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Late Pleistocene stratigraphy of south-central British Columbia
Geological Survey of Canada, 601 Booth Street, Ottawa, Onr., Canada KIA OE8
AND
Department of Geology, Ohio University, Athens, OH 45701, U.S.A. Received December 20, 1977
Accepted February 16, 1978
The late Pleistocene deposits of south-central British Columbia record two major glacial and two major nonglacial periods of deposition. The oldest recognized Pleistocene deposits, called Westwold Sediments, were deposited during a nonglacial interval more than 60000 years ago. Little information is available on the climate of this period, but permafrost may have been present at one time during final stages of deposition of Westwold Sediments. The latter part of this nonglacial period is probably correlative with the early Wisconsin Substage of the Great Lakes - St. Lawrence Valley area. However, deposition of the Westwold Sediments may have begun during the Sangamon Interglacial.
Okanagan Centre Drift is the name applied to sediments deposited during the glaciation that followed deposition of Westwold Sediments. Okanagan Centre Drift is known to be older than 43 800 years BP and probably is older than 51 000. It is considered to correlate with an early Wisconsin glacial period.
Bessette Sediments were deposited during the last major nonglacial period, which in south- central British Columbia persisted from at least 43 800 years BP (possibly more than 51 000) to about 19000 years BP. This episode corresponds to Olympia Interglaciation of the Pacific Coast region and the mid-Wisconsin Substage of the Great Lakes - St. Lawrence Valley area. During parts of Olympia Interglaciation the climate was probably as warm as the present-day climate in the interior of British Columbia. Information from coastal regions indicates that there may have been periods of cooler and moister climate.
Kamloops Lake Drift was deposited during the last major glaciation of south-central British Columbia. Ice occupied lowland areas from approximately 19000 to 10000 years BP. This period corresponds approximately to the Fraser Glaciation of the Pacific Coast region and the late Wisconsin Substage of central and eastern parts of North America.
Les dep8ts de la fin du Pleistocene dans le centre sud de la Colombie-Britannique enregistrent deux periodes majeurs de dep8t glaciaire et deux periodes majeures de depdt non glaciaire. Les depdts les plus anciens du Pleistocene, les sediments de Westwold, se sont deposes durant un intervalle non glaciaire il y a plus de 60000 ans. I1 y a peu d'information disponible sur le climat a cette periode, mais le pergelisol a pu 2tre present durant un certain temps au cours des derniers stades de dep8t des sediments de Westwold. La derniere partie de cette phase non glaciaire est probablement correlative des dep6ts du Wisconsin inferieur de la region des Grands-Lacs et de la vallee du Saint-Laurent. Toutefois, le dep8t des sediments de Westwold peut avoir debute durant l'interglaciaire du Sangamon.
Le drift de I'Okanagan Centre est le nom qu'on donne aux sediments deposes durant la glaciation qui a suivi le dep8t des sediments de Westwold. Ce drift est plus ancien que 43 800 ans avant le present et peut-2tre plus ancien que 51 000 ans. On considere qu'il est correlatif avec la p6riode glaciaire du debut du Wisconsin.
Les sedimentsde Bessette se sont deposes durant laderniere periode non glaciaire majeure qui a persiste dans le centre sud de la Colombie-Britannique entre 43 800 ans avant le present (possiblement 51 000 ans) et il y a environ 19000 ans. Cet episode correspond a I'interglaciaire d'olympia de la region de la Cbte du Pacifique et au Wisconsin moyen de la region des Grands- Lacs et de la vallee du Saint-Laurent. Durant des intervalles de l'interglaciaire d'olympia, le climat a ete probablement aussi doux qu'actuellement a I'interieur de la Colombie-Britannique. L'information provenant des regions cbtieres indique qu'il apu y avoir des p6riodes declimat plus froid et plus humide.
Le drift de Kamloops Lake s'est &pose durant laderniere glaciation majeure du centre sud de Colombie-Britannique. La glace a occupe les basses-terres d'environ 19000 a 10000 ans avant le present. Cette periode correspond approximativement a la glaciation de Fraser dans la region de la C8te du Pacifique e t a la fin du Wisconsin dans le centre et I'est de 1'Amerique du Nord.
