Ian Hutchinson: Abstracts

TITLE: Early growth of the Cordilleran ice sheet deduced from glacio-isostatic depression in southwest British Columbia, Canada.

AUTHORS: Clague, J.J., Froese, D., Hutchinson, I., James, T.S., and Simon, K.M.

ABSTRACT: Relative sea level at Vancouver, British Columbia rose from below the present datum about 30,000 cal yr BP to at least 18 m asl 28,000 cal yr BP. In contrast, eustatic sea level in this interval was at least 85 m lower than at present. The difference in the local and eustatic positions is attributed to glacio-isostatic depresssion of the crust in the expanding fore field of the Cordilleran ice sheet during the initial phase of the Fraser Glaciation. Our findings suggest that about 1 km of ice was present in the northern Strait of Georgia 28,000 cal yr BP, early during the Fraser Glaciation.

TITLE: Reconstruction of late Quaternary sea-level change in southwestern British Columbia from sediments in isolation basins.

AUTHORS: Hutchinson, I., James, T.S., Clague, J.J., Barrie, J.V., and Conway, K.W.

ABSTRACT: Bracketing ages on marine–freshwater transitions in isolation basins extending from sea level to 100 m elevation on Lasqueti Island, and data from shallow marine cores and outcrops on eastern Vancouver Island, constrain late Pleistocene and Holocene sea-level change in the central Strait of Georgia. Relative sea level fell from 150 m elevation to about –15 m from 14000 cal. yr BP to 11500 cal. yr BP. Basins at higher elevations exhibit abrupt changes in diatom assemblages at the marine–freshwater transition. At lower elevations an intervening brackish phase suggests slower rates of uplift. Relative sea level rose to about +1 m about 9000 cal. yr BP to 8500 cal. yr BP, and then slowly fell to the modern datum. The mean rate of glacio-isostatic rebound in the first millennium after deglaciation was about 0.11 m a^-1, similar to the peak rate at the centres of the former Laurentide and Fennoscandian ice complexes. The latter feature smooth, exponential-style declines in sea level up to the present day, whereas in the study area the uplift rate dropped to less than one-tenth of its initial value in only about 2500 years. Slower, more deeply seated isostatic recovery generated residual uplift rates of <0.01 m a^-1 in the early Holocene after the late-Pleistocene wasting of the Cordilleran ice sheet.

TITLE: Architecture and evolution of a fjord-head delta, western Vancouver Island, British Columbia

AUTHORS: Gutsell, J.E., Clague, J.J., Best, M.E., Bobrowsky, P.T., and Hutchinson, I.

ABSTRACT: The architectural framework and Holocene evolution of the Zeballos fjord-head delta on west-central Vancouver Island was established through a multidisciplinary field-based study. The Zeballos delta is a composite feature, consisting of an elevated, incised, late Pleistocene delta and an inset Holocene delta graded to present sea level. Both deltas have a classic Gilbert-type tripartite architecture, with nearly flat topset and bottomset units and an inclined foreset unit. Time domain electromagnetic (TDEM) and ground-penetrating radar (GPR) surveys, borehole data, and gravel pit exposures provided information on the internal form, lithologies and substrate of both deltas. Both sets of deltaic deposits coarsen upward from silt in the bottomset unit to gravel in the topset unit. The TDEM survey revealed a highly irregular, buried bedrock surface, ranging from 20 m to 190 m in depth, and it delineated saltwater intrusion into the deltaic sediments. Late Quaternary sea-level change at Zeballos was inferred from delta morphology and the GPR survey. The elevated, late Pleistocene delta was constructed when the sea was about 21 m higher relative to the land than it is today. It was dissected when sea-level fell rapidly as a result of glacio-isostatic rebound. Relative sea-level reached a position about 20 m below the present datum during the early Holocene. Foreset beds that overlap and progressively climb in a seaward direction and topset beds that thicken to 26 m landward imply that the delta aggraded and prograded into Zeballos Inlet during the middle and late Holocene transgression. Sea-level may have risen above the present datum during the middle Holocene, creating a delta plain at about 4 m a.s.l. Remnants of this surface are preserved along the valley margins.

TITLE: Marine and limnic radiocarbon reservoir corrections for studies of late- and post-glacial environments in Georgia Basin and Puget Lowland.

