Quantitative bedrock geology of Alaska and Canada

We quantitatively analyze the area-age distribution of sedimentary, igneous, and metamorphic bedrock on the basis of data from the most recent geologic maps of Alaska [Beikman, 1980] (1: 2,500,000) and Canada [Douglas, 1969] ( updated; 1: 5,000,000), made available in digital form by the U. S. Geologic Survey and the Geologic Survey of Canada ( National Resources Canada), respectively. Sediments cover 72.9% and 52.4% of the surface of Alaska and Canada. Volcanic rocks comprise 11.6% and 6% of the surface area in Alaska and Canada, respectively, whereas intrusive rocks cover 6.6% and 24% of the surface, respectively. Ultramafic rocks account for 0.20% and 0.08% of the bedrock area, whereas metamorphic rocks cover 3.4% and 16.1%. The average ages of major lithologic units, weighted according to bedrock area, are significantly younger in Alaska than in Canada: marine sediments (stratigraphic age of 206 Myr in Alaska versus 599 Myr in Canada), continental sediments (stratigraphic age of 81 Myr versus 150 Myr), volcanic rocks (126 Myr versus 1377 Myr), intrusive rocks (114 Myr versus 2109 Myr), ultramafic rocks (261 Myr versus 874 Myr), and metamorphic rocks (474 Myr versus 2545 Myr). This age difference reflects the contrast between the young active margin influence on the geology of Alaska and the old cratonic nature of much of the bedrock exposed in Canada. Glacial erosion, and the resulting exposure of Precambrian bedrock, primarily in Canada, contributes to this age contrast. The average temporal and spatial resolution of the digital data is sufficiently high to perform age-area analyses on individual river basins larger than similar to20,000 km(2) and to evaluate the relationship between bedrock geology and river chemistry.