THE rocks of Grand Canyon which have been assigned to the Algonkian age appear in many scattered localities over a distance of about 42 miles from east to west along the course of the Colorado River, and in several places they extend northward for some miles along the canyons of tributary streams. The lower half of the Algonkian rocks, known as the Unkar group, has been studied in detail in two of the four main exposures. Dr. C. D. Walcott1 measured the section in the eastern end of Grand Canyon in 1895 and Dr. L.F. Noble2 did likewise in the Shinumo Canyon some years later. The excellent exposure along Bright Angel Creek, however located nearly midway between these two, has until recently3 been neglected as far as detailed work is concerned, although it is today one of the most readily accessible and most frequently visited areas in the Grand Canyon. Another large exposure is at Tapeats Crock, and minor ones are at Vishnu, Clear and Crystal Creeks and several other places.
Because of the lack of fossils which can be used for correlation of the Algonkian beds, the changes in character of individual members must be traced by means of detailed sections measured at places as near each other as outcroppings permit. The Algonkian rocks of the Bright Angel Creek section show a similarity in structure to those of most other exposures in the Grand Canyon in that they dip in a general northeasterly direction, the result of one of a series of more or less parallel faults running in a north-south direction. In brief, the mountains which existed in this region during Algonkian time must have developed from a series of fault blocks and formed a surface not unlike that of our basin-ranges of today. (See illustration No. 1.) The major faults are crossed by minor ones in many places but the direction of dip is to the east or northeast in most areas so far examined.
That the course of Bright Angel Canyon was determined by a Cenozoic fault following closely the line of a great Pre-Cambrian fault was recognized by Ransome4 and later described in detail by the writer.5 A similar situation was noted in the eastern end of Grand Canyon by Walcott6. The Pre-Cambrian faults in these areas and in others, all nearly parallel, dropped the soft basal material of the Algonkian (Hakatai shale and Bass limestone) down below the level of the Archean peneplain so that they were protected under the base level of erosion, and thus the Algonkian wedges of Grand Canyon are accounted for. Interestingly enough, wherever these blocks were dropped sufficiently to leave the base of the hard cliff-forming Shinumo quartzite at or near the peneplain surface, that rock because of its resistant nature afterwards formed an island in the Cambrian sea. The relationship between the Algonkian monadnocks and the hardness of the Shinumo quartzite is a consistent feature in Grand Canyon, as shown in the accompanying block diagram.
1. Walcott, C. D., Algonkian Rocks of Grand Canyon of the Colorado, The Journal of Geology, Volume III No. 3, 1895.
2. Noble, L. F., The Shinumo Quadrangle, U.S.G.S. Bull. 549, 1914.
3. A measured section was made by the writer in 1932 and detailed work is being done there at present by Dr. N.E.A. Hinds and associates working under a grant from the Carnegie Institution.
4. Ransome, F. L., Pre-Cambrian Sediments and Faults in the Grand Canyon of the Colorado, Science, Vol. XXVII, No. 695, 1906.
5. McKee, E. D. The origin of Bright Angel Canyon, G.C. Nature Notes, Vol. 6, No. 2, Dec. 1931.
6. Walcott, C. D. Study of a Line of Displacement in the Grand Canyon of the Colorado. Bull. Geol. Soc. Am., Vol. 1, 1889.
|<<< Previous||> Cover <||Next >>>|