The Quaternary stratigraphic record of British Columbia-evidence for episodic sedimentation and erosion controlled by glaciation1

JOHN J. CLAGUE Geological Survey of Canada, 100 West Pender St., Vancouver, B.C. , Canada V6B IA8

Received April 29, 1985

Revision accepted December 17, 1985

The terrestrial Quaternary stratigraphic record of British Columbia is largely a product of brief depositional events separated by long periods of nondeposition and erosion. Thick, stratified Quaternary sediments are present mainly in valleys and coastal lowlands and accumulated during periods of growth and decay of the Cordilleran Ice Sheet. At glacial maxima, till was deposited over large areas of low and moderate relief. However, at the same time, much of the landscape was eroded by glaciers.

Sedimentation has been more restricted and has occurred at lower rates during nonglacial periods than during glaciations. On land, the only important sediment accumulation sites during nonglacials have been lakes, floodplains, and fans. However, large amounts of sediment have accumulated offshore, especially in fjords and basins such as the Strait of Georgia. Because of the restricted aspect of sedimentation during nonglacials, the stratigraphic record of these periods is meagre. In most places, tlue nonglacial units are thin and discontinuous, or they are absent altogether. Commonly, a nonglacial period is recorded only by an unconformity produced when streams incised valley fills shortly after the end of the preceding glaciation.

Le registre stratigraphique du Quaternaire de la Colombie-Britannique est domink par des Cvtnements de dkposition de courte durCe dparCs par de longues pCriodes de nondCposition et d'krosion. Des dCp6ts Cpais stratifiCs de sCdiments quater- naires sont pksents particuli&rement dans les vallees et les basses-terres littorales, oti ils se sont accumulCs durant les periodes de croissance et d'ablation de la calotte glaciaire cordillCrienne. Au point culminant de la glaciation, des dep6ts de till ont couvert de grandes rCgions de relief bas et moyen. Cependant, au mCme moment, une partie importante du terrain subissait 1'Crosion.

La ~Cdimentation Ctait plus restreinte, et les taux de sdimentation Ctaient plus lents durant les pCriodes nonglaciaires que durant les glaciations. Sur le continent et lors des intervalles glaciaires, les seuls lieux d'accumulation furent les lacs, les plaines de dtbordement et les c6nes de dkjection. Cependant, de grandes quantitks de sCdiments se sont accumulCes au large des cbtes, particulikrement dans les fjords et les bassins semblables & ceux du dCtroit de GCorgie. Le registre stratigraphique de ces intervalles glaciaires est pauvre ii cause de la faible accumulation de skdiments. Dans la majoritk des androits, les unites nonglaciaires authentiques sont minces et discontinues, ou encore elles sont absentes. En gCnCral, une pCriode nonglaciaire est reconnue uniquement par une discordance ksultant de I'encaissement des cours d'eau dans les sCdiments qui ont comb16 les vallCes peu de temps apks la phase finale de la glaciation pkctdente.

[Traduit par la revue] Can. 1. Earth Sci. 23, 885-894 (1986)

Introduction events and from climatic, land-use, and other environmental . . . the history of any one part of the earth, like the life of a changes; it also may occur when geomorphic thresholds intrin- soldier, consists of long periods of boredom and short periods sic to a particular drainage basin or geomorphic Process are of terror. exceeded.

-Ager (1973, p. 100) The concepts of incompleteness of the stratigraphic record

Earth scientists have long recognized that the stratigraphic record is incomplete, recording a relatively small part of the actual passage of time. Ager (1973), for example, in a thought- ful treatise on stratigraphy, argued that stratigraphic sequences comprise more "gaps" than "record" and attributed this to both nondeposition and erosion. This concept seems reasonable when one considers the complexity of external factors, such as climate and tectonism, that to a large extent determine the course of landscape evolution. In addition, various changes within geomorphic systems (i.e., basin morphology and sedi- ment loads) may trigger widespread erosion if thresholds are exceeded (Schumm 1973, 1975).

Stratigraphic records, in addition to being incomplete, are products of spasmodic sedimentation (Gage 1970; Ager 1973). Periods of slow sedimentation or nondeposition commonly are separated by brief periods of rapid sedimentation during which the bulk of a stratigraphic unit accumulates. Rapid sedimenta- tion may result from short-lived exceptional or catastrophic

'This paper was presented at the "W. H. Mathews Symposium: A celebration," held at the University of British Columbia, Vancouver, on October 10, 1984. Printed in Canada / Imprimt au Canada

and spasmodic sedimentation apply to the entire rock column, but perhaps they are best exemplified by terrestrial sedimen- tary deposits of the Quaternary Period. In this paper, I examine the terrestrial Quatemaly stratigraphic record of British Columbia in the context of these concepts. I attempt to demon- strate that most Quatemaly stratigraphic units in the province are products of ephemeral events separated by much longer periods of nondeposition or erosion. Sedimentation and erosion also are discussed in a spatial framework because geo- logically significant sedimentation (i.e., that which produces deposits capable of being preserved in the stratigraphic record) has been restricted to particular parts of the British Columbia landmass.