[Traduit par le journal]
Can. J . Earth Sci., 15,971-980(1978)
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972 CAN. J . EARTH SCI. VOL. 15, 1978
Introduction The chronology of geologic events for southern
British Columbia during the last 52000 years has been presented by Fulton (1971). Fyles (1963) has provided the stratigraphic framework for the Quaternary deposits of southwestern British Col- umbia and this scheme was further refined and in part redefined by Armstrong and Clague (1977). Armstrong and Hicock (1975) provide a strati- graphic subdivision for deposits of the Lower Fraser Valley. The present paper proposes a sub- division of the late Pleistocene glacial and nongla- cia1 deposits of the south-central part of the pro- vince (Fig. 1).
The late Pleistocene deposits of south-central British Columbia record two glacial and two non- glacial periods that span Wisconsin(an) time, and may include a portion of Sangamon time (as defined by Frye et al. (1965), p. 52). The stratigraphic ter- minology employed herein is that of Fyles (1963, p. 5) who applied the term "drift" to a group of de- posits recording a glaciation and the term "sedi- ments" to the deposits recording a nonglacial event. The stratigraphic units defined in this report are Westwold Sediments, Okanagan Centre Drift, Bessette Sediments, and Kamloops Lake Drift. Some of this terminology has been used (Fulton 1975, 1976) but the earlier reports did not include definitions of the lithostratigraphic units. Each drift unit and each sediments unit consists of several distinctive lithologic subdivisions each of which may ultimately be treated as 'formations'. How- ever, the work to date has not been of sufficient detail to permit setting up formal units at a more refined level.
Westwold Sediments The oldest sediments that can presently be
placed in a meaningful stratigraphic context are named Westwold Sediments after the type locality, which is located 1.5 km (1 mile) west of Westwold (Fig. 1). At the type locality, the unit consists of 1.8 m (6 ft) of marl, sand, silt, and clay that overlie 16.5 m (55 ft) of cross-stratified gravelly sand (Fig. 2).
The gravelly sand at the base of the unit is well sorted. The lower part of the sand contains large planar crossbeds, whereas the upper part is pre- dominantly horizontally stratified. Wedge-shaped disruptions in the upper 3-4 m (lo- 13 ft) of the sand may be ice-wedge casts. If this is the case, then the wedges were present at the time of sand deposition, because the cast tops are truncated at different levels and none extend into the overlying unit. If
these features are fossil ice-wedge casts, then at least the top part of the gravelly sand was deposited under permafrost conditions.
The complex unit overlying the gravelly sand consists of a basal poorly-sorted coarse-grained sand, which is overlain by a 0.6 m (2 ft) bed of marl containing fragments of tiny mollusk shells and im- pressions of plant material, a 0.3 m (1 ft) bed of clay and silt containing finely disseminated organic material, and an upper 20cm (8in.) layer of oxidized pebbly sand that contains vertebrate and insect remains. The marl and the clay are consi- dered to! have been deposited in a shallow pond. Organic remains from the oxidized sand have been identified by C. R. Harington, National Museums of Canada, as Bison sp. beetle, fish, and rodent (fragments are too small for more specific iden- tification). Because this thin bed contains a con- centration of bones, it seems likely that it marks an unconformity, and that the sand is a lag deposit that accumulated following disappearance of the pond. The fact that the wedge-shaped disruptions thought to be fossil ice wedges do not continue upward from the underlying unit might suggest that the climate during existence of the pond was warmer than dur- ing later stages of deposition of the underlying sand.
The base of the Westwold Sediments has thus far not been seen in exposed section, and drill informa- tion is not available to indicate the nature of the underlying material. The unit above the Westwold Sediments consists of 10.5 m (35 ft) of horizontally stratified silt, with minor clay and interbedded sand. Grain size generally increases upwards in this unit with the top part consisting dominantly of strat- ified sand and silt. Near the top of this succession, the silt is brecciated and sand extends into the fractures between silt blocks. The lack of discerni- ble organic material and the uniform grey colour might indicate that this material was deposited in a proglacial lake, and that the sand near the top of the unit could have been deposited as ice approached this locality. Therefore, this silt has been correlated with the lower stratified unit of Okanagan Centre Drift. Distribution
The one other locality in the study area where nonglacial sediments underlie Okanagan Centre Drift is at Okanagan Centre where 36m (120ft) of sand and gravel thought to be the same age as Westwold Sediments are exposed. Westwold Sed- iments may be present elsewhere, but statigraphic evidence necessary for their identification is not available.
