possibility that this incipient slide may further develop and eventually generate a massive (>107 m3) slide.

Flood-Induced Turbidites From Northern Hudson Bay And Western Hudson Strait: A Two-Pulse Record Of Lake Agassiz Final Outburst Flood?

In Hudson Bay and Hudson Strait, the rapid collapse of the Laurentide Ice Sheet (LIS) culminated in the catastrophic drainage of proglacial Lake Agassiz into the North Atlantic around 8500 cal BP. It has been suggested that this catastrophic event may have triggered the 8200 cal BP cold event recorded in Greenland ice cores. Evidence for that outburst flood was the identification of a centimeter to decimeter-thick hematiterich red layer that was observed in Hudson Strait sediments around 8000 yr BP. In this paper, we have identified a sequence of two flood-induced turbidites (i.e., hyperpycnites) in a reddish layer from two cores collected in northern Hudson Bay (core AMD0509-27bLEH) and western Hudson Strait (core AMD0509-28PC) in 2005 onboard the ice-breaker CCGS Amundsen. These two reddish layers can be correlated to a red bed previously identified as a regional isochron in Hudson Strait and associated with the final drainage of Lake Agassiz around 8500 cal BP. Regardless of the exact timing of the catastrophic drainage, the hyperpycnites described in this paper suggest that they were deposited following two pulses, which is in agreement with the one of the scenarios proposed by Clarke et al. (2003) [Science 301, 922-923] for the drainage of Lake Agassiz. Finally, this study demonstrates for the first time the turbiditic and the flood-induced nature of the Hudson Strait red bed isochron.

Shallow-water longshore drift-fed submarine fan deposition (Moisie River Delta, Eastern Canada)

Submarine canyons and associated submarine fans are in some cases located at the end of a littoral cell where they act as conduits for the transfer of eroded terrigenous sediments to the marine environment. Such fans are generally found in deep-water settings at >500 m water depth. Offshore the Moisie River Delta (NW Gulf of St. Lawrence, Eastern Canada), high-resolution multibeam bathymetry and seismic data led to the discovery of an unusually shallow submarine fan (≤60 m) located at the end of a littoral cell. Sediment is transported westward on the shallow coastal shelf, as demonstrated by the downcurrent displacement of oblique nearshore sandbars where the shelf narrows to less than 1 km. The steep slope near the end of the littoral cell is incised by a channel that feeds a submarine fan composed of smaller channels and depositional lobes. According to existing Holocene evolution models for the region, the fan formed within the last 5,000 years. Its evolution is largely due to the transport of sediment by longshore drift. Multibeam echosounder and seismic data also reveal that the gravity-driven accretion of the submarine fan is characterized mainly by two processes, i.e., frequent small-scale, downslope migration of sandwaves on the slope, and more episodic slumping/turbidity-current activity in the deeper part of the fan. This study documents that, besides their common deep-water location, smaller-scale submarine fans can occur also in very shallow water, implying that they could be more frequent than previously thought both in modern environments and in the rock record.

French Alpine Foreland Holocene Paleoseismicity Revealed by Coeval Mass Wasting Deposits in Glacial Lakes

The French alpine foreland area has been struck by several earthquakes with magnitudes above 5 on Richter scale in recent history. In this paper we document the regional impact of historical and Holocene earthquakes based on the identification of mass wasting deposits in glacial lakes at different settings. Lake Le Bourget and Lake Paladru are situated at low elevations (respectively 231 m–492 m) and Lake Blanc Huez is located at 2500 m altitude. Through the integration of high-resolution acoustic profiles and accurately dated sediment samples from cores, recent coeval mass wasting deposits in each lake were correlated with nearby historical earthquakes, whereas coeval mass wasting deposits around 5200 cal BP and 9550 cal BP in these three lakes were correlated to regional earthquakes events.

Late-Holocene Mass Movements in High Arctic East Lake, Melville Island (Western Canadian Arctic Archipelago)

East Lake, located at Cape Bounty (Melville Island, Canadian High Arctic), was mapped using a high-resolution swath bathymetric sonar and a 12 kHz sub-bottom profiler, allowing for the first time the imaging of widespread occurrence of mass movement deposits (MMDs) in a Canadian High Arctic Lake. Mass movements occurred mostly on steep slopes located away from deltaic sedimentation. The marine to lacustrine transition in the sediment favours the generation of mass movements where the underlying massive mud appears to act as a gliding surface for the overlying varved deposits. Based on acoustic stratigraphy, we have identified at least two distinct events that triggered failures in the lake during the last 2000 years. The synchronicity of multiple failures and their widespread distribution suggest a seismic origin that could be related to the nearby Gustaf-Lougheed Arch seismic zone. Further sedimentological investigations on the MMDs are however required to confirm their age and origin.

Morphosedimentology Of Submarine Mass-Movements And Gravity Flows Offshore Sept-Îles, Nw Gulf Of St. Lawrence (Québec, Canada)

Recent multibeam sonar and acoustic subbottom profiler surveys and sediment coring offshore the city of Sept-Iles (NW Gulf of St. Lawrence) reveal different types of submarine mass-movements and gravity flows in glaciomarine, paraglacial and postglacial deposits. These mass-movement and gravity flow features are slumps, gullies and channel-levee systems and fans. The key results of this study include: 1) slumps involve the entire deglacial and postglacial sequence, indicating their recent triggering; 2) identification of a 57-cm thick turbidite and several sand layers in sediment cores collected in a deep and isolated basin unaffected by fluvial inputs, along with 210 Pb measurements, indicate the recent activity of mass wasting events derived from slope instabilities; 3) important volumes of sediments are being transported from the coastal to the deeper marine environment by gravity flows processes on the prodelta of the Moisie River. Hypotheses for explaining the widespread occurrence of recent mass-movements due to slope instabilities in the area possibly include their possible triggering by the AD 1663 (M~7) or another large earthquake.

Late Wisconsinan grounding-zone wedges, northwestern Gulf of St Lawrence, eastern Canada

Grounding-zone wedges (GZWs) are formed on high-latitude continental shelves by the accumulation of subglacial sediments from fast-flowing ice at the grounding zone of an ice sheet during temporary stillstands (e.g. Mosola & Anderson 2006; Dowdeswell & Fugelli 2012; Batchelor & Dowdeswell 2015). GZWs are therefore recognized to record phases of episodic ice-sheet retreat (Dowdeswell et al. 2008). The morphology and distribution of a series of GZWs mapped using high-resolution multibeam sonar and airgun seismic data in the northwestern Gulf of St Lawrence, eastern Canada, are described here (Fig. 1a–d). These ice-contact submarine landforms were deposited when the marine-based southeastern margin of the Laurentide Ice Sheet stabilized for a few decades to centuries during its rapid retreat through the Gulf of St Lawrence. Fig. 1. Grounding-zone wedges (GZW) in the northwestern Gulf of St Lawrence, eastern Canada. ( a ) Location of study area (red box; map from GEBCO_08). ( b ) Oblique view of grounding-zone wedge at 180 m water depth (GZW 1) illustrating circular depressions along its steeper down-ice slope. ( c ) Seismic-reflection profile with two GZWs. Seismic profile shows bedrock surface which consists of gently dipping monoclinal Middle Ordovician sedimentary rocks of the St Lawrence Platform (Sanford 1993). Profile is located …