Methodological limitations Before proceeding, it is necessaly to mention some limita-

tions in the use of stratigraphic sequences in reconstructing past geomorphic systems. The stratigraphic record generally is difficult to interpret in terms of short-term events and geomor- phic controls (Johnson 1982). Most stratigraphic units repre- sent an integrated response of geomorphic systems to a variety of interdependent factors; the contribution of each of these

886 CAN. J. EARTH SCI. VOL. 23, 1986

e Q U E S N E L

e S P E N C E S BRIDGE

I T L A M VALLEY

FIG. 1. Locality index map.

factors to the genesis of a unit may be difficult or impossible to assess from a study of sediments alone. In this context, strati- graphic studies appear to be inferior to contemporary process studies of geomorphic systems. On the other hand, Quaternary stratigraphic records do provide information on sedimentation over intermediate and long time scales (10'- lo6 years) that obviously cannot be met by short-term process studies. Church (1980) concluded that contemporary records in most instances provide a misleading impression of the long-term variability of processes, especially where thresholds are important, and consequently may not be very useful in explaining landscape evolution.

Another limitation relates to the previously discussed con- cept of episodic and spasmodic sedimentation. Because most units are isolated "snapshots" in the long panorama of geologic time, they provide an incomplete picture of past geo- morphic systems and the total spectrum of changes that these systems have undergone. The problem is aggravated by the fact that many terrestrial sedimentary environments are poorly represented in Quaternary stratigraphic sequences.

The Quaternary Period in British Columbia In British Columbia, as in most other parts of Canada, the

Quaternary Period has been dominated by the repeated growth and decay of continental ice sheets. Isotopic and magnetic studies of deep-sea sediments have shown that there have been eight major climatic cycles in the last 800 ka, each about 100 ka in duration and each marked by sharp fluctuations in climate on shorter time scales; these cycles were preceded back to before the beginning of the Pleistocene by similar cycles of lesser magnitude but greater frequency (Shackleton and Opdyke 1973, 1976). Many or most of these climatic cycles were accompanied by widespread glaciation in British Colum- bia and by related isostatic crustal adjustments and sea-level change (Fulton 1971; Clague 1981, 1983; Clague et al. 1982).

The Quaternary in British Columbia was also a time of in- tense tectonic activity related to plate interactions in the North Pacific Ocean (Riddihough and Hyndman 1976; Riddihough 1977). Tectonism has played an important role in shaping the British Columbia landscape: it has affected Quaternary sedi- mentation and erosion through uplift, subsidence, faulting, earthquake-induced mass movements, and other phenomena (Ryder 1981~).

Although glaciation and tectonism have contributed greatly to the present landscape of British Columbia, physiography itself has influenced landscape evolution through its effects on sedimentation and erosion. For example, erosion in British Columbia during Quaternary glaciations has been significant in some physiographic settings, but negligible in others (see "Spatial Constraints on Sedimentation").

The following sequence of events characterized most Qua- ternary glaciations in British Columbia. At the end of each major nonglacial period, ice was restricted to high mountain areas, much as today. Climatic deterioration marking the in- ception of glaciation was accompanied by glacier expansion and by poorly understood vegetation and hydrologic changes. Valley glaciers advanced and eventually overtopped interval- ley ridges to coalesce as small mountain ice sheets (Davis and Mathews 1944). Eventually, glaciers spread across plateau areas of the interior and into lowlands along the coast. At the climaxes of great Quaternary glaciations, piedmont and valley glaciers from separate mountain ranges coalesced to bury the entire interior region and parts of the continental shelf and westernmost Interior Plains east of the Rocky Mountains (Fig. 1). The buildup of glacier ice triggered complex isostatic depression of the land surface (complicated by forebulge ef- fects and possible hydro-isostatic uplift of the continental shelf) (Clague 1983). It also led to a radical restructuring of the stream network that had been established during the preceding nonglacial period.

Glaciations terminated with sharp climatic amelioration and rapid melting of glaciers. The Cordilleran Ice Sheet decayed by downwasting and complex frontal retreat (Fulton 1967; Clague 1981). Isostatically depressed coastal lowlands were flooded by the sea as glaciers, destabilized by eustatically rising water levels, calved back into mountain valleys. In areas of low and moderate relief in the interior, deglaciation pro- gressed by stagnation, with uplands appearing through the ice cover first and dividing the ice sheet into a series of tongues that retreated in response to local conditions. Active ice even- tually became restricted to major mountain ranges. Deglacia- tion was accompanied by rapid isostatic uplift; areas at the periphery of the ice sheet rebounded first, whereas parts of the Interior Plateau and mountain ranges that supported glaciers until late in the glacial cycle experienced delayed rebound.

