| Popis: |
The rocks around Boulder Dam, which range from pre-Cambrian to Recent in age, are dominantly volcanic, but other rock types occur. Pre-Cambrian granitic rocks occur in the Black Mountains, to the east; at Saddle Ridge is a chloritic schist, also pre-Cambrian. No known Paleozoic or Mesozoic rocks occur within the area. The oldest of the early Tertiary (?) rocks is the Altered Monzonite Porphyry (?), which occupies a belt trending east-west across the area. This formation has been quite thoroughly hydrothermally altered. Intrusive into it is the Quartz-Monzonite, which occurs mainly south of Hemenway Wash. The Older Volcanic Series, the most extensive formation in the area, occurs on both sides of Hemenway Wash, and south of Boulder Dam. It consists of a tilted series of adesite flows and intercalated sedimentary breccias. Between it and the Younger Volcanic Series, in Black Canyon, occurs the sedimentary formation, the Dam Breccia. The Younger Volcanic Series occupies a depressed and tilted block, penetrated by Black Canyon, and rests uncomformably or by fault contact on the Older Volcanic Series, and the Dam Breccia. It consists of four volcanic extrusive and intrusive members, two tuff members, and two inter-volcanic sedimentary breccias. The volcanic members are all generally latitic in composition, being high in potassa and in total alkalies. These volcanic rocks are differentiation products of one primary magma. The Fortification Hill Basalt and the conformable underlying Muddy Creek Formation are Pliocene, or possible Pleistocene, in age. Quaternary deposits include terrace gravels of the Colorado River, and recent alluvium. |