AUTHORS: Hutchinson, I., James, T.S., Reimer, P.J., Bornhold, B., and Clague, J.J.

ABSTRACT: Models of late-glacial environmental change in coastal areas are commonly based on radiocarbon ages on marine shell and basal lake sediments, both of which may be compromised by reservoir effects. The magnitude of the oceanic reservoir age in the inland waters of the Georgia Basin and Puget Lowland of northwestern North America is inferred from radiocarbon ages on shell-wood pairs in Saanich Inlet and previously published estimates. The weighted mean oceanic reservoir correction in the early and mid Holocene is –720±90 yr, slightly smaller than, but not significantly different from the modern value. The correction in late-glacial time is –950±50 yr. Valley-head sites yield higher reservoir values (–1200±130 yr) immediately after deglaciation. The magnitude of the gyttja reservoir effect is inferred from pairs of bulk gyttja and plant macrofossil ages from four lakes in the region. Incorporation of old carbon into basal gyttja yields ages from bulk samples that are initially about 600 yr too old. The reservoir age declines to less than 100 yr after the first millennium of lake development. When these corrections are accounted for, dates of deglaciation and late-glacial sea-level change in the study area are pushed forward in time by more than 500 yr.

TITLE: Multiple sources for late Holocene tsunamis at Discovery Bay, Washington State, USA.

AUTHORS: Williams, H.F.L., Hutchinson, I., and Nelson, A.R.

ABSTRACT: Nine muddy sand layers interrupt a 2500-yr old sequence of peat deposits beneath a tidal marsh at the head of Discovery Bay on the south shore of the Strait of Juan de Fuca, Washington. An inferred tsunami origin for the sand beds is assessed by means of six criteria. Although all the sand beds contain marine diatoms and almost all the beds display internal stratification, the areal extent of the oldest beds is too limited to confirm their origin as tsunami deposits. The ages of four beds overlap with known late Holocene tsunamis generated by plate-boundary earthquakes at the Cascadia subduction zone. Diatom assemblages in peat deposits bracketing these four beds do not indicate concurrent change in elevation at Discovery Bay. Diatoms in the peat bracketing a tsunami layer deposited about 1000 cal. yr BP indicate a few decimeters of submergence, suggesting deformation on a nearby upper-plate fault. Other beds may mark tsunamis caused by more distant upper-plate earthquakes or local submarine landslides triggered by earthquake shaking. Tsunamis from both subduction zone and upper-plate sources pose a significant hazard to shoreline areas in this region

TITLE: Late Holocene environmental change at treeline in the northern Coast Mountains, British Columbia, Canada

AUTHORS:Clague, J.J., Wolfarth, B., Ayotte, J., Ericksson, M., Hutchinson, I., Mathewewes, R.W., Walker, I.R., and Walker, L.

ABSTRACT: An integrated stratigraphic, paleoecological, and geochronological study of lake and fen sediments just beyond the terminus of Berendon Glacier provides insights into late Holocene climate, vegetation, and glacier change in the northern Coast Mountains of British Columbia. Cores collected from two small lakes in the foreland of Berendon Glacier and pits dug in a nearby fen record Little Ice Age and older glacier advances. The first Little Ice Age advance in this area occurred more than 500 years ago and peaked in the early 17th century. An earlier Neoglacial advance began about 2800-3000 cal yr ago and may have lasted for hundreds of years. There is also evidence for an intervening advance of even smaller magnitude around 1200-1300 cal yr ago. The advances are broadly synchronous with those in other parts of western North America, indicating that they were caused by regional, possibly global, changes in climate. Plant communities within the study area did not change dramatically during the late Holocene. The ranges of some plants, however, likely retracted or extended near treeline in response to changes in mean temperatures of perhaps 1-2oC, as well as changes in summer snow cover. The greatest changes in vegetation occurred within and just beyond the forefields of Berendon, Frank Mackie, and other nearby glaciers. The largest climate shifts of the last 3000 years took place during the late Little Ice Age and the last century. Climate warmed about 1-2oC during the twentieth century, accompanied by a rise in treeline, an increase in coniferous tree cover in the subalpine zone, and an increase in the temperature and biological productivity of ponds. These trends are likely to continue if climate, as expected, continues to warm.

