University of Washington Publications in Geology The Geology of Mount Rainier National Park

THE PUGET GROUP

The oldest rocks are black argillites which have been so well indurated that it is difficult to scratch them with a knife. These are well exposed on the west bank of the Carbon River near the mouth of Cataract Creek. Individual beds vary in thickness from an inch to more than a foot and alternate with layers of a peculiar light-colored rock which closely resembles small porphyry sills or possibly silicified pyroclastics. This alternation of light and dark colored layers produces a most striking outcrop. A more detailed study of the rocks in this exposure is now being made by Professor G. E. Goodspeed of the University of Washington and the writer, and the results will be published in a subsequent paper.

In the Puget Sound region, immediately to the northwest of the Park, is a two-mile thick sequence of sandstones, shales and arkoses, with a considerable amount of carbonaceous matter and intercalated volcanics. To these beds, White (40) gave the name, Puget Group, and assigned to them a lower Tertiary age. Within 10 miles of the Carbon River locality, at Fairfax, coal has been mined from this sequence. Sediments definitely referable to the Puget Group are exposed 8 miles due west of the Carbon River locality. The carbonaceous argillites of the Carbon River region within the Park boundaries are tentatively assigned to the Puget Group because of their proximity to large, known masses of Puget rock, because of the carbonaceous content which is common to both and because the structural trend of the Carbon River outcrop parallels the general trend of the Puget beds. During the Miocene, the sediments and volcanics of the Puget Group were thrown into a series of folds trending, generally, in a northwest-southeast direction.