SURFICIAL GEOLOGY - Central Saanich
SURFICIAL GEOLOGY - Central Saanich
SURFICIAL GEOLOGY - Central Saanich
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<strong>SURFICIAL</strong> <strong>GEOLOGY</strong><br />
<strong>SURFICIAL</strong> <strong>GEOLOGY</strong> AND GEOMORPHOLOGY OF<br />
CENTRAL SAANICH PENINSULA, SOUTHEASTERN<br />
VANCOUVERISLAND<br />
1 2 3<br />
David Huntley, Katrina Bennett , Peter Bobrowsky and John<br />
4<br />
Clague<br />
1.WesternGeographicResearch,505-350DouglasStreet,Victoria,B.C.<br />
2.LatitudeGeographicsGroup,257MarketSquare,Victoria,B.C.<br />
3.BritishColumbiaGeologicalSurvey,MinistryofEnergyandMines,Victoria,B.C.<br />
4.DepartmentofEarthSciences,SimonFraserUniversity,Burnaby,B.C.<br />
INTRODUCTION<br />
ThissectionsummarisesthesurficialgeologyandQuaternaryhistoryof<br />
central <strong>Saanich</strong> Peninsula. The map, figures and text also provide<br />
baselineinformationforinterpretingthedistributionofgroundwaterand<br />
aggregateresources,potentiallyunstablelandareas,soilsandvegetation<br />
cover. Fieldwork and research was undertaken for Ocean Drilling<br />
Programleg169S andtheResourceAtlasProjectoftheEnvironmental<br />
AdvisoryCommissionoftheMunicipalityof<strong>Central</strong><strong>Saanich</strong>. Support<br />
wasprovidedbytheCentreforEarthandOceanResearch,Universityof<br />
Victoria, the British ColumbiaGeological Survey Branch andMinistry<br />
ofEnvironment,LandsandParks,theGeologicalSurveyofCanadaand<br />
NationalScienceandEngineeringResearchCouncilofCanada.<br />
Landformsandsurficialsedimentsweremappedfromobservationsat39<br />
fieldsites and from 1:23 000 and 1: 70 000 scale air photographs.<br />
Surficial units were defined on the basis of landform associations,<br />
texture, sorting, fissility, colour, sedimentary structures, consolidation<br />
andcontactrelationships.Icedispersalpatternshavebeeninferredfrom<br />
striae,flutes,drumlins,rochesmoutonnes,andlong-axisorientationsof<br />
clastsintill.Meltwaterflowdirectionswereinferredfromforeset-cross<br />
and ripple-bedding, clast imbrication and channel orientations.<br />
Additionalinformationwascompiledfromearlier regionalstudies(e.g.<br />
Fyles 1963, Halstead 1966, 1968, Clague 1976, 1977, Alley and<br />
Chatwin, 1979,Hicock andArmstrong 1983,Bornhold et al.1997 and<br />
1998); 1:63,360 and 1:50,000 scale surficial geology maps (Halstead<br />
1966,BlythandRutter1993,Huntleyunpublishedmapping);1:250,000<br />
and 1:100,000 scale geology maps (Muller 1980, Massey et al.1994);<br />
soils reports and accompanying 1: 100,000 and 1:20,000 scale maps<br />
(Jungen 1985, Jungen et al. 1985); and the Ministry of Environment,<br />
LandsandParkswaterwellandlakedatabases.<br />
PHYSIOGRAPHY,<strong>GEOLOGY</strong>ANDGEOMORPHOLOGY<br />
<strong>Central</strong> <strong>Saanich</strong> Peninsula lies on the southeastern margin of the<br />
Nanaimo Lowland and Vancouver Island Ranges (Holland 1980). The<br />
peninsula is bordered to the east by Cordova Channel and by <strong>Saanich</strong><br />
Inlet to the west. The area is mainly underlainby fractured and faulted<br />
volcanicandigneousrocksrangingfromTriassic(ca.230ma)toJurassic<br />
(ca. 170 ma) inage (Muller 1980, Massey et al. 1994). Mount Newton<br />
(307 masl), thehighestpointinthe study area, is underlainbyJurassic<br />
granodiorite. Steep, rollinguplandsof the Partridge Hills areunderlain<br />
byJurassicbasalts. Undulatingandgentlyrollinglowlandterraininthe<br />
vicinityofBrentwoodBay,KeatingandIslandViewBeachisunderlain<br />
by Triassic basalts and breccia and granodiorite. Permian to Triassic<br />
limestonesarefoundsouthofBrentwoodBay.<br />
Bedrockismantledbyasurficialsedimentcoverthatrangesinthickness<br />
fromlessthanametreonsteepuplandslopestogreaterthan40metresin<br />
some lowland areas. Seven surficial units are identified (informally<br />
designated units 1to 7, Table G2 and G3). Units are correlated with<br />
previously defined sequences in adjacent coastal areas by stratigraphic<br />
position, sedimentology,surfaceexpressionandlandformassociations.<br />
The dominantlandformsanddepositsare LateWisconsinan in age(ca.<br />
29to10ka),andrecordtheadvance,maximumandretreatphasesofthe<br />
last (Fraser) glaciation. Soil textures and hydrologic characteristics<br />
reflectunderlyingsurficialunitsandbedrock(TableG2).<br />
QUATERNARYSTRATIGRAPHYAND GEOMORPHOLOGY<br />
Pre-LateWisconsinan<br />
Theoldestsurficialsedimentsare exposedinaseacliff betweenIsland<br />
ViewBeachand Cowichan Head (Figure G-1, logA;refer toTable G1<br />
for key to faciescodes used instratigraphic columns).Abasal unit, the<br />
Muir Point Formation, consists of non-glacial marine, alluvial, fluvial<br />