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FULTON AND SMITH
1( rmnlorrm L m h m Or411
B a l l lmt t . g m d 1 m m m 1 1
0 O k P m P q m m Cmmlra D r i l l
W +m.trmtd S m a l n m n r l
o l a t o
FIG. 1 . Location map, south-central British Columbia. Radiocarbon dates and numbers mentioned in the text are shown in the boxes.
Age and Correlation The absolute age of Westwold Sediments is not
known because they pre-date the limits of radiocarbon dating. The overlying glacial deposits (Okanagan Centre Drift) are known to be older than
: 43 800 years BP and are probably older than 51 000 years BP.
Westwold Sediments do not contain unique time marker beds, and detailed paleoecological studies have not yet been carried out. Therefore, the corre- lation with deposits in other areas may only be based on relative stratigraphic position (see Table
I
1). On this basis, Westwold Sediments are corre- lated tentatively with Mapleguard Sediments of Vancouver Island (Fyles 1963, p. 15) and Highbury Sediments of the Lower Fraser Valley (Armstrong and Hicock 1975), as each underlies the till that underlies sediments of Olympia Interglacial age. If the overlying Okanagan Centre Drift is equivalent to Salmon Springs I1 of the Pacific northwest of the United States (Crandell 1965, p. 345) then Westwold Sediments would correlate with the Salmon Springs Interstadial deposits. If Okanagan Centre Drift correlates with both Salmon Springs I
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974 CAN. J . EARTH SC :I. VOL. 15, 1978
m c o v e r
1 B o u l d e r y g r a v e l , 6 1 p o o r l y w a s h e d a n d I I p o o r l y s o r t e d . I 3 m 7 D a r k g r e y sandy t i l l . I
S a n d , s i l t , c l a y a n d m a r l c o n t a i n i n g
p l a n t r e m o i n s o n d v e r t e b r a t e b o n e s .
10.Sm
S a n d , p e b b l y s a n d a n d s a n d y g r a v e l .
16.5m
b a s e o f e x p o s u r e
L a m i n a t e d s i l t a n d c l a y w i t h m i n o r i n t e r b e d d e d s a n d -
FIG. 2. Section exposed 1.5 km west of Westwold, British Columbia. Type section of Westwold Sediments.
and 11, then Westwold Sediments are equivalent to Puyallup Interglacial deposits (Crandell 1965, p. 443). If the overlying Okanagan Centre Drift is early Wisconsin in age, as is suggested below, then Westwold Sediments would be equivalent to San- gamon Interglacial deposits. The occurrence of f a - tures indicative of permafrost near but not at the top of Westwold Sediments suggests that deposi- tion of this unit continued in early Wisconsin time as is shown in Table 1.
Okanagan Centre Drift The till and associated deposits that overlie
Westwold Sediments and underlie the nonglacial Bessette Sediments are here named Okanagan Centre Drift. The type locality is an exposure on the east side of Okanagan Lake 32 km (20 mi) south of Vernon (Fig. 1). At the type section, the Okana- gan Centre Drift consists of 7.2m (24ft) of till, overlying 46 m (154 ft) of cobbly and bouldery gravel and overlain by 6.8m (22ft) of laminated pebbly silt and gravel (Fig. 3).
The lower stratified unit of Okanagan Centre Drift consists of poorly sorted and poorly stratified cobbly and bouldery gravel, which is considered to
be a deposit of advance outwash. The till (unstrat- ified unit of the Okanagan Centre Drift) is a light grey sandy, silty diamicton. The high content of sand and gravel in the till suggests that it was de- rived in large part from the underlying stratified materials. The upper unit of the drift consists of 5.5 m (18 ft) of laminated dark grey silt with 0.5 m (1.6ft) of gravel at the base and 0.8m (2.6ft) of gravel at the top. The laminated silt is considered to be a glacial lake deposit. The gravel at the base of the silt may be glaciofluvial, whereas the gravel at the top is interpreted as a beach deposit formed during drainage of the lake.