The actual pattern of ice-sheet growth and decay was proba- bly more complex than suggested above. There were brief intervals of glacier recession during periods of ice-sheet growth, as well as local stillstands and readvances during periods of ice-sheet decay. In addition, not all glaciations progressed to the continental ice-sheet stage of development during which most of British Columbia was covered by a sin- gle confluent mass of ice. Some, possibly most, glaciations climaxed with the ice cover consisting of a series of separate glacier complexes confined to, or spreading some distance beyond, mountain ranges; at such times, significant plateau areas, some coastal lowlands, and parts of the continental shelf remained ice free.

CLAGUE

Quaternary nonglacial periods varied in length and, like glacials, were marked by fluctuations in climate. In British Columbia, major nonglacials were times of limited ice cover; glaciers were restricted to mountain ranges, and lowland and plateau areas were continuously ice free. Physiography and sedimentary environments during the last nonglacial period (middle Wisconsinan) were similar to those of the present, and the same probably is true for earlier nonglacials as well.

Quaternary stratigraphy The terrestrial Quaternary stratigraphic record of British

Columbia is a product of the episodic growth and decay of the Cordilleran Ice Sheet. Most Quaternary units are of glacial origin and comprise (1) stratified sediments deposited in pro- glacial and ice-contact fluvial, marine, and lacustrine environ- ments and (2) till deposited subglacially and supraglacially. The thickest and most varied glacial sediments occur in valleys and coastal lowlands and commonly represent two or more glaciations. In any given area, the deposits of each glaciation generally are similar and comprise several units laid down in a relatively small number of sedimentary environments. How- ever, each of these environments is complex, thus the resulting sediments are extremely heterogeneous. For example, Pleisto- cene glaciolacustrine sediments in British Columbia include horizontally stratified clay and silt, foreset-bedded sand and gravel, and complexly intertonguing, folded, and faulted sand, gravel, and diamicton.

Glacial sequences are separated from one another by nongla- cial sediments or by an unconformity. Nonglacial sediments generally are thin and patchy, although thick fluvial, deltaic, and lacustrine deposits are present in a few areas (Fulton 1972). More commonly, unconformities separate major glacial sequences. These define former land surfaces similar in mor- phology and relief to the present and are products of glacial ind fluvial erosion.

All major Quaternary stratigraphic units in British Columbia are time transgressive. This is largely a result of diachronous glacier growth and decay during Pleistocene glaciations. Sedi- mentation and erosion during periods of glacier growth began first in the mountains and propagated outward onto the Interior Plateau, Interior Plains, -coasd lowlands, and continental shelf. In contrast, sedimentation during deglacial phases com- menced earlier in peripheral glaciated areas than in the core area of the Cordilleran Ice Sheet.

Excluding volcanics, most terrestrial Quaternary strati- graphic units in British Columbia record Wisconsinan and Holocene events. Pre-Wisconsinan terrestrial deposits have been positively identified in only a few interior valleys and beneath lowlands bordering the Strait of Georgia (Fulton and Smith 1978; Hicock and Armstrong 1983; Fulton 1984). Thick sediments of early and middle Quaternary age may underlie parts of the continental shelf at the periphery of the former Cordilleran Ice Sheet (Lutemauer and Murray 1983), but these probably are marine, rather than terrestrial, in origin. The pre- Wisconsinan terrestrial record is poor for two reasons: (1) Sangamonian and older deposits are covered by thick younger sediments and thus are not well exposed; and (2) most of these old deposits have been obliterated by glacial erosion during the Wisconsinan Stage.

Episodic sedimentation and erosion The Quaternary stratigraphic record of British Columbia

Q U A D R A S A N D I GLACIER

FIG. 2 . Origin of Quadra Sand. This unit was deposited as aprons in front of and along the margins of glaciers advancing down the Strait of Georgia during the early part of the Fraser Glaciation. The configu- ration shown in this diagram dates to about 25 -28 ka BP. Location is shown in Fig. 1.

provides evidence for brief intervals of deposition separated by long periods of nondeposition and erosion. The spasmodic character of sedimentation and erosion in British Columbia during the Quaternary is illustrated in this section, with exam- ples from various parts of the province.

Glaciations Thick sediments accumulated in British Columbia valleys

and coastal lowlands during periods of growth and decay of the Cordilleran Ice Sheet. Climatic deterioration and glacier ex- pansion during the early phase of each glaciation led to an increase in sediment production, especially in alpine areas. Initially, much of this sediment accumulated as outwash in mountain valleys. However, as glaciers advanced out of the mountains, large amounts of this material, as well as colluvial and fluvial sediments deposited during the preceding nongla- cial period, were flushed from these staging areas into inter- montane valleys and fjords. Streams, unable to transport the large amounts of sediment made available to them, aggraded their valleys, and as a result substantial amounts of outwash accumulated in short periods of time. Loci of deposition shifted as glaciers advanced; thus, aggradation occurred at different times in different places. As glaciers continued to expand, they increasingly disrupted the drainage, ponding large, rapidly evolving lakes in which significant quantities of clay, silt, and sand accumulated.

