EVIDENCE FOR A 5 MA EROSION SURFACE AND THE SUBSEQUENT RISE OF THE ANTICLINAL CASCADE RANGE, WASHINGTON

On the eastern flank of the central Cascade Range in WA, the 10 to 12 Ma volcanoclastic Ellensburg Formation unconformably overlies the Columbia River Basalt Group (CRBG). Unlike the present flora, the flora in the Ellensburg Formation do not indicate a rain shadow (Mustoe and Leopold, 2015); so, the topography of the range is < 10 Ma. The Cascade Range is a SSW plunging anticlinorium, defined, in part, by dips in CRBG. The Puget Lowland to the west is a synclinorium, defined, in part, by strata with ages < the Ellensburg Formation, including a thick Pleistocene section.

East and south of Mount Rainier (MR), areas with nearly concordant summits in the CRRG and older rocks imply the presence of a dissected 5 Ma erosion surface. The maximum altitudes of > 5 Ma rocks are 2368 m at Goat Rocks and 2358 m at MR. This surface is partially obscured by the following overlying units: (1) the 0.6 to 4.2 Ma basaltic rocks of the Simcoe Mountains, which dip < 5 degrees to the east, (2) the 0.6 to 3.0 Ma andesites of Goat Rocks, and (3) the < 1.0 Ma andesitic (and iconic) edifices of Mi. Adams and MR. In the Kittitas valley 80 km northeast of MR, the ≈ 4 Ma Thorp Gravel dips ≤ 5 degrees eastward.

By eliminating the peaks underlain by < 5 Ma units from the DEM, and by choosing a distance of > 4 km between the remaining peaks, the resultant second-order polynomial trend surface has a gentle antiformal shape that rends NNF. The crest of this trend surface is at an altitude > 2200 m, and its ESE limb rises 1.4 km in a horizontal distance of 85 km. This surface probaby approximates the folded shape and altitude of the 5 Ma erosion surface subsequent to some growth of the CRA.

Reiners et al. (2002) obtained exhumations ages from MR area and northward; these cluster at 12, 8, and 6 Ma. These ages likely formed during the pre-Ellensburg unconformity, the post-Ellensburg unconformity, the 5 Ma erosion surface, and, possibly, the growth and erosion of the CRA.

Growth of the CRA continues. In the Brightwater tunnel 20 km north pf Seattle, on the eastern limb of the Puget synclinorium, the 70 ka Possession Drift descends westward by 43 m in 11 km (McCormack and Troost, 2016).