At the type section, Okanagan Centre Drift is underlain by 36m (120ft) of sand and gravel (lower 32 m not illustrated in Fig. 3) tentatively assigned to the Westwold Sediments. Approximately 1.5 m (5ft) of massive sandy silt containing tephra (Westgate and Fulton 1975) and one or more paleosols overlies the drift. Mineralogical proper- ties of the tephra permit correlation of this unit with the Bessette Sediments.
Distribution Deposits correlated with the Okanagan Centre
Drift occur at several localities within the area of study. The correlations, however, are based on stratigraphic position rather than distinctive physi- cal properties of the drift units. Till, overlying laminated silt and underlying bouldery, poorly sorted gravel at Westwold (Fig. 2), is considered to be correlative with Okanagan Centre Drift. Col- luvium, eolian sediment, and floodplain deposits deposited during the Olympia Interglaciation in the fircell Trench (Fulton 1968) overlie tiil correlated with Okanagan CentFe Drift. Till, tentatively as- signed to the Okanagan Centre Drift, is also ex- posed on Peterson Creek at Kamloops (Armstrong and Fulton 1965, stop 12-1) and on Heffley Creek 21 km (13 mi) north of Kamloops. Gravels and laminated silt and clay considered to also be of a glacial origin overlie the till at Peterson Creek and are in turn overlain by nonglacial sediments of probable Olympia Interglacial age. The till at Heffley Creek is overlain by gravel that is overlain by sand of probable Olympia Interglacial age. Silty till, as much as 43 m (140ft) thick beneath gravel, sand, silt, and till at the mouth of Bonaparte River (Armstrong and Fulton 1965, stop 12-3), is corre- lated with the nonstratified unit of the Okanagan Centre Drift. This till locally overlies silt that may be equivalent to the lower stratified unit at Okana- gan Centre and is truncated by horizontally strat- ified coarse gravel that is probably of Olympia In- terglacial age.
Deposits considered to be equivalent to the
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976 CAN. J. EARTH SCI. VOL. 15, 1978
l e n s e s o f g r o v e l a n d s i l t
G r o v e l o n d s a n d , i n t e r b e d d e d
S o n d , m o s s i v e S i l t sond m o s s i v e o x l d z e d . t k p h r a
Lominated and pebbl) S ~ l t , g rove l
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P e b b l y a n d c o b b l y 1 4 . 4 + ~ g r o v e l , h o r i z o n t o l
s t r o t i f i c o t i o n
FIG. 3. Section exposed 32 km south of Vernon near Okana. gan Centre. Type section of Okanagan Centre Drift.
that of the younger stadial. Okanagan Centre Drift would correlate with one or more early Altonian units (Frye et al. 1965, p. 51) and with Guildwood Stadia1 deposits of southern Ontario and Quebec (Dreimanis and Karrow 1972, p. 8).
Bessette Sediments Bessette Sediments are the nonglacial deposits of
south-central British Columbia that overlie Okana- gan Centre Drift and underlie deposits of the last glaciation. The type section is exposed at Riggins Road on Bessette Creek 13 km (8 mi) east of Lumby (Smith 1969). An exposure at Duncan Dam borrow pit in Purcell Trench (Fulton 1968) is designated as a reference section. Correlation of the two sections has been established by tephra chronology (Westgate and Fulton 1975). At the type section Bessette Sediments consist of 22 m (74ft) of inter- bedded silt, sand, and gravel containing plant re- mains (Fig. 4). Two tephra units occur at about 11.5 m (37 ft) and 16 m (53 ft) below the top of the nonglacial sequence (Westgate and Fulton 1975). No information is presently available on the total thickness of this succession or on the material un- derlying it, because the bottom of the exposure is at
upper stratified unit of the Okanagan Centre Drift stream level. Laminated grey silt overlies Bessette Sediments in the type section. are exposed at Merritt (Armstrong and Fulton 1965, Two successions of Bessette Sediments are ex-
stop 11-31 and at Mission Flats 6 km (4 mi) west of posed in the reference section. One consists of in- Kamloops (Fulton and Halstead 1972, stop 5-7). At both localities, the deposits consist of grey lami- terbedded silt, sand, and gravel containing at least
nated silt or clayey silt interpreted as glacial lake one volcanic ash, plant remains, and peat. The other succession consists of colluvial sediments sediments. ~
and wind deposited siltv sand, containing organic Age and Correlation
Nonglacial sediments overlying Okanagan Centre Drift in the Purcell Trench were deposited more than 43 800 years ago (Fulton 1968, p. 1078). This date and the position of the sediments in the Quaternary succession suggest correlation with the Dashwood Drift of Vancouver Island (Table 1; Fyles 1963, p. 17). Because it appears that the material overlying Dashwood Drift was deposited more than 51 000 years ago (J. G. Fyles, personal communication, 1972; Fulton 1971, p. 8), Okana- gan Centre Drift is probably also at least this old.
