Quadra Sand and its relation to the late Wisconsin glaciation of southwest British Columbia Geological Survey of Canada, 100 West Pender Street, Vancouver, B.C., Canada V6B lR8 Received 19 December 1975 Revision accepted for publication 1 March 1976 Quadra Sand is a late Pleistocene lithostratigraphic unit with widespread distribution in the GeorgiaDepression, British Columbia and Puget Lowland, Washington. The unit consists mainly of horizontally and cross-stratified, well sorted sand. It is overlain by till deposited during the Fraser Glaciation and is underlain by fluvial and marine sediments deposited during the preceding nonglacial interval. Quadra Sand was deposited progressively down the axis of the Georgia-Puget Lowland from source areas in the Coast Mountains to the north and northeast. The unit is markedly diachron- ous; it is older than 29 000 radiocarbon years at the north end of the Strait of Georgia, but is younger than 15 000 years at the south end of Puget Sound. Aggradation of the unit occurred during the climatic deterioration at the beginning of the Fraser Glaciation. Thick, well sorted sand was deposited in part as distal outwash aprons at successive positions in front of, and perhaps along the margins of, glaciers advancing from the Coast Mountains into the Georgia-Puget Lowland during late Wisconsin time. The sand thus provides a minimum age for the initial climatic change accompanying the Fraser Glaciation. This change apparently occurred before 28 800 y BP, substantially earlier than glacial occupation of the southern Interior Plateau of British Columbia. Thus, several thousand years may have intervened between the alpine and ice-sheet phases of the Fraser Glaciation. Le Sable de Quadra est une unite lithostratigraphique de la fin du Pleistocene dont I'extension est vaste dans la depression de Georgie, en Colombie Britannique, et dans les basses-terres de Puget, dans l'etat de Washington. L'unite consiste principalement en un sable bien trie, a stratification horizontale on entre-croiske. Elle est surmontee par un till depose durant la Glacia- tion de la Fraser et repose sur des sediments fluviatiles et marins deposes durant I'intervalle non glaciaire precedent. For personal use only. Le Sable de Quadra fut depose progressivement le long de I'axe des basses-terres de Puget et de Gborgie; il provenait de la Chaine CBtiere situte au nord et au nord-est de cette region. L'unite manifeste un diachronisme marque: elle date de plus de 29 000 annees CI4 a I'extremite nord du Detroit de Georgie, mais est Bgee de moins de I5 000 ans a I'extremite sud de la baie de Puget. L'edification de I'unite se fit durant la deterioration climatique du debut de la Glaciation de la Fraser. Un sable epais, bien trie, fut depose, constituant les depBts distaux d'une plaine pro- glaciaire au front, et peut-Stre aux marges, des glaciers venant de la Chaine C6tikre qui s'avan~aientdans les basses-terres de Puget et de Georgie durant la fin du Wisconsin. Le sable fournit ainsi un ige minimum au changement climatique initial correlatif de la Glaciation de la Fraser. Ce changement debuta apparemment avant 28 800 ans AA, et donc nettement plus t8t que I'occupation glaciaire du sud du Plateau interieur de la Colombie Britan- nique. Ainsi plusieurs milliers d'annCes peuvent avoir separe la phase alpine de la Glaciation de la Fraser de la phase de glaciation continentale. [Traduit par le journal] Can. J. Earth Sci. 13,803-815 (1976) Introduction local lithostratigraphic units, from youngest to The Georgia Depression, an elongate sedimen- oldest: Salish Sediments, F'raser Glaciation drift,' and Cowichan Head Formation (Table I). Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Vancouver Island University on 11/06/12 tation basin now largely covered by sea water, is located between the Coast Mountains and the Older sediment units have been recognized at a Vancouver Island upland of British Columbia. A few localities (Fyles 1963; Armstrong 1975), variety of glacial and nonglacial deposits re- but their age and correlation are not well cording major Quaternary climatic oscillations occurs within the Georgia Depression and in the 'Drift deposited during the Fraser Glaciation may be subdivided into the lithostratigraphic units listed in contiguous Puget Lowland of Washington. Most Table 1. In this paper the drift is referred to informally as exposed sediments are assigned to the following Fraser Glaciation drift. 