Extension of the telluride erosion surface to Washington State, and its regional and tectonic significance

Abstract

Abstract The Wenatchee Formation of Oligocene age (34 m.y.) rests on a profound unconformity that developed during an extended period of erosion and tectonic quiescence from perhaps 40–34 m.y. B.P. Its occurrence strongly suggests that a stripped erosion surface in the northern Cascade Range, previously considered of Miocene age, is of Late Eocene—early Oligocene age and that it is part of a more widespread surface in the Pacific North-west. The Late Eocene—early Oligocene unconformity/erosion surface is apparently of continental extent, from British Columbia to Mexico. Where it is incised on crystalline basement rocks, it now may be exhumed and exposed as a plateau or level-crested divides of concordant elevation. Where it is developed on sedimentary rocks, it may be found as a profound unconformity. Widespread eruptions of pyroclastic volcanic rocks covered much of the surface in Oligocene time and tended to protect and preserve it. The surface is particularly well documented in the San Juan volcanic field of Colorado. There it was given the name “Telluride peneplain”, which is here broadened to the Telluride erosion surface of continental extent. A magmatic gap indicating a period of magmatic quiescence is generally associated with tectonic quiescence implied by the erosion surface, but the timing varies regionally. Development of the Telluride surface and associated magmatic gaps correlates with general plate reorganization during the Eocene. An erosion surface correlates with a global tectonic transition because erosion rates immediately dominate uplift rates when compressive deformation is slowed or shut off. An analysis of the dynamic balance between erosion rates and plate convergence rates suggests that erosion may maintain a continental surface of low relief if the relative convergence rate falls to about 0.5 cm/yr or less. Gaps in magmatic activity do not correlate as precisely with the 40 m.y. tectonic transition, in part because of the possibility of time lags. A great deal of scattered geologic evidence suggests a major global event at about 40 m.y. BP. The timing does not always precisely correlate with the timing of events in western North America, which suggests that the 40 m.y. tectonic transition was spread over several millions of years: A most impressive global impact of the 40 m.y. transition is the largest drop in eustatic sealevel at about 30 m.y. BP. The 40 m.y. tectonic transition profoundly altered geologic history, including the development of continental landscapes, global sea-level, and global climate. Classical stratigraphic studies remain the best way of documenting its effect on the continental surface. The small remnant of Oligocene rocks in central Washington, overlooked in early reconnaissance studies, is of great geologic significance. Other undiscovered remnants probably exist in western North America.