Stratigraphic succession and radiocarbon age of overlying materials would indicate correlation of Okanagan Centre Drift with the Semiahmoo Drift of the Fraser Lowland (Armstrong and Hicock 1975) and Salmon Springs Glacial deposits of the Puget Lowland (Crandell 1965, p. 344). However, the Salmon Springs Glaciation is subdivided into two glacial episodes separated by an interstadial, and it is not known whether Okanagan Centre de- posits represent all of Salmon Springs time or only
material and-well-develbped soil horizons (Fulton 1968). The colluvium-loess succession, which de- veloped on a slope, is underlain by, and in part derived from, Okanagan Centre Drift. It inter- tongues with, and is overlain by, the silt, sand, and gravel succession. The silt, sand, and gravel succession is overlain by till.
Bessette Sediments at both the type and refer- ence sections appear to have been deposited under conditions similar to those prevailing in the area today. At both localities modern analogues exist. Sand and gravel are being deposited in the channels of small streams, fine sediments and organic mate- rial are being deposited on floodplains, and soil and colluvium are developing on the slopes adjacent to the stream floodplains.
Organic material in the Bessette Sediments pro- vide finite radiocarbon dates. The oldest date for organic material from this unit is 43 800 t 800 years (GSC-740, Fulton 1968, Fig. 3). The youngest date is 19 100 k 240 years (GSC-913) from the type Bes- sette section (Fig. 4).
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FULTON AND SMITH 977
S i l t , l a m i n a t e d
~ C h e r r y v i l l e T e p h r o
~ G S C - 1 9 4 5 ~ 2 5 400 f 2 7 0 1 -.-. :.:* . ,- . . , , , ,:. C - G S C - 1 9 5 3 ; 2 5 300 f 3 2 0 1
. R i g g i n s R o a d T e p h r a
- 4 ~ s ~ - 1 9 3 8 ; 31 1 0 0 % 4 8 0 1
- 4 ~ ~ ~ - 2 0 3 1 ~ 3 1 2 0 0 + 9001
S t r e a m l e v e l
FIG. 4. Section exposed at Riggins Road on Bessette Creek, 13 km east of Lumby. Type section of Bessette Sediments.
Distribution Dated deposits of Bessette Sediments in south-
central British Columbia are located at the follow- ing localities: Celista on the north shore of Shuswap Lake; Gardom Lake, 13 km (8 mi) south of Salmon Arm; Rialto Creek, 6 km (4 mi) west of Castlegar; Rutland, 3 km (2mi) east of Kelowna; and Boat Encampment at the junction of Canoe and Colum- bia Rivers. Study of tephra deposits in south- central British Columbia (Westgate and Fulton 1975) indicates that several additional successions are correlative with Bessette Sediments. Bessette Sediments correlated on the basis of petrographi- cally similar tephra occur at Sweetsbridge, 34 km (21 mi) south of Salmon Arm; Mission Flats, 6 km (4 mi) west of Kamloops; Okanagan Centre, 32 km (20mi) south of Vernon; and Cherryville, 45km
(28 mi) east of Vernon. Deposits thought to be Bes- sette Sediments are exposed at many other localities, but cannot be correlated with the same degree of certainty, because they lack datable or- ganic material and correlatable tephra.
