John L. Luternauer

Holocene sponge bioherms on the western Canadian continental shelf

Abstract Bioherms dominated by hexactinellid sponges occur in the three major troughs which cross the north-central continental shelf of British Columbia, Canada. The preferred substrate of the bioherms is till, glaciomarine sediments or bedrock in 150 to 250 m water depth. These bioherms began to develop in the early Holocene and have expanded since that time to discontinuously mantle more than 700 km 2 today. The sponge colonies form mounds and sheet-like structures up to 15 m thick and several kilometres wide and accrete new material to the bioherm through the removal of silty clay from suspension by the baffling of bottom currents. The sedimentologic and morphologic characteristics of the bioherms are in part due to fused siliceous mainframe skeletons of the sponges that provide the framework for the construction of the bioherm. The locus of sponge habitation on the surface of the bioherms changes with time in response to sediment accretion and microtopographic changes which affect delivery of sediment and nutrients entrained in bottom currents.

Burrow distribution of thalassinidean shrimp on a Fraser Delta tidal flat, British Columbia

Burrows of the thalassinidean shrimp Callianassa californiensis and Upogebia pugettensis , which resemble the trace fossils Thalassinoides and Ophiomorpha , respectively, occur on the southern tidal flats at the western front of the Fraser Delta. Burrow densities for both shrimp species are highest at lower intertidal levels immediately inshore of an extensive bed of Zostera marina eelgrass, but the high-density shrimp populations are segregated: the highest densities of C. californiensis burrows (350–450 burrow openings/m2) occur in a strip of clean sandy sediments, whereas U. pugettensis burrows are most abundant (30–80 burrow openings/m2) in a patch of muddy sands and sandy muds. On the tidal flats of the Fraser Delta as a whole, however, textural properties of the sediment show no consistent correlation with the burrow density of either shrimp. In the eelgrass bed, eelgrass root mats appear to markedly restrict the density of C. californiensis burrows. Towards the shore, the well-lined dwelling burrows of U. pugettensis occur no higher than mean sea level where maximum continuous exposure is ≤0.5 days. In contrast, C. californiensis , which has an unlined feeding burrow and a higher tolerance for anoxia, is present up to the margin of the local salt marsh which lies near the mean higher high water level where the flats can be exposed continuously up to 5 days. In situ experiments indicate that C. californiensis extrudes 18 ± 9 ml of wet sediment/shrimp/ day onto the substrate surface, while U. pugettensis seldom forms a mound around its burrow entrance. Burial of filter-feeding postlarval U. pugettensis under the mounds produced by C. californiensis may increase postlarval mortality and reduce adult populations of U. pugettensis in areas of high C. californiensis burrow density. The results of this study suggest that in distinguishing Ophiomorpha from Thalassinoides more emphasis should be placed on the presence of an extensive burrow lining in Ophiomorpha than on the presence of a knobby burrow exterior, because the former characteristic has a more profound bearing on burrow function, physiology of the occupant organism, and, consequently, burrow distribution.

Late Pleistocene terrestrial deposits on the continental shelf of western Canada: Evidence for rapid sea-level change at the end of the last glaciation

A paleosol with in situ rooted plant remains has been found in a core at a water depth of 95 m on the central continental shelf of British Columbia. Fluvial sediments associated with the paleosol are sharply overlain by lagoonal or shallow pond sediments; these, in turn, are overlain by shallow-marine ediments. Radiocarbon dates on wood fragments and root recovered from the cored sediments indicate that relative sea level was at least 95 m lower 10,500 yr B.P. and that the core site was rapidly transgressed by the sea shortly thereafter. This rapid transgression was contemporaneous with an equally rapid regression at the heads of fiords on the British Columbia mainland to the east. The two are probably genetically linked and a result of late Pleistocene deglaciation and the migration and collapse of a glacial forebulge. Our evidence indicates that large areas of the British Columbia continental shelf were subaerially exposed 11-10 ka. This may have facilitated the southward migration of early humans from Beringia into mid-continental North America at the end of the Pleistocene.

Suspended sediment hysteresis in a salt-wedge estuary: Fraser River, Canada

Abstract Suspended sediment transport in the Fraser River estuary, British Columbia, Canada, is affected by fluvial and tidal time lag, or hysteresis. Fluvial hysteresis is evident after mobile sediment is exhausted early in the spring freshet and the peak in daily sediment concentration precedes the peak in river discharge. Tidal hysteresis is apparent over tidal cycles as the peak in sediment concentration generally follows the peak in velocity. This is a consequence of settling lag resulting from enhanced shear velocities during decelerating currents.

