Development of a Shear Wave Velocity Model of the Near-Surface Deposits of Southwestern British Columbia, Canada

Sufficient data currently exist to propose a shear-wave velocity model for the near-surface deposits in southwestern British Columbia. This model has been developed in order to estimate shear-wave velocity profiles where such data are lacking but where the stratigraphy is known, primarily for seismic microzonation mapping. In general, Pleistocene deposits that have been overridden by glaciers have shear-wave velocities greater than 400 m/sec. Consequently, little amplification of ground motion due to soil conditions would be expected in these deposits. However, Late Pleistocene deltaic and glaciomarine deposits that have not been overridden by glaciers, and Holocene deltaic, alluvial, and lacustrine, shoreline and organic deposits have average shear-wave velocities between 75 and 330 m/sec. Where sufficiently thick, these deposits are susceptible to moderate to high amplification of ground motion. In these Late Pleistocene and Holocene deposits, average shear-wave velocity increases with grain size. The data presented here are preliminary, and additional data are required for a reliable characterization of some environment

Quaternary rupture of a crustal fault beneath Victoria, British Columbia, Canada

The seismic potential of crustal faults within the forearc of the northern Cascadia subduction zone in British Columbia has remained elusive, despite the recognition of recent seismic activity on nearby fault systems within the Juan de Fuca Strait. In this paper, we present the first evidence for earthquake surface ruptures along the Leech River fault, a prominent crustal fault near Victoria, British Columbia. We use LiDAR and field data to identify >60 steeply dipping, semi-continuous linear scarps, sags, and swales that cut across both bedrock and Quaternary deposits along the Leech River fault. These features are part of an ~1-km-wide and up to >60-km-long steeply dipping fault zone that accommodates active forearc transpression together with structures in the Juan de Fuca Strait and the U.S. mainland. Reconstruction of fault slip across a deformed <15 ka colluvial surface near the center of the fault zone indicates ~6 m of vertical separation across the surface and ~4 m of vertical separation of channels incising the surface. These displacement data indicate that the Leech River fault has experienced at least two surface-rupturing earthquakes since the deglaciation following the last glacial maximum ca. 15 ka, and should therefore be incorporated as a distinct shallow seismic source in seismic hazard assessments for the region.This research was supported by an NSERC Discovery grant to KM and NSF EAR IRFP Grant #1349586 to CR

Pleistocene Stratigraphy of the Athabasca River Valley Region, Rocky Mountains, Alberta