Age, Climate, and Correlation Bessette Sediments are time equivalent to Olym-
pia Interglacial deposits of the Lower Fraser Valley -western Washington area (Armstrong et al. 1965). Macrofossils from Bessette Sediments in south- central British Columbia suggest that at times dur- ing deposition of Bessette Sediments, the climate may have been similar to present climate. How- ever, information from coastal British Columbia and Washington State suggests that Olympia In- terglacial time was cooler and more moist than present (Fyles 1963, p. 37; Easterbrook 1969; Han- sen and Easterbrook 1974). Heusser (1972, p. 199) indicates that on the west coast of the Olympic Peninsula there was considerable climatic variation during Olympia Interglacial time, with temperature being, in general, slightly cooler than present, oc- casionally cooling nearly to the minimum reached during the maximum of Fraser Glaciation, and at times warming to present levels (Heusser 1973, p. 301). Climatic conditions further north in British Columbia are for the most part unknown, but organic-rich sediments radiocarbon dated at 43 800 and 34000 years from the centre of the province at 55"N latitude indicate nonglacial deposition at this time (Harington et al. 1974).
The period of deposition of Bessette Sediments is equivalent to most of the Altonian Substage, all of the Farmdalian Substage and the early part of the Woodfordian Substage (Table I ; Frye et al. 1965, p. 51). The Bessette Sediments are time equivalents of Port Talbot Interstade, Cherrytree Stade, and Plum Point Interstade deposits.
Kamloops Lake Drift Kamloops Lake Drift (Figs. 5, 6) is the name
applied to the stratified and nonstratified deposits that overlie Bessette Sediments and underlie post- glacial deposits or the present erosion surface. This drift consists of three units: a lower unit of stratified silt, sand, and gravel; a middle unit of till; and an upper unit of stratified silt, sand, and gravel. The complete drift sequence is not well exposed in a single section. However, all units are well exposed in the vicinity of Kamloops Lake, which is desig- nated as the type area.
Lower Stratijied Unit The lower stratified unit consists of laminated,
locally rhythmically-bedded, silt and clay and
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l.Oml Sondy g r o v e l ond p e b b l y s a n d I
S i l t a n d c l a y , r y t h m i c 13-5m b e d d i n g c r u m p l e d
a n d d i s r u p t e d
- -. - - -. - 6 . 0 m S i l t a n d c l o y , I------I I r y t h m i c b e d d i n g
0 . . .. * . - . I G r o v e l , w e l l s o r t e d a . m . .
a - * - * 22.5m h o r ~ z o n t a l s t ro t i f~ca t im e m - .: minor s a n d a n d s i l t y . . a *.. s a n d l e n s e s 1 . ... - * r . * . . - .* .* . .- * .. .. *I.. a
....a .. . . e m . * . ma.... 1 bose o f e x p o s u r e
FIG. 5. Section of Kamloops Lake Drift as seen in an exposure at the mouth of Cherry Creek.
fine-grained ripple-laminated sand and pebbly sand and bouldery gravel. This unit is well exposed at the top of several gravel pits in the southwestern portion of the city of Kamloops, at the top of expo- sures near the mouth of Cherry Creek on Kamloops Lake about 23 km (14 mi) west of Kamloops, in the South Thompson River Valley between Neds Creek and Monte Creek, 35 km (22 mi) east of Kam- loops, and in the valleys of Paul and Heffley Creeks, 3 and 21 km (2 and 13 mi) north of Kam- loops. The thickness of silt and fine sand is vari- able, and in many places either the bottom or top of the unit is not exposed. In the South Thompson River Valley and west of Kamloops, the unit ex- ceeds 45 m (150ft) in thickness. On Peterson Creek, in Kamloops, the lower stratified unit consists of 23 m (76ft) of bouldery gravel that is poorly strat- ified and locally contains silt-enclosed pebbles and a silty matrix (Fig. 6). This is considered to be a local facies variation, but gravel interbeds do also occur locally in other areas at the base and top of the finer-grained facies of this unit.
SCI. VOL. 15, 1978
,.= - I- v LL -= - .- a Z 0
W - Y -
5 20 0 -
m 0 a - 0 z 0 - Z - a * Y 30
3 0 -I
BESSETTE SEDIMENTS
4 0
FIG. 6. Section exposing Kamloops Lake Drift on Peterson Creek in Kamloops.
