Turbidity Process And Sedimentary Environment Of South-East Canada(Atlantic) Continental Slope

Submarine canyons are pathways for the transfer of sediment from the shelf to deep water and sites of initiation of turbidity currents.The details of how this sediment transfer takes place is poorly understood,largely because in modern systems turbidity currents that pass through canyons and reach deep water(>3000 m)are rare.Turbidity current flow has been monitored in detail in only two marine systems: Monterey Canyon and Squamish prodelta channels.It is well known that the frequency of turbidity currents in canyons is higher than in deeper water.While monitoring of modern systems is providing critical information on flow behavior of turbidity currents,much of our understanding of these currents comes from study of their deposits.The role of canyons in trapping sediment from turbidity currents,both those that die within the canyon and those that reach deeper water,is poorly understood.The authoer have collected a targeted set of cores then enable us to track the depositional variation both down-canyon and with height above the talweg and make inferences on the behavior of flows larger than those monitored in modern canyons and prodeltas.Halibut Canyon is a large canyon on the SE Canadian continental margin(44 –45°N,52-55°W)that was highly erosional under glacial maximum conditions,but in the latest Pleistocene built an inner levee adjacent to the narrow talweg.It may be representative of many submarine canyons on glaciated margins in which meltwater has played an important role.Sediment cores across this inner levee,in transects at 20 km and 40 km from the canyon head,provide a ?19 ka record of turbidity current flow,encompassing periods of ice retreat and some sea level rise on the adjacent shelf.Part of the 1929 “Grand Banks” turbidity current flowed down at least West Halibut Canyon and the talwegs of both East and West Halibut canyons appear erosional.Modern densewater flows transport some sediment in the canyon head.The overall goals of this study were(1)to compare turbidity currents and turbidites at times of erosional meltwater discharges on the shelf with those from times of less energetic canyon head processes;(2)to relate the geometry of canyon fill to the inferred history of turbidity currents;(3)to reconstruct the sedimentary environment in Halibut Canyon over the last 19 ka and(4)to track down-canyon changes in turbidity canyon and current evolution and deposition.Units and lithofacies: Three distinct sediment units in Halibut Canyon can be defined based on lithofacies,physical properties(a*,grain size)and geochemistry(p XRF).Unit 3 is a thick olive grey mud unit of Holocene age with a distinctive geochemical composition,with contourite sediment mainly deposited from the Labrador Current.Unit 2 contains silty and sandy turbidite beds,diatom ooze beds,and interbedded brown mud are recorded,dating from the Younger Dryas to Heinrich event1.Underlying unit 1 has thicker and coarser turbidite beds of proglacial origin,compositionally similar to ice-proximal sand and mud beds sampled on the shelf.Radiocarbon dating provides a chronologic framework of continuous sediment records from before Heinrich Stage 1.The author describe and characterize both sandy and muddy turbidites and correlate some individual deposits both laterally and downcanyon.Sedimentary source and environment: Two main sediment transport pathways are expected: along-slope transport by the Labrador Current and down-slope transport by various gravity-related processes.Silt and sand beds,present in unit 2 and dominant in unit 1,are representative of down-slope transport,whereas muds may represent either down-slope or along-slope transport.Bulk chemical composition and other proxy data are used to identify changes in sources of turbidite sediment through time.Evolutional phases: Three stages of ice-margin canyon evolution are recognised,taking Halibut Canyon as an example: 1)canyon erosion stage,created by plunging hyperpycal meltwater flows of uncertain age when the ice stream reached the shelf edge;2)canyon active deposition stage that built inner levees and was supplied by meltwater from an ice margin on the outer shelf,with flows of variable size similar to those on modern deltas;and 3)the canyon passive deposition stage when downslope processes are of minor importance and sedimentation was dominated by the strengthened alongslope Labrador Current.