The Pleistocene stratigraphy of the central Canadian Rocky Mountains is described from a region where few studies of Late Quaternary deposits have been conducted. Six informal lithostratigraphic units are recognized from newly mapped exposures in Jasper National Park. The oldest deposits are interpreted as paleofan deposits (Unit 1) and they are overlain by glaciofluvial gravels and sands (Unit 2), glaciolacustrine sediments (Unit 3) and by a glacigenic diamicton sequence (Unit 4) that includes basal till, supraglacial deposits and ice-marginal debris flow sediments. Proximal glaciofluvial gravels, debris flow deposits and minor glaciolacustrine sediments (Unit 5) and paragiacial fan deposits and loess (Unit 6) cap the stratigraphic sequence. Limited chronologic control suggests that nonglacial fluvial and alluvial fan sedimentation began prior to 48 ka and continued throughout the Middle Wisconsinan. Braided stream deposits were accumulating in the Athabasca River valley near Jasper townsite about 29 ka. In the Late Wisconsinan, Rocky Mountain and Cordilleran glaciers advanced through the area, initially damming lakes in a number of Front Range tributary valleys. During déglaciation, ice-marginal glaciofluvial activity and paragiacial debris flows dominated sedimentation. Glacial lakes were limited in extent. A radiocarbon date on shells from one small ice-marginal lake indicates that glaciers were well in retreat by about 12 ka. Alpine glaciers in the region were at or near their present limits by 10 ka.On a reconnu six unités lithostratigraphiques informelles à partir d'affleurements récemment cartographies dans le parc national de Jasper. Les dépôts les plus anciens ont été interprétés comme étant d'anciens cônes de déjection (unité n°1), surmontés par des graviers et des sables fluvioglaciaires (unité n° 2), des sédiments glaciolacustres (unité n° 3) et par une séquence de diamictons (unité n° 4) qui comprend du till de fond, des dépôts supraglaciaires et des sédiments provenant de coulées boueuses de marge glaciaire. Des graviers fluvioglaciaires proximaux, des dépôts de coulée boueuse et des sédiments glaciolacustres (unité n° 5) ainsi que des dépôts de cône de déjection « paraglaciaires » et des loess (unité n° 6) complètent la séquence. L'imprécision chronologique découle du fait que la sédimentation non glaciaire de type fluviatile et alluvionnaire en cône de déjection a commencé avant 48 ka et s'est poursuivie tout au long du Wisconsinien moyen. Les dépôts de cours d'eau anastomosés se sont accumulés dans la vallée de l'Athabasca, près de Jasper, vers 29 ka. Au cours du Wisconsinien supérieur, les glaciers des Rocheuses et de la Cordillère ont couvert la région, édifiant d'abord des barrages dans un bon nombre de vallées tributaires du Front Range. À la déglaciation, l'activité fluvioglaciaire de marge glaciaire et les coulées boueuses « paraglaciaires » étaient les principales formes de sédimentation. Les lacs glaciaires étaient peu étendus. Une date au radiocarbone de coquillages recueillis dans un petit lac de marge glaciaire montre que le retrait glaciaire était en bonne voie vers 12 ka. Les glaciers alpins avaient dès 10 ka atteint à peu près leur limite actuelle.Die Stratigraphie der zentralen kanadischen Rocky Mountains im Pleistozàn wird fur eine Region beschrieben, fur die wenige Studien uber Ablagerungen im spàten Quaternar durchgefuhrt worden sind. Man hat inneu kartographierten Aufschlussen im Jasper Nationalpark sechs informelle lithostrati-graphische Einheiten identifiziert. Die àltesten Ablagerungen werden als Palàoschwemmfâcher gedeutet (Einheit l),und bie sind von glazifluvialem Kies und Sand uberlagert (Einheit 2), glaziallimnischen Sedimenten (Einheit 3) und einer glazigenen Diamiktit-Sequenz (Einheit 4), die Grundtill, supraglaziale Ablagerungen und Sedimente vom SchuttfluB am Eisrand enthàlt. Proximaler glazifluvialer Kies, SchuttfluBablagerungen und geringere glaziallimnische Sedimente (Einheit 5) sowie paraglaziale Schwemmfàcherablagerungen und LoB (Einheit 6) beschlieBen die stratigraphische Sequenz. Die begrenzte chronologische Kontrolle IaBt annehmen, daB die nichtglaziale fluviatile und Schwemmfàcher-Sedimentierung vor 48 ka begann und durch das ganze mittlere Wisconsinium fortdauerte. Verwilderte Stromablagerungen sammelten sich im AthabaskafluBtal in der Nâhe der Stadt Jasper urn etwa 29 ka an. Im spàten Wisconsinium ruckten die Gletscher der Rocky Mountains und der Kordilleren durch das Gebiet voran und dâmmten Seen in einer Reihe von tributàren Tàlern des Front Range. Wàhrend der Enteisung haben glazifluviale Eisrandaktivitàt und paraglazialer SchuttfluB die Sedimentierung beherrscht. Glaziale Seen waren in ihrer Ausdehnung begrenzt. Eine Radiokarbondatierung von Muscheln von einem kleinen Eisrandsee zeigt, daB der Gletscherruckzug urn etwa 12 ka schon eingetreten war. Alpine Gletscher des Gebiets befanden sich urn 10ka an Oder nahe bei ihren heutigen Grenzen

Glacial Stratigraphy of the Bulkley River Region: A Depositional Framework for the Late Pleistocene in Central British Columbia