Some of the most extensive and conspicuous Quaternary stratigraphic units in British Columbia formed during periods of growth of the Cordilleran Ice Sheet. The following are two examples of units deposited during the early part of the Fraser Glaciation (late Wisconsinan); the first is from coastal British Columbia, and the second is from the interior.

(1) Quadra Sand, an important lithostratigraphic unit in the Strait of Georgia region of southwestern British Columbia and adjacent Puget Lowland of Washington State (Fig. I), com- prises horizontally and cross-stratified, well sorted sand, minor silt, and gravel up to about 100 m thick (Clague 1976, 1977). These sediments were deposited as aprons in front of and at the margins of glaciers as they advanced southward from source areas in the southern Coast Mountains (Fig. 2). Much of the

888 CAN. J . EARTH SCI. VOL. 23. 1986

FIG. 3. Late Wisconsinan advance outwash (upper Nicoamen gravel of Ryder 1981b) in Thompson Valley south of Spences Bridge. These sediments are a remnant of a thick valley fill deposited during the early part of the Fraser Glaciation.

NORTH U N C - SOUTH

C

C L A Y m d T I L L

-1 SILT LANDSLIDE DEBRIS

[Fi GRAVEL BEDROCK

FIG. 4. Geologic section across Peace River valley near Fort St. John showing thick, stratified drift of probable early Fraser Glaciation age. The drift comprises a lower unit of outwash gravel and an upper unit of glaciolacustrine silt. In places, these sediments unconfomably overlie an older glacial sequence. Modified from Mathews (1978, Fig. 5) .

sand probably was recycled repeatedly by meltwater streams as the glaciers advanced. Aggradation may have been facilitated by a relative rise in sea level or by downwarping of the land surface to the north and northeast, both caused by glacio- isostatic depression of the crust. The sand probably accumu- lated both on delta-top floodplains and ponds and in shallow, subaqueous delta-front environments. Quadra Sand is mark- edly diachronous, ranging in age from about 29 ka at the north end of the Strait of Georgia to about 15 ka at the south end of Puget Lowland. At each site, however, the unit accumulated over a much shorter period of time, probably on the order of hundreds to at most a few thousand years (Clague 1977; Clague and Luternauer 1982).

(2) Thick, horizontally bedded gravel and subordinate sand underlie late Wisconsinan till in many valleys in the British Columbia interior (Figs. 3, 4) (Fulton 1975; Ryder 1976, 1981b; Mathews 1978). These sediments, which are up to 150 m thick and occur up to 150 m above present base level, are thought to be distal outwash deposited during the early part of the Fraser Glaciation. The coarseness of the sediments and their similarity to braided-stream deposits associated with pre-

sent-day glaciers argue for rapid aggradation at each site. These coarse-grained sediments in places are conformably overlain by glaciolacustrine clay, silt, and sand deposited when drainage was blocked by advancing glaciers or by rapidly accumulating drift (Figs. 4, 5).

Aggradation on a similar scale to that accompanying ice- sheet growth also occurred at the close of, and shortly after, each major glaciation (Church and Ryder 1972). During periods of deglaciation, large quantities of sediment were re- leased from wasting ice masses and discharged onto flood- plains and into glacial lakes. In addition, newly deposited, unstable drift was transported by running water and mass- wasting processes from slopes into valleys, where it accumu- lated on-floodplains and fans. Along the coast, glaciomarine sediments were laid down on isostatically depressed lowlands and in offshore areas (Fyles 1963; Armstrong 1981; Clague 1985). As a result of these processes, thick sediment fills accu- mulated in most interior and mountain valleys, on lowlands bordering the Strait of Georgia, in fjords, and in some offshore basins.

Remnants of fills dating to the end of the Fraser Glaciation

CLAGUE 889

FIG. 5. Interbedded silt and sand deposited in a lake dammed by glaciers or outwash during the advance phase of the Fraser Glaciation near Quesnel.

and the end of the penultimate glaciation are important strati- graphic units in British Columbia. They are disproportionately large in size and volume, considering the extremely brief periods of time in which they accumulated. Two examples illustrate this point.

(1) At the end of the Fraser Glaciation, glacial Lake Thomp- son formed in South Thompson Valley between ice tongues receding west and northeast (Fulton 1965). Meltwater streams carried enormous quantities of silt into the lake, and a fill up to 150 m thick accumulated in a period of only 100-200 years (Fig. 6). The rapidity of sedimentation is indicated by the fact that individual annual layers at the base of the "South Thomp- son silt" are more than 6 m thick (Fulton 1965). This lake is only one of many similar lakes that formed and disappeared in southern British Columbia valleys over a period of about 1000 years at the close of the Fraser Glaciation (Fulton 1969; Clague 1981). Significant sediment fills are associated with many of these short-lived lakes.

