Thermokarst sedimentology of the Tuktoyaktuk Coastlands, Northwest Territories.

• Main Author: Murton, Julian Baird.
• Format: Thesis
• Language: unknown
• Published: Université d'Ottawa / University of Ottawa 1993
• Subjects: Geology.; Northwest Territories; Tuktoyaktuk; Ice; permafrost; Thermokarst
• Online Access: https://dx.doi.org/10.20381/ruor-11419; http://www.ruor.uottawa.ca/handle/10393/6734

Thermokarst sedimentology is the study of the sedimentary processes and facies associated with thermokarst. Using facies analysis, oxygen isotopes and observations of processes in the Tuktoyaktuk Coastlands, this thesis (1) classifies frozen ground according to its cryostructures and cryofacies; (2) describes thermokarst facies, facies associations and sedimentary structures; (3) examines the sedimentary processes associated with thermokarst; (4) proposes thermokarst facies models; and (5) proposes criteria for identifying thermokarst-modified sediments. The thermokarst sedimentary system of the Tuktoyaktuk Coastlands comprises uplands, slopes and basins. Beneath ice-rich uplands, downwearing thermokarst produces a thick ($\le$c. 2.5m) thaw layer in which sediments melt-out from underlying permafrost. Ice-rich slopes are subdivided into steep icy slopes and retrogressive thaw slumps, the former occurring where the percentage of excess ice in upland materials is less than c. 30-40%, the latter where it exceeds this value. As slopes retreat by backwearing thermokarst, upland materials are redeposited by alluvial and colluvial processes. Thermokarst basins form where back- and downwearing thermokarst coincide. In deep non-oriented basins containing thermokarst lakes, three stages of basin infilling are identified. The first occurs during early and rapid basin expansion, when intense backwearing thermokarst at basin margins transports large quantities of upland materials into the basins. This pulse of resedimentation initiates sublacustrine benches. The second stage beings as the rate of basin expansion diminishes, reducing the influx of clastic sediment into lakes; thus the tops of sublacustrine benches are reworked and the main depositional process changes to suspension settling in basin centres. The final stage, commencing as lakes drain, involves basin infilling by peat accumulation and by gelifluction and aeolian deposition. Two sedimentary structures relating to thermokarst are frost-fissure pseudomorphs and thermokarst involutions. Frost-fissure pseudomorphs develop through thaw-modification processes: slow subsidence, thermal erosion, refreezing, loading, buoyancy, spreading, folding, shearing and mass movement. Thermokarst involutions form primarily by water-escape or by loading and buoyancy. Involutions within a thick palaeothaw layer probably reflect the massive scale of soft-sediment deformation that accompanies regional thermokarst, and they provide a potential analogue for some Pleistocene involutions in the mid-latitudes. Five criteria identify thermokarst-modified sediments in the Tuktoyaktuk Coastlands: (1) organic-rich (and sandy) diamicton; (2) granular mud aggregates in stratified facies; (3) impure sand ($\pm$diamicton); (4) frost-fissure pseudomorphs; and (5) thermokarst involutions.