andcolluvial sediments, inferred todate to the Sangamonian stage (ca.<br />
120ka;HicockandArmstrong1983). Unit1isoverlainbyglaciofluvial<br />
outwash, till and fossiliferous marinebeds assigned to the Dashwood<br />
Drift.The age of unit 2is uncertain,butisprobably EarlyWisconsinan<br />
(ca. 80 to 65 ka; Fyles 1963, Hicock and Armstrong 1983). Unit 3<br />
includesoxidisedfluvialsandandgravel,andorganic-richestuarinesilt<br />
and clay. These deposits are older than 23 ka and are assigned to the<br />
MiddleWisconsinanCowichanHeadFormation(ArmstrongandClague<br />
1977).<br />
Late Wisconsinan - Fraser Glaciation (glacial-advance<br />
phase)<br />
Fraser Glaciationadvance-phase sediments,termed QuadraSand, were<br />
deposited after 29 ka (Halstead 1966, 1968, Clague 1976, 1977). Two<br />
facies are recognised in the study area. Unit 4a is an ice-distal facies<br />
consistingofwell-sortedsand,withgravel,siltandclay.Itisrestrictedto<br />
theeastside of<strong>Saanich</strong>Peninsulaand islandseastof Cordova Channel<br />
(Figure G-1, log A). Unit 4b,informally referred to as the <strong>Saanich</strong>ton<br />
gravel (Halstead1968), isanice-proximal facies consisting of massive<br />
G-1<br />
<strong>SURFICIAL</strong><strong>GEOLOGY</strong>AND GEOMORPHOLOGY<br />
to stratified gravel and sand (Figure G-8). Palaeoflow indicators are<br />
consistent with meltwater and sediment transport from Koksilah and<br />
Cowichan valleys toward <strong>Saanich</strong> Peninsula (Figure G-2, log B).<br />
Advance-phase glaciofluvial outwash in central <strong>Saanich</strong> Peninsula<br />
occursuptoapproximately100masl.<br />
Late Wisconsinan - Fraser Glaciation (glacial-maximum<br />
phase)<br />
Glaciallysculptedbedrock,oldersedimentsandadvance-phasedeposits<br />
areunconformablyoverlainbymorainaldepositscomprisingtheVashon<br />
Drift (Hicock and Armstrong 1985). Deposition of glacial maximum<br />
sediments occurred between ca. 23 and 18 ka (Clague et al. 1980).<br />
Morainal deposits include diamictons interpreted as lodgement and<br />
melt-out tills (unit 5a), and interstratified glaciofluvial gravel and sand<br />
(unit5b).InthePartridgeHillsandontheflanksofMt.Newton,moraine<br />
is extensively colluviated and best preserved in glacially-scoured<br />
bedrock depressions (Figure G-3, log C). Along valley floors and in<br />
lowland areas, unit 5forms an extensive drumlinised ground moraine<br />
overlyingolderunitsandstreamlinedbedrock(FiguresG-8,FigureG-1<br />
and G-2 logs A, B). Till textures reflect underlying materials. In the<br />
<strong>Saanich</strong>tonandKeating areas, tillshaveasand-richmatrixthatreflects<br />
derivationfromunderlyingQuadraSandandearlierunits.Inthevicinity<br />
ofMountNewton, BrentwoodBayand PartridgeHills,tillsarederived<br />
from bedrock and matrices are predominantly clay and silt-rich. The<br />
distributionandorientationofstriae,crag-and-tails,drumlins,andflutes<br />
indicate that iceflow was directed southward toward the Victoria area.<br />
Tillclastsanderraticbouldersarepredominantlylocallyderivedorhave<br />
provenances in the Koksilah andCowichan valleys. Exotic clasts have<br />
sources in central Vancouver Island and coastal mainland British<br />
Columbia.<br />
LateWisconsinan-FraserGlaciation(glacial-retreatphase)<br />
Fraser Glaciationretreat-phase sequences, termed Capilano Sediments<br />
(Fyles1963,Halstead1966,1968,HicockandArmstrong1985),include<br />
colluvial(unit6a),glaciofluvial(unit6b),glaciolacustrine(unit6c)and<br />
glaciomarine deposits (unit 6d). In upland areas and on valley sides,<br />
bedrockandmorainaldepositsareoverlainbydiamictonsinterpretedas<br />
ice-contactandproglacialmass-movementdeposits(FigureG-4logD).<br />
TerracedgravelsandcolluviumonthesouthflankofMountNewton,and<br />
Figure G-1. Figure G-2. Figure G-3.<br />
Figure G-4.<br />
Municipality of <strong>Central</strong> <strong>Saanich</strong> ResourceAtlas
<strong>SURFICIAL</strong> <strong>GEOLOGY</strong><br />
ameltwater spillway in the Durrance Lake valley indicate astable<br />
recessionalicesurfaceovercentral<strong>Saanich</strong>Peninsulaatabout150masl.<br />
Theorientationofthesefeaturesisconsistentwithsoutheasttransportof<br />
meltwaterandsedimenttowardCordovaChannel.Below80masl,older<br />
units, moraine and bedrock are draped by raised glaciomarine debrisflowdiamictons,sandandgraveloutwash,andmassivetorhythmicallybeddedsilt-clay(FigureG-1,logA).Morainaldepositsarealsoterraced<br />
and winnowed,indicatingreworkingbysurfacerunoffandwaveaction<br />
(Figure G-9). The elevation range of unit 6d and wave-cut terraces<br />
delimits the extent of marine transgression over central <strong>Saanich</strong><br />