Late Quaternary geology and geochronology of the central continental shelf of western Canada

Piston cores and high-resolution seismic profiles from Queen Charlotte Sound have yielded new information on the late Quaternary history of this part of the western Canada continental shelf. Cores in three major troughs (Goose Island, Mitchells and Moresby), up to 130 km apart, sample a similar sedimentary sequence. This sequence comprises five units: The lowest unit (A), which is up to 40 m thick, is a laminated to massive mud containing coarse ice-rafted debris deposited between pre- 13.6 ka and 12.9 ka during deglaciation of Queen Charlotte Sound. This unit is overlain by a sequence of stratified sediments (units B1–B3) up to 20 m thick deposited during the transition from the Pleistocene to the Holocene. Unit B1 directly overlies, and is similar to, unit A but lacks ice-rafted detritus. Unit B2 is sandy mud (locally gravelly) deposited during a period of lagging and winnowing from about 12.9 to 10.2 ka when sea level in Queen Charlotte Sound was lower, relative to the land, than it is today. Unit B3 is a laminated to massive mud similar to Unit B1 deposited when water depths in Queen Charlotte Sound rapidly increased at the end of the Pleistocene due to the combined effect of eustatic sea-level rise and forebulge collapse. The sedimentary sequence is capped by up to 10 m of mud (unit C) which is more organic-rich and contains more clay than any of the other units. It was accumulating under present-day conditions as early as 9.7 k.y. ago.

Surficial geology of the northwestern Canadian continental shelf

Abstract The northwestern Canadian continental shelf, including Dixon Entrance, Hecate Strait and Queen Charlotte Sound, can be divided into four surficial geological units all overlying Tertiary bedrock within the Queen Charlotte Basin. They include: (1) glacial till, deposited primarily in the troughs between banks and considered to be late Wisconsinan in age; (2) glaciomarine mud, deposited in the troughs as ice retreated from the shelf; (3) Queen Charlotte sands and gravels, outwash deposition in the north and sublittoral sand deposition in the south during sea level low stand and subsequent reworking during transgression; and (4) Queen Charlotte muds, deposited within the troughs of the region from late Wisconsinan times until present. These units are a result of a glaciation ending 15,000–13,000 years BP followed by relative sea level lowering by approximately 100 m, that persisted until 10,500 years ago, and a subsequent transgression that occurred over approximately 1500 years in a series of steps.

Sediment Transport in a Submarine Channel System: Fraser River Delta, Canada

ABSTRACT The Sand Heads Seavalley is a submarine channel system seaward of the mouth of Fraser River, Canada. Soundings and sonographs reveal that the Seavalley is composed of tributary channels that coalesce downslope into a single sinuous channel that splits into distributary channels at the base of the slope. The channels are inferred to result from turbidity currents generated by periodic liquefaction of sandy sediment at the mouth of the river. Bathymetric surveys of a river mouth slope failure in 1985 are used to estimate sediment transport rates for a turbidity current generated by the failure. These rates are compared to predictions using a turbidity current model based on relationships between settling velocities of surface sediments and shear velocities required to transport the sediment as suspended and wash loads. Model predictions that assume suspended load transport are much lower than estimates from the landslide, but predictions based on wash load transport show reasonable agreement. The better agreement with the wash load model may be because grain sizes in the deposits are controlled by the sizes of sediment supplied to the distributary mouth bar by the river, rather than by the competency of the turbidity curre t itself.

A delta plain sheet sand in the Fraser River delta, British Columbia, Canada

Abstract Borehole data in the Fraser River delta demonstrate that a nearly continuous sand unit, generally 8 to 20 m thick, underlies the surficial silts and sands of most of the delta plain. The sand unit consists of one or more sharp-based fining-upward sequences and is interpreted to be a complex of distributary channel deposits. Distributary channel migration and the resulting accumulation of channel sands has occurred primarily in a tidal flat environment, due to the interaction of tidal and fluvial processes and the high proportion of sand in the sediment load. This sand unit thus provides a model for sand-rich, river and tide dominated delta plains. Jetties now maintain the position of the Main Channel of the Fraser River where it crosses the tidal flats, and prevent channel migration and further storage of sand in the lower delta plain. As a result, more of the available sand is being transported directly to the river mouth, where failures transport the sand downslope. Consequently, these structures contribute to instability at the river mouth.

Origin of modern heavy mineral deposits, northern British Columbia continental shelf

Abstract Bottom sediments of the northern British Columbia continental shelf contain, on average, 10% of heavy minerals by weight within the dominant sand fraction. Anomalous concentrations occur within selected sand samples from Queen Charlotte Sound (> 25%) and Hecate Strait (> 18%). The heavy mineral suite here is dominated by amphibole and the titanium minerals ilmenite, sphene and titaniferous magnetite. Dynamic modelling of the present oceanographic and sedimentary environment in Queen Charlotte Sound suggests that the anomalous offshore concentrations of heavy minerals result from continuous differential sorting during extreme yearly storms. The deposits are not considered to be relict.

Suspended sediment concentration in a buoyant plume: Fraser River, Canada

A study of suspended sediment concentration in the buoyant plume of the Fraser River, Canada, showed that unstratified flow conditions at the river mouth caused resuspension of sandy bed material and high concentrations of coarse sediment. When flow at the mouth was stratified, sediment was fine-grained and concentrations were low. Application of a multivariate model revealed that suspended sediment concentration along the plume axis was controlled primarily by distance seaward of the river mouth, secondly by tidal height, and least by sediment concentration in the river.