(2) At about the same time that sediment was accumulating in glacial Lake Thompson, gigantic ice-contact deltas were being built into the sea in the Terrace -Kitimat area of west- central British Columbia (Fig. 7) (Clague 1984, 1985). These deltas were constructed during successive stillstands of the glacier occupying the valley north of Kitimat. The largest delta, which is graded to a relative sea-level position 200 m above the present shore, covers an area of 60 krn2, consists of more than lo9 m3 of sand and gravel, and, judging from radio- carbon dates, formed in no more than a few hundred years (Clague 1985). Extensive deposits of sea-floor muds accumu-

lated at the same time in pro-delta positions (Fig. 7). Patterns of sedimentation and erosion at glacial maxima,

when most of the province was covered by ice, are more diffi- cult to characterize than those of periods of glacier growth and decay. Most areas, especially mountains, fjords, and valleys parallel to the ice-flow direction, were eroded by glaciers at these times, and their sediment cover was stripped away. How- ever, glaciers accomplished little erosion in some areas, and significant remnants of older sediments remained following deglaciation. In addition, many glacially eroded surfaces were covered by till before becoming deglaciated. Such tills are more widely distributed than the stratified valley and lowland fills described previously but generally are thinner. They con- sist of detritus recycled from older Quaternary sediments and eroded directly from bedrock.

Nonglacial periods Significant sedimentation has been much more limited

during nonglacial periods than during glaciations. Important nonglacial sedimentation sites on land are localized and of relatively small size. They include large lakes in river valleys, some flood~lains. and fans at the mouths of some streams. However, large amounts of sediment have accumulated in marine deltas and on the floors of fjords and basins such as the Strait of Georgia.

Early durini each nonglacial period, valley fills in most areas were deeply incised by streams (Fig. 8). This resulted mainlv from a drastic reduction in the amount of sediment supplied to the fluvial system as slopes stabilized and became

890 CAN. J . EARTH SCI. VOL. 23, 1986

FIG. 6. South Thompson silt near Karnloops. These sediments were deposited in a glacial lake in only 100-200 years at the end of the Fraser Glaciation. The layers in the lower photograph are varves (person at lower right provides scale).

vegetated. In addition, base level in coastal areas was lowered up to 200 m as a result of an isostatically induced fall in the level of the sea relative to the land. The effects of this base- level lowering probably were propagated slowly inland and may have contributed to valley incision by streams. However, other local base-level controls such as rock ledges and con- strictions probably played an equal or more important role in controlling downcutting. Whatever the causes, streams deeply incised deglacial and older fills before achieving quasi-equilib- rium at much lower levels. Baning tectonic uplift, subsidence, or renewed glaciation, streams tended to flow at or near these levels for long periods of time. In the case of the present nonglacial period (Holocene), most major streams in British Columbia were flowing near their present levels within several thousand years of the close of the Fraser Glaciation; sub- sequent changes in floodplain levels have been minor com- pared with those accompanying and immediately following deglaciation.

The stratigraphic record of Quaternary nonglacial periods in British Columbia is meagre because of the limited extent of aggradation at these times and because base level fell at the close of each glacial cycle. The Olympia Nonglacial Interval (middle Wisconsinan), which is known to span at least 40 ka

(Clague 1980), is represented in stratigraphic sections in most areas by an unconformity or by relatively thin stratified sedi- ments (Figs. 4, 9). In the Strait of Georgia region, for exam- ple, thin middle Wisconsinan nonglacial sediments (Cowichan Head Formation), which range from about 25 to >60 ka old, occur between much thicker drift units deposited over periods of hundreds of years to at most several thousand years (Fig. 9) (Armstrong and Clague 1977). At Dashwood on the west side of the Strait of Georgia, these nonglacial sediments are less than 10 m thick and are overlain by up to 60 m of Fraser Glaciation deposits, mainly Quadra Sand. In Coquitlarn Valley near Vancouver, the Cowichan Head Formation is only about 1 m thick and is overlain by up to 200 m of late Wisconsinan drift, at least the lower half of which, judging from the radio- carbon dates, was deposited in a few centuries.

In summary, the terrestrial Quaternary stratigraphic record of British Columbia is a product of brief sedimentation events separated by long intervals of nondeposition and erosion. Most sediments in valleys were deposited during the early and late stages of various Pleistocene glaciations (Fig. 10). In contrast, during nonglacial periods, sedimentation was more restricted and occurred at lower rates, and in fact the early part of each nonglacial period typically was characterized by a short epi-

CLAGUE

HOLOCENE n FLUVIAL. DELTAIC, COLLUVIAL, A N D

ORGANIC SEDlMENlS

LATE W I S C O N S I N A N

GLAClOMARlNE MUD (THICK, T H I N )

GLACIOFLUVIAL-DELTAIC GRAVEL A N D SAND

ICE-CONTACT SEDIMENTS AND SUBAQUEOUS OUTWASH

BEDROCK COVERED PARTLY WITH T H I N I+ TILL A N D COLLUVIUM

,/ ICE-FLOW DIRECTION

'14 ICE-CONTACT FACE

0 8 KETTLE

MELTWATER CHANNEL

I 2850'

FIG. 7. Geologic map showing one of three large ice-contact delta complexes in the valley between Terrace and Kitimat. This body of sand and gravel accumulated over a period of a few hundred years at the end of the Fraser Glaciation. Geology after Clague (1984).

sode of valley incision as streams adjusted to changing envi- ronmental conditions associated with the disappearance of the Cordilleran Ice Sheet. Sedimentation and erosion at the base of the ice sheet were more complex, with erosion in most places, but with deposition of glacially eroded debris in the form of till on ice-scoured surfaces over large areas of low to moderate relief such as the Interior Plateau.