Peninsula during deglaciation. Raised glaciomarine deposits are<br />
correlative to the earliest deglacial sediments in ODP cores1033 and<br />
1034,anddatebetween12and15ka(Mathewsetal.1970,Clagueetal.<br />
1982,Bornholdetal.1997and1998,Huntleyetal.inpress).<br />
Figure G-5. Figure G-6.<br />
Holocene(post-glaciation)<br />
Post-glacialsequences,termedSalishSediments(Fyles1963, Halstead<br />
1968), includecolluvial (unit 7a), fluvial (unit 7b), lacustrine (unit 7c)<br />
and marine deposits (unit 7d). On steep slopes, deglacial and older<br />
exposedsedimentsarepartlyreworkedbymass-movement,andlocally<br />
overlainbypostglacialcolluvium(FigureG-4,logD).Glaciomarineand<br />
morainal sediments are incised by streams. Mass-movement in the<br />
vicinityof<strong>Saanich</strong>tonislikelyattributedtoactivestreamincision.West<br />
of Sannichtonand Keating, eroded sediment is deposited along valley<br />
floors, in lakebasins, and in<strong>Saanich</strong> Inlet (Figure G-5 to G-7). Tothe<br />
east,mostsedimentisdepositedinCordovaChannelandHaroStrait.In<br />
situfossiltreestumpsexposedintheintertidalzoneatIslandViewBeach<br />
indicates marine transgression on the east coast of <strong>Saanich</strong> Peninsula<br />
overthelast2ka(Mathewsetal.1970).Sedimenterodedfromseacliffs<br />
at Cowichan Head is transported northward by long-shore drift and<br />
deposited mostly on Island View Beach. The intertidal zone bordering<br />
BrentwoodBayand<strong>Saanich</strong>Inletisdominatedbysteepbedrockslopes,<br />
andismantledbyathincolluvialcover.Theoldestpostglacialsediments<br />
in <strong>Saanich</strong> Inlet are diatomaceous muds and massive clays deposited<br />
undermoderatelyoxygenatedconditionswhensea-levelwaslowerthan<br />
present. The Mazama ash is adistinctive horizon in marine and lake<br />
sequences dating to about 7.6ka. Above the ash layer, varved<br />
diatomaceous muds were deposited under low-energy and anoxic<br />
conditions (Bornhold et al. 1997 and 1998). Interbedded with these<br />
sediments are massive muds, interpreted to be mass-flow deposits<br />
emplacedduringstormsandearthquakes(Blais-Stevensetal.1997).<br />
Figure G-7.<br />
Table G-1. Key tofacies codesused<br />
in stratigraphic columns.<br />
Facies code Description<br />
Dcm Diamicton, clast-supported, massive<br />
Dcs Diamicton, clast-supported, stratified<br />
Dmm Diamicton, matrix-supported, massive<br />
Dms Diamicton, matrix-supported, stratified<br />
Bm Boulders, massive<br />
Gm Gravel, massive<br />
Gs Gravel, stratified<br />
Gf Gravel, foreset-bedded<br />
Gt Gravel, trough cross-bedded<br />
Sm Sand, massive<br />
Sp Sand, planar<br />
St Sand, trough cross-bedded<br />
Sr Sand, ripple-bedded<br />
Fm(d) Silt and clay,massive(with dropstones)<br />
Fl(d) Silt and clay,laminated (withdropstones)<br />
G-2<br />
<strong>SURFICIAL</strong><strong>GEOLOGY</strong>AND GEOMORPHOLOGY<br />
Figure G-8. Vashon<br />
Drift,lodgement till<br />
(5a [sdMb])<br />
unconformably<br />
overlying<br />
drumlinised Quadra<br />
Sand, glaciofluvial<br />
outwash (4b [gsFb])<br />
exposed in the<br />
vicinity of Keating<br />
townsite. Seealso<br />
Figure G-2,log317,<br />
and map legend for<br />
description of<br />
terrain units.<br />
Figure G-9. Capilano Sediments, glaciomarine suspension and<br />
debris flow deposits (6d[zdMw]). Hagan Creekvalley,view<br />
toward Brentwood Baytownsite. See map legendfor<br />
description ofterrain units.<br />
Municipality of <strong>Central</strong> <strong>Saanich</strong> ResourceAtlas
<strong>SURFICIAL</strong> <strong>GEOLOGY</strong><br />
Map Legend A-Surficial Materials<br />
For Map G1. Surficial geology material, <strong>Central</strong><br />
<strong>Saanich</strong>.1998 (recordedon Map G1 as uppercase<br />
symbols).<br />
W Marine deposits:massive to stratifiedclay, silt, sand and<br />
gravel; generallymoderatelytowell-sorted,deposited in<br />
saltand brackishwater by settlingfromsuspension and<br />
submarine gravityflows, including turbiditycurrents, wave<br />
actionand longshore drift; may containshells and other<br />
organic material.<br />
O Organic deposits:peat and other vegetative materials;<br />
formed by theaccumulationof organic matter in<br />
depressions orlevelareas; includesbogs, fensand swamps.<br />
L Lacustrine deposits:massivetostratified fine sand, siltand<br />
clay;depositedfromsuspension and subaqueous gravity<br />
flows, includingturbidity currents and waveactioninfresh<br />
water..<br />
F Fluvialdeposits:gravelto silt-size sediment;generally<br />
well-sorted and stratified; clastswell-rounded;deposited<br />
byrivers, streams and creeks.<br />
C Colluvialdeposits:massive tostratified, clast- and matrixsupporteddiamicton<br />
