SURFICIAL GEOLOGY - Central Saanich

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SURFICIAL GEOLOGY Legend Map G-1-Surficial MaterialsandTextures Map G-1. Map coloursindication the type of surficial material present: Colluvium Fluvial Morainal Organic Marine Glacio-marine GEOMORPHIC SYMBOLS trimline crag-and-tail drumlin flute striation erratic boulders meltwaterchannel (flow directionknown) C F M O W WG meltwaterchannel (flow directionuncertain) terrace gully mass-movement (flow directionknown) surface seepage, spring drownedtrees See previous page foradescription forthemap symbols. Note: SurficialmaterialsL-Lacustrine, LG -Glaciolacustrine, FG - Glaciofluvialare notdisplayedon thismapbutdo occur in the studyarea at some locationsbelowthe surface. The stratigraphiclocations of some of these depositsoccur inthe GroundwaterAquifer section onthe crosssections;andtheyare alsorecorded inthe Surficial GeologyAppendix.. 464 000 469 000 Map G-1 Surficial GeologyMaterials of Central Saanich. 1000 m 0m 1000 m Scale 1:40000 2000 m G-4 SURFICIALMATERIALSANDTEXTURESMAP 464 000 469 000 474 000 Map Notes: This map displaysthe materialswhich lie at the surface of the land. Thisis onlyone type of display derived fromthe data base. In many areas, layers orstratasof other materials occuratdepth such as materials which identify gravellyaquifers. Municipality of Central Saanich ResourceAtlas 474 000
SURFICIAL GEOLOGY QUATERNARYHISTORY In this section, the late Pleistocene and early Holocene history of the centralSaanichPeninsulaisdiscussedinaregionalcontext.FiguresG- 10 to G-16 show a conceptual model that depicts the changing palaeogeography of southeastern Vancouver Island, showing central Saanich Peninsula from ca. 19to 4ka (calendar years before present). During this geomorphically dynamic interval, Saanich Inlet evolved from aglacierised fjord basin into amarine inlet dominated by nonglacial processes. At the same time, significant changes in sediment delivery rates,sea-level, water temperature, geochemistry and ecology occurredonthepeninsula. Changes in ice margins, drainage patterns and shoreline geometry are inferredfromthedistributionandelevationrangeofmoraine,proglacial colluvium, glaciofluvial and glaciomarine landforms (Huntley et al. in press). Landform and sediment assemblagesin the area aresimilar to thoseobservedincontemporarypalaeoglacialenvironmentsandmodern fjords dominated by retreating tidewater glaciers. The chronology of major geomorphic events and sea-level changes is constrained by radiocarbon dates from late Pleistocene and Holocene sequences in Saanich Inlet, adjacent parts of southern Vancouver Island and the Pacific northwest (Mathews et al. 1970, Alley and Chatwin, 1979, HicockandArmstrong1985,Bornholdetal.1997,PorterandSwanson 1998). Legendfor Quaternary History Diagrams Glacierice (with estimated ice thickness in metres) Deglaciated terrain Glaciofluvial outwash Sea and lake water Iceflow direction(determined from paleo-iceflow indicator) Meltwater flow (current direction known) Meltwater flow (current direction uncertain) Lateglacial-maximumphase(ca.19to17ka) Before 17 ka, central Saanich Peninsula was covered by an ice sheet flowing from montane sources on Vancouver Island and the adjacent BritishColumbiamainland.Bedrockandoldersedimentswereoverlain by subglacial morainal deposits consisting of lodgement till, melt-out till, and glaciofluvial sediments (Figure G-1). Streamlined bedforms indicate that during this phase, ice and subglacial meltwater-flow was directed south-to southeasttoward Juan de Fuca Strait and Haro Strait (Figure 2a). In the northwest, cirque headwalls above 1200 masl have been modified by frost-shatter and solifluxion. This relict periglacial terrain isinterpretedtorepresentaminimumlimitto glaciationpriorto the onset of deglaciation (Huntley et al. in press). Ice thicknesses over Saanich Peninsula at this time ranged from 800 to 1000 m. In Saanich Inlet,icethicknesswasgreaterthan1500m(FigureG-10). G-5 QUATERNARYHISTORY FigureG-10. Late glacial maximum phase ( ca. 19 to 15 ka). Figure G-11. Early deglacialphase ( ca. 15to 14 ka). Earlyglacial-retreatphase(ca.16to15ka) Deglaciationbeganbefore16ka(PorterandSwanson1998).Mountains anduplandsweredeglaciatedearlyastheregionalequilibriumlinerose above the elevationof localcirquebasins.As aresult, remnant ice was confined to valleys and coastal lowlands (Figure 2b). In proglacial settings, exposed glacial debris was extensively remobilised by meltwaterandmass-movement.Meltwateranderodedsedimentdrained southeast across Saanich Inlet. Hydrologic continuity between ice-free areas was likely maintained by eskers, subglacial sheet-flow and supraglacial channels. Glacio-isostatically depressed lowlands were inundatedastheeustaticsea-levelrose.Asicethinned,tide-waterglacier margins partly floated and retreated to grounding lines in southern SaanichInlet,SatelliteChannel,andoverSaanichPeninsula(Huntleyet al. in press). Ice, meltwater and sediment drained to alarge calving embaymentformedinHaroStrait,southernStraitofGeorgiaandeastern JuandeFucaStrait(FigureG-11). Municipality of Central Saanich ResourceAtlas
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SURFICIAL GEOLOGY - Central Saanich

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