J'aimerais apporter un supplément d’information à mon billet d’hier sur les glissements de terrain de l’an mille, à Quyon (Québec).
Le reportage de Tom Spears paru dans l’Ottawa Citizen du 18 septembre et qui avait été le prétexte de mon billet était avant tout un exposé des travaux de Gregory R. Brooks, chercheur à la Commission géologique du Canada. Je n’ai peut-être pas assez bien fait ressortir le fait hier. Pour ceux qui aiment aller à la source, une épreuve d'un article de M. Brooks à paraître dans le Quaternary Research est disponible dans Internet (ScienceDirect).
Disponible, à la condition de débourser 31,50 $, US ou CAN, à ce prix, ça ne fait pas très grande différence. Pour ma part, je préfère me contenter du résumé (gratuit) :
«A landslide debris field covering ~ 31 km2, the presence of large sediment blocks up to hundreds of meters long, and the exposure of deposits of a single landslide along the incised course of the Quyon River are evidence of a massive failure of sensitive Champlain Sea glaciomarine sediments along the lower Quyon Valley, southwestern Quebec, Canada. Seventeen radiocarbon ages indicate that the failure occurred between 980 and 1060 cal yr BP. Twenty-four additional radiocarbon ages reveal that nine landslides within a 65-km belt in the Quyon–Ottawa area also occurred at approximately this time. In combination, the contemporaneous occurrence of ten landslides between 980 and 1060 cal yr BP, the setting or morphology of five of the other failures, and the close proximity of two of the failures to the Quyon Valley landslide provide circumstantial evidence of a paleoearthquake-triggering mechanism. The paleoearthquake is estimated to be Mw ~ 6.1 or larger, with the epicenter within the West Quebec Seismic Zone. A common earthquake-triggering mechanism for the three largest landslides in eastern Canada suggests a close link between massive failures of sensitive glaciomarine sediments and the regional seismicity.»
«Figure 9. Map of the Ottawa Valley showing the locations of the Quyon Valley landslide (shaded polygon) and other landslides (white stars) mentioned in the text (a—Murphy Road, b—Alary Road, c—Luskville, d—upper Quyon River, e—Breckenridge Valley and f—Beacon Hill).» Tiré de Brooks (2013) (lien plus haut) : malgré les apparence, il s'agit de la version agrandie de la miniature disponible en ligne.
Pour les autres qui, comme moi, hésiteraient à dépenser et qui aimeraient quand même «voir», The Landslide Blog a commenté le travail de Brooks, avec, en prime une carte couleur tirée de son article (lien). Je la reprends ici à mon tour.
«Figure 1. A) Location of study area in eastern Canada. B) Shaded relief map showing the landslide source and depositional areas within the lower Quyon Valley, Quebec. The source area is subdivided into the upper and lower scar zones, based on the obvious constriction in the valley (marked by arrows). Arrows also mark the transition from the lower scar zone and depositional area. Also shown are three separate, but much smaller, landslide scars (labeled SC1 to SC3) and the village of Quyon. The shaded relief map is based on a LiDAR-derived DEM (LiDAR DEM, © Government of Quebec).» Tiré de Brooks (2013), dans The Landslide Blog (liens plus haut).
Le journal The Equity (Pontiac) a aussi fait écho aux travaux de Brooks :
«There are more than 250 landslide scars in the Ottawa Valley and most of these landslides occurred in prehistoric times.
As part of the Public Safety Geoscience program, the GSC, NSR is conducting a study to determine how long ago some of the larger ancient landslides happened in the greater Ottawa area including Pontiac. [...] The GSC team has approached landowners in Pontiac in the Breckenridge, Luskville, Eardley, Quyon and Clarendon area to ask if they could work on their land.» (Andrea Cranfield, The Equity, 19 sept. 2013.)