or rubble; resulting fromtheme<br />
chanicalor chemical weathering ofbedrockand surficial<br />
sediment, andthe down-slopemovement ofmaterialsby<br />
gravitational processesincluding creep, solifluxion, sliding,<br />
debris-flow, avalanching, topple androckfall.<br />
WG Glaciomarinedeposits:diamicton, sand, siltand clay<br />
depositedfromsuspension and submarine gravity flows,<br />
including turbidity currents, inproximity toglacier ice;<br />
alsoincludesice-rafted stones, till-like diamictonsand<br />
interstratifiedglaciofluvialmaterials;wave-cutterraces,<br />
abrupt changesin texture anddistortedbedding common;<br />
marine shellsand other organic materialmaybe present.<br />
LG Glaciolacustrine deposits: well-stratified sand, siltandclay,<br />
including minor gravel and diamicton;deposited deposited<br />
fromsuspension and subaqueousgravity flows, including<br />
turbidity currentsandwave action in lakesadjacent to<br />
glacial ice; slump structures,irregular topographyand<br />
kettlesindicative of collapsefrommeltingof buried ice<br />
maybe locallypresent.<br />
Table G-2. GeneticParent MaterialsandSoils Relationships.<br />
Ta ble G-2 Genetic Parent Materials and Soils Rela tionships<br />
FG Glaciofluvial deposits:massive to stratified boulders,cobbles,<br />
pebble-gravel, sand, siltand diamicton; sortinggoodto poor;<br />
depositedby riversandstreamsflowing from, or incontact<br />
with glacialice;includingspillways, outwash plains,kames<br />
andraised deltas;evidence for ice collapse including<br />
slumping, kettlesandirregular topography.<br />
M Morainal deposits: massiveto stratified, matrix-supported<br />
diamicton, till;sub-rounded andfaceted boulders, cobbles and<br />
pebblesin aclay, siltandsand matrix;interstratified with<br />
minor gravel, sand andsilt;depositedeither directlyby<br />
lodgement, basal meltout; subglacial meltwater;or sediment<br />
gravity-flow processes,including debrisflowsassociated with<br />
ice melting; overlies drumlinised olderglacialnonglacial<br />
sedimentsand glaciallysculpted bedrock;may contain kettle<br />
depressions.<br />
G-3<br />
<strong>SURFICIAL</strong>MATERIALSANDTEXTURES MAPLEGEND<br />
Unit Parent material Soil texture Soil classification Soil Names Drainage<br />
and Types classification<br />
unit 7d marine sands and gravels Regosols, saline phases Coastal Beach, well- to poorly-drained<br />
mud, rock Regosols, saline phases Tidal Flats, Rocky Coast well to poor<br />
unit 7c lacustrine/organic humic Terric Humisol Metchosin very poorly-drained<br />
unit 7b fluvial gravely, sandy loam Gleyed Dystric Brunisol Brigantine imperfectly-drained<br />
unit 7a colluvial/till gravely sandy loam Orthic Dystric Brunisol Rumsley, Mount Newton moderately well-drained<br />
gravely sandy loam Duric Dystric Brunisol Shawnigan moderately well-drained<br />
unit 6d glaciomarine gravely loam and silty Duric Dystric Brunisol Dashwood moderately well-drained<br />
gravely sandy loam Duric Sombric Brunisol Langford moderately well-drained<br />
silt, clay loam Orthic Sombric Brunisol <strong>Saanich</strong>ton moderately well-drained<br />
silty clay loam Orthic Humic Gleysol Tolmie; Cowichan poorly-drained<br />
unit 6c glaciolacustrine/organichumic Terric Humisol Metchosin very poorly-drained<br />
unit 6b glaciofluvial gravely, loamy sand Orthic Dystric Brunisol Qualicum well-drained<br />
unit 6a proglacial colluvial/till gravely sandy loam Orthic Dystric Brunisol Rumsley, Mount Newton moderately well-drained<br />
gravely sandy loam Duric Dystric Brunisol Shawnigan moderately well-drained<br />
unit 5 moraine gravely sandy loam Duric Dystric Brunisol Shawnigan moderately well-drained<br />
unit 4 glaciofluvial gravely, loamy sand Orthic Dystric Brunisol Qualicum well-drained<br />
unit 3 fluvial, lacustrine, marineexposed in sea-cliff Orthic Regosol Steep slope well-drained<br />
unit 2 glaciomarine, moraine exposed in sea-cliff Orthic Regosol, ancient phase poorly-drained<br />
unit 1 fluvial, lacustrine, marineexposed in sea-cliff Orthic Regosol, ancient phase poorly-drained<br />
and wave-cut platform<br />
Surficial Material Textures<br />
(recorded on Map G-1 as lower case<br />
symbols)<br />
cz clays with silts<br />
sd mixedfragmentwithsands<br />
d diamicton, mixed fragments<br />
sg gravelswith sands<br />
dz silts with mixed fragments<br />
sz silts with sands<br />
hz silts with humic materials<br />
zc clays with silts<br />
zd mixedfragments withsilts<br />
zh humicmaterialswithsilts<br />
s sands<br />
Municipality of <strong>Central</strong> <strong>Saanich</strong> ResourceAtlas
<strong>SURFICIAL</strong> <strong>GEOLOGY</strong><br />