S i l t y sond, massive
Sand and s i l t , i n t e r b e d d e d , f a u l t e d , c o n t o r t e d
T i l l , s a n d y , i n p a r t s t r a t i f i e d
G r o v e l , b o u l d e r y , p o o r l y s t r a t i f i e d , poor ly w a s h e d
S o n d , s i l t a n d g r a v e 1 , r u s t y c o l o r e d
The lower stratified Kamloops Lake Drift over- lies Bessette Sediments. In many places the two units are conformable, and the contact between the two is difficult to define. However, sand and silt containing traces of organic material, well-sorted, and well-stratified gravels and oxidized sands, silts, and gravels that underlie unoxidized units are con- sidered to be Bessette Sediments. The overlying materials without these features are referred to as Kamloops Lake Drift. The lower stratified Kam- loops Lake Drift is generally overlain by till (un- stratified Kamloops Lake Drift), but in several places, such as the South Thompson Valley be- tween Neds and Monte Creeks, it occurs as the surface material.
The lower stratified Kamloops Lake Drift is the material that was deposited during the last major ice advance. It would appear that most deposition took place in lakes impounded either by glaciers or by the trains of outwash sediments derived from advancing ice, or possibly by isostatic downwarp- ing due to the build-up of ice. The depositional sequence varies from place to place. In the area around Kamloops and at Merritt these sediments are mainly silt and fine-grained sand, and it appears that the supply of sediment decreases as the ice advanced. East of Lumby, at the type section of Bessette Sediments, however, initial sedimenta- tion of fine-grained lacustrine materials gave way to
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FULTON AND SMITH 979
deposition of sand that in turn gave way to deposi- glacial lakes that developed as the ice retreated. tion of gravel as the ice approached (Fig. 4). The gravel and sand were deposited in the form of
Unstrat$ed Kamloops Lake Drift glacial outwash and ice contact deposits and along
The middle unit of Kamloops Lake Drift is till. spillways leading from one glacial lake to another.
Texture of the till varies from place to place, but is generally silty and sandy. Till pebble composition and till texture are in many places related to local bedrock, but withinand on the southor east sides of large valleys it reflects the texture and composition of the sediments that occupied the valley at the time of ice advance (Fulton 1975). Typical till thickness is from 1 to 5 m (3- 15 ft), but in local sites the till is as much as 15 m (50ft) thick.
Kamloops Lake till is the surface unit over much of south-central British Columbia. In many areas it occurs as a blanket or veneer that fails to mask the gross topographic forms of the underlying bedrock. Drumlinoid forms are the most common deposi- tional landforms developed in till, but locally it has been shaped into minor ridges and hummocks. Most till in the area is considered to be lodgement till, but locally a thin ablation mantle is present, and in some valleys till may have been formed in part by deposition from floating ice. Locally this unit is overlain by upper stratified Kamloops Lake Drift and by postglacial sediments.
Upper Stratijied Kamloops Lake Drift Upper stratified Kamloops Lake Drift, consist-
ing of silt, sand, and gravel, generally lies at the present surface and overlies unstratified drift. The most common facies of this deposit is glaciolacus- trine silt that occurs in many of the valleys of the southern Interior Plateau. Widespread deposits oc- cupy the South Thompson River Valley east of Kamloops where silt and minor clay occur as rhythmites up to 3 m (loft) thick and comprise de- posits more than 90 m (300ft) thick (Fulton 1965). The sand and gravel, or glaciofluvial, facies of upper stratified Kamloops Lake Drift occurs as hummocky and kettled sand 1.5 km (1 mi) south of Kamloops, as kettled and nonkettled terraces of pebbly sand and gravel at the mouth of Campbell Creek, 16km (1Omi) east of Kamloops, as a large ridge of sand and pebbly sand at Barnhart Vale, 11 km (7 mi) east of Kamloops, and as a myriad of small ridges, hummocks, and terraces of gravel and sand on the uplands north and south of Kamloops. In the areally small deposits, the gravel and sand is rarely more than 4.5 m (15 ft) thick, but in the larger delta terraces and in ridges it may be more than 30 m (1OOft) thick.
