Depositional history of the Eocene Chumstick Formation: Implications of tectonic partitioning for the history of the Leavenworth and Entiat‐Eagle Creek fault systems, Washington

Abstract

The Chumstick basin opened as an extensional half‐graben prior to 51 Ma, and was subsequently modified by two episodes of tectonic partitioning of drainage prior to basin deformation. Initially, westward flowing fluvial systems formed a unified depositional system with the Swauk basin. Tectonic partitioning of drainage at 51–49 Ma and at 44–42 Ma was controlled by localized uplift on the Leavenworth (LFZ), Eagle Creek (ECFZ), and Entiat (EFZ) fault zones and led in each instance to the truncation of regional depositional systems, modification and reversal of paleoflow, and internal drainage. Relief on the LFZ at 51–49 Ma may be the result of isostatic uplift of the extensional footwall, producing the Swauk and Chumstick basins as a pair of west facing half grabens. The earliest convincing evidence for the onset of oblique slip in the region is at about 48 Ma (folding in the Swauk basin) or about 44–42 Ma (probable transpressive uplift at left‐stepping bends of the LFZ, development of a transtensional step‐over basin between the ECFZ and EFZ, horsetail splays in the ECFZ, and possible flower structures in the LFZ and ECFZ in the Chumstick basin). Each episode of tectonic partitioning was followed by proximal onlap and overtopping of fault zones, to reestablish regional flow systems. The Chumstick Formation was deformed by dextral transpression between 37–34 Ma, and is unconformably overlain by the Oligocene Wenatchee Formation. The Chumstick basin is an example of an extensional basin modified by subsequent strike‐slip tectonics, thus caution should be used in applying idealized basin models.

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

“…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. Despite the length of their discussion on this point, it is not really controversial in any way.…”

“…Litho facies are based upon depositional processes, and do not necessarily obey stratigraphic principles (for example, lithofacies are frequently repeated in a vertical sequence), with the exception of instances where stratigraphic units are composed of a single lithofacies or unique and nonrepeated collection of lithofacies. In this case, however, their approach does not work because each of the four members of the Chumstick Formation is a heterolithic combination of conglomerate, sandstone, and shale of different ages that represent complex fl uvial and/or lacustrine depositional environments (Evans, 1991a(Evans, , 1994(Evans, , 2000. The implications are very signifi cant-for example, careful stratigraphic work using marker beds, such as paleosols, tuffs (Evans, 1991a), and turbidites (Evans, 2000), or using lateral trends in paleobotanical assemblages (Evans, 1991b) can be used to show both vertical and lateral ( proximal-distal) relationships within each of these four stratigraphic units (Evans, 1994).…”

“…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. Later, I proposed subdividing the lower part of the Chumstick Formation into the Clark Canyon, Tumwater Mountain, and Deadhorse Canyon members (Evans, 1994). These are mappable units (Fig.…”

“…These are mappable units (Fig. 1), and each of the four members of the Chumstick Formation has a defi ned type section and published descriptions of lithology, age, and stratigraphic relationships, based on 4225 m of measured section at 27 local ities (Evans, 1994).…”

“…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.…”