Legend Map G-1-Surficial MaterialsandTextures Map G-1.<br />
Map coloursindication the type of surficial material present:<br />
Colluvium<br />
Fluvial<br />
Morainal<br />
Organic<br />
Marine<br />
Glacio-marine<br />
GEOMORPHIC SYMBOLS<br />
trimline<br />
crag-and-tail<br />
drumlin<br />
flute<br />
striation<br />
erratic boulders<br />
meltwaterchannel<br />
(flow directionknown)<br />
C<br />
F<br />
M<br />
O<br />
W<br />
WG<br />
meltwaterchannel<br />
(flow directionuncertain)<br />
terrace<br />
gully<br />
mass-movement<br />
(flow directionknown)<br />
surface seepage, spring<br />
drownedtrees<br />
See previous page foradescription forthemap symbols.<br />
Note: SurficialmaterialsL-Lacustrine, LG -Glaciolacustrine, FG -<br />
Glaciofluvialare notdisplayedon thismapbutdo occur in the studyarea<br />
at some locationsbelowthe surface. The stratigraphiclocations of some<br />
of these depositsoccur inthe GroundwaterAquifer section onthe crosssections;andtheyare<br />
alsorecorded inthe Surficial GeologyAppendix..<br />
464 000 469 000<br />
Map G-1 Surficial GeologyMaterials of <strong>Central</strong> <strong>Saanich</strong>.<br />
1000 m 0m<br />
1000 m<br />
Scale 1:40000<br />
2000 m<br />
G-4<br />
<strong>SURFICIAL</strong>MATERIALSANDTEXTURESMAP<br />
464 000 469 000 474 000<br />
Map Notes: This map displaysthe materialswhich lie at the<br />
surface of the land. Thisis onlyone type of display derived<br />
fromthe data base. In many areas, layers orstratasof other<br />
materials occuratdepth such as materials which identify<br />
gravellyaquifers.<br />
Municipality of <strong>Central</strong> <strong>Saanich</strong> ResourceAtlas<br />
474 000
<strong>SURFICIAL</strong> <strong>GEOLOGY</strong><br />
QUATERNARYHISTORY<br />
In this section, the late Pleistocene and early Holocene history of the<br />
central<strong>Saanich</strong>Peninsulaisdiscussedinaregionalcontext.FiguresG-<br />
10 to G-16 show a conceptual model that depicts the changing<br />
palaeogeography of southeastern Vancouver Island, showing central<br />
<strong>Saanich</strong> Peninsula from ca. 19to 4ka (calendar years before present).<br />
During this geomorphically dynamic interval, <strong>Saanich</strong> Inlet evolved<br />
from aglacierised fjord basin into amarine inlet dominated by nonglacial<br />
processes. At the same time, significant changes in sediment<br />
delivery rates,sea-level, water temperature, geochemistry and ecology<br />
occurredonthepeninsula.<br />
Changes in ice margins, drainage patterns and shoreline geometry are<br />
inferredfromthedistributionandelevationrangeofmoraine,proglacial<br />
colluvium, glaciofluvial and glaciomarine landforms (Huntley et al. in<br />
press). Landform and sediment assemblagesin the area aresimilar to<br />
thoseobservedincontemporarypalaeoglacialenvironmentsandmodern<br />
fjords dominated by retreating tidewater glaciers. The chronology of<br />
major geomorphic events and sea-level changes is constrained by<br />
radiocarbon dates from late Pleistocene and Holocene sequences in<br />
<strong>Saanich</strong> Inlet, adjacent parts of southern Vancouver Island and the<br />
Pacific northwest (Mathews et al. 1970, Alley and Chatwin, 1979,<br />
HicockandArmstrong1985,Bornholdetal.1997,PorterandSwanson<br />
1998).<br />
Legendfor Quaternary<br />
History Diagrams<br />
Glacierice (with estimated<br />
ice thickness in metres)<br />
Deglaciated terrain<br />
Glaciofluvial outwash<br />
Sea and lake water<br />
Iceflow direction(determined<br />
from paleo-iceflow indicator)<br />
Meltwater flow<br />
(current direction known)<br />
Meltwater flow<br />
(current direction uncertain)<br />
Lateglacial-maximumphase(ca.19to17ka)<br />
Before 17 ka, central <strong>Saanich</strong> Peninsula was covered by an ice sheet<br />
flowing from montane sources on Vancouver Island and the adjacent<br />
BritishColumbiamainland.Bedrockandoldersedimentswereoverlain<br />
by subglacial morainal deposits consisting of lodgement till, melt-out<br />
till, and glaciofluvial sediments (Figure G-1). Streamlined bedforms<br />
indicate that during this phase, ice and subglacial meltwater-flow was<br />
directed south-to southeasttoward Juan de Fuca Strait and Haro Strait<br />
(Figure 2a). In the northwest, cirque headwalls above 1200 masl have<br />
been modified by frost-shatter and solifluxion. This relict periglacial<br />
terrain isinterpretedtorepresentaminimumlimitto glaciationpriorto<br />
the onset of deglaciation (Huntley et al. in press). Ice thicknesses over<br />
<strong>Saanich</strong> Peninsula at this time ranged from 800 to 1000 m. In <strong>Saanich</strong><br />
Inlet,icethicknesswasgreaterthan1500m(FigureG-10).<br />
G-5<br />
QUATERNARYHISTORY<br />
