Geological Survey Professional Paper 669 The Colorado River Region and John Wesley Powell

During the years immediately preceding the establishment of the U.S. Geological Survey, and for several decades thereafter, extensive studies were made of the stratified rocks of Grand Canyon by John Wesley Powell and a small group of his Washington, D.C., colleagues. Especially prominent in this connection are the names of G. K. Gilbert, A. R. Marvine, C. E. Dutton, and C. D. Walcott. Gilbert and Marvine actually did their early work while serving as geologists on the Wheeler Survey, but those studies were continued under auspices of the U.S. Geological Survey.

Between 1870 and 1890, contributions to an understanding of the stratified rocks in Grand Canyon were largely in the form of information on distribution, thickness, and lithology of various units, the collection and study of fossils from several formations, refinement of rock subdivisions and age assignments and the application of local names for many of the rock units. Still lacking were any notable advances in the interpretation of depositional environments and in solving problems of genesis.

The first formal names to be given Grand Canyon rock units were the Tonto group, Redwall group, and Aubrey group, applied by G. K. Gilbert (1874; 1875a, p. 184) and later illustrated in graphic section (Marvine, 1875, fig. 82) as shown in figure 5 of the present report. According to Gilbert (1875a, p. 177), "it was found convenient by Mr. Marvine and myself, to attach local names to the more important subdivisions [on the Colorado Plateau]." A short time later, Powell (1876, p. 60) presented a similar but more complete list of rock units, as follows: Upper Aubrey Group, Lower Aubrey Group, Redwall Group, Tonto Group, Grand cañon Group, Grand cañon Schists. Still later, rocks of Devonian age were recognized by Walcott (1883, p. 438) and named Temple Butte Limestone (Walcott, 1890, p. 50), as shown in his charts reproduced as figures 6 and 7 of this publication. Meanwhile, rocks of late Precambrian age belonging to Powell's Grand cañon Group were studied in detail by Walcott (1883, p. 440; 1890, p. 50; 1894, p. 503; 1895, p. 329), who, after various revisions in classification and terminology, ultimately proposed (fig. 8) the adoption of Grand Canyon Series for the entire sequence and Unkar and Chuar for lower and upper subdivisions, called terranes by him (currently classified as groups).

FIGURE 5.—Stratigraphic section at the mouth of the Grand Canyon. From G. K. Gilbert (in Marvine, 1875, fig. 82). (click on image for an enlargement in a new window)

Age assignments for most of the Grand Canyon formations continued to fluctuate during this period as one worker after another obtained new fossil evidence or reinterpreted old correlations. Not until many years later did the geologic periods that were determined for various rock units finally become established, and not until relatively recent times have reliable data permitted assignment of rocks to smaller time units (geologic epochs) that could be accepted with confidence. For example, the Aubrey Limestone that had been considered Permian by Marcou and Carboniferous by Newberry, was classed as Permo-Carboniferous by Gilbert (1875a, p. 177) and by Marvine (1875, p. 213). Then, however, a detailed study of its fauna caused C. A. White (in Powell, 1876, p. 80) to state "* * * I therefore regard it as not improbable that the time of the Permian period may be represented in the Plateau Province by the Upper Aubrey Group * * *"; the age assignment, therefore, was changed back to the Permian where it remains today.

Other age assignments that changed repeatedly were those of the Tonto Group and the Grand Canyon Series. The Tonto was changed from Devonian ("Old Red") to Cambrian ("Potsdam"), to Primordial Silurian, to Carboniferous, and finally back to Cambrian, where it has remained since the middle 1880's. Assignment of its stratigraphic position within the Cambrian, however, continued to be changed until recent years, as shown in McKee and Resser (1945, p. 12), reproduced as figure 9 of this publication. Stratified rocks of the Precambrian Grand Canyon Series, first recorded by Powell (1875, p. 212) but not assigned an age, were later variously considered as Silurian (Powell, 1876; Dutton, 1882), Lower Cambrian (Walcott, 1883), and finally Precambrian (Walcott, 1886, p. 41). Today they are established as late Precambrian.

The most detailed stratigraphic work done in Grand Canyon during the last part of the 19th century was by Walcott. Not only did he measure numerous sections of various formations and make systematic collections of fossils that he later (1897-1925) described in 15 papers, but he also made many suggestions concerning paleogeography, sedimentology, and paleoecology. Among his most significant contributions to sedimentary stratigraphy are:

1. (Walcott, 1880). A description of the channeling and extensive erosion surface formed on Cambrian limestones before deposition of the Devonian.

2. (Walcott, 1883). An interpretation of the environment of deposition during late stages in developing the great pre-Paleozoic unconformity and a description of the buried knolls and ridges of Precambrian rocks projecting into the basal Cambrian strata that illustrate "the sea breaking off and burying with drifting sand, fragments of the rocky islands" (p. 439).

3. (Walcott, 1890). A detailed study of the fault movement that occurred after cessation of Precambrian sedimentation and before Cambrian sedimentation along the line referred to as the East Kaibab displacement. He pointed out that the downthrown side was to the west, rather than to the east, as in later movements, and that the movement in Precambrian time ranged from 400 to 4,000 feet.

4. (Walcott, 1894). A record of the volcanic history of late Precambrian time. True fissure eruptions were shown to be represented in the Unkar Group by dikes and by a series of lava flows interbedded with red sandstones. The dikes had been first noted by Powell (1875, p. 81) on his river trips.

5. (Walcott, 1895, p. 329). Recognition that the Grand Canyon Series, with its long history of "orographic movement and subsequent erosion" after deposition, must have formed in Precambrian time, as these events would have exceeded the time interval recognized as necessary for Lower Cambrian sedimentation in other regions.

6. (Walcott, 1895). The first detailed stratigraphic sections of upper Precambrian rocks in the Grand Canyon and measurements of the succession totalling 12,000 feet.