Drift Isopach Map, Ts'ude niline Tu'eyeta Candidate Protected area, Northwest Territories

This publication presents a model of interpolated and minimum estimates of drift thickness overlying bedrock of the Ts'ude niline Tu'eyeta Candidate Protected Area. Drift refers to all unconsolidated earth materials, and largely comprises glacial sediments of Late Wisconsinan age (25,000 - 10,000). Understanding the extent and thickness of the regional drift cover is important to studies of landslides, hydrogeology, permafrost and massive ground ice distribution, granular aggregate assessments, and drift geochemical exploration. Data used in the drift isopach model is principally derived from 30,006 seismic shothole drillers' logs (Smith et al., 2007; Smith and Lesk-Winfield, in press). These lithostratigraphic logs were created during geotechnical seismic exploration when holes were drilled to set explosive charges. A secondary source of data, 1216 data points from the borehole geotechnical database of Smith et al. (2005) were also included, as were 52 points from Janicki's (2005) formation top well database. Additionally, 35,277 points were interpolated from published surficial geology maps of Duk-Rodkin (1992, 2002a, b; 2005) and Duk-Rodkin and Hughes (1992a-h, 1993a, b). Based on their respective map legends, polygons of till and colluvial veneer were assigned maximum drift thicknesses of 2 or 3 m, while rock polygons were assigned drift thicknesses of 0 m.The Ts'ude niline Tu'eyeta Candidate Protected Area is characterized by prominent northwest - southeast oriented bands of thick (>14 m) drift oriented parallel or oblique to the Mackenzie River, interspersed with areas of thin drift, particularly in the southern regions where it rises towards the Mackenzie Mountains. In the northern part of the candidate protected area, drift appears to infill a valley that has a geometry suggesting it was a tributary channel of a previously connected preglacial Ontaratue and Ramparts river, that drained southeastward through a valley into a preglacial Mackenzie River. The southeastward and then abruptly northeast flow path of the modern Ontaratue River likely reflects a diversion along an eastward retreating ice margin during deglaciation. Thick packages of drift in the Fort Good Hope area may relate to the infill of former tributary channels or drainage paths of a preglacial Mackenzie River, and are comprised of both till and glaciofluvial sediments. Thick drift cover north of the candidate protected area is largely till (Smith and Lesk-Winfield, in press). Thick deposits along the west and southwest borders of the candidate protected area are considered to be related to ice marginal deposits emplaced along the mountain front during deglaciation. Areas of thin drift in the candidate protected area are largely comprised of till overlying shale bedrock. The abundance of small lakes, ponds, and wetlands in the central Ontaratue and lower Ramparts river areas reflect the impermeability of the clay and fine-silt rich till that underlies this terrain, and the shale bedrock that underlies the drift material. Surface ponding is also likely a reflection of the extensive discontinuous and intermediate discontinuous permafrost and associated high (>15%) to moderate (10-15%) ice contents that characterize the area (Heginbottom, 2000; Smith et al., 2007). As the thickness of permafrost in the area is poorly constrained, it is uncertain whether the thick valley fills operate as sub-surface aquifers, draining into the Mackenzie River. Isolated areas of thick drift east of Fort Good Hope appear to have significant granular aggregate potential (Duk-Rodkin, 1992a; Smith and Lesk-Winfield, 2009). Most of the east, north, and central terrain of the candidate protected area offer little granular aggregate potential; that which does exist is largely comprised of sand-rich deposits (Smith and Lesk-Winfield, 2009). Large, surface gravel, and mixed gravel and sand deposits are found along the lower Gayna and Mountain rivers, both within and outside the proposed boundaries of the candidate protected area. These deposits are associated with a series of glaciofluvial outwash terraces (Duk-Rodkin and Hughes, 1993a), and range in thickness between 5 and 10+ m (Smith and Lesk-Winfield, 2009).

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