Spatial constraints on sedimentation To a considerable degree, physiography has controlled sedi-

mentation in British Columbia during the Quaternary. The main terrestrial sediment "sinks" have been valleys and coastal lowlands (Table 1). In addition, some plateaus have been repositories of large quantities of till and ice-contact sedi- ments. In contrast, mountains and other highlands have been mainly areas of erosion during the Quaternary. Sediments were deposited locally in these areas during nonglacial periods but were removed during glaciations, accompanied by significant erosion of rock surfaces by glaciers. Sediment fills in valleys and coastal lowlands were partially eroded during nonglacials, and the erosion products were transferred to floodplains, lakes, and the sea. The large plateau areas of the British Columbia interior have been dominantly passive surfaces during nongla- cials, experiencing neither significant deposition nor erosion, except in the vicinity of streams and where relief is high. However, during each glaciation, large amounts of drift were

TABLE 1. Major Quaternary sediment sinks

Nonglacial intervals Glacial maxima Transitions -

Some lakesa Plateaus Valleys Some valleysb Plains Coastal lowlands Fjords Continental shelfd Fjords Inner shelf basinsc Abyssal plains and Inner continental shelf

basinsd

"Those lakes with significant fluvial input. bAggradational floodplains; active fans. 'Strait of Georgia, for example. dThose parts of the continental shelf and deep-sea floor beyond the limits of

the Cordilleran Ice Sheet.

removed from plateau surfaces by ice before a new mantle of drift was laid down.

The ultimate sediment sink for British Columbia Quaternary sediments is the sea. Much of the sediment produced during the Quaternary has already reached the sea, possibly after many cycles of erosion, transportation, and deposition. Over the long term (10" lo7 years), continued uplift of British Columbia resulting from lithospheric plate interactions in the northeast Pacific Ocean will ensure the destruction of much of the existing Quaternary cover. This material will be camed to the sea by streams and deposited in deltas, in fjords, on parts of the continental shelf, and on abyssal plains and basins beyond

892 CAN. 1. EARTH SCI. VOL. 23. 1986

FIG. 8. Incised valley fill, Fraser Valley west of Clinton. Fraser River trenched the valley fill during early Holocene time, shortly after the end of the Fraser Glaciation. Province of British Columbia photo BC1087-46.

the continental margin. However, sediments deposited on the sea floor inside the limit of glaciation could be removed by grounded ice if the Cordilleran Ice Sheet should ever form again. Furthermore, those occurring at depths of less than about 150 m might be eroded by waves and currents during periods of eustatic sea-level lowering.

Concluding remarks The Quaternary stratigraphic record is a complex response of

geomorphic systems to threshold events of varying magnitude. The forcing events have been mainly climatic, but local and regional physiography, geology, and other factors have also determined the character and distribution of stratigraphic units.

Fluvial systems in British Columbia became overloaded with sediment during periods of ice-sheet growth and decay, giving rise to significant stratigraphic units. Coarser sediment carried by streams was laid down close to glacier sources to produce deltas, outwash plains, and valley trains. Fine material was transported farther and deposited on the floors of lakes and the sea. Release of sediment directly from ice into lacustrine and marine environments also contributed to the formation of many stratigraphic units. Most units thus are products of unusual, short-lived events; although one hesitates to label such events catastrophes, this appellation may be warranted in a geologic sense.

A study of these stratigraphic units provides detailed infor- mation about unusual sedimentary environments for which

CLAGUE

D A S H W O O D

C O Q U I T L A M VALLEY

O K A N A G A N C E N T R E HOLOCENE N O N G L A C I A L SEDIMENTS

LATE W l S C O N S l N A N S T R A T I F I E D D R I F T (SAND. GRAVEL)

TILL ( INCLUDES GLACIOMAR INE SEDIMENTS AT . DASHWOOD)

MIDDLE W I S C O N S I N A N N O N G L A C I A L SEDIMENTS

EARLY W I S C O N S I N A N OR OLDER

S T R A T I F I E D D R I F T (SILT, SAND, GRAVEL)

TILL ( INCLUDES GLACIOMARINE SEDIMENTS AT DASHWOOD)

32.6 ko . . - RADIOCARBON AGE

FIG. 9. Stratigraphic sections from three sites in southern British Columbia showing the relationships of Fraser Glaciation drift, middle Wisconsinan nonglacial sediments, and early Wisconsinan or older drift. Although the nonglacial sediments record a lengthy period of time, they are much thinner than underlying and overlying drift units. Sources of information: sections-Fyles (1963), Fulton and Smith (1978), and Clague and Lutemauer (1982); radiocarbon dates-Clague (1980).