TITLE: Chronology and hazards of large debris flows in the Cheekye River basin, British Columbia, Canada

ABSTRACT: Stump Lake is strategically located to provide a sediment record of very large debris flows that travel down Cheekye River to lower Cheekye fan from the west flank of Mount Garibaldi, a Quaternary volcano in the southern Coast Mountains of British Columbia. Cores collected from Stump Lake span the last 11,500 years and consist largely of gyttja. However, a diamicton layer and an associated detrital organic layer, which is capped by a clay lamina, record a large debris flow derived from the headwaters of Cheekye basin. Radiocarbon ages indicate that this event occurred about 6,900 years ago. We estimated the peak discharge of the flow by measuring cross-sections of the Cheekye River channel adjacent to Stump Lake and by calculating flow velocity using a Newtonian flow model. Reasonable values of peak discharge, calculated in this way, were input into an empirical equation linking discharge and debris-flow volume, developed for non-granitic debris flows. The estimated minimum volume of the debris flow is 3 to 5X10⁶ m³. Another large debris flow (ca. 3X10⁶ m³), which occurred about 800 years ago, is not recorded in Stump Lake sediments and thus was too small to reach the lake. The Stump Lake sediment sequence indicates that no debris flows larger than several million cubic meters have reached lower Cheekye fan in post-glacial time.

TITLE: Improved Relative Sea-level Histories for Victoria and Vancouver from Isolation Basin Coring.

ABSTRACT: The freshwater deposits of low-elevation lakes and bogs in southwestern British Columbia are commonly underlain by marine and glaciomarine deposits. Radiocarbon ages of shells, plant macrofossils, and gyttja from the marine-freshwater transition define the time that the basin became isolated from the sea. We present data from five lakes and bogs in the Victoria area, which, when combined with earlier results, indicate that sea level fell from above 60 m asl to below present sea-level between 12,500 and 11,500 14C years ago. In the Fraser Lowland data from seven lakes and bogs show that sea level fell from above 180 m asl to about 80 m asl between 12,500 and 12,000 years ago. The rate of sea-level fall may have then briefly slowed, but continued to drop to 20-30 m by 11,000 and to about 10 m by 10,000 years ago. A secondary resubmergence of the Fraser Lowland by 100 m or more proposed by earlier workers is ruled out by the radiocarbon ages and by diatom analyses indicating that freshwater conditions, once established, continued uninterrupted to the present.

TITLE: When the mountain dwarfs danced: aboriginal traditions of paleoseismic events along the Cascadia subduction zone of western North America.

ABSTRACT: Geological evidence demonstrates that recurrent great earthquakes have been generated at the Cascadia subduction zone, off the west coast of North America. Such major earthquakes and associated tsunamis would have had devastating impacts on Native villages along this coastline. Native oral traditions of such disasters, along with earthquake figures in myth and ceremony, are examined for evidence of the nature of such past geological events and the impact they have had on human populations.

TITLE: Archaeological evidence of village abandonment associated with late Holocene earthquakes at the northern Cascadia subduction zone.

ABSTRACT: Geologic evidence suggests that great (magnitude 8 or larger) earthquakes, or series of such earthquakes, occurred six times in the last 3000 years at the northern Cascadia subduction zone. The archaeological record, and native oral traditions, demonstrate that native villages along the adjacent coasts of southern British Columbia and Washington State were periodically abandoned in the late Holocene as a result of these earthquakes and associated tsunamis. We infer the temporal pattern of village occupation and abandonment from midden stratigraphy and an activity index based on the probability distributions of radiocarbon ages at thirty archaeological sites in three regions of northern Cascadia. Deposits of probable tsunami origin are interbedded with, or bound, cultural strata at several sites. Hiatuses in occupation, or periods of low inferred human activity, postdate earthquakes at many sites. The strongest correlation between earthquake incidence and site abandonment occurs in the Nootka Sound region. Variations in tsunami impact among sites and regions likely reflect differences in village locations, coastal morphology, and late Holocene sea-level history.

