Structural study of the Beaver Creek (Yukon) recession moraine: What it is a cryostratigraphic ... sediments and to infer the frozen history of earth material.
Structural study of the Beaver Creek (Yukon) recession moraine: From boreholes to a map What it is a cryostratigraphic approach Cryostratigraphy is the study of frozen layers in the earth's crust (French and Shur, 2010). It is a branch of geocryology and differs from traditional stratigraphy by explicitly recognizing permafrost contains structures that are different from those found in unfrozen sediment and rock. Its aim is threefold:
Sliger, M. (1 et 3) et Fortier, D. (1, 2 et 3)
Actual setting and known history
1) Département de géographie - Université de Montréal (Montréal, QC) 2) Institute of Northern Engineering - University of Alaska (Fairbanks, AK)
From the surveys and the satellite image, the local ecological systems have been mapped. A quick analysis shown that:
The muskeg, which is a synonym for wetland, obviously fits with the negative topographic elements of the landscape. The boreal forest, which is a mesic environment, coalesce with the positive topographic elements.
Contrasted cryostratigraphic sequence A total of 25 new shallow boreholes have been described according to the cryostratigraphic approach (French and Shur 2010) and the result were integrated in a GIS to expand the former analysis of 22 deep boreholes by Stephani (2013). It resulted in the following general synthesis; basically, every units are thicker in the Type B cryostratigraphic sequence. Type B
The paleosol was unclear under the forest and the diamicton haven’t been found under the lowest surface of the muskeg. The basal diamicton includes significant organic and ice contents when close downstream from a slope break.
Mapping the most representative areas of the boreal forest allows to guess the cryostratigraphic sequence over Unit 1, where the ‘’muck’’ deposit could be as thin as 2-3m. Transition
Boreal forest
Transition
Muskeg
Boreal forest
Transition zone
Slope inflexion
Unit 5A Unit 5B
Transition
Convex slope
Concave slope
Unit 4
Unit 3 Unit 1A Unit 1A Unit 2
Unit 1B
Figure 5 - Idealized profile view of the deposit over the Beaver Creek recession moraine based on the forested ecosystem mapping. This view shows the transect between two forested ecosystemic islands, under the actual alignment of the highway at BC-RES.
Unit 5 — Contemporary syngenetic permafrosts (including the active layer)
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5
Units 2-5 are thicker away from the forested patch.
In between, patches of both ecosystems juxtapose to form a patchwork where forested stands progressively tend to dominate toward the tops of the hills.
The active layer is significantly thicker under the forest than under the muskeg while the or ganic cover follows the opposite trend, it is thicker under the muskeg and thinner under the boreal forest.
Type A
The contrasted cryostratigraphic sequences (Type A and B) appear linked with topography and ecosystems
Link between ecosystems and topography
The ecosystems are linked with topography.
3) Centre d’étude Nordique - Université Laval (Québec, QC)
1) To identify the genesis of perennially frozen sediments and to infer the frozen history of earth material. 2) To document the ground-ice conditions in different types of perennially-frozen rock and soil. 3) To correlate sequences of ground ice in contemporary permafrost regions with horizons of former ground ice in past permafrost regions. At the Beaver Creek road experimental site (BC-RES), this approach was used to document the ground ice genesis to allow understanding of its distribution in the landscape. To proceed, the use of the available data on paleoclimate and paleogeography at the regional scale were a key component. Some ecological considerations were necessary as well.
Type A was found close from the top of hills, its ice-rich transition layer is about 0.5m and the total unit’s ice content smoothly decrease with depth. Type B was found anywhere else in the muskeg. Its thick transition layer contrasts strongly with the type A’s one. Ice-wedges were found close from the surface.
Both idealized soil columns represent a specific point in the landscape. However, from the top of a toposequence to its bottom, someone should expect an irregular increase in this Unit’s thickness. (Photo: Larrivée, K. Scan: )
Unit 4 — Ice poor dense silt
Figure 1 - Beaver Creek is located in the extended discontinuous permafrost zone (Brown et al., 1998). The mean annual air temperature is –5.1°C. A prior study at the site shown that the ground temperature close to the zero annual amplitude depth was –3°C (Lipovsky and Yoshikawa, 2008). The dry continental climate provides about 400mm of precipitation yearly (Environment Canada, 2014).
1.35mm/10yr 20.00mm/10yr
(Vermaire and Cwynar, 2010)
Figure 3 - It was found in the Antifreeze Pond, at the site, that 2m of loess has been deposited in 1ky at the glacial-interglacial transition (1514kyBP), and another 2m accumulated more slowly later, during the Holocene. Pollen analysis showed a drastic ecological shift at the Holocene thermal maximum (118.5kyBP), suggesting sudden climate warming (Vermaire and Cwynar, 2010) which has likely led to a permafrost thawing event .
Figure 2 - The site has been affected by multiple glacial events. It now sits on the mid-Pleistocene recession moraine; the last front ended just upstream the BC-RES (Duk-Rodkin, 1999).
Figure 6 - Focusing on the forested ecosystemic islands, the post glacial sedimentary blanket thickness can be quickly guessed on the basis of the cryostratigraphic data.