R I V E R VALLEYS

AGGRADATION 4 E0Ul l lBRl"M

DEGRADATION )

P L A T E A U S 0 t

SEDIMENTATION 4 - ............................ ........ EOUILIBR1UM.-. ,

EROSION ) LrOSiON OF SfDIMCNT6

M O U N T A I N S 0 *

Eo"lLlBR,"M,",,. ..... ........... EROSION ) 1 I

N O N G L A C I A L - - - NONGLACIAL- - GLACIATION- -- h

TRANSITION TRANSITION T IME -

FIG. 10. Diagrammatic summary of patterns of sedimentation and erosion in British Columbia during the Quaternary. These patterns are depicted for a hypothetical glacial cycle during which the Cordilleran Ice Sheet grows and decays in a regular manner. The open arrows indicate the time of glacier overriding (earlier in mountains than else- where); the darkened arrows indicate the time of deglaciation. Ordi- nates are unit-free rate scales. In river valleys and on plateaus, a period of subglacial erosion is shown as preceding a later period of subglacial deposition, although other possibilities exist.

modem analogues are poor and much smaller in scale. On the other hand, the stratigraphic record is much less helpful in elucidating processes that are not directly controlled by glacia- tion, in other words, those prevailing during periods when most of British Columbia was free of glacier ice. An under- standing of the behaviour and short-term variability of such geomorphic systems and the mechanics and controls of pro- cesses operating within them probably is best gained from field observations of contemporary geomorphic processes.

Acknowledgments Seeds of the concepts presented in this paper were planted by

W. H. Mathews. The ideas were further developed in discus- sions with J. E. Armstrong, M. A. Church, R. J. Fulton, and J. M. Ryder. A benchmark paper on paraglacial sedimentation by Church and Ryder (1972) was particularly influential in developing my thinking on episodic sedimentation controlled by glaciation. M. A. Church, R. J. Fulton, L. E. Jackson, Jr., and J. M. Ryder kindly reviewed a draft of the manuscript.

AGER, D. V. 1973. The nature of the stratigraphical record. Macmil- Ian Press, London, UK, 114 p.

ARMSTRONG, J. E. 1981. Post-Vashon Wisconsin glaciation, Fraser Lowland, British Columbia. Geological Survey of Canada, Bulletin, 322, 34 p.

ARMSTRONG, J. E., and CLAGUE, J. J. 1977. Two major Wisconsin lithostratigraphic units in southwest British Columbia. Canadian Journal of Earth Sciences, 14, pp. 1471 - 1480.

CHURCH, M. 1980. Records of recent geomorphological events. In Timescales in geomorphology. Edited by R. A. Cullingford, D. A. Davidson, and J. Lewin. John Wiley & Sons, Chichester, UK, pp. 13-29.

CHURCH, M., and RYDER, J. M. 1972. Paraglacial sedimentation: consideration of fluvial processes conditioned by glaciation. Geo- logical Society of America Bulletin, 83, pp. 3059 -3072.

CLAGUE, J. J. 1976. Quadra Sand and its relation to the late Wiscon- sin glaciation of southwest British Columbia. Canadian Journal of Earth Sciences, 13, pp. 803 - 815.

1977. Quadra Sand: a study of the late Pleistocene geology and geomorphic history of coastal southwest British Columbia. Geological Survey of Canada, Paper 77-17, 24 p.

1980. Late Quaternary geology and geochronology of Brit- ish Columbia. Part 1: Radiocarbon dates. Geological Survey of Canada, Paper 80-13, 28 p.

1981. Late Quaternary geology and geochronology of Brit- ish Columbia. Part 2: Summary and discussion of radiocarbon-

894 CAN. 1. EARTH SCI. VOL. 23, 1986

dated Quaternary history. Geological Survey of Canada, Paper 80-35, 41 p.

1983. Glacio-isostatic effects of the Cordilleran Ice Sheet, British Columbia, Canada. In Shorelines and isostasy. Edited b y D. E. Smith and A. G. Dawson. Academic Press, London, UK, pp. 321 -343.

1984. Quaternary geology and geomorphology , Smithers - Terrace - Prince Rupert area, British Columbia. Geological Sur- vey of Canada, Memoir 413, 71 p.

1985. Deglaciation of the Prince Rupert - Kitimat area, British Columbia. Canadian Journal of Earth Sciences, 22, pp. 256-265.

CLAGUE, J. J., and LUTERNAUER, J. L. 1982. Excursion 30A: Late Quaternary sedimentary environments, southwestern British Columbia. International Association of Sedimentologists, 1 lth International Congress on Sedirnentology, Hamilton, Ont., Field Excursion Guide Book, 167 p.