Upper stratified Kamloops Lake Drift is material that was deposited during deglaciation of the area. The finer-grained facies accumulated in the many
Distribution Kamloops Lake Drift is found throughout the
area. The lower stratified unit has an irregular dis- tribution throughout the area, but is found in most large valleys, and occurs in many other areas where materials deposited during ponding, caused by the ice advance, were not removed by later glacial ero- sion. Till is widely distributed and forms the sur- face material throughout much of the area but is difficult to locate in some large valleys because it either has been buried by later deposits or may never have been deposited. The fine-grained facies of the upper stratified Kamloops Lake Drift is most extensive where large volumes of sediment were supplied to glacial lakes. Some of the areas of major deposition are: South Thompson Valley east of Kamloops, Nicola River Valley at Merritt, Okana- gan Valley north and south of Armstrong and in the vicinity of Penticton, and the Shuswap Lake area between Tappin and Blind Bay. Other parts of the same valleys that were not near a sediment source or were occupied by ice at the time of maximum sediment influx may contain little or no glacial lacustrine sediment. The coarse-grained facies of the upper stratified Kamloops Lake Drift is widely distributed throughout the area, occurring most ex- tensively where meltwater flow was concentrated at the margin of the ice or where large streams discharged onto ice tongues or into lakes.
Age and Correlation Kamloops Lake Drift is the material deposited in
south-central British Columbia during the last major advance and retreat of glaciers. The only radiocarbon date directly related to this period is 19 100 k 240 BP (GSC-913, Fig. 4), which is from the base of the lower stratified Kamloops Lake Drift unit at the type locality of the Bessette Sedi- ments. This date indicates that complete ice cover of south-central British Columbia did not occur until after 19 000 BP. Dates of 21 500 k 300 (GSC- 173) and 21 700 + 240 years (GSC-1258) on wood from lacustrine sediments in the Columbia Moun- tains at the junction of the Canoe and Columbia Rivers suggest that extensive ice build-up had not begun in the valleys of south-central British Col- umbia prior to 21 000 years BP. A build-up of ice could, however, have begun at high elevations prior to this time. The oldest postglacial date from south-central British Columbia is 11 000 k 180 years BP(GSC-909), obtained on marl at the base of
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980 CAN. .I. EARTH SCI. VOL. 15. 1978
a bog deposit in Arrow Lakes valley 32 km (20mi) northwest of Castlegar. Dates of 10270 + 190 years BP (GSC-719) for basal peat from a bog in Kootenay Lake Valley 96 km (60mi) north of Cres- ton and of 10500 k 170 years BP (GSC-1524) for basal peat from a bog 48 km (30 mi) east of Kam- loops indicate that deglaciation was underway throughout south-central British Columbia 11 000 years ago, and was probably complete before 10 000 years ago.
These dates relate deposition of Kamloops Lake Drift to the time of Fraser Glaciation (Armstrong et al. 1965). However, there is presently no evidence that stades and interstades of the Fraser Glaciation on the West Coast can be recognized in south- central British Columbia. As is shown on Table 1, the time of Kamloops Lake Drift deposition cor- responds to the late Wisconsin, Nissouri, Erie, Bruce Point, Mackinaw, Port Huron, and North Bay stadials and interstadials of Dreimanis and Karrow (1972, p. 8) and to the time referred to as Woodfordian, Two Creekan and early Valderan of Frye et al. (1965, p. 51).
Conclusions Deposits of two major glacial periods and two
major nonglacial periods are recognized in south- central British Columbia. Sediments of the last gla- cial episode may be correlated with deposits of the Fraser Glaciation and the late Wisconsin Substage. Radiocarbon dates and tephrostratigraphy facili- tate inter-valley correlations of deposits of the last major nonglacial period. These deposits are consi- dered to be time equivalent to deposits of the Olympia Interglaciation and the mid-Wisconsin Substage of other areas. Regional correlation of older deposits throughout south-central British Columbia is based on stratigraphic relationships with overlying materials. No absolute criteria are presently available to permit direct correlation of these older sediments with deposits outside of south-central British Columbia. However, on the basis of present knowledge it is considered unlikely that any of the deposits discussed in this paper predate Sangamonian time.
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1968. Olympia Interglaciation, Purcell Trench, British Columbia. Geological Society of America Bulletin, 79, pp. 1075- 1080.
1971. Radiocarbon geochronology of southern British Columbia. Geological Survey of Canada, Paper 71-37.28 p.
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