ABSTRACT: Thin sheets of sand and gravel occur within a sequence of fine organic-rich sediments at Catala Lake, off the west coast of Vancouver Island, British Columbia. The uppermost of these coarse sheets thins and fines landward away from the lake outlet, consistent with deposition by a tsunami. This coarse sediment sheet has been radiocarbon dated to some time after AD1655; we suggest that it was deposited in 1700 by the tsunami of the last great earthquake at the Cascadia subduction zone. Abundant plant macrofossils, derived from nearby forest, are present within and on top of the sand and gravel layer, suggesting that the tsunami transported forest-floor litter, mosses, and seeds into Catala Lake. Deposition coincided with abrupt changes in diatom and foramifera communities in the lake. The post-tsunami diatom assemblage is more marine in character than the immediate pre-tsunami asemblage, and the foraminifera community became more diverse afer the tsunami. These changes are due either to coseismic subsidence or erosion of the outlet by the tsunami, which increased tidal exchange between the sea and the lagoon that was the precursor to Catala Lake. Older coarser sediment layers in cores from Catala Lake and the bordering marsh may also be tsunami deposits. One of these layers is about 1,000 years old and dates to the time of the penultimate great Cascadia earthquake.

TITLE: Tsunamis and tectonic deformation at the northern Cascadia margin: a 3000 year record from Deserted Lake, Vancouver Island, British Columbia.

ABSTRACT: Three layers of sand occur within a sequence of muddy gyttja in a 3 m core recovered from Deserted Lake, which lies at the head of a fjord on the central west coast of Vancouver Island, British Columbia. The sedimentology and microfossil assemblages of these layers are consistent with deposition by tsunamis. Radiocarbon ages suggest that the tsunamis were generated by plate-boundary earthquakes at the Cascadia subduction zone about 2600, 1600, and 300 years ago. Diatom and protozoan contents of the gyttja layers indicate that relative sea level fell in the periods between tsunamis. The sea-level changes are likely a product of interseismic uplift of the North America plate margin. Post-tsunami diatom and protozoan assemblages are generally more marine than their pre-tsunami counterparts, suggesting that the study site subsided during earthquakes. Estimates of interseismic and coseismic deformation during one earthquake cycle indicate that less than half of the interseismic uplift was recovered during the earthquake, leading to cumulative, or permanent, uplift of the area.

TITLE: Pre-Younger Dryas resurgence of the southwestern margin of the Cordilleran Ice Sheet., British Columbia, Canada.

AUTHORS: Clague, J.J., Mathewes, R.W., Guilbault, J.P., Hutchinson, I., and Ricketts, B.D.

ABSTRACT: A lobe of the Cordilleran ice sheet readvanced into the central Fraser Lowland, southwestern British Columbia, Canada, on at least two occasions near the end of the last glaciation. This ice also flowed into the previously deglaciated, lower reaches of mountain valleys adjacent to the Fraser Lowland and into Washingon state. The first of these advances occurred before about 11 900 BP and ended with glacier retreat and the establishment of lodgepole pine forest on newly deglaciated terrain. Parts of this forest were overridden by ice during a second advance, shortly after 11 300 BP. The younger advance is most likely older than the Younger Dryas Chronozone (11 000 - 10 000 BP) and may correlate with an intra-Allerød cooling event (the Killarney-Gerzensee oscillation). The older advance may have occurred during the Oldest Dryas or Older Dryas cold period. Non-climatic factors could also be involved, as emergence of the Fraser Lowland before the older advance greatly reduced or eliminated calving at the glacier margin and thus altered the mass balance of the ice lobe.

TITLE: Plant macrofossil, pollen, diatom and foraminiferal biofacies of the Fraser Delta: an ecological perspective on palaeoecological investigations.

ABSTRACT: The Fraser River delta plain supports a diverse array of marine and terrestrial ecological communities. A mosaic of marshes, sand flats, and mud flats in the intertidal zone and wet meadows, shrub lands, bogs, and riparian woodlands above the limit of tides can be linked by a successional model that relates ecological communities to elevation and substrate characteristics. Paleoecological investigations of plant macrofossils, pollen, diatom, and foraminifers have proven useful in 1) reconstructing late-glacial and early Holocene conditions in the Strait of Georgia, 2) reconstructing small-scale changes in relative sea level, 3) assessing flood frequency, and 4) explaining autogenic changes related to fire occurrence in raised bogs on the delta plain.

TITLE: Reconstructing the tsunami record on an emerging coast: a case study of Kanim Lake, Vancouver Island, British Columbia, Canada.

