Depositional history of the southeastern outcrop belt of the Chuckanut Formation: implications for the Darrington – Devil's Mountain and Straight Creek fault zones, Washington (U.S.A.)

Abstract

The southeastern outcrop belt of the Eocene Chuckanut Formation contains the erosional remnants of a larger depositional system. In the study area, the Chuckanut Formation can be split into four units based upon differences in age, lithology, sedimentology, paleocurrents, and provenance relationships. The Coal Mountain unit (Early Eocene) represents a southwest-flowing fluvial system that shows no evidence for fault control of drainage. The overlying Higgins Mountain unit (early Middle Eocene) represents a northeast-flowing fluvial system east of the Devil's Mountain fault zone (DMFZ), with lithologies derived from western source areas. The Sperry Peak unit (early Middle Eocene) represents a fluvial system with a wide dispersion of paleocurrent azimuths and a possible mixture of sediment source areas. We believe the Sperry Peak unit was deposited in a fault-wedge graben at the junction of the DMFZ and Straight Creek fault zone (SCFZ), with sediment sources from both east and west. The Grade Creek unit (age unknown) is a fluvial unit found along the trace of the SCFZ with paleo-flow subparallel to the fault zone. The sedimentary evidence suggests that onset of Paleogene faulting on the DMFZ and SCFZ was at about 50–48 Ma, while movement on the Darrington fault zone may have been later (post-42 Ma).

The Basin and Range Province as a Composite Extensional Domain

“…Creek fault system of northwestern Washington and southwestern British Columbia during the interval 54-36 Ma (Evans, 1994;Evans and Ristow, 1994) was apparently coeval with clockwise (dextral) rotation of the Blue Mountains. The fault trace was locally obliterated at the international border by intrusion of the Late Oligocene (29-26 Ma) Chilliwack batholith (Richards and McTaggart, 1976), which places a temporal lid on the timing of strike slip.…”

Reply [to “Comment on ’Depositional history of the Eocene Chumstick Formation: Implications of tectonic partitioning for the history of the Leavenworth and Entiat‐Eagle Creek fault systems, Washington’”]

“…Johnson disagrees with this, and appears in this critique to be commenting on the paper by Evans and Ristow [1994]. For the benefit of those readers that have not read Evans and Ristow [1994], the argument is based on the evidence that the oldest sedimentary units in each basin have the following similarities: (1) they were avulsion-dominated, mixed-load fluvial depositional systems;…”

“…For the Pacific Northwest, the recognition of a episode of extension prior to 48 Ma, followed by the onset of strike-slip faulting commencing sometime after 48 Ma, has implications for the evolution of depositional systems. It has been proposed that there was an interval of early Eocene "regional" drainages, characterized by numerous fluvial systems draining source areas to the northeast [Evans, 1991;Evans and Ristow, 1994]. Johnson disagrees with this, and appears in this critique to be commenting on the paper by Evans and Ristow [1994].…”

“…It has been proposed that there was an interval of early Eocene "regional" drainages, characterized by numerous fluvial systems draining source areas to the northeast [Evans, 1991;Evans and Ristow, 1994]. Johnson disagrees with this, and appears in this critique to be commenting on the paper by Evans and Ristow [1994]. For the benefit of those readers that have not read Evans and Ristow [1994], the argument is based on the evidence that the oldest sedimentary units in each basin have the following similarities: (1) they were avulsion-dominated, mixed-load fluvial depositional systems;…”

“…(2) they had relatively high sediment accumulation rates; (3) each basin was characterized as "open" drainages, with west-southwest directed paleoflow crossing each basin (including the linking of the Chumstick and Swauk depositional systems); and (4) these units had broad similarities in sandstone petrofacies [Frizzell, 1979;Hartman, 1973] Finally, it should be observed that there are stratigraphic trends in sandstone compositions, as seen in the Clark Canyon, Tumwater Mountain, and Nahahum Canyon members of the Chumstick Formation [Evans, 1994], in "type !, 2, and 3" petrofacies from the northwestern outcrop belt of the Chuckanut Formation [Johnson, 1984], and in the Coal Mountain, Sperry Peak, and Higgins Mountain units from the southeastern outcrop belt of the Chuckanut Formation [Evans and Ristow, 1994]. Compositional changes in each basin form a stratigraphic trend toward the recycledorogen petrofacies field.…”