The Chiwaukum Structural Low: Cenozoic shortening of the central Cascade Range, Washington State, USA

Abstract

The central Cascade Range of Washington State has become a testing ground for theories surrounding the exhumation of deep-seated arcs, generation of both arc and fl ood-basalt volcanism, strike-slip faulting that translated crustal blocks along the Cordilleran margin, and development of fault-bounded basins. A central existing hypothesis is that the region underwent either regional extension or transtension during the Eocene. Both the extension and transtension models derive from the interpretation that clastic Eocene formations were deposited syntectonically in local basins. Geologic mapping and structural analyses presented here support an alternative hypothesis: that these formations are preserved in regional synclines, not in separate depositional basins. The type area for the Eocene history is the so-called Chiwaukum graben or Chumstick basin, here renamed the Chiwaukum Structural Low. The southwestern boundary of the Chiwaukum Structural Low includes post-depositional, northwest-striking reverse faults with adjacent northwest-striking folds. The reverse faults place the regionally extensive and arkosic, early Eocene Swauk Formation over arkosic, mid-Eocene strata that have previously been called the Chumstick Formation. Cataclastic structures provide independent evidence for the reverse faulting. Elsewhere, 39-42 Ma rocks unconformably overlie the folds. The reverse faults and fold hinges are cut by northerly striking strike-slip faults, which likely are of late Eocene age. The Eocene folds and faults were reactivated by deformation of the Miocene Columbia River Basalt Group, the gentler folding of which largely defi nes the regional map pattern around the Chiwaukum Structural Low. Instead of an extensional or transtensional history, the Eocene-to-Recent history of the Central Cascade region is characterized by multiple periods of folding and reverse faulting alternating with periods of strike-slip faulting.

The Chiwaukum Structural Low: Cenozoic shortening of the central Cascade Range, Washington State, USA: Comment

“…It is surprising, therefore, that in a paper focused on addressing this very point, Cheney and Hayman (2009) neglected to describe the existing stratigraphic framework. Both Gresens et al (1981) and Tabor et al (1987) subdivided the Chumstick Formation into an undivided lower part and an upper part called the Nahahum Canyon Member.…”

“…Despite the length of their discussion on this point, it is not really controversial in any way. Indeed, Cheney and Hayman (2009) should have acknowledged that their three, post-Chumstick Formation faults have been mapped by previous workers; for example, they are shown in fi gures 4-11 of Evans (1988) and fi gure 3 of Evans (1994) although not given names. Cheney and Hayman (2009) should have also acknowledged that the concept that folds in the Miocene Columbia River Basalt Group are fault-propagation structures was originally proposed by Plescia and Golombek (1986) and Reidel (1984) and that this information was used previously to interpret the history of the Eagle Creek fault zone (Evans, 1988(Evans, , 1994.…”

“…Their study provides valuable detailed geologic mapping, interpreted cross sections, and alternative views that will promote a lively discussion for years to come. Unfortunately, I fi nd myself misrepresented in both areas of agreement and disagreement with Cheney and Hayman (2009), and I wish to comment on these areas. Cheney and Hayman (2009) agree with previous workers (Gresens et al, 1981;Tabor et al, 1982Tabor et al, , 1984Tabor et al, , 1987Johnson, 1985;Evans 1988Evans , 1994Taylor et al 1988;Evans and Johnson, 1989) that the geographic region of the Chumstick basin was affected by postChumstick Formation (Late Eocene) strikeslip faulting and folding.…”

“…The controversial part of Cheney and Hayman (2009) is their assertion that the deposition of the Chumstick Formation was unaffected by syndepositional faulting. This is a worthy subject of investigation, and their use of geologic mapping is an appropriate tool.…”

Field-trip guide to subaqueous volcaniclastic facies in the Ancestral Cascades arc in southern Washington State—The Ohanapecosh Formation and Wildcat Creek beds

“…From 57 to 43 Ma (Cheney and Hayman, 2009), the faults promoted the formation of separate basins (Chuckanut Basin, PugetNaches Basin, Chiwaukum Graben, and Swauk Basin) that have distinct sedimentary and deformation histories (Johnson, 1984(Johnson, , 1985. The basins ( fig.…”