FigureG-10. Late glacial maximum phase ( ca. 19 to 15 ka).<br />
Figure G-11. Early deglacialphase ( ca.<br />
15to 14 ka).<br />
Earlyglacial-retreatphase(ca.16to15ka)<br />
Deglaciationbeganbefore16ka(PorterandSwanson1998).Mountains<br />
anduplandsweredeglaciatedearlyastheregionalequilibriumlinerose<br />
above the elevationof localcirquebasins.As aresult, remnant ice was<br />
confined to valleys and coastal lowlands (Figure 2b). In proglacial<br />
settings, exposed glacial debris was extensively remobilised by<br />
meltwaterandmass-movement.Meltwateranderodedsedimentdrained<br />
southeast across <strong>Saanich</strong> Inlet. Hydrologic continuity between ice-free<br />
areas was likely maintained by eskers, subglacial sheet-flow and<br />
supraglacial channels. Glacio-isostatically depressed lowlands were<br />
inundatedastheeustaticsea-levelrose.Asicethinned,tide-waterglacier<br />
margins partly floated and retreated to grounding lines in southern<br />
<strong>Saanich</strong>Inlet,SatelliteChannel,andover<strong>Saanich</strong>Peninsula(Huntleyet<br />
al. in press). Ice, meltwater and sediment drained to alarge calving<br />
embaymentformedinHaroStrait,southernStraitofGeorgiaandeastern<br />
JuandeFucaStrait(FigureG-11).<br />
Municipality of <strong>Central</strong> <strong>Saanich</strong> ResourceAtlas
<strong>SURFICIAL</strong> <strong>GEOLOGY</strong><br />
Figure G-12. Middle deglaciation ( ca. 14 to 13 ka).<br />
Figure G-13. Late deglacial phase ( ca. 13 to 12 ka).<br />
Middleglacial-retreatphase(ca.14to13ka)<br />
Alongvalleyfloors,ephemerallakeswereimpoundedbydebris-covered<br />
stagnant ice and retreat-phase glaciofluvial outwash. Meltwater and<br />
sediment were directed into interlobate basins formed along the lower<br />
reaches of Cowichan and Koksilah rivers, and an embayment<br />
encompassing<strong>Saanich</strong> Peninsula and <strong>Saanich</strong> Inlet(Figure G-12). Icecontact<br />
deltas graded to between 100 and 80 masl delimit stable<br />
recessional grounding-line positions for tide-water glaciers in <strong>Saanich</strong><br />
Inlet, Satellite Channel, Cowichan and Koksilah valleys, and over<br />
<strong>Saanich</strong>Peninsula.Below80masl,glaciomarinedebris-flows,sandand<br />
gravel outwash, and clay-silt rhythmites draped winnowed till and<br />
bedrock (Figure G-2). Raised glaciomarine deposits are correlative to<br />
earliestdeglacialsedimentsinODPcores1033and1034(Bornholdetal.<br />
1997). The elevation range of unit 6d delimits the extent of marine<br />
overlap attributed to glacio-isostatic depression and eustatic sea-level<br />
rise. The 80 mmarine transgression limit is probably equivalentto the<br />
post-14ka75mmarinelimitnearVictoria(Mathewsetal.1970,Clague<br />
etal.1982).<br />
Lateglacial-retreatphase(ca.13to12ka)<br />
By 13 ka, inland southeastern Vancouver Island was ice-free down to<br />
approximately400 masl (Alley and Chatwin 1979). In uplands and on<br />
valleysides,glacialdepositscontinuedtobereworkedbymeltwaterand<br />
mass-movement. Proglacial lakes drained as ice downwasted and<br />
outwash was degraded (Figure G-13). Extensional deformation<br />
structuresinglaciolacustrinesedimentsareattributedtochannelincision<br />
during rapid lake drawdown (Huntley et al. in press). Coarse outwash<br />
overlyingtruncateddeltaicandlakesedimentslikelyrepresentdeglacial<br />
flooddepositsrelatedtolakedrainage.<br />
G-6<br />
QUATERNARYHISTORY<br />
FigureG-14. Postglacial phase ( ca. 12 to 10 ka)<br />
Earlypost-glacialphase(ca.12to10ka)<br />
The flux of meltwater and sediment into <strong>Saanich</strong> Inlet decreased<br />
markedlyafter12kaasstagnant,debris-coveredvalleyicedisappeared<br />
(FigureG-14).SoutheastVancouverIslandwasdeglaciatedbefore11ka<br />
(AlleyandChatwin1979),anddeglaciatedslopesbecamevegetatedand<br />
stabilised. Plant macrofossils in early postglacial lake sediments<br />
indicate the presence of lodgepole pine and grass. This vegetation<br />
assemblage indicates relatively cool conditions (Hebda 1995). Raised<br />
wave-cutplatformsandstrandlinesindicatethatglacio-isostaticrebound<br />
continued after deglaciation, and that periodically, rate of rebound and<br />
sea-levelriseweresimilar.Between12and10ka,relativesea-levelwas<br />
closetothemoderndatum(Mathewsetal.1970,Clagueetal.1982),and<br />
theshore-lineof<strong>Saanich</strong>Inletwassimilartopresent(Figure2e).<br />
Municipality of <strong>Central</strong> <strong>Saanich</strong> ResourceAtlas
<strong>SURFICIAL</strong> <strong>GEOLOGY</strong><br />
Figure G-15. EarlyHolocene ( ca. 10 to 8ka)<br />
FigureG-16. Middle Holocene ( ca. 8to 4ka).<br />
EarlyHolocene(ca.10to8ka)<br />