CLAGUE, J., HARPER, J. R., HEBDA, R. J., and HOWES, D. E. 1982. Late Quaternary sea levels and crustal movements, coastal British Columbia. Canadian Journal of Earth Sciences, 19, pp. 597 -618.

DAVIS, N. F. G., and MATHEWS, W. H. 1944. Four phases of glacia- tion with illustrations from southwestern British Columbia. Journal of Geology, 52, pp. 403-413.

FULTON, R. J. 1965. Silt deposition in late-glacial lakes of southern British Columbia. American Journal of Science, 263, pp. 553 -570.

1967. Deglaciation studies in Karnloops region, an area of moderate relief, British Columbia. Geological Survey of Canada, Bulletin 154, 36 p.

1969. Glacial lake history, southern Interior Plateau, British Columbia. Geological Survey of Canada, Paper 69-37, 14 p.

197 1 . Radiocarbon geochronology of southern British Columbia. Geological Survey of Canada, Paper 71-37, 28 p.

1972. Stratigraphy of unconsolidated fill and Quaternary de- velopment of North Okanagan Valley. In Bedrock topography of the North Okanagan Valley and stratigraphy of the unconsolidated valley fill. Geological Survey of Canada, Paper 72-8B, pp. 9 - 17.

1975. Quaternary geology and geomorphology , Nicola - Vernon area, British Columbia. Geological Survey of Canada, Memoir 380, 50 p.

1984. Quaternary glaciation, Canadian Cordillera. In Qua- ternary stratigraphy of Canada-a Canadian contribution to IGCP Project 24. Edited by R. J. Fulton. Geological Survey of Canada, Paper 84-10, pp. 39-47.

FULTON, R. J., and SMITH, G. W. 1978. Late Pleistocene strati- graphy of south-central British Columbia. Canadian Journal of Earth Sciences, 15, pp. 971 -980.

FYLES, J. G. 1963. Surficial geology of Home Lake and Parksville map-areas, Vancouver Island, British Columbia. Geological Sur- vey of Canada, Memoir 318, 142 p.

GAGE, M. 1970. The tempo of geomorphic change. Journal of Geology, 78, pp. 619-625.

HICOCK, S. R., and ARMSTRONG, J. E. 1983. Four Pleistocene forma- tions in southwest British Columbia: their implications for patterns of sedimentation of possible Sangamonian to early Wisconsinan age. Canadian Journal of Earth Sciences, 20, pp. 1232 - 1247.

JOHNSON, W . H. 1982. Interrelationships among geomorphic inter- pretations of the stratigraphic record, process geomorphology and geomorphic models. In Space and time in geomorphology. Edited b y C. E. Thorn. George Allen & Unwin, London, UK, pp. 219-241.

LUTERNAUER, J. L., and MURRAY, J. W. 1983. Late Quaternary morphologic development and sedimentation, central British Columbia. Geological Survey of Canada, Paper 83-21, 38 p.

MATHEWS, W. H. 1978. Quaternary stratigraphy and geomorphology of Charlie Lake (94 A) map-area, British Columbia. Geological Survey of Canada, Paper 76-20, 25 p.

RIDDIHOUGH, R. P. 1977. A model for recent plate interactions off Canada's west coast. Canadian Journal of Earth Sciences, 14, pp. 384 - 396.

RIDDIHOUGH, R. P., and HYNDMAN, R. D. 1976. Canada's active western margin-the case for subduction. Geoscience Canada, 3, pp. 269-278.

RYDER, J. M. 1976. Terrain inventory and Quaternary geology, Ashcroft, British Columbia. Geological Survey of Canada, Paper 74-49, 17 p.

1981a. Geomorphology of the southern part of the Coast Mountains of British Columbia. Zeitschrift fiir Geomorphologie, Neue Folge, 37, pp. 120- 147.

1981b. Terrain inventory and Quaternary geology, Lytton, British Columbia. Geological Survey of Canada, Paper 79-25, 20 p.

SCHUMM, S. A. 1973. Geomorphic thresholds and complex response of drainage systems. In Fluvial geornorphology. Edited b y M. Morisawa. George Allen & Unwin, London, UK, pp. 299 - 3 10.

1975. Episodic erosion: a modification of the geomorphic cycle. In Theories of landform development. Edited by W. N . Melhorn and R. C. Flernal. George Allen & Unwin, London, UK, pp. 69-85.

SHACKLETON, N. J., and OPDYKE, N. D. 1973. Oxygen-isotope and palaeomagnetic stratigraphy of equatorial Pacific core V28-238: oxygen isotope temperatures and ice volumes on a 10' year and lo6 year scale. Quaternary Research, 3, pp. 39-55.

1976. Oxygen-isotope and paleomagnetic stratigraphy of Pacific core V28-239, late Pliocene to latest Pleistocene. In Inves- tigation of late Quaternary paleoceanography and paleoclimato- logy. Edited by R. M. Cline and J. D. Hayes. Geological Society of America, Memoir 145, pp. 449-464.