ABSTRACT: A pilot study was conducted at Kanim Lake on the emerging coast of western Vancouver Island, British Columbia, to assess the efficacy of using lake sediments to determine tsunami run-up and recurrence. Sediment sequences in lakes near the coast can complement tsunami records derived from deposits underlying intertidal marshes. Marshes on emerging coasts are uncommon, of limited areal extent, and, most importantly, their deposits have a short lifespan. Tsunami deposits in lakes are less susceptible to bioturbation and erosion and, generally, can be more accurately dated than similar deposits in marshes and other terrestrial settings. An inferred tsunami deposit in Kanim Lake has distinctive lithological characteristics and contains marine and brackish-water microfossils. Kanim Lake also illustrates some of the limitations in using lakes to reconstruct tsunami run-up and recurrence. Although the lake has been in the potential run-up zone for tsunamis triggered by great earthquakes on the nearby Cascadia subduction zone for the last 3500 - 4000 years, it apparently has been inundated by only one tsunami in this period. This event probably occurred about 2800 years ago. Tsunamis since that time have failed to reach Kanim Lake as the lake basin has continued to rise through the run-up zone and the distance to the sea has increased. The development of dense forest stands on the progressively widening reach between the sea and the lake has probably been the most important factor in limiting tsunami access to the site.

TITLE: Stratigraphic and microfossil evidence for late Holocene tsunamis at Swantown marsh, Whidbey Island, Washington State.

ABSTRACT: Four muddy sand sheets occur within a tidal marsh peat at Swantown on the west coast of Whidbey Island, Washington. The two largest sand sheets pinch out about 100 m inland and became thinner and finer-grained landward. All four sand sheets contain marine microfossils and have internal stratification. They record repeated inundation of the marsh over a short time period by distinct pulses of sediment-laden oceanic water, consistent with deposition by a tsunami wave train. The layers have been radiocarbon-dated to 1160-1310, 1395-1610, 1820-2010 and 1865-2105 cal yr B.P. The overlap in age between the two youngest layers and inferred great earthquake events at the Cascadia plate boundary, some 250 km to the west, suggests that were emplaced by tsunamis from this source area. The two older layers do not correlate with plate-boundary events. They may be products of tsunamis caused by earthquakes on local faults in the Strait of Juan de Fuca or by submarine landslides in this area.

ABSTRACT: Postglacial rebound is the response of the Earth to the decay of ice sheets. A postglacial rebound model explains crustal tilting and rapid uplift at the northern Cascadia subduction zone that occurred during retreat of the Cordilleran ice sheet. Observations explained by the model include the shoreline tilts of two proglacial lakes that formed at 13.514 ka (¹⁴ C years ago) and rapid sea-level fall (land uplift) at 12-12.5 ka. Modeled mantle viscosity values range from 5 X 10¹⁸ Pa s to 5 X 10¹⁹ Pa s, and are consistent with previous viscosity inferences from observations of crustal deformation following subduction zone earthquakes (10¹⁸ to 10¹⁹Pa s). No lower limit to subduction zone mantle viscosity is apparent from our model, but viscosity values equal to or larger than 10²⁰ Pa s are definitely ruled out. Our modelled subduction zone viscosity values are smaller than most upper mantle viscosity estimates derived from postglacial rebound studies of tectonically less active regions (10²⁰ to 10²¹ Pa s). The rapid observed uplift at 12 ka requires, in addition to a low mantle viscosity, rapid unloading from a sudden collapse of remaining coastal portions of the southern Cordilleran ice sheet. The sudden collapse provides 0.18 m of global eustatic sea-level rise, approximately 0.7% of the sea-level rise associated with melt-water pulse IA. Predictions of a global postglacial rebound model (ICE-3G) with a 10²¹ Pa s upper mantle viscosity were previously applied to geodetic data from this region to isolate signals associated with the earthquake cycle. Owing to the low viscosity values, and resulting rapid recovery of glacial deformation, our model predicts present-day postglacial rebound uplift rates at least 10 times smaller than ICE-3G (less than about 0.1 mm/yr). As the ICE-3G adjustments were substantial, this indicates the need for re-evaluation of the geodetic data.

TITLE: The geologic record of large tsunamis on Vancouver Island, British Columbia.