Glacio-isostaticreboundcontinuedduringtheearlyHolocene(Clagueet<br />
al. 1982). During this interval, Pleistocene valley fill was intensively<br />
dissected by degrading postglacial streams as relative sea-level fell.<br />
UnconformitiesinearlyHoloceneandlatePleistocenemarinesediments<br />
and submerged terraces in Juan de Fuca Strait around 50 mbsl likely<br />
representaminimumelevationforamarinestandca.9to8ka(Clagueet<br />
al. 1982).As sea fellbelow presentdatum, the palaeogeography of the<br />
<strong>Saanich</strong>Inletbasin changedsignificantly (Figure G-15). Baysentering<br />
the inlet were reduced in size and the entrance to Satellite Channel<br />
became constricted. The land area of <strong>Saanich</strong> Peninsula increased to<br />
include parts of Cordova Channel and James Island. These changes<br />
probably had an impact on oceanographic conditions within <strong>Saanich</strong><br />
Inlet. Lake elevations were at their minimum stands during the early<br />
Holocene.Vegetationassemblages,dominatedby grass, alder, Douglas<br />
fir and oak, correspond to axerothermic interval observed in southern<br />
British Columbia and the Pacific Northwest (Hebda 1995, Pellattet al.<br />
1998). Charcoal in peat deposits indicates that southern <strong>Saanich</strong><br />
PeninsulawasdisturbedbyfireduringtheearlyHolocene.<br />
MiddleHolocene(ca.8to4ka)<br />
After 8ka, eustatic sea-level rise was dominant (Clague et al. 1982).<br />
EarlyHolocenecoastalslopesweredrownedanderodedbywaveaction<br />
astheseatransgressed.ThelowerreachesofChemainus,Cowichanand<br />
Koksilahvalleyswere aggradedasthemodernCowichandeltaformed.<br />
Lakes surrounding <strong>Saanich</strong> Inlet were slowly infilled with silt-rich<br />
gyttja. Tephra, inferred to be Mazama ash, forms adistinct regional<br />
markerhorizondatedtoaround7.6kaforcoresrecoveredfrom<strong>Saanich</strong><br />
Inlet and lakes in the region (Bornhold et al. 1997, Huntley et al. in<br />
press). After 7.6 ka, Douglas fir, western hemlock, red cedar became<br />
dominant, reflecting the transition to amild and wet climatein the late<br />
Holocene (Hebda 1995, Pellatt et al. 1998). Modern drainage and<br />
sedimentation patterns were established in the study area after 4ka as<br />
relativesea-levelapproacheditspresentelevation(Mathewsetal.1970,<br />
Clagueetal.1982,FigureG-16).<br />
G-7<br />
QUATERNARYHISTORY<br />
Table G-3. Relationship of glacial eventsthroughtime<br />
togenetic materials andfacies units*.<br />
Stratigraphicunit Faciesunit Genesis Age(ka)<br />
Holocene unit 7d Nonglacialmarine,estuarine, ca.11katopresent<br />
SalishSediments lagoonal andbeachdeposits<br />
unit 7c Lacustrinedeposits ca.11katopresent<br />
unit 7b Channel, floodplainand ca.11katopresent<br />
deltaic deposits<br />
unit 7a Nonglacialhillslopecolluvium, ca.11katopresent<br />
talus andother<br />
mass-movement deposits<br />
LateWisconsinan unit 6d Ice-contact submarine debris ca. 14to 11ka<br />
Fraser Glaciation flows,glaciomarinesuspension<br />
Capilano Sediments andraisedbeachdeposits<br />
Retreat phase<br />
unit 6c Subaerial ice-contact and ca. 14to 11ka<br />
proglaciallakedeposits<br />
unit 6b Ice-contact andproglacial ca. 14to 11ka<br />
channel, braidplain, kame<br />
terraceanddeltadeposits<br />
unit 6a Ice-contact andproglacial ca. 15to 11ka<br />
colluvium,talus andother<br />
mass-movement deposits<br />
VashonTill unit 5b Subglacialfluvialdeposits ca. 23to 11ka<br />
Glacialmaximum phase<br />
unit 5a Lodgementtill, locally ca. 23to 12ka<br />
streamlinedandmelt-out till<br />
QuadraSand unit 4c Ice-contact andproximal ca. 29to 15ka<br />
Advance phase hillslopecolluvium, talus and<br />
other mass-movement deposits<br />
unit 4b Ice-proximalglaciofluvialfan- ca. 23to 17ka<br />
apron andbraidplain deposits<br />
unit 4a Ice-distalglaciofluvialfan- ca. 29to 23ka<br />
apronandbraidplaindeposits<br />
Olympia NonglacialInterval unit 3 Nonglacialmarine,estuarine, ca. 65to 23ka<br />
to earlyFraser Glaciation lagoonal andfluvialdeposits<br />
Cowichan Head Formation<br />
DashwoodDrift marine deposits<br />
Sangamonian Interglaciation unit 1 Marine, alluvial, fluvial ~125ka<br />
MuirPointFormation<br />
unit 2<br />
andcolluvial deposits<br />
*Ages of surficial units are shown in radiocarbon yearsbefore<br />
present. These depositsare discussed in the Quaternary<br />
Stratigraphy and Geomorphology section,and withinthe<br />
GroundwaterAquifer section with respectto texturalgroups as<br />
aquifers .<br />
Municipality of <strong>Central</strong> <strong>Saanich</strong> ResourceAtlas
<strong>SURFICIAL</strong> <strong>GEOLOGY</strong><br />
REFERENCES<br />
Alley, N.F. and Chatwin, S.C. 1979. Late Pleistocene history and<br />