804 CAN. 3. EARTH SCI. VOL. 13, 1976 I TABLE1. Stratigraphic framework of surficial sediments in the Georgia Depression Stratigraphic unit Geologic-climatic interval Salish Sediments Postglacial Fraser Glaciation drift Fraser Glaciation Sumas Drift Capilano Sediments Fort Langley Formation Vashon Drift Quadra Sand Cowichan Head Formation Olympia nonglacial interval Semiahmoo and Dashwood Drifts* Salmon Springs Glaciation Highbury and Mapleguard Sediments** ? Westlynn Drift ? - - - *Semiahmoo Drift (in Fraser Lowland) probably correlates with Dashwood Drift (on Vancouver Island). **Relation between Highbury Sediments (in Fraser Lowland) and Mapleguard Sediments (on Vancouver Island) is unknown. established. The Cowichan Head Formation, on tion from nonglacial to glacial conditions at the the basis of numerous radiocarbon dates (sum- beginning of the Fraser Glaciation. Evidence for marized in Fulton 1971), is assigned to the the unit having been laid down as outwash is Wisconsin Stage; the Salish Sediments are of presented in this paper. On the basis of this Holocene age. evidence, Quadra Sand is considered to be part Drift deposited during the Fraser Glaciation of Fraser Glaciation drift. in the Georgia Depression consists of three The distribution and character of Ouadra. Sand parts: a lower unit of stratified sediments laid are presented below, and the significance of the down prior to glacier overriding; a middle unit unit in terms of the timing of the late Wisconsin (mainly till) deposited when the area was covered glacial advance in southwest British Columbia is by ice; and an upper complex of diamicton and discussed. For personal use only. outwash formed during deglaciation. Fraser Glaciation drift overlies fluvial, estua- Distribution and Character of Quadra Sand I rine, and marine sediments deposited during the Figure 1 shows those areas in the Georgia Olympia nonglacial interval and termed the Depression known to be underlain by Quadra Cowichan Head Formation. The Cowichan Sand. Also shown are areas where stratified Head Formation is divisible into a lower unit of gravel is exposed beneath till. This gravel, marine clay, silt, and sand, and an upper unit of variously termed the Saanichton gravel (Hal- plant-bearing silt, sand, and gravel. The upper stead 1968; Fulton and Halstead 1972) and the unit includes channel, overbank, and bog sedi- Colebrook gravel (Armstrong 1956, 1957), is ments deposited on flood plains, and possibly advance outwash of the Fraser Glaciation. lagoonal and littoral sediments of transitional Although limited in distribution to small areas marine-terrestrial environments (Fyles 1963). near Vancouver and north of Victoria, the Thick, well sorted sand with silt and gravel gravel is thought to be a coarse upper facies of interbeds underlies till and overlies the Cowichan Quadra Sand. Head Formation. This unit is called the Quadra Quadra Sand occurs mainly below an eleva- Sand and is widely distributed around the tion of 100 m on both sides of the Strait of Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Vancouver Island University on 11/06/12 Georgia Depression. The type section is at the Georgia and on islands within the strait. Here, south end of Quadra Island (50q00' N, 125"10f the unit has a maximum thickness of about 75 m. i W), where cross-bedded, well sorted sand up to It also crops out at higher elevations in a few 40 m thick is exposed beneath till. Sediments in upland valleys extending into the adjacent 1 the Puget Lowland that correlate with Quadra mountainous regions. In one such valley near Sand have been described by Mullineaux et al. Upper Campbell Lake (49'57' N, 125O36' W), (1965) and Easterbrook (1969) and termed the Quadra Sand occurs to an elevation of about Esperance Sand Member of the Vashon Drift. 380 m. Quadra Sand was deposited during the transi- In general, Quadra Sand overlies the upper -LEGEND - a Inferred d~strtbut~onol Quadm Sand and Cow~chanHead Formallon Oulcrops of Quadra Sand an valleys adjacent lo Georgla Depresslo" Outcrops 01 Saanachton and Colebroak gravel lacles ot Quadra Sand Outcrops 01 sub-1111 nonglactal sedlmenls ol unhnown relation lo Cowlchan Head Formallon Radiocarbon locallly, Quadra Sand or Cow~chanHead Formatton FIG. 1. Distribution of Quadra Sand in the Georgia Depression. Radiocarbon localities and dates are listed in Table 2. member of the Cowichan Head Formation, but sediment and by the topography of the surface For personal use only. locally it rests upon older marine and glacio- upon which the sand was deposited. marine sediments and bedrock. The contact be- In many areas Quadra Sand crops out down to tween Quadra Sand and the Cowichan Head sea level, and probably occurs below. For Formation is sharp, the two units being dis- example, on Hernando Island (Fig. 1) an ap- tinguishable on lithologic criteria. Upper Cowi- parently continuous Quadra sequence was chan Head sediments comprise horizontal beds logged in a well to an elevation of -25 m of silt, sand, and gravel, commonly oxidized to (Erdman and Brown 1969). reddish hues and containing abundant organic Examples of stratigraphic sections showing the matter; cross-bedding is rare; and sand beds relationships of Quadra Sand to bounding strata include large amounts of detritus eroded from are presented in Figs. 2 and 3. volcanic and sedimentary rocks. In contrast, The unit consists mainly of well sorted, fine Quadra Sand consists largely of well sorted, to coarse grained sand (Fig.