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Cover. DGGS Geologist Mandy Willingham on an outcrop of Fortymile River metasediments near the Lake George-Fortymile detachment (photograph taken 07/04/2018 by K. R. Sicard).

Cover.Outcrop of the Nanushuk Formation along the Colville River's north bank at the informally named Colville incision locality. Sandstones of the Nanushuk serve as reservoirs at the Umiat oil field ~20 km to the northeast. Hammer is 31 cm long. Photograph by T.M. Herriott.

This map includes unconsolidated surficial deposits in the Tanacross D-1 Quadrangle, the eastern one-third of the Tanacross D-2 Quadrangle, the northern half of the Tanacross B-2 Quadrangle and the northeastern one-third of the Tanacross B-2 Quadrangle (sheet 1). To create this map we incorporated (1) geologic information from previous geologic mapping and published literature, (2) new interpretations using remotely sensed imagery, and (3) information gathered from limited field investigations. Previous geologic mapping of the unconsolidated deposits in the Tanacross Quadrangle includes work by Foster (1970) at a scale of 1:250,000, which provided a framework for our mapping and was instrumental in understanding the regional distribution and character of Quaternary deposits. Weber (1986) described the glacial history of the Yukon Tanana Upland and gave detailed descriptions of the extent and character of these deposits. Her work along with glacial limits mapped by Péwé and others (1967) helped us identify and map glacial deposits in the map area. Work by Pinney ( 2001) and Stevens and Burns (2010) in the Eagle A-1 and A-2 quadrangles enabled us to understand the distribution and character of deposits in adjoining areas to the north and enhanced our understanding of the morphology and distribution of alluvial terraces, many of which are gold bearing and extend southward into this map area. Mapping at a scale of 1:50,000 in the adjoining Borden Creek and Crag Mountain quadrangles by Jackson (2005aJackson ( , 2005b allowed comparisons with the distribution and character of deposits in Canada. Remotely Sensed ImagerySurficial geology was mapped by interpreting 2.5-m-resolution SPOT 5 color-infrared and natural color red-green-blue (RGB) imagery (©2013, Distribution SpotImage S.A., SICORP, USA, all rights reserved) collected in 2009 and 2010 and stereoscopic pairs of approximately 1:65,000-scale, false-color, infrared aerial photographs taken in 1978 and 1981. SPOT 5 imagery was interpreted in ArcGIS Pro by overlaying it on hillshade and slopeshade images derived from Interferometric Synthetic Aperture Radar (IFSAR) bare-earth digital-elevation models (DEMs) created using data collected in 2010. By using the pseudo-3-D functionality of ArcGIS Pro and adjusting the transparency of the SPOT 5 imagery and the transparency and color of hillshade and slopeshade images, we were able to visualize subtle differences in features and identify geologic landforms. We used aerial photographs to identify geologic features and check our interpretations. Geospatial polygon features that represent geologic units were drawn using onscreen digitizing techniques in ArcGIS Pro. Field WorkWe conducted helicopter-supported surficial geology field work June 20-23, 2017, and July 3-4, 2018. During the field campaign we visited exposures and dug soil pits to examine material and gather information for unit descriptions and to check geologic mapping. We also measured the thickness of the active layer at several locations.

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