geomorphology, southwestern Vancouver Island, British Columbia.<br />
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Armstrong, J.E. and Clague, J.J. 1977. Two major Wisconsinan<br />
lithostratigraphicunitsinsouthwestBritishColumbia.CanadianJournal<br />
ofEarth Sciences,v.14,pp.1471-1480.<br />
Blais-StevensA.,Clague,J.J.Bobrowsky,P.T.andPatterson,R.T.1997.<br />
LateHolocenesedimentationin<strong>Saanich</strong>Inlet,BritishColumbia,andits<br />
paleoseismic implications. Canadian Journal of Earth Sciences, v.34,<br />
pp.1345-1357.<br />
Blyth,H.E.andRutter,N.W.1993.Quaternarygeologyofsoutheastern<br />
VancouverIslandandGulfIslands(92B/5,6,11,13and14).Ministryof<br />
Energy, Mines and Petroleum Resources, Geological Fieldwork 1992,<br />
Paper1993-1,pp.407-413.<br />
Bornhold,B.D.,FirthJ.V.etal.1997and1998.ProceedingsoftheOcean<br />
DrillingProgram.InitialReports,v.169S,51p.<br />
Clague, J.J. 1976. Quadra Sandand itsrelationto the lateWisconsinan<br />
glaciation of southwest British Columbia. Canadian Journal of Earth<br />
Sciences,v.13,pp.803-815.<br />
Clague, J.J. 1977.QuadraSand:astudyof thelate Pleistocenegeology<br />
and geomorphic history of coastal southwest British Columbia.<br />
GeologicalSurveyPaper77-17,28p.<br />
Clague, J.J.,Armstrong, J.E. and MathewsW.H. 1980.Advance of the<br />
Late Wisconsinan Cordilleran Ice Sheet in Southern British Columbia<br />
since22,000yrB.P.QuaternaryResearch,v.13,pp.322-326.<br />
Clague, J.J., Harper, J.R., Hebda, R.J. and Howes, D.E. 1982. Late<br />
Quaternarysea-levelandcrustalmovements, coastalBritishColumbia.<br />
CanadianJournalofEarthSciences.v.19,pp.597-618.<br />
Fyles, J.G.1963.Surficial geology of Horne Lake and Parksville mapareas,<br />
Vancouver Island, British Columbia. Geological Survey of<br />
Canada,Memoir318,142p.<br />
Halstead,E.C.1966.SurficialgeologyofDuncanandShawniganmapareas,<br />
British Columbia. Geological Survey of Canada, Paper 65-24<br />
(1:63,360scale).<br />
Hebda,R.J.1995.BritishColumbiavegetationandclimatehistorywith<br />
focuson6kaB.P.GéographiephysiqueetQuaternaire,49,pp.55-79.<br />
Halstead, E.C. 1968. The Cowichan Ice tongue, Vancouver Island.<br />
CanadianJournalofEarthSciences,v.5,pp.1409-1415.<br />
Hicock, S.R. andArmstrong J.E. 1983 Four Pleistocene formations in<br />
southwestern British Columbia: their implications for patterns of<br />
sedimentation of possible Sangamonian to early Wisconsinan age.<br />
CanadianJournalofEarthSciences,v.22,pp.339-346.<br />
Hicock, S.R. andArmstrongJ.E. 1985. Vashon Drift:definition for the<br />
formation in the Georgia Depression, southwest British Columbia.<br />
CanadianJournalofEarthSciences,v.22,pp.748-757.<br />
Holland, S.S. 1980. Landforms of British Columbia, aphysiographic<br />
outline. British Columbia Ministry of Energy, Mines and Petroleum<br />
Resources,Bulletin48,138p.<br />
Howes,D.E.andKenk,E.1997.TerrainclassificationsystemforBritish<br />
Columbia(secondedition).MinistryofEnvironment,LandsandParks,<br />
Manual10,90p.<br />
Huntley,D.H.,BobrowskyP.T.andClagueJ.J.OceanDrillingProgram<br />
Leg 169S: Surficial geology, stratigraphy and geomorphology of the<br />
<strong>Saanich</strong> Inlet area, southeastern Vancouver Island, British Columbia<br />
MarineGeology.Inpress.<br />
Jungen, J.R. 1985. Soils of southern Vancouver Island. Ministry of<br />
Environment,TechnicalReport17.<br />
Jungen, J.R., Sanborn, P.and Christie, P.J. 1985. Soils of southeast<br />
Vancouver Island: Duncan-Nanaimo area. Ministry of Environment,<br />
TechnicalReport14.<br />
Massey,N.W., Desjardins, P.J. and Grunsky, E.C. 1994. Geological<br />
compilation Vancouver Island (NTS 92 B,C,E,G,K, L and 102I).<br />
MinistryofEnergy,MinesandPetroleumResources,OpenFile1994-6<br />
(1:250,000-scale).<br />
Mathews,W.H.,Fyles,J.G.andNasmith,H.W.1970.Post-glacialcrustal<br />
movementsinsouthwesternBritishColumbiaandadjacentWashington<br />
State. CanadianJournalofEarthSciences,v.7,pp.690-702.<br />
Muller, J.E. 1980. Geology of Victoria. Geological Survey of Canada.<br />
Map1553A(1:100000scale).<br />
Pellatt, M.G., Mathewes R.W.and Hebda R.J. 1998. High resolution<br />
palynological record from <strong>Saanich</strong> Inlet, British Columbia: postglacial<br />
vegetation and climate history. Geological Association of<br />
Canada/MineralogicalAssociationofCanada,AbstractVolume,Quebec<br />
City,Quebec,A142.<br />
Porter, S.C. and Swanson, T.W. 1998. Radiocarbon age constraints of<br />
advanceandretreatofthePugetLobeoftheCordilleranIceSheetduring<br />
thelastglaciation.QuaternaryResearch,50,pp.205-213.<br />
G-8<br />
Municipality of <strong>Central</strong> <strong>Saanich</strong> ResourceAtlas<br />
REFERENCES