ABSTRACT: Large tsunamis strike the British Columbia coast an average of once every several hundred years. Some of the tsunamis, including one from Alaska in 1964, are the result of distant great earthquakes. Most, however, are triggered by earthquakes at the Cascadia subduction zone, which extends along the Pacific coast from Vancouver Island to northern California. Evidence of these tsunamis has been found in tidal marshes and low-elevation coastal lakes on western Vancouver Island. The tsunamis deposited thin sheets of sand and gravel within peat and mud sequences in these low-energy environments. Sheets of tsunami sand and gravel commonly contain marine fossils and thin and fine landward, consistent with deposition by landward surges of water. They occur in settings where other possible depositional processes, such as stream flooding and storm surges , can be ruled out.

The last large tsunami generated by an earthquake at the Cascadia subduction zone occurred in January 1700. The spatial distribution of the deposits of the 1700 tsunami, together with theoretical numerical modelling, indicate wave run-ups of up to 5 m asl along the outer coast of Vancouver Island and up to 15-20m asl at the heads of some inlets. The waves attenuated as they moved eastward along Juan de Fuca Strait and into Puget Sound and the Strait of Georgia. No deposits of the 1700 event or, for that matter, any other tsunami, have yet been found in the Strait of Georgia, suggesting that waves were probably no more than 1 m high in this area.

If such a tsunami occurred today, communities along the outer Pacific coast from southern British Columbia to northern California would be severely damaged. There would be little time to evacuate coastal communities because the tsunami would strike the outer coast within minutes of the first ground shaking. Fortunately, such tsunamis are infrequent and perhaps as few as seven have occurred in the last 3500 years. Other tsunamis that are much smaller and more localized, although probably more frequent, are caused by local earthquakes and landslides along the British Columbia coast.

TITLE: A comparison of the vertical zonation of diatom, foraminifera, and macrophyte assemblages in a coastal marsh: implications for greater palaeo-sea level resolution.

Researchers generally only use one type of plant or animal to study a particular marsh. Consequently, it has been impossible to directly compare zonations obtained using different groups between sites. To facilitate such comparison, cluster analysis of foraminiferal, ditaom and macrophyte data collected in transects from a tidal marsh at Zeballos, northwestern Vancouver Island, British Columbia was carried out. These analyses yielded three, six, and four elevation-controlled assemblage zones, respectively. Physical parameters such as salinity and oxygen concentration affect the various taxa differently, resulting in significantly different assemblage boundaries between groups. A composite analysis of all groups yielded an assemblage zonation very similar to that obtained with the macrophytes alone. Although fewer assemblage zones were resolved with the composite analysis than with the diatom data alone, fewer sample misclassifications resulted in more precise elevation determinations. A second composite analysis using only foramiferal and diatom data, which is more useful to paleo-sea level researchers, also gave four elevation-controlled assemblage zones, although assemblage zone elevational boundaries differed slightly from those obtained with data from all groups. Our results will permit researchers working on diatoms, foraminifera or macrophytes to calibrate their zonations thus making it easier for workers in different fields to compare their results.

TITLE: On the annular growth pattern in Scirpus maritimus in an intertidal wetland: extension of the concept of cyclic development to within-clone spatial dynamics.

In a seminal work on community organization and succession, A.S. Watt presented evidence to show that some plant communities are in dynamic but cyclical equilibrium with their environment. Watt held that the resultant vegetation may have a patchy structure made up of different developmental phases ranging from "youngest" to "oldest" (termed "juvenile", "mature", "senile", and "slack"). At any given location, the vegetation repeatedly cycles throuigh these phases over time. We extend Watt's concept of cyclic succession to within-clone spatial dynamics in Scirpus maritmus L. (Cyperaceae) colonizing an early successional brackish-water environment. Specifically, we hypothesized that shoot populations of S. maritimus could be categorized into distinctive phases based on stem density and height and that differences in the developmental stage may be driven by a combination of biotic and abiotic controls. Data indeed show the existence of a strong annular pattern for stem density and height. The clone centres were highly truncated, typical of a phalanx growth strategy. Further analysis revealed that the chemical and physical environment varies between phases within clones, which may drive within-clone stem dynamics. We conclude with a model of phasic clonal growth for S. maritimus that posits the effects of demographic density-dependence and physico-environmental control on annular growth and within-clone spatial dynamics.