Comparisons of the 2005 Geologic Map of North America with the 1965 Map, Areas 5-7
7. High Plains, Laramide Rocky Mountains, Colorado Plateau and parts of the Basin and Range Province
In spite of more than three decades of geologic mapping and detailed studies, parts of the map in this view changed very little from the 1965 map. Understanding of the geology of this region has greatly improved, but the new map information when generalized to this scale results in surprisingly few changes. Locally the positions of some contacts have been slightly adjusted, but the general distribution and age assignments of most of the geologic units are little changed. The most significant changes are:
- Subdivision of Precambrian rocks in the cores of the Laramide uplifts, shown on the 1965 map simply as Precambrian, to reflect the results of extensive radiometric dating. The basement rocks in the uplifts in Wyoming (such as the Laramie Range, A) are now known to be largely Archean (2.5 billion years and older), while those to the south in Arizona, New Mexico, and Colorado (such as the Front Range, B) are all Proterozoic (1.8 to 1 billion years or younger). These two Precambrian provinces are juxtaposed along the Cheyenne Belt (C), a major suture zone in southern Wyoming that marks the southern edge of the continent during the Archean. The Archean rocks are subdivided into Late Archean granititic rocks and Late Archean gneiss; the Proterozoic rocks are chiefly Early Proterozoic metamorphic rocks and granites of three different ages, Early Proterozoic 1.8-1.6 billion years, Middle Proterozoic (about 1.4 billion years) and Middle Proterozoic (about 1.0 billion years). The Uinta Mountains (D) are cored by a great thickness of little-metamorphosed sedimentary rocks of Middle and Late Proterozoic age.
- Subdivision of the Tertiary rocks in the volcanic fields surrounding the Colorado Plateau, including the San Juan (E), Mogollon-Datil (F), and Marysville (G) fields, on the basis of age and lithology. All of these rocks were simply shown as "Tertiary volcanic rocks" on the 1965 map. Calderas are not shown on the 1965 map, but have been added on the new map.
Return to comparisons of map areas 5-7.
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