Its organic matter and ice contents are low, rare decayed roots can be found and some faults are found in association with oxidation around. The grain-size statistical distribution agrees with a typical eolian deposit (Seppäla, 2004). However, some gravel inclusions may indicate the proximity of coarse material (assuming the frost heave process is part of the cryoturbation phenomenon). This unit was affected by a partial thaw and an epigenetic permafrost freezeback event at the Holocene climatic optimum.
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(Photo: Stephani, 2013)
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Unit 3 — Organic and ice-rich syngenetic paleosol The high organic matter content generally appears as peat block inclusions in ice-rich silt conform to the one of Unit 2. A microlenticular cryostructure is associated with silt while visible macropore ice comes with silty peat. The contacts tilt and orientation between elements are rarely straight. Parts of an extended ice-wedges network are found in almost every deep borehole (under the road in the muskeg). Some wood pieces can be found, especially in the bottom.
(Photo: Stephani, 2013)
Conclusion and further Ground ice, being a thermally vulnerable sediment, has to be considered with care. The cryostratigraphic approach complementing a sufficient knowledge of the regional natural history and geography might be a key component of an efficient and respectful use of unstable terrains.
Unit 2 — Ice-poor stratified deposit
Figure 4 - The Wellesley Basin (circled) consists in a geologic depression hundreds of kilometers wide that has been extensively filled by the glacial drift (Bostock, 1948) from the St-Elias glacial lobe (arrow). From the geological map (Lipovsky and Bond, 2014), the whole area is classified as quaternary deposit. Thick unconsolidated sediments have been preserved around the area glaciated during the last glacial maximum .
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The stratified silt with organic-rich parallel layers of ~5mm is sometimes intersected by sand or fine gravel seams. Branches and/or roots are commonly found, but not always in relation with peat. The sediment appears to result from lacustrine, and sometimes fluvial, accumulation and the permafrost seems to be epigenetic.
Being the discipline that study northern geosystems at large, geocryology would use such an approach to contribute to northern engineering. Ground structure was the first step for a geosystem assessment. The next steps would include: (Photo: Stephani, 2013)
Unit
2
1 — Silty diamictons
Type A is massive and ice-poor, it is interpreted as a glacial diamicton (lodgement till) in which permafrost aggraded. The Mirror Creek glacial maximum hasn’t been dated yet.
Type B is the same material but includes a very high content of lenticular and ataxitic ice and massive peat beds with wood in the upper portion of the diamicton (reworked) . It is only found at the inflexion of a natural slope.
1) Study of the physical properties of each cryostratigraphic unit 2) Study of the thermo-hydrological behaviour of each eco-system 3) Integration of the complete knowledge by numerical modelling (structure/properties/behaviour) (Photo: Stephani, 2013)
1? RÉFÉRENCES
Bostock, H.S. 1948. PHYSIOGRAPHY OF THE CANADIAN CORDILLERA, WITH SPECIAL REFERENCE TO THE AREA NORTH OF THE FIFTY-FIFTH PARALLEL. Print#2483 CANADA DEPARTMENT OF MINES AND RESOURCES - MINES AND GEOLOGY BRANCH - BUREAU OF GEOLOGY AND TOPOGRAPHY, Ottawa, ON.
Duk-Rodkin, A.,1999. Glacial limits map of Yukon Territory. Commission géologique du Canada, Dossier public. Geological Survey of Canada, Ottawa, ON
Seppäla, M. 2004. Wind as a geomorphic agent in cold climates. Cambridge, UK: Cambridge University Press, 358p.
French, H., Shur, Y. 2010. The principles of cryostratigraphy. Earth-Science Reviews 101, 190-206
Brown, J., O.J. Ferrians, J., Heginbottom, J.A., Melnikov, E.S. 1998. Circum-arctic map of permafrost and ground ice conditions., in: Glaciology., National Snow and Ice Data Center/World Data Center for Glaciology (Ed.), Revised February 2001 Boulder, CO.
Lipovsky, P.S. & Bond, J.D. 2014. Yukon digital surficial geology compilation. Digital release 1, 08-Apr-2014, Yukon Geological Survey.
Stephani, E. 2013. PERMA FROST GEOSY STEM A SSESSMENT AT THE BEAV ER CREEK ROA D EX PERIMENTA L SITE (ALASKA HIGHWAY, YUKON, CANADA). Thesis from Institute of Northern Engineering. University of Alaska Fairbanks, Fairbanks, AK, p. 481.
Lipovsky, P.S., Yoshikawa. K. 2008. Initial results from the first year of the Permafrost Outreach Program, Yukon, Canada., in: Yukon Geological Research (Ed.), Yukon Exploration and Geologygy 2008. Geological survey of Yukon, Whitehorse.
Vermaire, J.C., Cwynar, L.C. 2010. A revised late-Quaternary vegetation history of the unglaciated southwestern Yukon Territory, Canada, from Antifreeze and Eikland ponds. Canadian Journal of Earth Sciences 47, 75-88.
Environment Canada. 2014. Canadian climate normals1981–2010, http://www.climat.meteo.gc.ca.
Figure 7 - The trend observed at the site could be reasonably extrapolated to all the recession moraine that wasn’t extensively reworked by glacio-fluvial processes (one distinct and uniform terrain). Numerous other areas seems to be of a similar geomorphological nature in the region.