A depositional framework for late Pleistocene sediments in central British Columbia was developed from the composite stratigraphy of glacial sediments found in the Bulkley River region. Nonglacial deposits correlated to the Olympia Nonglacial Interval, are overlain in succession by sub-till, ice-advance sediments, Late Wisconsinan (Fraser Glaciation) till, and late-glacial sediments. Due to local erosion and depositional variability, some of the units are not continuous throughout the region and differ locally in their thickness and complexity. At the onset of the Fraser Glaciation, ice advance was marked by rising base levels in rivers, lake ponding, and ice marginal subaqueous deposition. Physiography and glacier dynamics influenced the position of drainage outlets, direction of water flow, and ponding. The region was completely ice covered during this glaciation and ice-flow directions were variable, being dominantly influenced by the migrating position of ice divides. Deglaciation was marked by the widespread deposition of fine-grained sediments in proglacial lakes and glaciofluvial sands and gravels at locations with unrestricted drainage.Un scénario de sédimentation datant du Pléistocène tardif est élaboré à partir de la stratigraphie glaciaire observée dans la région de la rivière Bulkley. Les dépôts non-glaciaires attribués à l’interstade d’Olympia sont surmontés d’un till d’une avancée glaciaire, d’un till datant du Wisconsinien supérieur (glaciation du Fraser) et de sédiments tardiglaciaires. Plusieurs unités ne sont pas continues dans la région et diffèrent par leur épaisseur et leur complexité, en raison de l’érosion locale et des taux de sédimentation variables. Au début de la glaciation du Fraser, l’avancée des glaces fut accompagnée d’une augmentation du niveau de base des rivières, la création d’étangs et d’une sédimentation près des marges glaciaires. La physiographie et la dynamique glaciaire influence l’emplacement des exutoires de drainage, la direction de l’écoulement des eaux et la création des bassins. La région fut alors entièrement couverte de glace et les directions d’écoulement glaciaire, très variables, furent fortement contrôlées par la migration des lignes de partage glaciaires. La déglaciation se caractérise par la sédimentation de sédiments fins dans les lacs proglaciaires, et des sables et des graviers fluvioglaciaires dans les zones de drainage libres

Reconstructing the confluence zone between Laurentide and Cordilleran ice sheets along the Rocky Mountain Foothills, southwest Alberta

Landform mapping near the Athabasca valley was fundamental to determining whether the Late Wisconsinan Laurentide and Cordilleran Ice Sheets (LIS, CIS) coalesced. In this paper we examine the detailed landform record using new LiDAR digital elevation and hillshade models of the area of confluence and eventual desuturing of these ice sheets. This work reveals an earlier more extensive Cordilleran advance before coalescence with the LIS. When the ice sheets coalesced, the flow pattern was dominated by ice flow along the mountain front, with Cordilleran ice flowing out of major trunk valleys but Laurentide ice flowing into the Foothills up smaller valleys. This flow pattern implies that when the ice sheets coalesced the CIS was already waning, or at least was not at its maximum. Deglaciation was interrupted by several re-advances, probably related to the destabilizing effect of proglacial lakes along the ice margin

Controls on modern tributary-junction alluvial fan occurrence and morphology: High Atlas Mountains, Morocco

Modern tributary-junction alluvial fans (cone-shaped depositional landforms formed in confined valley settings) were analysed from a 20-km-long reach of the Dades River in the distal part of the fold-thrust belt region in the south-central High Atlas Mountains of Morocco. Here, a deeply dissected network of ephemeral tributary streams and a perennial trunk drainage characterised by an arid mountain desert climate are configured onto a folded and thrust faulted Mesozoic sedimentary sequence. Out of 186 tributary streams, only 29 (16%) generated alluvial fans at their tributary junctions. The fan-generating catchments possess higher relief, longer lengths, lower gradients, and larger areas than nonfan-generating catchments. Whilst geologically, fan-generating catchments are underlain by folded / steeply dipping weak bedrock conducive to high sediment yield. Tributary-junction fans are built from debris flow or fluvial processes into open or confined canyon trunk valley settings. The proximity of the perennial trunk drainage combined with the valley morphology produces lobate or foreshortened trimmed fan forms. Analysis of fan (area, gradient, process), catchment (area, relief, length, gradient), and tributary valley (width) variables reveals weak morphometric relationships, highlighted by residual plots that show dominance of smaller and lower gradient than expected fan forms. These morphometric relationships can be explained by interplay between the catchment and trunk drainage geology, morphology, climate, and flood regime that are combined into a conceptual ‘build and reset’ model. Ephemeral tributary-junction fans develop progressively during annual localised winter-spring storm events, attempting to build toward a morphological equilibrium. However, the fans never reach an equilibrium morphological form as they are reset by rare (>10 year) large floods along the River Dades that are linked to regional incursions of Atlantic low pressure troughs. The model highlights the spatial and temporal variability of tributary-junction fan building and illustrates the connectivity / coupling importance of such features in dryland mountainous terrains