Methods of terrain evaluation,Mackenzie Transportation Corridor,N.W.T., Canada

In 1971 and 1972, the Terrain Sciences Division of the Geological Survey of Canada carried out the largest mapping program in its history of the distribution and characteristics of terrain types. Over 140.000 mile² (360,000 km²) of the Mackenzie Transportation Corridor was evaluated. This information was supplied to the public, industry and government in order to plan and design projects properly, by efficiently utilizing natural resources and minimizing environmental damage. The requirements of potential users varied to such a degree that it was decided to map the ‘basic’ geology and other terrain characteristics so that derivative maps could be constructed for specific uses. A mapping system was devised for surficial deposits using genesis as the primary unit, modified by texture and morphology. Organic and permafrost terrain were included, factors not ordinarily a part of traditional geological mapping. The system was comprehensive and flexible enough to include such aspects as the percentages of minor units, unit thickness, and slope and other morphological characteristics. In bedrock and unglaciated areas, a second system based upon the same principles as the first but arranged in a different manner was devised. In this case, the primary unit is based upon relief, with additional information on lithology of the bedrock, texture of overlying debris, and broad morphological and slope characteristics. Comprehensive legends were prepared. Using the mapping units as the base, the legends were divided into various categories, providing information on the composition, ground ice content, thickness, and engineering characteristics of the deposits, and surface characteristics including topography, drainage pattern, vegetation, and dominant soils (pedologic sense).     

Reply to Lowe,D.J., Wilson,C.J.N., Newnham,R.M., Hogg,A.G. comment on Grapes, R., Rieser,U., Wang,N., 2010. Optical luminescence dating of a loess section containing a critical tephra marker horizon,SW North Island of New Zealand. Quaternary Geochronology 5, doi:10.1016/j.quageo.2009.01.004, 2010

The arguments presented by Lowe et al. [Lowe, D.J., Wilson, C.J.N., Newham, R.M., Hogg, A.G., 2010. Dating the Kawakawa/Oruanui eruption: comment on “optical luminescence dating of a loess section containing a critical tephra marker horizon, SW North Island of New Zealand” by R. Grapes et al. Quaternary Geochronology 5(4), 493–496] against our IRSL results, which suggested that the widespread Kawakawa tephra (KkT) could be considerably younger than the generally accepted 27.1 ka cal BP age, are unsustainable. We discuss the points raised by Lowe et al., in terms of: 1) Presentation and analysis of luminescence ages (comparison between reporting and error margins of luminescence and ¹⁴C ages, statistical treatment of age data); 2) Possible sources of error (“upbuilding pedogenesis” and its affect on U and Th distribution in loess, effect of biotubation, variation of K in loess, single grain luminescence dating of quartz, probability of luminescence age underestimation in dating tephra); 3) Stratigraphic and paleoenvironmental considerations (ages of tephras overlying KkT, timing of the end of Ohakea loess deposition and its distribution; 4) Radiocarbon-based ages of KkT (problems with the currently accepted ¹⁴C 27.1 ka cal BP age of KkT). We stress that our study was not to establish a new benchmark age for the KkT, but to open debate about the currently accepted benchmark age of the KkT, which we deem to be erroneous.