Ajout (20 sept. 2013)
On peut aussi consulter le document publié par Russel et al. (2011) auquel a collaboré Brooks et intitué Deglacial history of the Champlain Sea basin and implications for urbanization. La section «Earth Flow Scars of Breckenridge Valley», rédigée par Brooks et Medioli, décrit les cicatrices de glissements de terrain préhistoriques et historiques (le plus récent de la dernière catégorie datant de 2008) à Breckenridge, entre Quyon et Gatineau. Voir ci-dessous la figure 2-7 et les extraits du texte :
«Breckenridge Creek is a tributary of the Ottawa River, located approximately 14 km northwest of Aylmer, Gatineau, Quebec. Draining ~66 km2, the creek watershed encompasses i) Precambrian bedrock, which forms the steep western side of the Gatineau Hills, ii) a quasi-flat plain underlain by fine-grained Champlain Sea deposits, and, along its lower 2 km, iii) an erosional terrace of the Ottawa River (Fig. 2-7). Across the surfaces of the Champlain Sea plain and the terrace, the stream network has eroded steep-sided valleys that are incised up to about 30 m deep (shallower on the terrace) into Champlain Sea sediments. [...] Breckenridge Valley is particularly notable because of the large clustering of earth flow scars along the stream network where it crosses the Champlain Sea plain. Twenty six of these scars (numbered 1 to 26 in Fig. 2-7) represent large prehistoric failures that occurred along Breckenridge Creek or one of its larger tributaries. The scars range from 13 000 to 252 000 m2 and have retrogression distances of 50 to 920 m. [...] Three radiocarbon dates from [layer of organic material buried beneath prehistoric-aged spoil] range between 1115 to 1205* yr BP (Brooks unpublished data). A second buried layer of organic material, including large logs, can be found at the upstream end of the spoil on the north side of the creek. [...] The ages of two wood samples from near the top of the bank range from 7000 to 7105 yr BP (Brooks unpublished data) and are evidently part of the same layer that contained wood aged 7050 ± 80 (GSC-6233), 7030 ± 70 (GSC-6243) and 6980 ± 80** (GSC-6246) yr BP that was collected by the GSC in 1996. The presence of these two buried organic layers of markedly different age within the spoil and the backscarp reveal that the 2008 failure occurred through the remobilization of older earth flow spoil and that one of the older failures occurred at about 7000 yr BP.» (J'ai retiré du texte les réf. à des figures non reprises ici.)
* Dates proches de celles du glissement de terrain de Quyon.
** Dates près de celle de 7060 BP établie par Aylsworth et Lawrence (2003) pour d'autres glissements de terrain à l'est de Gatineau (voir dans le billet précédent, lien plus haut).
Tout ça fait un peu compilation à la hâte. Mais c'est précisément ce que je voulais faire.
Références
- Gregory R. Brooks, 2013 – «A massive sensitive clay landslide, Quyon Valley, southwestern Quebec, Canada, and evidence for a paleoearthquake triggering mechanism», Quaternary Research, Corrected Proof, available online 24 August 2013.
- Brooks, G.R. and Medioli, B.A., 2011. «Stop 2-3: Earth flow scars of Breckenridge Valley». In: Deglacial history of the Champlain Sea basin and implications for urbanization. Russell, H.A.J., Brooks, G.R. and Cummings, D.I. (Editors), Joint annual meeting GAC-MAC-SEG-SGA, Ottawa, Ontario, May 25-27, 2011, Field Guide Book, p. 57-61. Geological Survey of Canada, Open File 6947*.
- Nouveau (ajout 30 sept. 2013) Brooks, G.R., Medioli, B.E., Aylsworth, J.M., and Lawrence, D.E., 2013 – A compilation of radiocarbon dates relating to the age of sensitive clay landslides in the Ottawa Valley, Ontario-Québec. Geological Survey of Canada, Open File 7432, 62 p. doi:10.4095/292913*.
- Russell, H.A.J., Brooks, G.R., and Cummings, D.I. (ed.), 2011. Deglacial history of the Champlain Sea basin and implications for urbanization; Joint annual meeting GAC-MAC-SEG-SGA, Ottawa, Ontario, May 25–27, 2011; Fieldtrip guidebook; Geological Survey of Canada, Open File 6947, 96 p. doi:10.4095/289555*
* Publications disponible gratuitement dans Géoscan ; taper le doi de la publication qui figure à la fin de la notice bibliographique dans l'outil recherche.
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